U.S. patent application number 10/800134 was filed with the patent office on 2004-11-11 for immunoneutral silk-fiber-based medical devices.
This patent application is currently assigned to Tissue Regeneration, Inc.. Invention is credited to Altman, Gregory H., Chen, Jingsong, Horan, David, Horan, Rebecca.
Application Number | 20040224406 10/800134 |
Document ID | / |
Family ID | 21734509 |
Filed Date | 2004-11-11 |
United States Patent
Application |
20040224406 |
Kind Code |
A1 |
Altman, Gregory H. ; et
al. |
November 11, 2004 |
Immunoneutral silk-fiber-based medical devices
Abstract
Silk is purified to eliminate immunogenic components
(particularly sericin) and is used to form fabric that is used to
form tissue-supporting prosthetic devices for implantation. The
fabrics can carry functional groups, drugs, and other biological
reagents. Applications include hernia repair, tissue wall
reconstruction, and organ support, such as bladder slings. The silk
fibers are arranged in parallel and, optionally, intertwined (e.g.,
twisted) to form a construct; sericin may be extracted at any point
during the formation of the fabric, leaving a construct of silk
fibroin fibers having excellent tensile strength and other
mechanical properties.
Inventors: |
Altman, Gregory H.; (Dedham,
MA) ; Chen, Jingsong; (Malden, MA) ; Horan,
Rebecca; (Westfield, MA) ; Horan, David;
(Westfield, MA) |
Correspondence
Address: |
David S. Resnick, Esq.
NIXON PEABODY LLP
100 Summer Street
Boston
MA
02110
US
|
Assignee: |
Tissue Regeneration, Inc.
Medford
MA
|
Family ID: |
21734509 |
Appl. No.: |
10/800134 |
Filed: |
March 11, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10800134 |
Mar 11, 2004 |
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10008924 |
Nov 16, 2001 |
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60453584 |
Mar 11, 2003 |
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Current U.S.
Class: |
435/395 ;
442/128 |
Current CPC
Class: |
A61L 27/386 20130101;
A61P 21/00 20180101; C07K 14/43518 20130101; A61L 27/3895 20130101;
A61F 2/00 20130101; A61L 27/3804 20130101; Y10T 442/2566 20150401;
A61L 27/3608 20130101; A61P 19/00 20180101; C12N 5/0663 20130101;
A61L 27/227 20130101; A61L 27/48 20130101; A61L 2430/10 20130101;
C12N 2533/50 20130101; A61P 9/00 20180101; A61K 35/28 20130101;
A61K 38/17 20130101; A61P 19/04 20180101; A61L 27/3662 20130101;
Y10T 428/249921 20150401; A61F 2/08 20130101; A61L 27/3834
20130101; A61L 27/3821 20130101; C12N 5/066 20130101; C08L 89/00
20130101; A61K 35/12 20130101; A61K 9/70 20130101; Y10T 428/298
20150115; A61L 27/48 20130101 |
Class at
Publication: |
435/395 ;
442/128 |
International
Class: |
C12N 005/02; B32B
027/04 |
Claims
1. A fabric comprising: a yarn, said yarn comprising one or more
sericin-extracted fibroin fibers, said fibers being biocompatible
and non-randomly organized, wherein said yarn promotes ingrowth of
cells around said fibroin fibers and is biodegradable.
2. The fabric as recited in claim 1, wherein the sericin-extracted
fibroin fibers comprises fibroin fibers obtained from Bombyx mori
silkworm.
3. The fabric of claim 1, wherein the sericin-extracted fibroin
fibers retain their native protein structure and have not been
dissolved and reconstituted.
4. The fabric of claim 1, wherein the fabric is
non-immunogenic.
5. The fabric of claim 1, wherein the sericin-extracted fibroin
fibers include less than 20% sericin by weight.
6. The fabric of claim 1, wherein the sericin-extracted fibroin
fibers include less than 10% sericin by weight.
7. The fabric of claim 1, wherein the sericin-extracted fibroin
fibers include less than 1% sericin by weight.
8. The fabric of claim 1, wherein the yarn has an ultimate tensile
strength of at least 0.52 N per fiber.
9. The fabric of claim 8, wherein the yarn has a stiffness between
about 0.27 and about 0.5 N/mm per fiber.
10. The fabric of claim 9, wherein the yarn retains 80% of its UTS
when tested wet.
11. The fabric of claim 9, wherein the yarn has an elongation at
break between about. 10% and about 50%.
12. The fabric of claim 11, wherein the yarn has a fatigue life of
at least 1 million cycles at a load of about 20% of the yarn's
ultimate tensile strength.
13. The fabric of claim 1, wherein the yarn comprises parallel or
intertwined sericin-extracted fibroin fibers.
14. The yarn of claim 13, wherein said yarn comprises at least
three aligned sericin-extracted fibroin fibers.
15. The yarn of claim 14, wherein the aligned sericin-extracted
fibroin fibers are intertwined.
16. The yarn of claim 15, wherein the yarn is a braid, textured
yarn, twisted yarn, cabled yarn, and combinations thereof.
17. The yarn of claim 16, wherein the aligned sericin-extracted
fibroin fibers are twisted or cabled about each other at 0 to 11.8
twists per cm.
18. The fabric of claim 1, further comprising a yarn having a
single-level hierarchical organization, said single-level
hierarchical organization comprising a group of parallel or
intertwined yarns.
19. The fabric of claim 1, further comprising a yarn having a
two-level hierarchical organization, said two-level hierarchical
organization comprising a bundle of intertwined groups.
20. The fabric of claim 1, further comprising a yarn having a
three-level hierarchical organization, said three-level
hierarchical organization comprising a strand of intertwined
bundles.
21. The fabric of claim 1, further comprising a yarn having a
four-level hierarchical organization, said four-level hierarchical
organization comprising a cord of intertwined strands.
22. The fabric of claim 1, wherein the yarn is twisted at or below
30 twists per inch.
23. The fabric of claim 1, wherein a plurality of the yarns are
intertwined to form a fabric.
24. The fabric as recited in claim 1, wherein the fabric comprises
a composite of the sericin-extracted fibroin fibers and one or more
degradable polymers selected from group consisting of Collagens,
Polylactic acid or its copolymers, Polyglycolic acid or its
copolymers, Polyanhydrides, Elastin, Glycosamino glycans, and
Polysaccharides.
25. The fabric of claim 20, wherein a plurality of yarns are
non-randomly organized into a fabric selected from the group
consisting of, woven fabrics, knit fabrics, warp knit fabrics,
bonded fabrics, coated fabrics, dobby fabrics, laminated fabrics,
mesh and combinations thereof.
26. The fabric of claim 20, wherein a plurality of yarns are
randomly organized into a non-woven fabric.
27. The fabric of claim 1, further comprising a drug associated
with the fabric.
28. The fabric of claim 1, further comprising a cell-attachment
factor associated with the fabric.
29. The fabric of claim 28, wherein the cell-attachment factor is
RGD.
30. The fabric of claim 1, wherein the fabric is treated with gas
plasma.
31. The fabric of claim 1, further comprising biological cells
seeded onto the fabric.
32. A method for forming a fabric comprising: a. aligning fibroin
fibers in parallel or intertwined with other fibroin fibers to form
a yarn, b. substantially removing sericin from the fibroin fibers
without substantially altering the native structure of fibroin in
the fibers, c. and organizing a plurality of yarns to form a
fabric.
33. The method of claim 32, further comprising intertwining the
parallel silk fibers before the sericin is extracted.
34. The method of claim 32, further comprising intertwining the
parallel silk fibers after the sericin is extracted.
35. The method of claim 32, further comprising aligning multiple
fibroin fibers into yarns, wherein each yarn comprises at least
three parallel or intertwined fibers.
36. The method of claim 35, wherein the fibroin fibers of each yarn
are twisted about each other at 0 to 11.8 twists per cm.
37. The method of claim 32, wherein multiple yarns are twisted
about each other at 0 to 11.8 twists per cm.
38. The method of claim 32, wherein sericin is extracted from no
more than about 50 parallel or intertwined fibroin fibers.
39. The method of claim 32, wherein the yarn is twisted at or below
30 twists per inch.
40. The method of claim 32, further comprising forming a knit or
woven fabric from a plurality of non-randomly organized yarns.
41. The method of claim 32, further comprising forming a non-woven
fabric from a plurality of randomly organized yarns.
42. The method of claims 40 and 41, wherein the fabric is formed
after sericin is extracted from the fibers in the yarns.
43. The method of claims 40 and 41, wherein the fabric is formed
before sericin is extracted from the fibers in the yarns.
44. The method of claims 40 and 41, wherein the yarn is exposed to
a force no greater than its yield point.
45. The method of claim 32, further comprising associating a drug
with the fabric.
46. The method of claim 32, further comprising associating a
cell-attachment factor with the fabric.
47. The method of claim 46, further comprising associating RGD with
the fabric.
48. The method of claim 32, further comprising treating the fabric
with gas plasma.
49. The method of claim 32, further comprising: sterilizing the
fabric.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Ser. No.
10/008,924, filed Nov. 16, 2001, the entire teachings of which are
incorporated herein by reference. Priority to provisional
application U.S.S.No. 60/453,584, filed Mar. 11, 2003, is also
claimed, and its entire contents are also incorporated by reference
herein.
BACKGROUND
[0002] Disease, aging, trauma or chronic wear often lead to tissue
or organ failure. In treating such failures, the goal of many
clinical procedure is restoration of function. A patient often
requires additional support, beyond the body's own means of
healing, such as surgery or the implantation of a medical device.
Such procedures are often needed to combat permanent disability and
even death. The fields of biomaterials and tissue engineering are
providing new options to gradually restore native tissue and organ
function through the research and development of temporary
scaffolds, matrices, and constructs (i.e., devices) that initially
support a disabled tissue or organ, but eventually allow for the
development and remodeling of the body's own biologically and
mechanically functional tissue.
[0003] The responsibilities or design requirements of such a
scaffold include: (i) the ability to provide immediate mechanical
stabilization to the damaged or diseased tissue, (ii) support cell
and tissue ingrowth into the device, (iii) communicate the
mechanical environment of the body to the developing tissue; such
is achieved through the proper mechanical and biological design of
the device, (iv) degrade at such a rate that the ingrowing cells
and tissue have sufficient time to remodel, thus creating new
autologous function tissue that can survive the life of the
patient. In certain instances, the device should mimic the correct
three-dimensional structure (e.g., a bone scaffold) of the tissue
it is attempting to support. In other instances, the device may
serve as a temporary ligature (e.g., a flat mesh for hernia repair
or a hemostat for bleeding) to a three-dimensional tissue
(abdominal wall muscle in the case of hernia). Regardless of
application, the present direction of the medical device field is
the complete restoration of bodily function through the support of
autologous tissue development.
[0004] Unfortunately, most biomaterials-available today do not
possess the mechanical integrity of high load demand applications
(e.g., bone, ligaments, tendons, muscle) or the appropriate
biological functionality; most biomaterials either degrade too
rapidly (e.g., collagen, PLA, PGA, or related copolymers) or are
non-degradable (e.g., polyesters, metal), where in either case,
functional autologous tissue fails to develop and the patient
suffers disability. In certain instances a biomaterial may
misdirect tissue differentiation and development (e.g., spontaneous
bone formation, tumors) because it lacks biocompatibility with
surrounding cells and tissue. As well, a biomaterial that fails to
degrade typically is associated with chronic inflammation, where
such a response is actually detrimental to (i.e., weakens)
surrounding tissue.
[0005] If properly designed, silk may offer new clinical options
for the design of a new class of medical devices, scaffolds and
matrices. Silk has been shown to have the highest strength of any
natural fiber, and rivals the mechanical properties of synthetic
high performance fibers. Silks are also stable at high
physiological temperatures and in a wide range of pH, and are
insoluble in most aqueous and organic solvents. Silk is a protein,
rather than a synthetic polymer, and degradation products (e.g.,
peptides, amino acids) are biocompatible. Silk is non-mammalian
derived and carries far less bioburden than other comparable
natural biomaterials (e.g., bovine or porcine derived
collagen).
[0006] Silk, as the term is generally known in the art, means a
filamentous fiber product secreted by an organism such as a
silkworm or spider. Silks produced from insects, namely (i) Bombyx
mori silkworms, and (ii) the glands of spiders, typically Nephilia
clavipes, are the most often studied forms of the material;
however, hundreds to thousands of natural variants of silk exist in
nature. Fibroin is produced and secreted by a silkworm's two silk
glands. As fibroin leaves the glands, it is coated with sericin, a
glue-like substance. However, spider silk is valued (and
differentiated from silkworm silk) as it is produced as a single
filament lacking any immunogenic contaminates, such as sericin.
[0007] Unfortunately, spider silk can not be mass produced due to
the inability to domesticate spiders; however, spider silk, as well
as other silks can be cloned and recombinantly produced, but with
extremely varying results. Often, these processes introduce
bioburdens, are costly, cannot yield material in significant
quantities, result in highly variable material properties, and are
neither tightly controlled nor reproducible.
[0008] As a result, only silkworm silk has been used in biomedical
applications for over 1,000 years. The Bombyx mori specie of
silkworm produces a silk-fiber (known as a "bave") and uses the
fiber to build its cocoon. The bave, as produced, includes two
fibroin filaments or "broins", which are surrounded with a coating
of gum, known as sericin-the silk fibroin filament possesses
significant mechanical integrity. When silk fibers are harvested
for producing yarns or textiles, including sutures, a plurality of
fibers can be aligned together, and the sericin is partially
dissolved and then resolidified to create a larger silk fiber
structure having more than two broins mutually embedded in a
sericin coating.
[0009] As used herein, "fibroin" includes silkworm fibroin (i.e.
from Bombyx mori) and fibroin-like fibers obtained from spiders
(i.e. from Nephila clavipes). Alternatively, silk protein suitable
for use in the present invention can be obtained from a solution
containing a genetically engineered silk, such as from bacteria,
yeast, mammalian cells, transgenic animals or transgenic plants.
See, for example, WO 97/08315 and US Pat. No. 5,245,012.
[0010] Silkworm silk fibers, traditionally available on the
commercial market for textile and suture applications are often
"degummed" and consist of multiple broins plied together to form a
larger single multi-filament fiber. Degumming here refers to the
loosening of the sericin coat surrounding the two broins through
washing or extraction in hot soapy water. Such loosening allows for
the plying of broins to create larger multifilament single fibers.
However, complete extraction is often neither attained nor desired.
Degummed silk often contains or is recoated with sericin and/or
sericin impurities are introduced during plying in order to congeal
the multifilament single fiber. The sericin coat protects the frail
fibroin filaments (only .about.5 microns in diameter) from fraying
during traditional textile applications where high-through-put
processing is required. Therefore, degummed silk, unless explicitly
stated as sericin-free, typically contain 10-26% (by weight)
sericin (see Tables 1 & 2).
[0011] When typically referring to "silk" in the literature, it is
inferred that the remarks are focused to the naturally-occurring
and only available "silk" (i.e., sericin-coated fibroin fibers)
which have been used for centuries in textiles and medicine.
Medical grade silkworm silk is traditionally used in only two
forms: (i) as virgin silk suture, where the sericin has not been
removed, and (ii) the traditional more popular silk suture, or
commonly referred to as black braided silk suture, where the
sericin has been completely removed, but replaced with a wax or
silicone coating to provide a barrier between the silk fibroin and
the body tissue and cells. Presently, the only medical application
for which silk is still used is in suture ligation, particularly
because silk is still valued for it mechanical properties in
surgery (e.g., knot strength and handlability).
[0012] Despite virgin silk's use as a suture material for thousands
of years, the advent of new biomaterials (collagen, synthetics)
have allowed for comparisons between materials and have identified
problems with sericin. Silk, or more clearly defined as Bombyx mori
silkworm silk, is non-biocompatible. Sericin is antigenic and
elicits a strong immune, allergic or hyper-T-cell type (versus the
normal mild "foreign body" response) response. Sericin may be
removed (washed/extracted) from silk fibroin; however, removal of
sericin from silk changes the ultrastructure of the fibroin fibers,
exposing them, and results in loss of mechanical strength, leading
to a fragile structure.
[0013] Extracted silk structures (i.e., yarns, matrices) are
especially susceptible to fraying and mechanical failure during
standard textile procedures due to the multifilament nature of the
smaller diameter (.about.5 um) fibroin filaments. The extracted
fibroin's fragility is the reason that when using silk in the
design and development of medical devices, following extraction, it
is typically taught (Perez-Rigueiro, J. AppI. Polymer Science, 70,
2439-2447, 1998) that you must dissolve and reconstitute silk using
standard methods (U.S. Pat. No. 5,252,285) to gain a workable
biomaterial. The inability to handle extracted silk fibroin with
present-day textile methods and machinery has prevented the use of
non-dissolved sericin-free fibroin from being explored as a medical
device.
[0014] Additional limitations of silk fibroin, whether extracted
from silkworm silk, dissolved and reconstituted, or produced from
spiders or insects other than silkworms include (i) the hydrophobic
nature of silk, a direct result of the beta-sheet crystal
conformation of the core fibroin protein which gives silk its
strength, (ii) the lack of cell binding domains typically found in
mammalian extracellular matrix proteins (e.g., the peptide sequence
RGD), and (iii) silk fibroin's smooth surface. As a result, cells
(e.g., macrophages, neutrophils) associated with an inflammatory
and host tissue response are unable to recognize the silk fibroin
as a material capable of degradation. These cells thus opt to
encapsulate and wall off the foreign body (see FIG. 18A) thereby
limiting (i) silk fibroin degradation, (ii) tissue ingrowth, and
(iii) tissue remodeling. Thus, silk fibroin filaments frequently
induce a strong foreign body response (FBR) that is associated with
chronic inflammation, a peripheral granuloma and scar encapsulation
(FIG. 18A).
