U.S. patent application number 12/875821 was filed with the patent office on 2010-12-30 for directed assembly of three-dimensional structures with micron-scale features.
This patent application is currently assigned to Board of Trustees of University of Illinois. Invention is credited to Gregory Gratson, Jennifer A. Lewis.
Application Number | 20100330220 12/875821 |
Document ID | / |
Family ID | 33551382 |
Filed Date | 2010-12-30 |
United States Patent
Application |
20100330220 |
Kind Code |
A1 |
Gratson; Gregory ; et
al. |
December 30, 2010 |
DIRECTED ASSEMBLY OF THREE-DIMENSIONAL STRUCTURES WITH MICRON-SCALE
FEATURES
Abstract
Polyelectrolyte inks comprising a solvent, a cationic
polyelectrolyte dissolved in the solvent, and an anionic
polyelectrolyte dissolved in the solvent, are described. The
concentration of at least one of the polyelectrolytes in the
solvent is in a semidilute regime.
Inventors: |
Gratson; Gregory; (Urbana,
IL) ; Lewis; Jennifer A.; (Urbana, IL) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Assignee: |
Board of Trustees of University of
Illinois
Urban
IL
|
Family ID: |
33551382 |
Appl. No.: |
12/875821 |
Filed: |
September 3, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11560610 |
Nov 16, 2006 |
7790061 |
|
|
12875821 |
|
|
|
|
10463834 |
Jun 17, 2003 |
7141617 |
|
|
11560610 |
|
|
|
|
Current U.S.
Class: |
425/150 |
Current CPC
Class: |
B33Y 70/00 20141201;
C09D 11/03 20130101 |
Class at
Publication: |
425/150 |
International
Class: |
B29C 47/92 20060101
B29C047/92 |
Claims
1. An apparatus for the fabrication of microstructures, the
apparatus comprising: (1) a nozzle comprising an opening having a
diameter of at most 10 microns; (2) a stage below the nozzle for
supporting a substrate; (3) a micropositioner operably connected to
at least one of the nozzle and the stage; and (4) an ink reservoir
in fluid communication with the nozzle.
2. The apparatus of claim 1, further comprising a pressure
dispensing system operably connected to the ink reservoir.
3. The apparatus of claim 1, wherein the micropositioner is a
computer-controlled micropositioner.
4. The apparatus of claim 1, wherein the opening of the nozzle has
a diameter of at most 1 micron.
5. The apparatus of claim 1, wherein the opening of the nozzle has
a diameter of at least 0.1 micron.
6. The apparatus of claim 1, further comprising a plurality of
nozzles.
7. The apparatus of claim 6, further comprising a plurality of
substrates on the stage.
8. The apparatus of claim 1, further comprising a substrate on the
stage, the substrate comprising a deposition bath.
9. The apparatus of claim 8, wherein the deposition bath is
configured to induce gelation of an ink flowed through the
nozzle.
10. The apparatus of claim 8, wherein the deposition bath comprises
an aqueous solvent.
11. The apparatus of claim 8, wherein the deposition bath comprises
an apolar solvent.
12. The apparatus of claim 11, wherein the apolar solvent comprises
an alcohol.
13. The apparatus of claim 12, wherein the deposition bath
comprises a mixture of isopropyl alcohol and water.
Description
RELATED APPLICATIONS
[0001] The present patent document is a divisional of U.S. patent
application Ser. No. 11/560,610, which was filed on Nov. 16, 2006,
which is a divisional of U.S. patent application Ser. No.
10/463,834, which was filed on Jun. 17, 2003. The preceding patent
documents are hereby incorporated by reference in their
entirety.
BACKGROUND
[0002] Three-dimensional structures with micron-scale features have
many potential applications, for example as photonic band gap
materials, tissue engineering scaffolds, biosensors, and drug
delivery systems. Consequently, several assembly techniques for
fabricating complex three-dimensional structures with features
smaller than 100 microns have been developed, such as
microfabrication, holographic lithography, two-photon
polymerization and colloidal self assembly. However, all these
techniques have limitations that reduce their utility.
