U.S. patent application number 11/858062 was filed with the patent office on 2008-03-20 for degradable thiol-ene polymers.
This patent application is currently assigned to The Regents of the University of Colorado. Invention is credited to Kristi Anseth, Christopher Bowman, Bilge Hacioglu, Charlie Nuttelman.
Application Number | 20080070786 11/858062 |
Document ID | / |
Family ID | 23281918 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080070786 |
Kind Code |
A1 |
Bowman; Christopher ; et
al. |
March 20, 2008 |
Degradable Thiol-ene Polymers
Abstract
A thiol-ene polymeric material is disclosed. The material is
produced by the photopolymerization of reactants having thiol and
olefin moieties. The material can incorporate encapsulated
components, including cells. Additionally, the material can be
derivatized by reacting the polymeric material with components such
as proteins.
Inventors: |
Bowman; Christopher;
(Boulder, CO) ; Anseth; Kristi; (Boulder, CO)
; Hacioglu; Bilge; (Boulder, CO) ; Nuttelman;
Charlie; (Boulder, CO) |
Correspondence
Address: |
SHERIDAN ROSS PC
1560 BROADWAY
SUITE 1200
DENVER
CO
80202
US
|
Assignee: |
The Regents of the University of
Colorado
Denver
CO
|
Family ID: |
23281918 |
Appl. No.: |
11/858062 |
Filed: |
September 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10269916 |
Oct 10, 2002 |
7288608 |
|
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11858062 |
Sep 19, 2007 |
|
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60328669 |
Oct 10, 2001 |
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Current U.S.
Class: |
504/359 ;
435/182; 514/772.3; 524/21; 528/373 |
Current CPC
Class: |
C08G 75/045 20130101;
C08J 3/075 20130101; C08L 83/00 20130101; A61L 27/54 20130101; C05G
5/37 20200201; A01N 25/04 20130101; A61L 27/18 20130101; C08G
81/025 20130101; C08G 81/027 20130101; C08J 2329/04 20130101; C08L
81/02 20130101; C08G 81/00 20130101; C08J 2371/02 20130101; C08L
2666/22 20130101; C08L 71/02 20130101; A61L 27/38 20130101; A61L
27/16 20130101; C08L 89/00 20130101; Y10S 524/916 20130101; A61L
2300/62 20130101; A61L 27/52 20130101; C08J 2367/04 20130101; C08L
81/02 20130101; C08L 2205/05 20130101; C08L 71/02 20130101; C05G
5/23 20200201; C08G 2261/126 20130101; Y10S 525/936 20130101; A61K
47/34 20130101 |
Class at
Publication: |
504/359 ;
435/182; 514/772.3; 524/021; 528/373 |
International
Class: |
C08G 75/00 20060101
C08G075/00; A01N 25/28 20060101 A01N025/28; C12N 11/04 20060101
C12N011/04; A61K 47/30 20060101 A61K047/30 |
Goverment Interests
GOVERNMENT INTEREST
[0002] This invention was made with government support under
National Science Foundation Grant No. CTS 945-3369. The U.S.
government has rights in the invention.
Claims
1. A polymeric material comprising repeating units of the formula:
--[--S--R.sub.1--S--C--C--R.sub.2--C--C]-- wherein R.sub.1 and
R.sub.2 are independent linkers, at least one of R.sub.1 and
R.sub.2 are degradable, and the polymeric material comprises at
least one biologically active component encapsulated within the
polymeric material.
2. The polymeric material, as claimed in claim 1, wherein the
biologically active component is selected from the group consisting
of cells, tissues, and tissue aggregates.
3. The polymeric material, as claimed in claim 1, wherein the
biologically active component is selected from the group consisting
of chondrocytes, immortalized cell lines, stem cells,
hormone-producing cells, and fibroblasts.
4. A polymeric material comprising repeating units of the formula:
--[--S--R.sub.1--S--C--C--R.sub.2--C--C]-- wherein R.sub.1 and
R.sub.2 are independent linkers, at least one of R.sub.1 and
R.sub.2 are degradable, and the polymeric material is derivatized
with a functional molecule.