[0015] In addition to the biological disadvantages of silk, the
multifilament nature of silk (e.g., as sutures) as well as the
small size of the fibroin filaments can lead to a tightly packed
structure. As such, silk may degrade too rapidly. Proteases
(enzymes) produced from the stimulated cells found within the
peripheral encapsulation can penetrate the implanted structure (see
FIG. 11A and FIG. 11B), but cells depositing new tissue (e.g.,
fibroblasts) which may reinforce the device (in this case a black
braided suture) during normal tissue remodeling cannot. Therefore,
the interior of non-treated or non-modified fibroin devices does
not come in contact with the host foreign body response and tissue
(led and produced by fibroblasts) and as a result, the capacity of
the device to direct tissue remodeling is limited. Host cell and
tissue growth is limited and degradation is not normally
possible.
[0016] In the case of sutures, it is thought that these problems
can be managed by treating fibroin sutures with cross-linking
agents or by coating the sutures with wax, silicone or synthetic
polymers, thereby shielding the material from the body. Coatings,
such as sericin, wax or silicone, designed to add mechanical
stability to the fibroin (combating its fragility while providing a
barrier between the body and the fibroin), limits cell attachment,
recognition and infiltration and tissue ingrowth and fibroin
degradation. As a result, silk is traditionally thought of as a
non-degradable material.
[0017] Classification as a non-degradable may be desirable when
silk is intended for use as a traditional suture ligation device,
i.e., cell and tissue ingrowth into the device are not desirable.
Therefore, cell attachment and ingrowth (which lead to matrix
degradation and active tissue remodeling) is traditionally
prevented by both the biological nature of silk and the structure's
mechanical design. In fact, a general belief that silk must be
shielded from the immune system and the perception that silk is
non-biodegradeable have limited silk's use in surgery. Even in the
field of sutures, silk has been displaced in most applications by
synthetic materials, whether biodegradable or permanent.
[0018] Therefore, there exists a need to generate sericin-extracted
silkworm fibroin fibers that are biocompatible, promote ingrowth of
cells, and are biodegradable.
SUMMARY
[0019] Natural silk fibroin fiber constructs, disclosed herein,
offer a combination of high strength, extended fatigue life, and
stiffness and elongation at break properties that closely match
those of biological tissues. The fibers in the construct are
non-randomly aligned into one or more yarns. The fiber constructs
are biocompatible (due to the extraction of sericin from the
silkworm silk fibers) and substantially free of sericin. The fiber
constructs are further non-immunogenic; i.e., they do not elicit a
substantial allergic, antigenic, or hyper T-cell response from the
host, diminishing the injurious effect on surrounding biological
tissues, such as those that can accompany immune-system responses
in other contexts. In addition, the fiber constructs promote the
ingrowth of cells around said fibroin fibers and are
biodegradable.
[0020] Indications that the fiber construct is "substantially free"
of sericin mean that sericin comprises less than 20% sericin by
weight. Preferably, sericin comprises less than 10% sericin by
weight. Most preferably, sericin comprises less than 1% sericin by
weight (see Table 2). Furthermore, "substantially free" of sericin
can be functionally defined as a sericin content that does not
elicit a substantial allergic, antigenic, or hyper T-cell response
from the host. Likewise, indication that there is less than a 3%
change in mass after a second extraction would imply that the first
extraction "substantially removed" sericin from the construct and
that the resulting construct was "substantially free" of sericin
following the first extraction (see Table 2 and FIG. 1F).
[0021] Methods of this disclosure extract sericin from the
construct much more thoroughly than do the typical "degumming"
procedures that characterize traditional processing practices for
the production of silk textiles for non-surgical applications (see
above for definition). FIG. 1A shows an image of a degummed fiber
where fibroin filaments were plied together forming a larger fiber
re-encased with sericin. This "degummed" fiber contains .about.26%,
by weight, sericin. In a preferred embodiment, the
sericin-extracted silkworm fibroin fibers retain their native
protein structure and have not been dissolved and
reconstituted.
[0022] "Natural" silk fibroin fibers are produced by an insect,
such as a silkworm or a spider and possess their native, as formed,
protein structure. Preferably, the silk fibroin fiber constructs
are non-recombinant (i.e., not genetically engineered) and have not
been dissolved and reconstituted. In a preferred embodiment, the
sericin-extracted fibroin fibers comprised fibroin fibers obtained
from Bombyx mori silkworm. Further, the term, "biodegradable," is
used herein to mean that the fibers are degraded within one year
when in continuous contact with a bodily tissue. In addition, our
data suggests (FIG. 13 A-E, FIG. 18 A-C & FIG. 19 A-D) that the
rate of degradation can be influenced and enhanced by surface
modification of the fibroin (FIG. 13A-D & FIG. 18A-C) as well
as the geometric configuration of the yarn and/or fabric (FIG.
19A-D). In one embodiment, silk fibroin yarn lost 50% of its
ultimate tensile strength within two weeks following implantation
in vivo (FIG. 12) and 50% of its mass within approximately 30 to 90
days in vivo, depending on implantation sight (FIG. 13A-D). The
choice of implantation site in vivo (e.g., intra-muscular versus
subcutaneous) was shown to significantly influence the rate of
degradation (FIG. 13A-D).
[0023] Textile-grade silk" is naturally occurring silk that
includes a sericin coating of greater than 19%-28% by weight of the
fiber. "Suture silk" is silk that either contains sericin ("virgin
silk suture") or is coated with a hydrophobic composition, such as
bee's wax, paraffin wax, silicon, or a synthetic polymeric coating
("black braided silk suture"). The hydrophobic composition repels
cells or inhibits cells from attaching to the coated fiber. Black
braided silk is a suture silk in which sericin has been extracted
and replaced with additional coating. Suture silk is typically
non-biodegradable.
[0024] Due to the absence of a protective wax or other hydrophobic
coating on the fibers the silk fibroin constructs described are
biologically (coupling of cell binding domains) and/or mechanically
(increase silk surface area and decrease packing density) designed
to promote increased cell infiltration compared to textile-grade
silk or suture silk when implanted in bodily tissue. As a result,
the silk fibroin constructs support cell ingrowth/infiltration and
improved cell attachment and spreading, which leads to the
degradation of the silk fibroin construct thereby essentially
creating a new biodegradable biomaterial for use in medical device
and tissue engineering applications. The ability of the fiber
construct to support cell attachment and cell and tissue
ingrowth/infiltration into the construct, which in return supports
degradation, may be further enhanced through fibroin surface
modification (peptide coupling using RGD, chemical species
modification and increasing hydrophilicity through gas plasma
treatment) and/or the mechanical design of the construct thereby
increasing material surface area thus increasing its susceptibility
to those cells and enzymes that possess the ability to degrade
silk. The silk fibers are optionally coated with a hydrophilic
composition, e.g., collagen or a peptide composition, or
mechanically combined with a biomaterial that supports cell and
tissue ingrowth to form a composite structure. The choice of
biomaterial, amount and mechanical interaction (e.g., wrapped or
braided about a core of silk fibroin) can be used to alter and/or
improve rates of cell ingrowth and construct degradation.
[0025] Fibers in the construct are non-randomly aligned with one
another into one or more yarns. Such a structure can be in a
parallel, braided, textured, or helically-organized (twisted,
cabled (e.g., a wire-rope)) arrangement to form a yarn. A yarn may
be defined as consisting of at least one fibroin fiber. Preferably,
a yarn consists of at least three aligned fibroin fibers. A yarn is
an assembly of fibers twisted or otherwise held together in a
continuous strand. An almost infinite number of yarns may be
generated through the various means of producing and combining
fibers. A silk fiber is described above; however, the term fiber is
a generic term indicating that the structure has a length 100 times
greater than its diameter.
[0026] When the fibers are twisted or otherwise intertwined to form
a yarn, they are twisted/intertwined enough to essentially lock in
the relative fiber positions and remove slack but not so much as to
plastically deform the fibers (i.e., does not exceed the material's
yield point), which compromises their fatigue life (i.e., reduces
the number of stress cycles before failure). The sericin-free
fibroin fiber constructs can have a dry ultimate tensile strength
(UTS) of at least 0.52 N/fiber (Table 1, 4), and a stiffness
between about 0.27 and about. 0.5 N/mm per fiber. Depending on
fiber organization and hierarchy, we have shown that fibroin
construct UTSs can range from 0.52 N/fiber to about 0.9N/fiber.
Fibroin constructs described here retained about 80% of their dry
UTS and about 38% of their dry stiffness, when tested wet (Table
5). Elongations at-break between about 10% and about 50% were
typical for fibroin constructs tested in both dry and wet states.
Fibroin constructs typically yielded at about 40 to 50% of their
UTS and had a fatigue life of at least 1 million cycles at a load
of about 20% of the yarns ultimate tensile strength.
[0027] In one embodiment of the present invention, the aligned
sericin-extracted silkworm fibroin fibers are twisted about each
other at 0 to 11.8 twists per cm (see Table 6 & 7).
[0028] The number of hierarchies in the geometrical structure of
the fiber construct as well as the number of
fibers/groups/bundles/strands/cords within a hierarchical level,
the manner of intertwining at the different levels, the number of
levels and the number of fibers in each level can all be varied to
change the mechanical properties of the fiber construct (i.e.,
yarn) and therefore, fabric (Table 4 & 8). In one embodiment of
the present invention, the fiber construct (i.e. yarn) is organized
in a single-level hierarchical organization, said single-level
hierarchical organization comprising a group of parallel or
intertwined yarns. Alternatively, the fiber construct (i.e. yarn)
organized in a two-level hierarchical organization, said two-level
hierarchical organization comprising a bundle of intertwined
groups. In another embodiment of the present invention, the fiber
construct (i.e. yarn) is organized into a three-level hierarchical
organization, said three-level hierarchical organization comprising
a strand of intertwined bundles. Finally, another embodiment of the
present invention, the fiber construct (i.e. yarn) is organized
into a four-level hierarchical organization, said four-level
hierarchical organization comprising a cord of intertwined
strands.
[0029] The sericin can be removed from the fibroin fibers before
the alignment into a yarn or at a higher level in the hierarchical
geometry of the fiber construct. The yarn is handled at low tension
(i.e., the force applied to the construct will never exceed the
material's yield point during any processing step) and with general
care and gentleness after the sericin is removed. Processing
equipment is likewise configured to reduce abrasiveness and sharp
angles in the guide fixtures that contact and direct the yarn
during processing to protect the fragile fibroin fibers from
damage; extraction residence times of 1 hour are sufficient to
extract sericin but slow enough as not to damage the exposed
filaments. Interestingly, when a silk fiber construct consisting of
multiple fibers organized in parallel has been extracted under
these conditions, a "single" larger sericin free yarn resulted
(i.e., individual fibers cannot be separated back out of the
construct due to the mechanical interaction between the smaller
fibroin filaments once exposed during extraction). Furthermore, as
a result of the mechanical interplay between the sericin-free micro
filaments, extraction of twisted or cabled yarns has typically
resulted in less "lively" yarns and structures. As a result of this
phenomenon, a greater degree of flexibility existed in the design
of the yarns and resulting fabrics; for example, higher twist per
inch (TPI) levels can be used, which would normally create lively
yarns that would be difficult to form into fabrics. The added
benefit of higher TPIs was the reduction in yarn and fabric
stiffness. (i.e. matrix elasticity can be increased)(Tables 6 and
7; FIG. 6A and FIG. 6B).
[0030] A plurality of yarns are intertwined to form a fabric.
Fabrics are generated through the uniting of one or more individual
yarns whereby the individual yarns are transformed into textile and
medical device fabrics. In one embodiment of the present invention,
the yarn is twisted at or below 30 twists per inch. Fabrics are
produced or formed by non-randomly combining yarns: weaving,
knitting, or stitch bonding to produce completed fabrics. In one
embodiment, this combining of yarns to form a fabric is done on a
machine. However, it is very important to note that the end fabric
product is distinct based on the yarn type used to make it thus
providing tremendous power through yarn design to meet clinical
needs. A fabric can be, but is not limited to, woven, knit,
warp-knit, bonded, coated, dobby, laminated, mesh, or combinations
thereof.
[0031] Of note, the textile methods of braiding, in addition to
making yarns, can also be used to make fabrics, such as a flat
braided fabric or a larger circular braid (FIG. 4A). Inversely,
weaving and knitting, two fabric forming methods, although not
commonly used, can also be used to make yarns. In such instances,
the differentiation between a "yarn" and a "fabric" is not entirely
apparent, and the homogeneity should be used to make clear
distinctions, i.e., a yarn is typically more homogeneous in
composition and structure than a fabric.
[0032] In one embodiment of the present invention, multiple
silkworm silk fibers may be organized helically (e.g., twisted or
cabled) or in parallel, in a single hierarchical level or in
multiple levels, extracted, and used to create a braided suture for
tissue ligation. In another embodiment, the mechanical interaction
of extracted fibroin filaments in a twisted or cabled configuration
following extraction can be used as a medical suture.
[0033] Non-woven fabrics may be formed by randomly organizing a
plurality of yarns, or a single yarn cut into many small length
pieces. Non-limiting examples include a fabric for hemostat or bone
scaffold. All fabrics can either derive from a single yarn
construct (homogenous) or multiple yarns constructs
(heterogeneous). The ability to design for a variety of silk
fibroin yarn structures, as described in detail below, dramatically
increases fabric design potential when considering a heterogeneous
fabric structure.
[0034] In one embodiment of the present invention, the fabric is a
composite of the sericin-extracted fibroin fibers or yarns and one
or more degradable polymers selected from the group consisting of
Collagens, Polylactic acid or its copolymers, Polyglycolic acid or
its copolymers, Polyanhydrides, Elastin, Glycosamino glyccands, and
Polysaccharides. Furthermore, the fabric of the present invention
may be modified to comprise a drug associated or a cell-attachment
factor associated with fabric (i.e. RGD). In one embodiment of the
present invention, the fabric is treated with gas plasma or seeded
with biological cells.
[0035] Additional aspects of this disclosure relate to the repair
of specific bodily tissues, such as hernia repair, urinary bladder
tissues and slings, pelvic floor reconstruction, peritoneal wall
tissues, vessels (e.g., arteries), muscle tissue (abdominal smooth
muscle, cardiac), hemostats, and ligaments and tendons of the knee
and/or shoulder as well as other frequently damaged structures due
to trauma or chronic wear. Examples of ligaments or tendons that
can be produced include anterior cruciate ligaments, posterior
cruciate ligaments, rotator cuff tendons, medial collateral
ligaments of the elbow and knee, flexor tendons of the hand,
lateral ligaments of the ankle and tendons and ligaments of the jaw
or temporomandibular joint. Other tissues that may be produced by
methods of this disclosure include cartilage (both articular and
meniscal), bone, skin, blood vessels, stents for vessel support
and/or repair, and general soft-connective tissue.
[0036] In other aspects, silkworm fibroin fibers, in the form of a
yarn or of a larger construct of yarns, now termed a device, is
stripped of sericin, and made (e.g., woven, knitted, non-woven wet
laid, braided, stitch bonded, etc.) into a fabric, sterilized and
used as an implantable supporting or repair material that offers a
controllable lifetime (i.e., degradation rate) and a controllable
degree of collagen and/or extracellular matrix deposition. The
support or repair material can be used for any such purpose in the
body, and in particular can be used for hernia repair,
reconstruction of body walls, particularly in the thorax and
abdominal cavity, and support, positioning or immobilization of
internal organs, including, without limitation, the bladder, the
uterus, the intestines, the urethra, and ureters. Alternatively,
silkworm fibroin fibers may be stripped of sericin and organized
into a non-woven fabric. Such non-woven fabric can be used as an
implantable supporting or repair material as above, but more
specifically for applications where a sponge formation would be
useful.
[0037] The purified silk can be purified by any of a variety of
treatments that remove the sericin proteins found in the native
fibrils. Sericin has been removed sufficiently when implants of
purified silk elicit only a mild, transient foreign body reaction
in the absecense of an antigenic (B-cell, T-cell) response, i.e.,
are biocompatible. A foreign body reaction is characterized by an
inner layer of macrophages and/or giant cells with a secondary zone
of fibroblasts and connective tissue. The degree of foreign body
response has been shown to be controllable through fibroin
modification (FIG. 13A-D & FIG. 18A-C) and yarn design (FIG.
19A-D). Sericin can be removed from individual silkworm fibroin
fibers, a group of silkworm fibroin fibers (i.e. a yarn), having an
organized orientation (e.g., parallel or twisted), or form a fabric
or other construct comprising a plurality of yarns. The construct
can then be sterilized and implanted in an organism as a medical
device.
[0038] Other features and advantages of the invention will be
apparent from the following description of preferred embodiments
thereof.
BRIEF DESCRIPTION OF THE FIGURES
[0039] FIG. 1A is a scanning electron microscopy (SEM) image of a
single native degummed and plied 20/22 denier silk fiber having a
sericin coating.
[0040] FIG. 1B illustrates SEM of the silk fiber of FIG. 1A
extracted for 60 min at 37.degree. C.
[0041] FIG. 1C illustrates SEM of the silk fiber of FIG. 1A
extracted for 60 min at 90.degree. C. and illustrating complete
removal of the sericin coating.
[0042] FIG. 1D is a chart showing ultimate tensile strength (UTS)
and stiffness (N/mm for a 3 cm length matrix) as a function of
extraction conditions.
[0043] FIG. 1E illustrates SEM of a raw silk fibroin. FIG. 1F
illustrates a first extraction at 90.degree. for 60 min. FIG. 1G
illustrates a second extraction under identical conditions. These
figures show mechanical damage to the filaments that results in a
typical 3% mass loss following the second extraction. Therefore, as
long as the % mass loss does not change more than 3% from the first
to the second extraction (90.degree. C., 1 hr, standard detergent
and salt), it is assumed that complete extraction has been
achieved. The utility of a 3% loss in total mass loss reflects the
variability in the measurements, assays and mechanical damage
resulting in mass loss of the yarn following the second
extraction.
[0044] FIG. 2A is a representative 3-D model of a (cable or
twisted) yarn depicting its 5 levels of hierarchy (single fiber
level not shown). Depending on the number of fibers used in each
level, the cord could serve as either a yarn for knitting a hernia
repair mesh or as a cord to be used in parallel with other cords to
form an ACL matrix.
