U.S. patent application number 16/887223 was filed with the patent office on 2020-11-19 for tubular tissue construct and a method of printing.
This patent application is currently assigned to President and Fellows of Harvard College. The applicant listed for this patent is President and Fellows of Harvard College. Invention is credited to Kimberly A. Homan, David B. Kolesky, Jennifer A. Lewis, Mark A. Skylar-Scott, Ryan L. Truby.
Application Number | 20200360567 16/887223 |
Document ID | / |
Family ID | 1000004993382 |
Filed Date | 2020-11-19 |
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United States Patent
Application |
20200360567 |
Kind Code |
A1 |
Lewis; Jennifer A. ; et
al. |
November 19, 2020 |
TUBULAR TISSUE CONSTRUCT AND A METHOD OF PRINTING
Abstract
A 3D printed tubular construct, such as a nephron, with or
without embedded vasculature as well as methods of printing tubular
tissue constructs are described.
Inventors: |
Lewis; Jennifer A.;
(Cambridge, MA) ; Homan; Kimberly A.; (Somerville,
MA) ; Kolesky; David B.; (Cambridge, MA) ;
Truby; Ryan L.; (Boston, MA) ; Skylar-Scott; Mark
A.; (Brookline, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
President and Fellows of Harvard College |
Cambridge |
MA |
US |
|
|
Assignee: |
President and Fellows of Harvard
College
Cambridge
MA
|
Family ID: |
1000004993382 |
Appl. No.: |
16/887223 |
Filed: |
May 29, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15567570 |
Oct 18, 2017 |
10702630 |
|
|
PCT/US2016/030710 |
May 4, 2016 |
|
|
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16887223 |
|
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62157286 |
May 5, 2015 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61L 27/56 20130101;
A61L 27/3878 20130101; A61L 27/50 20130101; A61L 2430/32 20130101;
A61L 27/3834 20130101; A61L 2300/64 20130101; B29C 64/106 20170801;
B33Y 80/00 20141201; A61L 27/3633 20130101; A61L 27/3675 20130101;
A61L 2400/08 20130101; A61F 2/04 20130101; A61L 27/3808 20130101;
B33Y 10/00 20141201; A61L 2300/62 20130101; A61L 27/18 20130101;
A61L 2430/26 20130101; A61F 2240/002 20130101; A61L 27/3804
20130101; A61L 27/54 20130101; A61L 27/3891 20130101 |
International
Class: |
A61L 27/38 20060101
A61L027/38; A61L 27/56 20060101 A61L027/56; A61L 27/36 20060101
A61L027/36; A61L 27/50 20060101 A61L027/50; A61L 27/54 20060101
A61L027/54; B33Y 10/00 20060101 B33Y010/00; B33Y 80/00 20060101
B33Y080/00; B29C 64/106 20060101 B29C064/106; A61L 27/18 20060101
A61L027/18 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0003] This invention was made with Government support under
contract number CMMI-1548261 awarded by the National Science
Foundation EAGER. The Government has certain rights in this
invention.
Claims
1.-58. (canceled)
59. A 3D printed nephron comprising: one or more functional
channels comprising a patterned cell layer thereon along the length
of the functional channel, wherein the patterned cell layer
comprises renal proximal tubule cells over a first predetermined
length of the one or more functional channels, loop of Henle cells
over a second predetermined length of the one or more functional
channels, and renal distal tubule cells over a third predetermined
length of the one or more functional channels; and an extracellular
matrix composition at least partially surrounding the one or more
functional channels.
60. The 3D printed nephron of claim 59, further comprising one or
more tissue patterns, each tissue pattern comprising a plurality of
viable cells of one or more predetermined cell types, wherein the
extracellular matrix composition at least partially surrounds the
one or more tissue patterns.
61. The 3D printed nephron of claim 59, further comprising an
interpenetrating network of tubes in the extracellular matrix
composition.
62. The 3D printed nephron of claim 61, wherein the
interpenetrating network of tubes comprises a vascular pattern
interpenetrating the functional channels.
63.-93. (canceled)
94. The 3D printed nephron of claim 59, wherein the extracellular
matrix composition comprises one or more of gelatin, fibrin,
gelatin methacrylate, collagen I, collagen III, collagen IV,
fibrinogen, matrigel, laminin, carbopol, N-isopropylacrylamide,
polyethylene glycol, gelatin methacrylate,
polyhydroxyethylmethacrylate, silk, hyaluronic acid, proteoglycans,
and/or combinations thereof.
Description
RELATED APPLICATIONS
[0001] The present patent document claims the benefit of the filing
date under 35 U.S.C. .sctn. 119(e) of Provisional U.S. Patent
Application Ser. No. 62/157,286, filed May 5, 2015, which is hereby
incorporated by reference.
[0002] All patents, patent applications and publications, and other
literature references cited herein are hereby incorporated by
reference in their entirety. The disclosures of these publications
in their entireties are hereby incorporated by reference into this
application in order to more fully describe the state of the art as
known to those skilled therein as of the date of the invention
described and claimed herein.
BACKGROUND
[0004] The present disclosure is related generally to tissue
engineering and more particularly to fabricating tubular tissue
constructs including embedded vasculature and/or tubules.
[0005] The ability to create three-dimensional (3D) vascularized
tissues on demand could enable scientific and technological
advances in tissue engineering, drug screening, toxicology, 3D
tissue culture, and organ repair. To produce 3D engineered tissue
constructs that mimic natural tissues and, ultimately, organs,
several key components--cells, extracellular matrix (ECM),
epithelium, and vasculature--may need to be assembled in complex
arrangements. Each of these components plays a vital role: cells
are the basic unit of all living systems, ECM provides structural
support, epithelium provides a stromal functional unit, and
vascular networks provide efficient nutrient and waste transport,
temperature regulation, delivery of factors, and long-range
signaling routes. Without perfusable vasculature within a few
hundred microns of each cell, three-dimensional tissues may quickly
develop necrotic regions. The inability to embed vascular networks
in tissue constructs has hindered progress on 3D tissue engineering
for decades.
[0006] The need to produce tubular tissue constructs is applicable
to both embedded vasculature and embedded epithelial tissue. Our
method extends broadly to epithelial networks in the body, such as
nephrons, that include multiple types of cells along their
lengths.
SUMMARY
[0007] Certain embodiments relate to a method of printing a tubular
tissue construct. The method includes depositing one or more
sacrificial filaments on and/or in a substrate to form a functional
channel pattern. Each sacrificial filament comprises a fugitive ink
and a plurality of predetermined types of viable cells, wherein
each predetermined type of viable cells is deposited at a different
predetermined location along a length of the sacrificial filament.
The method also includes the steps of at least partially
surrounding the functional channel pattern with an extracellular
matrix composition and removing the fugitive ink to create one or
more functional channels in the extracellular matrix composition.
At least a portion of each different predetermined type of viable
cells remains at the different predetermined location after removal
of the fugitive ink, thereby forming a tubular tissue construct.
The substrate may be a perfusable chip. The tubular construct may
be exposed to one or more biological agents, a biological agent
gradient, a pressure, and/or an oxygen tension gradient, wherein
the one or more biological agents, the biological agent gradient,
the pressure, and/or the oxygen tension gradient may further direct
development, differentiation, and/or functioning of the tubular
tissue construct. The tubular tissue construct may be a nephron or
the tubule portion of the nephron, wherein the plurality of
predetermined types of viable cells may include renal proximal
tubule cells, loop of Henle cells, renal distal tubule cells,
collecting duct cells, mesangial cells, renal microvascular cells,
renal cell progenitors, pluri or multipotent stem cells, other
endothelial lineage cells, fenestrated glomerular endothelial
cells, induced pluripotent stem cells (iPSCs), and/or iPSCs-derived
patent-specific pro-kidney or individual cell lines present in a
nephron or the tubule portion of the nephron. Alternatively, the
tubular tissue construct may be intestine, milk duct, sweat gland,
colon, esophagus, stomach, airway epithelium, epididymus, urethra,
liver bile duct, pancreatic duct, or lymph. The method may further
include depositing one or more sacrificial filaments on and/or in
the substrate to form an interpenetrating network of tubes, each of
the sacrificial filaments comprising a second fugitive ink, and
removing the second fugitive ink to create the interpenetrating
network of tubes in the tubular tissue construct. The
interpenetrating network of tubes may form vascular channels.
[0008] Certain further embodiments relate to another method of
printing a tubular tissue construct. The method includes depositing
one or more sacrificial filaments on and/or in a substrate to form
a functional channel pattern. Each sacrificial filament comprises a
fugitive ink and a plurality of predetermined types of binding
domains, wherein each predetermined type of binding domain is
deposited at a different predetermined location along a length of
the sacrificial filament and is capable of binding to a
predetermined type of target cell. The method also includes the
steps of at least partially surrounding the functional channel
pattern with an extracellular matrix composition and removing the
fugitive ink to create one or more functional channels in the
extracellular matrix composition. At least a portion of the
different predetermined types of binding domains remains at the
different predetermined locations after removal of the fugitive
ink. The method also includes a step of injecting a suspension
comprising at least one predetermined type of target cells into the
functional channel, wherein the target cells bind to corresponding
predetermined types of binding domains, thereby forming a tubular
tissue construct. In certain embodiments, the suspension comprises
multiple predetermined types of target cells. The binding domains
for the target cells may be proteins, e.g., antibodies; DNA; RNA;
aptamers; nanoparticles; bacteria; or combinations thereof. The
substrate may be a perfusable chip. The tubular construct may be
exposed to one or more biological agents, a biological agent
gradient, a pressure, and/or an oxygen tension gradient, wherein
the one or more biological agents, the biological agent gradient,
the pressure, and/or the oxygen tension gradient may further direct
development, differentiation, and/or functioning of the tubular
tissue construct. The tubular tissue construct may be a nephron,
wherein the at least one predetermined type of target cells is
selected from the group consisting of renal proximal tubule cells,
loop of Henle cells, renal distal tubule cells, collecting duct
cells, fenestrated glomerular endothelial cells, induced
pluripotent stem cells (iPSCs), and/or iPSCs-derived
patent-specific pro-kidney or individual cell lines present in a
nephron. In certain embodiments, the tubular tissue construct may
be any other tubular organ, such as intestine, milk duct, sweat
gland, colon, esophagus, stomach, airway epithelium, epididymus,
urethra, liver bile duct, pancreatic duct, or lymph. The at least
partial surrounding of the functional channel pattern with the
extracellular matrix composition may occur during deposition of the
one or more sacrificial filaments, the one or more functional
channel patterns thereby being formed and embedded simultaneously
in the extracellular matrix composition. The method may further
include depositing one or more sacrificial filaments on and/or in
the substrate to form a vascular pattern interpenetrating the
functional channel pattern, each of the sacrificial filaments
comprising a second fugitive ink, and removing the second fugitive
ink to create vascular channels in the extracellular matrix
composition, thereby forming an interpenetrating vascular network
in the tubular tissue construct. The method may further include
injecting a suspension of viable epithelial cells into the one or
more vascular channels.
[0009] Yet further embodiment relates to a printed tubular tissue
construct that includes one or more functional channels comprising
a patterned cell layer thereon along the length of the functional
channel, the patterned cell layer comprising one or more types of
viable cells, each type of viable cells being positioned along a
different predetermined location of the functional channel and an
extracellular matrix composition at least partially surrounding the
one or more functional channels. The patterned cell layer may
include a plurality of viable cells of at least two predetermined
types. The patterned cell layer may include renal proximal tubule
cells, loop of Henle cells, distal tubule cells, collecting duct
cells, fenestrated glomerular endothelial cells, induced
pluripotent stem cells (iPSCs), and/or iPSCs-derived
patent-specific pro-kidney or individual cell lines present in a
nephron, each distributed along a different predetermined location
of the nephron. The tubular tissue construct may be a nephron. In
alternative embodiment, the tubular tissue construct may be any
other tubular organ, such as intestine, milk duct, sweat gland,
colon, esophagus, stomach, airway epithelium, epididymus, urethra,
liver bile duct, pancreatic duct, or lymph. The extracellular
matrix composition at least partially surrounds the one or more
tissue patterns. The printed tubular tissue construct may further
include a network of vascular channels in the extracellular matrix
composition. Alternatively, the printed tubular tissue construct
may further include a channel or a network of epithelial channels
in the extracellular matrix composition.
[0010] Certain further embodiments relate to a printed tubular
tissue construct with embedded vasculature that includes one or
more functional channels comprising a patterned cell layer thereon
along the length of the functional channel, the patterned cell
layer comprising one or more types of viable cells, each type of
viable cells being positioned along a different predetermined
location of the functional channel. The printed tubular tissue
construct with embedded vasculature also includes a network of
vascular channels interpenetrating the one or more functional
channels and an extracellular matrix composition at least partially
surrounding the one or more functional channels and the network of
vascular channels. The patterned cell layer may include a plurality
of viable cells of at least two predetermined cell types, wherein
the extracellular matrix composition at least partially surrounds
the one or more tissue patterns. The printed tubular tissue
construct may be a nephron, wherein the patterned cell layer
comprises at least two of renal proximal tubule cells, loop of
Henle cells, renal distal tubule cells, collecting duct cells,
mesangial cells, renal microvascular cells, renal cell progenitors,
pluri or multipotent stem cells, other endothelial lineage cells,
fenestrated glomerular endothelial cells, induced pluripotent stem
cells (iPSCs), and/or iPSCs-derived patent-specific pro-kidney or
individual cell lines present in a nephron. In certain alternative
embodiments, the printed tubular tissue construct with embedded
vasculature may be wherein the tubular tissue construct is
intestine, milk duct, sweat gland, colon, esophagus, stomach,
eustachian tube, airway epithelium, epididymis, seminiferous
tubules, urethra, liver bile duct, pancreatic duct, common bile
duct, cerebro-spinal ventricles and aquaducts, parotid glands, oral
mucosa, fallopian tube, vas deferens, or lymph.
[0011] Yet another embodiment relates to a method of printing a
nephron, the method including depositing one or more continuous
sacrificial filaments on and/or in a substrate to form a functional
channel. Each sacrificial filament comprises a first fugitive ink
formulation over a first length of the sacrificial filament, a
second ink formulation over a second length of the sacrificial
filament, and a third ink formulation over a third length of the
sacrificial filament, wherein the first fugitive ink formulation
comprises a fugitive ink and renal proximal tubule cells, the
second fugitive ink formulation comprises the fugitive ink and loop
of Henle cells, and the third fugitive ink formulation comprises
the fugitive ink and renal distal tubule cells. The method also
includes the steps of at least partially surrounding the functional
channel pattern with an extracellular matrix composition and
removing the fugitive ink to create one or more functional channels
in the extracellular matrix composition, at least a portion of the
renal proximal tubule cells remaining along the first length of the
one or more functional channels after removal of the first ink, at
least a portion of the loop of Henley cells remaining in the second
length of the one or more functional channels after removal of the
second ink, and at least a portion of the distal tubule cells
remaining in the third length of the one or more functional
channels after removal of the third ink, thereby forming a nephron.
The substrate may be a perfusable chip. The nephron may be exposed
to one or more biological agents, a biological agent gradient, a
pressure, and/or an oxygen tension gradient, wherein the one or
more biological agents, the biological agent gradient, the
pressure, and/or the oxygen tension gradient may further direct
development, differentiation, and/or functioning of the nephron.
The depositing one or more continuous sacrificial filaments on
and/or in a substrate to form a functional channel may be through a
single printhead. The step of depositing one or more continuous
sacrificial filaments on and/or in the substrate to form a
functional channel includes providing a nozzle body comprising at
least a first ink delivery channel, a second ink delivery channel
and a third ink delivery channel in fluid communication with a
nozzle outlet, and forcing the first ink formulation to flow
through the first ink delivery channel while preventing the second
ink formulation and the third ink formulation from flowing through
the second delivery channel and the third delivery channel,
respectively, thereby extruding through the nozzle outlet the
continuous sacrificial filament comprising the first ink
formulation over the first predetermined length thereof. The method
also includes the step of applying a withdrawal pulse to the first
ink delivery channel while applying an infusion pulse to the second
ink delivery channel, thereby forcing the second ink formulation to
flow through the second ink delivery channel while preventing the
first ink formulation and the third ink formulation from flowing
through the first delivery channel and the third delivery channel,
respectively, thereby extruding through the nozzle outlet the
continuous sacrificial filament comprising the second ink
formulation over the second predetermined length thereof. The
method further includes the step of applying a withdrawal pulse to
the second ink delivery channel while applying an infusion pulse to
the third ink delivery channel, thereby forcing the third ink
formulation to flow through the third ink delivery channel while
preventing the first ink formulation and the second ink formulation
from flowing through the first delivery channel and the second
delivery channel, respectively, thereby extruding through the
nozzle outlet the continuous sacrificial filament comprising the
third ink formulation over the third predetermined length thereof,
thereby 3D printing the one or more continuous sacrificial
filaments comprising multiple cell types over different
predetermined lengths of the filaments.
[0012] A further embodiment relates to a 3D printed nephron
comprising one or more functional channels comprising a patterned
cell layer thereon along the length of the functional channel,
wherein the patterned cell layer includes renal proximal tubule
cells over a first predetermined length of the one or more
functional channel, loop of Henle cells over a second predetermined
length of the one or more functional channel, and renal distal
tubule cells over a third predetermined length of the one or more
functional channel, and an extracellular matrix composition at
least partially surrounding the one or more functional channels.
The 3D printed nephron may further include collecting duct cells,
fenestrated glomerular endothelial cells, induced pluripotent stem
cells (iPSCs), and/or iPSCs-derived patent-specific pro-kidney or
individual cell lines present in a nephron along different lengths
of the function channel. The 3D printed nephron may further include
one or more tissue patterns, each tissue pattern including a
plurality of viable cells of one or more predetermined cell types,
wherein the extracellular matrix composition at least partially
surrounds the one or more tissue patterns. The 3D printed nephron
may further include a network of vascular channels in the
extracellular matrix composition.
[0013] Yet another embodiment relates to a method of printing a
nephron, the methods includes depositing one or more continuous
sacrificial filaments on and/or in a substrate to form a functional
channel, each sacrificial filament comprising a first fugitive ink
formulation over a first length of the sacrificial filament, a
second ink formulation over a second length of the sacrificial
filament, and a third ink formulation over a third length of the
sacrificial filament, wherein the first fugitive ink formulation
comprises a fugitive ink and a first predetermined type of binding
domains to target renal proximal tubule cells, the second fugitive
ink formulation comprises the fugitive ink and a second
predetermined type of binding domains to target loop of Henle
cells, and the third fugitive ink formulation comprises the
fugitive ink and a third predetermined type of binding domains to
target renal distal tubule cells, at least partially surrounding
the functional channel pattern with an extracellular matrix
composition, removing the fugitive ink to create one or more
functional channels in the extracellular matrix composition, at
least a portion of the first predetermined type of binding domains
remaining along the first length of the one or more functional
channels after removal of the ink, at least a portion of the second
predetermined type of binding domains remaining in the second
length of the one or more functional channels after removal of the
ink, and at least a portion of the third predetermined type of
binding domains remaining in the third length of the one or more
functional channels after removal of the ink, and injecting a
suspension comprising at least one of renal proximal tubule cells,
loop of Henle cells and renal distal tubule cells into the
functional channel, wherein the cells bind to their corresponding
predetermined binding domains, thereby forming a nephron. The
suspension may include renal proximal tubule cells, loop of Henle
cells and renal distal tubule cells. The binding domains may be
peptides, proteins, e.g., antibodies; DNA; RNA; aptamers;
nanoparticles; small molecules, chemical functional groups,
bacteria; or a combination thereof. In certain embodiments, the
extracellular matrix surrounding the patterned sacrificial
filaments contains predetermined coupling moieties to capture the
binding domains from the sacrificial filament. The coupling
moieties are chemically reactive to the binding domains, thereby
locally capturing said binding domains upon contact before, during,
or subsequent to the evacuation of the sacrificial filament. The
coupling moieties comprise native extracellular matrix binding
domains, antibodies, peptides, proteins, DNA, RNA, aptamers,
nanoparticles, small molecules, chemical functional groups, and
bacteria. The method may further include the steps of depositing
one or more sacrificial filaments on or in the substrate to form a
vascular pattern interpenetrating the functional channel pattern,
each of the sacrificial filaments comprising a second fugitive ink
and removing the second fugitive ink to create vascular channels in
the extracellular matrix composition, thereby forming an
interpenetrating vascular network in the tubular tissue construct
and injecting a suspension of viable epithelial cells into the one
or more vascular channels. The substrate may be a perfusable chip.
The nephron may be exposed to one or more biological agents, a
biological agent gradient, a pressure, and/or an oxygen tension
gradient, wherein the one or more biological agents, the biological
agent gradient, the pressure, and/or the oxygen tension gradient
may further direct development, differentiation, and/or functioning
of the nephron.
[0014] Yet another embodiment relates to a method of printing a
tubular tissue construct, where the method includes depositing one
or more cell-laden filaments each comprising a plurality of
predetermined types of viable cells on and/or in a substrate to
form one or more tissue patterns, each of the tissue patterns
comprising at least two predetermined cell types, wherein each
predetermined type of viable cells is deposited at a different
predetermined location along a length of the cell-laden filament.
The method also includes depositing one or more sacrificial
filaments on and/or in the substrate to form a functional channel
pattern interpenetrating the one or more tissue patterns, each of
the sacrificial filaments comprising a fugitive ink, at least
partially surrounding the one or more tissue patterns and the
functional channel pattern with an extracellular matrix
composition, and removing the fugitive ink to create functional
channels in the extracellular matrix composition, thereby forming
an interpenetrating channel network in a tissue construct. The
substrate may be a perfusable chip. The tubular construct may be
exposed to one or more biological agents, a biological agent
gradient, a pressure, and/or an oxygen tension gradient, wherein
the one or more biological agents, the biological agent gradient,
the pressure, and/or the oxygen tension gradient may further direct
development, differentiation, and/or functioning of the tubular
tissue construct. The one or more cell-laden filaments may include
an extracellular matrix material, such as gelatin, fibrin, gelatin
methacrylate, collagen I, collagen III, collagen IV, fibrinogen,
matrigel, laminin, carbopol, N-Isopropylacrylamide (NIPAAM),
Polyethylene glycol (PEG), gelatin methacrylate (GelMA),
Polyhydroxyethylmethacrylate (PHEMA), silk, hyaluronic acid, growth
factors, proteoglycans like heparin sulfate or others and/or
combinations thereof. In the method, at least one of the one or
more cell-laden filaments may further include one or more
functional chemical substances selected from the group consisting
of: drugs, small molecules, toxins, proteins, and hormones. The
tubular tissue construct may be a nephron, where the predetermined
types of viable cells include renal proximal tubule cells, loop of
Henle cells, renal distal tubule cells, collecting duct cells,
fenestrated glomerular endothelial cells, induced pluripotent stem
cells (iPSCs), and/or iPSCs-derived patent-specific pro-kidney or
individual cell lines present in a nephron. Alternatively, the
tubular tissue construct may be intestine, milk duct, sweat gland,
colon, esophagus, stomach, eustachian tube, airway epithelium,
epididymis, seminiferous tubules, urethra, liver bile duct,
pancreatic duct, common bile duct, cerebro-spinal ventricles and
aquaducts, parotid glands, oral mucosa, fallopian tube, vas
deferens, or lymph. The method may further include depositing one
or more sacrificial filaments on and/or in the substrate to form a
vascular pattern interpenetrating the functional channel pattern,
each of the sacrificial filaments comprising a second fugitive ink,
and removing the second fugitive ink to create vascular channels in
the extracellular matrix composition, thereby forming an
interpenetrating vascular network in the tubular tissue construct.
The method may further include injecting a suspension of viable
epithelial cells into the one or more vascular channels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawings will be provided by the Office upon
request and payment of the necessary fee.
[0016] FIG. 1A shows an illustration of a bioprinting concept in
which vasculature, an extracellular matrix, and cells may be
printed in combination.
[0017] FIG. 1B shows a schematic of 3D printed heterogeneous tissue
construct that includes vasculature and multiple cell types
precisely placed in three dimensions.
[0018] FIG. 2A is a cross-sectional schematic showing a 2D tissue
construct including two tissue patterns and an interpenetrating
vascular network.
[0019] FIG. 2B shows an example of a 3D tissue construct including
a tissue pattern and an interpenetrating vascular network.
[0020] FIG. 2C shows an example of a 3D tissue construct including
two tissue patterns and an interpenetrating vascular network.
[0021] FIGS. 3A and 3B show examples of 3D tissue constructs
including two tissue patterns and an interpenetrating vascular
network.
[0022] FIGS. 4 shows four print heads (nozzles) mounted onto a
custom 3D printer where each z-axis is controlled
independently.
[0023] FIG. 5A also shows four print heads (nozzles), and FIG. 5B
shows a four-layer microstructure where complex ink patterns are
deposited sequentially from four nozzles to form the four-layer
microstructure with varied composition.
[0024] FIG. 5C shows sequential fabrication images of each layer of
the deposition process, where each inset illustrates the geometry
of each layer.
[0025] FIG. 6A shows a schematic of the sol-gel transition of
Pluronic F127.
[0026] FIG. 6B shows the temperature dependence on the shear moduli
(G' and G'') of 40 wt. % Pluronic F127 ink.
[0027] FIG. 6C shows a schematic of the helix-to-coil transition
characteristic of GelMA.
[0028] FIG. 6D shows the thermal dependence of GelMA shear
moduli.
[0029] FIG. 6E shows GelMA laden with cells.
[0030] FIG. 6F shows the shear moduli as function of temperature
for GelMA laden with 10T1/2 fibroblast cells.
[0031] FIGS. 7A and 7B show ID channels formed in GelMA, where
diameters range from 115 .mu.m to 500 .mu.m.
[0032] FIG. 7C shows the channels perfused with a water-soluble
fluorescent dye for visualization.
[0033] FIGS. 7D-7E show a 2D hierarchical branching network with
curvilinear filaments printed using a single 30 .mu.m glass
capillary.
[0034] FIG. 7F shows the structure perfused with red fluorescent
dye for visualization.
[0035] FIGS. 7G-7I show a highly periodic 3D lattice printed from
sacrificial filaments to create a 3D vascular pattern that can be
perfused after evacuation of the fugitive ink.
[0036] FIG. 7J shows an optical image of representative
microchannel within a 2D vascular network perfused with a HUVEC
suspension.
[0037] FIG. 7K shows a confocal image of the microchannel shown in
FIG. 7J with live HUVEC cells lining the microchannel walls.
[0038] FIG. 8A shows representative cross-sections of various
channel diameters created by depositing sacrificial filaments and
removing the fugitive ink.
[0039] FIG. 8B is a plot showing the swelling ratio
(D.sub.final/D.sub.initial) of various printed sacrificial
filaments comprising Pluronic F127. Each data point is an average
of six samples, each deposited at a fixed speed, pressure, and
nozzle height (z axis). The diameters are measured directly after
printing and again after evacuation via top-down optical
microscopy.
[0040] FIGS. 9A-9B show that endothelialized vascular channels can
be created in fibrin gel, as shown.
[0041] FIGS. 9C and 9D show before and after photographs of animal
blood infiltration in a fabricated bifurcating vascular
network.
[0042] FIG. 10A provides a top-down view of a final 3D printed
heterogeneous tissue constructs structure that is printed from four
separate inks.
[0043] FIG. 10B provides an angled view of the complex tool-path
used to create the tissue construct shown in FIG. 10A, where the
green filaments comprise GFP HNDF-laden GelMA, the blue filaments
comprise 10T1/2 fibroblast-laden GelMA, and the red filaments
comprise the Pluronic ink that may be endothelialized with RFP
HUVECs. The gray shaded region corresponds to pure GelMA matrix
that encapsulates the 3D printed construct.
[0044] FIG. 10C is a bright field microscopy image overlaid with
the green fluorescent channel of the structure of FIG. 10A directly
after printing.
[0045] FIG. 10D is a photograph illustrating the spanning and
out-of-plane nature of the printed structure.
[0046] FIG. 10E shows a demonstration of the fugitive ink
evacuation process.
