U.S. patent application number 14/463161 was filed with the patent office on 2014-12-04 for interpenetrating polymer network hydrogel.
The applicant listed for this patent is The Board of Trustees of the Leland Stanford Junior University. Invention is credited to Curtis W. Frank, Laura Hartmann, David Myung, Jaan Noolandi, Christopher N. Ta.
Application Number | 20140357559 14/463161 |
Document ID | / |
Family ID | 39887743 |
Filed Date | 2014-12-04 |
United States Patent
Application |
20140357559 |
Kind Code |
A1 |
Myung; David ; et
al. |
December 4, 2014 |
Interpenetrating Polymer Network Hydrogel
Abstract
A strain-hardened interpenetrating polymer network (IPN)
hydrogel is provided. The interpenetrating polymer network hydrogel
is based on two different networks. The first network is a
non-silicone network of preformed hydrophilic non-ionic telechelic
macromonomers chemically cross-linked by polymerization of its
end-groups. The second network is a non-silicone network of
ionizable monomers. The second network has been polymerized and
chemically cross-linked in the presence of the first network and
has formed physical cross-links with the first network. An aqueous
salt solution having a neutral pH is used to ionize and swell the
second network in the interpenetrating polymer network. The
swelling of the second network is constrained by the first network,
and this constraining effect results in an increase in effective
physical cross-links within the interpenetrating polymer network,
and, in turn, an increase its elastic modulus. The strain-hardened
interpenetrating polymer network hydrogel is attractive and useful
for medical, industrial, and personal hygiene purposes.
Inventors: |
Myung; David; (San Jose,
CA) ; Hartmann; Laura; (Berlin, DE) ;
Noolandi; Jaan; (La Jolla, CA) ; Ta; Christopher
N.; (Palo Alto, CA) ; Frank; Curtis W.;
(Cupertino, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Board of Trustees of the Leland Stanford Junior
University |
Palo Alto |
CA |
US |
|
|
Family ID: |
39887743 |
Appl. No.: |
14/463161 |
Filed: |
August 19, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12070336 |
Feb 15, 2008 |
8821583 |
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14463161 |
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11243952 |
Oct 4, 2005 |
7857849 |
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12070336 |
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11636114 |
Dec 7, 2006 |
7857447 |
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12070336 |
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11409218 |
Apr 20, 2006 |
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12070336 |
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11639049 |
Dec 13, 2006 |
7909867 |
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12070336 |
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60901805 |
Feb 16, 2007 |
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60616262 |
Oct 5, 2004 |
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60673172 |
Apr 20, 2005 |
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60843942 |
Sep 11, 2006 |
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60783307 |
Mar 17, 2006 |
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60673600 |
Apr 21, 2005 |
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60843942 |
Sep 11, 2006 |
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Current U.S.
Class: |
514/9.6 ;
514/1.1; 514/20.9; 514/23; 514/44R; 514/772.6 |
Current CPC
Class: |
A61K 47/60 20170801;
A61K 47/58 20170801; A61K 47/6903 20170801; A61L 27/38 20130101;
A61L 27/52 20130101 |
Class at
Publication: |
514/9.6 ;
514/772.6; 514/1.1; 514/23; 514/44.R; 514/20.9 |
International
Class: |
A61K 47/48 20060101
A61K047/48 |
Claims
1. A pre-stressed interpenetrating polymer network hydrogel,
comprising: (a) a first non-silicone network of self-linked
non-ionic telechelic macromonomers covalently bonded to themselves
and to others through their end-groups in said first network; (b) a
second non-silicone network physically entangled with said first
network wherein said second network is a network of crosslinked
charged polymers formed through free radical polymerization; and
(c) an aqueous salt solution having a neutral pH, wherein said
aqueous salt solution has charged and swollen said second network
in said interpenetrating polymer network hydrogel, wherein said
swelling of said second network is constrained by said first
network, and wherein said swelling of said second network induces
an osmotic pressure within said first network resulting in said
pre-stressed interpenetrating polymer network hydrogel.
2. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein each of the self-linked non-ionic macromonomers in
said first network has a molecular weight between about 275 Da to
about 20,000 Da.
3. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said aqueous salt solution has a pH in the range
of about 6 to 8.
4. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said first network comprises at least about 50% by
dry weight telechelic macromonomers.
5. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said first network comprises monomers grafted onto
said first network.
6. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said second network further comprises
macromonomers grafted onto said second polymer network.
7. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said pre-stressed interpenetrating polymer network
hydrogel has a tensile strength of at least about 1 MPa.
8. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said pre-stressed interpenetrating polymer network
hydrogel has an initial Young's modulus of at least about 1
MPa.
9. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said pre-stressed interpenetrating polymer network
hydrogel has an oxygen permeability of at least 15 Barrers.
10. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said pre-stressed interpenetrating polymer network
hydrogel has an equilibrium water content of at least 50%.
11. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein said pre-stressed interpenetrating polymer network
hydrogel is at least about 70% transparent.
12. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein the coefficient of friction of said pre-stressed
interpenetrating polymer network hydrogel in an aqueous solution is
less than 0.2.
13. The interpenetrating polymer network hydrogel as set forth in
claim 1, further comprising biomolecules tethered to the surface of
said pre-stressed interpenetrating polymer network hydrogel.
14. The interpenetrating polymer network hydrogel as set forth in
claim 1, further comprising biomolecules that support cell
adhesion.
15. The interpenetrating polymer network hydrogel as set forth in
claim 1, wherein the degree of chemical crosslinks in said second
network is less than the degree of chemical crosslinks in said
first network.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/070,336 filed Feb. 15, 2008, now U.S. Pat.
No. 8,821,583 issued on Sep. 2, 2014, which is incorporated herein
by reference.
[0002] U.S. patent application Ser. No. 12/070,336 filed Feb. 15,
2008 claims priority from U.S. Provisional Application Nos.
60/901,805, filed on Feb. 16, 2007, which is incorporated herein by
reference.
[0003] U.S. patent application Ser. No. 12/070,336 filed Feb. 15,
2008, now U.S. Pat. No. 8,821,583 issued on Sep. 2, 2014, is a
continuation-in part of U.S. patent application Ser. No.
11/243,952, filed Oct. 4, 2005, which is incorporated herein by
reference. U.S. patent application Ser. No. 11/243,952, filed Oct.
4, 2005 claims the benefit of U.S. Provisional Applications
60/616,262 filed on Oct. 5, 2004 and 60/673,172 filed on Apr. 20,
2005.
[0004] U.S. patent application Ser. No. 12/070,336 filed Feb. 15,
2008, now U.S. Pat. No. 8,821,583 issued on Sep. 2, 2014, is also a
continuation-in-part of U.S. application Ser. No. 11/636,114, filed
Dec. 7, 2006, which is incorporated herein by reference. U.S.
application Ser. No. 11/636,114, filed Dec. 7, 2006 claims the
benefit of U.S. Provisional Application Nos. 60/843,942, filed on
Sep. 11, 2006, and 60/783,307, filed Mar. 17, 2006.
[0005] U.S. patent application Ser. No. 12/070,336 filed Feb. 15,
2008, now U.S. Pat. No. 8,821,583 issued on Sep. 2, 2014, is
further a continuation-in-part of U.S. application Ser. No.
11/409,218, filed Apr. 20, 2006, which is incorporated herein by
reference. U.S. application Ser. No. 11/409,218, filed Apr. 20,
2006 claims the benefit of U.S. Provisional Application No.
60/673,600, filed Apr. 21, 2005.
[0006] U.S. patent application Ser. No. 12/070,336 filed Feb. 15,
2008, now U.S. Pat. No. 8,821,583 issued on Sep. 2, 2014, is
further a continuation-in-part of U.S. application Ser. No.
11/639,049, filed Dec. 13, 2006, which is incorporated herein by
reference. U.S. application Ser. No. 11/639,049, filed Dec. 13,
2006 claims the benefit of U.S. Provisional Application No.
60/843,942, filed on Sep. 11, 2006.
FIELD OF THE INVENTION
[0007] The present invention relates generally to interpenetrating
polymer network hydrogels. More particularly, the present invention
relates to materials useful for medical, industrial, and personal
hygiene purposes including but not limited to orthopedic
prostheses, ophthalmic implants and lenses, artificial tissues and
organs, cell scaffolds, transplantation vehicles, absorbent
diapers, feminine hygiene products, biosensors, surface coatings,
shock-absorbing materials, and lubricating materials.
BACKGROUND OF THE INVENTION
[0008] Hydrogels are water-swollen polymers that are useful in a
variety of biomedical device applications due to their
biocompatibility, high water content, and in some cases,
responsiveness to stimuli. Unfortunately, the mechanical fragility
of most hydrogels poses a formidable obstacle to their application
in many applications, which require a high elastic modulus and high
mechanical strength. Although a number of strategies--such as high
crosslinking density, fiber-reinforcement, and
copolymerization--can be used to improve the strength of hydrogels,
the enhancement afforded by these often involves some compromise in
the desired characteristics of the original material, such as
hydrophilicity, transparency, or permeability. For many tissue
replacement applications, maintenance of these properties is
critical to their performance/in vivo/. Accordingly, there is a
need in the art to develop hydrogels with high values for Young's
modulus and tensile strength that would at least overcome some of
these disadvantages. The present invention addresses these needs
and provides a strain-hardened interpenetrating polymer network
hydrogel with high elastic modulus and a method for fabricating
this material.
SUMMARY OF THE INVENTION
[0009] The present invention provides a strain-hardened
interpenetrating polymer network (IPN) hydrogel. The
interpenetrating polymer network hydrogel is based on two different
networks. The first network is a non-silicone network of preformed
hydrophilic non-ionic telechelic macromonomers chemically
cross-linked by polymerization of its end-groups. The second
network is a non-silicone network of ionizable monomers. The second
network has been polymerized and chemically cross-linked in the
presence of the first network and has formed physical cross-links
with the first network. Within the interpenetrating polymer
network, the degree of chemical cross-linking in the second network
is less than the degree of chemical cross-linking in the first
network. An aqueous salt solution having a neutral pH is used to
ionize and swell the second network in the interpenetrating polymer
network. The swelling of the second network is constrained by the
first network, and this constraining effect results in an increase
in effective physical cross-links within the interpenetrating
polymer network. The strain-induced increase in physical
cross-links is manifested as a strain-hardened interpenetrating
polymer network with an increased initial Young's modulus, which is
larger than the initial Young's modulus of either (i) the first
network of hydrophilic non-ionic telechelic macromonomers swollen
in pure water or in an aqueous salt solution, (ii) the second
network of ionized monomers swollen in pure water or in an aqueous
salt solution, or (iii) the interpenetrating polymer network
hydrogel formed by the combination of the first and second network
swollen in pure water. The observed increase in Young's modulus as
a result of strain (induced herein by swelling) is caused by an
increase in the number of physical cross-links within the
interpenetrating polymer network. For the purposes of the present
invention, strain-hardening is defined as an increase in the number
of physical cross-links and Young's modulus with applied
strain.