5. The polymeric material, as claimed in claim 4, wherein the
polymeric material is derivatized through a thiol linkage.
6. The polymeric material, as claimed in claim 4, wherein the
material is derivatized with a protein.
7. The polymeric material, as claimed in claim 6, wherein the
protein is selected from the group consisting of adhesion peptides,
growth factors, hormones, antihormones, signaling compounds, serum
proteins, albumins, macroglobulins, globulins, agglutinins,
lectins, antibodies, antigens, enzymes, and extracellular matrix
proteins.
8. A polymeric material comprising repeating units of the formula:
--[--S--R.sub.1--S--C--C--R.sub.2--C--C]-- wherein R.sub.1 and
R.sub.2 are independent linkers, at least one of R.sub.1 and
R.sub.2 are degradable, and at least one agricultural chemical is
encapsulated within the polymeric material.
9. The polymeric material, as claimed in claim 8, wherein the
agricultural chemical is selected from the group consisting of
fungicides, herbicides, fertilizers, pesticides, carbohydrates,
nucleic acids, organic molecules, and inorganic biologically active
molecules.
10. A polymeric material comprising repeating units of the formula:
--[--S--R.sub.1--S--C--C--R.sub.2--C--C]-- wherein R.sub.1 and
R.sub.2 are independent linkers, at least one of R.sub.1 and
R.sub.2 are degradable, and at least one pharmacologically active
agent is encapsulated within the polymeric material.
11. The polymeric material, as claimed in claim 10, wherein the
pharmacologically active agent is selected from the group
consisting of analgesics, antipyretics, nonsteriodal
antiinflammatory drugs, antiallergics, antibacterial drugs,
antianaemia drugs, cytotoxic drugs, antihypertensive drugs,
dermatological drugs, psychotherapeutic drugs, vitamins, minerals,
anorexiants, dietetics, antiadiposity drugs, carbohydrate
metabolism drugs, protein metabolism drugs, thyroid drugs,
antithyroid drugs, and coenzymes.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/269,916, filed Oct. 10, 2002, which claims priority
under 35 U.S.C. .sctn.119(e) from U.S. Provisional Application Ser.
No. 60/328,669 filed Oct. 10, 2001, the complete disclosures of
these priority documents are incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The invention is directed to the production of degradable
thiol-ene based polymers via photopolymerization.
BACKGROUND OF THE INVENTION
[0004] Recent approaches in the field of tissue engineering involve
the use of polymeric biomaterials as cell scaffolds, which provide
cells with a three-dimensional support material on which to grow.
Despite a recent expansion in the design and development of
suitable scaffold materials, there is still a lack of suitable
scaffold materials with systematically variable properties. Without
suitable materials available with a wide range of properties to
serve as scaffolds for tissue engineering, it is unlikely that the
field will achieve its full potential.
[0005] Advances in polymer chemistry and materials science have
spawned the development of numerous biomaterials and scaffolding
methods that have potential uses in a wide range of tissue
engineering applications. Several criteria must be achieved in the
design of a biomaterial. First, the material must be biocompatible.
That is, it must not promote an immune, allergenic, or inflammatory
response in the body. Also, a method must exist to reproducibly
process the material into a three-dimensional structure. Adhesive
properties of the surface of the biomaterial must permit cell
adhesion and promote growth. In addition, the biomaterial should
have a high porosity to facilitate cell-polymer interactions,
improve transport properties, and provide sufficient space for
extracellular matrix generation. Finally, depending upon the
particular application, the biomaterial should be biodegradable
with an adjustable degradation rate so that the rate of tissue
regeneration and the rate of scaffold degradation can be
matched.