[0003] Two-photon polymerization is capable of creating
three-dimensional structures with sub-micron features, but from
precursors that are not biocompatible. Many techniques have been
developed to fabricate three-dimensional photonic crystals, but
they rely on expensive, complicated equipment or time-consuming
procedures. Colloidal self-assembly has also been utilized to make
three-dimensional periodic structures, but controlling the
formation of defects is difficult.
[0004] One fabrication technique relies on the deposition of
viscoelastic colloidal inks, usually by a robotic apparatus. These
inks flow through a deposition nozzle because the applied pressure
shears the interparticle bonds, inducing a breakdown in the elastic
modulus. The modulus recovers immediately after leaving the nozzle,
and the ink solidifies to maintain its shape and span unsupported
regions. The particles in the ink have a mean diameter of about 1
micron, meaning that it would be impossible for the ink to flow
through a 1 micron diameter deposition nozzle without clogging or
jamming. In practice, nanoparticle inks (mean diameter.about.60 nm)
also tend to jam nozzles smaller than 30 microns, limiting the
applicability of viscoelastic colloidal inks to this length
scale.
[0005] Polymeric solutions are used in nature to fabricate thin
filaments. Spiders, for example, derive their silk fibers from a
concentrated protein biopolymer solution that solidifies as it is
drawn to form an extremely strong filament. The extensional flow of
the solution aligns liquid crystal sheets in the polymer, and the
solution gels by adding ions as it leaves the spinneret. This
process was artificially recreated by the deposition of the
recombinant spider silk biopolymer into a polar "deposition bath"
to produce filament fibers with comparable properties.
SUMMARY
[0006] In a first aspect, the disclosure provides polyelectrolyte
inks comprising a solvent, a cationic polyelectrolyte, dissolved in
the solvent, and an anionic polyelectrolyte, dissolved in the
solvent. The concentration of at least one of the polyelectrolytes
in the solvent is in a semidilute regime.
[0007] In a second aspect, the disclosure provides a solid filament
comprising a complex of a cationic polyelectrolyte and an anionic
polyelectrolyte. The filament has a diameter of at most 10
microns.
[0008] In a third aspect, the disclosure provides a method of
making a polyelectrolyte ink comprising mixing together ingredients
that comprise a solvent, a cationic polyelectrolyte, and an anionic
polyelectrolyte. The concentration of at least one of the
polyelectrolytes in the solvent is in a semidilute regime.
[0009] In a fourth aspect, the disclosure provides a method for
fabricating a filament, comprising flowing the polyelectrolyte ink
through a nozzle, and contacting the ink with a deposition bath.
The polyelectrolyte ink gels in the deposition bath.
[0010] In a fifth aspect, the disclosure provides a method of
forming three-dimensional structure, comprising fabricating a
plurality of filaments, each filament fabricated by the method set
forth in the fourth aspect.
DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 shows viscosity and elastic modulus of
polyelectrolyte mixtures as a function of the mixing ratio of
ionizable groups at a constant polymer volume fraction
(.PHI..sub.poly=0.4).
[0012] FIG. 2 shows the elastic modulus of the ink reacted in a
water/IPA deposition bath as a function of IPA concentration in the
deposition reservoir.
[0013] FIGS. 3A, 3B and 3C are electron micrographs of structures
fabricated through the directed assembly of polyelectrolyte inks.
(A) Four-layer microstructure with a missing rod that may be
utilized as a waveguide in a photonic crystal. (B) Eight-layer
structure with walls showing the ink's ability to form spanning and
space-filling elements. (C) Radial structure showing the inks
ability to turn sharp and broad angles.
DETAILED DESCRIPTION
[0014] The present disclosure provides a method of microstructure
fabrication via deposition of inks that flow through a deposition
nozzle of 10 micron or less, without clogging or jamming. When
deposited in a deposition bath the inks solidify after leaving the
nozzle. The resulting microstructures have features in the micron
scale and are amenable to fabrication with biocompatible materials,
and are relatively easy and inexpensive to make.