[0010] The interpenetrating polymer network of the present
invention could be varied according to the following embodiments
either by themselves or in any combinations thereof. For example,
the hydrophilic non-ionic macromonomer in the first network has a
molecular weight between about 275 Da to about 20,000 Da, about
1000 Da to about 10,000 Da, or about 3000 Da to about 8000 Da. In
another example, the molar ratio between the ionizable monomers and
the hydrophilic non-ionic telechelic macromonomers is greater than
or equal to 1:1 or greater than 100:1. In still another example,
the aqueous salt solution has a pH in the range of about 6 to 8. In
still other examples, the first network has at least about 50%, at
least 75% or at least 95% by dry weight telechelic macromonomers.
In still another example, the first network has hydrophilic
monomers grafted onto the first network. In still another example,
the second network further has hydrophilic macromonomers grafted
onto the second polymer network. In still another example, the
strain-hardened interpenetrating polymer network hydrogel has a
tensile strength of at least about 1 MPa. In still another example,
the strain-hardened interpenetrating polymer network hydrogel has
an initial Young's modulus of at least about 1 MPa. In still
another example, the strain-hardened interpenetrating polymer
network hydrogel has an oxygen permeability of at least 15 Barrers.
In still another example, the strain-hardened interpenetrating
polymer network hydrogel has an equilibrium water content of at
least 50%. In still another example, the strain-hardened
interpenetrating polymer network hydrogel is at least about 70%
transparent. In still another example, the coefficient of friction
of the strain-hardened interpenetrating polymer network hydrogel in
an aqueous solution is less than 0.2. In still another example,
biomolecules are tethered to the surface of the strain-hardened
interpenetrating polymer network hydrogel using azide-active-ester
linkages. In one example, the biomolecules could be used to support
cell adhesion.
[0011] At least some of the characteristics of the strain-hardened
interpenetrating polymer network hydrogel make this new hydrogel
advantageous over conventional hydrogels. Accordingly, the
strain-hardened interpenetrating polymer network hydrogel is
attractive and useful for medical, industrial, and personal hygiene
purposes including but not limited to orthopedic implants,
ophthalmic implants and lenses, contact lenses, artificial corneas,
artificial cartilage, artificial tissues and organs, cell
scaffolds, transplantation vehicles, absorbent diapers, feminine
hygiene products, biosensors, surface coatings, shock-absorbing
materials, and lubricating materials.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 shows according to an embodiment of the present
invention a mechanically enhanced interpenetrating polymer network
(IPN) hydrogel based on an end-linked first network and an ionized
second network.
[0013] FIG. 2 shows the steps required for synthesis of an IPN
hydrogel according to the present invention. [0014] A. The starting
material for the hydrogel is a solution of telechelic macromonomers
(left) with functional end groups (circles) dissolved in water. The
telechelic macromonomers are polymerized to form a first,
water-swollen polymer network (right). [0015] B. hydrophilic,
ionizable monomers (stars) mixed with water are added to the first
polymer network (left) along with a photoinitiator and a
crosslinking agent. The hydrophilic, ionizable monomers are then
photopolymerized and cross-linked in the presence of first polymer
network to form second polymer network in the presence of the
first. This results in formation of a water-swollen IPN hydrogel
(FIG. 2B, right). [0016] C. The water-imbibed IPN is then immersed
in a salt-containing solution at a typical pH of 7.4 and is swollen
to equilibrium, yielding an unusual simultaneous increase in both
the water content and Young's modulus of the IPN. Despite having
higher water content, the IPN on the right has a higher modulus
compared to the IPN on the left due strain hardening induced by
swelling of the second network within constraint posed by the
highly crosslinked first network.
[0017] FIG. 3 A. shows according to an embodiment of the present
invention method steps of forming a telechelic macromonomeric first
network and linear macromolecules and/or biomacromolecules. A
mixture of the first and second polymeric components is made, and
then the telechelic macromonomers are reacted under UV light to
form the first network in the presence of the second. If the second
network is crosslinked chemically, then it is a fully
interpenetrating network. If it is not (and only physically
crosslinked), then it is a semi-interpenetrating network. [0018] B.
shows according to an embodiment of the present invention method
steps of how a first network is formed from monomers (stars).
Exposure to UV light in the presence of a photoinitiator and
crosslinker (not shown) leads to polymerization and crosslinking to
form a network. [0019] C. shows according to an embodiment of the
present invention method steps of how an IPN is formed from a
monomer-based first network. The first network is swollen with the
second network precursor monomers (stars), a crosslinking agent
(not shown) and a photoinitiator (not shown). Exposure to UV light
initiates polymerization and crosslinking of the second network in
the presence of the first to form the IPN. [0020] D. shows
according to an embodiment of the present invention method steps of
how an IPN is formed from monomer-based first network and linear
macromolecules and/or biomacromolecules. A mixture of the monomers
and macromolecules is made, and then the monomers are reacted under
UV light to form the first network in the presence of the second.
If the second network is crosslinked chemically, then it is a fully
interpenetrating network. If it is not (and only physically
crosslinked), then it is a semi-interpenetrating network.
[0021] FIG. 4 shows according to embodiments of the present
invention: (A) an IPN with two different polymers, differentiated
by black lines (410) and grey lines (420), (B) an IPN with a
graft-copolymer (430) of the two polymers in the first network and
a homopolymer in the second network (420), (C) an IPN with a
homopolymer (410) in the first network and a graft-copolymer of the
two polymers in the second network (440), and (D) an IPN with
graft-copolymers (430, 440) of the two polymers in both the first
and the second networks.
[0022] FIG. 5 shows according to an embodiment of the present
invention a schematic of the synthesis of telechelic PEG-diacrylate
from a PEG-diol macromonomer. To generate PEG-dimethacrylate,
methacryloyl chloride would be reacted with the PEG-diol instead of
acryloyl chloride.
[0023] FIG. 6 shows according to an embodiment of the present
invention a schematic of the synthesis of telechelic
PEG-diacrylamide from a PEG-diol macromonomer. To generate
PEG-dimethacrylamide, methacryloyl chloride would be reacted with
the PEG-diol instead of acryloyl chloride.
[0024] FIG. 7 shows according to an embodiment of the present
invention a schematic of the synthesis of telechelic PEG-allyl
ether from a PEG-diol macromonomer.
[0025] FIG. 8 A. shows according to an embodiment of the present
invention true stress versus true strain curves for PEG-DA single
networks of MW 3400 (.tangle-solidup.), 4600 ( ), 8000
(.box-solid.), and 14000 (). [0026] B. shows according to an
embodiment of the present invention true stress versus true strain
curves for PEG/PAA IPNs with PEG MW 3400 (.tangle-solidup.), 4600 (
), 8000 (.box-solid.), and 14000 (). The intersection (*) between
the initial and final tangents to each curve defines the critical
strain (.epsilon..sub.crit) for strain hardening in each IPN.
[0027] FIG. 9 A. shows according to an embodiment of the present
invention true stress-true strain curves for PEG(8.0 k)/PAA IPN,
PEG(8.0 k)-PAA copolymer, PEG(8.0 k), and PAA networks. [0028] B.
shows according to an embodiment of the present invention
normalized true stress-true strain curves for PEG(8.0 k)/PAA IPN,
PEG(8.0 k)-PAA copolymer, PEG(8.0 k), and PAA networks.
[0029] FIG. 10 shows according to an embodiment of the present
invention: (a) pH-dependence of the stress at break
(.sigma..sub.break) and water content for PEG(8.0 k)/PAA IPNs and
PAA single networks, (b) pH-dependence of the strain at break
(.epsilon..sub.break) and water content for PEG(8.0 k)/PAA IPNs and
PAA single networks, (c) pH-dependence of the initial modulus
(E.sub.o) and water content for PEG(8.0 k)/PAA IPNs and PAA single
networks.
[0030] FIG. 11 shows according to an embodiment of the present
invention: A. true stress per unit polymer versus true strain
curves for PAA in pH 3-6, B. true stress per unit polymer versus
true strain curves for PEG(8.0 k)/PAA in pH 3-6.
[0031] FIG. 12 shows according to an embodiment of the present
invention show a PEG/PAA hydrogel in (a) the dry state, (b) the
partially-swollen state, and (c) the fully, equilibrium-swollen
state.
[0032] FIG. 13 shows according to an embodiment of the present
invention: A. appearance of a PEG/PAA IPN based on PEG MW 4600 in
the dried state. B. appearance of the PEG/PAA IPN shown in FIG. 13A
after being immersed for 40 minutes in PBS, pH 7.4.
[0033] FIG. 14 shows according to an embodiment of the present
invention time-dependence of the water content of a single network
PEG hydrogels and PEG/PAA IPNs with different amounts of acrylic
acid (AA) at the time of polymerization. The hydrogels were placed
in deionized water in the dry state at time=0 and then weighed at
regular intervals.
[0034] FIG. 15 shows according to an embodiment of the present
invention true stress versus true strain curves of the PEG(4.6
k)/PAA IPN in PBS and deionized water, as well as the PEG and PAA
single networks in PBS and deionized water. The PEG(4.6 k) network
is unaffected by the change from water to PBS. The arrow indicates
the shift in the stress-strain profile of the IPN after it has been
strain-hardened by swelling to equilibrium in PBS.
[0035] FIG. 16 shows according to an embodiment of the present
invention the stress-strain profile of a PEG/PAA IPN prepared from
a PEG-diacrylamide first network and a PAA second network
crosslinked with N,N'-(1,2-dihydroxyethylene)bisacrylamide and
swollen to equilibrium in PBS at pH 7.4. The strain-hardened
mechanical properties of this IPN are similar to those of
acrylate-based IPN system in FIG. 15.
[0036] FIG. 17 shows according to an embodiment of the present
invention effect of ionic strength on the water content of the
PEG(8 k)/PAA IPN
[0037] FIG. 18 shows according to an embodiment of the present
invention the effect of ionic strength on the stress-strain
behavior of PEG(8.0 k)/PAA IPNs
[0038] FIG. 19 shows according to an embodiment of the present
invention the effect of varying the acrylic acid (AA) volume
fraction in the preparation of PEG(3.4 k)-based IPNs. Increasing
the AA volume fraction from 0.5 (.box-solid.) to 0.7 ( ) and 0.8
(.tangle-solidup.) in the IPN leads to an increase in Young's
modulus.
[0039] FIG. 20 shows according to an embodiment of the present
invention the ffect of the mass fraction of AA monomer in the
second network precursor solution on the volume change in the
resultant IPN. The vertical dotted line indicates the point of
equimolar amounts of AA and ethylene glycol (EG) monomer units in
the IPN, while the horizontal dotted line indicates where the PEG
network and the PEG/PAA IPN have the same volume.
[0040] FIG. 21 shows according to an embodiment of the present
invention the dependence of the fracture stress and Young's modulus
of the PEG/PAA IPN on the mass fraction of AA in the IPN. The
vertical dotted line indicates the point of equimolar amounts of AA
and ethylene glycol (EG) monomer units in the IPN.
[0041] FIG. 22 shows according to an embodiment of the present
invention the effect of copolymerizing HEA into the second network
of PEG(8000)/PAA IPNs on stress-strain behavior.