[0045] FIG. 2B is a schematic depicting the generation of a
two-level hierarchical twisted or cabled yarn containing 36 fibers
before being plied in parallel to form an ACL matrix or used to
generate a weave-or knit fabric for tissue engineering and tissue
repair (e.g. hernia mesh). The schematic representations visually
define two very popular forms of fabric formations: a "weave" and a
"knit."
[0046] FIG. 2C illustrates a single cord of yarn having a geometry
that is helically organized about a central axis and composed of
two levels of twisting hierarchy. When six cords are used in
parallel (e.g., Matrix 1), the yarn has mechanical properties
similar to a native ligament.
[0047] FIG. 2D illustrates a single cord of yarn having a geometry
that is helically organized about a central axis and composed of
three levels of twisting hierarchy. When six cords are used in
parallel (e.g., Matrix 2), the matrix has mechanical properties
similar to a native ligament.
[0048] FIG. 3A illustrates load-elongation curves for five samples
(n=5) of Matrix 1 formed from six parallel silk fibroin cords
illustrated in FIG. 2A.
[0049] FIG. 3B is a chart of cycles to failure at UTS, 1680N, and
1200N loads (n=5 for each load) illustrating Matrix 1 fatigue data.
Regression analysis of Matrix 1 fatigue data, when extrapolated to
physiological load levels (400 N) to predict number of cycles to
failure in vivo, indicates a matrix life of 3.3 million cycles.
[0050] FIG. 3C illustrates load-elongation curves for three samples
(n=3) of Matrix 2 (n=3) formed from six parallel silk fibroin cords
as illustrated in FIG. 2B.
[0051] FIG. 3D is a chart of cycles to failure at UTS,2280N, 2100N
and 1800N loads (n=3 for each load) illustrating Matrix 2 fatigue
data. Regression analysis of Matrix 2 fatigue data, when
extrapolated to physiological load levels (400 N) to predict number
of cycles to failure in vivo, indicates a matrix life of greater
than 10 million cycles.
[0052] FIG. 4A shows images of multiple yarn and fabric forms
generated in our laboratories. Several different yarn structures,
including various types of braids (i, ii, iv), a flat braid (iii),
a varying diameter or taper braid (v), a larger (.about.250 fibers)
cabled two-level bundle (vi), a parallel plied and bonded (swaged)
yarn consisting 24-12-fiber textured yarns (vii), a variety of
twisted yarns (viii-xi), and a parallel plied and bonded (swaged)
yarn consisting 24-12-fiber two level cabled yarns (xii).
[0053] FIG. 4B is a chart of load-elongation curves for (I) a braid
(48 fibers, a 4 carrier braider using twisted extracted 12 fiber
yarn) and textured yarns (48 fibers total) and (II) twisted
compared to cabled yarns, 12 fibers in total-all samples were 3 cm
in length.
[0054] FIG. 4C is a chart of fatigue data for small yarns, 3 cm in
length, as compared to 3B and 3D for (I) a small cable of 36 fibers
and (II) a small textured yarn of 60 fibers).
[0055] FIG. 5A provides strength and stiffness data for a 36 fiber
yarn as a function of 6 different strain rates at which they were
tested (N=5 per group).
[0056] FIG. 5B shows load-elongation curves for a 36-fiber yarn, 3
cm long, tested at 2 of the 6 different strain rates. The data
represents the effect of the testing procedures (here, specifically
strain rate) on the reported mechanical properties (e.g. UTS) of
the yarn structure.
[0057] FIG. 6A is a chart of UTS as a function of twists per inch
(TPI); trend lines were generated to extrapolate data to a 4.sup.th
order polynomial--TPIs from 0-15 are shown. A maximum was observed
indicating an ordered structure where individual filaments are
working in unison.
[0058] FIG. 6B is a chart of stiffness (for a 3 cm length sample)
as a function of twists per inch (TPI); trend lines were generated
to extrapolate data to a 5.sup.th order polynomial--TPIs from 0-15
are shown. A maximum was observed indicating that TPI could be used
as a tool to design for a specific UTS or stiffness.
[0059] FIG. 7A illustrates SEM of extracted silk fibroin prior to
seeding with cells.
[0060] FIG. 7B illustrates SEM of bone marrow stromal cells seeded
and attached on silk fibroin immediately post seeding.
[0061] FIG. 7C illustrates SEM of bone marrow cells attached and
spread on silk fibroin 1 day post seeding.
[0062] FIG. 7D illustrates SEM of bone marrow stromal cells seeded
on silk fibroin 14 days post seeding forming an intact
cell-extracellular matrix sheet.
[0063] FIG. 8A illustrates a 3 cm length of the silk fibroin cord
illustrated in FIG. 2C and seeded with bone marrow stromal cells,
cultured for 14 days in a static environment and stained with MTT
to show even cell coverage of the matrix following the growth
period.
[0064] FIG. 8B illustrates a control strand of silk fibroin cord 3
cm in length stained with MTT.
[0065] FIG. 9A is a chart illustrating bone marrow stromal cell
proliferation on silk fibroin Matrix 1 determined by total cellular
DNA over 21 day culture period indicating a significant increase in
cell proliferation after 21 days of culture.
[0066] FIG. 9B is a bar graph illustrating bone marrow stromal cell
proliferation on silk fibroin Matrix 2 determined by total cellular
DNA over 14 day culture period indicating a significant increase in
cell proliferation after 14 days of culture.
[0067] FIG. 10 illustrates the ultimate tensile strength of a 30
silk fiber extracted construct that is either seeded with bone
marrow stromal cells or non-seeded over 21 days of culture in
physiological growth conditions.
[0068] FIG. 11A is a chart of UTS as a function of in vitro
enzymatic degradation; no strength loss was observed in the
negative control, PBS. Silk lost 50% of its strength after 21 days
in culture. A 1 mg/ml solution of Protease XIV from Sigma was
used.
[0069] FIG. 11B is a chart of mass loss as a function of in vitro
enzymatic degradation; no strength loss was observed in the
negative control, PBS. 50% mass loss was observed after 41 days in
culture.
[0070] FIG. 12 is a chart of UTS loss as function of in vivo
degradation following RGD-modified matrix implantation into a
non-loaded subcutaneous rat model for 10, 20 and 30 days. 50%
strength loss was observed after 10 days in vivo in a non-loaded
environment.
[0071] FIG. 13A shows histological sections of 12(0).times.3(8)
non-modified and RGD-modified sericin-free silk fibroin matrices
after 30 days of subcutaneous implantation in a Lewis rat. Row I is
H&E staining at 40.times., row II is H&E staining at
128.times., row III is collagen trichrome staining at 128.times.,
row IV is collagen backed out of the row III images to allow for
collagen ingrowth quantification and row V are the pixels
associated with the cross-sections of remaining silk fibroins
backed out to allow for quantification of degradation. Upon
qualitative assessment, in the subcutaneous environment, both the
non-treated and modified groups supported cell ingrowth and
collagen deposition within the matrix itself with limited
peripheral encapsulation.
[0072] FIG. 13B quantitatively represents a 36% decrease in
RGD-modified silk cross-sectional area after 30 days of
subcutaneous implantation indicating a significant improvement in
the ability of the host to degrade the surface modified silk
fibroin matrices compared to non-treated controls.
[0073] FIG. 13C quantitatively shows a significant 63% increase in
collagen deposition within the RGD-modified fibroin matrices as
compared to the non-treated controls again demonstrating the
ability of the modified silk matrix to support host cell and tissue
ingrowth.
[0074] FIG. 13D shows H&E staining of an extracted 36 fiber
fibroin yarn implanted intra-muscularly in the abdominal Was of a
Lewis rat. Images are shown at 40.times. and 128.times. for both
non-modified and RGD-modified matrices. Results show,
qualitatively, that RGD-modification dramatically increased cell
and tissue infiltration within 30 days in vivo. Unlike black
braided silk suture or virgin silk suture, no peripheral
encapsulation or plasma cells were observed. Compared to the
subcutaneous implants, little to no cell infiltration and collagen
deposition was observed in the non-treated controls indicating the
effect of implantation site in addition to surface
modification.
[0075] FIG. 13E is a numerical representation of mass loss in vivo
from the two different modification groups compared to non-treated
controls. RGD modification, followed by gas plasma modification
significantly (p<0.05) increased the extent of degradation after
90 days of intramuscular implantation. However, it appears
degradation was more aggressive in the subcutaneous environment as
compared to the intra-muscular environment, as was expected.
[0076] FIG. 14 illustrates gel eletrophoretic analysis of RT-PCR
amplification of selected markers over time. The gel shows
upregulation in both collagen types I and III expression levels
normalized to the housekeeping gene, GAPDH by bone marrow stromal
cell grown on Matrix 2 over 14 days in culture. Collagen type
II.(as a marker for cartilage) and bone sialoprotein (as a marker
of bone tissue formation) were not detected indicating a ligament
specific differentiation response by the BMSCs when cultured with
Matrix 2.
[0077] FIGS. 15A and FIG. 15B illustrates a single cord of Matrix 1
(not seeded at the time of implantation) following six weeks of
implantation in vivo and used to reconstruct the medial collateral
ligament (MCL) in a rabbit model. FIG. 15A shows Matrix 1 fibroin
fibers surrounded by progenitor host cells and tissue ingrowth into
the matrix and around the individual fibroin fibers visualized by
hematoxylin and eosin staining. FIG. 15B shows collagenous tissue
ingrowth into the matrix and around the individual fibroin fibers
visualized by trichrome staining.
[0078] FIGS. 16A, 16B and 16C illustrate bone marrow stromal cells
seeded and grown on collagen fibers for 1 day (FIG. 16A) and 21
days (FIG. 16B); RT-PCR (FIG. 16C) and gel electrophoretic analysis
of collagen I and III expression vs. the housekeeping gene GAPDH:
a=Collagen I, day 14; b=Collagen I, day 18; c=Collagen III, day 14;
d=Collagen III, day 18; e=GAPDH, day 14; f=GAPDH, day 18. Collagen
type II (as a marker for cartilage) and bone sialoprotein (as a
marker of bone tissue formation) were not detected indicating a
ligament specific differentiation response.
[0079] FIG. 17 illustrates real-time quantitative RT-PCR at 14 days
that yielded a transcript ratio of collagen I to collagen III,
normalized to GAPDH, of 8.9:1.
[0080] FIG. 18A and FIG. 18B are H&E stained cross-sections of
of 6 bundles of (A) 2-0 black braided silk suture and (B)
RGD-surfaced modified silk (36 fibers/bundle), respectively, 30
days following intra-muscular implantation. 18C is RGD-modified
silk pre-seeded with BMSCs for 4 weeks prior to implantation. FIG.
18A shows a typical and extensive foreign body reaction to
commercially available (Ethicon, Inc.) black braided silk suture
where no ingrowth or cell infiltration can be observed. FIG. 18B
demonstrates the engineered silk's ability to promote cell and
tissue ingrowth. FIGS. 18A, 18B and 18C illustrate tissue response
to silk fiber constructs that are coated in wax (FIG. 18A),
stripped of sericin and coated with RGD (FIG. 18B), and stripped of
sericin and seeded with progenitor adult stem cells (FIG. 18C).
[0081] FIGS. 19A-D shows H&E stained cross sectional images at
40.times. (top row, FIG. 19A & FIG. 19B) and 128.times. (bottom
row, FIGS. 19C and 19D) of two yarns (4.times.3.times.3 and
12.times.3), each containing the same number of fibers, but
organized differently with specific hierarchies following
implantation in a rat model for 30 days. Results indication that
yarn design and structure can influence the extent of cell and
tissue ingrowth as the 12.times.3 yarn construct allowed for
ingrowth, while it appears the 4.times.3.times.3 thwarted it.
[0082] FIGS. 20A, B and C are pictures of (A) single fiber wet laid
non-woven fabric extracted post fabric formation (fibers can first
be extracted and formed into the non-woven--data not shown), (B) a
knit fabric produced from a form of chain stitching using 12-fiber
yarn extracted post fabric formation, and (C) a woven fabric
produced from pre-extracted 12-fiber yarn with a 36-fiber
pre-extracted yarn running in the weft direction.
[0083] FIG. 21 is a schematic flow chart of the various methods and
sequences that can be employed to create a biocompatible and
biodegradable silk fibroin matrix. For example, extract single
fiber, twist into yarns and knit into fabrics OR ply yarns, twist
plied yarns, form fabric and then extract. An almost infinite
number of combination exists, but all will be dependent on the
hierarchy of the yarn, the number of fibers per level and the TPI
per level as shown in Tables 4, 6, 7, and 8.
DETAILED DESCRIPTION
[0084] In methods described in greater detail, below, silk fibroin
fibers are aligned in a parallel orientation; the fibers can remain
in a strictly parallel orientation, or they can be twisted or
otherwise intertwined to form a yarn. The yarn can include any
number of hierarchies, beginning at fiber level and expanding
through bundle, strand, cord, etc., levels. Intertwining can be
provided at each level. Furthermore, sericin is extracted from the
silk fibers at any point in the hierarchy up to the point where the
number of fibers exceeds that at which the extracting solution can
penetrate throughout the yarn. The maximum number of silkworm
fibroin fibers (20/22 denier as purchased) that can be combined and
successfully extracted is about 50 (Table 4). These yarns can then
be used as a fiber construct for, e.g., ligament or tissue
reconstruction, or can be incorporated into a fabric for use, e.g.,
in the generation of soft tissue mesh for repairs such as hernia
repair, abdominal floor reconstruction and bladder slings.
Formation of fiber constructs will be discussed in the context of
exemplary applications, below.
[0085] Although much of the discussion that follows is directed to
a silk-fiber-based matrix (i.e. construct, scaffold) for producing
an anterior cruciate ligament (ACL), a variety of other tissues,
such as other ligaments and tendons, cartilage, muscle, bone, skin
and blood vessels, can be formed using a novel silk-fiber based
matrix. In the case of the ACL, a large yarn (540-3900 fibers per
yarn, before plying in parallel; see Table 8 & 11) with
multiple hierarchical levels of intertwining and relevant
physiological properties was described. In addition to a
silk-fiber-based ACL matrix, multiple smaller yarn configurations
(1-50 silk fibers) (Table 1, 4 & 5) with relevant physiological
properties after combining either in parallel or into a specific
fabric formation, can serve as tissue matrices for guided tissue
formation (FIG. 2A-B). In addition to silk matrices for guided
tissue formation or engineering, this work is specifically directly
to producing a variety of silk-fiber based matrices tissue support
structures for guided tissue repair (e.g., hernia repair, bladder
slings for urinary stress incontinence) (FIG. 2A-B & FIG.
20A-C).
[0086] Constructs (i.e. fabrics or yarns) can be surface modified
or seeded with the appropriate cells (FIG. 7A-D, FIG. 8A-B &
FIG. 16A-C) and exposed to the appropriate mechanical stimulation,
if necessary, for proliferating and differentiating into the
desired ligament, tendon or other tissue in accordance with the
above-described techniques.
[0087] Additionally, the present invention is not limited to using
bone marrow stromal cells for seeding on the fiber construct, and
other progenitor, pluripotent and stem cells, such as those in
bone, muscle and skin for example, may also be used to
differentiate into ligaments and other tissues.
[0088] Fabrics can also be formed from similar constructs of
purified filaments, and used in various applications. Fabrics can
be divided into various classes, including woven, non-woven,
knitted fabrics, and stitch-bonded fabrics, each with numerous
subtypes. Each of these types may be useful as an implant in
particular circumstances. In discussing these silk-based fabrics,
we describe the natural silk, e.g., of Bombyx mori, as a "fibroin
fiber." The fibers should be at least one meter long, and this
length should be maintained throughout the process to facilitate
their handling during processing and incorporation into a fabric.
Given that a yarn may be defined as an assembly of fibers twisted
or otherwise held together in a continuous strand and that a single
fibroin fiber, as defined above, is comprised of multiple plied
broins, sometimes from multiple cocoons, a single fibrion fiber may
be termed a "yarn." As well, fibroin fibers are twisted together or
otherwise intertwined to form a "yarn." Yarns are used to weave or
knit fabrics for use in the invention. In an alternative procedure,
silk yarns are disaggregated into shorter (5 mm to 100 mm) lengths
or into silk fibroin filaments. These filaments may then be (wet)
laid to form a non-woven fabric (FIG. 20A).
[0089] When the yarns are formed into a fabric, the tension (force)
exerted on the yarns (typically, via machinery) is no greater than
the yarn's yield point (FIG. 3A-D). Accordingly, the yarns are
handled at lower speeds and under smaller loads than are yarns that
are typically used in, e.g., textile manufacturing when forming the
fabric so as to preserve the integrity of the exposed fragile
fibroin fibers. Likewise, contact points between handling machinery
and the yarn are designed to avoid sharp angles and high-friction
interactions so as to prevent lousing and fraying of fibers around
the perimeter of the yarn (FIG. 4A-C).
[0090] Numerous applications of fabrics as implants are known in
the medical and surgical arts. One example is as a support in
hernia repair. For such repair, a fabric, most typically a
warp-knit with a desired stitch (e.g., an atlas stitch designed to
prevent unraveling of the mesh during cutting), is sewn (or
sometimes stapled or glued) or simply laid in place without
tensioning, onto the inside of the abdominal wall after it is
repaired with conventional sutures. One function of the warp knit
fabric is to provide short-term support for the repair. In a
preferred embodiment of the present invention, the fibroin fibers
within the fabric promote ingrowth of cells and subsequent tissue
growth into fabric itself (FIGS. 13A & 13D) as well as through
the fabric's interstices formed during knitting and into the region
in need of repair. This embodiment aims to permanently strengthen
the injured area through functional tissue ingrowth and remodeling
as the silk matrix degrades (FIGS. 13A, B & C).