[0015] The present disclosure includes the three-dimensional
fabrication of structures with micron-scale features by making use
of an ink. An applied pressure forces the ink through a deposition
nozzle that is attached to a moving x-y-z micropositioner, into a
deposition bath that gels the ink in situ as the micropositioner
moves to form a two-dimensional pattern on the substrate. The
nozzle then incrementally rises in the z (vertical) direction for
the next layer of the pattern. This process is repeated until the
desired three-dimensional structure has been created. With this
technique, any three-dimensional structure can be defined and
fabricated.
[0016] The inks of the present disclosure are concentrated mixtures
of oppositely charged polyelectrolytes, also referred to as
polyelectrolyte complexes (PEC). The PEC contains two oppositely
charged polyelectrolytes (e.g. poly(acrylic acid) and
poly(ethylenimine)). One polyelectrolyte is preferably larger than
the other, and the concentration of the larger polyelectrolyte is
preferably within the semidilute regime: the concentration is above
the concentration c* that separates the dilute from the semidilute
concentration regime. Below c*, in the dilute regime, the mixture
of polyelectrolytes forms particles rather than the single phase
fluid needed for the deposition of a continuous filament. Above c*,
in the semidilute regime, polymer coils strongly overlap with each
other, and the mixture of electrolytes may be used for structure
deposition.
[0017] The ink viscosity is preferably in the range that allows
consistent, controllable flow at a modest applied pressure.
Preferred viscosity values vary between at least 0.05 Pa*sec to at
most 600 Pa*sec. More preferred viscosity values are at least 0.1
Pa*sec to at most 150 Pa*sec. Yet more preferred viscosity values
are at least 1 Pa*sec to at most 20 Pa*sec. Moreover, the ink
undergoes a rapid solidification reaction when it comes in contact
with the deposition bath that allows the extruded filament to
maintain its shape while spanning unsupported regions of the
structure.
[0018] Examples of polyelectrolytes that may be used in PEC are
poly(acrylic acid), poly(ethylenimine), poly(styrene sulfonate)
poly(allylamine) hydrochloride, poly(diallyldimethyl ammonium
chloride), poly(4-vinyl pyridine), and cationic or anionic
surfactants. Electrically or optically active classes of polymers,
for example polyacetylene, polyaniline, polypyrrole, polythiophene,
poly(3,4 ethylenedioxythiophene) (PEDOT), NAFION.RTM. (Du Pont,
Wilmington, Del.), polyphenylene vinylene, polyphenylbenzenamine,
sulfonated poly-p-phenylene azobenzene dye and other organic dyes
may be used, and are well suited to applications involving organic
LEDs and circuits. The parent polymers of some of these classes of
polymers do not contain charged groups, however, copolymers and
derivatives of these classes do; for example charged groups may be
introduced through monomers containing substituents (which may be
protected until after synthesis of the polymer), or by derivatizing
reactive groups (such as hydroxyl groups, or electrophilic addition
on phenyl rings).
[0019] For biochemical, molecular biological and biomedical
applications, such as biocatalysis, gene manipulation and tissue
engineering, biological electrolytes may be used. Example
biological polyelectrolytes are polynucleotides, such as DNA and
RNA, peptides, proteins, peptide nucleic acids, enzymes,
polysaccharides such as starch and cellulose, acidic
polysaccharides such as hemicelluloses (for example
arabinoglucuronoxylan), basic polysaccharides such as poly-(1,4)
N-acetyl-D-glucosamine (chitosan), galactans such as agarose,
polyuronides such as alginic acid, carrageenans, hyaluronic acid,
collagen, fibrin, proteoglycans, polylactic acid, polyglycolic
acid, copolymers of organic acids, cationic lipids. Biological
polyelectrolytes with both positive and negative charges, for
instance zwitterions such as polycarboxybetaine, may also be
included in the ink compositions.
[0020] Bioactive molecules may also be incorporated in the ink, for
example charged or neutral nutrient molecules, molecular messengers
such as growth stimulants, and cellular adhesion molecules.
Molecular probes for biomolecules such as cellular lipids or
cellular membrane proteins, cellular components such as ion
channels and receptors, or organelles such as mitochondria or
lysosomes may also be added.