[0042] FIG. 23 shows according to an embodiment of the present
invention the effect of neutralizing the AA monomer solution prior
to polymerization ("pre-neutralized") on the stress-strain behavior
of a PEG(3.4 k)/PAA IPN (black) compared to a PEG(3.4 k)/PAA IPN
prepared under acidic conditions and neutralized after
polymerization ("post-neutralized").
[0043] FIG. 24 shows according to an embodiment of the present
invention the effect of the extension rate on the stress-strain
behavior of post-neutralized PEG(3.4 k)/PAA IPNs.
[0044] FIG. 25 shows according to an embodiment of the present
invention method steps of how the IPN could function as an
absorbent material for infant diapers or feminine hygiene products.
Exposure of the dried hydrogel to bodily fluids such as blood or
urine causes the gel to soak up the water and solutes, leading to a
swollen, expanded hydrogel.
[0045] FIG. 26 shows according to an embodiment of the present
invention illustrations and photos (insets) of the structures of
natural cartilage (left) and PEG/PAA (right).
[0046] FIG. 27 shows according to an embodiment of the present
invention an example showing that because PEG/PAA imitates the
structure and properties of natural cartilage, it should recreate
the moist lubricity of a cartilaginous joint, which is mediated by
a persistent fluid film at the joint interface. The persistence of
this film is made possible by movement of water out of hydrated
joint tissue, the constituents of the synovial fluid, and by the
abundance of negative charges.
[0047] FIG. 28 shows according to an embodiment of the present
invention PEG/PAA and natural cartilage contain similar amounts of
water.
[0048] FIG. 29 shows according to an embodiment of the present
invention indentation force versus time profiles for the PEG/PAA
hydrogel during indentation.
[0049] FIG. 30 shows according to an embodiment of the present
invention a comparison of dynamic coefficients of friction between
PEG/PAA, cartilage, and UHMWPE under various conditions.
[0050] FIG. 31 A. shows according to an embodiment of the present
invention the true tensile stress-strain profile of a PEG/PAA
hydrogel with 65% water. [0051] B. shows according to an embodiment
of the present invention the tensile creep profile of a PEG/PAA
hydrogel with 65% water.
[0052] FIG. 32 shows according to an embodiment of the present
invention an example showing that like cartilage, PEG/PAA is
extremely strong despite being made of mostly water (65%). Shown
here is an unconfined compression test in which a load of over 900
N (>200 pounds) was applied to a small cylindrical specimen (14
mm in diameter, 6.0 mm thick) without causing it to fracture.
[0053] FIG. 33 shows according to an embodiment of the present
invention compressive stress versus compressive strain of (a)
PEG/PAA IPNs and (b) PEG single networks.
[0054] FIG. 34 shows according to an embodiment of the present
invention a stress-strain profile of a PEG(3.4 k)/PAA IPN under
unconfined compression.
[0055] FIG. 35 shows according to an embodiment of the present
invention a PEG/PAA IPN under confined compression.
[0056] FIG. 36 shows according to an embodiment of the present
invention observations of PEG/PAA (left) and UHMWPE (right) after a
pin-on-disc wear test.
[0057] FIG. 37 shows according to an embodiment of the present
invention photographs of PEG/PAA after 100,000 cycles in a custom
made, hydrogel-on-hydrogel wear-tester.
[0058] FIG. 38 shows according to an embodiment of the present
invention examples of a PEG/PAA IPN molded into a hemispherical
shape. The curved hydrogel was molded by photopolymerization in a
curved plastic mold and then swollen to equilibrium in phosphate
buffered saline (pH 7.4).
DETAILED DESCRIPTION
[0059] The present invention provides an interpenetrating polymer
network (IPN) hydrogel network based on a neutral cross-linked
network of end-linked macromonomers in the first network and an
ionizable crosslinked polymer in the second network. In one of the
embodiments, the first network is composed of end-linked
poly(ethylene glycol) macromonomers with defined molecular weight.
The second network is, in contrast, a loosely crosslinked,
ionizable network of poly(acrylic acid) (PAA). A photograph of the
swollen PEG/PAA hydrogel is shown in FIG. 1. This PEG/PAA IPN has
high tensile strength, high compressive strength, and a low
coefficient of friction when swollen in phosphate buffered saline
at a pH of 7.4.
[0060] Homopolymer networks of PEG and PAA are both relatively
fragile materials (the former is relatively brittle, the latter is
highly elastic), so neither would be expected to make the sole
contribution to mechanical strength enhancement. However, the two
polymers can form complexes through hydrogen bonds between the
ether groups on PEG and the carboxyl groups on PAA. This
interpolymer hydrogen bonding enhances their mutual miscibility in
aqueous solution, which, in turn, yields optically clear,
homogeneous polymer blends. By loosely cross-linking (instead of
densely cross-linking) the ionizable network (PAA, pk.sub.a, =4.7),
large changes in its network configuration can be induced by
changing the pH of the solvent without affecting the neutral PEG
network. At a pH greater than 4.7, the PAA network becomes charged
and swells; at a pH lower than 4.7, the PAA network is protonated
and contracts.
[0061] FIG. 2A shows the steps required for synthesis of an IPN
hydrogel according to the present invention. The starting material
for the hydrogel is a solution of telechelic macromonomers (left)
with functional end groups (circles) dissolved in water (not
shown). The telechelic macromonomers are polymerized to form a
first, water-swollen polymer network (right). Next, (FIG. 2B)
hydrophilic, ionizable monomers (stars) mixed with water are added
to the first polymer network (left) along with a photoinitiator and
a crosslinking agent. The hydrophilic, ionizable monomers are then
photopolymerized and cross-linked in the presence of first polymer
network to form second polymer network in the presence of the
first. This results in formation of a water-swollen IPN hydrogel
(FIG. 2B, right). The water-imbibed IPN is then immersed in a
salt-containing solution at pH 7.4 (FIG. 2C), and is swollen to
equilibrium, yielding an unusual simultaneous increase in both the
water content and Young's modulus of the IPN. Despite having higher
water content, the IPN on the right in FIG. 2C has a higher Young's
modulus compared to the IPN on the left. This increase in modulus
as a result of strain (induced in this case by swelling) is caused
by an increase in the number of physical crosslinks within the IPN.
For the purpose of the present invention, strain hardening is
defined as an increase in physical crosslinks and Young's modulus
with applied strain. When the IPN is strain-hardened, it is
effectively "pre-stressed" in the sense that stress is built into
the IPN network, yielding a material with an increased Young's
modulus relative to its unstrained state.
[0062] Any hydrophilic telechelic macromonomer may be used to form
the first polymer network. In a preferred embodiment, preformed
polyethylene glycol (PEG) macromonomers are used as the basis of
the first network. PEG is biocompatible, soluble in aqueous
solution, and can be synthesized to give a wide range of molecular
weights and chemical structures. The hydroxyl end-groups of the
bifunctional glycol can be modified into crosslinkable end-groups.
End-group or side-group functionalities to these macromolecules and
biomacromolecules may include, but are not limited to, acrylate
(e.g. PEG-diacrylate), methacrylate, vinyl, allyl, N-vinyl
sulfones, methacrylamide (e.g. PEG-dimethacrylamide), and
acrylamide (e.g. PEG-diacrylamide). For instance, PEG macromonomers
can be chemically modified with endgroups such as diacrylates,
dimethacrylates, diallyl ethers, divinyls, diacrylamides, and
dimethacrylamides. These endgroups can be added to other
macromonomers, such as polycarbonate, poly(N-vinyl pyrrolidone),
polyurethane, poly(vinyl alcohol), polysacchrarides (e.g. dextran),
biomacromolecules (e.g. collagen) and derivatives or combinations
thereof. The first network can also be copolymerized with any
number of other polymers including but not limited to those based
on acrylamide, hydroxyethyl acrylamide, N-isopropylacrylamide,
polyurethane, 2-hydroxyethyl methacrylate, polycarbonate,
2-hydroxyethyl acrylate or derivatives thereof
[0063] FIG. 3A shows a schematic of how an IPN is formed from a
telechelic macromonomeric first network and linear macromolecules
and/or biomacromolecules. A mixture of the first and second
polymeric components is made, and then the telechelic macromonomers
are reacted under UV light to form the first network in the
presence of the second. If the second network is crosslinked
chemically, then it is a fully interpenetrating network. If it is
not (and only physically crosslinked), then it is a
semi-interpenetrating network.
[0064] FIG. 3B shows a schematic of how a first network is formed
from monomers (stars). Exposure to UV light in the presence of a
photoinitiator and crosslinker (not shown) leads to polymerization
and crosslinking to form a network.
[0065] FIG. 3C shows a schematic of how an IPN is formed from a
monomer-based first network. The first network is swollen with the
second network precursor monomers (stars), a crosslinking agent
(not shown) and a photoinitiator (not shown). Exposure to UV light
initiates polymerization and crosslinking of the second network in
the presence of the first to form the IPN.
[0066] FIG. 3D shows a schematic of how an IPN is formed from
monomer-based first network and linear macromolecules and/or
biomacromolecules. A mixture of the monomers and macromolecules is
made, and then the monomers are reacted under UV light to form the
first network in the presence of the second. If the second network
is crosslinked chemically, then it is a fully interpenetrating
network. If it is not (and only physically crosslinked), then it is
a semi-interpenetrating network.
[0067] Preferably, the hydrophilic monomer in the second network is
ionizable and anionic (capable of being negatively charged). In a
preferred embodiment, poly(acrylic acid)(PAA) hydrogel is used as
the second polymer network, formed from an aqueous solution of
acrylic acid monomers. Other ionizable monomers include ones that
contain negatively charged carboxylic acid or sulfonic acid groups,
such as methacrylic acid, 2-acrylamido-2-methylpropanesulfonic
acid, hyaluronic acid, heparin sulfate, chondroitin sulfate, and
derivatives, or combinations thereof. The second network monomer
may also be positively charged or cationic. The hydrophilic monomer
may also be non-ionic, such as acrylamide, methacrylamide,
N-hydroxyethyl acrylamide, N-isopropylacrylamide,
methylmethacrylate, N-vinyl pyrrolidone, 2-hydroxyethyl
methacrylate, 2-hydroxyethyl acrylate or derivatives thereof. These
can be copolymerized with less hydrophilic species such as
methylmethacrylate or other more hydrophobic monomers or
macromonomers. Crosslinked linear polymer chains (i.e.
macromolecules) based on these monomers may also be used in the
second network, as well as biomacromolecules such as proteins and
polypeptides (e.g. collagen, hyaluronic acid, or chitosan).
[0068] Adding a photoinitiator to an aqueous solution of the
end-linkable macromonomers in water and exposing the solution to UV
light results in the crosslinking of the PEG macromonomers, giving
rise to a PEG hydrogel. Polymerizing and crosslinking a second
network inside the first network will give rise to the IPN
structure. Preparing IPN hydrogels through free-radical
polymerization has the additional advantage that it will enable the
use of molds to form hydrogels of desired shape. The free-radical
polymerization can be initiated through UV irradiation--in which
case transparent molds can be used--or through other means such as
thermal-initiation in which non-transparent molds can be used.
Preferably, the first polymer network contains at least 50%, more
preferably at least 75%, most preferably at least 95% of the
telechelic macromonomer by dry weight. Other solutions including
buffers and organic solvents (or mixtures thereof) may also be used
to dissolve the first network macromonomers or second network
monomers.