[0047] FIG. 10F provides a composite image of the three fluorescent
channels: 10T1/2 fibroblasts (blue), HNDFs (green), HUVECs (red)
from the structure of FIG. 10A.
[0048] FIG. 10G shows cell-viability assay results of printed
10T1/2 fibroblasts compared with a non-printed control.
[0049] FIG. 11A shows a schematic of a three-layered structure
containing multiple cell-laden filaments and sacrificial filaments
comprising a fugitive ink.
[0050] FIG. 11B shows a photograph of the evacuated microstructure
before endothelialization.
[0051] FIG. 11C shows an epifluorescent image of GFP HNDFs (green)
and RFP HUVECs (red) after two days in culture.
[0052] FIGS. 12A-12C show a schematic and images that illustrate
the shaping of growth factor (GF) gradients by direct printing of a
GF-laden extracellular matrix material comprising
fibrin-gelatin.
[0053] FIG. 12D show an exemplary mold for perfusion of parallel
printed vascular channels with fluorescently labeled BSA in only
one channel.
[0054] FIGS. 12E and 12F show that a nearly linear gradient is
generated between the two channels of FIG. 12D in 24 hours.
[0055] FIGS. 13A-13D illustrate the synthesis of a fibrin-gelatin
interpenetrating polymer network. First, gel precursors are mixed
together with transglutaminase (TG). Then, by polymerizing
fibrinogen via the enzyme thrombin, a fibrin network is formed. The
second phase is then formed around the fibrin gel, and the two
phases are slowly crosslinked together via TG.
[0056] FIGS. 13E-13F show mechanical properties of the
fibrin-gelatin matrix material.
[0057] FIGS. 13G-13J show the diversity of the fibrin-gelatin
matrix adhesivity for fibroblasts (connective tissue), smooth
muscle cells, endothelial cells, and renal proximal tubule
(epithelial) cells, respectively.
[0058] FIG. 14 illustrates the influence of TG incubation time on
the optical properties of the fibrin-gelatin interpenetrating
polymer network. Transparency is determined by the final pore
architecture of the fibrin gel, which is visualized using a
rhodamine tagged fibrinogen and confocal microscopy.
[0059] FIGS. 15A-15D show an exemplary mold for passive rocking
perfusion of a tissue construct.
[0060] FIGS. 15E-15G show exemplary molds for active pump-based
perfusion of a tissue construct.
[0061] FIGS. 16A-16C show schematically the deposition of
endothelial cells within a sacrificial filament formed from a
fugitive ink, encapsulation of the sacrificial filament with an
extracellular matrix composition, and evacuation of the fugitive
ink to form a channel with endothelial cells lining the channel
wall.
[0062] FIG. 16D shows an as-printed fugitive ink (Pluronic F127)
comprising a dispersion of HUVECs; FIG. 16E shows the fugitive ink
after casting and liquefying; FIG. 16F shows the vascular network
after 1 day of incubation of the HUVECs; and FIG. 16G shows the
vascular network after active perfusion for 24 hours.
[0063] FIG. 17A illustrates the creation of one or more functional
channels in an extracellular matrix composition to form a
functional channel network in a tissue construct (specifically, in
this example, an epithelial tissue construct). The steps include
deposition of one or more sacrificial filaments comprising a
fugitive ink (which may be a cell-laden fugitive ink) to form a
functional channel pattern, at least partial encapsulation of the
channel pattern with an extracellular matrix composition, removal
of the fugitive ink to form the functional channels, and an
optional seeding approach for lining the functional channels with
epithelial cells.
[0064] FIGS. 17B-17C shows two functional channels in an
extracellular matrix composition where the channels are lined with
epithelial cells.
[0065] FIGS. 17D-17F show various confocal microscopy images of
PTEC-lined channels and immunofluorescence images with various
cell-specific proteins being expressed, including Na/K ATPase.
[0066] FIGS. 17G-I show the renal proximal tubule (circled in
black) with epithelial cells (FIG. 17G). Printing of a convoluted
path to make the proximal tubule is shown in FIG. 17H and a close
up view of a typical epithelialized tubule is shown stained for
nuclei (blue) and ATPase (green) is shown in FIG. 17I.
[0067] FIGS. 17J-N show proximal tubule epithelial cells (PTECs)
stained using immunofluorescence for several functional properties
on a 2D dish (AQPI (17J), ATPase (17K), Doming (17L), Cilia (17M),
ZO-1 (17N)) and the presence of domes in culture.
[0068] FIGS. 17O-T show perfused tubule constructs stained using
immunofluorescence for several functional properties on a 2D dish
(AQPI (17O), ATPase (17P), Doming (17R), Cilia (17S), ZO-1 (17T)).
The properties were retained or enhanced in the perfused tubule
constructs.
[0069] FIGS. 17 U-X show SEM and TEM images of the tubules
highlighting the confluent nature of the cells in the constructs,
primary cilia, and formation of a brush border.
[0070] FIGS. 18A-18C highlight the initially uniform distribution
of HUVECs (red) and HNDFs (green) at 3 days post-seeding. FIGS.
18D-18E shows the same channel after eight days, at which time the
channel comprises a distinct outer stromal (HNDF) layer and a
confluent endothelial (HUVEC) layer.
[0071] FIGS. 19A-19F show that, after printing a fugitive ink
directly onto a cell-laden matrix, encapsulating with more
cell-laden matrix, evacuating the fugitive ink to form vascular
channels, and seeding the vascular channels with HUVECs, the
endothelial cells form confluent layers and assemble into capillary
structures over time.
[0072] FIGS. 20A-20C are confocal microscopy images that show
spontaneous neovasculature formation in a printed cell-laden
filament comprising two cell types (HNDFs and HUVECs dispersed
within a gelatin-fibrin extracellular matrix material).
[0073] FIG. 21A shows a schematic depicting an embedded printing
process.
[0074] FIG. 21B shows a schematic of an extracellular matrix
composition comprising a semi-interpenetrating polymer network
(IPN) (e.g., PAA-GelMA) suitable for embedded printing.
[0075] FIG. 21C illustrates a complex heterogeneous structures with
arbitrary 3D shape that may be constructed by embedded
printing.
[0076] FIG. 21D provides representative rheological measurements of
ink and matrix rheology appropriate for embedded printing.
[0077] FIG. 21E is a photograph of a vascular cube including a
vascular network formed by embedded printing.
[0078] FIG. 21F shows a printed cell-laden filament within a
semi-IPN extracellular matrix composition.
[0079] FIG. 22 demonstrates that the transparency of a semi-IPN
extracellular matrix composition may be tuned by degree of
substitution (DS).
[0080] FIG. 23A is an illustration of a nephron.
[0081] FIGS. 23B-E are photographs depicting cross-sectional views
of the nephron showing types of cells present in the nephron.
[0082] FIG. 23F is a photograph of an embedded nephron print
(pluronic with fluorescent dye in Polydimethylsiloxane (PDMS)).
[0083] FIGS. 24A-C depict images of fibrin structure within the
printed ink and matrix.
[0084] FIG. 24D depicts photographs showing TG preincubation time
before adding thrombin; the TG concentration used in these gels is
0.2%wt, scale bar=50 .mu.m.
[0085] FIG. 25A depicts a graph showing results of gelatin
processing time on cell viability exhibiting a lower plateau shear
elastic modulus.
[0086] FIG. 25B depicts a graph showing results of gelatin
processing time on cell viability exhibiting a lower shear yield
stress.
[0087] FIGS. 25C and 25D show photographs showing that printed
cell-laden filaments are less uniform if the gelatin is solubilized
at 70.degree. C. (FIG. 25C) compared to those produced by gelatin
solubilized at 95.degree. C. (FIG. 25D), Scale bars=250 .mu.m.
[0088] FIGS. 26A-26S depict a schematic illustration of the
construction of a silicone interface chip and perfusion of a thick
vascularized tissue on the chip.
[0089] FIGS. 27A-27F depict 3D vascularized tissue fabrication.
[0090] FIG. 27A depicts a schematic illustration of the tissue
manufacturing process.
[0091] FIG. 27B depict a picture of HUVECs growing on top of the
matrix in 2D, scale bar=50 .mu.m.
[0092] FIG. 27C depicts a picture of HNDFs growing inside the
matrix in 3D, scale bar=50 .mu.m.
[0093] FIG. 27D depicts a picture of hMSCs growing on top of the
matrix in 2D, scale bar=50 .mu.m.
[0094] FIGS. 27E and 27F depict images of printed hMSC-laden ink
prepared using 95.degree. C. processed gelatin as printed (FIG.
27E) and after 3 days in the 3D printed filament where actin
(green) and nuclei (blue) are stained (FIG. 27F).
[0095] FIG. 27G shows that gelatin processing temperature affects
the plateau modulus and cell viability after printing. Higher
temperatures lead to lower modulus and higher HNDF viability
post-printing.
[0096] FIG. 27H is a photograph of interpenetrated sacrificial
(red) and cell inks (green) as printed on chip, scale bar=2 mm.
[0097] FIG. 27I depicts a top-down bright field image of
sacrificial and cell inks, scale bar=50 .mu.m.
[0098] FIG. 27J depicts a photograph of a completed construct
housed within a perfusion chamber.
[0099] FIGS. 27K and 27L depict corresponding cross sections of a
completed construct housed within a perfusion chamber, scale bars=5
mm.
[0100] FIG. 28A depicts a schematic showing the three-step gelation
of gelatin-fibrin.
[0101] FIG. 28B depicts a fluorescent image of rhodamine labeled
fibrinogen within a gelatin-fibrin IPN demonstrating that a dense
fibrillar network of fibrin is formed upon addition of thrombin,
scale bar=50 .mu.m.
[0102] FIG. 28C depict a graph showing that, at 37.degree. C.,
fibrinogen increases the solution viscosity of gelatin upon
addition, yet only marginally alters the shear plateau modulus of
the resulting gel at room temperature.
[0103] FIG. 28D depict a graph showing that viscoelastic behavior
of gelatin fibrin inks is highlighted by several orders of
magnitude of shear elastic modulus. At room temperature (22.degree.
C.) and below, the modulus of printable gelatin-fibrinogen inks is
between IE4-IE5 Pa. When the temperature is increased above
30.degree. C. the solution is rheologically suitable for cell
dispersion and casting--the solution exists in the viscous state
with a shear elastic modulus approaching zero.
[0104] FIGS. 28E and 28F depict graphs showing that the dynamic
shear elastic modulus of fibrin-gelatin rapidly increases with time
upon addition of thrombin, indicating fibrin network formation,
while the plateau elastic modulus increases with increasing fibrin
content (FIG. 28E) within a fibrin-gelatin composite, the fibrin
phase of the IPN imparts more stiffness to the resulting gel (FIG.
28F).
[0105] FIG. 29A depicts a table showing the adhesion behavior and
relative cell spreading on various matrix formulations.
[0106] FIG. 29B shows fluorescent micrographs of RFP-HUVECs showing
examples of low, medium, and high levels of cell spreading on gels.
Fibrinogen is gelled at pH 7 to generate an opaque fibrin gel, and
at pH 7.5 to generate a transparent or "clear" fibrin gel. pH is
adjusted using 1M NaOH, scale bar=50 .mu.m.
[0107] FIG. 30A is a schematic depicting a single HUVEC-lined
vascular channel supporting a fibroblast cell-laden matrix and
housed within a perfusion chip.
[0108] FIGS. 30B and 30C are confocal microscopy images of the
vascular network after 42 days, CD-31 (red), vWF (blue), and
VE-Cadherin (magenta). scale bars=100 .mu.m.
[0109] FIG. 30D shows a long-term perfusion of HUVEC-lined (red)
vascular network supporting HNDF-laden (green) matrix shown by
top-down (left) and cross-sectional confocal microscopy at 45 days
(right), scale bar=100 .mu.m.
[0110] FIG. 30E shows a bar graph showing quantification of barrier
properties imparted by endothelial lining of channels, demonstrated
by reduced diffusional permeability of FITC-dextran.
[0111] FIG. 30F depicts GFP-HNDF distribution within the 3D matrix
shown by fluorescent intensity as a function of distance from
vasculature.
[0112] FIG. 31A depicts images of FITC-labeled Dextran (70 kDa)
perfused through channels with, and without a lining of HUVECs. The
fluorescence signal was recorded every 5 minutes up to 30 minutes
to measure the degree of diffusion into the gel.
[0113] FIG. 31B depicts a bar graph of calculated diffusional
permeability with and without the cell lining.
[0114] FIG. 31C depicts FITC-dextran fluorescence signatures with
cell lining demonstrating the slow spreading of the fluorescence
signal with time, scale bar=500 .mu.m.
[0115] FIG. 32 depicts fibroblast cell proliferation within
vascularized tissues. A GFP-expressing fibroblast laden
gelatin-fibrin hydrogel is perfused via four printed channels that
are lined with RFP-expressing HUVECs. The cell density, as measured
by GFP levels, is maximal nearest to the channels, and decreases in
the central region far from the channels, scale bar=500 .mu.m.
[0116] FIG. 33A depicts a Schematic depicting the geometry of the
printed heterogeneous tissue within the customized perfusion chip,
wherein the branched vascular architecture pervades hMSCs that are
printed into a lattice architecture, and HNDFs are cast to fill the
interstitial space.
[0117] FIG. 33B depicts photographs of a printed tissue construct
within and removed from the customized perfusion chip, from the
side.
[0118] FIG. 33C depicts a confocal microscopy image through a
cross-section of 1 cm thick vascularized osteogenic tissue
construct after 30 days of active perfusion and in situ
differentiation, scale bar=1.5 mm.
[0119] FIG. 33D depicts osteocalcin intensity across the thick
tissue sample inside the red lines shown in FIG. 33C.
[0120] FIG. 33D is a high resolution image showing oscteocalcin
(purple) localized within hMSCs and they appear to take on
symmetric osteoblast-like morphologies after 30 days, scale bar=100
.mu.m.
[0121] FIGS. 33F-33G show images of thick tissue constructs stained
for collagen-I (yellow), which appears localized near hMSCs, scale
bars=200 .mu.m.
[0122] FIG. 33H shows an image of alizarin red used to stain CaP
deposition and fast blue is used to stain alkaline phosphatase,
indicating tissue maturation and differentiation over time, scale
bar=200 .mu.m.
[0123] FIG. 34A shows images of hMSC's cultured in a polystyrene
well in the presence of different media conditions. At day 14, the
cells were stained with fast blue and alizarin red to visualize
osteocytes and deposited minerals, respectively; scale bar=2
mm.
[0124] FIG. 34B depicts a graph showing average absorbance at 500
nm at 9 different locations within each well. 500 nm absorbance is
a measure of the amount of alizarin red in the wells. Base
media=Rooster Media. Growth media=Rooster media+GTX media booster.
Osteo supplement=10 mM beta-glycerophosphate and 50 .mu.g mL.sup.-1
L-ascorbic acid. BMP-2 concentration=100 ng mL.sup.-1.
[0125] FIG. 35A depicts images of maturation of thick vascularized
tissues differentiated toward an osteogenic lineage in situ. Thick
vascularized tissue is analyzed at various time points to visualize
maturation. Collagen-I (pink) deposition is not present in
acelullar scaffolds and is instead secreted by the cells in the
tissue, after 3 days there is very little collagen in regions near
hMSCs and HNDFs, however, by day 30, printed hMSCs have produced
significant collagen in both filaments and circumferentially around
the vascular channels, scale bars=100 .mu.m.
[0126] FIG. 35B depicts images of maturation of thick vascularized
tissues differentiated toward an osteogenic lineage in situ. The
delivery of an osteogenic cocktail through the vascular network has
led to first the osteogenic lineage commitment of hMSCs and then
the deposition of mineral over the course of 30 days. This is
evidenced by appearance of alkaline phosphatase expression, which
is observable via fast blue, and subsequent CaP
mineralization--observable via alizarin red stain; scale bars=100
.mu.m.
[0127] FIG. 36A depicts a Schematic of a nephron highlighting the
convoluted proximal tubule.
[0128] FIGS. 36B and 36C depict schematics and images of different
steps in the fabrication of 3D convoluted, perfusable proximal
tubules, in which a fugitive ink is first printed on a
gelatin-fibrinogen extracellular matrix, ECM (i), additional ECM is
cast around the printed feature (ii), the fugitive ink is evacuated
to create an open tubule (iii), and PTEC cells are seeded within
the tubule and perfused for long time periods (iv).
[0129] FIG. 36D depicts a 3D rendering of the printed convoluted
proximal tubule acquired by confocal microscopy.
[0130] FIG. 36E is a higher magnification view of the region in
FIG. 36D denoted by the white rectangle, scale bar=200 .mu.m.
[0131] FIG. 36F depicts a schematic showing the result of the
fabrication process where an open lumen circumscribed with an
epithelial lining is created in 3D and directionally perfused on
chip.
[0132] FIG. 37 depicts a 3D proximal tubule lined with PTEC cells
and embedded in a fibroblast-laden extracellular matrix (phase
contrast image of a 3D PT grown to a confluent epithelium, in which
fibroblasts thrive in the surrounding ECM, scale bar=100
.mu.m).
[0133] FIG. 38 depicts a PT model constructed with 3 layers of
independently addressable perfusable tubes.
[0134] FIGS. 39A-39K depict a 3D proximal tubule maturation
process.
[0135] FIG. 39A is a photo of a mature (fully confluent)
tubule.
[0136] FIG. 39B depicts PTEC loading at Day 0, scale bar=500
.mu.m.
[0137] FIG. 39C depicts higher magnification view of PTEC loading
of FIG. 39B, scale bar=300 .mu.m.
[0138] FIG. 39D shows PTECs adhering to the tubule at Day 1 after
non-adherent cells are flushed away, scale bar=200 .mu.m.
[0139] FIG. 39E depicts a low magnification view of PTECs growing
into the tubule at Day 2, scale bar=500 .mu.m.
[0140] FIG. 39F is an image at Day 4 where cells grow from colonies
or clusters, scale bar=100 .mu.m.
[0141] FIG. 39G shows an image at Day 4 where cells are near
confluency, scale bar=100 .mu.m.
[0142] FIG. 39H shows an image of a mature tubule at Day 38, scale
bar=500 .mu.m.
[0143] FIG. 39I shows a higher magnification view of the confluent
tubule at Day 38 shown in FIG. 39H, scale bar=100 .mu.m.
[0144] FIG. 39J shows an image of the tubule, which approaches
within 350 .mu.m of itself due to its convoluted architecture,
scale bar=100 .mu.m.
[0145] FIG. 39K shows a timeline of construction and maturation of
the PT model.
[0146] FIGS. 40A-40D depict brightfield images of Cyclosporine
A-induced cytotoxicity.
[0147] FIGS. 40E-40H depict 3D renderings of Cyclosporine A-induced
cytotoxicity.
[0148] FIGS. 40I-40L sow high magnification images of printed and
perfused 3D PTs dosed with varying concentrations of Cyclosporine A
for 24 h, where actin (green) and nuclei (blue) are stained, scale
bars=200 .mu.m (a-h) and scale bars=20 .mu.m (i-l),
respectively.
[0149] FIG. 40M depicts a graph showing Diffusional permeability
measurements taken after dosing with Cyclosporine A, *p <0.003,
**p<0.02.
[0150] FIG. 40N depicts a graph showing cell viability measured for
the 2D control (on bare dish) after dosing with Cyclosporine A (all
populations shown are statistically significantly different with a
p<0.005).
[0151] FIGS. 41A-41D depict fluorescent images captured at varying
times: t=0 min (41A), t=45 min (41B), and t=0 min (41C) and 5 min
(41D) (for control samples composed of a bare 3D PT (without
PTECs), in which the FITC-labeled dextran diffuses much faster into
the surrounding ECM) following perfusion with FITC-labeled dextran
(70 kDa) solution through the 3D PT lined with confluent PTECs,
scale bars=200 .mu.m.
[0152] FIG. 41E depicts a bar graph showing measured diffusional
permeability of 3D PT channels with and without proximal tubule
epithelium.
[0153] FIG. 42A depicts an SEM image of 6 PTs printed adjacent to
one another (i.e., Multiplexed 3D proximal tubules), scale bar=500
.mu.m.
[0154] FIG. 42B shows a high magnification image taken inside the
larger 3D PT shown in the background, scale bar=50 .mu.m.
[0155] FIG. 43A depicts a Schematic representation of the ECM
constituents and their gelation and cross-linking as a function of
different stimuli.
[0156] FIG. 43B depicts relative mRNA levels of 33 selected genes
related to renal epithelial function, transport, endocytosis,
hormone response, injury response, and cell differentiation for
three cell lines (primary renal PTEC, PTEC-TERT1, and the A498
cancer renal cell line). PTEC-TERT1 cells are transcriptionally
similar to primary PTEC and different from the A498 renal cancer
epithelial cell line.
[0157] FIGS. 44A-44C depict bar graphs of the relative
concentration of IL-6 (FIG. 44A), IL-8 (FIG. 44B), and MCP-1 (FIG.
44C) following a 3D proximal tubule perfusate analysis.
[0158] FIG. 45A depicts a phase contrast image of a mature 3D PT
construct taken at 6 weeks, scale bar=500 .mu.m.
[0159] FIG. 45B depicts a phase contrast image of the 3D PT
construct at 6 weeks, scale bar=250 .mu.m.
[0160] FIG. 45C depicts a TEM image of the PTECs within the tubule
at 5 weeks, scale bar=5 .mu.m.
[0161] FIG. 45D depicts a TEM image of the PTECs grown on a 2D dish
coated with ECM with no perfusion, scale bar=5 .mu.m.
[0162] FIG. 45E depicts a schematic view of the columnar epithelium
seen in native tissue, in which PTECs pack together closely and
exhibit a dense brush border on the apical side, tight junctions,
and a solid basement membrane, reprinted with permission (Mescher,
A. in Junqueira's Basic Histology: Text and Atlas, Edn. 13 85
(McGraw-Hill Education, 2013)).
[0163] FIG. 45F depicts a PTEC cell height as measured from TEM
images of the 3D PT constructs (3DP) as well as three 2D controls
(2DP=PTECs on ECM in 2D with perfusion, 2D=PTECs on ECM in 2D not
perfused, Dish=bare tissue culture dish not perfused), *p<0.001,
**p<0.02.
[0164] FIG. 45G depicts SEM images at low (scale bar=50 .mu.m) and
higher (scale bar=20 .mu.m) magnifications showing a confluent
layer of PTECs within the 3D PT, white arrows highlight the
presence of primary cilia at a density of one per cell.
[0165] FIG. 45H depicts a 3D rendering of a partial tubule showing
the apical side, which highlights the primary cilia (red), scale
bar=20 .mu.m.
[0166] FIG. 45I depicts an image of the PT highlighting the
presence of Na/K ATPase in green, scale bar=100 .mu.m.
[0167] FIG. 45J depicts an image of the 3D PT highlighting the
presence of AQPI in yellow, scale bar=100 .mu.m.
[0168] FIG. 45K depicts a high magnification view of the image in
FIG. 25J highlighting actin in red and showing AQPI in yellow,
scale bar=20 .mu.m.
[0169] FIG. 46A depicts a TEM image of the brush border on the
apical side of PTECs at 6 weeks, scale bar=1 .mu.m.
[0170] FIG. 46B depicts a TEM image of the basal side of PTECs at 6
weeks highlighting the presence of the engineered extracellular
matrix (ECM), basement membrane proteins secreted by the cells
(BM), basolateral interdigitations (BI), and circular invaginations
in the membrane marked with white arrows, scale bar=1 .mu.m.
[0171] FIG. 46C depicts PTECs at 6 weeks showing the basement
membrane proteins the cells secreted, namely laminin (predominant
protein in red) and collagen IV (green), scale bar=10 .mu.m.
[0172] FIG. 46D depicts a tight junction (white arrow) between
PTECs in the bioprinted tubule, scale bar=500 nm.
[0173] FIG. 46E depicts the cell junction protein K Cadherin
(magenta) stained in the PT, scale bar=10 .mu.m.
[0174] FIGS. 46F and 46G depict a microvilli length (46F) and
microvilli density (46G) quantified through TEM images of the 3D PT
constructs (3DP) as well as three 2D controls (2DP=PTECs on ECM in
2D with perfusion, 2D=PTECs on ECM in 2D without perfusion,
Dish=bare tissue culture dish without perfusion), p<0.001.
[0175] FIG. 47A depicts shows a 3D reconstruction of PTECs stained
for Na.sup.+/K.sup.+ ATPase (green) and acetylated tubulin (red)
where basal-lateral expression of Na.sup.+/K.sup.+ ATPase is
apparent and two primary cilia are visible on the apical side,
scale bar=10 .mu.m.
[0176] FIG. 47B depicts a TEM image of primary cilia, scale bar=1
.mu.m.
[0177] FIG. 48A is a graph showing the flow cytometry data
comparing the fluorescence intensity of PTECs fed FITC-labeled
human serum albumin for 2 h under several conditions, including 2D
controls on bare (blue) and ECM-coated (green) plastic dishes and
in 3D PTs perfused for 65 days (magenta).
[0178] FIG. 48B depicts a graph of the flow cytometry data
comparing the fluorescence intensity of megalin for the same PTEC
samples as shown in FIG. 48A.
[0179] FIG. 48C depicts a fluorescence image of the 3D PT
constructs stained for FITC-labeled albumin (red), scale bars=20
.mu.m.
[0180] FIG. 48D depicts a fluorescence image of the 3D PT
constructs stained for FITC-labeled megalin (blue), scale bars=20
.mu.m.
[0181] FIG. 48E depicts a fluorescence image of the 3D PT
constructs stained for FITC-labeled albumin and FITC-labeled
megalin (blue), scale bars=20 .mu.m.
[0182] FIG. 49A depicts a brightfield image of a healthy proximal
tubule at 4 weeks, scale bar=100 .mu.m.
[0183] FIG. 49B depicts a brightfield image of a tubule after 24 h
of cyclosporine A exposure, scale bar=100 .mu.m.
[0184] FIG. 49C depicts an image showing live (green) and dead
(red) staining of the tubule at 24 h after cyclosporine A exposure
showing that <5% of the total cells are dead, scale bar=100
.mu.m.
[0185] FIG. 49D depicts a high magnification image showing damage
observed following dosing with with 10 .mu.M cyclosporine A, where
actin (green) and nuclei (blue) are stained, scale bar=20
.mu.m.
DETAILED DESCRIPTION
[0186] A printed tissue construct including an interpenetrating
vasculature and a method of printing such a tissue construct are
described herein. FIG. 1A provides an illustration of the
bioprinting concept. The printing method may enable the fabrication
of heterogeneous 2D and 3D tissue constructs including cells,
vasculature, epithelial ducts, and extracellular matrix in
predetermined locations for applications ranging from 3D tissue
culture and drug screen to organ transplants.
[0187] A printed tubular tissue construct, such as a nephron
including at least two types of viable cells, each type of viable
cells being positioned along a different predetermined location of
the tubular tissue construct and methods of 3D printing such
tubular tissue constructs are also described. FIGS. 23A-F provide
illustrations of a nephron with the various types of cells along
its length (A-E) as well as an illustration of an embedded nephron
print (F). The constructs may be used, for example, in drug
toxicology, whole organ printing, organ supplement, and/or dialysis
replacement/supplement.
[0188] FIG. 1B and FIGS. 2A-2C provide schematics showing exemplary
printed tissue constructs that include vasculature and multiple
cell types precisely placed in three dimensions. Referring to FIG.
2A or 2C, an exemplary printed tissue construct 100 comprises a
first tissue pattern 115a and a second tissue pattern 115b, where
each of the first and second tissue patterns 115a, 115b comprises a
plurality of viable cells of one or more predetermined cell types.
For example, the first tissue pattern 115a may include cell types A
and B, and the second tissue pattern 115b may include cell type C.
An arrangement of one or more cell-laden filaments 105 comprising
the viable cells and having the predetermined cell types may define
each tissue pattern 115a, 115b. In this example, the cell-laden
filaments 105 that define the first tissue pattern 115a include
cell types A and cell B, and the cell-laden filaments that define
the second tissue pattern 115b include cell type C. A network of
vascular channels 135 interpenetrates the tissue patterns 115a,
115b. An extracellular matrix composition 130 at least partially
surrounds the one or more tissue patterns 115a, 115b and the
network of vascular channels 135.