[0021] Smaller organic and inorganic species can also be
incorporated into the inks, to amounts that do not deleteriously
affect the rheological properties of the ink. Examples include
nanoparticles, quantum dots, charge neutral polymers,
organometallic precursors and biomolecules. These species may
interact with the polyelectrolytes to aid in the gelation or remain
inert in the ink, depending on their ionic nature. Also, many other
polymers may be made into polyelectrolytes through functionalizing
the polymer backbone with charged moieties, for example amino
groups, sulfonate groups, and carboxylic groups.
[0022] The molecular weight of the larger polyelectrolyte is
preferably high enough to facilitate chain overlap (preferably at
least 5000 daltons) but also low enough to form a concentrated ink
with a viscosity that enables flow at moderate pressures
(preferably at most 100,000 daltons). The concentration of the ink
is preferably high to avoid deformation of the structures upon
drying. A typical polymer concentration ranges from at least 5% to
at most 95% by weight. More preferably, the concentration varies
from at least 25% to at most 75% by weight. Yet more preferably,
the concentration varies from at least 35% to at most 45% by
weight. Most preferable concentrations range from at least 38% to
at most 42% by weight.
[0023] The larger polyelectrolyte and the smaller polyelectrolyte
are preferably mixed together in a ratio such that one of the
charge groups is in excess (usually the charge group of the larger
polymer), yielding a mixture away from a stoichiometric (1:1)
cationic:anionic group ratio. In the vicinity of this ratio, the
strong interactions between complementary polyelectrolytes may lead
to the formation of kinetically stable, inhomogeneous aggregates,
and the complex may form two phases, a polymer-rich aggregate and a
polymer-poor fluid.
[0024] Once the polyelectrolytes and solvent have been chosen, a
phase diagram can be developed relating the ratio of cationic to
anionic groups as a function of overall polyelectrolyte
concentration (in the chosen solvent), with the goal of determining
the range for homogeneous inks. This range will be above the
dilute/semidilute transition of the larger polymer and away from a
stoichiometric (1:1) cationic:anionic group ratio. The viscosity of
the homogeneous inks increases as the polymer concentration
increases and as the mixing ratio approaches 1:1. The viscosity may
thus be controlled for the deposition of inks through a variety of
nozzle sizes.
[0025] A deposition bath is selected to fabricate three-dimensional
structures through a rapid solidification reaction. In these
polyelectrolyte inks, the reaction will occur by increasing the
strength of the attractions between the oppositely charged
polyelectrolytes. This can be achieved, for example, through pH
changes, ionic strength changes, solvent composition changes, or
combinations of more than one change. The reaction produces a
filament that is strong enough to maintain its shape while spanning
unsupported regions in the structure, but also soft enough to allow
the filament to adhere to the substrate and flow through the nozzle
consistently.
[0026] Deposition baths that induce gelation through pH changes are
generally used when the polyelectrolytes contain acidic and/or
basic charged groups. The pH change eliminates the excess of one of
the charge groups, for instance by ionizing acidic groups that are
neutral at the pH of the ink. This yields a mixture with a
stoichiometric (1:1) cationic:anionic group ratio that gels into a
filament.
[0027] In addition, the pH of the deposition bath may be selected
in order to induce partial dissolution of the deposited filament,
while the shape is maintained. The pH of the bath lowers the bond
strength between the oppositely charged polyelectrolytes, leading
to the dissolution. The structures have a residual charge on the
surface, and may be used for the adsorption of charged
nanoparticles.
[0028] Coagulation may also be achieved through changes in solvent
composition. For example, an aqueous ink may be deposited in a
deposition bath containing a relatively apolar solvent such as an
alcohol. The resulting drop in dielectric constant leads to an
increase in the coulombic attractions between the polyelectrolytes.
Also, an apolar solvent that is a poor solvent for the
polyelectrolytes may be chosen, leading to increased
polyelectrolyte/polyelectrolyte bonding. The reaction yields a
polyelectrolyte complex precipitate with a positive:negative charge
ratio closer to 1:1 than in the unreacted ink, but not to the
extent of the pH induced reaction. The structures have a residual
charge on the surface, and may be used for the adsorption of
oppositely charged nanoparticles.