[0069] Any type of compatible cross-linkers may be used such as,
for example, ethylene glycol dimethacrylate, ethylene glycol
diacrylate, diethylene glycol dimethacrylate (or diacrylate),
triethylene glycol dimethacrylate (or diacrylate), tetraethylene
glycol dimethacrylate (or diacrylate), polyethylene glycol
dimethacrylate, or polyethylene glycol diacrylate, methylene
bisacrylamide, N,N'-(1,2-dihydroxyethylene) bisacrylamide,
derivatives, or combinations thereof. Any number of photoinitiators
can also be used. These include, but are not limited to,
2-hydroxy-2-methyl-propiophenone and
2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone.
[0070] The following description refers to an exemplary embodiment
of a strain-hardened interpenetrating polymer network hydrogel with
PEG as a first network polymer and PAA as a second network polymer.
The IPN hydrogel is synthesized by a (two-step) sequential network
formation technique based on UV initiated free radical
polymerization. A precursor solution for the first network is made
of purified, telechelic PEG dissolved in phosphate buffered saline
(PBS) solution, water, or an organic solvent with, either
2-hydroxy-2-methyl-propiophenone or
2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone.as the
UV sensitive free radical initiator. The types of telechelic PEG
macromonomers used were PEG-diacrylate, PEG-dimethacrylate,
PEG-diacrylamide, and PEG-diallyl ether. In other embodiments,
either network can be synthesized by free radical polymerization
initiated by other means, such as thermal-initiation and other
chemistries not involving the use of ultraviolet light. In the case
of UV polymerization, the precursor solution is cast in a
transparent mold and reacted under a UV light source at room
temperature. Upon exposure, the precursor solution undergoes a
free-radical induced gelation and becomes insoluble in water. The
mold is fabricated in such a way that yields hydrogels at
equilibrium swelling desired dimensions.
[0071] To incorporate the second network, the PEG-based hydrogels
are removed from the mold and immersed in the second monomer
solution, such as an aqueous solution of (10-100% v/v) acrylic acid
containing a photo-initiator and a cross-linker, from 0.1% to 10%
by volume triethylene glycol dimethacrylate (TEGDMA), triethylene
glycol divinyl ether, N,N-methylene bisacrylamide, and
N,N'-(1,2-dihydroxyethylene)bisacrylamide, for 24 hours at room
temperature. The swollen gel is then exposed to the UV source and
the second network is polymerized and crosslinked inside the first
network to form an IPN structure. Preferably, the molar ratio of
the first network telechelic macromonomer to the second network
monomer ranges from about 1:1 to about 1:5000. Also preferably, the
weight ratio of the first network to the second network is in the
range of about 10:1 to about 1:10. In another embodiment of the
present invention, the IPNs have a molar ratio of the second
monomer ingredient to the first macromonomer ingredient higher than
100:1.
[0072] Key characteristics of hydrogels such as optical clarity,
water content, flexibility, and mechanical strength can be
controlled by changing various factors such as the second monomer
type, monomer concentration, molecular weight and UV exposure
time.
[0073] A range of hydrogels of the preferred embodiment (PEG/PAA
IPN) have been developed. Specifically, IPNs of PEG-diacrylate
(PEG-DA) and poly(acrylic acid) from PEG of molecular weights 275
to 14000 have been synthesized. It was found that the low molecular
weight PEG-DA (<1000) gave rise to gels that were opaque or
brittle, whereas the hydrogels made from the higher molecular
weight PEG-DA (>1000) were transparent and flexible. In
addition, IPNs with PEG-dimethacrylate, PEG-diacrylamide,
PEG-diallyl ether, and combinations of these (and with varying
molecular weight) in the first network have been developed.
[0074] In one example, we fixed the concentration of PEG-DA (MW
3400-14000 Da) to 50%(wt/wt) in PBS for the first network and
changed concentrations of acrylic acid from 15%(v/v) to 100% (v/v).
The cross-linking density of the IPN hydrogel increased as
molecular weight of PEG decreased and concentration of acrylic acid
increased. We made a mechanically strong and transparent hydrogel
when the concentration of acrylic acid was in the range of 20%(v/v)
to 100% (v/v). In this range of concentration of acrylic acid, the
weight ratio of first and second network was varied over a wide
range.
[0075] In one embodiment of the present invention, grafted polymers
are used to form the IPN. FIG. 4A shows a standard IPN according to
the present invention, with first polymer network (black lines) and
second polymer network (grey lines). FIG. 4B shows an IPN in which
first polymer network is grafted with hydrophilic polymer. Any of
the aforementioned macromonomers, monomers, or combinations of
macromonomers and monomers may be used to get a grafted structure.
FIG. 4C shows an IPN in which second polymer network is grafted
with a hydrophilic macromonomer. FIG. 4D shows an IPN in which
first polymer network is grafted with hydrophilic monomer and
second polymer network is grafted with a hydrophilic macromonomer.
The grafted networks are made by polymerizing aqueous mixtures of
the two components in ratios that yield a network that is
predominantly made from one polymer but has grafted chains of the
second polymer.
Examples of First Network Telechelic Macromonomers
[0076] Telechelic PEG macromonomers with acrylate or methacrylate
endgroups were synthesized in the following manner. PEG was dried
from Toluene, redissolved in THF (per 100 g 550 mL) and kept under
Nitrogen. Distilled Triethylamine (2.5 eq per OH group) was added
slowly. Then acryloyl chloride (or methacryloyl chloride) was added
via dropping funnel (diluted with THF) over 30 min, room
temperature. The reaction (FIG. 5) was allowed to proceed
overnight. Filtration was carried out to remove the formed salt.
The volume of the solvent was reduced using a Rotavap, and
precipitation was carried out in diethylether. As an alternative to
extraction, filtration via cellulose membrane has also been
performed. The raw product was dried after precipitation from
diethylether in a vacuum, then dissolved in MeOH and dried in a
Rotavap. It is then dissolved in water and filtrated through a
membrane, and finally freeze-dried.
[0077] To attain a PEG-diacrylamide first network, the following
procedure was followed. PEG mol wt 3400 (100 g, 58.8 mmol--OH) was
azeotropically distilled in 700 mL toluene under nitrogen, removing
about 300 mL of toluene. The toluene was then evaporated completely
and then the PEG re-dissolved in anhydrous tetrahydrofuran. The
triethylamine was distilled prior to use. The excess of
Mesylchloride used was 3 eq per OH endgroup. The solution was
cooled in a room temperature bath under Nitrogen and then cooled in
an ice bath. Anhydrous dichloromethane (Aldrich) was added until
the solution became clear (about 100 mL). Triethylamine (24.6 mL,
176.5 mmol, Aldrich) was then added dropwise with stirring,
followed by the dropwise addition of 13.65 mmol mesyl chloride
(176.5 mmol, Aldrich). The reaction proceeded overnight under
argon. The solution was filtered through paper under vacuum until
clear, followed by precipitation in diethyl ether. The product was
then collected by filtration and dried under vacuum. The
PEG-dimesylate product was added to 400 mL 25% aqueous ammonia
solution in a 1 L bottle. The lid was tightly closed and sealed
with Parafilm, and the reaction was vigorously stirred for 4 days
at room temperature. The lid was then removed and the ammonia
allowed to evaporate for 3 days. The pH of the solution was raised
to 13 with 1 N NaOH, and the solution was extracted with 100 mL
dichloromethane. For the extraction with dichloromethane, NaCl was
added to the water-phase (.about.5 g) and the water-phase was
extracted several times with 150 mL of dichloromethane. The
dichloromethane washes were combined and concentrated in vacuo. The
product was precipitated in diethyl ether, and dried under vacuum:
PEG-diamine mol wt 3400 (20 g, 11.76 mmol amine) was then
azeotropically distilled in 400 mL of toluene under Nitrogen,
removing about 100 mL of toluene. The toluene was then evaporated
completely and then the PEG re-dissolved in anhydrous
tetrahydrofuran. The solution was cooled in a room temperature bath
under Nitrogen and then cooled in an ice bath Anhydrous
dichloromethane (Aldrich) was added until the solution become
clear, about 50 mL. Triethylamine (2.46 mL, 17.64 mmol, Aldrich)
was added dropwise with stirring, followed by the dropwise addition
of 1.43 mL of acryloyl chloride (17.64 mmol). The reaction (FIG. 6)
proceeded overnight in the dark under Nitrogen. The solution was
then filtered through paper under vacuum until clear, followed by
precipitation in diethyl ether. The product was collected by
filtration and dried under vacuum. The product was then dissolved
in 200 mL of deionized water, with 10 g of sodium chloride. The pH
was adjusted to pH 6 with NaOH and extracted 3 times with 100 mL of
dichloromethane (with some product remaining in the water phase as
an emulsion). The dichloromethane washes were combined and the
product was precipitated in diethyl ether, and dried under vacuum.
Alternatively, PEG-diacrylamide has been precipitated from
Diethylether once, redissolved in MeOH, dried from MeOH and then
purified by centrifugal filtration in water through a cellulose
membrane (MWCO:3000). Freeze drying was used to attain the desired
product.
[0078] Macromonomers containing diols can be converted into allyl
ethers. Difunctional allyl ether macromonomers were synthesized
from (PEG) using the following procedure. Fresh anhydrous
tetrahydrofuran (THF) (100 mL) was added to every 10 g of PEG
(Aldrich). This mixture was gently heated until the PEG dissolved
and then cooled in an ice bath before sodium hydride (Aldrich) was
slowly added in multiple portions (1.05 molar equiv. NaH for the
PEG ReOH groups). After the release of H.sub.2 gas ceased, the
system was purged with argon and allyl chloride or allyl bromide
(1.1 molar equiv. per PEG OH-group, diluted 1:10 in THF) was added
dropwise using an addition funnel, after which the reaction mixture
(FIG. 7) was transferred to an 85 degrees Celsius oil bath and
refluxed overnight. Vacuum filtration was used to remove the sodium
bromide side products and rotary evaporation was used to reduce the
concentration of THF before the PEG-allyl ether products were
precipitated from solution using iced diethyl ether (Fisher
Scientific, 10:1 v:v diethyl ether:THF solution).
Strain-Hardening
[0079] The following pertains to studies performed by the inventors
on the relationship between the network structure and the
mechanical properties of PEG/PAA IPN hydrogels with acrylate
crosslinking in the first network and triethylene glycol
dimethacrylate crosslinking in the second network as an exemplary
embodiment. Hydrogels based on poly(ethylene glycol) (PEG) and
poly(acrylic acid) (PAA) have properties such as biocompatibility,
hydrophilicity, transparency, permeability, and resistance to
protein adsorption, all of which are advantageous in a variety of
biomedical applications. PEG, for instance, is widely utilized as a
surface coating for intravenous and intraperitoneal catheters due
to its ability to prevent the adhesion of thrombogenic and
immunogenic proteins. PAA is an ionizable polymer (pk.sub.a, =4.7)
that is the absorbent material. In addition, it undergoes large
volume changes in response pH changes. At a pH greater than 4.7,
the PAA network is negatively charged and swollen. At a pH lower
than 4.7, the PAA network is protonated and contracted. To date,
however, the relative fragility of both PEG and PAA has precluded
them from serving as the primary material for tissue replacement or
augmentation applications that require high mechanical
strength.