[0189] A pattern or network that "interpenetrates" another pattern
or network in a printed tissue construct may be understood to
comprise one or more filaments, channels or portions that are
layered with, partially or completely overlapping, partially or
completely underlapping, surrounding, embedded within, and/or
interwoven with one or more filaments, channels or portions of the
other pattern or network. A filament "deposited on a substrate" may
be understood to be deposited directly on the substrate, directly
in the substrate, or directly on another filament, channel or
portion previously deposited or formed on the substrate.
[0190] Referring now to FIG. 2B, a tissue construct comprising an
embedded vasculature may be printed by depositing one or more
cell-laden filaments 105, where each cell-laden filament 105
comprises a plurality of viable cells, on a substrate 110 to form
one or more tissue patterns 115 (one tissue pattern in this
example). The tissue pattern 115 comprises cells of one or more
predetermined cell types. One or more sacrificial filaments 120,
each comprising a fugitive ink, are also deposited on the substrate
110 to form a vascular pattern 125 that interpenetrates the one or
more tissue patterns 115. The one or more tissue patterns 115 and
the vascular pattern 125 are partially or fully surrounded by an
extracellular matrix composition 130. The fugitive ink is then
removed to create a network of vascular channels 135 in the
extracellular matrix composition 130. Thus, an interpenetrating
vascular network is formed in the tissue construct 100.
[0191] The tissue construct may include up to n different
predetermined cell types. For example, n may satisfy
1.ltoreq.n.ltoreq.300, 2.ltoreq.n.ltoreq.200, or
2.ltoreq.n.ltoreq.100. More typically, n is no more than 50, no
more than 30, or no more than 20. For example, there may be 2 or
more, 4 or more, 8 or more, 16 or more, or 20 or more predetermined
cell types in the tissue construct. Further, one cell type may be
printed, such as a multi- or pluripotent stem cells, of any origin,
that could turn into 20 or more cells types as directed by either
perfusion, matrix cues, or perfused growth factors, small
molecules, or other agents.
[0192] As illustrated by the examples of FIGS. 2A-2C, each tissue
pattern comprises or is defined by a two- or three-dimensional
arrangement of one or more cell-laden filaments, and each tissue
pattern (and thus each arrangement of cell-laden filaments) may
comprise a different subset of the predetermined cell types. For
example, in a tissue construct that includes 5 different
predetermined cell types (e.g., cell types A, B, C, D, and E) and 3
different tissue patterns (e.g., tissue patterns 1, 2, and 3),
tissue pattern 1, which is defined by a first arrangement of one or
more cell-laden filaments, may comprise cell type A; tissue pattern
2, which is defined by a second arrangement of one or more
cell-laden filaments, may comprise cell types B and C; and tissue
pattern 3, which is defined by a third arrangement of one or more
cell-laden filaments, may comprise cell types A and E.
[0193] In addition to the viable cells, the one or more cell-laden
filaments may comprise a synthetic or naturally-derived
biocompatible material that may be referred to as an extracellular
matrix material. Each of the one or more cell-laden filaments may
also or alternatively comprise one or more functional chemical
substances (e.g., drugs, toxins, proteins and/or hormones) as
described below. Each tissue pattern may include one layer or
multiple layers of the cell-laden filament(s), which may in some
embodiments be at least partially coalesced at regions of contact
therebetween. For example, adjacent layers formed from one or more
cell-laden filaments may be partially or fully coalesced depending
on filament composition and the deposition (or post-deposition)
conditions.
[0194] The arrangement of the cell-laden filaments in the tissue
construct may be continuous or discontinuous. In a continuous
arrangement, the cell-laden filaments of an exemplary tissue
pattern (and comprising one or more predetermined cell types) may
form a single interconnected tissue network in the tissue
construct. For example, a single cell-laden filament comprising
viable cells of the predetermined cell type(s) may be deposited in
a single layer or in multiple layers to form the continuous
arrangement. Alternatively, a plurality of cell-laden filaments
comprising viable cells of the predetermined cell type(s) may be
deposited in a single layer or in multiple layers to form the
continuous arrangement, where each of the cell-laden filaments is
in physical contact with, and possibly at least partially coalesced
with, another cell-laden filament comprising the same predetermined
cell type(s).
[0195] In a discontinuous arrangement of cell-laden filaments
comprising one or more predetermined cell types, a single
interconnected tissue network of the predetermined cell type(s) is
not formed within the tissue construct. Instead, the cell-laden
filaments comprising the predetermined cell type(s) may be
dispersed uniformly or nonuniformly throughout the tissue
construct. Consequently, the cells corresponding to the
predetermined cell type(s) may also be dispersed uniformly or
nonuniformly (e.g., in clumps) throughout the tissue construct. In
this embodiment, some, all or none of the cell-laden filaments of a
given tissue pattern and cell type(s) may be in physical contact
with another cell-laden filament comprising cells of the same cell
type(s).
[0196] Each of the one or more cell-laden filaments includes at
least one viable cell and may include a large number of viable
cells. For example, each of the cell-laden filaments may have a
cell concentration of at least about 100 cells/ml, at least about
1000 cells/ml, at least about 10.sup.4 cells/ml, at least about
10.sup.5 cells/ml, at least about 10.sup.6 cells/ml, at least about
10.sup.7 cells/ml, or at least about 10.sup.8 cells/ml. Typically,
the cell concentration is no higher than about 10.sup.9 cells/ml,
or no higher than about 10.sup.8 cells/ml. Consistent with this,
the one or more tissue patterns of the tissue construct may have a
cell concentration of at least about 100 cells/ml, at least about
1000 cells/ml, at least about 10.sup.4 cells/ml, at least about
10.sup.5 cells/ml, at least about 10.sup.6 cells/ml, at least about
10.sup.7 cells/ml, or at least about 10.sup.8 cells/ml. Typically,
the cell concentration in the tissue pattern is no higher than
about 10.sup.9 cells/ml, or no higher than about 10.sup.8
cells/ml.
[0197] The cell concentration may be substantially uniform (e.g.,
within .+-.10%, within .+-.5%, or within .+-.1%) throughout each of
the cell-laden filaments, and the cell concentration may also be
substantially uniform throughout each of the tissue pattern(s).
Alternatively, it is possible to deposit cell-laden filaments that
include aggregates or clusters of cells that may range in size from
about 10 cells/cluster to about 1000 cells/cluster, or from about
10 cells/cluster to about 100 cells/cluster. Such clusters may be
dispersed uniformly or non-uniformly within the cell-laden
filaments (and thus uniformly or non-uniformly throughout the one
or more tissue patterns). Overall, the cell concentration may be
substantially uniform throughout the tissue construct, or the cell
concentration may include predetermined inhomogeneities within the
tissue construct that may be defined by the location and morphology
of the one or more tissue patterns, and/or by the cell distribution
within the one or more tissue patterns.
[0198] The vascular network that interpenetrates the one or more
tissue patterns is a two- or three-dimensional interconnected
arrangement of vascular channels. The network may include one or
more--furcations (e.g., bifurcations, trifurcations, etc.) from a
parent vascular channel to a plurality of branching vascular
channels. The network may have a hierarchical branching structure,
where larger diameter channels branch into smaller diameter
channels. Some or all of the vascular channels may follow a curved
path, and thus may be considered to be curvilinear. All of the
vascular channels in the network may have the same diameter, or at
least one, some, or all of the vascular channels may have a
different diameter. In some cases, one or more of the vascular
channels may have a nonuniform diameter along a length thereof.
[0199] It is beneficial for the cells of the tissue construct to be
close enough to the interpenetrating network of vascular channels
to remain viable. One major problem with previous attempts to
create tissue and organ-like structures is that necrotic regions
could develop in areas without accessible perfusable vasculature.
In the present work, each cell-laden filament, and thus each cell,
may be placed in a location near to the vascular network, or near
to where the vascular network may be formed. For example, at least
a portion of the one or more cell-laden filaments forming each
tissue pattern, and thus some or all of the viable cells, may be no
more than about 1 mm away, no more than about 500 microns away, no
more than about 300 microns away, no more than about 200 microns
away, no more than about 100 microns away, no more than about 50
microns away, and/or no more than about 10 microns away from a
vascular channel. One or more of the cell-laden filaments and thus
at least some of the viable cells may be deposited so as to be in
direct contact with a vascular channel. It is envisioned that some
portion of the vascular network, for example the smallest
capillaries, may be formed by angiogenesis and/or tubulogenesis
after deposition of the sacrificial filaments and removal of the
fugitive ink. For example, cell-laden filaments comprising
endothelial cells may be deposited adjacent to the fugitive network
to encourage tubulogenesis and/or angiogenesis to generate new
capillaries.
[0200] Because the printing process described below for deposition
of the cell-laden (and other) filaments allows for a high
positional accuracy, the placement of the viable cells and/or the
extracellular matrix material within the tissue construct may be
controlled to within .+-.200 microns, within .+-.100 microns,
within .+-.50 microns, within .+-.10 microns, or within .+-.1
micron.
[0201] Different types of cells may be placed in close proximity to
one another by depositing a cell-laden filament that includes cells
of more than one cell type, as discussed above. It is also
contemplated that, in addition to the interpenetrating vasculature,
one or more of the tissue patterns may interpenetrate one or more
of the other tissue patterns so that certain types of cells may be
positioned in close proximity to another. For example, one or more
cell-laden filaments comprising a first type of cells (e.g.,
epithelial or endothelial cells) may be layered with, partially or
completely overlapping, partially or completely underlapping,
surrounding, embedded within, and/or interwoven with one or more
cell-laden filaments comprising a second type of cells (e.g.,
smooth muscle cells). In some embodiments, all of the tissue
patterns may interpenetrate at least one other tissue pattern, and
it is also contemplated that all of the tissue patterns may
interpenetrate all of the other tissue patterns.
[0202] The extracellular matrix composition may partially or fully
surround the one or more tissue patterns, where a tissue pattern
that is fully surrounded includes no exposed cell-laden filaments.
The extracellular matrix composition may also partially or fully
surround the network of vascular channels, where a vascular network
that is fully surrounded includes no exposed vascular channels. For
example, the network of vascular channels may be fully surrounded
by the extracellular matrix composition, while the tissue pattern
may be only partially surrounded by (e.g., adjacent to) the
extracellular matrix composition. In such an example, the
cell-laden filaments may be deposited after the vascular pattern is
encapsulated. In some embodiments, the extracellular matrix
composition may comprise additional viable cells and/or one or more
functional chemical substances, as described below, which may be
deposited along with the extracellular matrix composition. Such an
extracellular matrix composition may be referred to as a cell-laden
matrix. As described below, the extracellular matrix composition
may be printed, cast or formed by another method known to one of
ordinary skill in the art.
[0203] The tissue construct may have any desired 2D or 3D shape.
For example, the tissue construct may have a planar geometry
constructed from a single layer or multiple layers of cell-laden
filaments and an interpenetrating vascular network. Such structures
may have any desired height ("thickness"). Typically, the tissue
construct has a height of about 100 cm or less, about 10 cm or
less, about 1 cm or less, about 1 mm or less, about 500 microns or
less, or about 100 microns or less, and typically at least about 10
microns, at least about 100 microns, at least about 200 microns, or
at least about 1 mm, with applications ranging from tissue cultures
and drug screening to skin constructs and corneal replacements. In
certain alternative embodiments, the tissue construct may have
thickness of at least 1 cm and, preferably, exceed the thickness of
1 cm. The term "thick" in reference to a tissue construct means
thicker than 1 mm.
[0204] Alternatively, the tissue construct into which a vascular
network is embedded may have an arbitrary or application-dependent
3D size and shape. The tissue construct may have a solid structure,
a porous structure, and/or a hollow structure (e.g., tubular or
nontubular) and may be fabricated to mimic the morphology and
function of particular organ. For example, the tissue construct may
have the size and shape of a kidney, heart, pancreas, liver,
bladder, vagina, urethra, trachea, esophagus, skin or other bodily
organ. The 3D size and shape may in some cases be determined by a
mold, as described below.
[0205] In general, in a three-dimensional arrangement of cell-laden
filaments with an interpenetrating vascular pattern, the
sacrificial filaments may have portions that overlie or underlie
portions of the cell-laden filaments, and the sacrificial and
cell-laden filaments may or may not be confined to an XY plane
normal to the vertical direction (as defined by the force of
gravity). The sacrificial filaments may be in physical contact with
some or all of the cell-laden filaments, and, in some embodiments,
the filaments may be partially or fully coalesced at the regions of
contact. Both the sacrificial and cell-laden filaments may have
spanning portions that extend unsupported between points of
contact.
[0206] FIGS. 2A-2C show exemplary tissue constructs 100 each
comprising one or more tissue patterns 115 interpenetrated by a
vascular pattern 125 or a network of vascular channels 135. In FIG.
2A, two tissue patterns 115a, 115b each comprising two cell-laden
filaments 105 are deposited on the substrate 110 in a single layer.
Adjacent to and/or in physical contact with the cell-laden
filaments 105 are sacrificial filaments 120 of the vascular pattern
125, where each sacrificial filament 120 comprises a fugitive ink.
After encapsulation with the extracellular matrix composition 130,
the fugitive ink may be removed to form the network of vascular
channels 135.
[0207] FIG. 2B shows a schematic of a tissue construct 100
comprising a 3D lattice structure 115 of cell-laden filaments 105
alternating with sacrificial filaments 120 of an interpenetrating
vascular pattern 125. The fugitive ink making up the sacrificial
filaments 120 is ultimately removed to create the network of
vascular channels 135, which may also be visualized in FIG. 2B.
[0208] The tissue construct 100 of FIG. 2C includes two tissue
patterns 115a, 115b each defined by a curvilinear cell-laden
filament 105 that are interpenetrated by a network of vascular
channels 135 (or by a vascular pattern 125 comprising sacrificial
filaments 120 if the fugitive ink has not yet been removed). The
vascular network 135 has a hierarchical branching structure
including curvilinear channels of various lengths and diameters. A
solid substrate 110 is shown underlying the tissue construct 100;
however, in this and in the other exemplary figures, the underlying
solid substrate 110 may not be present.
[0209] FIG. 3A shows a top view of two exemplary tissue patterns
115a, 115b comprising cell-laden filaments 105 in a semi-woven
configuration with sacrificial filaments 120 of a vascular pattern
125. FIG. 3B shows multiple layers of the same tissue patterns
115a, 115b and vascular pattern 125 (or network of vascular
channels 135 if the fugitive ink has been removed) surrounded by an
extracellular matrix composition 130.
[0210] The viable cells and the predetermined cell types in the
tissue construct may include any mammalian cell type selected from
cells that make up the mammalian body, including germ cells,
somatic cells, and stem cells. Depending on the type of cell, cells
that make up the mammalian body can be derived from one of the
three primary germ cell layers in the very early embryo: endoderm,
ectoderm or mesoderm. The term "germ cells" refers to any line of
cells that give rise to gametes (eggs and sperm). The term "somatic
cells" refers to any biological cells forming the body of a
multicellular organism; any cell other than a gamete, germ cell,
gametocyte or undifferentiated stem cell. For example, in mammals,
somatic cells make up all the internal organs, skin, bones, blood
and connective tissue. As such, a cell may include any somatic cell
isolated from mammalian tissue, including organs, skin, bones,
blood and connective tissue (i.e., stromal cells). Examples of
somatic cells include fibroblasts, chondrocytes, osteoblasts,
tendon cells, mast cells, wandering cells, immune cells, pericytes,
inflammatory cells, endothelial cells, myocytes (cardiac, skeletal
and smooth muscle cells), adipocytes (i.e., lipocytes or fat
cells), parenchyma cells (neurons and glial cells, nephron cells
(i.e., renal proximal tubule cells, loop of Henle cells, renal
distal tubule cells, collecting duct cells, and fenestrated
glomerular endothelial cells (i.e., podocytes)), hepatocytes,
pancreatic cells, lung parenchyma cells) and non-parenchymal cells
(e.g., sinusoidal hepatic endothelial cells, Kupffer cells and
hepatic stellate cells). The term "stem cells" refers to cells that
have the ability to divide for indefinite periods and to give rise
to virtually all of the tissues of the mammalian body, including
specialized cells. The stem cells include pluripotent cells, which
upon undergoing further specialization become multipotent
progenitor cells that can give rise to functional or somatic cells.
Examples of stem and progenitor cells include hematopoietic stem
cells (adult stem cells; i.e., hemocytoblasts) from the bone marrow
that give rise to red blood cells, white blood cells, and
platelets; mesenchymal stem cells (adult stem cells) from the bone
marrow that give rise to stromal cells, fat cells, and types of
bone cells; epithelial stem cells (progenitor cells) that give rise
to the various types of skin cells; neural stem cells and neural
progenitor cells that give rise to neuronal and glial cells; and
muscle satellite cells (progenitor cells) that contribute to
differentiated muscle tissue.
[0211] In some embodiments, induced pluripotent stem cells (iPSCs)
may be used to as starting materials to derive patient-specific
predetermined cell types for use in the tissue construct
development. iPSCs are a type of pluripotent stem cell that can be
generated directly from adult cells, self-renew indefinitely and
have unlimited developmental potential. For example, iPSCs can be
used to derive patient-specific pro-kidney or individual cell lines
present in a nephron (e.g., renal proximal tubule cells, loop of
Henle cells, renal distal tubule cells, collecting duct cells, and
fenestrated glomerular endothelial cells (i.e., podocytes)).
[0212] The tissue construct may also include one or more functional
chemical substances selected from among drugs, toxins, proteins
and/or hormones, including, but not limited to: growth factors,
growth inhibitors, cytokines, steroids, and/or morphogens. Some
cell specific examples include: bone morphogenic protein, vascular
endothelial growth factor, fibroblast growth factors, including but
not limited to VEGF, EGF, TGF-beta. The one or more functional
chemical substances may be deposited with the cell-laden
filament(s) and/or the sacrificial filaments and may diffuse into
the surrounding extracellular matrix composition.
[0213] Such an approach may be used to generate gradients of cues
within the extracellular matrix composition. Cells respond to
gradients of fixed and diffusible chemical cues during development,
wound healing and inflammatory responses that can direct cell
migration, proliferation and differentiation. One method to
introduce gradients of cues is to directly print cell-laden
filaments preloaded with cues of interest, as illustrated in FIGS.
12A-12C, which may diffuse out upon encapsulation with an
extracellular matrix composition to generate concentration
gradients. Such gradients may or may not be anchored to the
scaffold by action of transglutaminase. Alternatively, to generate
fixed, long-term gradients, the channels formed by removing
fugitive ink can be used to introduce factors that may diffuse into
the surrounding extracellular matrix composition. For example, the
formation of a linear gradient of fluorescently labeled BSA is
demonstrated in FIGS. 12D-12F by creating a pair of parallel
channels and flowing the fluorescent BSA through only one channel.
At 24 hours, a near-linear gradient is apparent between the two
channels (FIGS. 12E-12F).
[0214] The extracellular matrix material of the cell-laden
filaments and the extracellular matrix composition that at least
partially surrounds the tissue and vascular patterns may comprise a
synthetic or naturally derived biocompatible material. The
extracellular matrix material and the extracellular matrix
composition may comprise the same or different biocompatible
materials. Because the cell-laden filaments and, in some
embodiments, the extracellular matrix composition may be deposited
in a 3D printing process that entails extrusion through a
micronozzle, as described below, it may be beneficial for one or
both of the extracellular matrix material and the extracellular
matrix composition to: (1) exhibit shear thinning behavior; (2)
exhibit a defined yield stress T.sub.y; and/or (3) have a shear
elastic modulus G' and a shear viscous modulus G'' modulus where
G'>G'' at room temperature.
[0215] In one example, the extracellular matrix material and/or the
extracellular matrix composition may comprise a gel. An ideal gel
for bioprinting applications may exhibit a rapid transition from a
low viscosity solution to a solid-like gel, which may be seen by an
initial increase in shear elastic modulus. Rapid, controllable
gelation may enhance printed structure fidelity by minimizing or
obviating swelling and dissociation typical of slow gelation
processes. The term "gel" may refer to a semi-solid substance that
may comprise a gelling agent to provide viscosity or stiffness. The
gel may be formed upon use of a gelling agent, such as a thickening
agent, crosslinking agent or a polymerization agent, and may
comprise a cross-linked structure or a non-cross-linked structure.
The gel may be hydrophobic or hydrophilic. Some examples of
suitable gels include a hydrogel, thermo-reversible gel, a
photo-sensitive gel, a pH sensitive gel, a peptide gel, or a cell
type specific gel. Additional examples of gels include silica gel,
silicone gel, aloe vera gel, agarose gel, nafion, polyurethane,
elastomers (thermoplastic, mineral-oil thermoplastic, etc.),
ion-exchange beads, organogels, xerogels and hydrocolloids.
Hydrogels include those derived from collagen, hyaluronate, fibrin,
alginate, agarose, chitosan, gelatin, matrigel, glycosaminoglycans,
and combinations thereof. In one example, the gel may comprise
gelatin methacrylate (GelMA), which is denatured collagen that is
modified with photopolymerizable methacrylate (MA) groups. Suitable
hydrogels may comprise a synthetic polymer. In certain embodiments,
hydrogels may include those derived from poly(acrylic acid) and
derivatives thereof, poly(ethylene oxide) and copolymers thereof,
poly(vinyl alcohol), polyphosphazene, and combinations thereof. The
extracellular matrix material and/or the extracellular matrix
composition may comprise a naturally derived biocompatible
material, such as one or more extracellular matrix components,
including collagen (e.g., I, III, and IV), fibrin, fibronectin,
fibrinogen, gelatin (e.g., low and high bloom gelatin and/or
temperature treated), laminin, hyaluronates (e.g., hyaluronic
acid), elastin, and/or proteoglycans. Other suitable biocompatible
materials for the extracellular matrix material and/or the
extracellular matrix composition may include variations and/or
combinations of cellulose, Matrigel, acrylates, acrylamides,
polylactic co-glycolic acid, epoxies, aldehydes, ureas, alcohols,
polyesters, silk, carbopol, proteins, glycosaminoglycans,
carbohydrates, minerals, salts, clays, hydroxyapatite, and/or
calcium phosphate. Further examples may include variations and/or
combinations of N-Isopropylacrylamide (NIPAAM), Polyethylene glycol
(PEG), gelatin methacrylate (GelMA), Polyhydroxyethylmethacrylate
(PHEMA). Combinations of the above listed materials are also
contemplated for use as the extracellular matrix material and/or
the extracellular matrix composition.
[0216] In a preferred embodiment, the extracellular matrix material
and/or the extracellular matrix composition may comprise gelatin
and fibrin. The gelatin and fibrin may form an interpenetrating
polymer network that mimics natural extracellular matrix (ECM) and
may be optimized for cell attachment, bioprinting, transparency,
and biocompatibility. The fibrin-gelatin interpenetrating polymer
network may be created by mixing solutions of fibrinogen and
gelatin with transglutaminase (TG), a slow-acting Ca.sup.2+
dependent enzyme, to create a gel-precursor solution that may later
be mixed with bovine thrombin to create a fibrin gel backbone, as
illustrated in FIGS. 13A-13D. Fibrin may be made from a
concentrated fibrinogen solution that has been activated by bovine
thrombin and calcium chloride. Fibrin is a rapidly coagulating
phase that permits rapid, controllable gelation of a printed
structure. Advantageously, fibrin and gelatin can be welded
together via mobile surface chain entanglement, while forming a
strong interface. Creating monolithic gels of this nature is
possible due to the slow crosslinking kinetics of transglutaminase
(TG). Although thrombin rapidly induces fibrin gel formation, the
gelatin present in the IPN allows one to print sacrificial ink on
the already cast layer, and, ultimately, to encapsulate with liquid
gelatin-fibrin. The two phases may weld together, creating a
monolithic gel. This material system, which is discussed further
below in the Examples, can be readily tailored to modify gelation
kinetics, interface adhesion, mechanical properties, optical
properties, and cell-material interactions.
[0217] As described above, one or more sacrificial filaments
comprising a fugitive ink may be deposited on and/or in a substrate
to form a vascular pattern that interpenetrates one or more tissue
patterns. The vascular pattern comprises a two- or
three-dimensional interconnected arrangement or network of the one
or more sacrificial filaments. Removal of the fugitive ink after
partial or complete encapsulation with the extracellular matrix
composition creates a perfusable network of vascular channels in
the tissue construct. Because, like the cell-laden filaments, the
sacrificial filaments may be deposited in a 3D printing process
that involves extrusion through a micronozzle, it may be
advantageous for the fugitive ink to: (1) exhibit shear thinning
behavior; (2) exhibit a defined yield stress T.sub.y; and/or (3)
have a shear elastic modulus G' and a shear viscous modulus G''
modulus where G'>G'' at room temperature.
[0218] The substrate for deposition typically comprises a material
such as glass or other ceramics, silicone, PDMS, acrylic,
polyurethane, polystyrene or other polymers. In some embodiments,
the substrate may comprise living tissue or dehydrated tissue, or
one of the extracellular matrix compositions described above. The
substrate may be cleaned and surface treated prior to printing. For
example, glass substrates may undergo a silane treatment to promote
bonding of the cell-laden filaments to the glass substrate. In some
embodiments, it is envisioned that the substrate may not be a
solid-phase material but may instead be in the liquid or gel phase
and may have carefully controlled rheological properties, as
described, for example, in W. Wu et al., Adv. Mater. 23 (2011)
H178-H183, which is hereby incorporated by reference. In the work
of Wu et al., a fugitive ink was printed directly into synthetic
hydrogels to create network structures. However, these synthetic
materials do not support cell attachment and proliferation,
limiting their use to non-biological applications. In the present
disclosure, an extracellular matrix composition that facilitates
cell attachment, migration, proliferation, and tissue-specific
function while maintaining the appropriate rheology for printing is
described. The cell-laden and sacrificial filaments are embedded in
the extracellular matrix composition during printing, and thus the
at least partial surrounding of the tissue and vascular patterns
with the extracellular matrix composition occurs during deposition
of each of the cell-laden and sacrificial filaments, as shown
schematically in FIG. 21A. This includes arbitrarily complex 3D
structures that may require support material during printing, as
shown for example in FIG. 21C. When the forming and embedding of
the tissue and vascular patterns occurs simultaneously, as
described above, the substrate onto which and/or into which
deposition occurs may be considered to be the container that holds
the extracellular matrix composition or the extracellular matrix
composition itself.
[0219] To form the extracellular matrix composition, a microgel
(e.g., a poly(acrylic acid) (PAA) microgel) may be used as a
rheological modifier and blended with one or more extracellular
matrix materials, as set forth previously, such as gelatin
methacrylate. A semi-interpenetrating polymer network (semi-IPN)
may be formed, as shown schematically in FIG. 21B. Microgels may be
understood to comprise colloidal gel particles that are composed of
chemically cross-linked three-dimensional polymer networks.
Microgels may act as sterically stabilized colloids with only a
shell and no core. They can vary in composition and may include
PAA, polystyrenes, PEG, and/or other biomaterials. It is
contemplated that a natural extracellular matrix or biomaterial may
be converted into a microgel form to impart the ideal rheology.
Examples of suitable biomaterials include hyaluron, collagen,
alginate, fibrin, albumin, fibronectin, elastin, or matrigel.
Alternatively, synthetic materials such as PEG, acrylates,
urethanes, or silicones may be modified in a similar manner.
[0220] Representative rheological measurements of ink and matrix
rheology that are appropriate for embedded printing are shown in
FIG. 21D. In one example, a high molecular weight (>1.25 MDa)
PAA microgel is used as a rheological modifier and blended with
gelatin-methacrylate (GelMa) to create an extracellular matrix
composition that supports the creation of complex 3D vascular
networks, which can be endothelialized as described previously. The
transparency of the extracellular matrix composition may be altered
by varying the degree of substitution and mesh size, as shown in
FIG. 22. FIG. 21E shows a vascular cube demonstrating the control
over the embedded printing of 3D vascular networks, and FIG. 21F
shows a printed cell-laden filament within a PAA-GelMA
extracellular matrix composition.