[0091] Repair-strengthening fabrics are used in similar situations
for repair or support of any part of the abdominal wall,
particularly in hernia repair and abdominal floor reconstruction,
or in repair or support of other walls and septa in the body, for
example of the chest, or of organs such as the heart or the
bladder, particularly after surgery or tumor removal. Implantable
fabrics can also be used to support bladders or other internal
organs (included but not limited to the intestines, the ureters or
urethra, and the uterus) to retain them in their normal positions
after surgery, damage or natural wear as a result of age or
pregnancy, or to position them in an appropriate location. "Organ"
here includes both "solid" organs, such as a liver, and tubular
organs such as an intestine or a ureter. Fabrics, especially bulky
fabrics such as some non-woven types or those that can be created
through 3-dimensional knitting or braiding (FIG. 4A-C), can be used
to fill cavities left by surgery to provide a fiber construct onto
which cells can migrate or to which cells can be pre-attached (e.g.
to improve the rate of repair). Usage sites include cavities in
both soft tissues and hard tissues such as bone. In other cases,
fabrics are used to prevent adhesions, or to prevent the attachment
and/or ingrowth of cells; this may be achieve through surface
modification of the silk fibroin matrix or through the attachment
of a drug or factor to the matrix.
[0092] The silk-fibroin-based fabrics of the invention can easily
be modified in several ways to enhance healing or repair at the
site. These modifications may be used singly or in combination. The
silk-fibroin-based fabrics of the invention can be surface modified
to support cell attachment and spreading, cell and tissue ingrowth
and remodeling, and device biodegradation through the use of RGD
peptide coupling or gas plasma irradiation (FIGS. 13A-E). The
fabrics can be modified to carry cell-attachment factors, such as
the well-known peptide "RGD" (arginine-glycine-aspartic acid)
or-any of the many natural and synthetic attachment-promoting
materials, such as serum, serum factors and proteins including
fibronectin, blood, marrow, groups, determinants, etc., known in
the literature. Such materials can be in any of the usual
biochemical classes of such materials, including without limitation
proteins, peptides, carbohydrates, polysaccharides, proteoglycans,
nucleic acids, lipids, small (less than about 2000 Daltons) organic
molecules and combinations of these. Such plasma modification can
improve the fabric's surface functionality and/or charge without
affecting the materials bulk mechanical properties. Fabrics can be
gas plasma irradiated after sericin extraction without compromising
the integrity of the sericin-extracted silk fibroin fibers (Table
9).
[0093] Additionally, the fabric can be treated so that it delivers
a drug. Attachment of the drug to the fabric can be covalent, or
covalent via degradable bonds, or by any sort of binding (e.g.,
charge attraction) or absorption. Any drug can be potentially used;
non-limiting examples of drugs include antibiotics, growth factors
such as bone morphogenic proteins (BMPs) or growth differentiation
factors (GDFs), growth inhibitors, chemo-attractants, and nucleic
acids for transformation, with or without encapsulating
materials.
[0094] In another modification, cells can be added to the fabric
before its implantation (FIG. 7A-D, FIG. 8A-B, and FIG. 9A-B).
Cells can be seeded/absorbed on or into the fabric. Cells can also
or in addition be cultivated on the fabric, as a first step towards
tissue replacement or enhancement. The cells may be of any type,
but allogenous cells, preferably of the "immune protected," immune
privileged," or stem cell types are preferred, and autologous cells
are particularly preferred. The cells are selected to be able to
proliferate into required cell types on or in the fiber construct
(FIG. 9A-B).
[0095] Another class of modification is incorporation of other
polymers (e.g. in fiber or gel form) into the fabric, to provide
specific structural properties or to modify the native surfaces of
the silk fibroin and its biological characteristics (see FIG.
16A-C: seeding of collagen fibers with BMSCs). In one type of
incorporation, fibers or yarns of silk and of another material are
blended in the process of making the fabric. In another type, the
silk-based fibers, yarns or fabrics are coated or over-wrapped with
a solution or with fibers of another polymer. Blending may be
performed (i) randomly, for example by plying (1 or multiple fibers
of) both silk and the polymer together in parallel before twisting
or (ii) in an organized fashion such as in braiding where fibers or
yarns being input into the larger yarn or fabric can alternate
machine feed positions creating a predicable outcome. Coating or
wrapping may be performed by braiding or cabling over a central
core, where the core can be the polymer, the silk fibroin or a
composit of both, depending on the desired effect. Alternatively,
one yarn can be wrapped in a controlled fashion over the other
polymer, where the wrapping yarn can be used to stabilize the
structure. Any biocompatible polymer is potentially usable.
Examples of suitable polymers include proteins, particularly
structural proteins such as collagen and fibrin, and
strength-providing degradable synthetic polymers, such as polymers
comprising anhydrides, hydroxy acids, and/or carbonates. Coatings
may be provided as gels, particularly degradable gels, formed of
natural polymers or of degradable synthetic polymers. Gels
comprising fibrin, collagen, and/or basement membrane proteins can
be used. The gels can be used to deliver cells or nutrients, or to
shield the surface from cell attachment. Further, proteins or
peptides can be covalently attached to the fibers or the fibers can
be plasma modified in a charged gas (e.g., nitrogen) to deposit
amine groups; each of these coatings supports cell attachment and
ingrowth, as silk is normally hydrophobic, and these coatings make
the fibers more hydrophilic.
[0096] Non-limiting examples of some of these embodiments are
described in examples, below.
[0097] Wet laydown was selected for a prototype of fabric formation
because it is the simplest procedure. The non-woven product (FIG.
20A) was created from a single silk fibroin fiber prior to
extraction at the fabric level. The product is correspondingly a
relatively inexpensive material, and can be used in applications
where its low tensile strength would be satisfactory. When more
tensile strength is needed, a non-woven material could be bonded
together, as is well known for fabrics and paper or mineralized for
bone repair. Alternatively, silk yarn material produced by
extraction of the sericin can be formed into a variety of more
complex yarns, as described above. The size and design of the yarn
can be used to control porosity, independent of non-woven machine
capabilities. The yarns can also be knit (FIG. 20B) or woven (FIG.
20C) into a fabric. One type of fabric of interest is a simple
mesh, similar to gauze, which can be used by itself (e.g. as a
hemostat), or to deliver cells or drugs (e.g. a clotting factor) to
a site, in a situation where flexibility is important.
[0098] When strength is important, a warp knit fabric (FIG. 20B),
including the familiar tricots and jerseys, having an elasticity
that can be controlled through the helical design of the yarn used
in the fabric, and typically substantial tensile strength, can be
very useful for applications (e.g., hernia repair, bladder slings,
pelvic floor reconstructions, etc.) requiring provision of
mechanical support for a significant length of time, such as
months.
[0099] In other applications, the material should have little
elasticity and great strength. For such fabrics, a dense weave of
thick yarns is appropriate, producing a material similar to
standard woven fabrics (FIG. 20C). Such a material can optionally
be supplemented by, a coating treatment or a heat treatment to bond
the crossovers of the yarn segments, thereby preventing both
raveling and stretching. Heat treatment must not entirely denature
the silk protein. The fabric can optionally be sewn, glued or
stapled into place, as is currently done with polypropylene mesh.
The implant, like any of the other types discussed, can be coated
with various materials to enhance the local healing and tissue
ingrowth process, and/or with a coating to prevent adhesion of the
repair site to the viscera.
[0100] In another alternative, the fabric, mesh, non-woven, knit or
other repair material can be made of unextracted silk, and then the
finished fabric can be extracted as described herein (FIG. 21 )(for
example, with alkaline soap solution at elevated temperature) to
remove the immunomodulatory sericins from the material. As a
further alternative, the extraction of the sericin can take place
at an intermediate stage, such as extraction of the formed yarn,
bundle, or strand, in so far as the number of fibers does not
exceed that at which the extracting solution can penetrate
throughout the fibers (see FIG. 21 for non-limiting options).
[0101] The above discussion has described making fabrics composed
of yarns, where the most-typical form of yarn in the fabric
formations discussed about would derive from twisting silkworm
fibroin fibers together in an organized manner and extracting
sericin. Many yarn geometries and methods of yarn formation may
also be used as described (Tables 4, 5, 6, 7 & 8). Such methods
may include the formation of non-twisted bundles of fibroin fibers,
bound together by wrapping the bundles with silk or another
material as discussed above. Any of these yarns could, as described
above, be formed by blending silk fibers with other materials.
Further still, the fibers can be intertwined, e.g., cabled,
twisted, braided, meshed, knitted, etc. (see FIGS. 2A&B and
21). The term, "intertwined," is used herein to indicate an
organized (i.e., non-random) repeating structure in terms of how
the fibers contact and bind one another.
[0102] Blending could also be done at higher levels of
organization, such as the use of filaments of different materials
to form a thicker yarn, or using yarns of differing materials in
weaving or knitting. In each case, the final material would include
purified, essentially sericin-free silk as a significant component,
used for one or all of its strength and biocompatibility and (e.g.,
long-term) degradation characteristics (FIG. 11A-B). The other
polymer or polymers are selected for their biocompatibility,
support (or inhibition through rapid tissue formation at
desired-locals) of cell attachment or infiltration (FIG. 16A-C),
degradation profile in vivo, and mechanical properties.
Biodegradable polymers include any of the known biodegradable
polymers, including natural products such as proteins,
polysaccharides, glycosaminoglycans, and derivatized natural
polymers, e.g., celluloses; and biodegradable synthetic polymers
and copolymers including polyhydroxy acids, polycarbonates,
polyanhydrides, some polyamides, and copolymers and blends thereof.
In particular, collagen and elastin are suitable proteins.
[0103] Silk-containing fabric constructs/matrices used for tissue
repair may be treated so that they contain cells at the time of
implantation (FIG. 7A-D, FIG. 8A-B, FIG. 9A-B, & FIG. 18C) to
improve tissue outcomes in vivo. The cells may be xenogenic, more
preferably allogenic, and most preferably autologous. Any type of
cell is potentially of use, depending on the location and the
intended function of the implant. Pluripotent cells are preferred
when the appropriate differentiation cues are present or provided
in the environment. Other cell types include osteogenic cells,
fibroblasts, and cells of the tissue type of the implantation
site.
[0104] While silk from Bombyx mori and other conventional silkworms
has been described, any source of silk or silk-derived proteins can
be used in the invention, as long as it provokes no more than a
mild foreign body reaction on implantation (i.e., is
biocompatible)(see FIGS. 18B & C). These include without
limitation silks from silkworms, spiders, and cultured cells,
particularly genetically engineered cells, and transgenic plants
and animals. Silk produced by cloning may be from full or partial
sequences of native silk-line genes, or from synthetic genes
encoding silk-like sequences.
[0105] While in many cases only a single fabric type will be used
in formation of a medical device or prosthesis, it may be useful in
some cases to use two or more types of fabric in a single device.
For example, in hernia repair, it is desirable to have the
tissue-facing side of the repair fabric attract cells, while the
peritoneal face should repel cells, to prevent adhesions. This
effect can be achieved by having one layer of silk that does not
attract cells, and another layer that does (for example, an
untreated layer and an RGD-containing layer, as in the example,
below). Another example includes formation of a bladder sling. The
basic sling should be conforming and somewhat elastic, and have a
long projected lifetime. However, the face of the sling closest to
the bladder should have as little texture as feasible. This can be
accomplished by placing a layer of thin but tightly woven,
non-woven or knitted fabric, fabricated from a yarn having a small
diameter (e.g., a single fiber), of the invention in the sling
where it will contact the bladder. The non-woven fabric should be
of as small a gauge (denier) as feasible. Numerous other situations
needing two or more types of fabric are possible.
[0106] Examples of the above-described structures were fabricated
and evaluated in a series of tests. In a first example, a fabric
was formed from purified silk fibrils. First, raw silk was
processed into purified fibroin fibrils. Raw silkworm fibers were
extracted in an aqueous solution of 0.02 M Na2CO3 and 0.3% w/v
IVORY soap solution for 60 minutes at 90 degrees. C. The extracted
fibers were rinsed with water to complete the extraction of the
glue-like sericin protein. The resulting suspension of fibrils was
wet-laid on a screen, needle-punched, and dried (FIG. 20A). The
resulting fleecy material felt somewhat like wool to the touch, and
was very porous. It was sufficiently interbonded by entanglement
and needling that it was easily handled and cut to a desired
shape.
[0107] In another example, the purified silk fibroin fibrils were
treated with cell attracting agents (Table 9). First, yarns were
made by twisting purified fibers of silk fibroin together. Some
yarns were made of filaments that were derivatized with the peptide
RGD to attract cells, using procedures described in Sofia et al, J.
Biomed. Mater. Res. 54: 139-148, 2001. Sections of treated and
untreated (black braided silk suture) yarns were implanted in the
abdominal wall of rats (FIG. 18A-C). After 30 days of implantation,
the black braided sutures contained compact fibril bundels, with
cell infiltration between fibril bundles but not within them. In
contrast, the RGD treated fibril bundles were extensively invaded
by host cells, and were expanded and non-compact (FIG. 13A-E, 18B),
but were not yet significantly degraded (FIG. 13A-E).
[0108] This example illustrates the use of derivatization to
control the rate of degradation of implanted silk fibroin fibrils,
as well as demonstrating the ability of derivatized fibrils to
recruit cells to a fabric-like structure. Clearly, greater
specificity of recruitment can be obtained by using a more specific
attractant. Similar techniques (chemical derivatization) or simpler
methods such as absorption, adsorption, coating, and imbibement,
can be used to provide other materials to the implantation
site.
[0109] Each of the samples reported in the Tables below, was
prepared in accordance with the above description, wherein sericin
was removed over 60 minutes at a temperature of
90.degree..+-.2.degree. C. Using a temperature in this range for a
sufficient period of time has been found to produce fibers from
which sericin is substantially removed (FIG. 1A-C, Table 1, 2,
3)(to produce a fiber construct that is substantially free of
sericin so as not to produce a significant immunological response
and not to significantly impede the biodegradeability of the fiber)
while substantially preserving the mechanical integrity of the
fibroin (Table 1). Note that when temperatures reach 94.degree. C.
(Table 1), UTS was not dramatically affected; however, stiffness
significantly declined indicating a silk thermo sensitive at
temperatures of 94.degree. C. and above. The fibers in each group
were manually straightened (i.e., made parallel) by pulling the
ends of the fibers; alternatively, straightening could easily have
been performed via an automated process. The force applied was
marginally greater than what was required to straighten the
group.
[0110] The sample geometry designations in all Tables reflect the
following constructs: # of fibers (tpi at fiber level in S
direction).times.# of groups (tpi at group level in Z
direction).times.# of bundles (tpi at bundle level in S
direction).times.# of strand (tpi at stand level in Z
direction).times. etc., wherein the samples are twisted between
levels unless otherwise indicated. The twist-per-inch designation,
such as 10 s.times.9 z tpi, reflects (the number of twists of the
fibers/inch within the group).times.(the number of twists of the
groups/inch within the bundle). In each sample, the pitch of the
twist is substantially higher than is ordinarily found in
conventional yarns that are twisted at a low pitch intended merely
to hold the fibers together. Increasing the pitch of the twists
(i.e., increasing the twists per inch) decreases the tensile
strength, but also further decreases the stiffness and increases
the elongation at break of the construct.
[0111] The ultimate tensile strength (UTS), percent elongation at
break (% Elong), and stiffness were all measured using an INSTRON
8511 servohydraulic material testing machine with FAST-TRACK
software, which strained the sample at the high rate of .about.100%
sample length per second in a pull-to-failure analysis. In other
words, up to the point of failure, the sample is stretched to
double its length every second, which greatly restricts the
capacity of the sample to relax and rebound before failure.
However, FIG. 5A-B demonstrates the effect of strain rate can have
on observed mechanical properties as well as wet or dry testing
conditions which were shown (FIG. 6A-B) to have a dramatic effect
on silk matrix UTS and stiffness. Consistency is needed if
comparisons are to be made between data sets. The resulting data
was analyzed using Instron Series IX software. Ultimate tensile
strength is the peak stress of the resulting stress/strain curve,
and stiffness is the slope of the stress/strain plot up to the
yield point. Unless specified, at least an N=5 was used for all
tested groups to generate averages and standard deviations.
Standard statistical methods were employed to determine if
statistically significant differences existed between groups, e.g.,
Student's t-test, one-way ANOVA.
[0112] The fibroin fibers in the samples in all of the above Tables
and Figures (and throughout this disclosure) are native (i.e., the
fibers are not dissolved and reformed); dissolution and
reformulation of the fibers results in a different fiber structure
with different mechanical properties after reforming. Surprisingly,
these samples demonstrate that yarns of silk fibroin fibers, from
which sericin has been completely or nearly completely removed, can
possess high strengths and other mechanical properties that render
the yarns suitable for various biomedical applications (Table 4,
FIG. 2A-D & FIG. 20A-C), such as for forming a fiber construct
or support for ligament replacement, hernia repair or pelvic floor
reconstruction. Previously, it was believed that fibroin needed to
be dissolved and extruded into a reformulated fiber to provide
desired mechanical properties. Fatigue strength has generally been
found to suffer in such reformed fibroin fibers. The methods of the
present invention, allow for sericin removal without a significant
loss of strength (Tables 1 & 4; FIGS. 3A-D & 4A-B).
[0113] In Table 8, samples 1 and 2 compare the properties of a
3-fiber group (sample 1) with those of a 4-fiber group (sample 2).
Sample 2 had a square configuration of fibers, while the fibers
of-sample 1 had a triangular configuration. As shown in the Table,
the addition of the extra fiber in sample 2 lowered the per-fiber
stiffness of the sample demonstrating the ability to control yarn
and fabric properties through hierarchical design.
[0114] Table 4 illustrates the effects of different configurations
of cabled-fiber constructs and a twisted-fiber geometry. Note, in
particular, samples 7 and 8 include the same number of fibers and
the same number of geometrical levels. The twisted-fiber geometry
of sample 8 offers greater UTS and greater stiffness, while the
cabled geometry of sample 7 has lower strength and lower stiffness.
Of samples 7-9, the cabled geometry of sample 7 has the highest
strength-to-stiffness ratio; for use as an ACL fiber construct, a
high strength-to-stiffness ratio is desired (i.e., possessing a
high strength and low stiffness).