[0080] This present invention is based on loosely crosslinking PAA
within a preexisting, highly crosslinked neutral PEG network
results in an IPN with unusually high mechanical strength.
Moreover, these PEG/PAA IPNs were found to exhibit very different
mechanical behavior in pure water and buffered saline, indicating
that both pH and salt concentration play important roles in
defining the relative network configuration. The controllable
swelling of PAA within the confines of the more rigid, neutral PEG
network provided a convenient first step for studying the effect of
relative chain configuration and topological interactions on the
properties of the IPN. The use of defined, telechelic macromonomers
in the first network facilitated tuning of the mesh size of the
first network while placing a three-dimensional constraint on the
swelling of the second network.
[0081] The experimental focus of this section is on the strain
hardening observed in this system by testing how it manifests
through uniaxial tensile tests under various conditions of first
and second network crosslinking and swelling. Swelling data were
used to calculate the equilibrium water and polymer content of the
networks, which were correlated with Young's modulus, true
stress-at-break, and true strain-at-break. The results indicate
that strain hardening is derived from physical entanglements
between the PEG and PAA networks that are intensified by bulk
deformation. Under conditions that promote hydrogen bonding (when
the pH is at or below 4.7, the pk.sub.a, of PAA), these
entanglements are reinforced by interpolymer complexes between PEG
and PAA, leading to an increase in the fracture strength of the
IPN. Under conditions that promote ionization of PAA (when the pH
is above 4.7), increased steric interactions (i.e. physical
crosslinks) between the swelling PAA network and static, telechelic
PEG macromonomer network lead to a dramatic increase in Young's
modulus.
Hydrogel Synthesis and Swelling
[0082] All hydrogels were formed by photopolymerization with UV
light using the water-soluble photoinitiator,
2-hydroxy-2-methyl-propiophenone. Before the IPNs were prepared,
single network hydrogels based on PEG and PAA were synthesized
separately to confirm the formation of gels of each composition and
to investigate the physical properties of the single networks. For
the PEG single network, a range of hydrogels that varied between
275 and 14000 for the MW of the PEG macromer was synthesized. It
was found that low MW PEG macromonomers gave rise to gels that were
transparent but brittle, whereas the hydrogels made from higher
molecular weight PEG-DA (3400) were transparent and flexible when
swollen in deionized water.
[0083] Based on these results, a range of different MWs of PEG
(3400, 4600, 8000, and 14000) were chosen as macromonomers for the
first hydrogel network. A series of IPNs was synthesized by
polymerizing and crosslinking a PAA network within each type of PEG
network. The resultant IPNs were transparent and had significantly
greater mechanical strength compared with single network
hydrogels.
Effect of Changing the MW of the PEG-DA Macromonomer
[0084] To explore the effect of the molecular weight of the
telechelic PEG-DA macromonomer on IPN mechanical strength, PEG
chains with MWs 3400 Da, 4600 Da, 8000 Da, and 14000 Da were used
in the first network while keeping the acrylic acid polymerization
conditions constant (50% v/v in deionized water with 1% v/v
crosslinker and 1% v/v photoinitiator with respect to the monomer).
The resulting IPNs were characterized in terms of their water
content, tensile properties, and mesh size in deionized water.
[0085] Changing the MW of the PEG-DA macromonomer led to a change
in the moduli of the PEG-DA single networks, as shown in FIG. 8A.
This effect was magnified in the PEG/PAA IPNs (FIG. 8B), where the
IPNs initial and final moduli get increasingly higher as the
networks are prepared from lower molecular weight PEG-DA
macromonomers.
[0086] Of note, there was little increase in strength when the PEG
MW is increased above 8000, indicating that a contrast between the
molecular weight between crosslinks of the PEG and PAA networks is
important for strength enhancement. Moreover, the molecular weight
of the PEG macromonomer was strongly correlated to the critical
strain (.epsilon..sub.crit) at which the stress-strain curve makes
the transition from the initial modulus to the strain-hardened
final modulus (FIG. 8B). The .epsilon..sub.crit was smaller for the
IPNs prepared from lower MW PEG macromonomers, meaning that these
networks strain-harden more rapidly in response to deformation.
[0087] The significance of forming an interpenetrating structure
rather than a copolymeric structure was explored by synthesizing a
PEG-co-PAA copolymer hydrogel and testing its tensile properties.
Its stress-strain profile was then juxtaposed with those of the IPN
and the PEG and PAA single networks. In FIG. 9A, a representative
true stress (.sigma..sub.true) versus true strain
(.epsilon..sub.true) profile of the PEG(8.0 k)/PAA IPN is compared
to those of the PEG(8.0 k)-PAA copolymer and their component
PEG(8.0 k) and PAA networks. The IPN exhibits strain-hardening
behavior with a stress-at-break that is greater than four times
that of the copolymer and single network. However, since each of
the materials tested has different water content, the stress data
were normalized on the basis of polymer content to determine the
true stress per unit polymer in each hydrogel.
[0088] In FIG. 9B, the true stress per unit polymer
(.epsilon..sub.true unit polymer) is plotted against true strain
for PEG(8.0 k)-DA, PAA, PEG(8.0 k)/PAA, and the PEG(8.0 k)-PAA
copolymer. The initial moduli of the PEG single network, the
copolymer, and IPN are identical (E.sub.o per unit polymer=0.91
MPa), while that of the PAA single network is lower (E.sub.o per
unit polymer=0.55 MPa). Near the break point of the PEG network,
.epsilon..sub.true.about.0.6, the copolymer continues to be
elongated with a modulus that is intermediate between the PEG and
PAA single networks, of which it is equally composed by weight.
Ultimately, it fails at a strain that is also intermediate between
the .epsilon..sub.break values of the two single networks. In stark
contrast, just beyond the failure point of the PEG network, the
PEG/PAA IPN manifests a dramatic strain hardening effect in which
its modulus increases by 30 fold, and breaks at .epsilon..sub.true
.about.1.0 under a mean maximum stress per unit solid of 10.6 MPa.
Without normalization for polymer content, .sigma..sub.break for
the IPN (20% solid) and copolymer (51% solid) are 3.5 MPa and 0.75
MPa, respectively.
pH Dependence of IPN and PAA Physical Properties
[0089] To explore the role of interpolymer hydrogen bonding, the pH
of the hydrogel swelling liquid was varied to change the ionization
state of the PAA network. Since the equilibrium swelling of PAA is
sensitive to variations in pH, a change in the pH was expected to
have an impact on the mechanical properties of PEG/PAA IPNs. After
synthesis, the water-swollen PAA single networks and PEG(8.0 k)/PAA
IPNs were placed in buffers of pH 3-6 and constant ionic strength
(I) of 0.05. In both the PAA network and the IPN, the equilibrium
water content increased as the pH was increased from 3 to 6 (FIG.
10a-c, upper plots) In the case of the PAA networks, those at pH 3
and 4 were moderately swollen, while those at pH 5 or 6 were highly
swollen due to ionization of PAA above its pK.sub.a (4.7). The IPNs
also achieved different levels of swelling depending on the pH;
those at pH 3 and 4 were moderately swollen, while those at pH 5 or
6 were highly swollen due to ionization of PAA above its pK.sub.a
(4.7). Of note, at both pH 3 and 4, the IPN achieved a lower
equilibrium water content than PAA alone. This can be explained, in
part, by the fact that PEG and PAA complex with each other via
hydrogen bonds in an acidic environment, leading to a more compact,
less hydrated interpenetrating network structure. At pH above 4.7,
the PEG and PAA chains dissociate as the PAA becomes ionized and
counterions (along with water) enter the hydrogel to maintain
charge neutrality, leading to a high degree of swelling.
Nevertheless, the IPNs swell to a slightly lower extent (1.0-1.5%)
than the PAA single networks due to the constraint that the PEG
network places on PAA swelling.
[0090] FIG. 10a also shows that the stress-at-break
(.sigma..sub.break), or tensile strength, of the PEG/PAA IPN is
nearly an order of magnitude greater in its less-swollen state at
pH 3 (.sigma..sub.break=8.2 MPa) than in its more swollen state at
pH 6 (.sigma..sub.break=0.86 MPa). A similar phenomenon is observed
in the PAA network, but the absolute values for .sigma..sub.break
are only 0.38 MPa at pH 3 and 0.05 MPa at pH 6. At every pH, then,
the IPN has greater tensile strength than the PAA network, and this
difference is intensified at lower pH.
[0091] In contrast to the differences in the stress-at-break, the
trends in the strain-at-break values of the IPN and PAA networks
are roughly equivalent (FIG. 10b), changing from
.epsilon..sub.break values of .about.1.2 at pH 3 to .about.0.55 at
pH 6. This result confirms the observation made in FIGS. 9a-b, in
which the extensibility of the IPN seems to be due to the presence
of the PAA network, which has a higher .epsilon..sub.break (0.9)
than PEG (0.6). The mere presence of the PAA network in the IPN
appears to enhance the uniaxial extensibility of the network. In
the context of the stress-at-break data (FIG. 10a), however, the
load-bearing capacity at higher extensions is dramatically greater
in the presence of hydrogen bonding at low pH than it is in the
absence of hydrogen bonding at high pH.
[0092] In contrast, FIG. 10c indicates that the pH dependence of
the initial Young's moduli (E.sub.o) of the IPN and PAA networks is
less straightforward. The modulus of the PAA network exhibits a
small drop from 0.09 MPa to 0.05 MPa as the pH is increased from 3
to 6. On the other hand, the modulus of the IPN does not decrease
at all, but rather increase when the pH is changed from 3 to 6. Of
note, the pH-dependence of the IPN does not follow the trend
exhibited by the PAA single network, in which the modulus drops by
approximately one-half when transitioning from pH 4 to pH 5. This
decrease in modulus is correlated with an increase in water content
of the PAA single network (FIG. 10c, upper plots). Moreover, the
apparent preservation of the modulus in the IPN despite an increase
in water content and loss of hydrogen bonding is paradoxical in the
context of the sharp declines observed in the .epsilon..sub.break
and .sigma..sub.break values.
[0093] The strain-hardening per unit polymer is shown in the
stress-strain profiles in FIGS. 11A and 11B. FIG. 11A plots the
true stress per unit polymer versus strain in the PAA gels. In
contrast to the non-normalized data in which the initial modulus of
PAA at pH 3-4 is higher than it is at pH 5-6 (not shown), the
initial moduli of PAA at all pHs converge when the stress data are
corrected for differences in polymer content. This indicates that
the reduction in mechanical strength that accompanies an increase
in pH in PAA networks is largely due to the swelling of the
network. Correcting the stress data for polymer content in the IPNs
yields the graph shown in FIG. 11B. In this plot, the difference in
the onset of strain hardening between the low pH and high pH
regimes is accentuated. The stress-strain curves of the high pH
IPNs proceed beyond their uncorrected maximum stresses and end
parallel to those of the low pH regimes. This result suggests that,
while the more swollen IPNs have lower tensile strength, this may
be a side effect of the accelerated strain hardening that leads to
greater load bearing at smaller strains (due to a higher modulus),
and, in turn, an earlier strain-at-break.