[0221] The method may further include, prior to surrounding or
encapsulating the tissue and vascular patterns with the
extracellular matrix composition, depositing one or more structural
filaments layer by layer on and/or in the substrate in a
predetermined pattern to form a mold. The structural filaments may
comprise one or more structural materials selected from among the
exemplary extracellular matrix compositions or extracellular matrix
materials provided above. The mold may hold the extracellular
matrix composition during the encapsulation and may remain as part
of the tissue construct, or it may be removed after processing. The
structural filaments may define the perimeter of the tissue
construct on and/or in the substrate and all or at least a portion
of the three-dimensional shape of the tissue construct out of the
XY plane.
[0222] The mold may also have other functionalities besides
defining the shape of the tissue construct. For example, the mold
may serve as an interface for perfusion of channels in a printed
tissue construct. FIGS. 15A-15D and 15E-15G show exemplary designs
of printed molds or interface structures. The exemplary mold shown
in FIGS. 15A-15D is designed for passive rocking perfusion. The
mold, which may also be referred to as an interface structure, can
hold vascularized tissue in place during rocking by immobilizing
the tissue construct between a base portion of the mold, which may
comprise PDMS, and an overlying cover, which may comprise glass.
The perfusion may be for a short time periods (i.e., 14 days). In
certain embodiments, vascularized tissues may be perfused on chip
for long time periods (>3 months).
[0223] In certain embodiments, as described in the Examples
section, the tubular tissue construct may be printed and/or
embedded into vascularized matrix on a mold, e.g., perfusable chip
for arterial and venous circulation. To manufacture the customized
perfusion chips, the silicone ink may be loaded into a syringe,
centrifuged to remove air bubbles, and deposited through a tapered
nozzle (e.g., 410 .mu.m); printed using a direct ink writing (see
FIGS. 26A-B). The gasket design may be created using custom MATLAB
software and the structures may be printed onto, e.g., 50
mm.times.75 mm glass slides. After printing, the chips may be cured
at 80.degree. C. in an oven for >1 hour and stored at room
temperature.
[0224] In certain further embodiments, printing of the tubular
tissue construct on a perfusable chip allows for exposing the
tubular tissue construct to one or more biological agents, a
biological agent gradient, a pressure, and/or an oxygen tension
gradient. The one or more biological agents, the biological agent
gradient, the pressure, and/or the oxygen tension gradient further
direct development, differentiation, and/or functioning of the
tubular tissue construct.
[0225] The mold designs of FIGS. 15E-15G enable active pump-based
perfusion of a tissue construct and include flow channels that are
in fluid communication with (e.g., contiguous with) the vascular
channels of the tissue construct. Conduits that serve as flow
channels may be partially or fully embedded in the mold itself and
hollow pins (e.g., metal pins) may be used to interface with the
vascular channels, as shown in FIGS. 15F-15G. The exemplary mold
shown in FIG. 15E has a wall with multiple buttresses that contain
the flow channels, which include hollow pins extending into the
interior of the mold, where the tissue construct is fabricated. The
vascular channels of the tissue construct may be contiguous with
apertures of the hollow pins to enable flow to be introduced into
the vascular channels from tubing connected to the flow channels,
and fluid may be removed from the vascular channels through one or
more other apertures.
[0226] In one example, the mold may be formed of an elastomeric
silicone, a structural material known to be viscoelastic,
non-toxic, biocompatible, and capable of forming reversible
press-to-fit seals. The structural material may be 3D printed to
form one or more uncured structural filaments comprising one or
more of silicone, epoxies, esters of acrylic acid, or one of the
extracellular matrix compositions provided above. After printing is
complete, the structural filament(s) may be cured (e.g. by heating
or photopolymerizing) for a suitable time duration (e.g., about one
hour or more), after which the mold may exhibit the desired
material properties.
[0227] The encapsulation of the tissue and vascular patterns may
comprise casting a liquified matrix precursor into the mold and
gelling the matrix precursor to form the extracellular matrix
composition. Casting of the matrix precursor may take place at a
temperature of from about 25.degree. C. to about 40.degree. C. For
example, gelatin methacrylate, or GelMA, may be cast at a
temperature of about 37.degree. C. After casting, the matrix
precursor may be cooled (e.g., to about 15.degree. C. in the case
of GelMA or 4.degree. C. in the case of Gelbrin) to form a rigid
physical gel. Alternatively, the encapsulation may occur during
deposition of the tissue and vascular patterns in an embedded or
omni-directional 3D printing process, as indicated above. It is
also contemplated that the extracellular matrix composition may be
deposited by filament deposition, similar to the cell-laden and
sacrificial filaments. For example, one or more ECM filaments
comprising the extracellular matrix composition may be extruded
from a nozzle and deposited on and/or in the substrate layer by
layer to build up the desired 3D geometry, as described below. In
such a case, it may not be necessary to employ a mold to contain
the extracellular matrix composition.
[0228] The extracellular matrix composition may be cured before or
after removal of the fugitive ink to form a permanently chemically
cross-linked structure. Depending on the extracellular matrix
composition, the curing may entail heating, UV radiation or
chemical additives (e.g., enzymatic curing).
[0229] Any or all of the filaments deposited on and/or in the
substrate--including the cell-laden filaments defining the one or
more tissue patterns, the one or more sacrificial filaments
defining the interpenetrating vascular pattern or a functional
channel pattern, the one or more structural filaments that may
define the mold, and/or the one or more ECM filaments that may
yield the extracellular matrix composition--may be extruded from a
nozzle before being deposited on and/or in the substrate. In the
discussion of the extrusion process that follows, the sacrificial
filaments, the cell-laden filaments, the structural filaments
and/or the ECM filaments may be collectively referred to as "the
filaments" since the processing steps may be applicable to any or
all of the filament compositions.
[0230] FIG. 4 shows four exemplary nozzles or print heads that may
be employed to extrude the filaments and deposit them on and/or in
the substrate. The nozzles shown are part of a custom-built 3D
printer comprising a large build platform (750 mm.times.650 mm)
equipped with four independent z-axes. FIGS. 5A-5C provide a
demonstration of the 3D printing of a four-layer, multimaterial
construct by sequential deposition of a filament of a different
composition from each of the four nozzles. The insets of FIG. 5C
show, for each layer, the repeating unit of the 3D structure.
[0231] Although there are four nozzles for the exemplary printer of
FIGS. 4 and 5A, the number of nozzles employed to form the tissue
construct by 3D printing may be lower or higher. In general, 1 or
more, 2 or more, 3 or more, 4 or more, 5 or more and up to N
nozzles may be used for extruding the filaments, where
1.ltoreq.N.ltoreq.1024, and more typically N is no more than 512, N
is no more than 256, N is no more than 128, or N is no more than
64. The filaments may be extruded from the N nozzles sequentially
in a serial deposition process or simultaneously in a parallel
deposition process, where each nozzle may contain a different
precursor ink (e.g., a cell-laden ink comprising one or more
predetermined cell types, a fugitive ink, a structural ink, or an
ECM ink). It is also contemplated that the deposition may include
both parallel and serial deposition steps. To facilitate sequential
or serial printing, the nozzles can be independently controlled in
the z-direction, as shown in FIG. 4.
[0232] Each nozzle may have an inner diameter of from about 1
micron to about 1 mm in size, and more typically from about 50
microns to about 500 microns. The size of the nozzle may be
selected depending on the desired filament diameter. Depending on
the injection pressure and the nozzle translation speed, the
deposited filament may have a diameter ranging from about 1 micron
to about 10 mm, and more typically from about 100 microns (0.1 mm)
to about 1 mm. The inks fed to the nozzles may be housed in
separate syringe barrels that may be individually connected to a
nozzle for printing by way of a Luer-Lok.TM. or other connector.
The extrusion of each of the filaments may take place under an
applied pressure of from about 1 psi to about 200 psi, from about
10 psi to about 80 psi, or from about 20 psi to about 60 psi. The
pressure during extrusion may be constant or it may be varied. By
using alternative pressure sources, pressures of higher than 100
psi or 200 psi and/or less than 1 psi may be applied during
printing. A variable pressure may yield a filament having a
diameter that varies along the length of the filament. Such an
approach may be used, for example, to form the branching,
hierarchical vascular network shown in FIGS. 2C and in FIGS. 7E-7F,
which is formed from sacrificial filaments of various lengths and
diameters. The extrusion may be carried out at ambient or room
temperature conditions (e.g., from about 18.degree. C. to about
25.degree. C.).
[0233] During the extrusion and deposition of each filament, the
nozzle may be moved along a predetermined path (e.g., from
(x.sub.1, y.sub.1, z.sub.1) to (x.sub.2, y.sub.2, z.sub.2)) with a
positional accuracy of within .+-.100 microns, within .+-.50
microns, within .+-.10 microns, or within .+-.1 micron.
Accordingly, the filaments may be deposited with a positional
accuracy of within .+-.200 microns, within .+-.100 microns, within
.+-.50 microns, within .+-.10 microns, or within .+-.1 micron. The
nozzles may be moved and the filaments may be deposited at speeds
as high as about 3 m/s (e.g., from about 1 cm/s to about 3 m/s),
and are more typically in the range of from about 1 mm/s to about
500 mm/s, from about 1 mm/s to about 100 mm/s, or from about 1 mm/s
to about 10 mm/s.
[0234] The predetermined path of the nozzle may have an XY boundary
area of at least about 2400 cm.sup.2, at least about 2700 cm.sup.2
and up to about 1 m.sup.2 as determined by the size of the build
platform of the printer. For example, the build platform may have a
length of from about 60 cm to about 100 cm and a width of from
about 40 cm to about 100 cm. Each print head may be moved in the
z-direction a distance from about 10 cm to about 50 cm, or about 15
to about 30 cm.
[0235] The deposited filaments are formed from precursor inks
(e.g., cell-laden inks comprising one or more predetermined cell
types, fugitive inks, structural inks, or ECM inks) having a
suitable composition and rheological properties. The precursor inks
may be viscoelastic and comprise a viscosity with a non-linear
shear dependence. The viscosity of the precursor inks may fall in
the range of from about 0.001 Pa-sec to about 10,000 Pa-sec. The
precursor inks may optionally include viscosifiers to help control
the rheological properties. Each cell-laden ink, and optionally,
the fugitive and/or ECM ink, may include one or more cells of one
or more predetermined cell types in a carrier that may be a liquid
or a gel. The carrier may include, in addition to an extracellular
matrix material as described above, one or more functional chemical
substances as described above. The carrier may also or
alternatively include a cell culture medium designed to support the
growth of cells. In one example, to form a cell-laden ink
comprising viable cells mixed with a hydrogel, a predetermined
amount of a hydrogel precursor powder is mixed with a cell culture
medium to form a solution of an appropriate composition. The cells
of interest are then dispersed in the solution at the desired cell
concentration (e.g., any of the cell concentrations set forth above
for the cell-laden filaments), and mixed thoroughly. Steps to
prepare exemplary cell-laden GelMA inks, cell-laden gelatin-fibrin
inks, Pluronic F127 fugitive inks, and PDMS structural inks are
described in the Examples below.
[0236] After encapsulation of the tissue and vascular patterns, the
fugitive ink may be removed to form a network of vascular channels
in the extracellular matrix composition. The fugitive ink may
comprise a biocompatible material and may be designed for
compatibility with the cell-laden formulations and the
extracellular matrix composition during room temperature
deposition. Suitable fugitive inks may include, for example,
Pluronic F127, Pluronic F123, agarose, sugar, wax, and fatty oils
(e.g., animal fat derived oils such as Crisco). If a hydrogel is
employed for the extracellular matrix composition (and/or the
extracellular matrix material), and a hydrogel such as Pluronic
F127 is employed as the fugitive ink, it may be advantageous for
the fugitive ink and the matrix hydrogel to have similar water
contents (e.g., within .+-.30%) to avoid distortion of the fugitive
ink after printing. The fugitive ink and the extracellular matrix
composition may also be selected to have complementary thermal
transitions, as discussed further below.
[0237] Pluronic F127 is an FDA-approved material that is
biologically inert to multiple cell types over the short time
periods needed to complete the fabrication process. The material
includes a hydrophobic poly(propylene oxide) (PPO) segment and two
hydrophilic poly(ethylene oxide) (PEO) segments arranged in a
PEO-PPO-PEO configuration. Pluronic F127 undergoes thermally
reversible gelation above a critical micelle concentration (CMC;
about 21 wt. %) and the gelation temperature. The gelation
temperature decreases from approximately 10.degree. C. to 4.degree.
C. as the PEO-PPO-PEO concentration increases. When both of these
critical parameters are exceeded, micelles form as the hydrophilic
PEO segments self-assemble into corona that are well solvated by
water, while the hydrophobic PPO segments tightly associate within
the micelle cores. However, below the gelation temperature, the
hydrophobic PPO units are hydrated, such that individual
PEO-PPO-PEO species become soluble in water giving rise to a
gel-to-fluid transition for systems whose concentration exceeds the
CMC. Thus, the material liquifies upon cooling below the gel
point.
[0238] It is important that the patterned cells and surrounding
extracellular matrix composition are not damaged during deposition
of the sacrificial filaments or removal of the fugitive ink, and
thus it is preferred that harsh solvents and/or elevated
temperatures are not utilized during the removal process. With
proper selection of the fugitive ink and the extracellular matrix
composition/material, the fugitive ink may be removed without
damage to the tissue construct. For example, if the fugitive ink
undergoes a gel-to-fluid transition as described above, cooling of
the vascular pattern after encapsulation may be effective for
removal of the fugitive ink. To remove Pluronic F127, the vascular
pattern may be cooled to a temperature of no more than about
1.degree. C., depending on the concentration. It is also
contemplated that the fugitive ink may be dissolved in a suitable
aqueous solution for removal. Once the fugitive ink is liquefied or
dissolved, a vacuum may be applied to an exposed end of the
vascular pattern to extract the ink.
[0239] Advantageously, the tissue constructs may be designed to
support the attachment and proliferation of endothelial cells,
which line vascular channels providing a barrier to fluid
diffusion, while simultaneously facilitating homeostatic functions
and helping establish vascular niches specific to the various
tissues. To promote endothelialization, in some embodiments the
sacrificial filament(s) comprising the fugitive ink may further
include a plurality of endothelial cells or other viable cells. The
cells may be deposited along with the sacrificial filament and may
remain in the vascular channels after removal of the fugitive ink,
as illustrated in FIGS. 16A-16C. Direct cellularization of the
channels can be achieved if the cells adsorb to the channel walls
after liquidation of the fugitive ink. This approach may allow one
to incorporate viable cells into highly tortuous networks or small
channels that may be difficult to infill using direct injection due
to an increased resistance to flow. An exemplary printed tissue
construct including a channel formed by evacuation of a fugitive
ink comprising endothelial cells and Pluronic F127 is shown in
FIGS. 16D-16G, and is further described in the Examples. In another
example, epithelial cells may be delivered in a fugitive ink and
used to create tubular epithelial tissues present in the mammary
gland, kidney or liver.
[0240] In addition to or as an alternative to depositing
endothelial and/or other viable cells with the fugitive ink,
endothelialization may be effected by injecting a suspension of
viable cells (e.g., endothelial cells) into the vascular channels
after removing the fugitive ink. Using one or both of these
approaches, an endothelial layer having up to 100% confluency may
be formed lining the wall of one or more of the vascular channels,
where 100% confluency means that the wall is completely covered by
endothelial cells. Each endothelial layer formed in the network of
vascular channels may have a confluency of at least about 80%, at
least about 90%, at least about 95%, at least about 98%, at least
about 99%, or 100%, so that the vascular channels may function as
actual blood vessels. As described in the Examples below, it has
been shown that representative hierarchical bifurcating networks
may be successfully injected with a human umbilical vein
endothelial cell (HUVEC) suspension followed by gentle rocking (see
FIG. 7J). After 48 h, these cells retained greater than 95%
viability and assembled into a nearly confluent layer, as
determined by live/dead staining coupled with confocal imaging
within a representative, bifurcated microchannel.
[0241] Multiple types of cells may be injected into the vascular
channels. In vivo, every blood vessel having a diameter larger than
a capillary contains an outer fibrous tissue layer, a smooth muscle
layer, and an inner layer of endothelial cells. One or more other
types of cells, such as fibroblasts, may be injected into the
vascular channels along with the endothelial cells after removing
the fugitive ink. As described in the Examples below, the vascular
channels may be co-seeded with fibroblasts and HUVECs, two cell
types which may self-assemble into stromal and endothelial layers,
respectively, mimicking the anatomy of native blood vessels.
[0242] It is also contemplated that the same or a different
fugitive ink may be deposited as a sacrificial filament and removed
as described above to form channels, ducts and/or compartments in
addition to or in lieu of the vascular channels within the tissue
construct. In other words, one or more additional sacrificial
filaments may be deposited to form a functional channel pattern on
and/or in the substrate, either in addition to or in lieu of the
vascular pattern. This is shown schematically in FIG. 17A.
[0243] Each additional sacrificial filament may comprise a second
fugitive ink, which is the same as or different from the fugitive
ink used to define the vascular pattern (if the vascular pattern is
present). After deposition, the functional channel pattern may be
at least partially surrounded with the extracellular matrix
composition (e.g., the hydrogel solution shown in step III of FIG.
17A), as described above in reference to the vascular pattern. It
is also contemplated that the at least partial surrounding of the
functional channel pattern with the extracellular matrix
composition may occur during deposition of the one or more
sacrificial filaments, such that the one or more functional channel
patterns are simultaneously formed and embedded in the
extracellular matrix composition. When the forming and embedding of
the functional channel patterns occurs simultaneously, the
substrate onto which and/or into which deposition occurs may be
considered to be the container that holds the extracellular matrix
composition or the extracellular matrix composition itself. The
second fugitive ink may then be removed, as illustrated in step V
of FIG. 17A and as described above in reference to the vascular
channels, to create one or more functional channels in the
extracellular matrix composition. Thus, a functional channel
network may be formed in the tissue construct, which in this
example is an epithelial tissue construct, as shown in FIGS. 17A
(center) and 17B. One or more types of viable cells (e.g.,
epithelial cells) may be deposited with the additional sacrificial
filaments, and at least a portion of the viable cells may remain in
the one or more functional channels after removal of the second
fugitive ink. Also or alternatively, after removing the second
fugitive ink, a suspension of viable cells (e.g., epithelial cells)
may be injected into the functional channels, as shown in step VI
of FIG. 17A.
[0244] The functional channels may define tubular tissues or tissue
components. Examples of tubular structures that can be formed via
3D printing and epithelialization include, but are not limited to,
a nephron (of the kidney), the tubule portion of the nephron (of
the kidney), intestine, milk duct, sweat gland, colon, esophagus,
stomach, eustachian tube, airway epithelium, epididymis,
seminiferous tubules, urethra, liver bile duct, pancreatic duct,
common bile duct, cerebro-spinal ventricles and aquaducts, parotid
glands, oral mucosa, fallopian tube, vas deferens, or lymph. Such a
printed epithelial tissue construct may comprise one or more
functional channels comprising an epithelial layer thereon, and an
extracellular matrix composition may at least partially surround
the one or more functional channels, as illustrated in FIGS. 17A
(center) and 17B. A stromal layer may also be present on the
epithelial layer. The printed epithelial tissue construct may
further comprise one or more tissue patterns, each comprising a
plurality of viable cells of one or more predetermined cell types,
in the extracellular matrix composition, as set forth above. The
viable cells and the one or more predetermined cell types may
comprise epithelial cells and/or another cell type described
previously. The printed epithelial tissue construct may further
comprise a network of vascular channels in the extracellular matrix
composition, also as described above.
[0245] For example, a network of vessels (channels) of the
lymphatic system may be created using sacrificial filaments
comprising a fugitive ink. In another example, compartments of any
desired geometry may be embedded within the tissue construct by
depositing a predetermined arrangement of sacrificial filaments.
Such embedded compartments may be used for containing growth
factors, additional cells and/or supplemental scaffold materials
that may in some embodiments be deposited with the sacrificial
filaments to direct cell behavior, differentiation, function,
movement and/or growth.
[0246] A printed epithelial tissue construct comprising a
functional channel that is subsequently seeded with epithelial
cells (epithelialization) is shown in FIGS. 17A-17F. Referring to
FIGS. 17A-17B, the printed tissue construct is a proximal
convoluted tubule, a portion of the nephron. It can be printed in a
simple or convoluted shape and seeded with epithelial cells, which
thrive and circumscribe the functional channels, as shown in FIGS.
17C-17F. The cells employed are human renal proximal tubule cells
(PTEC); however, this approach may be applied to any of a number of
types of epithelial tissue. For example, in vitro models may be
fabricated for tissue-specific disease and toxicity studies. This
type of functional human tissue mimic can be used as a building
block for the growth of larger organs or for high throughput drug
toxicity and screening.
[0247] A printed tubular tissue construct, such as a nephron
described below, including at least two types of viable cells, each
type of viable cells being positioned along a different
predetermined location of the tubular tissue construct and methods
of 3D printing the tubular tissue construct are described.
[0248] A kidney consists of two regions: an outer region (cortex)
and an inner region (medulla). The cortex and medulla are composed
on nephrons (which are the functional units of the kidney), blood
vessels, lymphatics, and nerves. Each kidney contains approximately
1.2 million nephrons, which are hollow tubes composed of a single
cell layer. As shown in FIG. 23A, each nephron consists of a renal
corpuscle, a proximal tubule, a loop of Henle, a distal tubule, and
a collecting duct system.
[0249] The first step in urine formation begins with the
ultrafiltration of plasma across the glomerular capillaries (i.e.,
glomerulus). The glomerulus consists of network of capillaries
supplied by the afferent arteriole and drained by the efferent
arteriole. The capillaries are covered by epithelial cells, called
podocytes, which form the visceral layer of Bowman's capsule. The
visceral cells are reflected at the vascular pole to form the
parietal layer of Bowman's capsule. The space between the visceral
layer and the parietal layer is called Bowman's space, which at the
urinary pole of glomerulus, becomes the lumen of the proximal
tubule. The endothelial cells of glomerular capillaries are covered
by a basement membrane, which is surrounded by podocytes.
[0250] The proximal tubule initially forms several coils, followed
by a slight piece that descends toward the medulla. The next
segment is Henle's loop (noted as "intermediate tubule" in the
figure), which is composed of the straight part of the proximal
tubule, the descending thin limb, the ascending thin limb (only in
nephrons with long loops of Henle), and the thick ascending limb.
The last segment before the collecting duct system is the distal
tubule.
[0251] Each nephron segment is composed of cells that are uniquely
suited to perform specific transport functions (FIGS. 23B-E).
Proximal tubule cells have an extensively amplified apical membrane
(the urine side of the cell, called the brush border), which is
only present in the proximal tubule. The basolateral membrane (the
blood side of the cell) is highly invaginated. These invaginations
contain many mitochondria. In contrast, the descending thin limb
and ascending thin limb of Henle's loop have poorly developed
apical and basolateral surfaces and few mitochondria. The cells of
the thick ascending limb and the distal tubule have abundant
mitochondria and extensive infoldings of the basolateral membrane.
The collecting duct is composed of two types of cells: principal
cells and intercalated cells. Principal cells have a moderately
invaginated basolateral membrane and contain few mitochondria.
Intercalated cells have a high density of mitochondria. The final
segment of the nephron, the inner medullary collecting duct, is
composed of inner medullary collecting duct cells.
[0252] A printed tubular tissue construct that, in certain
embodiments, resembles and functions as a nephron, is described.
The printed tubular tissue construct includes one or more
functional channels comprising a patterned cell layer thereon along
the length of the functional channels, where the patterned cell
layer includes the one or more types of viable cells, each type of
viable cells positioned along different predetermined location of
the functional channel. The patterned cell layer may include a
plurality of viable cells of at least two predetermined types (as
described, e.g., immediately above); alternatively, a plurality of
viable cells of at least three predetermined types; and
alternatively, a plurality of viable cells of at least four or more
predetermined types. In certain embodiments, the patterned cell
layer may include renal proximal tubule cells, loop of Henle cells
and/or renal distal tubule cells, collecting duct cells, and
fenestrated glomerular endothelial cells (i.e., podocytes)),
mesangial cells, renal microvascular cells, renal cell progenitors,
pluri or multipotent stem cells, other endothelial lineage cells,
each distributed along a different predetermined location of the
construct, thereby forming a nephron. In certain other embodiments,
patient-specific iPSCs may be used as starting materials to derive
patient-specific predetermined cell types for use in the tissue
construct development. For example, patient specific iPSCs can be
used to derive a pro-kidney or individual cell lines present in a
nephron (e.g., renal proximal tubule cells, loop of Henle cells
and/or renal distal tubule cells, collecting duct cells, and
fenestrated glomerular endothelial cells (i.e., podocytes)).
iPSCs-derived cells lines may then be used as the predetermined
types of cells for printing patient-specific tubular tissue
constructs.
[0253] FIG. 23F depicts a picture of an embedded nephron print
(pluronic with fluorescent dye in PDMS).
[0254] Specifically, in certain embodiments, a printed tubular
tissue construct includes functional channels comprising a
patterned cell layer thereon along the length of the functional
channel, the patterned cell layer comprising one or more types of
viable cells, each type of viable cells being positioned along a
different predetermined location of the functional channel. The
printed tubular tissue construct also includes an extracellular
matrix composition at least partially surrounding the functional
channels. The patterned cell layer may include a plurality of
viable cells of at least two predetermined types (as described
above). In certain embodiments, the extracellular matrix
composition at least partially surrounds the one or more tissue
patterns. The extracellular matrix composition may include a
naturally derived biocompatible material, such as one or more
extracellular matrix components, including collagen (e.g., I, III,
and IV), fibrin, fibronectin, fibrinogen, gelatin (e.g., low and
high bloom gelatin and/or temperature treated), laminin,
hyaluronates (e.g., hyaluronic acid), elastin, and/or
proteoglycans. Other suitable biocompatible materials for the
extracellular matrix composition may include variations and/or
combinations of cellulose, Matrigel, acrylates, acrylamides,
polylactic co-glycolic acid, epoxies, aldehydes, ureas, alcohols,
polyesters, silk, carbopol, proteins, glycosaminoglycans,
carbohydrates, minerals, salts, clays, hydroxyapatite, and/or
calcium phosphate. Further examples may include variations and/or
combinations of N-Isopropylacrylamide (NIPAAM), Polyethylene glycol
(PEG), gelatin methacrylate (GelMA), Polyhydroxyethylmethacrylate
(PHEMA). In certain embodiments, the printed tubular tissue
construct may also include a network of vascular channels in the
extracellular matrix composition. In certain embodiments, the
functional channels may also include immune cells, fibroblast, stem
cells, iPSCs, or the like, which could lead to improved physiology
or disease state modeling.