[0115] Tables 1 and 4 demonstrate the effect of sericin extraction
on the fibers. All samples were immersed in an extraction solution,
as described in Table 1. Samples 1-5 were immersed in a bath at
room temperature, at 33.degree. C. and 37.degree. C. These
temperatures are believed to be too low to provide significant
sericin extraction. Samples 6-9 were extracted at 90.degree. C.,
where complete sericin extraction is believed to be attainable, but
for varying times. Similarly sample 10 was extracted-at the
slightly higher temperature of 94.degree. C. The data suggests that
30 to 60 min at 90.degree. C. is sufficient to significantly remove
sericin (see Tables 2&3) and that 94.degree. C. may be damaging
the protein structure of silk as shown by a dramatic decrease in
stiffness.
[0116] Finally, samples 11 to 16 have comparable cabled geometries;
the fibers of samples 12, 14, and 16 were extracted, whereas the
fibers of samples 11, 13, and 15 were not. As can be seen in the
Table, the extraction appears to have had little effect on (high)
ultimate tensile strengths per fiber.
[0117] The fibers of sample 10 of Table 4 were subject to a
curl-shrinking procedure, wherein the fibers were twisted in one
direction and then in the opposite direction, rapidly; the fibers
where then heated to lock in the twist structure and tested
non-extracted. The strength and stiffness of the resulting yarn
were comparatively lower than most of the other non-extracted yarns
tested. However, Tables 6&7 show the fibroins remarkable
ability, post extraction, to withstand up to 30 TPI. Table 6 shows
the ordering effect TRI has on silk matrices likely due to the
ordering of the multifilament structure following extraction.
[0118] FIG. 10 demonstrates the properties of a group of 30
parallel fibroin fibers seeded and non-seeded in culture conditions
for 21 days. These three samples exhibited very similar mechanical
properties, thereby reflecting little if any degradation of silk
matrices due to cell growth thereon or due to time in vitro.
Stiffness values are likely much lower in this experiment in
comparison with the other samples as a result of the 21 day wet
incubation prior to mechanical testing (see Table 5).
[0119] Table 4, samples 14-16 are all braided samples. The fibers
of sample 14 were braided from eight carriers, with a spool mounted
on each carrier, wherein two fibers were drawn from each spool. The
fibers of sample 15 were drawn from 16 carriers, with a spool
mounted on each carrier; again, two fibers were drawn from each
spool. Finally, sample 16 was formed from 4 yarns, each yarn
comprising 3 twisted groups of four fibers (providing a total of 12
fibers per yarn); each of the yarns was drawn from a separate spool
and carrier.
[0120] Table 9 demonstrates the effect of surface modification. The
designation, "PBS," reflects that the samples were immersed in a
phosphate-buffered saline solution for about 24 hours before
testing. The effect of exposing the samples to the saline solution
was measured and provided an indication that the fiber construct
can maintain its mechanical properties and substantially preserve
the inherent protein structure in a saline environment (e.g.,
inside a human body). The "RGD" designation reflects that the
samples were immersed in an arg-gly-asp (RGD) saline solution for
about 24 hours before testing. RGD can be applied to the construct
to attract cells to the construct and thereby promote cell growth
thereon. Accordingly, any effect of RGD on the mechanical
properties of the construct is also of interest, though no
significant degradation of the construct was apparent. Accordingly,
these samples offer evidence that prolonged exposure to a saline
solution or gas ethylene oxide sterilization or to an RGD solution
results in little, if any, degradation of the material properties
of the fiber constructs. Though, the data associated with samples
28 and 29, wherein the geometrical hierarchy was extended to a
higher level, reveal that the UTS/fiber drops as higher levels (and
increased overall fiber count) are reached. This is an effect of
heiarchical design (Table 8) rather than surface modification.
[0121] Table 4, samples 18 through 23 were tensioned under 6 pounds
of constant force for 1, 2, 3, 4, 5 and 6 days, respectively,
before testing to evaluate the effect of tension on the mechanical
properties over time. From the data, there does not appear to be
much if any change in the material properties of the construct as
the pretension procedure is extended over longer periods of time.
Sample 25 was also "pre-tensioned" (after twisting) at 6 pounds
force for a day before testing; for comparison, sample 24, which
had an identical geometrical configuration was not pre-tensioned.
Samples 24 and 25 accordingly reveal the effect of pre-tensioning
the construct to remove the slack in the structure, which results
in a slight reduction in both the-construct's UTS and its
elongation at break.
[0122] The silk-fiber-based construct serves as a matrix for
infiltrating cells or already infiltrated or seeded with cells,
such as progenitor, ligament or tendon fibroblasts or muscle cells,
which can proliferate and/or differentiate to form an anterior
cruciate ligament (ACL) or other desired tissue type. The novel
silk-fiber-based construct is designed having fibers in any of a
variety of yarn geometries, such as a cable, or in an. intertwined
structure, such as twisted yarn, braid, mesh-like yarn or knit-like
yarn. The yarn exhibits mechanical properties that are identical or
nearly identical to those of a natural tissue, such as an anterior
cruciate ligament (see Table 4, 1, infra); and simple variations in
fiber construct organization and geometry can result in the
formation of any desired tissue type (see Table 10, infra).
Alternatively, a plurality of yarns can be formed into a fabric or
other construct that is implanted to position or support an organ.
Additionally, the construct can be used to fill internal cavities
after surgery or to prevent tissue adhesions or promote the
attachment or ingrowth of cells.
[0123] Pluripotent bone marrow stromal cells (BMSCs) that are
isolated and cultured as described in the following example can be
seeded on the silk-fiber construct and cultured in a bioreactor
under static conditions. The cells seeded onto the fiber construct,
if properly directed, will undergo ligament and tendon specific
differentiation forming viable and functional tissue. Moreover, the
histomorphological properties of a bioengineered tissue produced in
vitro generated from pluripotent cells within a fiber construct are
affected by the direct application of mechanical force to the fiber
construct during tissue generation. This discovery provides
important new insights into the relationship between mechanical,
stress, biochemical and cell immobilization methods and cell
differentiation, and has applications in producing a wide variety
of ligaments, tendons and tissues in vitro from pluripotent
cells.
[0124] A fiber construct comprising silk fibers having a cable
geometry, is illustrated in FIGS. 2C and 2D. The fiber construct
comprises a hierarchy in terms of the way that fibers are grouped
in parallel and twisted and how the resultant group is grouped and
twisted, etc., across a plurality of levels in the hierarchy, as is
further explained, below. The silk fibers are first tensioned in
parallel using, for example, a rack having spring-loaded clamps
that serve as anchors for the fibers. The rack can be immersed in
the sericin-extraction solution so that the clamps can maintain a
constant tension on the fibers through extraction, rinsing and
drying.
[0125] The extraction solution can be an alkaline soap solution or
detergent and is maintained at about 90.degree. C. The rack is
immersed in the solution for a period of time (e.g., at least 0.5
to 1 hr, depending on solution flow and mixing conditions) that is
sufficient to remove all (.+-.0.4% remaining, by weight) or
substantially all sericin (allowing for possible trace residue)
from the fibers. Following extraction, the rack is removed from the
solution and the fibers are rinsed and dried. Computer-controlled
twisting machines, each of which mounts the fibers or constructs of
fibers about a perimeter of a disc and rotates the disc about a
central axis to twist the fibers (i.e. cabling) or constructs of
fibers twisted about each other according to standard processes
used in the textile industry, though at a higher pitch rate for the
twists (e.g., between about 0 and about 11.8 twists per cm) than is
typically produced in traditional yarns. The cabling or twist rate,
however, should not be so high as to cause plastic deformation of
the fibers as a result of the balloon tension created as the yarn
is let-off from the feed spool prior to twisting or cabling.
[0126] Extraction can be performed at any level of the construct
provided that the solution can penetrate through the construct to
remove the sericin from all fibers. It is believed that the upper
limit for the number of fibers in a compact arrangement that can
still be fully permeated with the solution is about 20-50 fibers.
Though, of course, those fibers can be arranged as one group of 20
parallel fibers or, for example, as 4 groups of 5 parallel fibers,
wherein the groups may be twisted, or even a construct comprising a
still higher level such as 2 bundles of 2 groups of 5 fibers,
wherein the groups and bundles may be twisted. Increasing the
number of hierarchical levels in the structure can also increase
the void space, thereby potentially increasing the maximum number
of fibers from which sericin can be fully extracted from 20 to 50
fibers.
[0127] Because the sericin, in some cases, is removed from the
construct after fibers are grouped or after a higher-level
construct is formed, there is no need to apply wax or any other
type of mechanically protective coating on the fibers or in order
to also form a barrier to prevent contact with sericin on the
fibers; and the construct can be free of coatings, altogether
(particularly being free of coatings that are not fully degraded by
the body or cause an inflammatory response).
[0128] As described in the examples below, mechanical properties of
the silk fibroin (as illustrated in FIGS. 1A, 1B and 1C) were
characterized, and geometries for forming applicable matrices for
ACL engineering were derived using a theoretical computational
model (see FIG. 1D). A six-cord construct was chosen for use as an
ACL replacement to increase matrix surface area and to enhance
support for tissue in-growth. Two construct geometrical hierarchies
for ACL repair comprise the following:
[0129] Matrix 1: 1 ACL yarn=6 parallel cords; 1 cord=3 twisted
strands (3 twists/cm); 1 strand=6 twisted bundles (3 twists/cm); 1
bundle=30 parallel washed fibers; and
[0130] Matrix 2: 1 ACL yarn=6 parallel cords; 1 cord=3 twisted
strands (2 twists/cm); 1 strand=3 twisted bundles (2.5 twists/cm);
1 bundle=3 groups (3 twists/cm); 1 group=15 parallel extracted silk
fibroin fibers.
[0131] The number of fibers and geometries for Matrix 1 and Matrix
2 were selected such that the silk prostheses are similar to the
ACL biomechanical properties in ultimate tensile strength, linear
stiffness, yield point and % elongation at break, serving as a
solid starting point for the development of a tissue engineered
ACL. The effects of increasing number of fibers, number of levels,
and amount of twisting on each of these biomechanical properties
are shown in Table 8 and Tables 6&7, respectively.
[0132] The ability to generate two matrices with differing
geometries both resulting in mechanical properties that mimic
properties of the ACL indicates that a wide variety of geometrical
configurations exist to achieve the desired mechanical properties.
Alternative geometries for any desired ligament or tendon tissue
may comprise any number, combination or organization of cords,
strands, bundles, groups and fibers (see Table 10, infra) that
result in a fiber construct with applicable mechanical properties
that mimic those of the ligament or tendon desired. For example,
one(1) ACL prosthesis may have any number of cords in parallel
provided there is a mean for anchoring the final fiber construct in
vitro or in vivo. Further, various numbers of twisting levels
(where a single level is defined as a group, bundle, strand or
cord) for a given geometry can be employed provided the fiber
construct results in the desired mechanical properties.
Furthermore, there is a large degree of freedom in designing the
fiber construct geometry and organization in engineering an ACL
prosthesis; accordingly, the developed theoretical computational
model can be used to predict the fiber construct design of a
desired ligament or tendon tissue (see the example, below). For
example when multiple smaller matrix bundles are desired (e.g., 36
fibers total) with only two levels of hierarchy to promote
ingrowth; a TPI of 8-11 or -3-4 twists per cm is required and can
be predicted by the model without the need for empirical work.
[0133] Consequently, a variation in geometry (i.e., the number of
cords used to make a prosthesis or the number of fibers in a group)
can be used to generate matrices applicable to most ligaments and
tendons. For example, for smaller ligaments or tendons of the hand,
the geometry and organization used to generate a single cord of
Matrix 1 (or two cords or three cords, etc.) may be appropriate
given the fiber construct's organization results in mechanical
properties suitable for the particular physiological environment.
Specifically, to accommodate a smaller ligament or tendon compared
to Matrix 1 or Matrix 2, less fibers per level would be used to
generate smaller bundles or strands. Multiple bundles could then be
used in parallel. In the case of a larger ligament such as the ACL,
it might be desirable to have more smaller bundles twisted at
higher TPIs to reduce stiffness and promote ingrowth then to have
fewer larger bundles where ingrowth cannot occur thereby limited
degradation of the matrix.
[0134] The invention is not, however, limited with respect to the
cable geometry as described, and any geometry or combination of
geometries (e.g., parallel, twisted, braided, mesh-like) can be
used that results in fiber construct mechanical properties similar
to the ACL (i.e., greater than 2000 N ultimate tensile strength,
between 100-600 N/mm linear stiffness for a native ACL or commonly
used replacement graft such as the patellar tendon with length
between 26-30 mm) or to the desired ligament and tendon that is to
be produced. The number of fibers and-the geometry of both Matrix 1
and Matrix 2 were selected to generate mechanically appropriate ACL
matrices, or other desired ligament or tendon matrices [e.g.,
posterior cruciate ligament (PCL)]. For example, a single cord of
the six-cord Matrix 1 construct was used to reconstruct the medial
collateral ligament (MC) in a rabbit (see FIG. 15A and FIG. 15B).
The mechanical properties of the silk six-cord constructs of Matrix
1 and Matrix 2 are described in Table 10 and in FIGS. 3A-3D, as is
further described in the example, infra. Additional geometries and
their relating mechanical properties are listed in Table 11 as an
example of the large degree of design freedom that would result in
a fiber construct applicable in ACL tissue engineering in
accordance with methods described herein.
[0135] Advantageously, the silk-fiber based fiber construct can
consist solely of silk. Types and sources of silk include the
following: silks from silkworms, such as Bombyx mori and related
species; silks from spiders, such as Nephila clavipes; silks from
genetically engineered bacteria, yeast mammalian cells, insect
cells, and transgenic plants and animals; silks obtained from
cultured cells from silkworms or spiders; native silks; cloned full
or partial sequences of native silks; and silks obtained from
synthetic genes encoding silk or silk-like sequences. In their raw
form, the native silk fibroins obtained from the Bombyx mori
silkworms are coated with a glue-like protein called sericin, which
is completely or essentially completely extracted from the fibers
before the fibers that make up the fiber construct are seeded with
cells.
[0136] The fiber construct can comprise a composite of: (1) silk
and collagen fibers; (2) silk and collagen foams, meshes, or
sponges; (3) silk fibroin fibers and silk foams, meshes, or
sponges; (4) silk and biodegradable polymers [e.g., cellulose,
cotton, gelatin, poly lactide, poly glycolic,
poly(lactide-co-glycolide), poly caproloactone, polyamides,
polyanhydrides, polyaminoacids, polyortho esters, poly acetals,
proteins, degradable polyurethanes, polysaccharides,
polycyanoacrylates, Glycosamino glycans (e.g., chrondroitin
sulfate, heparin, etc.), Polysaccharides (native, reprocessed or
genetically engineered versions: e.g., hyaluronic acid, alginates,
xanthans, pectin, chitosan, chitin, and the like), elastin (native,
reprocessed or genetically engineered and chemical versions), and
collagens (native, reprocessed or genetically engineered versions],
or (5) silk and non-biodegradable polymers (e.g., polyamide,
polyester, polystyrene, polypropylene, polyacrylate, polyvinyl,
polycarbonate, polytetrafluorethylene, or nitrocellulose material.
The composite generally enhances fiber construct properties such as
porosity, degradability, and also enhances cell seeding,
proliferation, differentiation or tissue development. FIGS. 16A,
16B and 16C illustrate the ability of collagen fibers to support
BMSC growth and ligament specific differentiation.
[0137] The fiber construct can also be treated to enhance cell
proliferation and/or tissue differentiation thereon. Exemplary
fiber construct treatments for enhancing cell proliferation and
tissue differentiation include, but are not limited to, metals,
irradiation, crosslinking, chemical surface modifications [e.g.,
RGD (arg-gly-asp) peptide coating, fibronectin coating, coupling
growth factors], and physical surface modifications.
[0138] A second aspect of this disclosure relates to a mechanically
and biologically functional ACL formed from a novel
silk-fiber-based fiber construct and autologous or allogenic
(depending on the recipient of the tissue) bone marrow stromal
cells (BMSCs) seeded on the fiber construct. The silk-fiber-based
fiber construct induces stromal cell differentiation towards
ligament lineage without the need for any mechanical stimulation
during bioreactor cultivation. BMSCs seeded on the silk-fiber-based
fiber construct and grown in a petri dish begin to attach and
spread (see FIGS. 7A-D); the cells proliferate to cover the fiber
construct (see FIGS. 8A-B, FIG. 9A and FIG. 9B) and differentiate,
as shown by the expression of ligament specific markers (see FIG.
14). Markers for cartilage (collagen type II) and for bone (bone
sialoprotein) were not expressed (see FIG. 14). Data illustrating
the expression of ligament specific markers is set forth in an
example, below.
[0139] Another aspect of this disclosure relates to a method for
producing an ACL ex vivo. Cells capable of differentiating into
ligament cells are grown under conditions that simulate the
movements and forces experienced by an ACL in vivo through the
course of embryonic development into mature ligament function.
Specifically, under sterile conditions, pluripotent cells are
seeded within a three-dimensional silk-fiber-based fiber construct
to which cells can adhere and which is advantageously of
cylindrical shape. The three-dimensional silk-fiber-based fiber
construct used in the method serves as a preliminary fiber
construct, which is supplemented and possibly even replaced by
extracellular fiber construct components produced by the
differentiating cells. Use of the novel silk-fiber-based fiber
construct may enhance or accelerate the development of the ACL. For
instance, the novel silk-fiber-based fiber construct can be
designed to possess specific mechanical properties (e.g., increased
tensile strength) so that it can withstand strong forces prior to
reinforcement from extracellular (e.g., collagen and tenascin)
fiber construct components. Other advantageous properties of the
novel silk-fiber based preliminary fiber construct include, without
limitation, biocompatibility and susceptibility to
biodegradation.