[0094] To investigate the consequence of relative network moduli
even further, the swelling of PAA within the IPNs was maximized.
The experimental data shown in FIG. 10c indicated that the modulus
of the IPN was not negatively affected by the increased swelling.
We hypothesized that the PEG network acts as a constraint on the
swelling of PAA in a way that leads to additional interpolymer
interactions and a corresponding increase in the IPN modulus. In
particular, we postulated that increasing the constraining effect
of the neutral PEG network on PAA swelling would increase the
intensity and number of physical entanglements in the IPN and, in
turn, lead to the strain hardening behavior observed in the
IPN.
[0095] To test this hypothesis, the IPNs with first network MW PEG
3400, 4600, and 8000 and constant PAA network conditions were
placed in phosphate buffered saline (PBS, pH 7.4, I=0.15) in order
to induce maximal swelling under physiologic conditions.
[0096] Table 1 shows the equilibrium water content and
corresponding swelling ratios for networks prepared from PEG
macromonomers with each of these molecular weights, juxtaposed with
the water content of the water-swollen and PBS-swollen IPNs.
Increasing the size of the first PEG network from 3400 Da to 4600
Da and 8000 Da increases the degree to which the IPN is able to
swell.
TABLE-US-00001 TABLE 1 Equilibrium water content (%) and swelling
ratio (q)* of PEG and PAA. single networks and PEG/PAA IPNs under
varying swelling conditions. Swelling Specimen Conditions Water
Content (%) Ratio (q) PAA dH.sub.20** 90.0 .+-. 1.7 10.0 PAA pH
7.4, I = 0.15 95.5 .+-. 1.7 22.1 PEG(3.4k) dH.sub.20 79.3 .+-. 2.1
4.8 PEG(3.4k)/PAA dH.sub.20 56.3 .+-. 3.3 2.4 PEG(3.4k)/PAA pH 7.4,
I = 0.15 68.7 .+-. 1.6 3.2 PEG(4.6k) dH.sub.20 84.5 .+-. 0.4 6.5
PEG(4.6k)/PAA dH.sub.20 57.0 .+-. 0.6 2.3 PEG(4.6k)/PAA pH 7.4, I =
0.15 77.0 .+-. 1.2 4.4 PEG(8.0k) dH.sub.20 90.5 .+-. 1.2 10.5
PEG(8.0k)/PAA dH.sub.20 80.2 .+-. 1.5 5.1 PEG(8.0k)/PAA pH 7.4, I =
0.15 90.9 .+-. 0.1 11.0 *q = W.sub.s/W.sub.d = ratio of swollen
weight and dry weight. **dH.sub.20 = deionized water
[0097] Specifically, while the PEG(3.4 k)/PAA IPN swells to only
70% water when ionized, the PEG(4.6 k)/PAA IPN swells to 77% water
and the PEG(8.0 k)/PAA IPN swells to 90% water (nearly the same
water content as the PEG(8.0 k) single network) when ionized. Of
note, the equilibrium water content values of the PEG(3.4 k) and
PEG(4.6 k)-based IPNs do not approach those of their component
PEG-DA networks (79.3% and 84.5%, respectively).
[0098] FIGS. 12a-c show a PEG/PAA hydrogel in (a) the dry state,
(b) the partially-swollen state, and (c) the fully,
equilibrium-swollen state. These photographs were taken by drying
the hydrogel in a dessicator, then placing it in deionized water,
and then removing it and patting it dry before taking pictures.
FIGS. 13A-B shows a disc-shaped PEG/PAA hydrogel next to a coin in
(d) the dried state, and (e) the swollen state, after being
immersed in phosphate buffered saline (pH 7.4) for 40 minutes.
[0099] The water content of the hydrogels was evaluated in terms of
the swollen-weight-to-dry-weight ratio. The dry hydrogel was
weighed and then immersed in water as well as phosphate buffered
saline. At regular intervals, the swollen gels were lifted, patted
dry, and weighed until equilibrium was attained. The percentage of
equilibrium water content (WC) was calculated from the swollen and
dry weights of the hydrogel:
WC = W s - W d W s .times. 100 ##EQU00001##
where W.sub.s and W.sub.d are the weights of swollen and dry
hydrogel, respectively.
[0100] FIG. 14 shows the time-dependent swelling behavior of an IPN
hydrogel composed of PEG and two different amounts of acrylic acid
in the second network (25% and 50%). The single network IPN gels
were dried in a desiccator, placed in deionized water, and then
weighed at regular time intervals. In both hydrogels, the majority
of swelling took place within 5-10 minutes and equilibrium swelling
was achieved within 30-40 minutes.
[0101] The parameters varied to obtain hydrogels with differing
water content were the molecular weight of the PEG macronomonomer,
the weight fraction of PAA in the second network, as well as the
amount of crosslinking agent (e.g. triethylene glycol
dimethacrylate, or low molecular weight PEG-DA) added to the first
or second networks.
[0102] Table 2 shows the effect of varying the concentration of
acrylic acid monomer used to prepare the second network on the
equilibrium water content of PEG/PAA IPNs. In general, lower
concentrations of acrylic acid monomer leads to hydrogels with
higher equilibrium water content.
TABLE-US-00002 TABLE 2 Equilibrium Water Content of PEG(8.0k)/PAA
hydrogels with varying preparation concentration of acrylic acid
(AA) monomer Concentration of AA Equilibrium Water Content in the
preparation state of PEG/PAA IPN 30% 99% 40% 91% 50% 83%
[0103] Hydrogels according to the present invention made from these
hydrogels, preferably have an equilibrium water content of between
about 15%-95% and more preferably between about 50%-90%.
[0104] Because different MWs of PEG and different starting
concentrations of acrylic acid result in different amounts of
equilibrium water content, the final amount of PEG and PAA in the
hydrogel varies depending on the MW of the starting PEG used and
the concentration of acrylic acid used. Examples of compositions of
varying weight ratios of PEG and PAA that have been made according
to the present invention are shown in Table 2. The compositions in
this table were all made using a starting concentration of 50% PEG
macromonomers in water.
TABLE-US-00003 TABLE 3 Compositions of PEG(8.0k)/PAA IPNs with
varying preparation concentration of AA monomer Concentration of AA
in Dry Wt. % Dry Wt. % (Dry Wt. PEG)/ the preparation state PEG in
IPN PAA in IPN (Dry Wt. PAA) 30% 23.5% 76.5% 0.30 40% 17.5% 82.5%
0.20 50% 13.0% 87.0% 0.15
[0105] We found that swelling of the PAA network within the
confines of a more densely crosslinked PEG network (by lowering the
MW of the PEG macromonomer) has dramatic consequences on the
resulting IPN modulus. Specifically, FIG. 15 shows that the
accelerated strain hardening due to elevated pH, as demonstrated in
FIG. 11b, is accentuated even further when a PEG network with lower
MW (4600 rather than 8000) is used to constrain PAA. These more
tightly crosslinked IPNs were placed in phosphate buffered saline
to examine them under physiologic conditions (pH 7.4, ionic
strength=0.15) where the PAA network is greater than 99% ionized.
The PEG(4.6 k)/PAA IPN was first swollen to equilibrium in pure
deionized water (pH 5.5, salt-free); it was then switched to the
ionizing conditions of phosphate buffered saline (pH 7.4, I=0.15)
and again swollen to equilibrium. The increase in the pH to 7.4 and
the addition of salt caused the PAA network (but not the PEG
network) to swell. The result of this differential swelling within
the IPN was a dramatic upward shift in the stress-strain profile
that included the initial portion of the curve. In other words,
there was an increase in not only the rate of strain hardening, but
also in the initial modulus.
Effects of Alternate Crosslinking in the IPN
[0106] FIG. 16 shows according to an embodiment of the present
invention the stress-strain profile of a PEG/PAA IPN prepared from
a PEG-diacrylamide first network and a PAA second network
crosslinked with N,N'-(1,2-dihydroxyethylene)bisacrylamide and
swollen to equilibrium in PBS at pH 7.4. The strain-hardened
mechanical properties of this IPN are similar to those of
acrylate-based IPN system in FIG. 15. These results demonstrated
that alternate crosslinking strategies can be employed to create
the strain-hardened IPNs based on telechelic macromonomer-based
first networks and ionized second networks without deviating from
the essence of the present invention.
Ionic Strength Effects
[0107] PEG/PAA IPNs were swollen to equilibrium in a series of PBS
solutions of varying ionic strength (0.15 M, 0.30 M, 0.75 M, and
1.5 M) and their equilibrium water content and stress-strain
properties were measured. FIG. 17 shows that the water content of
the IPN is reduced with higher salt concentration in the swelling
medium, from over 90% at I=0.15 to less then 78% at I=1.5. This
result is expected, since increased salt in the buffer screens the
negative charges on the PAA chains, reducing electrostatic
repulsion and, in turn, swelling of the networks.
[0108] Ionic strength had a modest effect on the stress-strain
properties. FIG. 18 shows that stress-strain profiles of the IPNs
at I=0.15 to I=0.75 were roughly equivalent. The IPN swollen in
buffer with I=1.5 showed a slight enhancement in the strain
hardening at higher strains. This result is consistent with the
water content data, since the hydrogels with higher solids content
(the IPN at higher ionic strength conditions) should have greater
mechanical strength. Of note, the the final modulus of the IPN in
the solution with the highest ionic strength (I=1.5) appeared to be
higher than those at lower ionic strength. However, the difference
was small and was not found to be statistically significant.
Effect of PAA Content in Phosphate Buffered Saline
[0109] To increase the quantity of topological interactions between
the PAA and PEG networks, the polymer content of PAA was varied
inside of a PEG(3.4 k) first network. The volume fraction of
acrylic acid in solution at the time of the second network
polymerization was varied between 0.5 and 0.8 prior to
polymerization. After polymerization, the IPNs were swollen to
equilibrium in PBS, as was the IPN described in FIG. 15. The
resultant hydrogels had different water content, from 62% in the
PEG(3.4 k)/PAA[0.8] IPN to 65% in the PEG(3.4 k)/PAA[0.7] IPN and
77% in the PEG(3.4 k)/PAA[0.5] IPN. Of note, the IPNs with
increased acrylic acid concentration had lower water content, which
in light of the super-absorbency of PAA is a counterintuitive
result. The true stress-true strain profiles for these IPNs are
shown in FIG. 19. The IPN with the highest PAA content had the
highest stress-at-break and modulus, while the one with the lowest
PAA content had the lowest stress-at-break and strain-at-break.
Notably, the initial modulus values for these samples varied
significantly, from 3.6 MPa in the PEG(3.4 k)/PAA[0.5] to 12 MPa in
the PEG(3.4 k)/PAA[0.7] IPN and 19.6 MPa in PEG(3.4 k)/PAA[0.8]
IPN.
Effect of PAA Content on IPN Swelling in Pure Water
[0110] PEG(4600) single networks were prepared and imbibed with
varying concentrations of AA in the second network in the presence
of the photoinitiator and crosslinker. IPNs based on these
AA-swollen PEG networks were then formed by UV-initiated
polymerization. The IPNs were then removed from their molds,
immersed in deionized water, and allowed to reach equilibrium. The
volume of the IPNs relative to the PEG single networks were then
measured and compared. The results are plotted in FIG. 20.