[0006] Natural materials, such as collagen and many
polysaccharides, generally exhibit a limited range of physical
properties, are difficult to isolate, and cannot be manufactured
with a high degree of reproducibility. However, natural materials
often are more biocompatible and may even have specific biologic
activity. Synthetic materials, on the other hand, can be cheaply
and reproducibly processed into a variety of structures and the
mechanical strength, hydrophilicity, and degradation rates of
synthetic scaffolds are more readily tailored. However, synthetic
polymers can cause inflammatory responses when implanted in the
host. Recent tissue engineering endeavors have attempted to combine
properties of both natural and synthetic polymers in the design of
a suitable scaffold.
[0007] Polylactide (PLA), polyglycolide (PGA) and their copolymers
(PLGA) are polyesters based on naturally occurring lactic and
glycolic acids (cc-hydroxy acids). They have been used as
biodegradable sutures and implantable materials for more than two
decades. They are biocompatible and biodegradable, and these
polymers have a history of use as polymer scaffolds in tissue
engineering. However, their highly crystalline and hydrophobic
nature makes it difficult to control their biodegradation process
and mechanical properties. Moreover, because of the lack of pendant
functional groups, it is extremely difficult to modify the surface
chemistry of PLA and PGA. For example, proteins and other molecules
that may facilitate cell adhesion and growth cannot be easily
attached to the backbone of these polymers because there is no
chemical "handle" with which to derivatize these substrates.
Attempts to introduce functional groups into these types of
polymers include copolymerization of the lactide and glycolide
cyclic monomers with more easily derivatizable monomers such as
cyclic lysine monomers modified by peptide attachments.
[0008] Recently, alternating copolymers of .alpha.-hydroxy acids
and .alpha.-amino acids (polydepsipeptides) have been obtained with
functional side groups. Additionally, poly(L-lactides) containing
.beta.-alkyl .alpha.-malate units have been prepared by ring
opening copolymerization of L-lactide with a cyclic diester. Major
drawbacks remain with these lactide based copolymers including the
difficulty in synthesis of cyclic monomers that are used in the
copolymerization with lactide and the generally low reaction
yields. Thus, the difficult synthesis and the low reaction yields
make the commercialization of the modified polylactide biomaterials
improbable and make it nearly impossible to tailor chemical,
physical, and degradation properties of the final polymer.
[0009] Photopolymerization systems have numerous advantages for
matrix production. First, photoinitiation allows facile control
over the polymerization process with both spatial and temporal
control. For example, a liquid macromer solution can be injected
into an area of the body, formed into a particular shape, and
photopolymerized on demand using a light source. The final polymer
hydrogel maintains the shape of that specific area of the body,
allowing intimate control over the final shape of the hydrogel and
improved adhesion and integration. In addition, the
photocrosslinking chemistry creates covalently crosslinked networks
that are dimensionally stable.
[0010] Known photopolymerization processes, however, suffer from a
number of drawbacks, including: the use of a separate initiator
specie that is cytotoxic at relatively low concentrations, the
difficulty in polymerizing thick samples because of light
attenuation by the initiator, the inhibition of the radical
polymerization by oxygen present in the air (which slows the
polymerization), and the ability to fabricate gels with a diverse
range of properties, especially gels with a high water content
while maintaining high mechanical strength. Thus, there exists a
need for biocompatible hydrogels which can polymerize in the
absence of cytotoxic initiators and which can be tailored to have
specific chemical, physical, and degradation properties under
physiological conditions.
SUMMARY OF THE INVENTION
[0011] One embodiment of the present invention is a polymeric
material having repeating units of the formula:
--[--S--R.sub.1--S--C--C--R.sub.2--C--C--]--, wherein R.sub.1 and
R.sub.2 are independent linkers, and at least one of R.sub.1 and
R.sub.2 are degradable. R.sub.1 and R.sub.2 can be independently
selected from poly(lactic acid), poly(ethylene glycol), poly(vinyl
alcohol), and mixtures thereof, and one or both of R.sub.1 and
R.sub.2 can have a degree of branching of greater than two. The
polymeric material is preferably biocompatible, and can have a
minimum dimension of at least about 4 cm.