[0029] Moreover, the mechanical properties of the deposited ink are
dependent on the composition of the deposition bath. As illustrated
in FIG. 2, different percentages of apolar solvents generally yield
filaments of varying stiffness.
[0030] An apparatus for depositing the ink may be manufactured by
connecting a deposition nozzle with a diameter of preferably at
least 0.1 microns to at most 10 microns to a micropositioner, for
example a computer controlled piezoelectric micropositioner, and an
ink reservoir. These micropositioners are used in a variety of
devices, such as scanning tunneling microscopes, and are
commercially available. Pressure pushes the ink through the nozzle,
or two or more nozzles, and the micropositioner controls the
deposition pattern of the filament. Alternatively, the nozzle (or
nozzles) may be static, while the stage holding the substrate on
which the microstructure is formed may be controlled by the
micropositioner. In another configuration, both the nozzle and the
stage may each be controlled by its own micropositioner. Multiple
substrates are also possible when multiple nozzles are present. The
assembly of structures is then preferably performed using patterns
created in a computer aided design computer program coupled to the
micropositioner.
[0031] There are many applications for solid PEC structures
fabricated with these materials and methods. The structure may be
infiltrated with a high refractive index material and subsequently
the PEC structure redissolved to form photonic crystals. The
ability of the ink to span distances renders it possible to
engineer defects (for example cavities or waveguides) into the
structure for functional photonic band gap materials. The highly
porous structures could be used for membranes that selectively
allow small molecules to flow through at a faster rate. Also,
screens may be prepared that do not allow cells, or cells larger
than a certain size, to flow through. Screens of this type may be
used, for instance, to separate smaller cells from larger cells in
a blood sample. They may also be used for drug delivery systems
where a range of porosities is necessary for controlled
release.
[0032] The charged complexes may be used as tissue engineering
scaffolds for cell adhesion and growth. For instance, copolymers of
poly(L-lactic acid) and poly(L-glycolic acid), both anionic
polyelectrolytes that have been FDA approved as biodegradable
polymers, may be combined with one or more cationic polymers, such
as chitosan, to form a biocompatible ink for tissue engineering
applications. To promote cell growth throughout the structure,
proteins and sugars may be added to the ink to be released as the
polyelectrolytes dissolve.
[0033] Biologically interesting molecules may also be attached to
the microstructures. Examples of these molecules include nucleic
acids, polypeptides and other organic molecules. Nucleic acids
include polynucleotides (having at least two nucleic acids),
deoxyribonucleic acid (DNA), such as expressed sequence tags
(ESTs), gene fragments or complementary DNA (cDNA), or intron
sequences that may affect gene transcription, such as promoters,
enhancers or structural elements. However, the nucleic acid need
not be related to a gene or gene expression, as aptamers (small
nucleic acid sequences that specifically bind to a target molecule)
can also be used. Ribonucleic acids (RNA) may also be used, such as
messenger RNA (mRNA), transfer RNA (tRNA) or ribosomal RNA (rRNA).
Nucleic acids may also be modified; for example, such as
substituting a nucleic acid with a non-naturally occurring one,
such as inosine. Chemical modifications of nucleic acids, such as
those that may confer stability or facilitated immobilization upon
a substrate, may also be used. Another example of a modified
nucleic acid is a peptide nucleic acid (PNA), which is a nucleic
acid mimic (e.g., DNA mimic) in that the deoxyribose phosphate
backbone is replaced by a pseudopeptide backbone and only the four
natural nucleobases are retained. The neutral backbone of PNAs
allows for specific hybridization to DNA and RNA under conditions
of low ionic strength. The synthesis of PNA oligomers can be
performed using standard solid phase peptide synthesis protocols.
Nucleic acids attached to the microstructures may be used, for
example, in diagnostic and prognostic assays, gene expression
arrays, pharmacogenomic assays, etc.
[0034] Polynucleotides may be linked to the microstructures through
thiol-mediated self-assembly attachment to gold nanoparticles
incorporated into the microstructures. The gold may be incorporated
into the microstructures either by addition to the undeposited ink
mixture or by attachment to the microstructures after deposition,
for example via sulfhydril groups present in the ink.