[0140] The pluripotent cells may be seeded within the preliminary
fiber construct either pre- or post-fiber construct formation,
depending upon the particular fiber construct used and upon the
method of fiber construct formation. Uniform seeding is usually
preferable. In theory, the number of cells seeded does not limit
the final ligament produced; however, optimal seeding may increase
the rate of generation. Optimal seeding amounts will depend on the
specific culture conditions. The fiber construct can be seeded with
from about 0.05 to 5 times the physiological cell density of a
native ligament.
[0141] One or more types of pluripotent cells are used in the
method. Such cells have the ability to differentiate into a wide
variety of cell types in response to the proper differentiation
signals and to express ligament specific markers. More
specifically, the method uses cells, such as bone marrow stromal
cells, that have the ability to differentiate into cells of
ligament and tendon tissue. If the resulting bioengineered ligament
is to be transplanted into a patient, the cells should be derived
from a source that is compatible with the intended recipient.
Although the recipient will generally be a human, applications in
veterinary medicine also exist. The cells can be obtained from the
recipient (autologous), although compatible donor cells may also be
used to make allogenic ligaments. For example, when making
allogenic ligaments (e.g., using cells from another human such as
bone marrow stromal cells isolated from donated bone marrow or ACL
fibroblasts isolated from donated ACL tissue), human anterior
cruciate ligament fibroblast cells isolated from intact donor ACL
tissue (e.g., cadaveric or from total knee transplantations),
ruptured ACL tissue (e.g., harvested at the time of surgery from a
patient undergoing ACL reconstruction) or bone marrow stromal cells
may be used. The determination of compatibility is within the means
of the skilled practitioner.
[0142] Ligaments or tendons including, but not limited to, the
posterior cruciate ligament, rotator cuff tendons, medial
collateral ligament of the elbow and knee, flexor tendons of the
hand, lateral ligaments of the ankle and tendons and ligaments of
the jaw or temporomandibular joint other than ACL, cartilage, bone
and other tissues may be engineered with the fiber construct in
accordance with methods of this disclosure. In this manner, the
cells to be seeded on the fiber construct are selected in
accordance with the tissue to be produced (e.g., pluripotent or of
the desired tissue type). Cells seeded on the fiber construct, as
described herein, can be autologous or allogenic. The use of
autologous cells effectively creates an allograft or autograft for
implantation in a recipient.
[0143] As recited, to form an ACL, cells, such as bone marrow
stromal cells, are seeded on the fiber construct. Bone marrow
stromal cells are a type of pluripotent cell and are also referred
to in the art as mesenchymal stem cells or simply as stromal cells.
As recited, the source of these cells can be autologous or
allogenic. Additionally, adult or embryonic stem or pluripotent
cells can be used if the proper environment (either in vivo or in
vitro), seeded on the silk-fiber based fiber construct, can
recapitulate an ACL or any other desired ligament or tissue in
extracellular fiber construct composition (e.g., protein,
glycoprotein content), organization, structure or function.
[0144] Fibroblast cells can also be seeded on the inventive fiber
construct. Since fibroblast cells are often not referred to as
pluripotent cells, fibroblasts are intended to include mature human
ACL fibroblasts (autologous or allogenic) isolated from ACL tissue,
fibroblasts from other ligament tissue, fibroblasts from tendon
tissue, from neonatal foreskin, from umbilical cord blood, or from
any cell, whether mature or pluripotent, mature dedifferentiated,
or genetically engineered, such that when cultured in the proper
environment (either in vivo or in vitro), and seeded on the
silk-fiber based fiber construct, can recapitulate an ACL or any
other desired ligament or tissue in extracellular fiber construct
composition (e.g., protein, glycoprotein content), organization,
structure or function.
[0145] The faces of the fiber construct cylinder are each attached
to anchors, through which a range of forces is to be applied to the
fiber construct. To facilitate force delivery to the fiber
construct, the entire surface of each respective face of the fiber
construct can contact the face of the respective anchors. Anchors
with a shape that reflects the site of attachment (e.g.,
cylindrical) are best suited for use in this method. Once
assembled, the cells in the anchored fiber construct are cultured
under conditions appropriate for cell growth and regeneration. The
fiber construct is subjected to one or more mechanical forces
applied through the attached anchors (e.g., via movement of one or
both of the attached anchors) during the course of culture. The
mechanical forces are applied over the period of culture to mimic
conditions experienced by the native ACL or other tissues in
vivo.
[0146] The anchors must be made of a material suitable for fiber
construct attachment, and the resulting attachment should be strong
enough to endure the stress of the mechanical forces applied. In
addition, the anchors can be of a material that is suitable for the
attachment of extracellular fiber construct that is produced by the
differentiating cells. The anchors support bony tissue in-growth
(either in vitro or in vivo) while anchoring the developing
ligament. Some examples of suitable anchor material include,
without limitation, hydroxyappatite, Goinopra coral, demineralized
bone, bone (allogenic or autologous). Anchor materials may also
include titanium, stainless steel, high density polyethylene,
DACRON and TEFLON.
[0147] Alternatively, anchor material may be created or further
enhanced by infusing a selected material with a factor that
promotes either ligament fiber construct binding or bone fiber
construct binding or both. The term infuse is considered to:
include any method of application that appropriately distributes
the factor onto the anchor (e.g., coating, permeating, contacting).
Examples of such factors include without limitation, laminin,
fibronectin, any extracellular fiber construct protein that
promotes adhesion, silk, factors that contain
arginine-glycine-aspartate (RGD) peptide binding regions or the RGD
peptides themselves. Growth factors or bone morphogenic protein can
also be used to enhance anchor attachment. In addition, anchors may
be pre-seeded with cells (e.g., stem cells, ligament cells,
osteoblasts, osteogenic progenitor cells) that adhere to the
anchors and bind the fiber construct, to produce enhanced fiber
construct attachment both in vitro and in vivo.
[0148] An exemplary anchor system is disclosed in applicant's
co-pending application U.S. Ser. No. 09/950,561, which is
incorporated herein by reference in its entirety. The fiber
construct is attached to the anchors via contact with the anchor
face or alternatively by actual penetration of the fiber construct
material through the anchor material. Because the force applied to
the fiber construct via the anchors dictates the final ligament
produced, the size of the final ligament produced is, in part,
dictated by the size of the attachment site of the anchor. An
anchor of appropriate size to the desired final ligament should be
used. An example of an anchor shape for the formation of an ACL is
a cylinder. However, other anchor shapes and sizes will also
function adequately. For example, anchors can have a size and
composition appropriate for direct insertion into bone tunnels in
the femur and tibia of a recipient of the bioengineered
ligament.
[0149] Alternatively, anchors can be used only temporarily during
in vitro culture, and then removed when the fiber construct alone
is implanted in vivo.
[0150] Further still, the novel silk-fiber-based fiber construct
can be seeded with BMSCs and cultured in a bioreactor. Two types of
growth environments currently exist that may be used in accordance
with methods of this disclosure: (1) the in vitro bioreactor
apparatus system, and (2) the in vivo knee joint, which serves as a
"bioreactor" as it provides the physiologic environment including
progenitor cells and stimuli (both chemical and physical) necessary
for the development of a viable ACL given a fiber construct with
proper biocompatible and mechanical properties. The bioreactor
apparatus provides optimal culture conditions for the formation of
a ligament in terms of differentiation and extracellular fiber
construct (ECM) production, and which thus provides the ligament
with optimal mechanical and biological properties prior to
implantation in a recipient. Additionally, when the silk-fiber
based fiber construct is seeded and cultured with cells in vitro, a
petri dish may be considered to be the bioreactor within which
conditions appropriate for cell growth and regeneration exist,
i.e., a static environment.
[0151] Cells can also be cultured on the fiber construct fiber
construct without the application of any mechanical forces, i.e.,
in a static environment; For example, the silk-fiber based fiber
construct alone, with no in vitro applied mechanical forces or
stimulation, when seeded and cultured with BMSCs, induces the cells
to proliferate and express ligament and tendon specific markers
(see the examples, described herein). The knee joint may serve as a
physiological growth and development environment that can provide
the cells and the correct environmental signals (chemical and
physical) to the fiber construct fiber construct such that an ACL
technically develops. Therefore, the knee joint (as its own form of
bioreactor) plus the fiber construct (either non-seeded, seeded and
not differentiated in vitro, or seeded and differentiated in vitro
prior to implantation) will result in the development of an ACL, or
other desired tissue depending upon the cell type seeded on the
fiber construct and the anatomical location of fiber construct
implantation. FIG. 15A-B illustrates the effects of the medial
collateral knee joint environment on medial collateral ligament
(MCL) development when only a non-seeded silk-based fiber construct
with appropriate MCL mechanical properties is implanted for 6 weeks
in vivo. Whether the cells are cultured in a static environment
with no mechanical stimulation applied, or in a dynamic
environment, such as in a bioreactor apparatus, conditions
appropriate for cell growth and regeneration are advantageously
present for the engineering of the desired ligament or tissue.
[0152] In experiments described in the examples, below, the applied
mechanical stimulation was shown to influence the morphology, and
cellular organization of the progenitor cells within the resulting
tissue. The extracellular fiber construct components secreted by
the cells and the organization of the extracellular fiber construct
throughout the tissue was also significantly influenced by the
forces applied to the fiber construct during tissue generation.
During in vitro tissue generation, the cells and extracellular
fiber construct aligned along the axis of load, reflecting the in
vivo organization of a native ACL that is also along the various
load axes produced from natural knee joint movement and function.
These results suggest that the physical stimuli experienced in
nature by cells of developing tissue, such as the ACL, play a
significant role in progenitor cell differentiation and tissue
formation. They further indicate that this role can be effectively
duplicated in vitro by mechanical manipulation to produce a similar
tissue. The more closely the forces produced by mechanical
manipulation resemble the forces experienced by an ACL in vivo, the
more closely the resultant tissue will resemble a native ACL.
[0153] When mechanical stimulation is applied in vitro to the fiber
construct via a bioreactor, there exists independent but concurrent
control over both cyclic and rotation strains as applied to one
anchor with respect to the other anchor. Alternatively, the fiber
construct alone may be implanted in vivo, seeded with ACL cells
from the patient and exposed in vivo to mechanical signaling via
the patient.
[0154] When the fiber construct is seeded with cells prior to
implantation, the cells are cultured within the fiber construct
under conditions appropriate for cell growth and differentiation.
During the culture process, the fiber construct may be subjected to
one or more mechanical forces via movement of one or both of the
attached anchors. The mechanical forces of tension, compression,
torsion and shear, and combinations thereof, are applied in the
appropriate combinations, magnitudes, and frequencies to mimic the
mechanical stimuli experienced by an ACL in vivo.
[0155] Various factors will influence the amount of force that can
be tolerated by the fiber construct (e.g., fiber construct
composition, cell density). Fiber construct strength is expected to
change through the course of tissue development. Therefore, applied
mechanical forces or strains will increase, decrease or remain
constant in magnitude, duration, frequency and variety over the
period of ligament generation, to appropriately correspond to fiber
construct strength at the time of application.
[0156] When producing an ACL, the more accurate the intensity and
combination of stimuli applied to the fiber construct during tissue
development, the more the resulting ligament will resemble a native
ACL. Two issues must be considered regarding the natural function
of the ACL when devising the in vitro mechanical force regimen that
closely mimics the in vivo environment: (1) the different types of
motion experienced by the ACL and the responses of the ACL to knee
joint movements and (2) the extent of the mechanical stresses
experienced by the ligament. Specific combinations of mechanical
stimuli are generated from the natural motions of the knee
structure and transmitted to the native ACL.
[0157] To briefly describe the motions of the knee, the connection
of the tibia and femur by the ACL provides six degrees of freedom
when considering the motions of the two bones relative to each
other. The tibia can move in three directions and can rotate
relative to the axes for each of these three directions. The knee
is restricted from achieving the full ranges of these six degrees
of freedom due to the presence of ligaments and capular fibers and
the knee surfaces themselves (Biden et al., "Experimental Methods
Used to Evaluate Knee Ligament Function," Knee Ligaments:
Structure, Function, Injury and Repair, Ed. D. Daniel et al., Raven
Press, pp.135-151, 1990). Small translational movements are also
possible. The attachment sites of the ACL are responsible for its
stabilizing roles in the knee joint. The ACL functions as a primary
stabilizer of anterior-tibial translation, and as a secondary
stabilizer of valgus-varus angulation, and tibial rotation
(Shoemaker et al., "The Limits of Knee Motion," Knee Ligaments:
Structure, Function, Injury and Repair, Ed. D. Daniel et al., Raven
Press, pp.1534-161, 1990). Therefore, the ACL is responsible for
stabilizing the knee in three of the six possible degrees of
freedom. As a result, the ACL has developed a specific fiber
organization and overall structure to perform these stabilizing
functions. These conditions are simulated in vitro to produce a
tissue with similar structure and fiber organization.
[0158] The extent of mechanical stresses experienced by the ACL can
be similarly summarized. The ACL undergoes cyclic loads of about
400 N between one and two million cycles per year (Chen et al., J.
Biomed. Mat. Res. 14: 567-586, 1980). Also considered are linear
stiffness (.about.182 N/mm), ultimate deformation (100% of ACL) and
energy absorbed at failure (12.8 N-m) (Woo et al., The tensile
properties of human anterior cruciate ligament (ACL) and ACL graft
tissues, Knee Ligaments: Structure, Function, Injury and Repair,
Ed. D. Daniel et al. Raven Press, pp.279-289, 1990) when developing
an ACL surgical replacement.
[0159] The examples section, below, details the production of a
prototype bioengineered anterior cruciate ligament (ACL) ex vivo.
Mechanical forces mimicking a subset of the mechanical stimuli
experienced by a native ACL in vivo (rotational deformation and
linear deformation) were applied in combination, and the resulting
ligament that was formed was studied to determine the effects of
the applied forces on tissue development. Exposure of the
developing ligament to physiological loading during in vitro
formation induced the cells to adopt a defined orientation along
the axes of load, and to generate extracellular matrices along the
axes as well. These results indicate that the incorporation of
complex multi-dimensional mechanical forces into the regime to
produce a more complex network of load axes that mimics the
environment of the native ACL will produce a bioengineered ligament
that more closely resembles a native ACL.
[0160] The different mechanical forces that may be applied include,
without limitation, tension, compression, torsion, and shear. These
forces are applied in combinations that simulate forces experienced
by an ACL in the course of natural knee joint movements and
function. These movements include, without limitation, knee joint
extension and flexion as defined in the coronal and sagittal
planes, and knee joint flexion. Optimally, the combination of
forces applied mimics the mechanical stimuli experienced by an
anterior cruciate ligament in vivo as accurately as is
experimentally possible. Varying the specific regimen of force
application through the course of ligament generation is expected
to influence the rate and outcome of tissue development, with
optimal conditions to be determined empirically. Potential
variables in the regimen include, without limitation: (1) strain
rate, (2) percent strain, (3) type of strain (e.g., translation and
rotation), (4) frequency, (5) number of cycles within a given
regime, (6) number of different regimes, (7) duration at extreme
points of ligament deformation, (8) force levels, and (9) different
force combinations. A wide variety of variations exist. The regimen
of mechanical forces applied can produce helically organized fibers
similar to those of the native ligament, described below.
[0161] The fiber bundles of a native ligament are arranged into a
helical organization. The mode of attachment and the need for the
knee joint to rotate .about.140.degree. of flexion has resulted in
the native ACL inheriting a 90.degree. twist and with the
peripheral fiber bundles developing a helical organization. This
unique biomechanical feature allows the ACL to sustain extremely
high loading. In the functional ACL, this helical organization of
fibers allows anterior-posterior and posterior-anterior fibers to
remain relatively isometric in respect to one another for all
degrees of flexion, thus load can be equally distributed to all
fiber bundles at any degree of knee joint flexion, stabilizing the
knee throughout all ranges of joint motion. Mechanical forces that
simulate a combination of knee joint flexion and knee joint
extension can be applied to the developing ligament to produce an
engineered ACL that possesses this same helical organization. The
mechanical apparatus used in the experiments presented in the
examples, below, provides control over strain and strain rates
(both translational and rotational). The mechanical apparatus will
monitor the actual load experienced-by the growing ligaments,
serving to `teach` the ligaments over time through monitoring and
increasing the loading regimes.
[0162] Another aspect of this disclosure relates to the
bioengineered anterior cruciate ligament produced by the
above-described methods. The bioengineered ligament produced by
these methods is characterized by cellular orientation and/or a
fiber construct crimp pattern in the direction of the mechanical
forces applied during generation. The ligament is also
characterized by the production/presence of extracellular fiber
construct components (e.g., collagen type I and type III,
fibronectin, and tenascin-C proteins) along the axis of mechanical
load experienced during culture. The ligament fiber bundles can be
arranged into a helical organization, as discussed above.
[0163] The above methods using the novel silk-fiber-based fiber
construct are not limited to the production of an ACL, but can also
be used to produce other ligaments and tendons found in the knee
(e.g., posterior cruciate ligament) or other parts of the body
(e.g., hand, wrist, ankle, elbow, jaw and shoulder), such as for
example, but not limited to posterior cruciate ligament, rotator
cuff tendons, medial collateral ligament of the elbow and knee,
flexor tendons of the hand, lateral ligaments of the ankle and
tendons and ligaments of the jaw or temporomandibular joint. All
moveable joints in a human body have specialized ligaments that
connect the articular extremities of the bones in the joint. Each
ligament in the body has a specific structure and organization that
is dictated by its function and environment. The various ligaments
of the body, their locations and functions are listed in Anatomy,
Descriptive and Surgical (Gray, H., Eds. Pick, T. P., Howden, R.,
Bounty Books, New York, 1977), the pertinent contents of which are
incorporated herein by reference. By determining the physical
stimuli experienced by a given ligament or tendon, and
incorporating forces which mimic these stimuli, the above-described
method for producing an ACL ex vivo can be adapted to produce
bioengineered ligaments and tendons ex vivo that simulates any
ligament or tendon in the body.