[0111] FIG. 20 shows that the volume of the IPN is increased with
increased amount of AA monomer in the second network. This is
consistent with the understanding that PAA absorbs water, and
therefore increased PAA content in the IPN should lead to increased
water absorption. Of note, however, is the fact that the IPN
deswells relative to the PEG single network when the AA:EG monomer
ratio is less than unity, and swells relative to the PEG network
when AA is in excess to EG monomers.
Effect of PAA Content on IPN Mechanical Properties in Pure
Water
[0112] The same PEG/PAA IPNs of varying AA monomer content were
tested by uniaxial tensile measurements. The results are shown in
FIG. 21. In this figure, both the fracture stress and Young's
modulus are plotted as functions of AA mass fraction at the time of
polymerization. Young's modulus exhibited a modest monotonic
increase as the AA concentration increased. In contrast, the
fracture stress exhibited a dramatic increase in magnitude when the
AA:EG ratio was increased beyond unity. As the AA monomer
concentration increased, however, the fracture stress exhibited a
monotonic decline.
Effect of P(AA-Co-HEA) Copolymerization on IPN Mechanical
Properties in Pure Water
[0113] To demonstrate that an ionizable monomer is important in the
second network, a series of IPNs were prepared under conditions
that disrupted the degree of ionizability in the second
network.
[0114] The first method used was copolymerization of the second
network with non-ionic monomers. AA monomers in the second network
were mixed in three different concentrations relative to the HEA
monomers: 10:1, 3:1, and 1:1. Uniaxial tensile testing experiments
(FIG. 22) of the hydrogels swollen in deionized water showed that
the PEG/P(AA-co-HEA) IPNs with the highest ratio of AA:HEA in the
second network had significantly enhanced mechanical strength in
terms of its stress-at-break and strain-at-break, while the IPNs
with higher relative HEA content exhibited almost no enhancement in
mechanical properties. This result demonstrates that complexation
between PEG and the PAA networks (due to the presence of ionizable
carboxyl acid groups in PAA donating hydrogen bonds) is necessary
process step in the production of enhanced mechanical properties in
these systems. Either externally applied strain or ionization of
the PAA in the second leads to increased physical crosslinking
between the polymeric constituents within the IPN.
Effect of AA Neutralization on IPN Mechanical Properties
[0115] In this set of experiments, PEG networks were immersed in AA
solutions (containing photoinitiator and crosslinker) that were
partially neutralized to pH 5.5 by titration with sodium hydroxide.
The monomer-swollen PEG networks were then exposed to UV light to
form a partially neutralized PAA network within the PEG network.
These "pre-neutralized" PEG/PAA IPNs were then washed in PBS and
subjected to uniaxial tensile tests. FIG. 23 shows that
neutralizing the AA solution prior to polymerization and then
forming the second network leads to an IPN with the same elastic
modulus, but with dramatically reduced fracture strength. The
stress-at-break is reduced from nearly 4 MPa--in the case of the
IPNs prepared under acidic conditions and then neutralized in PBS
buffer--to roughly 0.5 MPa. This demonstrates the importance of the
fabrication process in creating these strain-hardened IPNs; that
is, in the preferred embodiment, ionization of the second network
should be carried out after the IPN is fully formed. FIG. 24 shows
that the stress-strain behavior of the strain-hardening IPN is not
dependent on the extension rate of the applied uniaxial
deformation.
[0116] The IPNs presented in this invention swell substantially
from the dry state in the presence of water or saline, and as such
are useful for use as an absorbent material for diapers and
feminine hygiene products. The majority of diapers produced today
make use of poly(acrylic acid) polymers as the absorbent material.
While these can work well, the distribution of the polymeric
material is not always uniform, and can lead to leaks. A
homogeneous lining or series of thin but resilient linings made
from a PEG/PAA IPN may be more efficacious in uniformly absorbing
and containing urine and waste matter in the diaper. Similarly, if
used in combination with another paper or cotton-based absorbent
material, the hydrogel can be used as a component of a tampon or
pad for feminine hygiene by soaking up the aqueous part of blood. A
schematic of these applications is shown in FIG. 25. Molding the
gels into shapes such as cylinders or rectangular sheets is easily
accomplished by casting precursor solutions in molds prior to
initiating hydrogel polymerization.
Compressive Strength and Surface Friction
[0117] The mechanical properties of the IPN hydrogels of the
present invention can be "tuned" to yield initial Young's modulus
values (.about.10 MPa) that rival those of natural articular
cartilage. This is a significant finding in light of the fact that
hydrogels have long been thought of as potentially useful materials
for the replacement of cartilage, but have suffered from a lack of
mechanical strength. In fact, the most common way that hydrogels
are investigated in orthopaedics is in the form of soft, often
degradable scaffolds for chondrocytes to grow and eventually
regenerate cartilage. A handful of cell-free, purely synthetic
hydrogels are now being used in the repair of joints, but only for
focal or localized regions in joints such as the knee or the
vertebrae (e.g. nucleus pulposus). This is because the mechanical
properties and surface characteristics of most hydrogels preclude
their use in more than a small area on the joint interface. For a
hydrogel to completely and functionally replace natural cartilage,
it should (almost exactly) match the complex biomechanical
properties of natural cartilage. In doing so, it would restore the
physiologic distribution of loads to the adjacent bone, which is
known to be extremely sensitive to its stress environment.
[0118] The section discussed the characterization of the PEG/PAA
hydrogels in the context of the already well-known properties of
articular cartilage as a prerequisite to its application as a joint
surface restoration material. Experimental data are presented on
the biomechanical properties of these materials that enable this
material to functionally restore a cartilaginous joint.
[0119] PEG/PAA behaves as a synthetic analog of natural cartilage.
FIG. 26 juxtaposes the structures of the two materials. Natural
cartilage is a highly negatively charged, water-absorbing network
of glycosaminoglycans swollen within a rigid framework of collagen.
Similarly, PEG/PAA is a highly negatively charged, water-absorbing
polymer network of poly(acrylic acid) swollen within a rigid,
neutral poly(ethylene glycol) framework. The third (and most
prominent) component of both of these materials is not shown:
water. The striking structural similarity between cartilage and
PEG/PAA yields an equally striking functional similarity between
the two materials.
[0120] Because PEG/PAA is a "biphasic" material like cartilage, the
actual compressive loads are taken up by the fluid in the gel, thus
relieving the stress on the actual solid portion of the gel. The
abundance of negative charge combined with movement of this fluid
in and out of cartilage results in a persistent lubricating film of
fluid between cartilage surfaces in a joint. PEG/PAA, because it
mimics the water content, negative charge, and elasticity of
natural cartilage, has the potential to recreate a physiologic
joint interface, as shown in FIG. 27.
Water Content
[0121] One of the defining characteristics of natural cartilage is
that it is made up of mostly water. The water content of cartilage
is critical because movement of fluid out of cartilage upon
loading, in conjunction with an abundance of negatively charged
functional groups, is believed to be the reason for the high
lubricity observed in diarthroidal joints. A water content between
65% and 75% along with low hydraulic permeability is believed to
provide a surface "weeping lubrication" mechanism that has been
found in cartilage and is thought to be the reason the coefficient
of friction of cartilage is very low. Therefore, the measured the
equilibrium water content of PEG/PAA was compared to that of
natural human cartilage, as shown in FIG. 28.
Indentation and Hydraulic Permeability
[0122] Indentation experiments were carried out using a custom-made
indenting apparatus. In these experiments, a constant displacement
is applied and the reaction force is measured over time. The test
was carried out with two different ramp times (0.5 sec and 12 sec),
and the indentation force versus time data (FIG. 29) was used to
extract a hydraulic permeability for the PEG/PAA hydrogel. The
permeability of PEG/PAA was calculated to be 2.times.10.sup.-17
m.sup.4/N*s. This value is roughly an order of magnitude lower than
those reported in the literature for natural cartilage
(1-5010.sup.-16 m.sup.4/N*s).
Coefficient of Friction Measurements
[0123] In these experiments, the coefficient of friction of PEG(4.6
k)/PAA fully interpenetrating networks were compared to that of
ultra-high-molecular-weight polyethylene (UHMWPE, Orthoplastics,
UK), a material currently used in total knee replacement and
artificial disc prosthetics. These materials were also compared to
a transparency sheet, which would be expected to have a higher and
consistently measurable friction coefficient. The materials were
placed between a sled (mass 200 g) and a glass surface cleaned and
wetted with deionized water. An Instron 5844 materials tester
equipped with a 10 N load cell was used to pull the sled using an
Instron coefficient of friction fixture wire/pulley system that
conforms to ASTM D1894 standards. The average load detected during
motion of the sample was used to calculate the kinetic coefficient
of friction (.mu..sub.k) of the samples using the equation:
.mu..sub.k=A.sub.k/B
where A.sub.k is the average load reading obtained during sliding
and B is the sled weight.
[0124] FIG. 30 shows the mean coefficient of friction values for
PEG/PAA, UHMWPE, and a transparency sheet. The coefficients of
friction of PEG/PAA (3 samples) and UHMWPE (2 samples) were
comparable (0.056 and 0.065, respectively), while the transparency
sheet (3 samples) was much higher (0.38). The fact that the
coefficient of friction of the PEG/PAA hydrogel is similar to that
of the UHMWPE is favorable, especially in light of the fact that
PEG/PAA is both elastic and strong when subject to compressive
loads, indicating that it is a simultaneously lubricious and
"cushioning" surface. In contrast, UHMWPE is lubricious but
extremely rigid. The combination of lubricity and cushioning in our
PEG/PAA IPNs is advantageous in joint applications, where both
friction and load should be accommodated.
[0125] FIG. 30 shows data on the dynamic friction coefficient of
PEG/PAA on both itself (PEG/PAA on PEG/PAA) as well as on wetted
glass (PEG/PAA-on-glass). These data are juxtaposed with literature
data on natural cartilage on both itself and on glass, as well as
experimental data on ultra high molecular weight polyethylene
(UHMWPE) on glass. The results indicate that PEG/PAA surface
properties are within the range of values obtained for natural
cartilage in an in vitro setting.
Tensile Measurements
[0126] One of the defining features of the strain-hardened IPN is
its extremely high Young's modulus under tension--the highest
modulus among hydrogels described in the scientific literature.
FIG. 31a presents the stress-strain profile of a PEG/PAA IPN with a
water content of 65%. Young's modulus of this material is 10 MPa,
and the maximum tensile strength is also about 10 MPa, both of
which are similar to the respective values of natural cartilage.
Most hydrogels, including the ones being tested for orthopaedic
applications, have a low modulus (0.2-2.0 MPa) and are relatively
fragile. FIG. 31b presents the creep behavior of the same PEG/PAA
IPN (water content 65%). With an applied load of 4.5 N over 15
hours, the strain on the hydrogel increased from 20% to 30%, with
equilibrium strain being achieved at about 13.3 hours.