[0255] A printed tubular tissue construct with embedded vasculature
may include functional channels comprising a patterned cell layer
thereon along the length of the functional channel. The patterned
cell layer includes one or more types of viable cells, each type of
viable cells being positioned along a different predetermined
location of the functional channel. In certain embodiments, the
patterned cell layer includes a plurality of viable cells of at
least two predetermined cell types. The extracellular matrix
composition at least partially surrounds the tissue patterns. In
certain embodiments, the patterned cell layer includes a plurality
of viable cells of at least two predetermined cell types, wherein
the extracellular matrix composition at least partially surrounds
the one or more tissue patterns. The printed tubular tissue
construct also includes a network of vascular channels
interpenetrating functional channels and an extracellular matrix
composition at least partially surrounding the functional channels
and the network of vascular channels. The extracellular matrix
composition may include a naturally derived biocompatible material,
such as one or more extracellular matrix components, including
collagen (e.g., I, III, and IV), fibrin, fibronectin, fibrinogen,
gelatin (e.g., low and high bloom gelatin and/or temperature
treated), laminin, hyaluronates (e.g., hyaluronic acid), elastin,
and/or proteoglycans. Other suitable biocompatible materials for
the extracellular matrix composition may include variations and/or
combinations of cellulose, Matrigel, acrylates, acrylamides,
polylactic co-glycolic acid, epoxies, aldehydes, ureas, alcohols,
polyesters, silk, carbopol, proteins, glycosaminoglycans,
carbohydrates, minerals, salts, clays, hydroxyapatite, and/or
calcium phosphate. Further examples may include variations and/or
combinations of N-Isopropylacrylamide (NIPAAM), Polyethylene glycol
(PEG), gelatin methacrylate (GelMA), Polyhydroxyethylmethacrylate
(PHEMA). In certain embodiments, the tubular tissue construct is a
nephron, where the patterned cell layer includes at least two cell
types selected from renal proximal tubule cells, loop of Henle
cells, renal distal tubule cells, collecting duct cells,
fenestrated glomerular endothelial cells, mesangial cells, renal
microvascular cells, renal cell progenitors, pluri or multipotent
stem cells, other endothelial lineage cells, induced pluripotent
stem cells (iPSCs), and/or iPSCs-derived patent-specific pro-kidney
or individual cell lines present in a nephron, or a combination
thereof. In certain alternative embodiments, the printed tubular
tissue construct with embedded vasculature may be intestine, milk
duct, sweat gland, colon, esophagus, stomach, eustachian tube,
airway epithelium, epididymis, seminiferous tubules, urethra, liver
bile duct, pancreatic duct, common bile duct, cerebro-spinal
ventricles and aquaducts, parotid glands, oral mucosa, fallopian
tube, vas deferens, or lymph. In certain embodiments, the
functional channels may also include immune cells, fibroblast, stem
cells, iPSCs (including patient-specific iPSCs), or the like, which
could lead to improved physiology or disease state modeling.
[0256] Different approaches may be used to generate tubular tissue
constructs, such as nephrons, that include a plurality of multiple
cell types along their lengths. In this regard, 3D printing methods
to pattern functional cell types along the length of a single tube
are described.
[0257] The first approach relates to printing a tubular tissue
construct by directly depositing a plurality of cells types via a
fugitive ink. Specifically, the method of printing a tubular tissue
construct includes depositing sacrificial filaments on and/or in a
substrate to form a functional channel pattern, each sacrificial
filament comprising a fugitive ink and a plurality of predetermined
types of viable cells. Each predetermined type of viable cells is
deposited at a different predetermined location along a length of
the sacrificial filament. Next, the functional channel pattern is
at least partially surrounded with an extracellular matrix
composition. Next, the fugitive ink can be removed to create
functional channels in the extracellular matrix composition, at
least a portion of each different predetermined type of viable
cells remaining at the different predetermined location after
removal of the fugitive ink, thereby forming a tubular tissue
construct. Additional steps may include depositing one or more
sacrificial filaments on and/or in the substrate to form an
interpenetrating network of tubes, each of the sacrificial
filaments comprising a second fugitive ink, and removing the second
fugitive ink, thereby forming the interpenetrating network of tubes
in the tubular tissue construct. A suspension of viable epithelial
or endothelial cells may be injected into the one or more tubes. In
certain embodiments, the interpenetrating network of tubes
comprises a vascular channel pattern interpenetrating the
functional channel pattern. The term "interpenetrating," in the
context of the present invention means that the channel patterns
may or may not be in fluid contact with each other. For example,
the functional channel pattern may be in close proximity to the
vascular pattern but not in fluid contact with each other (i.e.,
independently addressable interpenetrating networks). In certain
embodiments, the at least partial surrounding of the functional
channel pattern with the extracellular matrix composition may occur
during deposition of the one or more sacrificial filaments, the
functional channel patterns thereby being formed and embedded
simultaneously in the extracellular matrix composition. As
described previously, the fugitive ink can be removed by cooling
the sacrificial filaments. In additional, optional step of the
method, cell-laden filaments, each comprising a plurality of viable
cells may be deposited on and/or in the substrate to form tissue
patterns, each of the tissue patterns comprising predetermined cell
types.
[0258] In certain embodiments, the sacrificial filaments can be
extruded through a single printhead before being deposited on
and/or in the substrate. Methods of extruding sacrificial filaments
using a single printhead were previously described in U.S.
Provisional Patent Application Ser. No. 62/133,039, entitled
"Printhead and Method for 3D Printing of Multiple Materials," filed
Mar. 13, 2015, which is incorporated herein in its entirety.
Alternatively, the sacrificial filaments can be extruded through
multiple printheads. Methods of extruding sacrificial filaments
using multiple printheads were previously described in U.S. Pat.
Pub. No. 2014/0314951, which is incorporated by reference in its
entirety.
[0259] Specifically, to print a nephron, continuous sacrificial
filaments are deposited on and/or in a substrate to form functional
channels. Each sacrificial filament includes a first fugitive ink
formulation over a first length of the sacrificial filament, a
second ink formulation over a second length of the sacrificial
filament, and a third ink formulation over a third length of the
sacrificial filament. Additional fugitive ink formulations may also
be used. The first fugitive ink formulation comprises a fugitive
ink and a first type of cells, e.g., renal proximal tubule cells;
the second fugitive ink formulation comprises the fugitive ink and
a second type of cells, e.g., loop of Henle cells; and the third
fugitive ink formulation comprises the fugitive ink and a thirst
type of cells, e.g., renal distal tubule cells, etc. Additional
fugitive ink formulations may include collecting duct cells,
fenestrated glomerular endothelial cells, mesangial cells, renal
microvascular cells, renal cell progenitors, pluri or multipotent
stem cells, other endothelial lineage cells, induced pluripotent
stem cells (iPSCs), and/or iPSCs-derived patent-specific pro-kidney
or individual cell lines present in a nephron. Next, the functional
channel pattern is at least partially surrounded with an
extracellular matrix composition. The fugitive ink is then removed
to create functional channels in the extracellular matrix
composition, at least a portion of the renal proximal tubule cells
remaining along the first length of the one or more functional
channels after removal of the first ink, at least a portion of the
loop of Henley cells remaining in the second length of the one or
more functional channels after removal of the second ink, and at
least a portion of the renal distal tubule cells remaining in the
third length of the one or more functional channels after removal
of the third ink, thereby forming a nephron. In certain
embodiments, continuous sacrificial filaments are deposited on
and/or in a substrate through a single printhead.
[0260] To deposit sacrificial filaments that include a plurality of
multiple cell types along their lengths through a single printhead
a nozzle body comprising multiple ink delivery channels (e.g., a
first ink delivery channel, a second ink delivery channel, a third
ink delivery channel, etc.) in fluid communication with a nozzle
outlet is used. The first ink formulation is forced to flow through
the first ink delivery channel while preventing the second ink
formulation and the third ink formulation from flowing through the
second delivery channel and the third delivery channel,
respectively, thereby extruding through the nozzle outlet the
continuous sacrificial filament comprising the first ink
formulation over the first predetermined length thereof. Once a
sufficient first length of the continuous sacrificial filament
comprising the first ink formulation is deposited, a withdrawal
pulse is then applied to the first ink delivery channel while
applying an infusion pulse to the second ink delivery channel,
thereby forcing the second ink formulation to flow through the
second ink delivery channel while preventing the first ink
formulation and the third ink formulation from flowing through the
first delivery channel and the third delivery channel,
respectively, thereby extruding through the nozzle outlet the
continuous sacrificial filament comprising the second ink
formulation over the second predetermined length thereof. Once a
sufficient second length of the continuous sacrificial filament
comprising the second ink formulation is deposited, a withdrawal
pulse is applied to the second ink delivery channel while applying
an infusion pulse to the third ink delivery channel, thereby
forcing the third ink formulation to flow through the third ink
delivery channel while preventing the first ink formulation and the
second ink formulation from flowing through the first delivery
channel and the second delivery channel, respectively, thereby
extruding through the nozzle outlet the continuous sacrificial
filament comprising the third ink formulation over the third
predetermined length thereof, thereby 3D printing the continuous
sacrificial filaments comprising multiple cell types over different
predetermined lengths of the filaments. In an alternative
embodiment, the continuous sacrificial filaments can be deposited
on and/or in a substrate through multiple printheads. The method of
printing tissue constructs using multiple printheads was previously
described in U.S. Pat. Pub. No. 2014/0314954, which is incorporated
by reference in its entirety.
[0261] The second approach to generate tubular tissue constructs,
such as nephrons, that include a plurality of multiple cell types
along their lengths includes direct depositing specific "binding
domains" or "ligands" for various target cells to be deposited in
the tubular tissue construct. There are various types of binding
domains or ligands that may be used to target specific cells for
placement in a tubular tissue construct. Examples of specific
binding domains or ligands include peptides, proteins, e.g.,
antibodies, small peptides, amino acids, DNA, RNA, aptamers,
nanoparticles, small molecules, chemical functional groups, and/or
bacteria.
[0262] In the context of this invention, a binding domain is a part
of a molecule or structure with physico-chemical features or
properties allowing the molecule or structure to bind with a site
on a protein, a specific atom or molecule (i.e., "receptor")
expressed by or associated with a predetermined type of target
cells. A receptor is a chemical group or molecule (as a protein) on
the surface or in the interior of a predetermined type of cells
that has an affinity for a specific binding domain or ligand, such
as a chemical group, molecule, bacteria, or virus. The receptor
specifically binds, with high affinity, to its ligand.
[0263] All proteins bind to other molecules. The ability of a
protein to bind to or interact with a ligand depends on the
formation of weak, non-covalent bonds between them. This process
relies on the sequence of amino acids in each protein and the way
in which their side chains (or R groups) interact with each other.
Different side chains can form different bonds. Because of this,
protein interactions can be very specific. As such, proteins may be
deposited as binding domains or ligands to bind to a site on a
protein ("receptor") expressed or associated with a predetermined
type of target cells via protein-protein interaction or binding.
One example of proteins that may be deposited as binding domains
includes antibodies.
[0264] Antibodies are "Y" shaped proteins that recognize and bind
to other proteins (intracellular and extracellular proteins),
called "antigens." Antigens, in the context of this invention, are
differentiation molecules that are expressed by or associated with
a predetermined type target cells. Antibodies display remarkable
specificity for antigens. The antibody systems that may be used to
bind target cells include antibodies that will bind directly with
an antigen present on the target cell. Different types of cells
express different combinations of antigens and/or differentiation
molecules on their surface, and produce different intracellular and
secretable proteins, which can be targeted by the antibodies.
Specifically, each tip of the "Y" of an antibody contains a
paratope that is specific for one particular epitope (similarly
analogous to a key) on a differentiation molecule, allowing these
two structures to bind together with precision. Using this binding
mechanism, an antibody can identify or immobilize target cells.
Antibodies for direct binding cells may be purchased directly from
various antibody suppliers.
[0265] Receptors expressed by different types of cells can also
bind to and interact with DNA or RNA. As such, in certain
embodiments, DNA and/or RNA can function as binding domains.
Helical DNA's backbone is made of sugar-phosphate-sugar linkages,
where phosphates in di-ester groups have negative charges. Base
pairs A-T and G-C in a specific sequence provide interactive
surfaces in the major groove and less commonly in the minor groove.
Though the N and O atoms are involved in base pairing to each other
by hydrogen bonds, the other groups NH and O groups do provide
atomic surfaces for the side chains of amino acids in proteins.
Proteins have charged amino acids, which can bind to specific bases
or to charged phosphates or to both. But the binding to phosphate
group in the back bone is not sequence specific but just
interactive charges. As such, in certain embodiments, DNA molecules
can be deposited as binding domains for a predetermined type of
cells. In certain alternative embodiments, RNA molecules can be
deposited as binding domains for a predetermined type of cells.
[0266] A binding domain may also be a small molecule, such as a
pharmaceutical drug, or parts of the outside of a virus or
microbe.
[0267] The biding domains may also include chemical functional
groups.
[0268] In certain embodiments, aptamers, which are oligonucleotide
(DNA and RNA) or peptide molecules that bind to a specific target
molecules expressed by or associated with a predetermined type of
cells or even whole cells, may be used as binding domains for the
predetermined type of cells. Aptamers are usually created by
selecting them from a large random sequence pool. Aptamers show
robust binding affinities (meaning that they discriminate for their
specific targets with high precision--once they have determined
their target, they then bind to it with a strong bond) for their
various targets. One example of peptide aptamers includes affimers,
which are small, highly stable proteins engineered to display
peptide loops which provide a high affinity binding surface for a
specific target protein/receptor, which in the context of this
invention, are expressed by or associated with a predetermined type
of cells.
[0269] Nanoparticles may also be used as binding domains for the
predetermined type of cells. In some embodiments, nanoparticles can
include coatings and/or can be linked to biological molecules that
can act as binding domains (such as monoclonal antibodies,
aptamers, streptavidin or peptides) to allow for cell binding.
Multivalent nanoparticles, bearing multiple targeting groups, can
cluster receptors, which can give stronger anchoring. As such, in
certain embodiments, cell-targeting nanoparticles that include
ligands (coated on or attached to the outsides of nanoparticles)
may be used as binding domains for a predetermined type of cells.
These ligands are special in that they can recognize and bind to
complementary molecules, or receptors, found on the surface of
cells.
[0270] In certain further embodiments, the extracellular matrix
surrounding the patterned sacrificial filaments may include
predetermined coupling moieties to capture the binding domains from
the sacrificial filament. The coupling moieties are chemically
reactive to the binding domains, thereby locally capturing said
binding domains upon contact before, during, or subsequent to the
evacuation of the sacrificial filament. The coupling moieties
comprise native extracellular matrix binding domains, antibodies,
peptides, proteins, DNA, RNA, aptamers, nanoparticles, small
molecules, chemical functional groups, and bacteria.
[0271] As mentioned previously, the second approach to generate
tubular tissue constructs, such as nephrons, includes depositing
one or more sacrificial filaments on and/or in a substrate to form
a functional channel pattern, each sacrificial filament comprising
a fugitive ink and a plurality of predetermined types of binding
domains. The various types of binding domains were described in
detail above. Each of the predetermined type of binding domain is
deposited at a different predetermined location along a length of
the sacrificial filament and is capable of binding to a
predetermined type of target cell. The functional channel pattern
can then be surrounded (partially or wholly) with an extracellular
matrix composition (as described in detail above). In certain
embodiments, the functional channel pattern can be surrounded with
the extracellular matrix composition during deposition of the one
or more sacrificial filaments, the one or more functional channel
patterns thereby being formed and embedded simultaneously in the
extracellular matrix composition. Once the extracellular matrix
composition is in place, the fugitive ink can be removed to create
one or more functional channels in the extracellular matrix
composition. It is important that at least a portion of the
different predetermined types of binding domains remains at the
different predetermined locations after removal of the fugitive
ink. Next, a suspension comprising at least one predetermined type
of target cells can be injected into the functional channel. The
target cells will bind to corresponding predetermined types of
binding domains, thereby forming a tubular tissue construct, such
as a nephron. In certain embodiments, the suspension can include
multiple (3 or more) predetermined types of target cells. The cells
may be individual cell lines normally present in the tubular organ
or a portion of the tubular organ to be printed and/or iPSC-derived
cells. As discussed in detail above, the binding domains for the
target cells can be proteins, e.g., antibodies; DNA; RNA; aptamers;
nanoparticles; bacteria; or any other suitable binding domain, or a
combination thereof.
[0272] In certain embodiments, to create a vascular pattern
interpenetrating the functional channel pattern, sacrificial
filaments can be deposited on and/or in the substrate, where each
of the sacrificial filaments comprises a second fugitive ink. Next,
the second fugitive ink can be removed to create vascular channels
in the extracellular matrix composition, thereby forming an
interpenetrating vascular network in the tubular tissue construct,
such as a nephron. A suspension of viable epithelial cells may be
injected into the vascular channels. In certain embodiments,
wherein the at least partial surrounding of the functional channel
pattern with the extracellular matrix composition occurs during
deposition of the sacrificial filaments, the functional channel
patterns can be thereby formed and embedded simultaneously in the
extracellular matrix composition. In certain embodiments,
cell-laden filaments each comprising a plurality of viable cells
may also be deposited on and/or in the substrate to form tissue
patterns, where each of the tissue patterns includes one or more
predetermined cell types.
[0273] To print a nephron using an approach that utilizes binding
domains, one or more continuous sacrificial filaments are deposited
on and/or in a substrate to form a functional channel, where each
sacrificial filament includes: a first fugitive ink formulation
over a first length of the sacrificial filament, a second ink
formulation over a second length of the sacrificial filament, and a
third ink formulation over a third length of the sacrificial
filament. The first fugitive ink formulation includes, for example,
a fugitive ink and a first predetermined type of binding domains to
target a first types of cells, e.g., renal proximal tubule cells.
The second fugitive ink formulation includes, for example, the
fugitive ink and a second predetermined type of binding domains to
target a second type of cells, e.g., loop of Henle cells. The third
fugitive ink formulation includes, for example, the fugitive ink
and a third predetermined type of binding domains to target a third
type of cells, e.g., renal distal tubule cells. Next, the
functional channel pattern is at least partially (or wholly)
surrounded with an extracellular matrix composition. The fugitive
ink is then removed to create functional channels in the
extracellular matrix composition. At least a portion of the first
predetermined type of binding domains remains along the first
length of the one or more functional channels after removal of the
ink. At least a portion of the second predetermined type of binding
domains remains in the second length of the one or more functional
channels after removal of the ink. At least a portion of the third
predetermined type of binding domains remains in the third length
of the one or more functional channels after removal of the ink.
Next, a suspension comprising at least one type of predetermined
cells, such as renal proximal tubule cells, loop of Henle cells and
renal distal tubule cells, collecting duct cells, fenestrated
glomerular endothelial cells is injected into the functional
channel, wherein the cells bind to their corresponding
predetermined binding domains, thereby forming a nephron. In
certain embodiments, the suspension includes some or all of the
predetermined types of cells. The binding domains may be peptides
proteins, e.g., antibodies; DNA; RNA; aptamers; nanoparticles;
small molecules, chemical functional groups, bacteria or any other
suitable binding domain, or a combination thereof.
[0274] Also, sacrificial filaments comprising a second fugitive ink
can be deposited on and/or in the substrate to form a vascular
pattern interpenetrating the functional channel pattern. The second
fugitive ink may then be removed to create vascular channels in the
extracellular matrix composition, thereby forming an
interpenetrating vascular network in the tubular tissue construct.
A suspension of viable epithelial cells can be injected into the
vascular channels. A single printhead or multiple printheads may be
used to deposit continuous sacrificial filaments on and/or in a
substrate.
[0275] In another approach of printing a tubular tissue construct,
cell-laden filaments, each including a plurality of predetermined
types of viable cells, can be deposited on and/or in a substrate to
form tissue patterns. Each of the tissue patterns includes at least
two predetermined cell types, wherein each predetermined type of
viable cells is deposited at a different predetermined location
along a length of the cell-laden filament. The predetermined cell
types can be any cells described herein, including kidney cells
(e.g., renal proximal tubule cells, loop of Henle cells and renal
distal tubule cells, collecting duct cells, fenestrated glomerular
endothelial cells) and/or iPSC-derived patient-specific pro-kidney
or patient-specific cell lines. The sacrificial filaments including
a fugitive ink can also be deposited on and/or in the substrate to
form a functional channel pattern interpenetrating the tissue
patterns. Next, the tissue patterns and the functional channel
pattern can be at least partially or wholly surrounded with an
extracellular matrix composition and the fugitive ink can be
removed to create functional channels in the extracellular matrix
composition, thereby forming an interpenetrating channel network in
a tissue construct. In certain embodiments, the cell-laden
filaments may also include an extracellular matrix material. The
extracellular matrix material can include gelatin, fibrin, gelatin
methacrylate, collagen I, collagen III, collagen IV, fibrinogen,
matrigel, laminin, carbopol, N-isopropylacrylamide, polyethylene
glycol, gelatin methacrylate, polyhydroxyethylmethacrylate, silk,
hyaluronic acid, and/or combinations thereof. The cell-laden
filaments may also include functional chemical substances selected
from the group consisting of: drugs, small molecules, toxins,
proteins, and hormones. In certain embodiments, the sacrificial
filaments including a second fugitive ink can be deposited on
and/or in the substrate to form a vascular pattern interpenetrating
the functional channel pattern, and the second fugitive ink can be
removed to create vascular channels in the extracellular matrix
composition, thereby forming an interpenetrating vascular network
in the tubular tissue construct. A suspension of viable epithelial
cells can be injected into the vascular channels. The tissue
construct may be a nephron, and the plurality of predetermined
types of viable cells can include renal proximal tubule cells, loop
of Henle cells, renal distal tubule cells, collecting duct cells,
fenestrated glomerular endothelial cells, iPSCs, and/or
iPSCs-derived patient-specific pro-kidney or individual cells lines
present in a nephron. In certain other embodiments, the tubular
tissue construct may be intestine, milk duct, sweat gland, colon,
esophagus, stomach, eustachian tube, airway epithelium, epididymis,
seminiferous tubules, urethra, liver bile duct, pancreatic duct,
common bile duct, cerebro-spinal ventricles and aquaducts, parotid
glands, oral mucosa, fallopian tube, vas deferens, or lymph.
[0276] Additional embodiments contemplate integrating (by 3D
printing) lymphatics and nerves to improve physiology of the 3D
printed tubular structures, such as nephrons.
[0277] As shown in FIGS. 17G-I, the ability to epithelialize is
demonstrated using the renal proximal tubule (FIG. 17G; circled in
black). Printing of a convoluted path to make the proximal tubule
is shown below (FIG. 17H; bottom left) and a close up view of a
typical epithelialized tubule is shown stained for nuclei (blue)
and ATPase (green) (FIG. 17I).
[0278] FIGS. 17J-N show proximal tubule epithelial cells (PTECs)
stained using immunofluorescence for several functional properties
on a 2D dish (AQPI, ATPase, Cilia, ZO-I) and the presence of domes
in culture.
[0279] FIGS. 17O-T show perfused tubule constructs stained using
immunofluorescence for several functional properties on a 2D dish
(AQPI, ATPase, Cilia, ZO-I). The properties were retained or
enhanced in the perfused tubule constructs.
[0280] FIGS. 17U-X show SEM and TEM images of the tubules
highlighting the confluent nature of the cells in the constructs,
primary cilia, and formation of a brush border.
EXAMPLES
Example 1
[0281] Fugitive Ink
[0282] Referring to FIG. 6A, highly concentrated (40 wt. %)
Pluronic F127A, which exhibits a strong shear-thinning response
when the applied shear stress exceeds the shear yield stress
(.tau..sub.y) (e.g., during printing), as well as a plateau shear
elastic modulus (G') that exceeds the shear viscous modulus (G'')
when the applied shear stress is below .tau..sub.y (e.g., after
printing), is selected as the fugitive ink for an exemplary system.
The fugitive ink elasticity is found to be about 2.times.10.sup.4
Pa at 22.degree. C., as shown in FIG. 6B. Below the CMT (about
4.degree. C.), the ink liquefies and its elasticity decreases by
several orders of magnitude, thereby facilitating its removal from
the tissue construct.
[0283] As described above, the sacrificial filaments formed from
the fugitive ink may include one or more additional cells, growth
factors, drugs, etc. For example, endothelial, epithelial and/or
other cells may be dispersed within the fugitive ink and deposited
with the sacrificial filaments. When the fugitive ink is removed to
form the vascular (or other) channels, the cells may remain, lining
walls of the channels.
[0284] This approach is demonstrated with a highly concentrated
endothelial cell-laden fugitive (pluronic) ink (1.times.10.sup.7
cells/ml). The fugitive ink is deposited and encapsulated with an
extracellular matrix composition. Upon removal of the fugitive ink
to form vascular channels, the endothelial cells remain affixed to
walls of the channels, as shown schematically in FIGS. 16A-16C and
experimentally in FIGS. 16D-16G. FIG. 16G shows a simple channel
created using this approach that has been perfused for over 24
hours. The endothelial cells appear to line the channel and look
qualitatively similar to those created using a conventional seeding
approach. This technique provides an alternative to seeding
existing vascular channels with endothelial cells, particularly in
the case of highly branched vascular networks where cells may clog
and inhibit flow, leading to non-uniform seeding.
[0285] Extracellular Matrix Composition and Material
[0286] As set forth above, an interpenetrating polymer network
based on gelatin and fibrin has been developed that mimics natural
ECM, and which may be used for the extracellular matrix composition
and/or the extracellular matrix material of the tissue
construct.
[0287] FIGS. 13A-13D show fabrication of a gelatin-fibrin
interpenetrating polymer network, or gelatin-fibrin matrix. First,
the gel precursors are first mixed together. Polymerizing
fibrinogen via the enzyme thrombin forms a fibrin gel or network.
This phase provides initial mechanical strength and rigidity, as
indicated by an increase in shear elastic modulus. The second phase
(gelatin) is then formed around the fibrin gel, and the two phases
are slowly crosslinked together via transglutaminase (TG). FIGS.
13E and 13F show shear modulus versus time (G' and G'') and a
stress-strain curve for the gelatin-fibrin interpentrating polymer
network.
[0288] TG is a naturally occurring enzymatic protein crosslinker
with myriad biological functions; for example, it may be
up-regulated during wound healing in vivo. By varying TG incubation
time, the optical properties (e.g., transparency) of the fibrin gel
can be tailored. The transparency is dictated by the final pore
architecture of the fibrin gel, which is visualized using a
rhodamine-tagged fibrinogen and confocal microscopy. It is also of
interest to determine if TG and gelatin disrupt natural fibrin
polymerization. Confocal microscopy images reveal that the
fibrillar nature of fibrin is preserved and can be precisely tuned
by varying different processing conditions, such as incubation
time, as illustrated in FIG. 14. A longer fibrin-TG incubation time
leads to a more dense fibrillar network and, subsequently, higher
optical transparency.
[0289] Besides fabrication considerations, cell
material-interactions play an important role in materials
selection. The gelatin-fibrin matrix has been shown to be
compatible with many different cell types, including fibroblasts
(connective tissue), smooth muscle cells, endothelial cells, and
renal proximal tubule cells (epithelial). The adhesivity of the
gelatin-fibrin matrix has been quantified by comparing the
projected area of cells on various substrates. The gelatin-fibrin
matrix outperformed all other materials including native fibrin,
tissue culture polystyrene (TCPS), and gelatin methacrylate
(GelMa). FIGS. 13G-13I show various cells essential for creating
blood vessels--fibroblasts, endothelial cells, and smooth muscle
cells--on an exemplary gelatin-fibrin matrix surface. To highlight
the diversity of the adhesivity, tissue-specific epithelial cells
were grown on the surface, as illustrated in FIG. 13J.
[0290] In a second example, gelatin methacrylate (GelMA), which is
biocompatible, easily processed and inexpensive, is selected for
use as both the extracellular matrix material for the cell-laden
formulation and as the extracellular matrix composition for the
encapsulation step. GelMA is denatured collagen that is modified
with photopolymerizable methacrylate (MA) groups, which allows the
matrix to be covalently cross-linked by UV light after printing.
Physical gelation arises from the assembly of intermolecular triple
helices that possess a structure similar to collagen, as
illustrated in FIG. 6C. By varying the concentration, degree of
methacrylation, and temperature, the shear yield stress and elastic
modulus of aqueous GelMA systems can be systematically tuned.
[0291] The extracellular matrix composition is produced by
dissolving 15 wt. % GelMA in cell culture media. Above
approximately 25.degree. C., the composition is a low viscosity
fluid with a G' value below 10.sup.-1 Pa. Upon cooling below
25.degree. C., the composition undergoes gelation, yielding a
clear, viscoelastic extracellular matrix material. The elasticity
of the extracellular matrix composition increases with decreasing
temperature, with G' values of about 10.sup.3 Pa and
2.times.10.sup.4 Pa observed at 22.degree. C. and 2.degree. C.
(FIG. 6D), which correspond to typical conditions for printing and
fugitive ink removal, respectively.