[0035] Polypeptides, having at least two amino acid residues, may
find use on or within the substrates of the application. Examples
of classes of polypeptides include antibodies and derivatives,
protein hormones (e.g., human growth hormone and insulin),
extracellular matrix molecules, such as laminin, collagen or
entactin; polypeptides involved in signaling, such as phosphatases
and kinases; receptors, such as dopamine receptors and hormone
receptors (advantageously may be attached in the native format, or
in the case of homodimers, trimers, etc., mixed with other
polypeptide chains or as single chains), etc. Attaching
polypeptides to the substrates of the disclosure allow for a wide
variety of applications, including drug screening, diagnostic and
prognostic assays, assays that resemble enzyme-linked immunosorbent
assays (ELISAs), proteomic assays and even cellular adhesion
studies.
[0036] Organic molecules also find use on the microstructures. For
example, steroid hormones, such as estrogen and testosterone may be
attached. Such couplings facilitate for screens for molecules that
bind these molecules, such as antibodies or aptamers. Likewise,
candidate small molecule antagonists or agonists may be attached to
facilitate pharmaceutical screening.
[0037] Entities such as prions, viruses, bacteria, and eukaryotic
cells may also be attached. Prions are misfolded protein aggregates
that can propagate their misfolded state onto native proteins;
examples include those aggregates that cause mad cow disease
(bovine spongiform encephalopathy (BSE)) or Creutzfeldt-Jacob
disease. Examples of viruses include herpes simplex,
orthopoxviruses (smallpox) or human immunodeficiency virus.
Bacteria that are of interest may include Vibrio cholera,
Clostridium perfringens, or Bacillus anthracis (anthrax).
Eukaryotic cells, such as those isolated as primary cultures from
subjects or plants, or from cell lines (e.g., those available from
the American Type Culture Collection (ATCC); Manassus, Va.), may be
immobilized onto the microstructures for a variety of purposes,
including screens for pharmaceuticals, investigations into
cell-substrate adhesion, or for the binding of various
molecules.
[0038] Any ink that gels through a solvent change may be used to
assemble three-dimensional structures from electrically, optically
or biologically active polymers. Inorganic structures may also be
fabricated by using sol-gel precursors to produce, for example,
sensors or template-free photonic band gap materials.
EXAMPLES
1) Ink Mixtures
[0039] A linear polyacid, poly(acrylic acid) (MW.about.10,000) and
a highly branched polybase, poly(ethylenimine) were combined in an
aqueous solvent, yielding solutions with a polymer fraction
.PHI..sub.poly=0.4. When these polyions were combined, the
carboxylate groups of poly(acrylic acid) (PAA) form ionic bonds to
the amine groups of poly(ethylenimine) (PEI). The polymers were
initially mixed under mildly acidic conditions (pH.about.3.6),
where the partial charge on the PAA let only a fraction of the
potentially ionizable groups participate in complexation. The
.PHI..sub.poly was maintained constant, and different PAA to PEI
ratios yielded mixtures with varying rheological properties, as
illustrated in the phase diagram of FIG. 1. In this figure, the
values on the left and the right y-axes indicate values for pure
PAA and pure PEI at .PHI..sub.poly=0.4 The dilute-semidilute
crossover concentration c* for PAA is indicated on the bottom
x-axis. The two-phase region consists of a dense, polymer-rich
phase, and a fluid-like, polymer-poor phase, and data could not be
obtained in this regime. As the ratio approaches the two-phase
region, the elastic modulus and the viscosity of the mixtures
increases.
[0040] A homogeneous, single phase was observed at mixing ratios in
the PAA and PEI rich regions. The charge imbalance forms a
non-stoichiometric, hydrophilic complex. The two-phase region, near
stoichiometric mixing ratios, comprises a dense, polymer-rich phase
with a stoichiometric, hydrophobic complex and a fluid-like,
polymer-poor phase.
[0041] The viscosity differences observed at different mixing
ratios may be utilized to assemble structures at different length
scales. At small nozzle sizes, a lower viscosity ink may be
deposited at modest applied pressures, whereas larger nozzle sizes
generally require more viscous inks in order to obtain flows with
controlled rates.