[0012] The polymeric materials of the present invention can be
produced by a process that includes combining a first reactant of
the formula R.sub.1--(C.dbd.C).sub.n with a second reactant of the
formula R.sub.2--(SHF).sub.m, wherein n and m are independently
integers greater than one and R.sub.1 and R.sub.2 are as described
above. The combined reactants are then irradiated with light to
cause reaction between the first and second reactants and
eventually between the formed products to obtain the polymeric
material. This process can include irradiating the reactants in the
absence of a chemical initiator.
[0013] In a further embodiment, the polymeric material can include
at least one biologically active component encapsulated within it.
The biologically active component can be selected from the group
consisting of cells, tissues, and tissue aggregates, such as
chondrocytes, immortalized cell lines, stem cells,
honnone-producing cells, or fibroblasts. Additionally, the
biologically active component can include pharmacologically active
agents or agricultural chemicals. Pharmacologically active agent
functional molecules can include analgesics, antipyretics,
nonsteriodal antiinflammatory drugs, antiallergics, antibacterial
drugs, antianaemia drugs, cytotoxic drugs, antihypertensive drugs,
dermatological drugs, psychotherapeutic drugs, vitamins, minerals,
anorexiants, dietetics, antiadiposity drugs, carbohydrate
metabolism drugs, protein metabolism drugs, thyroid drugs,
antithyroid drugs, or coenzymes. Agricultural chemical functional
molecules can include fungicides, herbicides, fertilizers,
pesticides, carbohydrates, nucleic acids, organic molecules, or
inorganic biologically active molecules.
[0014] In another embodiment, the polymeric material can be
derivatized with a fanctional molecule, for example, by forming the
polymeric material with excess thiol groups and reacting the
functional molecule with such excess thiol groups. The functional
molecules can be, for example, proteins, agricultural chemicals, or
pharmacologically active agents. Protein functional molecules can
include adhesion peptides, growth factors, hormones, antihormones,
signaling compounds, serum proteins, albumins, macroglobulins,
globulins, agglutinins, lectins, antibodies, antigens, enzymes, or
extracellular matrix proteins. The polymeric material of the
present invention can also be configured to form a degradable
commodity plastic.
[0015] A further embodiment of the present invention includes a
thiol-ene hydrogel having poly(lactic acid), poly(ethylene glycol),
and poly(vinyl alcohol) polymeric segments, wherein at least one of
the segments has a degree of branching of greater than two. In this
embodiment, the thiol-ene hydrogel has a modification selected from
encapsulation of at least one biologically active component within
the thiol-ene hydrogel and derivatization of the thiol-ene hydrogel
with a functional molecule.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows the general scheme for thiol-ene
polymerization.
[0017] FIG. 2 shows a scheme for the formation of a thiol-ene
hydrogel formed from derivatized PLA, PEG and PVA monomers.
[0018] FIG. 3 shows schemes for derivatizations of poly(vinyl
alcohol).
[0019] FIG. 4 shows schemes for derivatizations of poly(lactic
acid).
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention is directed to a novel class of
degradable scaffolds which are biocompatible thiol-ene hydrogels
built upon degradable materials, such as PVA, PEG and PLA blocks,
that can incorporate chemicals and live cells within the polymer
matrix
[0021] Thiol-ene polymerizations are photochemically initiated,
step growth, free-radical processes that take place between thiols
and olefins via a sequential propagation/chain-transfer process.
For polymerization to occur, each thiol-containing component must
have an average of at least two thiol groups and each
olefin-containing component must have at least two ene functional
groups, (i.e. the monomer must contain two or more double bonds).