[0292] The same aqueous GelMA composition is used to create
cell-laden inks that contain viable cells for printing. Prior
studies have shown that cells adhere, remodel, and migrate through
GelMA due to the presence of integrin-binding motifs and matrix
metal-proteinase sensitive groups. It is found that the
incorporation of a moderate concentration, e.g., 2.times.10.sup.6
cells/mL, of 10T1/2 fibroblast cells into the 15 wt. % GelMA ink
(FIGS. 6E and 6F) does not significantly alter the temperature at
which gelation ensues or the elasticity of the composition over the
temperature range of interest, e.g., 2.degree. C. to 40.degree. C.
Hence, both pure and cell-laden GelMA inks can be printed and
further processed, as needed, in the same manner.
[0293] The differences in thermally reversible gelation observed
for the fugitive Pluronic F127, pure GelMA, and cell-laden GelMA
inks give rise to three distinct processing windows. Between
approximately 4.degree. C. and 25.degree. C., each ink is stiff and
exhibits a solid-like response, where G'>G''. At
T.gtoreq.25.degree. C., the Pluronic F127 fugitive ink is stiff and
solid-like (G'>C''), while the pure and cell-laden GelMA inks
are liquids that flow readily. Below about 4.degree. C., the
Pluronic F127 fugitive ink is a liquid that flows readily, while
the pure and cell-laden GelMA inks are stiff and solid-like
(G'>G'').
[0294] Printing of Vascular Patterns
[0295] The complimentary thermal behavior described above for the
Pluronic F127-GelMA system is exploited to print representative
vascular patterns comprising a plurality of sacrificial filaments
which are then encapsulated in an acellular extracellular matrix
composition (pure GelMA). FIGS. 7A-7K illustrate the formation of
1D, 2D and 3D vascular networks and endothelialization of the
channels, with schematic views and corresponding optical images of
each vascular network design. After removing the fugitive ink, each
vascular network is perfused with a fluorescent red dye to aid in
visualization (FIGS. 7C, 7F and 7I). Within each tissue construct,
the diameter of the sacrificial filaments can be altered as desired
by modifying the printing pressure, speed, and/or nozzle height.
For example, 1-D microchannel arrays with diameters increasing from
45 .mu.m to 500 .mu.m are printed using a single 30 .mu.m nozzle
simply by increasing the printing pressure and nozzle height in a
stepwise fashion between each printed feature (FIGS. 7A-7B).
[0296] After photopolymerizing the GelMA matrix, the fugitive ink
is removed by cooling the printed constructs below 4.degree. C.,
yielding open 1-D microchannels. Representative cross-sectional
images of these 1-D channels, shown in FIG. 8A, reveal that their
final diameters range from about 100 .mu.m to about 1 mm. Since the
GelMA ink has a higher water content than the fugitive ink, the
printed vascular features may swell as water diffuses into the
fugitive ink (Pluronic F127) from the surrounding matrix. Indeed,
the diameters nearly double in size, with a swelling ratio that is
independent of initial microchannel diameter (FIG. 8B) for this
material system.
[0297] The 2-D vascular network design mimics the hierarchical,
bifurcating motifs found in biological systems, large channels
bifurcate to form smaller channels that maximize efficient blood
flow, nutrient transport, and waste removal while minimizing the
metabolic cost. These 2D hierarchical vascular networks are printed
using a single nozzle of 30 microns (e.g., FIGS. 7E and 7F). The
as-printed, largest channels (650 .mu.m in diameter) provide a
single inlet and outlet for perfusion, while the smallest channels
(45 .mu.m) in diameter) reduce the characteristic diffusion
distance between adjacent conduits. Finally, a 3D microvascular
network design, which is shown in FIGS. 7G, 7H and 7I and includes
a 3D periodic array of uniform microchannels, is printed. Because
the embedded microchannels are interconnected in all three
dimensions, the fugitive ink can be removed from the surrounding
GelMA matrix quickly and with high fidelity.
Seeding of Vascular Channels
[0298] Multiple types of fluids may be flowed through embedded
vascular networks to demonstrate their perfusable nature. For
example, the 2D hierarchical bifurcating networks are injected with
a human umbilical vein endothelial cell (HUVEC) suspension followed
by gentle rocking. After 48 h, it is found that the cells retained
greater than 95% viability and assembled into a nearly confluent
layer, as determined by live/dead staining coupled with confocal
imaging within a representative, bifurcated microchannel.
[0299] The vascular channels may be seeded with multiple cell
types, such as fibroblasts or smooth muscle cells in addition to
HUVECs. Here, fibroblasts are co-seeded with HUVECs. It is found
that, after about one week of perfusion culture, uniformly
co-seeded endothelial cells and fibroblasts self assemble into two
distinct layers of outer enveloping stroma (human dermal
fibroblasts; HNDFs) and confluent inner endothelium (HUVECs). FIGS.
18A-18C highlight the initially uniform distribution of HUVECs
(red) and HNDFs (green) at 3 days post-seeding. FIGS. 18D-18E shows
the same channel after eight days, at which time the channel
comprises a distinct outer stromal (HNDF) layer and a confluent
endothelial (HUVEC) layer. The confluent endothelium is visualized
using immunohistochemistry to stain for vascular endothelial
cadherin (VE-Cad), an endothelial-specific junction protein
expressed when cells form confluent networks (shown in
magenta).
[0300] To further promote the attachment and proliferation of the
endothelial cells along the fabricated channel walls, the interior
of the walls may be coated by perfusing a fibronectin solution
through the channels prior to introducing the HUVEC suspension, as
shown in FIGS. 9A-9D. Also, when animal blood is directly injected
into the inlet of the 2-D vascular network it rapidly flows through
the entire network to outlet. These initial demonstrations
illustrate the potential to create perfusable vasculature of nearly
any arbitrary design. Printing of Cell-Laden Filaments Including
More than One Cell Type
[0301] The printing of cell co-culture inks that allow the delivery
of two or more cell types within a single ink filament is
demonstrated. It is observed that a cell-laden ink including, in
this example, a dispersion of HNDFs and HUVECs in a gelatin-fibrin
matrix material, leads to spontaneous neovasculature formation in
the printed filament, as evidenced in FIGS. 20A-20C. This is not
observed in printed filaments based on a monoculture of HUVECs.
This approach suggests cell-laden filaments comprising more than
one cell type, including tissue-specific cell types (e.g.,
hepatocytes, islets, podocytes, neurons, etc.) or stem cells (e.g.,
iPSCs, MSCs, etc.), may be used to achieve desired heterogeneity
and also to enhance function.Printing of Fugitive Ink onto a
Cell-Laden Matrix
[0302] As in the previous example, HNDFs and HUVECs are dispersed
within an extracellular matrix composition (specifically, a
gelatin-fibrin matrix material) to form a cell-laden matrix. A
fugitive ink is printed directly onto the cell-laden matrix and
then encapsulated by the gelatin-fibrin matrix material. The
fugitive ink is evacuated to form vascular channels, and the
vascular channels are seeded with HUVECs. Over time, it is found
that the HUVECs assemble into capillary structures within the
printed cell-laden filament. FIGS. 19A-19D show that the
endothelial cells become attached to the vascular channels and form
confluent layers, and FIGS. 19E-19F show evidence of angiogenic
sprouting of small capillaries from the confluent blood vessels
indicating that the process is conducive to cellular remodeling and
higher-level biological processes.
[0303] Two effects are hypothesized to contribute to this observed
behavior. First, fibroblasts have been shown numerous times to be
pro-angiogenic support cells in vitro through specific chemical
cues such as fibroblast growth factor (FGF), often leading to
neovascularization processes. Additionally, the concentrated
population of proliferative cells within the matrix has extensive
metabolic requirements that are likely not met by diffusion alone.
It is widely accepted that cells that are not within a few 100
microns of blood vessel will become oxygen stressed and eventually
necrotic. In vivo, the recruitment of host vasculature into
avascular structures to prevent necrosis has been observed.
Printing of Tissue Constructs Including Interpenetrating
Vasculature
[0304] To demonstrate the fabrication of tissue constructs replete
with blood vessels, multiple types of cells, and an extracellular
matrix composition, 3D heterogeneous structures of varying design
are printed.
[0305] The first structure is composed of semi-woven features
printed in and out of plane (FIGS. 10A-10G). This four-layer tissue
construct includes two tissue patterns each comprising a different
cell type and a vascular pattern formed from sacrificial filaments
comprising a fugitive ink. The tissue construct is produced in a
layer by layer build sequence by printing four inks: PDMS, fugitive
Pluronic F127 and two different cell-laden GelMA inks, followed by
depositing pure GelMA ink at 37.degree. C. to fully encapsulate the
printed features, and finally photopolymerization to cross-link the
GelMA matrix. This 3D architecture was conceived and fabricated to
demonstrate the printing capabilities and also facilitate confocal
imaging through the entire 4-layer, printed construct.
[0306] As indicated previously, the PDMS ink is first printed in
the form of a high-aspect ratio border that surrounds each tissue
construct and serves as a mold for the pure GelMA ink used for the
encapsulation step. The fugitive ink and both cell-laden GelMA
inks, which contain either green fluorescent protein expressing
human neonatal dermal fibroblasts (HNDFs) or non-fluorescent
10T1/2s, an established mouse fibroblast line, are co-printed at
concentrations of 2.times.10.sup.6 cells/mL through 200 .mu.m
nozzles in a predefined sequential process. FIG. 10C shows an image
of the 3D structure directly after printing. Only the green
fluorescent channel is overlaid onto the bright field image so that
the printed cell channel can be easily visualized. After
fabrication, the fugitive ink is liquefied and removed from the 3D
construct. The evacuation procedure involves placing empty syringe
tips into the inlet and outlet microchannels and then suctioning
out the entire vascular network under a modest vacuum. The removal
process is rapid and yields a high fidelity, interpenetrating
vasculature, which is then endothelialized as described above.
[0307] Characterization of the Tissue Constructs
[0308] Using microscopy, the locations of the three cell types that
are independently stained (green-GFP HNDFs, blue-10T1/2, and
red-HUVECs) are identified. The semi-woven nature of this
engineered tissue construct is clearly visible in the schematics
and images shown in FIGS. 10B-10D. Using confocal laser scanning
microscopy, it is possible to fully interrogate this 3D tissue
construct and determine the precise locations of each cell.
Confocal microscopy images of the 3-D printed structure in XY, XZ,
and YZ after 2 days of culture are shown in FIG. 10G. To
demonstrate the versatility of this approach, other 3D tissue
construct were also designed and printed. Although it is difficult
to obtain confocal images due the dense, interpenetrating nature of
the cell-laden filaments, both the green fluorescent protein
expressing HNDFs in GelMA and the red-HUVECs that line the 3-D
vasculature network are visible.
[0309] Investigation of Cell Viability
[0310] As a final step, the viability of the printed 10T1/2
fibroblast cells over the course of one week was investigated. At
Day 0, the cell viability was 61%; however, it increased to 82%
after 7 days. While there is lower initial cell viability compared
with the control (78% on Day 0), the printed cells do proliferate
and spread over time leading to similar levels of viability after 1
week in culture. The decreased initial viability could arise from
the shear or extensional stress experienced by the cells during the
printing process. Applied pressure, nozzle diameter, cell type, and
environmental conditions may affect cell viability after printing.
Another critical parameter is the total build time required to
print the desired engineered tissue construct. There may be a
maximum time over which the cell-laden inks can be stored in the
ink reservoir prior to being harmed. However, implementation of
multinozzle print heads that were reported previously (J. A. Lewis
et al., "Multinozzle Deposition System for Direct Write
Applications," International Patent Application No.
PCT/US2012/044794, filed Jun. 29, 2012, which is hereby
incorporated by reference) for high-throughput, multimaterial
printing, may reduce the characteristic build times by two orders
of magnitude in comparison with single nozzle printing. For
example, printing an engineered tissue construct with a volume of
1000 cm.sup.3, comparable to a typical adult human liver, could
require approximately 72 h using a single 200 .mu.m nozzle at
typical printing speeds. However, implementation of a
64-multinozzle array may reduce the respective build time to about
1 h.
Example 2
3D Bioprinting of Thick Living Tissues
[0311] Central to the fabrication of thick living tissues is the
design of biological, sacrificial, and elastomeric inks for
multimaterial 3D bioprinting.
[0312] Methods
[0313] Solution Preparation:
[0314] Matrix and ink precursor solutions were prepared before
creating tissue engineered constructs. A 15 wt/v % gelatin solution
(Type A, 300 bloom from porcine skin, Sigma) was produced by
warming in DPBS (1X Dulbelco's phosphate buffered saline without
calcium and magnesium) to 70.degree. C. (unless otherwise noted)
and then gelatin powder was added to the solution while vigorously
stirring. The gelatin was allowed to fully dissolve by stirring for
12 h at 70.degree. C. (unless otherwise noted), and the pH was then
adjusted to 7.5 using 1M NaOH. The warm gelatin solution was
sterile filtered and stored at 4.degree. C. in aliquots for later
usage (<3 months).
[0315] Fibrinogen solution (50 mg mL.sup.-1) was produced by
dissolving lyophilized bovine blood plasma protein (Millipore) at
37.degree. C. in sterile DPBS without calcium and magnesium. The
solution was held at 37.degree. C. undisturbed for 45 minutes to
allow complete dissolution.
[0316] The transglutaminase (TG) solution (60 mg mL.sup.-1) was
prepared by dissolving lyophilized powder (Moo Glue) in DPBS
without calcium and magnesium and gently mixing for 20 sec. The
solution was then placed at 37.degree. C. for 20 minutes and
sterile filtered before using.
[0317] A 250 mM CaCl.sub.2 stock solution was prepared by
dissolving CaCl.sub.2 powder in DPBS without calcium and magnesium
(Corning).
[0318] To prepare stock solution of thrombin, lyophilized thrombin
(Sigma Aldrich) was reconstituted at 500 U mL.sup.-1 using sterile
DPBS, aliquotted and stored at -20.degree. C. The thrombin aliquots
were thawed immediately prior to use.
[0319] Matrix Formulations:
[0320] To create gelatin-fibrin IPNs, solutions of fibrinogen,
gelatin, calcium and TG were mixed together at various
concentrations at 37.degree. C. A typical final concentration was
10 mg mL.sup.-1 fibrinogen, 7.5 wt % gelatin, 2.5 mM CaCl.sub.2 and
0.2 wt % TG. For printing the large-scale tissues (FIG. 33), 1 wt %
TG was used to account for diffusion and dilution into printed cell
filaments. The amount of time these components were allowed to sit
before mixing with thrombin determined matrix optical clarity (FIG.
24A through 24D). This wait step, termed "TG preincubation time,"
was typically 15-20 minutes. After TG preincubation time, the
solution was rapidly mixed with thrombin at a ratio of 500:1
resulting in a final concentration of 1 U mL.sup.-1. After mixing,
the matrix was cast within 30 sec, as irreversible, enzymatically
driven polymerization of fibrinogen into fibrin gel occurs
rapidly.
[0321] Ink Formulations:
[0322] Several inks were created for 3D bioprinting of thick
vascularized tissues.
[0323] The first ink, which is used to create customized perfusion
chips, was composed of a two-part silicone elastomer (SE 1700, DOW
Chemical) with a 10:1 base to catalyst (by weight) that was
homogenized using a mixer (2000 speed, AE-310, Thinky Corp, Japan).
The silicone ink was printed within two hours of mixing with
catalyst.
[0324] The second ink, which was a fugitive ink used to print the
vasculature, was composed of 38 wt % Pluronic F127 (Sigma) and 100
U mL.sup.-1 thrombin in deionized, ultrafiltrated (DIUF) water. To
prepare this ink, 40% stock Pluronic F127 was homogenized using a
Thinky mixer until the powder was fully dissolved, and subsequently
stored at 4.degree. C. Prior to use, a 2000 U mL.sup.-1 thrombin
solution was added to the fugitive (Pluronic) ink at a ratio of
1:200, and homogenized using a Thinky mixer. This ink was then
loaded in a syringe (EFD Inc., East Providence, R.I.) at 4.degree.
C. and centrifuged to remove any air bubbles. Before printing, this
ink was brought to room temperature.
[0325] The third ink, a cell-laden printable ink, was composed of
7.5 wt/vol % gelatin and 10 mg ml.sup.-1 fibrinogen, unless
otherwise noted. Notably, ink stiffness was tuned by varying the
gelatin process temperature (70.degree. C-95.degree. C.) (FIG. 25A
though 25I). This ink was prepared similarly to the IPN matrix,
however, no TG or thrombin was used in this formulation.
[0326] Crosslinking of printed ink was achieved through diffusion
of thrombin and TG from the surrounding matrix after casting. To
uniformly disperse cells into the ink, the fibrinogen-gelatin blend
was maintained in a liquid state at 37.degree. C. and then cell
suspensions with concentrations greater than 2.times.10.sup.6 Cells
mL.sup.-1 were mixed via gentle pipetting. After the ink was
thoroughly mixed, the ink was held at 4.degree. C. for 15 min to
drive thermal gelation of the gelatin phase. Next, the ink was
removed from the refrigerator and allowed to equilibrate to room
temperature for at least 15 min, mounted to the 3D bioprinter and
used immediately for up to 2 h.
[0327] Fibrinogen-Fluorophore Conjugation:
[0328] To visualize the fibrin network in printed filaments and the
casting matrix, fibrinogen was conjugated to two fluorophores.
Specifically, 1 g of bovine fibrinogen was dissolved in 100 ml of
50 mM borate buffer, pH 8.5 (Thermo Scientific) to form a 10 mg
ml.sup.-1 solution. To this solution, N-hydroxysuccinimide (NHS),
conjugated with either fluorescein or rhodamine was added at a
molar ratio of 10:1 dye:fibrinogen. After reacting for 2 h at room
temperature, the labeled fibrinogen was separated from unconjugated
dye by dialysis using 10 kDa MWCO dialysis tubing in a 2 L bath
against PBS for 3 days, changing the PBS in the bath twice daily.
After dialysis was complete, the fluorescently conjugated
fibrinogen was frozen at -80.degree. C., lyophilized, and stored at
-20.degree. C. before use.
[0329] Rheological Characterization:
[0330] The ink rheology is measured using a controlled stress
rheometer (DHR-3, TA Instruments, New Castle, Del.) with a 40 mm
diameter, 2.degree. cone and plate geometry. The shear storage (G')
and loss (G'') moduli are measured at a frequency of 1 Hz and an
oscillatory strain (.gamma.) of 0.01. Temperature sweeps were
performed using a Peltier plate over the range from -5.degree. C.
to 40.degree. C. Samples were equilibrated for 5 min before testing
and for 1 min at each subsequent temperature to minimize thermal
gradients throughout the sample. Time sweeps were conducted by
rapidly placing a premixed solution onto the temperature-controlled
Peltier plate held at 37.degree. C. or 22.degree. C., unless
otherwise noted. It was important to minimize bubble generation
during mixing, because the solution rapidly gels upon casting.
[0331] Cell Culture and Maintenance:
[0332] Human bone-marrow derived mesenchymal stem cells (hMSCs)
(Rooster Bio) were cultured in Booster Media (Rooster Bio) and were
not used beyond 2 passages.
[0333] Green fluorescent protein-expressing human neonatal dermal
fibroblast cells (GFP-HNDFs, Angio-Proteomie) were cultured in
Dulbelco's modified Eagle medium containing high glucose and sodium
pyruvate (DMEM) (GlutaMAX.TM., Gibco) and supplemented with 10%
fetal bovine serum (FBS) (Gemini Bio-Products).
[0334] Primary red fluorescent protein expressing human umbilical
vein endothelial cells (RFP-HUVECs) (AngioProteomie) were cultured
in EGM-2 media (complete EGM.TM.-2 BulletKit.TM., Lonza).
[0335] All the cell cultures were passaged per the respective
vendor's instructions. GFP-HNDFs and RFP HUVECs were not used
beyond the fifteenth and ninth passages, respectively.
[0336] 3D Perfusion Chip Fabrication:
[0337] All vascularized tissues were created on a custom-designed
multimaterial 3D bioprinter equipped with four independently
addressable printheads mounted onto a 3-axis, motion-controlled
gantry with build volume 725 mm.times.650 mm.times.125 mm (AGB
10000, Aerotech Inc., Pittsburgh, Pa. USA). Each ink was housed
inside a separate syringe barrel to which nozzles of varying size
(i.e., 100 .mu.m-410 .mu.m diameter) were attached via a leur-lock
(EFD Inc., East Providence, R.I., USA). Inks were extruded through
each deposition nozzle by applying air pressure (800 Ultra
dispensing system, EFD Inc., East Providence, R.I., USA), ranging
from 10-140 psi, corresponding to print speeds from 1 mm s.sup.-1
to 5 cm s.sup.-1. Prior to printing, the relative X-Y offsets of
the four printheads were determined using orthogonally mounted
optical micrometers (LS-7600 series, Keyence, Japan).
[0338] To manufacture the customized perfusion chips, the silicone
ink was loaded into a 10 ml syringe, centrifuged to remove air
bubbles, and deposited through a tapered 410 .mu.m nozzle. The
gasket design was created using custom MATLAB software and the
structures were printed onto 50 mm.times.75 mm glass slides. After
printing, the chips were cured at 80.degree. C. in an oven for
>1 hour and stored at room temperature.
[0339] To produce thick vascularized tissues, multiple inks were
sequentially co-printed within the customized perfusion chips. To
form a base layer, a thin film of gelatin-fibrin matrix, containing
0.1% wt TG, was cast onto the base of the perfusion chip and
allowed to dry. Next, the fugitive Pluronic F127 and cell-laden
inks were printed onto the surface. The fugitive (Pluronic F127)
and cell-laden inks were printed using 200 .mu.m straight and
tapered nozzles, respectively. After printing, stainless metal
tubes were fed through the guide channels of the perfusion chip and
pushed into physical contact with printed vertical pillars composed
of the fugitive ink, which were positioned at the inlets and
outlets (FIGS. 26A-26S). Prior to encapsulation, TG was added to
the molten 37.degree. C. gelatin-fibrin matrix solution and
preincubated for 2-20 min depending on the desired matrix
transparency (FIG. 24D). To form a cell-laden matrix, the molten
37.degree. C. gelatin-fibrin matrix was first mixed with HNDF-GFP
cells and then mixed with thrombin. Next, this matrix was
immediately cast within the perfusion chip to fully encapsulate the
printed tissue, where it underwent rapid gelation due to thrombin
activity. The tissues were then stored at 37.degree. C. for 1 h to
allow fibrin polymerization to terminate and TG to crosslink the
network. The entire structure was then cooled to 4.degree. C. to
liquefy the printed fugitive ink, which was then flushed through
the device using cold cell media, leaving behind open conduits that
serve as the desired vascular network embedded within the printed
tissue and surrounding cell-laden matrix.
[0340] The 3D perfusion chips were loaded onto a machined stainless
steel base, and a thick acrylic lid was placed on top. The lid and
base were clamped together by four screws, forming a seal around
the silicone 3D printed gasket top. Next, sterile two-stop
peristaltic tubing (PharMed BPT) was filled with media and
connected to the outlet of a sterile filter that was attached to a
10 ml syringe (EFD Nordson), which served as a media reservoir.
Media that has been equilibrating for >6 h in an incubator at
37.degree. C., 5% CO.sub.2 was added to the media reservoir, and by
means of gravity, was allowed to flow through the filter and
peristaltic tubing, until all the air was displaced, before
connecting the peristaltic tubing to the inlet of each perfusion
chip. To complete the perfusion circuit, silicone tubing was
connected between the outlet of the perfusion chip and an inlet at
the top of the media reservoir. Hose pinch-off clamps were added at
the inlet and outlet of the perfusion chip to prevent uncontrolled
flow when disconnected from the peristaltic pump, which can damage
the endothelium or introduce air bubbles to the vasculature. The
media reservoir was allowed to equilibrate with atmospheric
pressure at all times by means of a sterile filter connecting the
incubator environment with the reservoir.
[0341] Endothelialization of Vascular Networks:
[0342] With the peristaltic tubing removed from the chip outlet,
50-500 .mu.L HUVEC suspensions (1.times.10.sup.7 cells mL.sup.-1)
were injected via pipette to fill the vascular network. Then, the
silicone tubing was replaced on the chip outlet and both the outlet
and inlet pinch-clamp were sealed. The perfusion chip was incubated
at 37.degree. C. to facilitate cell adhesion to the channels under
zero-flow conditions. After 30 min, the chip was flipped
180.degree. to facilitate cell adhesion to the other side of the
channel, and achieve circumferential seeding of cells in the
channel. Finally, the cells were further incubated for between 5 h
and overnight at 37.degree. C. before commencing active
perfusion.
[0343] Active Perfusion:
[0344] After endothelial cell seeding, the peristaltic tubing was
affixed to a 24-channel peristaltic pump (Ismatec), after which the
inlet and outlet hose clamps were unclipped to prepare for
perfusion. The peristaltic pump was then started at a desired
perfusion rate. For single vascular channels, the perfusion rate
was set at 13 .mu.L min.sup.-1, while for the thick, large-scale
tissues it was set at 27 .mu.L min.sup.-1.
[0345] Cell Viability Assay:
[0346] Cell viability was determined post-printing by printing inks
with 2.times.10.sup.6 cells mL.sup.-1 for each condition. For
printed cell viability, the printed filaments were deposited onto a
glass substrate and then stained using calcein-AM ("live", 1 .mu.L
mL.sup.-1, Invitrogen) and ethidium homodimer ("dead", 4 .mu.L
mL.sup.-1, Invitrogen) for 20 minutes prior to imaging via confocal
microscopy (n=3 unique samples, imaged n=10 times). Live and dead
cell counts were obtained using the 3D objects counter plugin in
ImageJ software. The results were averaged and standard deviations
determined for each sample.
[0347] Imaging and Analysis:
[0348] Macroscopic photographs and videos of printing and assembly
of tissues were collected using a DSLR camera (Canon EOS, 5D Mark
II, Canon Inc., USA). Fluorescent dyes were used to improve
visualization of Pluronic F127 (Red, Risk Reactor) and
gelatin-fibrin ink (Fluorescein, Sigma Aldrich). Various
microscopes were used to visual printed tissue structures including
a Keyence Zoom (VHX-2000, Keyence, Japan), an inverted fluorescence
(Axiovert 40 CFL, Zeiss), and an upright confocal microscope
(LSM710, Zeiss). ImageJ was used to generate composite microscopy
images by combining fluorescent channels. 3D rendering and
visualization of confocal stacks was performed in Imaris 7.6.4,
Bitplane Scientific Software and ImageJ software. Cell counting was
performed using semi-automated native algorithms in Imaris and
ImageJ counting and tracking algorithms.
[0349] Immuno-Staining:
[0350] Immuno-staining and confocal microscopy were used to assess
the 3D vascularized tissues. Printed tissues were first washed with
phosphate buffered saline (PBS) via perfusion for several minutes.
Next, 10% buffered formalin was perfused through the 3D tissue for
10 to 15 min. The tissue was removed from the perfusion chip and
bathed in 10% buffered formalin. Time of fixation with formalin
varied with tissue construct thickness; approximately 2 h of
fixation was required for a 1 cm thick tissue. The 3D tissues were
then washed in PBS for several hours and blocked overnight using 1
wt % bovine serum albumin (BSA) in PBS. Primary antibodies to the
cell protein or biomarker of interest were incubated with the
constructs for 2 days at the dilutions listed in Table 1 below in a
solution of 0.5 wt % BSA and 0.125 wt % TritonX.
TABLE-US-00001 TABLE 1 Antibody or stain: Source Catalog # Host
Species & Reactivity Concentration CD31 abcam ab24590 Mouse
anti-human 1:200 VECadherin Cell Signaling Technology D67F2 Rabbit
anti-human 1:250 von Willebrand Factor abcam ab194405 Mouse
anti-human 1:250 Alpha-smooth muscle actin abcam ab18147 Mouse
anti-human 1:250 Osteocalcin abcam ab13416 Mouse anti-human 1:200
Collagen I abcam ab34710 Rabbit anti-human 1:400 ActinGreen Life
Technologies R37110 N/A 2 drops per mL NucBlue Life Technologies
R37605 N/A 2 drops per mL Fast blue Sigma Aldrich B5655 N/A 20 mg
per mL Alizarin Red Sigma Aldrich A5533 N/A 50 .mu.g per mL
[0351] Removal of unbound primary antibodies was accomplished using
a wash step against a solution of PBS or 0.5 wt % BSA and 0.125 wt
% TritonX in PBS for 1 day. Secondary antibodies were incubated
with the constructs for 1 day at the dilutions listed in Table 1
above in a solution of 0.5 wt % BSA and 0.125 wt % TritonX in PBS.