Unconfined Compression Tests
[0127] FIG. 32 shows an unconfined compression test of the IPN of
the present invention. Unconfined compression tests were done (data
shown in FIG. 33 and FIG. 34) to determine the material's reaction
to high compressive loads. In one embodiment of the invention (FIG.
33) where a macromonomer molecular weight of 4600 Da was used in
the 1.sup.st network and 50% v/v acrylic acid was used to prepare
the second network, the failure stress of the IPN in PBS (a) was
near 7 MPa, while the corresponding PEG-only homopolymer in PBS (b)
had a failure stress of about 1 MPa. In another embodiment (FIG.
34) where the PEG molecular weight was 3400 Da and 70% v/v acrylic
acid was used to prepare the second network, the unconfined
compressive strength in PBS was found to be about 18 MPa, with a
failure strain under compression of over 0.8.
Confined Compression Measurements
[0128] Confined compression experiments were carried out on plugs
of PEG/PAA confined to a cylindrical chamber, and the displacement
of the material was monitored as a function of time. The indenting
device was made out of sintered stainless steel and is permeable to
fluids; therefore, when the sample is compressed, fluid seeping out
of it can pass through the indenter. From the displacement versus
time data for 7 N of load (FIG. 35), we found that the aggregate
(equilibrium) modulus of the material is 2.1 MPa, which is similar
to that of healthy, natural cartilage (.about.1.0-2.0 MPa).
Wear Measurements
[0129] PEG/PAA was subjected to 250,000 cycles in a pin-on-disc
wear tester following ASTM G99 specifications. A 10-mm diameter
ball-tipped pin was placed onto a PEG/PAA sample under a load of
6.0 N. After first equilibrating the hydrogel sample (2 mm thick)
in bovine serum for 2 hours, it was rotated at a constant velocity
of 300 rpm at 37.degree. C. in a bovine serum bath over a track
radius of 10 mm. A 2.0 mm-thick piece of ultra high molecular
weight polyethylene (UHMWPE, Orthoplastics, UK) was also tested
under the same conditions. Neither sample showed any detectable
mass loss after 250,000 cycles. Both PEG/PAA and UHMWPE had
physical evidence of pin movement on their surface. The hydrogel
sample actually showed a slight increase (.about.2%) in mass (by
gravimetric measurement), possibly due to diffusion of serum
proteins into the hydrogel. FIG. 36 shows the appearance of PEG/PAA
and a sample of UHMWPE after the aforementioned wear test.
[0130] PEG/PAA was then tested for 3,000,000 cycles at .about.1 Hz
using the custom-made tri-pin-on-disc, hydrogel-on-hydrogel tester
(FIG. 37) in phosphate buffered saline. For 2 million cycles, a
load of 15 pounds was applied and for the remaining 1 million
cycles, a load of 25 pounds was applied. Gross observation revealed
evidence of a raceway made by the hydrogel-on-hydrogel
configuration. However, the concentric marks appear to have been
the result of foreign body abrasion by dust or debris that entered
the fluid chamber; this is supported by the fact that there were
numerous additional marks with random orientations in the vicinity
of the raceway. Profilometry revealed an average roughness of only
0.5 .mu.m along the raceway, indicating that the material is highly
wear-resistant in a hydrogel-on-hydrogel configuration.
Hydrogel Molding
[0131] IPN hydrogels were cast within rounded molds to prove that
curved geometries are achievable with this material. Photographs of
these hydrogels are shown in FIG. 38.
Applications of the Strain-Hardened IPN Hydrogel
[0132] The IPN of the present invention has the advantage of
attaining the following characteristics simultaneously: (1) high
tensile and compressive strength, (2) low coefficient of friction
on its surface, (3) high water content and swellability, (4) high
permeability, (5) optical transparency, and (6) biocompatibility.
For instance, it possesses the high compressive strength and
lubricity necessary to serve as a replacement for articular
cartilage, intervertebral discs (lumbar or cervical), bursae,
menisci, and labral structures in the body. The types of
orthopaedic devices for which this invention is potentially useful
includes total or partial replacement or resurfacing of the knee
(the tibial, femoral, and/or patellar aspect), hip, shoulder,
hands, fingers (e.g. carpometacarpal joint), feet, ankle, and toes.
It is also useful in replacement or repair of intervertebral discs
or facets. In the knee, the hydrogel can also serve as a meniscus
replacement or a replacement material for the bursae in any joint
such the elbow or shoulder. It also would be useful as a lining
material for diapers by lending more uniform protection from
leakage and a neater, more compact arrangement of absorbent matter.
The material also is highly transparent, has high oxygen and
glucose permeability, and is resistant to protein adsorption,
making it suitable for ophthalmic lens and implant
applications.
Photochemical Surface Modification
[0133] Materials according to the present invention could have
biomolecules covalently linked to the IPN hydrogels. Any suitable
biomolecules may be covalently linked to the IPN hydrogel.
Preferably, the biomolecules are at least one of proteins,
polypeptides, growth factors (e.g. epidermal growth factor) amino
acids, carbohydrates, lipids, phosphate-containing moieties,
hormones, neurotransmitters, or nucleic acids. any combination of
small molecules or biomolecules can be used, including, but not
limited to, drugs, chemicals, proteins, polypeptides,
carbohydrates, proteoglycans, glycoproteins, lipids, and nucleic
acids. This approach may rely, for example, on (a) photoinitiated
attachment of azidobenzamido peptides or proteins, (b)
photoinitiated functionalization of hydrogels with an
N-hydroxysuccinimide ester, maleimide, pyridyl disulfide,
imidoester, active halogen, carbodiimide, hydrazide, or other
chemical functional group, followed by reaction with
peptides/proteins, or (c) chemoselective reaction of aminooxy
peptides with carbonyl-containing polymers. These biomolecules may,
e.g., promote epithelial cell adhesion and proliferation on the
nonadhesive hydrogel surface. Preferably, the heterobifunctional
crosslinker used to modify the IPN hydrogel surfaces are based on
azide-active-ester linkages, through molecules such as
5-azido-2-nitrobenzoyloxy-N-hydroxysuccinimide ester or its
derivatives such as its sulfonated and/or its chain-extended
derivatives. However, any coupling strategy can be used to create
strain-hardened IPN hydrogels with bioactive surfaces. Most
preferably in the case of ophthalmic applications, the biomolecules
attached are at least one biomolecules found in the cornea and/or
aqueous humor (e.g. collagen type I) or derivatives thereof. In
addition, polymeric tethers (such as poly(ethylene glycol) chains)
can be used as intervening spacer arms between polymer surfaces and
biomolecules and also between biomolecules.
Transport Properties: Oxygen Permeability
[0134] IPN hydrogels composed of a PEG first network with MW 8000
and concentration of 50% w/v in dH.sub.2O in the preparation state,
and a second network of polyacrylic acid with 50% v/v in dH.sub.2O
in the preparation state were used to test oxygen permeability. The
hydrogels were first rinsed in distilled water, then soaked in
phosphate buffer solution for at least 24 hrs. The harmonic
thickness of the hydrogel was then measured using Electronic
thickness gauge Model ET-3 (Rehder Development company). The
hydrogel was then soaked again in phosphate buffered saline
solution for at least 24 hrs. Next, an electrode assembly (Rehder
Development company) was attached to a polarographic cell and
electrical cables were attached between the electrode assembly and
a potentiostat. About 1.5 L of buffer solution was then saturated
with air for at least 15 minutes and preheated to 35.degree. C.
Next, the hydrogel was carefully placed onto the electrode, the gel
holder was placed over the hydrogel, and a few drops of buffer
solution were placed on top of the hydrogel to keep the hydrogel
saturated with buffer solution. The central part of the cell was
then attached onto the cell bottom and the top part of the cell,
containing the stirring rod, impeller, and coupling bushing, was
attached to the top part of the cell. Air saturated buffer solution
at 35.degree. C. was then poured into the assembled cell and filled
almost to the top. Next, heating coiled tubing was placed around
the cell, the tubing was connected to the heating bath, insulation
was wrapped around and on top of the cell, and the flow of heating
fluid was turned on. The speed was then set at 400 rpm and current
data was collected until the steady state was reached. The speed
was then reset in 100 rpm increments up to 1200 rpm, and data was
again collected. This data was then used to get the oxygen
permeability by plotting the inverse of steady current versus the
Reynolds number to the minus 2/3. An oxygen permeability of
95.9.+-.28.5 Barrers was obtained for the PEG/PAA IPN based on a
PEG macromonomer with molecular weight 8000. An average oxygen
permeability of 45 Barrers was obtained for the PEG/PAA IPN based
on a PEG macromonomer with molecular weight 4600. For use as a
contact lens material, the hydrogels according to the present
invention preferably have an oxygen permeability of more than about
15 Barrers, more preferably more than 40 Barrers.
Transport Properties: Optical Clarity
[0135] The percentage (%) of light transmittance of IPN hydrogels
composed of PEG with molecular weight 8000 Da (50% w/v in
dH.sub.2O) in the preparation state of the first network and
poly(acrylic acid) (50% v/v in dH.sub.2O) at 550 nm was also
measured using a Varian Cary 1E/Cary 3E UV-Vis spectrophotometer
following the method described by Saito et al (Saito et al,
"Preparation and Properties of Transparent Cellulose Hydrogels",
Journal of Applied Polymer Science, Vol. 90, 3020-3025 (2003)). The
refractive index of the PEG/PAA hydrogel (with PEG MW 8000) was
measured using an Abbe Refractometer (Geneq, Inc., Montreal,
Quebec). The percentage of light transmittance was found to be 90%,
and the refractive index was found to be 1.35.
Transport Properties: Glucose Permeability
[0136] We studied the glucose permeability across PEG/PAA IPNs, PEG
polymers, of varying molecular weight, PAA polymers, and PHEMA
polymers, as well as human, bovine, and pig corneas in vivo using a
modified blind well chamber apparatus developed in our laboratory.
In these experiments, non-porous mylar and dialysis membranes (MWCO
12 kD-14 kD) were used as negative and positive controls,
respectively. Glucose diffusion coefficients for PEG/PAA were
calculated using Fick's law and taking into account the sample
thicknesses. Similarly, glucose diffusion coefficients for human,
bovine, and pig corneas were also calculated taking into account
corneal thicknesses. Our results indicate that PEG/PAA IPNs have D
values between about 1.0.times.10.sup.-06 cm.sup.2/s and
3.0.times.10.sup.-06 cm.sup.2/s depending on the molecular weight
of the PEG macromonomer. This is consistent with the published
values of the diffusion coefficient of the human, bovine, rabbit
and pig corneas we have measured in vitro, which are all on the
order of D .about.10.sup.-06 cm.sup.2/sec.
[0137] We next compared PEG/PAA IPNs made with different MW PEG to
single networks made of PEG or PAA. The results show that glucose
permeability changes depending on the MW of PEG in the network. The
threshold of permeability should be between 10.sup.-05-10.sup.-07
cm.sup.2/sec, which is the physiologic range necessary to sustain
healthy corneal tissue.
[0138] As one of ordinary skill in the art will appreciate, various
changes, substitutions, and alterations could be made or otherwise
implemented without departing from the principles of the present
invention. Accordingly, the scope of the invention should be
determined by the following claims and their legal equivalents.
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