Polymerization of a dithiol and a diene results in the formation of
a linear polymer, rather than a crosslinked polymer. Crosslinked
gels can be readily formed by increasing the functionality, i.e.,
increasing the degree of branching, of one or both of the monomers
to be greater than two. Thiol-ene polymerizations have a number of
significant and unique advantages that make them particularly
beneficial. These benefits include a step growth polymerization
that causes the molecular weight to build up more slowly, the
ability to photoinitiate the sample without any need for a distinct
(and possibly cytotoxic) initiator specie, the ability to
polymerize extremely thick (more than 30 cm) samples because of a
self-eliminating light intensity gradient, the very low radical
concentration present during polymerizations producing less
cellular damage from the free radicals, the lack of oxygen
inhibition and the ease with which monomers of significantly
varying chemistry can be copolymerized.
[0022] Thiol-ene systems form ground state charge transfer
complexes, and therefore photopolymerize even in the absence of
initiators in reasonable polymerization times. Since the complex
which absorbs the light is consumed by the polymerization, the
polymer itself does not absorb light. Thus, polymerization can
proceed to extremely great depths, and no potentially toxic
initiator is required to initiate the polymerization. The polymer
properties can be tailored by appropriate monomer choices since the
products are regular, alternating copolymers. Nearly any
unsaturated monomer can polymerize via this mechanism in the
presence of a suitable polythiol and light.
[0023] The scheme shown in FIG. 1 is the general polymerization
mechanism. The charge transfer complex forms by the interaction of
the thiol group with the double bond of the ene followed by
electron transfer and formation of a thiyl radical upon exposure to
light. The thiyl radical then initiates the polymerization.
Termination involves radical-radical combinations of either
.beta.-carbon radicals or thiyl radicals.
[0024] One embodiment of the present invention is a polymeric
material comprising repeating units of the formula:
--[--S--R.sub.1--S--C--C--R.sub.2--C--C--]-- wherein R.sub.1 and
R.sub.2 are independent linkers, and at least one of R.sub.1 and
R.sub.2 are degradable. Thus, the chemical natures of R.sub.1 and
R.sub.2 are independent, that is, they can be the same or
different. R.sub.1 and R.sub.2 function as linkers to link together
the thiol-ene junctures. In accordance with the present invention,
the polymeric material is preferably produced by a process of
combining a first reactant of the formula R.sub.1--(C.dbd.C), with
a second reactant of the formula R.sub.2--(SH).sub.m, wherein n and
m are independently integers greater than one and R.sub.1 and
R.sub.2 are as defined above. The first and second reactants are
then irradiated with light to cause reaction between the first and
second reactants to form the polymeric material. In alternative
embodiments, the polymeric material of the present invention can
include additional linker segments, R.sub.3 . . . R.sub.n. Such
additional linker segments meet the requirements set forth herein
for R.sub.1 and R.sub.2 For example, a polymeric material having
the repeating unit described above can further comprise repeating
units of the formula:
--[--S--R.sub.3--S--C--C--R.sub.4--C--C--]--
[0025] wherein R.sub.3 and R.sub.4 are independent linkers.
[0026] As used herein, the term "degradable," with reference to the
R.sub.1 and R.sub.2 segments and the polymeric materials of the
present invention refers to a segment or material having a
molecular structure which can decompose to smaller molecules. Such
degradation or decomposition can be by various chemical mechanisms.
For example, a degradable polymer can be hydrolytically degradable
in which water reacts with the polymer to form two or more
molecules from the polymer by chemical bonds in the molecule being
hydrolyzed, thus producing smaller molecules. In a further
embodiment of the present invention, the segments or materials are
biodegradable. Biodegradability refers to a compound which is
subject to enzymatic decomposition, such as by microorganisms, or
to a compound, portions of which are subject to enzymatic
decomposition, such as by microorganisms.