Samples were counter-stained with NucBlue or ActinGreen for 2 h and
then washed for 1 day in PBS prior to imaging. Confocal microscopy
was performed using an upright Zeiss LSM 710 with water immersion
objectives ranging from 10X to 40X employing spectral lasers at
405, 488, 514, 561, and 633 nm wavelengths. Image reconstructions
of z-stacks were performed in ImageJ using the z-project function
with the maximum pixel intensity setting. Three dimensional image
reconstructions were performed using Imaris software.
[0352] hMSC Staining:
[0353] Fast Blue (Sigma Aldrich) and alizarin red (SigmaFast, Sigma
Aldrich) were used to visualize AP activity and calcium deposition.
One tablet of Fast Blue was dissolved in 10 mL of DI water. This
solution was stored in the dark and used within 2 hours. Cells were
washed using 0.05% Tween 20 in DPBS without calcium and magnesium
and fixed as described above. The samples were then covered with
Fast Blue solution and incubated in the dark for 5-10 min and
washed using PBS-Tween buffer. To assess mineralization, 2%
alizarin red solution was dissolved in DI water, mixed vigorously,
filtered, and used within 24 hours. Samples were equilibrated in DI
water and incubated with Alizarin Red solution for 2-5 minutes,
then the staining solution was removed, and samples were washed 3X
in DI water or until background dye was unobservable.
[0354] FITC-Dextran Permeability Testing:
[0355] To assess barrier function of the printed vasculature,
diffusional permeability was quantified by perfusing culture media
in the vascular channel, while alive, containing 25 .mu.g/mL
FITC-conjugated 70 kDa dextran (FITC-Dex, Sigma product 46945) at a
rate of 20 .mu.L min.sup.-1 for 3 min and 1 .mu.L min.sup.-1
thereafter for .about.33 min. The diffusion pattern of FITC-Dex was
detected using a wide-field fluorescent microscope (Zeiss Axiovert
40 CFL). Fluorescence images were captured before perfusion and
every 3 to 5 min after for 33 min. Diffusional permeability of
FITC-Dex was calculated by quantifying changes of fluorescence
intensity over time using the following equation:
P d = 1 I 1 - I b ( I 2 - I 1 t ) d 4 ##EQU00001##
[0356] where Pd is the diffusional permeability coefficient,
I.sub.1 is the average intensity at an initial time point, I.sub.2
is an average intensity after some time (t, .about.30 min), I.sub.b
is background intensity (image taken before perfusion of FITC-Dex),
and d is diameter of the channel (Price G, Tien J (2011) Methods in
Molecular Biology ed khademhosseini A (Humana Press)).
[0357] The permeability measurement was performed on two types of
channel structures: 1) perfused channel with cell lining, 2)
perfused channel without cell lining (empty channel). For each
type, the diffusional permeability was calculated from the
measurement of three independent samples (n=3).
[0358] Discussion
[0359] To satisfy the concomitant requirements of processability,
heterogeneous integration, biocompatibility, and long-term
stability, we first developed new printable cell-laden ink and
castable extracellular matrix (ECM) comprising a blend of gelatin
and fibrinogen (Lee KY, Mooney DJ (2001) Hydrogels for Tissue
Engineering. Chem Rev 101(7):1869-1880).
[0360] Specifically, these materials form a tough interpenetrating
polymer network (IPN) composed of gelatin-fibrin cross-linked by a
dual-enzyme, thrombin and transglutaminase, strategy (FIGS. 26, and
27A though 27L). The cell-laden inks must simultaneously facilitate
printing of self-supporting filamentary features under ambient
conditions and subsequent infilling of the printed tissue
architectures by casting without dissolving or distorting the
patterned cell-laden and fugitive (vasculature) inks (FIG. 27A).
The thermally reversible gelation of the gelatin-fibrinogen network
enables its use in both printing and casting, where gel and fluid
states are required, respectively (FIGS. 28A though 28F). Thrombin
is used to rapidly polymerize fibrinogen (Mosesson MW (1998), Semin
Thromb Hemost 24(2):169-174), while TG is a slow-acting
Ca.sup.2+-dependent enzymatic crosslinker that imparts mechanical
and thermal stability to the gel in culture (Chen R-N, Ho H-O, Sheu
M-T (2005) Characterization of collagen matrices crosslinked using
microbial transglutaminase. Biomaterials 26(20):4229-4235),
enabling long term perfusion. Notably, the printed cell-laden ink
does not contain either enzyme to prevent polymerization during
printing. While the castable (matrix) ink contains both thrombin
and TG, which diffuse into adjacent printed filaments forming a
continuous interpenetrating polymer network (IPN), in which the
native fibrillar structure of fibrin is preserved (FIG. 24).
Importantly, our approach allows arbitrarily thick tissues to be
fabricated, since these inks do not require UV irradiation to cure,
which has a low penetration depth within scattering tissue
(Jayakumar MKG, Idris NM, Zhang Y (2012) Remote activation of
biomolecules in deep tissues using near-infrared-to-UV upconversion
nanotransducers. Proceedings of the National Academy of Sciences
109(22):8483-8488).
[0361] The gelatin-fibrin matrix supports multiple cell types of
interest to both 2D and 3D culture conditions, including human
umbilical vein endothelial cells (HUVECs), human neonatal dermal
fibroblasts (HNDFs), and human bone marrow-derived mesenchymal stem
cells (hMSCs) (FIGS. 27B-D and FIGS. 29A-B).
[0362] We found that endothelial cells express vascular
endothelial-cadherin (VE-Cad) (FIG. 27B), and HNDFs (FIG. 27C) and
hMSCs (FIG. 27D) proliferate and spread on this matrix surface and
in bulk.
[0363] Moreover, the printed cell viability can be as high as 95%
depending on how the gelatin is preprocessed. At higher
temperatures, the average molecular weight of gelatin is reduced
(69 kDa at 70.degree. C. processing to 32 kDa at 95.degree. C.
processing) resulting in softer gels with lower viscosity, shear
yield stress, and shear elastic modulus that can be printed with
ease (FIGS. 27E and FIG. 25). Upon printing, hMSCs within this soft
gelatin-fibrinogen matrix continue to spread, proliferate, and
contract into dense, cellular architectures that align along the
printing direction (FIG. 26F), likely arising due to cellular
confinement (Klumpers DD, Zhao X, Mooney DJ, Smit TH (2013) Cell
mediated contraction in 3D cell-matrix constructs leads to
spatially regulated osteogenic differentiation. Integr Biol
5(9):1174-1183) and contraction via the Poisson-effect (Oster GF,
Murray JD, Harris AK (1983) Mechanical aspects of mesenchymal
morphogenesis. J Embryol Exp Morphol: 83-125).
[0364] To construct thick living tissues within perfusion chips, we
co-printed cell-laden, fugitive, and silicone inks (FIGS.
27H-27I).
[0365] First, the silicone ink is printed on a glass substrate and
cured to create customized perfusion chips, as shown in FIGS.
26A-26S. Next, the cell-laden and fugitive inks are printed on
chip, and then encapsulated in the castable ECM matrix (FIGS.
27J-27L). The fugitive ink, which defines the vascular network, is
composed of a triblock copolymer (i.e., polyethylene oxide
(PEO)-polypropylene oxide (PPO)-PEO). This ink can be removed from
the fabricated tissue upon cooling to roughly 4.degree. C., where
it undergoes a gel-to-fluid transition, as previously shown. This
process yields a pervasive network of open channels, which are then
lined with HUVECs. The vasculature is then perfused over long time
periods using an external pump (not shown) that generates a smooth
flow over a wide range of flow rates (Giulitti S, et al. (2013)
Optimal periodic perfusion strategy for robust long-term
microfluidic cell culture. Lab on a Chip 13(22):4430).
[0366] To demonstrate the formation of stable vasculature, we
printed a simple tissue construct composed of two parallel channels
embedded within a fibroblast cell-laden matrix (FIGS. 30A-30F). The
channels are lined with HUVECs and subsequently perfused with
endothelial growth media to form a confluent monolayer that lines
each blood vessel (FIG. 30A).
[0367] Importantly, after six weeks of active perfusion, these
endothelial cells maintain endothelial phenotype and remain
confluent, characterized by expression of CD31, von-Willebrand
Factor (vWF), and vascular endothelial cadherin (VECad) (FIGS.
30B-30C). The cross-sectional view of a representative vessel
reveals lumen formation (FIG. 30D). Confirming the barrier function
of the endothelium, we measured a five-fold reduction in the
diffusional permeability compared to unlined (bare) channels (FIG.
30E and FIGS. 31A-31C). Stromal HNDFs residing within the
surrounding matrix exhibit cell spreading and proliferative
phenotypes localized to regions within .about.1 mm of the
vasculature (FIG. 30F and FIG. 32); cells further away from these
regions become quiescent likely due to an insufficient nutrient
supply. The perfusable vasculature is critical to support living
tissues thicker than 1 mm over long time periods.
[0368] To explore emergent phenomena in complex microenvironments,
we created a heterogeneous tissue architecture (>1 cm thick and
10 cm.sup.3 in volume) by printing a hMSC ink into a 3D lattice
geometry along with an interpenetrating, branched vascular network
lined with HUVECs, and infilled with an HNDF-laden extracellular
matrix uniformly distributed within the interstitial space (FIG.
33A). The fibroblast-laden matrix forms a connective tissue that
both supports and binds the printed stem cell-laden tissue and
vascular network. The embedded vascular network is designed with a
single inlet and outlet that provides an interface between the
printed tissue and the perfusion chip. This network is
symmetrically branched to ensure uniform perfusion throughout the
tissue, including deep within its core. In addition to providing
transport of nutrients, oxygen, and waste materials, the perfused
vasculature is used to deliver specific differentiation factors to
the tissue in a more uniform manner than bulk delivery methods, in
which cells at the core of the tissue are starved of factors
(Griffith LG and Swartz MA (2006) Capturing complex 3D tissue
physiology in vitro. Nat Rev Mol Cell Biol 7(3):211-224). This
versatile platform (FIG. 33A) is used to precisely control growth
and differentiation of the printed hMSCs. Moreover, both the
printed cellular architecture and embedded vascular network are
visible macroscopically with this thick tissue (FIG. 33B).
[0369] To develop a dense osteogenic tissue, we transvascularly
delivered growth media to the tissue during an initial
proliferation phase (6 days) followed by an osteogenic
differentiation cocktail that is perfused for several weeks. Our
optimized cocktail is composed of BMP-2, ascorbic acid, and
glycerophosphate, to promote mineral deposition and alkaline
phosphatase (AP) expression (FIGS. 34A-34B).
[0370] To assess tissue maturation, changes in cell function and
matrix composition are observed over time. In good agreement with
prior studies (Klumpers DD, et al. (2013) Cell mediated contraction
in 3D cell-matrix constructs leads to spatially regulated
osteogenic differentiation. Integr Biol 5(9):1174-1183), we found
that AP expression in hMSCs occurs within 3 days, while mineral
deposition does not become noticeable until 14 days, which
coincides with visible collagen-I deposition by hMSCs (FIGS.
35A-35B). The phenotype of differentiating hMSCs varies across the
printed features: cells within the filament core are close-packed,
compacted, and associated with high mineralization, while those in
the periphery are more elongated, encapsulating the printed
filaments, and exhibit less mineralization. We observe that
sub-populations of both HNDFs and hMSCs migrate from their initial
patterned geometry towards the vascular networks and wrap
circumferentially around the channel (FIG. 33C). After 30 days, the
printed hMSCs express osteocalcin within the tissue, and
osteocalcin expression is proportional to distance from the nearest
vessel (FIG. 3d). Further, we find that collagen deposition is
localized within printed filaments and around the circumference of
the vasculature (FIGS. 33E-33H).
[0371] In summary, thick, vascularized human tissues with
programmable cellular heterogeneity that are capable of long-term
(>30 days) perfusion on chip have been fabricated by
multimaterial bioprinting. The ability to recapitulate
physiologically relevant, 3D tissue microenvironments enables the
exploration of emergent biological phenomena, as demonstrated by
our observations of the in-situ development of hMSCs within tissues
containing a pervasive, perfusable, endothelialized vascular
network. Our 3D tissue manufacturing platform opens new avenues for
fabricating and investigating human tissues for both ex vivo and in
vivo applications.
Example 3
Bioprinting of Convoluted 3D Renal Proximal Tubules on Perfusable
Chips
[0372] Extracellular Matrix Preparation and Rheology
[0373] The ECM is comprised of a network of gelatin and fibrin was
prepared as described in Example 2 above.
[0374] A controlled stress rheometer (DHR-3, TA Instruments, New
Castle, Del.) with a 40 mm diameter, 2.degree. cone and plate
geometry was used for ink rheology measurements. The shear storage
(C') and loss (G'') moduli were measured at a frequency of 1 Hz and
an oscillatory strain (.gamma.) of 0.01. Time sweeps were conducted
by rapidly placing a premixed ECM solution that contains thrombin
onto the Peltier plate held at 37.degree. C.
[0375] Ink Formulations
[0376] Two inks were required for 3D bioprinting of perfusable PT
models. The two inks were prepared as described in Example 2
above.
[0377] Bioprinting of 3D Perfusable Proximal Tubule Constructs
[0378] 3D PT constructs were fabricated using a custom-designed,
multimaterial 3D bioprinter equipped with four independently
addressable printheads mounted onto a 3-axis, motion-controlled
gantry with a build volume of 725 mm.times.650 mm.times.125 mm (AGB
10000, Aerotech Inc., Pittsburgh, Pa. USA). Inks were housed in
separate syringe barrels to which nozzles of varying size (i.e., 50
.mu.m-410 .mu.m diameter) were attached via a luer-lock (EFD Inc.,
East Providence, R.I., USA). Inks were extruded through deposition
nozzles by applying air pressure (800 Ultra dispensing system, EFD
Inc., East Providence, R.I., USA), ranging from 10-90 psi,
corresponding to print speeds between 1 mm/s and 5 cm/s.
[0379] To create the customized perfusion chip gasket, the silicone
ink was deposited through a tapered 410 .mu.m nozzle onto 50
mm.times.75 mm glass slides. The gasket design was created using a
custom MATLAB script that generated G-code for a final gasket
structure. After printing, the perfusion chip gasket was cured at
80.degree. C. in an oven for >1 h and stored at room temperature
prior to use.
[0380] Patterning 3D PTs within the perfusion chip required a
combination of casting the ECM and printing the fugitive ink.
First, an ECM solution composed of 10 mg/mL fibrinogen, 7.5 wt %
gelatin, 2.5 mM CaCl.sub.2 and 0.2 wt % TG was equilibrated at
37.degree. C. for 15-20 min before use to improve optical clarity
of the ECM, as described above in Example 2. Next, the solution was
rapidly mixed with thrombin at a ratio of 500:1, resulting in a
final thrombin concentration of 1 U/mL. Immediately after rapid
mixing, the ECM solution was cast onto the base of the perfusion
chip. Within 2 min at 37.degree. C., polymerization of fibrinogen
into fibrin gel ensued. The base ECM layer was then allowed to dry
slightly under nitrogen, such that it formed a flat surface. The
fugitive Pluronic F127 ink (with 100 U/mL thrombin) was printed on
the base ECM layer in the form of a convoluted filament (tubule)
using a tapered 200 .mu.m nozzle. A custom Python script (MeCode)
was used to specify the toolpath in G-code. Directly after fugitive
ink printing, metal hollow perfusion pins interfaced through the
silicone gasket were brought into contact with the printed ink. A
top layer of ECM was then formed by casting the ECM solution over
the printed tubule, as described above. If cells, such as HNDFs,
were incorporated in the ECM (FIG. 37), they were mixed in directly
after the equilibration period, prior to thrombin mixing and
subsequent casting. After the top ECM layer was cast, the construct
was held at 3.degree. C. for 1 h to allow fibrin polymerization to
terminate and TG to crosslink the network. The construct was then
cooled to 4.degree. C. for 15-20 min to liquefy the printed
fugitive ink, which was flushed out of the device using cold cell
media, leaving behind open conduits that serve as the desired
tubular network embedded within the ECM with or without cells in
the extratubular ECM space.
[0381] Further, these tubular architectures can be assembled
layer-by-layer to create 3D architectures. For example, the three
layer structure shown in FIG. 38 was constructed using all the same
methods, except that each layer was given only 20 min to gel at
37.degree. C. before the next layer fugitive ink was printed and
more ECM was cast. The insert of FIG. 38 shows the 3 pins connected
to 3 separate tubes perfused with fluorescent dyes. This
multi-layer model is a demonstration showing how bioprinting can be
combined with microfluidics to interface vascular layers and
proximal tubules in 3D. Furthermore, all layers were evacuated
after construction. Aqueous based risk reactor dyes were perfused
in through the channel and excited with UV light for
visualization.
[0382] Each 3D PT construct was placed onto a machined stainless
steel base and a thick acrylic lid was placed on top. The lid and
base were clamped together by four screws, forming a seal around
the printed silicone gasket. Next, sterile two-stop peristaltic
tubing (PharMed BPT, 0.25 mm internal diameter) was filled with
media and connected to the outlet of a sterile filter that was
attached to a 10 ml syringe barrel (EFD Nordson), which served as a
media reservoir. PTEC media (designed for growth, so ATCC
formulation plus 1% FBS, 1% aprotinin, and 1% anti-anti) that has
been equilibrating for >3 h in an incubator at 37.degree. C., 5%
CO.sub.2 was added to the media reservoir, and tubing from the
reservoir was connected to the outlet of the chip (metal hollow
perfusion pin). To complete the perfusion circuit, silicone tubing
from the reservoir was connected to the inlet metal perfusion pin
on the chip. Hose pinch-off clamps were added at the inlet and
outlet of the perfusion chip to prevent uncontrolled flow when
disconnected from the peristaltic pump, which can damage the
epithelium or introduce air bubbles into the system. The media
reservoir was equilibrated with atmospheric conditions in the
incubator at all times by means of a sterile filter on top of the
media reservoir.
[0383] Cell Culture
[0384] Human immortalized PTECs (RPTEC/TERT1, ATCC CRL-4031) were
cultured per ATCC's instructions and were used for all PT model
studies up to passage 20. For gene expression analysis, human
primary RPTEC (Cell Science), immortalized PTECs (RPTEC-TERT1,
Evercyte) and A498 (ATCC HTB-44) renal cancer cells were used and
cultured per supplier's instructions. Human neonatal dermal
fibroblasts (HNDF), GFP expressing (Angio-Proteomie) were cultured
per supplier's instructions and used up to passage 15.
[0385] Gene Expression Analysis
[0386] Human primary RPTEC (Cell Science), immortalized RPTEC-TERT1
(Evercyte) and A498 (ATCC HTB-44) renal cancer cells were grown in
96-well plates according to supplier's instructions and collected
at Day 3 post-confluency by replacing culture medium with 100
.mu.l/well of 1.times. RNA lysis mixture (QuantiGene.RTM. Sample
Processing Kit, QS0101). Then 40 .mu.l of lysate was mixed with an
mRNA-capture magnetic bead set (Panomics QuantiGene.RTM. Plex Set
12631, catalog number 312631), incubated overnight, processed for
branched DNA amplification, and analyzed according to the
manufacturer's instructions (Panomics QuantiGene.RTM. Plex Assay
kit, QP1015). The PPIB probe was used as a housekeeping gene for
normalization. Fluorescence Intensity (FI) data were presented as
average and standard deviation of 3 biological replicates.
[0387] Cytokine Analysis of Media Perfusate
[0388] Media perfusate was collected from a tubule over a period of
25 days post cell seeding and stored at -80.degree. C. prior to
analysis. For cytokine profiling, supernatants were thawed on ice,
diluted 2.times. in sample dilution buffer (BioRad catalog
#M60-009RDPD) and analyzed by Luminex technology-based ELISA using
the Bio-Plex Pro.TM. Human Chemokine IL-6 (Set #171BK29MR2), IL-8
(Set #171-BK31MR2) and MCP-1 (Set #171-BK36MR2) and the
Bio-Plex.RTM. 200 Systems (BioRad), according to the manufacturer's
instructions. Data were reported as average cytokine concentrations
and standard deviations of technical triplicates.
[0389] Epithelialization and Longitudinal Culture
[0390] Each printed 3D PT construct was perfused for several hours
with PTEC media in the incubator prior to cell loading/seeding.
PTECs (PTEC/TERT1, ATCC) were trypsinized from their culture dish
and concentrated in media to .about.2.times.10.sup.7 cells/mL. The
cell suspension was then loaded into the perfusion chip through the
outlet (FIGS. 39B and 39C). The loaded construct was placed
laterally in the incubator for several hours and then flipped
180.degree. and incubated in the tubule with no flow overnight. The
next day, non-adherent cells were flushed out of the tubule under
flow by gravity. Perfusion of fresh media was then started and the
remaining cells began to cluster and then grew from those colonies
(FIG. 39F) until they reached confluency at around 3 weeks post
seeding (FIG. 39K). During the growth phase, PTECs were fed PTEC
media prepared per ATCC guidelines plus 1% aprotinin (EMD
Millipore, used to slow down the degradation of the ECM), 1% fetal
bovine serum (FBS), and 1% antibiotic-antimycotic (Gibco). After
maturation, FBS was removed, and PTECs packed into a tight
epithelial monolayer (not shown). At Day 1 post-seeding, the PTECs
were exposed to continuous, unidirectional flow at 1 .mu.l/min,
equating to shear stresses that varied between 0.1 and 0.5
dynes/cm.sup.2 depending on the tubule cross section. Media was fed
via a peristaltic pump in a closed loop circuit and changed every 2
days.
[0391] Albumin Uptake Study
[0392] Albumin uptake is assessed for the printed 3D PT models as
well as 2D controls. The first control consists of PTECs grown on
tissue culture plastic, while the second control consists of PTECs
grown on our ECM. In each case, PTECs are grown to confluency and
allowed to mature in serum free media. Human serum albumin
conjugated with FITC (HSA-FITC, Abcam ab8030) is suspended in PTEC
media at 50 .mu.g/mL. All samples are incubated with HSA-FITC in
their media for 2 h (in the case of perfusion, it is perfused
through the open lumen). After exposure, all samples are washed
with 3.times. volume and then trypsinized with 10.times. trypsin to
collect the individual cells. Cells are fixed and counterstained
with primary and secondary antibodies for megalin. Table 2 lists
the specific antibodies used.
TABLE-US-00002 TABLE 2 Host Species & Antibody or stain: Source
Catalog # Reactivity Concentration Megalin abcam ab76969 Rabbit
anti-human 1:300 AQPI Santa Cruz SC25287 Mouse anti-human 1:300
Na/K ATPase abcam ab76020 Rabbit anti-human 1:400 Acetylated alpha
tubulin abcam ab24610 Mouse anti-human 1:300 Antibody to laminin
abcam ab11575 Rabbit anti-human 1:250 K Cadherin abcam ab133632
Rabbit anti-human 1:200 ActinGreen Life Technologies R37110 N/A 2
drops per mL NucBlue Life Technologies R37605 N/A 2 drops per
mL
[0393] Cells from those samples, and naked cells, are analyzed by
flow cytometry (BD LSR Fortessa) and data is collected from
n=10,000 cells per sample. To obtain images of HSA-FITC and megalin
in PTECs, samples are fixed in place with formalin instead of being
trysinized after the wash step. Those samples are counterstained
for megalin and imaged using confocal microscopy (Zeiss
LSM710).
[0394] Cyclosporine A Testing
[0395] The toxic effect of CysA on PTECs in both the 2D controls
and printed 3D PTs was explored. In 2D, cells were seeded in a
96-well format on tissue culture plastic and grown to confluency.
They were fed media per ATCC's guidelines. CysA (Sigma-Aldrich,
SML1018) was suspended in their media at various concentrations and
incubated with cells for 24 h. A viability assay using
(3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl-
)-2H-tetrazolium) in the presence of phenazine methosulfate (MTS)
was run at the 24 h mark post exposure. This assay was completed on
PTECs at early confluency, by giving CysA to the cells on the day
they reached confluency, as well as late confluency, by giving CysA
several days after they reached confluency. Notably, the toxicity
results were similar for each case (FIG. 40N). For 3D PTs, CysA was
fed at various concentrations through the open lumen of mature
tubules after reaching confluency (at .about.3 week mark), where no
serum was included in the media for a minimum of 10 days. At the 24
h mark post CysA exposure, a FITC-dextran leak test (described
below) was performed to assess and quantify perturbations to the
barrier function of PTECs. Directly following, the PT was fixed
using 10% buffered formalin for 1 h and counterstained for actin
and DAPI as provided in Table 2 above (specific stains).
[0396] Diffusional Permeability Measurements
[0397] To assess barrier function of the epithelium in 3D,
diffusional permeability was quantified by perfusing PTEC media in
the open lumen containing 25 .mu.g/mL FITC-conjugated 70 kDa
dextran (FITC-Dex, Sigma product 46945) at a rate of 15 .mu.L/min
for 3 min and 1 .mu.L/min thereafter for .about.30-45 min. The
entire test was performed under live cell imaging with both the
tubule and the surrounding ECM in the field of view (FIGS.
41A-41E). The diffusion pattern of FITC-Dex was detected using a
wide-field fluorescent microscope (Zeiss Axiovert 40 CFL).
Fluorescence images were captured before perfusion and every 3 to 5
min over a 30-45 min period. Diffusional permeability of FITC-Dex
was calculated by quantifying changes in fluorescence intensity
over time using the following equation (Price, G. & Tien, J. in
Biological Microarrays, Vol. 671. (eds. A. Khademhosseini, K.-Y.
Suh & M. Zourob) 281-293 (Humana Press, 2011));
P d = 1 I 1 - I b ( I 2 - I 1 t ) d 4 ##EQU00002##
[0398] P.sub.d is the diffusional permeability coefficient, I.sub.1
is the average intensity at an initial time point, I.sub.2 is an
average intensity at t.about.30-45 min, I.sub.b is background
intensity (image taken before perfusion of FITC-Dex), and d is the
diameter of the channel.
[0399] Electron Microscopy
[0400] For transmission electron microscopy (TEM), PTECs in 2D or
3D architectures were fixed using 2.5% glutaraldehyde, 1.25%
paraformaldehyde, and 0.03% picric acid in 0.1 M sodium cacodylate
buffer (pH 7.4) for a minimum of several hours. Small samples (1
mm.times.1 mm) were removed and washed in 0.1 M cacodylate buffer
and bathed in 1% osmiumtetroxide (OsO.sub.4) (EMS) and 1.5%
potassiumferrocyanide (KFeCN.sub.6) (Sigma) for 1 h, washed in
water 3.times. and incubated in 1% aqueous uranyl acetate (EMS) for
1 h followed by 2 washes in water and subsequent dehydration in
varying grades of alcohol (10 min each; 50%, 70%, 90%, 2.times.10
min 100%). The samples were then placed in propyleneoxide (EMS) for
1 h and incubated overnight in a 1:1 mixture of propyleneoxide and
TAAB Epon (Marivac Canada Inc. St. Laurent, Canada). The following
day the samples were embedded in TAAB Epon and polymerized at
60.degree. C. for 48 h. Ultrathin sections (about 60 nm) were cut
on a Reichert Ultracut-S microtome, placed on copper grids stained
with lead citrate and examined in a JEOL 1200EX Transmission
electron microscope and images were recorded with an AMT 2k CCD
camera. Image analysis was performed using ImageJ software.