[0164] The specific type of ligament or tendon to be produced is
predetermined prior to tissue generation since several aspects of
the method vary with the specific conditions experienced in vivo by
the native ligament or tendon. The mechanical forces to which the
developing ligament or tendon is subjected during cell culture are
determined for the particular ligament or tendon type being
cultivated. The specific conditions can be determined by studying
the native ligament or tendon and its environment and function. One
or more mechanical forces experienced by the ligament or tendon in
vivo are applied to the fiber construct during culture of the cells
in the fiber construct. The skilled practitioner will recognize
that a ligament or tendon that is superior to those currently
available can be produced by the application of a subset of forces
experienced by the native ligament or tendon. However, optimally,
the full range of in vivo forces will be applied to the fiber
construct in the appropriate magnitudes and combinations to produce
a final product that most closely resembles the native ligament or
tendon. These forces include, without limitation, the forces
described above for the production of an ACL. Because the
mechanical forces applied vary with ligament or tendon type, and
the final size of the ligament or tendon will be influenced by the
anchors used, optimal anchor composition, size and fiber construct
attachment sites are to be determined for each type of ligament or
tendon by the skilled practitioner. The type of cells seeded on the
fiber construct is obviously determined based on the type of
ligament or tendon to be produced.
[0165] Other tissue types can be produced ex vivo using methods
similar to those described above for the generation of ligaments or
tendons ex vivo. The above-described methods can also be applied to
produce a range of engineered tissue products that involve
mechanical deformation as a major part of their function, such as
muscle (e.g., smooth muscle, skeletal muscle, cardiac muscle),
bone, cartilage, vertebral discs, and some types of blood vessels.
Bone marrow stromal cells possess the ability to differentiate into
these as well as other tissues. The geometry of the silk-based
fiber construct or composite fiber construct can easily be adapted
to the correct anatomical geometrical configuration of the desired
tissue type. For example, silk fibroin fibers can be reformed in a
cylindrical tube to recreate arteries.
[0166] The results presented in the examples, below, indicate that
growth in an environment that mimics the specific mechanical
environment of a given tissue type will induce the appropriate cell
differentiation to produce a bioengineered tissue that
significantly resembles native tissue. The ranges and types of
mechanical deformation of the fiber construct can be extended to
produce a wide range of tissue structural organization. The cell
culture environment can reflect the in vivo environment experienced
by the native tissue and the cells it contains, throughout the
course of embryonic development to mature function of the cells
within the native tissue, as accurately as possible. Factors to
consider when designing specific culture conditions to produce a
given tissue include, without limitation, the fiber construct
composition, the method of cell immobilization, the anchoring
method of the fiber construct or tissue, the specific forces
applied, and the cell culture medium. The specific regimen of
mechanical stimulation depends upon the tissue type to be produced,
and is established by varying the application of mechanical forces
(e.g., tension only, torsion only, combination of tension and
torsion, with and without shear, etc.), the force amplitude (e.g.,
angle or elongation), the frequency and duration of the
application, and the duration of the periods of stimulation and
rest.
[0167] The method for producing the specific tissue type ex vivo is
an adaptation of the above-described method for producing an ACL.
Components involved include pluripotent cells, a three-dimensional
fiber construct to which cells can adhere, and a plurality of
anchors that have a face suitable for fiber construct attachment.
The pluripotent cells (such as bone marrow stromal cells) are
seeded in the three dimensional fiber construct by means to
uniformly immobilize the cells within the fiber construct. The
number of cells seeded is also not viewed as limiting, however,
seeding the fiber construct with a high density of cells may
accelerate tissue generation.
[0168] The specific forces applied are to be determined for each
tissue type produced through examination of native tissue and the
mechanical stimuli experienced in vivo. A given tissue type
experiences characteristic forces that are dictated by location and
function of the tissue within the body. For instance, cartilage is
known to experience a combination of shear and compression/tension
in vivo; bone experiences compression.
[0169] Additional stimuli (e.g., chemical stimuli, electromagnetic
stimuli) can also be incorporated into the above-described methods
for producing bioengineered ligaments, tendons and other tissues.
Cell differentiation is known to be influenced by chemical stimuli
from the environment, often produced by surrounding cells, such as
secreted growth or differentiation factors, cell-cell contact,
chemical gradients, and specific pH levels, to name a few. Other
more unique stimuli are experienced by more specialized types of
tissues (e.g., the electrical stimulation of cardiac muscle). The
application of such tissue specific stimuli (e.g., 1-10 ng/ml
transforming growth factor beta-1 (TGF-.beta.1) independently or in
concert with the appropriate mechanical forces is expected to
facilitate differentiation of the cells into a tissue that more
closely approximates the specific natural tissue.
[0170] Tissues produced by the above-described methods provide an
unlimited pool of tissue equivalents for surgical implantation into
a compatible recipient, particularly for replacement or repair of
damaged tissue. Engineered tissues may also be utilized for in
vitro studies of normal or pathological tissue function, e.g., for
in vitro testing of cell- and tissue-level responses to molecular,
mechanical, or genetic manipulations. For example, tissues based on
normal or transfected cells can be used to assess tissue responses
to biochemical or mechanical stimuli, identify the functions of
specific genes or gene products that can be either over-expressed
or knocked-out, or to study the effects of pharmacological agents.
Such studies will likely provide more insight into ligament, tendon
and tissue development, normal and pathological function, and
eventually lead toward fully functional tissue engineered
replacements, based in part on already established tissue
engineering approaches, new insights into cell differentiation and
tissue development, and the use of mechanical regulatory signals in
conjunction with cell-derived and exogenous biochemical factors to
improve structural and functional tissue properties.
[0171] The production of engineered tissues, such as ligaments and
tendons, also has the potential for applications such as harvesting
bone marrow stromal cells from individuals at high risk for tissue
injury (e.g., ACL rupture) prior to injury. These cells could be
either stored until needed or seeded into the appropriate fiber
construct and cultured and differentiated in vitro under mechanical
stimuli to produce a variety of bioengineered prosthetic tissues to
be held in reserve until needed by the donor. The use of
bioengineered living tissue prosthetics that better match the
biological environment in vivo and that provide the required
physiological loading to sustain, for example, the dynamic
equilibrium of a normal, fully functional ligament should reduce
rehabilitation time for a recipient of a prosthesis from months to
weeks, particularly if the tissue is pre-grown and stored. Benefits
include a more rapid regain of functional activity, shorter
hospital stays, and fewer problems with tissue rejections and
failures.
[0172] Additional aspects of this invention are further exemplified
in the following examples. It will be apparent to those skilled in
the art that many modifications, both to the materials and methods,
may be practiced without departing from the invention.
[0173] In a first example, raw Bombyx mori silkworm fibers, shown
in FIG. 1A, were extracted to remove sericin, the glue-like protein
coating the native silk fibroin (see FIGS. 1A-C). The appropriate
number of fibers per group were arranged in parallel and extracted
in an aqueous solution of 0.02 M Na2CO3 and 0.3% (w/v) IVORY soap
solution for 60 minutes at 90.degree. C., then rinsed thoroughly
with water to extract the glue-like sericin proteins.
[0174] Costello's equation for a three-strand, helical rope
geometry was derived to predict mechanical properties of the
silk-fiber-based construct. The derived model is a series of
equations that when combined, take into account extracted silk
fiber material properties and desired fiber construct geometrical
hierarchy to compute the overall strength and stiffness of the
fiber construct as a function of pitch angle for a given level of
geometrical hierarchy.
[0175] The material properties of a single silk fiber include fiber
diameter, modulus of elasticity, Poisson's ratio, and the ultimate
tensile strength (UTS). Geometrical hierarchy may be defined as the
number of twisting levels in a given fiber construct level. Each
level (e.g., group, bundle, strand, cord, ligament) is further
defined by the number of groups of fibers twisted about each other
and the number of fibers in each group of the first level twisted
where the first level is define as a group, the second level as a
bundle, the third as a strand and the fourth as a cord, the fifth
as the ligament.
[0176] The model assumes that each group of multiple fibers act as
a single fiber with an effective radius determined by the number of
individual fibers and their inherent radius, i.e., the model
discounts friction between the individual fibers due to its limited
role in given a relatively high pitch angle.
[0177] Two applicable geometries (Matrix 1 and Matrix 2) of the
many fiber construct geometrical configurations (see Table 10,
supra) computed to yield mechanical properties mimicking those of a
native ACL were derived for more detailed analysis. A six-cord
construct was selected for use as the ACL replacement. Matrix
configurations are as follows: Matrix 1: 1 ACL prosthesis=6
parallel cords; 1 cord=3 twisted strands (3 twists/cm); 1 strand=6
twisted bundles (3 twists/cm); 1 bundle=30 parallel washed fibers;
and Matrix 2: 1 ACL matrix=6 parallel cords; 1 cord=3 twisted
strands (2 twists/cm); 1 strand=3 twisted bundles (2.5 twists/cm);
1 bundle=3 groups (3 twists/cm); 1 group=15 parallel extracted silk
fibroin fibers. The number of fibers and geometries were selected
such that the silk prostheses are similar to the ACL biomechanical
properties in UTS, linear stiffness, yield point and % elongation
at break (see Table 10, supra), thus serving as a solid starting
point for the development of a tissue engineered ACL.
[0178] Mechanical properties of the silk fibroin were characterized
using a servohydraulic Instron 8511 tension/compression system with
Fast-Track software (Instron Corp., Canton, Mass., USA) (see FIG.
1D). Single pull-to-failure and fatigue analyses were performed on
single silk fibers, extracted fibroin and organized cords. Fibers
and fibroin were organized in both the parallel helical geometries
of Matrix 1 (see FIG. 2C) and of Matrix 2 (see FIG. 2D) for
characterization. Single pull to failure testing was performed at a
strain rate of 100%/sec; force elongation histograms were generated
and data analyzed using Instron Series 1X software. Both Matrix 1
and Matrix 2 yielded similar mechanical and fatigue properties to
the ACL in UTS, linear stiffness, yield point and percent
elongation at break (see Table 10 and FIGS. 3A-D).
[0179] Fatigue analyses were performed using a servohydraulic
Instron 8511 tension/compression system with Wavemaker software on
single cords of both Matrix 1 and Matrix 2. Data was extrapolated
to represent the 6-cord ACL prostheses, which is shown in FIGS. 3B
and 3D. Cord ends were embedded in an epoxy mold to generate a
3-cm-long construct between anchors. Cycles to failure at UTS's of
1,680 N and 1,200 N (n=5 for each load) for Matrix 1 (see FIG. 3B)
and at UTS's of 2280 N, 2100 N and 1800 N loads (n=3 for each load)
for Matrix 2 (see FIG. 3D) were determined using a H-sine wave
function at 1 Hz generated by Wavemaker 32 version 6.6 software
(Instron Corp.). Fatigue testing was conducted in a neutral
phosphate buffered saline (PBS) solution at room temperature.
[0180] Complete sericin removal was observed after 60 min at
90.degree. C. as determined by SEM (see FIGS. 1A-C). Removal of
sericin from silk fibers altered the ultrastructure of the fibers,
resulting in a smoother fiber surface, and the underlying silk
fibroin was revealed (shown in FIGS. 1A-C), with average diameter
ranging between 20-40 .mu.m. The fibroin exhibited a significant
15.2% decrease in ultimate tensile strength (1.033.+-.0.042 N/fiber
to 0.876.+-.0.1 N/fiber) (p<0.05, paired Students t-test) (see
FIG. 1D). The mechanical properties of the optimized silk matrices
(see FIG. 2A-D & FIG. 3A-D) are summarized in Table 11 above
and in FIG. 3A (for Matrix 1) and in FIG. 3C (for Matrix 2). It is
evident from these results that the optimized silk matrices
exhibited values comparable to those of native ACL, which have been
reported to have an average ultimate tensile strength (UTS) of
.about.2100 N, stiffness of .about.250 N/nm, yield point
.about.2100 N and 33% elongation at break (See Woo, S L-Y, et al.,
The Tensile Properties of Human Anterior Cruciate Ligament (ACL)
and ACL Graft Tissue in Knee Ligaments: Structure, Function, Injury
and Repair, 279-289, Ed. D. Daniel et al., Raven Press 1990).
[0181] Regression analysis of fiber construct fatigue data, shown
in FIG. 3B for Matrix 1 and in FIG. 3D for Matrix 2, when
extrapolated to physiological load levels (400 N) predict the
number of cycles to failure in vivo, indicate a fiber construct
life of 3.3 million cycles for Matrix 1 and a life of greater than
10 million cycles for Matrix 2. The helical fiber construct design
utilizing washed silk fibers resulted in a fiber construct with
physiologically equivalent structural properties, confirming its
suitability as a scaffold for ligament tissue engineering.
[0182] In another example involving cell isolation and culture,
bone marrow stromal cells (BMSC), pluripotent cells capable of
differentiating into osteogenic, chondrogenic, tendonogenic,
adipogenic and myogenic lineages, were chosen since the formation
of the appropriate conditions can direct their differentiation into
the desired ligament fibroblast cell line (Markolfet al., J. Bone
Joint Surg. 71A: 887-893, 1989; Caplan et al., Mesenchymal stem
cells and tissue repair, The Anterior Cruciate Ligament: Current
and Future Concepts, Ed. D. W. Jackson et al., Raven Press, Ltd,
New York, 1993; Young et al., J. Orthopaedic Res. 16: 406-413,
1998).
[0183] Human BMSCs were isolated from bone marrow from the iliac
crest of consenting donors at least 25 years of age by a commercial
vendor (Cambrex, Walkersville, Md.). Twenty-two milliliters of
human marrow was aseptically aspirated into a 25 ml syringe
containing three milliliters of heparinized (1000 units per
milliliter) saline solution. The heparinized marrow solution was
shipped overnight on ice to the laboratory for bone marrow stromal
cells isolation and culture. Upon arrival from the vendor, the
twenty-five milliliter aspirates were resuspended in Dulbecco's
Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine
serum (FBS), 0.1 mM nonessential amino acids, 100 U/ml penicillin,
100 mg/L streptomycin (P/S), and 1 ng/ml basic fibroblast growth
factor (bFGF) (Life Technologies, Rockville, Md.) and plated at
8-10 microliters of aspirate/cm2 in tissue culture flasks. Fresh
medium was added to the marrow aspirates twice a week for up to
nine days of culture. BMSCs were selected based on their ability to
adhere to the tissue culture plastic; non-adherent hematopoietic
cells were removed during medium replacement after 9-12 days in
culture. Medium was changed twice per week thereafter. When primary
BMSC became near confluent (12-14 days), they were detached using
0.25% trypsin/1 mM EDTA and replated at 5.times.103 cells/cm2.
First passage (P1) hBMSCs were trypsinized and frozen in 8%
DMSO/10% FBS/DMEM for future use.
[0184] Frozen P1 hBMSCs were defrosted, replated at 5.times.103
cells/cm2 (P2), trypsinized when near confluency, and used for
fiber construct seeding. Sterilized (ethylene oxide) silk matrices
(specifically, single cords of Matrices 1 and 2, bundles of 30
parallel extracted silk fibers, and helical ropes of collage
fibers) were seeded with cells in customized seeding chambers (1 ml
total volume) machined in Teflon blocks to minimize cell-medium
volume and increase cell-fiber construct contact. Seeded matrices,
following a 4 hour incubation period with the cell slurry
(3.3.times.106 BMSCs/ml) were transferred into a petri dish
containing an appropriate amount of cell culture medium for the
duration of the experiments.
[0185] To determine the degradation rate of the silk fibroin,
ultimate tensile strength (UTS) was measured as a function of
cultivation period in physiological growth conditions, i.e., in
cell culture medium. Groups of 30 parallel silk fibers 3 cm in
length were extracted, seeded with hBMSCs, and cultured on the
fibroin over 21 days at 37.degree. C. and 5% CO2. Non-seeded
control groups were cultured in parallel. Silk fibroin UTS was
determined as a function of culture duration for seeded and
non-seeded groups.
[0186] The response of bone marrow stromal cells to the silk fiber
construct was also examined.
[0187] BMSCs readily attached and grew on the silk and collagen
matrices after 1 day in culture (See FIG. 7A-C and FIG. 16A), and
formed cellular extensions to bridge neighboring fibers. As shown
in FIG. 7D and FIG. 16B, a uniform cells sheet covering the
construct was observed at 14 and 21 days of culture, respectively.
MTT analysis confirmed complete fiber construct coverage by seeded
BMSCs after 14 days in culture (see FIG. 8A-B). Total DNA
quantification of cells grown on Matrix 1 (see FIG. 9A) and Matrix
2 (see FIG. 9B) confirmed that BMSCs proliferated and grew on the
silk construct with the highest amount of DNA measured after 21 and
14 days, respectively, in culture.
[0188] Both BMSC-seeded or non-seeded extracted control silk
fibroin groups of 30 fibers, maintained their mechanical integrity
as a function of culture period over 21 days (see FIG. 10).
[0189] RT-PCR analysis of BMSCs seeded on cords of Matrix 2
indicated that both collagen I & III were upregulated over 14
days in culture (FIG. 14). Collagen type II and bone sialoprotein
(as indicators of cartilage and bone specific differentiation,
respectively) were either not detectable or negligibly expressed
over the cultivation period. Real-time quantitative RT-PCR at 14
days yielded a transcript ratio of collagen I to collagen III,
normalized to GAPDH, of 8.9:1 (see FIG. 17). The high ratio of
collagen I to collagen III indicates that the response is not wound
healing or scar tissue formation (as is observed with high levels
of collagen-type III), but rather ligament specific; the relative
ratio of collagen I to collagen III in a native ACL is .about.6.6:1
(Amiel et al., Knee Ligaments: Structure, Function, Injury, and
Repair, 1990).
[0190] Additionally, studies are conducted to provide insight into
the influence of directed multi-dimensional mechanical stimulation
on ligament formation from bone marrow stromal cells in the
bioreactor system. The bioreactor is capable of applying
independent but concurrent cyclic multi-dimensional strains (e.g.,
translation, rotation) to the developing ligaments. After a 7 to 14
day static rest period (time post seeding), the rotational and
translation strain rates and linear and rotational deformation are
kept constant for 1 to 4 weeks. Translational strain (3.3%-10%, 1-3
mm) and rotational strain (25%, 90.degree.) are concurrently
applied at a frequency of 0.0167 Hz (one full cycle of stress and
relaxation per minute) to the silk-based matrices seeded with
BMSCs; an otherwise identical set of bioreactors with seeded
matrices without mechanical loading serve as controls. The
ligaments are exposed to the constant cyclic strains for the
duration of the experiment days.