2) Ink Deposition Apparatus
[0042] Inks prepared as described in example 1 were loaded in a
deposition apparatus for microstructure fabrication. The apparatus
comprised a NanoCube.TM. XYZ NanoPositioning System (Polytec PI,
Auburn, Mass.) controlling .mu.-Tip (World Precision Instruments,
Sarasota, Fla.) deposition nozzles, and the ink was dispensed from
the apparatus by a Model 800 ULTRA Dispensing System with a 3 ml
ULTRA Barrel Reservoirs (EFD, Providence, R.I.).
3) Fabrication of Structures in an Isopropanol and Water
[0043] An ink prepared according to the procedure of example 1,
with a .PHI..sub.poly=0.4, a PAA:PEI ratio of 5.7:1, was deposited,
at a velocity of 20 microns/second and through a 1 micron nozzle,
in a deposition bath containing a mixture of isopropanol (IPA) and
water. The gelation occurs due to a decrease in solvent quality for
the polyelectrolytes and an increase in the coulombic attractions
between the ionizable groups, yielding a reacted ink filament. NMR
spectroscopic data (not shown) showed no discernable difference in
the types of bonds in the reacted and unreacted complexes,
indicating that the reaction only causes a change in the number and
strength of the bonds. The mechanical properties of the reacted ink
were highly dependent on the deposition bath, as illustrated in
FIG. 2.
4) Fabrication of Structures in a Water Deposition Bath
[0044] The experiment of Example 3 was repeated, this time using an
ink with a PAA:PEI ratio of .about.4.8 and a deposition bath of
deionized water. The pH change eliminated the excess of the groups
bearing a positive charge by ionizing acidic groups that were
neutral at the pH of the ink. This yielded a mixture with a nearly
stoichiometric cationic:anionic group ratio that gelled into a
filament (Table 1).
TABLE-US-00001 TABLE 1 (-:+) charge Polyelectrolyte Carbon Hydrogen
Nitrogen ratio PAA 39.48 4.80 NA PEI 52.04 11.78 31.68 NA Unreacted
PEC 40.12 5.21 2.27 4.8:1 Reacted PEC 41.92 5.58 2.90 1.1:1
4) Microstructures
[0045] FIGS. 3A to 3C show structures fabricated with a 1
micrometer nozzle in an 83% IPA (balance water) deposition bath.
FIG. 3A shows an FCT structure with a missing filament in the
middle that could be used as a waveguide in a photonic crystal.
FIG. 3B shows an 8-layer structure with walls, showing the ability
to form solid structures as well as spanning elements. FIG. 3C
shows a radial structure with porosity at multiple length scales.
The reacted complex is capable of spanning lengths much greater
than the filament diameter. Waveguides and radial structures were
fabricated, showing the ability to fabricate structural features
with tight or broad angles. Periodic structures with different
feature sizes were also fabricated, wherein the feature size of a
structure is the diameter of its thinnest filament, obtained with
different nozzles and inks (.PHI..sub.poly=0.4 and PAA:PEI
ratio.about.5.7:1; .PHI..sub.poly=0.4 and PAA:PEI ratio.about.5:1;
.PHI..sub.poly=0.43 and PAA:PEI ratio.about.2:1).
[0046] If the deposition occurs in a slightly acidic reservoir,
partial dissolution of the complex occurs, while the shape is
maintained, leading to highly porous structures with lower
elasticity. The structures have a residual negative charge on the
surface, and may be used for the adsorption of nanoparticles.
[0047] The microstructures may also undergo thermal treatment and
maintain their integrity. For example, the microstructures were
heated at 5.degree. C./min to 240.degree. C. in air. The
temperature was held at 240.degree. C. for 30 minutes, and then
cooled at 5.degree. C./min. The structures maintained their
original shape and became harder than prior to the heating. This
hardening may be due also to heat-induced inter-polyelectrolyte
bond formation, for example amide bonds formed between the carboxyl
groups of the PAA and the amine groups in the PEI.
* * * * *