[0401] For scanning electron microscopy (SEM), perfused PTECs in 3D
were fixed using 10% buffered formalin for 1 h. The samples were
thinly sliced (.about.1 mm thick) to expose cells circumscribing
the open lumen. The fixative was washed away using PBSx2 and
subsequent dehydration in varying grades of ethanol (20 min each;
30%, 50%, 70%, 90%, 3.times.20 min 100%). The samples were then
placed in 50% ethanol and 50% hexamethyldisilazane (HMDS) for 30
min followed by 100% HMDS 3.times.30 min. All steps were performed
in a closed and sealed glass container. After the final washing
with HMDS, the samples were removed and placed in an open container
under N.sub.2 in the fume hood to dry. Dried samples were mounted
to aluminum pin mounts using conductive carbon tape, sputter coated
with gold, and imaged with a Tescan Vega SEM.
[0402] Immunostaining
[0403] Immunostaining followed by confocal microscopy was used to
assess the cellular localization of proteins in 2D and 3D PTEC
models. Prior to immunostaining, each construct was washed with PBS
and then fixed for 20 min to 1 h using 10% buffered formalin. The
fixative was removed using several washes in PBS for several hours
and then blocked overnight using 1 wt % bovine serum albumin (BSA)
in PBS. Primary antibodies to the cell protein or biomarker of
interest were incubated with the constructs for 1 day at the
dilutions listed in Table 2 above in a solution of 0.5 wt % BSA and
0.125 wt % Triton X-100. Removal of unbound primary antibodies was
accomplished using a wash step against a solution of PBS or 0.5 wt
% BSA and 0.125 wt % Triton X-100 in PBS for 1 day. Secondary
antibodies were incubated with the constructs for 1 day at the
dilutions listed in the Table above in a solution of 0.5 wt % BSA
and 0.125 wt % Triton X-100 in PBS. Samples were counter-stained
with NucBlue or ActinGreen for 2 h and then washed for 1 day in PBS
prior to imaging.
[0404] Image Rendering and Analysis
[0405] Phase contract microscopy was performed using an inverted
Leica DM IL scope with objectives ranging from 1.25X to 40X.
Confocal microscopy was performed using an upright Zeiss LSM 710
with water immersion objectives ranging from 5X to 40X employing
spectral lasers at 405, 488, 514, 561, and 633 nm wavelengths.
Image reconstructions of z-stacks were performed in ImageJ using
the z-projection function with the maximum pixel intensity setting.
Any increases in brightness were performed uniformly across an
entire z-projected image. 3D image reconstructions were performed
using Imaris software. The new CytoSMART (Lonza) in incubator
system was used to capture time-lapse imaging (not shown). Image
analysis for quantification of diffusional permeability was
performed using custom MATLAB scripts employing previously reported
methods (Price, G. & Tien, J. in Biological Microarrays, Vol.
671. (eds. A. Khademhosseini, K.-Y. Suh & M. Zourob) 281-293
(Humana Press, 2011))._TEM image analysis was performed using
ImageJ software to measure cell height (n.gtoreq.50), microvilli
density (n.gtoreq.25), and microvilli length (n.gtoreq.200) over at
least 3 independent samples for each condition.
[0406] Statistical Analysis
[0407] Data were expressed as means.+-.standard deviation.
Statistical analysis was performed using MATLAB and statistical
significance was determined at a value of p <0.05 as determined
by an ANOVA using Tukey's multiple pairwise comparison test.
Different significance levels (p values) were indicated with
asterisks and specific p values were provided in each figure
legend.
[0408] Results
[0409] Printing, Seeding, and Longitudinal Culture of 3D Proximal
Tubules
[0410] The described bioprinting method was used to construct a 3D
convoluted proximal tubule segment of a nephron, as depicted in
FIG. 36A. First, as shown in FIGS. 36B and 36C, a silicone gasket
was printed on a glass slide that demarcates the outer border of
the 3D tissue chip. A layer of engineered extracellular matrix
(ECM), which was composed of a gelatin-fibrin hydrogel (Kolesky, D.
B., et al., Proc. Natl. Acad. Sci. U.S.A. (2016)), was then evenly
deposited within the gasket. Next, a fugitive ink, shown in pink,
was printed onto the ECM layer. The term "fugitive ink" refers to a
printed material that will ultimately be liquefied and removed from
the final 3D PT construct. After printing, the fugitive ink was
connected to hollow metal pins interfaced through the gasket walls
and additional ECM was cast over the printed structure. The 3D
tissue model was then housed within a perfusable chip, where it was
cooled to 4.degree. C. to liquefy and subsequently remove the
fugitive ink yielding an open convoluted tubular channel embedded
within the ECM. Finally, cell media was perfused through the 3D
convoluted tubular architecture on chip via an external peristaltic
pump. Notably, the described method can create 3D proximal tubule
models in myriad configurations with precisely controlled size,
curvature, and location. For instance, if multiple tubules are
required to increase statistical relevance of an assay or provide
basal-side access channels, they can be printed alongside one
another (see FIGS. 42A and 42B) and can be perfused independently
or through a single inlet. As shown in FIGS. 42A and 42B, multiple
PTs can be printed in parallel and lined with PTEC cells that grow
to confluency.
[0411] The composition and rheological properties of the ECM and
fugitive ink were specifically tailored for the described
biofabrication method. The ECM consisted of fibrinogen, gelatin,
and two enzymes (thrombin and transglutaminase), as described in
Example 2. The dual enzyme scheme enabled rapid solidification of
the ECM around printed features, through thrombin action on
fibrinogen to make fibrin. The second enzyme, transglutaminase,
provided a slower crosslinking of gelatin with fibrin, enabling a
seamless integration of the upper and lower ECM layers during
assembly (see FIG. 43A). Furthermore, the elastic modulus of the
ECM (.about.3.5 kPa) mimiced that of the cortex of a healthy kidney
(.about.4 kPa) (Bensamoun, S. F., Robert, L., Leclerc, G. E.,
Debernard, L. & Charleux, F. Stiffness imaging of the kidney
and adjacent abdominal tissues measured simultaneously using
magnetic resonance elastography. Clin. Imaging 35, 284-287 (2011));
both matrix stiffness and composition were important for the
retention of tissue-specific cell functionality (Jansen, J. et al.
Biotechnological challenges of bioartificial kidney engineering.
Biotechnol Adv 32, 1317-1327 (2014); and Furness, P. N.
Extracellular matrix and the kidney. J. Clin. Pathol. 49, 355-359
(1996)). The fugitive ink was composed of a triblock copolymer of
polyethylene-polypropylene-polyethylene (Pluronic.RTM. F127), which
formed a viscoelastic gel above a critical micelle concentration in
water at room temperature. This ink exhibited a gel-to-fluid
transition as the perfusable tissue chip was cooled to 4.degree.
C., enabling its removal from the ECM under those conditions
(Kolesky, D. B. et al., Adv. Mater. 26, 3124-3130 (2014); and Wu,
W., et al, Adv Mater 23, H178-183 (2011)). The fugitive ink also
contained a high concentration of thrombin (100 U/mL). When this
ink was surrounded by ECM during the casting process, soluble
fibrinogen was rapidly transformed to insoluble fibrin, templing
fibrin around the lumen and facilitating the desired, long-term
perfusion of cell media.
[0412] Prior to introducing cells, the 3D tissue chip was perfused
with cell media overnight at 37.degree. C. to remove any residual
fugitive ink or enzymes and equilibrate the matrix at 37.degree. C.
and 5% CO.sub.2 in the incubator. PTEC-TERT1 cells were introduced
that consisted of human proximal tubular cells immortalized through
stable expression of the catalytic subunit of human telomerase
reverse transcriptase (TERT) (Wieser, M. et al., American journal
of physiology. Renal physiology 295, F1365-1375 (2008)). PTEC-TERT1
were developed as a cell model that maintains morphological and
functional properties of primary PTEC cells with an additional
replicative advantage over primary cells that have a finite
lifespan in vitro due to telomere shortening. Genomic stability of
PTEC-TERT1 up to 90 population doublings has been demonstrated
(Wieser, M. et al. (2008)). The PTEC-TERT1 was profiled by carrying
out gene expression analysis on 33 key PTEC genes and compared them
with primary PTEC and the renal cancer cell line A498 (FIG. 43B).
The mRNA levels demonstrated that PTEC-TERT1 cells were
transcriptionally close to primary renal PTEC cells. Given the need
for scalable, stable cellular systems in drug discovery and safety
platforms, we optimized the described 3D PT model with PTEC-TERT1
(hereby referred to as PTECs).
[0413] To circumscribe the convoluted tubules with a confluent PTEC
monolayer, the cells were first trypsinized from a tissue culture
plastic dish, concentrated, and perfused into the open lumen of the
printed structure. The cells were incubated in the tubule overnight
with no flow to facilitate adherence to the ECM and are then
flushed lightly at Day 1 to remove any non-adherent cells. A time
sequence of their maturation process in the tubule is provided in
FIGS. 39A-39K. Notably, PTECs grow to confluency within the tubule,
circumscribing the open lumen in 3D over a period of approximately
3 weeks (FIG. 39K). Furthermore, since PTECs actively participate
in pro-inflammatory cytokine production in vivo and in vitro, we
measured the accumulation of IL6, IL-8 and MCPI in the tubule
perfusate over time. The cytokine profile showed distinct
concentrations in the growth and maturation phase, suggesting the
tubule stabilized after confluency (FIGS. 44A-44C). The light grey
bars in FIGS. 44A-44C represent the growth phase of the tubule. At
Day 12, the tubule is near confluency, FBS is removed from the
media, and the profile of the confluent tubule is shown in dark
grey bars. Note that once confluency is reached and FBS is removed,
cytokine levels stabilize. Moreover, the decrease of IL-6
concentration after serum removal was consistent with the
previously reported inductive effect of albumin on IL-6 production
in primary human PTECs (Pearson, A. L., Colville-Nash, P., Kwan, J.
T. & Dockrell, M. E. Albumin induces interleukin-6 release from
primary human proximal tubule epithelial cells. JN journal of
nephrology 21, 887 (2008)).
[0414] For increasing levels of complexity, support cells, such as
fibroblasts or immune cells, can be suspended in the ECM
surrounding the printed tubules. As shown in FIG. 37, fibroblasts
can survive adjacent to the tubule in the extratubular space of the
ECM. Tubule diameters ranging from 150 .mu.m to 700 .mu.m have been
printed; assays and quantitative measurements shown here were
carried out on PTs with diameters ranging from 400 .mu.m to 550
.mu.m under a flow rate of .about.1 .mu.L/min. Images of a mature
PT at low (FIG. 36D) and higher magnifications (FIGS. 36E and 36F)
reveal that PTECs circumscribe the lumen and adopt a cuboidal
morphology, as expected for their in vivo phenotype.
[0415] Specifically, FIG. 36D depicts_a 3D rendering of the printed
convoluted proximal tubule acquired by confocal microscopy_(actin
is stained in red and nuclei are blue); the white dotted line
denotes the location of the cross-sectional view shown below in
which PTEC cells circumscribe the open lumens in 3D, scale bar=500
.mu.m. FIG. 36E is a higher magnification view of the region in (d)
denoted by the white rectangle, scale bar=200 .mu.m. FIG. 36F is a
schematic showing the result of the fabrication process where an
open lumen circumscribed with an epithelial lining is created in 3D
and directionally perfused on chip.
[0416] These engineered 3D convoluted PTs were maintained
longitudinally by feeding media in a closed loop system. Media was
replaced every two days and the tubules remain viable for extended
periods; the longest period tested exceeds two months (65
days).
[0417] 3D Proximal Tubules Form a Tissue-Like Polarized
Epithelium
[0418] After PTECs were seeded and grown to maturity in the tubule,
a combination of light microscopy, scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) were used to
characterize the printed and perfused 3D PT (FIGS. 45A-46K and
46A-46G).
[0419] FIGS. 45A-45K depict proximal tubule morphology and
molecular markers: (A) A phase contrast image of a mature 3D PT
construct taken at 6 weeks, scale bar=500 .mu.m, (B) phase contrast
image of the 3D PT construct at 6 weeks, scale bar=250 .mu.m, (C)
TEM image of the PTECs within the tubule at 5 weeks, scale bar=5
.mu.m, (D) TEM image of the PTECs grown on a 2D dish coated with
ECM with no perfusion, scale bar=5 .mu.m, (E) schematic view of the
columnar epithelium seen in native tissue, in which PTECs pack
together closely and exhibit a dense brush border on the apical
side, tight junctions, and a solid basement membrane, reprinted
with permission (Mescher, A. in Junqueira's Basic Histology: Text
and Atlas, Edn. 13 85 (McGraw-Hill Education, 2013)) (F) PTEC cell
height as measured from TEM images of the 3D PT constructs (3DP) as
well as three 2D controls (2DP=PTECs on ECM in 2D with perfusion,
2D=PTECs on ECM in 2D not perfused, Dish=bare tissue culture dish
not perfused), *p<0.001, **p<0.02, (G) SEM images at low
(scale bar=50 .mu.m) and higher (scale bar=20 .mu.m) magnifications
showing a confluent layer of PTECs within the 3D PT, white arrows
highlight the presence of primary cilia at a density of one per
cell, (H) 3D rendering of a partial tubule showing the apical side,
which highlights the primary cilia (red), scale bar=20 .mu.m, (I)
image of the PT highlighting the presence of Na/K ATPase in green,
scale bar=100 .mu.m, (J) image of the 3D PT highlighting the
presence of AQPI in yellow, scale bar=100 .mu.m, (K) high
magnification view of the image in (j) highlighting actin in red
and showing AQPI in yellow, scale bar=20 .mu.m.
[0420] Specifically, low (FIG. 45A) and high (FIG. 45B)
magnification views in phase microscopy reveal that PTECs grow
throughout the tubule packing together in a columnar fashion. TEM
images of the tubule cross-section further show that PTECs assemble
into a tightly packed, columnar renal tubular epithelium (FIGS. 45C
and 45D). As shown schematically in FIG. 45E, the epithelium should
have a basement membrane formed on the basal side and a brush
border of microvilli on the apical side facing the open lumen with
cells in a columnar morphology. From the TEM images, the increase
in cell height was quantified, owing to the columnar cell
morphology within the 3D proximal tubule (FIG. 45C) compared to the
same cells grown for the same duration in 2D on ECM without
perfusion (FIG. 45D). Importantly, the PTECs in the printed and
perfused 3D PT constructs exhibited a two-fold increase in cell
height relative to the planar controls without perfusion and a 40%
increase relative to perfused 2D controls on our ECM (FIG.
45F).
[0421] SEM images of the apical side of the 3D PT (FIG. 45G) reveal
the formation of a confluent cell layer and the presence of primary
cilia (one per cell, akin to that observed in vivo). The primary
cilium is a sensory organelle that extends into the open lumen and
responds to shear stress; it is important for the maintenance of
the epithelial cell phenotype and is often lost once cells are
isolated and cultured in 2D in the absence of shear stress) Jang,
K. J. et al. Human kidney proximal tubule-on-a-chip for drug
transport and nephrotoxicity assessment. Integrative biology:
quantitative biosciences from nano to macro 5, 1119-1129 (2013)).
Primary cilia were also observed in the described PT using
immunofluorescence, by staining for acetylated tubulin (shown in
red in the 3D rendering in FIG. 45H and FIGS. 47A and B).
Furthermore, we confirmed the expression of the epithelial marker
Na.sup.+/K.sup.+ ATPase (FIGS. 45H and 45I), and its appropriate
sub-cellular localization to the basolateral plasma membrane (FIG.
47A), which is again akin to the in vivo PTEC phenotype. The
proximal tubule-specific (versus distal tubule) water channel
Aquaporin I (AQPI) was also predominant throughout the tubule (FIG.
45J) and the AQPI staining at higher magnification has a speckled
pattern on the membrane surface (FIG. 45K).
[0422] Cell polarity is a fundamental feature of epithelial cells
and is crucial to ensure vectorial transport by PTECs. Hence, we
explored PTEC polarity by first characterizing the apical side of
our 3D PT using TEM (FIG. 46A). At the apical surface, microvilli
were present and formed a brush border that is more pronounced than
in 2D (compare FIG. 46A and FIG. 45C with FIG. 45D). At the basal
(FIG. 46B) surface, basolateral interdigitations (BI) are
prominent. These BI extend the surface area of the lateral and
basal borders in vivo; cells in 2D (FIG. 45D) were mainly flat and
lacked BI. Presence of circular invaginations in the lateral
membrane, annotated with white arrows in FIG. 46B, suggests that
mechanisms of active transport are present at the lateral surface.
Furthermore, there is a distinct difference between the morphology
of the ECM and basement membrane (BM) proteins deposited by the
PTECs. Further exploration of the BM protein composition revealed
that in mature 3D PT constructs, PTECs deposited laminin and
collagen IV (FIG. 46C). Other important properties observed were
the presence of tight junctions between neighboring cells in TEM
(FIG. 46D) and the presence of cell-cell junction proteins, such as
K cadherin in FIG. 46E, that link cells in a characteristic
cobblestone pattern. Lastly, properties of the brush border were
quantified by TEM image analysis. The average microvilli length in
the 3D printed and perfused PTs was found to be .about.200% longer
than the 2D non-perfused and .about.40% higher than the 2D perfused
controls (FIG. 46F). Concurrently, microvilli density was also
significantly higher for the printed and perfused 3D PT constructs
compared to all 2D control conditions (all 2D controls were
statistically similar) (FIG. 46G).
[0423] PTECs will form near leak tight barriers against the traffic
of certain proteins, like high molecular weight dextran, when
healthy and confluent. Barrier function was assessed by traditional
methods (Price, G. & Tien, J. in Biological Microarrays, Vol.
671. (eds. A. Khademhosseini, K.-Y. Suh & M. Zourob) 281-293
(Humana Press, 2011)). Specifically, FITC-labeled dextran (70 kDa)
was perfused through the open lumen of mature PTs and the intensity
of fluorescence was captured using a wide-field fluorescence scope
with time. From the FITC intensity values, the diffusional
permeability was calculated and compared against a 3D tubule
without epithelial lining (FIGS. 41A-41E). The drastic reduction in
the diffusional permeability coefficient (greater than an order of
magnitude) indicated that the epithelial barrier in the printed and
perfused 3D PT construct was nearly leak tight and functional.
[0424] Albumin Uptake
[0425] Receptor-mediated endocytosis by PTEC cells is essential for
body fluid homeostasis. Reabsorption of plasma proteins from the
glomerular filtrate relies partially on the megalin-cubilin complex
located in the brush border (Cui, S., et al. Megalin/gp330 mediates
uptake of albumin in renal proximal tubule. American Journal of
Physiology-Renal Physiology 271, F900-F907 (1996); Gekle, M. Renal
proximal tubular albumin reabsorption: daily prevention of
albuminuria. Physiology 13, 5-11 (1998); and Norden, A. G. et al.
Urinary megalin deficiency implicates abnormal tubular endocytic
function in Fanconi syndrome. J. Am. Soc. Nephrol. 13, 125-133
(2002))_and can be modeled in vitro by monitoring albumin uptake by
PTECs. The ability of PTECs, grown either on perfused 3D PT
constructs or 2D controls, to uptake FITC-labeled human serum
albumin (HSA) was tested. After exposure to FITC-HSA for 2 h, PTECs
were collected, stained for megalin expression, and analyzed by
flow cytometry. The results for albumin uptake are provided in FIG.
48A. Large populations of cells in the 2D controls exhibit
fluorescence intensity similar to the non-fluorescent control,
whereas cells lining the perfused 3D PT constructs exhibit a
significant increase in the FITC-HSA intensity. Results for
megalin, one of the transporters for albumin, show that its
expression was also highest in the 3D PT (FIG. 48B). Mean values
for the fluorescence intensity of the populations analyzed by flow
cytometry are listed in Table 3.
TABLE-US-00003 TABLE 3 Mean Intensity Albumin Megalin 2D on Plastic
201 571 2D on Printing Matrix 310 1127 3D Printed (Perfused) 1452
1670
[0426] Contrary to the 2D controls, enhanced megalin expression was
strongly correlated with superior albumin functional uptake in the
perfused 3D PTs, suggesting that both 3D architecture and perfusion
improve epithelial function likely due to enhanced cell polarity
and brush border (FIGS. 46A-46G). Lastly, images of FITC-HSA (FIG.
48C), megalin (FIG. 48D), and the combination thereof (FIG. 48E)
reveal an overlapping distribution of albumin and megalin in PTECs
that line the 3D PT. Thus, the described, engineered 3D PT
constructs exhibit superior albumin uptake function relative to
either 2D control.
[0427] Drug Toxicity Testing
[0428] Cyclosporine A, a drug commonly given following transplant
surgery to prevent rejection, is a known nephrotoxin that damages
proximal tubule cells. To study its effect on the perfused 3D PT,
3D PT were exposed to various concentrations of Cyclosporine A
(CysA) and alterations of cell morphology and cytoskeleton
organization were monitored by immunostaining of actin filaments.
Bright field images of the tubules (FIGS. 40A to 40D) and
corresponding 3D renderings of actin staining (FIGS. 40E to 40H and
FIGS. 40I to 40L) reveal dose-dependent manifestation of
CysA-induced damage. Minor breaks in cell-cell junctions (FIG.
49A-D) and reorganization of actin (FIG. 40J) were observed at 10
.mu.M CysA, whereas discrete areas devoid of cells were readily
evident at 50 .mu.M CysA (FIGS. 40G and 40K) and exacerbated at 100
.mu.M CysA (FIGS. 40D, 40H, and 40L). We also note that cell layers
tightened and buckled at 50 and 100 .mu.M CysA (FIGS. 40G, 40K and
FIG. 39). Finally, CysA-induced disruption of the epithelial
barrier function was assessed by quantifying the diffusional
permeability of FITC-dextran (70 kDa) in treated tubules. As shown
in FIG. 40M, exposure to 50 and 100 .mu.M CysA increased the
epithelial barrier permeability by almost 4-fold and 6-fold,
respectively. It was found that the respective cell viability of
PTECs grown on 2D culture plastic dishes decreased by 40% and 60%
after treatment with 50 and 100 .mu.M CysA (FIG. 40N). Overall,
these results indicate that the 3D PT constructs can be used to
qualitatively (immunostaining) and quantitatively (diffusional
permeability measurements) assess nephrotoxicity.
[0429] Discussion
[0430] Recent advances in bioprinting have enabled the creation of
pervasive and interconnected channels within engineered
extracellular matrices (Kolesky, D. B., et al., Three-dimensional
bioprinting of thick vascularized tissues. Proc. Natl. Acad. Sci.
U.S.A. (2016); Kolesky, D. B. et al. 3D Bioprinting of
Vascularized, Heterogeneous Cell-Laden Tissue Constructs. Adv.
Mater. 26, 3124-3130 (2014); and Miller, J. S. et al. Rapid casting
of patterned vascular networks for perfusable engineered
three-dimensional tissues. Nature materials 11, 768-774 (2012)). It
was previously shown that these channels can be lined with
endothelial cells and perfused to create tissues with embedded
vasculature (Kolesky, D. B., et al. (2016) and Kolesky, D. B., et
al. (2014)). A pathway for fabricating perfusable, convoluted 3D
proximal tubules via in vitro epithelialization is now developed.
Importantly, a controlled unidirectional flow was incorporated to
enable longitudinal studies at physiologically relevant shear
stresses in our 3D PT model. By simultaneously integrating 3D cell
culture, tissue-on-chip, and bioprinting methods, a facile,
customizable platform for producing 3D tissues and organ models on
chip was created. The ability to programmably define tubule size
and geometry, including convolution, surpasses existing 3D PT
models that rely on pin pull out generate straight tubules in gels.
Moreover, the described, engineered 3D bioprinted PTs enable the
collection of hundreds of thousands of cells for analysis, e.g.,
far greater than the 10,000 cells required for accurate sampling
via flow cytometry. By contrast, the quantitative study of high
cell populations was previously difficult to achieve in planar
microfluidic-based devices (Adler, M. et al. A quantitative
approach to screen for kidney toxic compounds in vitro. J. Am. Soc.
Nephrol. (2015); and Adler, M. et al. A quantitative approach to
screen for kidney toxic compounds in vitro. J. Am. Soc. Nephrol.
(2015)).
[0431] Based on this study, more complex constructs in which
multiple tubules are patterned alongside one another to facilitate
basal side access, vascularization, or the study of crosstalk
between PTECs in adjacent channels may be created (FIGS. 42A-B and
38). For example, 3D PTs along with embedded vasculature with
integrated podocytes and mesangial cells may be co-printed.
iPSC-derived renal progenitors could also be seeded for maturation
under 3D perfused conditions. The described, versatile bioprinting
method also allows one to incorporate multiple cells types in the
extratubular space (FIG. 37), thereby programming predefined levels
of complexity required for studying cell-cell interactions.
[0432] The engineered ECM, based on enzymatic crosslinking of
fibrinogen and gelatin, enables both the formation of tubular
printed structures and favorable PTEC matrix interactions. PTECs
form a confluent layer that can be sustained for >60 days and
exhibit the hallmarks of functional cells in vivo, including many
important morphological features and functional markers.
Interestingly, 3D printed and perfused constructs show enhanced
microvilli length, density, and cell height in comparison to 2D
perfused controls. While more extensive studies are required to
fully elucidate the origin of these enhancements in the 3D case,
several factors such as apical constriction, cell-cell signaling,
or shear stress profiles could be investigated.
[0433] The described 3D PT constructs can be used to elucidate
mechanisms of drug-induced tubule damage prior to cell death, such
as weakening of cell-cell junctions, monitoring of real-time wound
healing, and studying cellular and molecular aspects of drug
transport through the renal barrier. The morphology and function of
PTECs seeded within printed 3D tubules whose outer diameter and
curvature more closely mimics in vivo PTs may be investigated to
probe whether further improvements to the epithelium structure and
function are possible (.about.60 .mu.m diameter is physiological).
It is believed that 3D tubules with smaller diameters may further
enhance the observed brush border. Currently, the microvilli are
1.3.+-.0.3 .mu.m in our 3D PTs, which is significantly higher than
the planar controls and approaching the microvilli length found in
a rat kidney is 2.73.+-.0.13 .mu.m.
[0434] In summary, the described method combines 3D cell culture,
tissue-on-chip, and bioprinting methods to create 3D, convoluted
renal proximal tubules embedded within an extracellular matrix on
customized perfusion chips. These 3D PT models promote the
formation of a tissue-like epithelium with in vivo phenotypic and
functional properties relative to the same cells grown in 2D
controls. The described bioprinting method opens new avenues for
creating 3D tissues on chip that better recapitulate in vivo
microenvironments, which could enable advances in drug screening,
mechanistic drug studies, disease models, and ultimately,
regenerative medicine.
[0435] A new approach has been developed and described in the
present disclosure for creating vascularized, heterogeneous tissue
constructs with epithelium on demand via 3D bioprinting. This
highly scalable platform enables the fabrication of engineered
tissue constructs in which vasculature, epithelium, multiple cell
types and optionally other functional chemical substances, such as
drugs, toxins, proteins and/or hormones, are programmably placed at
desired locations within an extracellular matrix. This technique
may lead to the rapid manufacturing of functional 3D tissues and
organs needed for transplant.
[0436] The following patents and patent application publications
are hereby incorporated by reference in their entirety:
International Application No. PCT/US2014/063810, entitled "Method
of printing a Tissue Construct with Embedded Vasculature," filed
Nov. 4, 2014; International Application No. PCT/US2012/044794,
entitled "Multinozzle Deposition System for Direct Write
Applications," filed Jun. 29, 2012; U.S. Patent Application
Publication No. 2013/0084449, entitled "Viscoelastic Ink for Direct
Writing of Hydrogel Structures," which was filed as PCT/US201
1/29429 on Mar. 22, 2011; and U.S. Pat. No. 8,101,139, entitled
"Microcapillary Networks," filed on Jun. 5, 2008.
[0437] Although the present invention has been described in
considerable detail with reference to certain embodiments thereof,
other embodiments are possible without departing from the present
invention. The spirit and scope of the appended claims should not
be limited, therefore, to the description of the preferred
embodiments contained herein. All embodiments that come within the
meaning of the claims, either literally or by equivalence, are
intended to be embraced therein.
[0438] Furthermore, the advantages described above are not
necessarily the only advantages of the invention, and it is not
necessarily expected that all of the described advantages will be
achieved with every embodiment of the invention.
* * * * *