[0191] Following the culture period, ligament samples, both the
mechanically challenged as well as the controls (static) are
characterized for: (1) general histomorphological appearance (by
visual inspection); (2) cell distribution (image processing of
histological and MTT stained sections); (3) cell morphology and
orientation (histological analysis); and (4) the production of
tissue specific markers (RT-PCR, immunostaining).
[0192] Mechanical stimulation markedly affects the morphology and
organization of the BMSCs and newly developed extracellular fiber
construct, the distribution of cells along the fiber construct, and
the upregulation of a ligament-specific differentiation cascade;
BMSCs align along the long axis of the fiber, take on a spheroid
morphology similar to ligament/tendon fibroblasts and upregulate
ligament/tendon specific markers. Newly formed extracellular fiber
construct (i.e., the composition of proteins produced by the cells)
is expected to align along the lines of load as well as the long
axis of the fiber construct. Directed mechanical stimulation is
expected to enhance ligament development and formation in vitro in
a bioreactor resulting from BMSCs seeded on the novel silk-based
fiber construct. The longitudinal orientation of cells and newly
formed fiber construct is similar to ligament fibroblasts found
within an ACL in vivo (Woods et al., Amer. J. Sports Med. 19:
48-55, 1991). Furthermore, mechanical stimulation maintains the
correct expression ratio between collagen type I transcripts and
collagen type III transcripts (e.g., greater than 7:1) indicating
the presence of newly formed ligament tissue versus scar tissue
formation. The above results will indicate that the mechanical
apparatus and bioreactor system provide a suitable environment
(e.g., multi-dimensional strains) for in vitro formation of tissue
engineered ligaments starting from bone marrow stromal cells and
the novel silk-based fiber construct.
[0193] The culture conditions used in these preliminary experiments
can be further expanded to more accurately reflect the
physiological environment of a ligament (e.g., increasing the
different types of mechanical forces) for the in vitro creation of
functional equivalents of native ACL for potential clinical use.
These methods are not limited to the generation of a bioengineered
ACL. By applying the appropriate magnitude and variety of forces
experienced in vivo, any type of ligament in the body as well as
other types of tissue can be produced ex vivo by the methods of
this disclosure.
[0194] Other embodiments are within the following claims.
1TABLE 1 Ultimate tensile strength and stiffness (N/mm given a 3 cm
long sample) as a function of sericin extraction from a 10-fiber
silkworm silk yarn with 0 twists per inch (i.e., parallel) and (i)
temperature and (ii) time. Repeat samples were processed two years
after initial samples with no significant change in properties. N =
5 for all samples. # of UTS Stiff UTS/ Stiffness/fiber Yarn fibers
Temp Time (N) stdev (N/mm) stdev fiber (N) (N/mm) 10(0) 10 RT 60
min 10.74 0.83 6.77 0.65 1.07 0.68 10(0) 10 RT 60 min (repeat)
10.83 0.28 6.36 0.14 1.08 0.64 10(0) 10 33 C. 60 min 10.44 0.17
6.68 0.55 1.04 0.67 10(0) 10 37 C. 60 min 9.60 0.84 6.09 0.59 0.96
0.61 10(0) 10 37 C. 60 min (repeat) 9.54 0.74 5.81 0.67 0.95 0.58
10(0) 10 90 C. 15 min 9.22 0.55 4.87 0.62 0.92 0.49 10(0) 10 90 C.
30 min 8.29 0.19 4.91 0.33 0.83 0.49 10(0) 10 90 C. 60 min 8.60
0.61 4.04 0.87 0.86 0.40 10(0) 10 90 C. 60 min (repeat) 8.65 0.67
4.55 0.69 0.87 0.46 10(0) 10 94 C. 60 min 7.92 0.51 2.42 0.33 0.79
0.24 9(12s) .times. 3(9z) 27 non-extracted 24.50 0.38 8.00 0.49
0.91 0.30 9(12s) .times. 3(9z) 27 90 C. 60 min 21.88 0.18 7.38 0.34
0.81 0.27 9(6s) .times. 3(3z) 27 non-extracted 24.94 0.57 9.51 0.57
0.92 0.35 9(6s) .times. 3(3z) 27 90 C. 60 min 21.36 0.40 7.95 1.00
0.79 0.29 9(12s) .times. 3(6z) 27 non-extracted 24.69 0.65 9.08
0.56 0.91 0.34 9(12s) .times. 3(6z) 27 90 C. 60 min 21.80 0.47 7.48
0.97 0.81 0.28
[0195]
2TABLE 2 Mass loss as a function of sericin extraction. +/-0.43%
standard deviation of an N = 5, reflects the greatest accuracy that
can be achieved when confirming sericin removal, i.e., 0.87 or 1%
error will always be inherent to the methods used and a mass loss
of about 24% represents substantially sericin free constructs.
non-extracted extracted and % mass yarn and dried (mg) dried (mg)
loss 9(12) .times. 3(6) 57.6 43.6 24.31 9(12) .times. 3(6) 58.3
43.9 24.70 9(12) .times. 3(6) 57.0 42.9 24.74 9(12) .times. 3(6)
57.2 42.7 25.35 average 57.53 43.28 24.77 stdev 0.57 0.57 0.43
[0196]
3TABLE 3 Illustrates the change in mass as a function of a second
sericin extraction. Correlated to FIG. 1E-1G, less than a 3% mass
loss is likely indicative of fibroin mass loss due to mechanical
damage during the 2.sup.nd extraction. mass after 1x mass after 2x
extraction, extracted, dried % mass yarn dried (mg) (mg) loss 9(12)
.times. 3(6) 42.5 41.7 1.88 9(12) .times. 3(6) 43.1 42 2.55 9(12)
.times. 3(6) 43.1 42.1 2.32 9(12) .times. 3(6) 42.5 41.7 1.88 9(12)
.times. 3(6) 42.6 42.4 0.47 9(12) .times. 3(6) 43.7 42.4 2.97 9(12)
.times. 3(6) 43.4 42.9 1.15 9(12) .times. 3(6) 43.7 43.1 1.37 9(12)
.times. 3(6) 44 43.2 1.82 average 43.18 42.39 1.82 stdev 0.56 0.57
0.76
[0197]
4TABLE 4 # of Total # UTS UTS % % Stiffness Stiffness UTS Stiffness
Ply levels of of average stdev Elong Elong avg stdev per per
Geometry Method Condition plying Fibers (N) (N) average stdev
(N/mm) (N/mm) fiber fiber 1(0) .times. 3(10) cable extracted 2 3
1.98 0.05 10.42 1.63 2.17 0.51 0.66 0.72 1(0) .times. 4(10) cable
extracted 2 4 2.86 0.14 11.98 1.54 2.08 0.31 0.72 0.52 3(0) .times.
3(3) cable extracted 2 9 6.72 0.17 12.30 0.72 4.54 0.16 0.75 0.50
1(0) .times. 3(10) .times. 3(9) cable extracted 3 9 6.86 0.23 13.11
1.45 4.06 0.36 0.76 0.45 2(0) .times. 6(11) cable 2 12 7.97 0.26
10.05 0.91 0.66 4(6) .times. 3(3) twist non-extracted 2 12 10.17
0.18 19.86 1.16 0.85 1(0) .times. 3(10) .times. 4(9) cable
extracted 3 12 9.29 0.19 14.07 0.98 5.10 0.31 0.77 0.43 1(0)
.times. 4(11) .times. 3(11) twist extracted 3 12 9.70 0.14 12.56
1.03 7.60 0.33 0.81 0.63 1(0) .times. 4(10) .times. 3(9) cable
extracted 3 12 8.78 0.17 14.25 1.09 5.10 0.32 0.73 0.43
15(textured) textured non-extracted, 1 15 10.62 0.68 10.76 1.70
4.75 0.30 0.71 0.316 dry 30(0) parallel extracted, wet 1 30 20.24
1.46 26.32 3.51 1.14 0.15 0.67 0.038 30(0) parallel incubated 21 1
30 19.73 2.10 20.70 6.03 0.66 days, wet 30(0) parallel cell-seeded
21 1 30 20.53 1.02 29.68 7.08 0.68 days, wet 2 fibers/carrier in an
8 braid extracted, dry 2 16 10.93 0.13 6.96 1.14 0.68 0.435 4
fibers/carrier in an 8 braid extracted, dry 2 32 24.60 0.22 12.39
0.53 0.77 0.387 4(6) .times. 3(3) in 4 braid extracted, dry 3 48
37.67 0.18 22.38 0.98 0.78 carrier 15(0) .times. 3(12) cable dry 2
45 27.39 0.62 31.68 1.35 4.63 0.49 0.61 0.102889 15(0) .times.
3(12) .times. 3(10) cable non-extracted, 3 135 73.61 6.00 33.72
5.67 12.33 1.53 0.55 0.091333 1 day after manufacturing 15(0)
.times. 3(12) .times. 3(10) cable non-extracted, 3 135 72.30 5.68
31.18 4.35 0.54 2 days after manufacturing 15(0) .times. 3(12)
.times. 3(10) cable non-extracted, 3 135 70.74 2.97 29.50 4.47 0.52
3 days after manufacturing 15(0) .times. 3(12) .times. 3(10) cable
non-extracted, 3 135 75.90 1.57 34.57 4.12 0.56 4 day after
manufacturing 15(0) .times. 3(12) .times. 3(10) cable
non-extracted, 3 135 71.91 5.71 36.72 3.75 0.53 5 days after
manufacturing 15(0) .times. 3(12) .times. 3(10) cable
non-extracted, 3 135 74.57 1.45 37.67 4.27 0.55 6 days after
manufacturing 13(0) .times. 3(11) .times. cable non-extracted, 4
351 189.01 14.00 45.87 3.72 0.54 3(10) .times. 3(0) dry 13(0)
.times. 3(11) .times. cable non-extracted, 4 351 170.12 7.37 39.95
1.37 0.48 3(10) .times. 3(0) cycled 30.times. to pretension,
dry
[0198]
5TABLE 5 Comparison of UTS and stiffness between wet (2 hr
incubation in PBS at 37.degree. C.) and dry mechanical testing
conditions. N = 5. Results show approximately a 17% drop in UTS as
a function of testing wet. # of Yarn Test UTS UTS Stiffness
Stiffness UTS/fiber Stiffness/fiber Yarn Fibers Conditions (N)
stdev (N/mm) stdev (N) (N/mm) 9(12s) .times. 3(9z) 27 extracted -
dry 21.88 0.18 7.38 0.34 0.81 0.27 9(12s) .times. 3(9z) 27
extracted - wet 18.52 0.25 2.56 0.31 0.69 0.09 9(6s) .times. 3(3z)
27 extracted - dry 21.36 0.40 7.95 1.00 0.79 0.29 9(6s) .times.
3(3z) 27 extracted - wet 17.94 0.30 2.40 0.28 0.66 0.09 9(12s)
.times. 3(6z) 27 extracted - dry 21.80 0.47 7.48 0.97 0.81 0.28
9(12s) .times. 3(6z) 27 extracted - wet 18.74 0.22 2.57 0.11 0.69
0.10 12(0) .times. 3(10s) 36 extracted - dry 30.73 0.46 16.24 0.66
0.85 0.45 12(0) .times. 3(10s) 36 extracted - wet 25.93 0.29 6.68
0.70 0.72 0.19 4(0) .times. 3(10s) .times. 3(9z) 36 extracted - dry
30.07 0.35 15.49 1.06 0.84 0.43 4(0) .times. 3(10s) .times. 3(9z)
36 extracted - wet 22.55 0.66 7.63 1.00 0.63 0.21
[0199]
6TABLE 6 Effect of TPI on UTS and Stiffness. N = 5 UTS Stiffness
UTS/fiber Stiffness/fiber Yarn TPI (N) stdev (N/mm) stdev (N)
(N/mm) 12(0) .times. 3(2) 2 23.27 0.28 6.86 0.60 0.65 0.19 12(0)
.times. 3(4) 4 24.69 0.31 7.61 1.17 0.69 0.21 12(0) .times. 3(6) 6
25.44 0.42 6.51 1.35 0.71 0.18 12(0) .times. 3(8) 8 25.21 0.23 5.80
0.67 0.70 0.16 12(0) .times. 3(10) 10 25.94 0.24 6.45 0.77 0.72
0.18 12(0) .times. 3(12) 12 25.87 0.19 6.01 0.69 0.72 0.17 12(0)
.times. 3(14) 14 22.21 0.58 5.63 0.71 0.62 0.16
[0200]
7TABLE 7 Additional tpi data to verify that up to 30 tpi can be
used without causing damage to the yarn that would result in a
dramatic decrease in UTS and stiffness; note, all matrices (N = 5
per group) were twisted. # of UTS stdev Stiffness stdev UTS/fiber
Stiffness/fiber Yarn fibers (N) (N) (N/mm) (N/mm) (N) (N/mm)
Conditions 1(30) .times. 6(20) .times. 3(4.5) 18 10.92 0.44 1.21
0.02 0.61 0.07 non-extracted, wet 1(30) .times. 6(20) .times. 3(10)
18 11.48 0.37 1.25 0.06 0.64 0.07 non-extracted, wet 1(30) .times.
6(6) 6 3.83 0.24 0.37 0.04 0.64 0.06 non-extracted, wet 15(20) 15
13.19 0.27 6.03 0.67 0.88 0.40 extracted, dry
[0201]
8TABLE 8 Effect of yarn hierarchy on mechanical properties (i.e.
the number of levels and the number of fibers per level can
significantly influence yarn and fabric outcomes. # of Total # UTS
% % Stiffness Stiffness UTS Stiffness levels of of UTS stdev Elong
Elong avg stdev per per Geometry Condition plying Fibers (N) (N)
average stdev (N/mm) (N/mm) fiber fiber 1(0) .times. 3(10)
extracted 2 3 1.98 0.05 10.42 1.63 2.17 0.51 0.66 0.72 1(0) .times.
3(10) .times. 3(9) extracted 3 9 6.86 0.23 13.11 1.45 4.06 0.36
0.76 0.45 1(0) .times. 3(10) .times. 4(9) extracted 3 12 9.29 0.19
14.07 0.98 5.10 0.31 0.77 0.43 1(0) .times. 4(10) extracted 2 4
2.86 0.14 11.98 1.54 2.08 0.31 0.72 0.52 1(0) .times. 4(10) .times.
3(9) extracted 3 12 8.78 0.17 14.25 1.09 5.10 0.32 0.73 0.43 15(0)
.times. 3(12) non-extracted dry 2 45 27.39 0.62 31.68 1.35 4.63
0.49 0.61 0.10 15(0) .times. 3(12) .times. 3(10) non-extracted dry
3 135 73.61 6.00 33.72 5.67 12.33 1.53 0.55 0.09
[0202]
9TABLE 9 Surface modification (RGD and ETO gas sterilization)
effects on extracted silk matrix mechanical properties; PBS was
used as a negative control during modification treatments. # of
Surface Modification/ UTS Stiffness UTS/fiber Stiffness/fiber Yarn
fibers Sterilization (N) stdev (N/mm) stdev (N) (N/mm) 12(0)
.times. 3(10s) 36 Non-treated 25.94 0.24 6.45 0.77 0.72 0.18 12(0)
.times. 3(10s) 36 RGD 23.82 2.10 3.79 2.06 0.66 0.11 12(0) .times.
3(10s) .times. 3(9z) 108 Non-treated 48.89 4.84 9.22 0.84 0.45 0.09
12(0) .times. 3(10s) .times. 3(9z) 108 RGD 55.28 3.28 8.17 0.81
0.51 0.08 4(11s) .times. 3(11z) .times. 36 ETO 18.72 0.45 5.52 0.42
0.52 0.15 3(10s) 4(11s) .times. 3(11z) .times. 36 RGD + ETO 19.30
0.62 4.67 0.3 0.54 0.13 3(10s)
[0203]
10TABLE 10 UTS Stiffness Yield Pt. Elongation (N) (N/mm) (N) (%)
Silk matrix 1 2337 +/- 72 354 +/- 26 1262 +/- 36 38.6 +/- 2.4 Silk
Matrix 2 3407 +/- 63 580 +/- 40 1647 +/- 214 29 +/- 4 Human ACL
2160 +/- 242 +/- 28 .about.1200 .about.26-32% 157 Mechanical
properties for two different cords based on a cord length of 3 cm
as compared to human ACL properties.
[0204]
11TABLE 11 Twisting Level Matrix Matrix Matrix Matrix Matrix Matrix
Matrix (# of twists/cm) 1 2 3 4 5 6 7 # fibers per group 30 15 1300
180 20 10 15 (0) (0) (0) (0) (0) (0) (0) # groups per bundle 6 3 3
3 6 6 3 (3) (3) (2) (3.5) (3) (3) (3) # bundles per strand 3 6 1 3
3 3 3 (3) (2.5) (0) (2) (2) (2.5) (2.5) # strands per cord 6 3 -- 2
3 3 3 (0) (2.0) (0) (1) (2) (2) # cords per ACL -- 6 -- -- 3 6 12
(0) (0) (0) (0) UTS (N) 2337 3407 2780 2300 2500 2300 3400
Stiffness (N/mm) 354 580 300 350 550 500 550 Examples of several
geometry hierarchies that would result in suitable mechanical
properties for replacement of the ACL. Note: Matrix 1 and 2 have
been developed as shown in the examples; Matrix 3 would yield a
single bundle prosthesis, Matrix 4 would yield a 2 strand
prosthesis, Matrix 5 would yield a 3 cord prosthesis, Matrix 6 is
another variation of a 6 cord prosthesis, and Matrix 7 will yield a
12 cord prosthesis.
* * * * *