[0027] R.sub.1 and R.sub.2, while at least one is degradable, can
be chemically diverse. In preferred embodiments, R.sub.1 and
R.sub.2 can be selected from poly(lactic acid) (PLA), polyglycolide
(PGA), copolymers of PLA and PGA (PLGA), poly(vinyl alcohol) (PVA),
poly(ethylene glycol) (PEG), poly(ethylene oxide), poly(ethylene
oxide)-co-poly(propylene oxide) block copolymers (poloxamers,
meroxapols), poloxamines, polyanhydrides, polyorthoesters,
poly(hydroxy acids), polydioxanones, polycarbonates,
polyaminocarbonates, poly(vinyl pyrrolidone), poly(ethyl
oxazoline), carboxymethyl cellulose, hydroxyalkylated celluloses
such as hydroxyethyl cellulose and methylhydroxypropyl cellulose,
and natural polymers such as polypeptides, polysaccharides or
carbohydrates such as polysucrose, hyaluranic acid, dextran and
similar derivatives thereof, heparan sulfate, chondroitin sulfate,
heparin, or alginate, and proteins such as gelatin, collagen,
albumin, or ovalbumin, or copolymers, or blends thereof. In
particularly preferred embodiments, R.sub.1 and R.sub.2 can be
selected from poly(lactic acid) (PLA), poly(vinyl alcohol) (PVA),
and poly(ethylene glycol) (PEG). PLA monomers provide degradability
to the system while PVA and PEG enhance the hydrophilic nature of
the hydrogel and provide for the possibility of further
derivatization and/or extensive crosslinking.
[0028] R.sub.1 and R.sub.2 can vary in size depending upon desired
properties for the resulting polymeric material. More particularly,
the molecular weight for R.sub.1 and R.sub.2 can range from about
30 DA to about 50000 Da. Prior to formation of the polymeric
material of the present invention, R.sub.1 and R.sub.2 are
derivatized to include thiol or olefin moieties such that they can
participate in photo-initiated thiol-ene polymerization. Thiolated
macromers such as poly(ethylene glycol) dithiol are available
commercially. The olefin moieties can be selected from any suitable
compound having a carbon double bond. For example, the olefin
moiety can be selected from any suitable ethylenically unsaturated
group such as vinyl, acetyl, vinyl ether, allyl, acrylate,
methacrylate, maleimide, and norbornene. Thus, it will be
appreciated that in the repeating unit shown above, the carbons can
be CH.sub.2 or can be substituted at one or more than one of the
carbons within the repeating group, even including ring structures
incorporating a double bond. If each of R.sub.1 and R.sub.2 are
derivatized with either two thiol or two olefin moieties, the
resulting thiol-ene polymer would be a linear copolymer composed of
alternating R.sub.1 and R.sub.2 segments. However, the thiol-ene
polymeric material is preferably formed to contain cross-linking
and branching. Thus, the derivatized R.sub.1 and R.sub.2 segments
preferably have more than two thiol or olefin moieties per molecule
that can participate in crosslinking and polymerization. The extent
of the branching and crosslinking can be controlled by the use of
differently derivatized R.sub.1 and R.sub.2 segments and control
over the concentration of the starting materials.
[0029] By photoinitiation of the thiol-ene polymerization reaction
with these monomeric, oligomeric or polymeric starting materials,
high molecular weight, crosslinked networks are obtainable in the
presence or absence of a chemical initiator within reasonable
reaction times. This is a very important property inherent to the
polymerization reactions, which can eliminate the adverse effects
of chemical initiators and still obtain rapid curing. Because of
the step growth nature of the polymerization, these polymers have
significantly lower glass transition temperatures and higher
degrees of swelling than homopolymer diacrylate analogues. Thus,
simple changes in molecular weight, number of functional groups,
and the chemistry of the monomer between the functional groups
allow facile control of the polymer properties over a wide
range.
[0030] FIG. 2 illustrates an example of a thiol-ene hydrogel that
can be formed from derivatized PLA, PEG and PVA monomers. The
resulting hydrogel is formed from the PLA triene (e.g., made from
glycerol with three lactide arms and subsequent ene attachment),
the PEG dithiol (such as the commercially available PEG dithiol ),
and the partially acrylated PVA derivatized to include the
well-known RGD adhesion sequence. Hydrogel matrices of this type
facilitate independent control of the (i) mechanical properties by
adjusting the PVA and PLA functionality, (ii) swelling through
adjustments to the relative amount of PEG, (iii) the degradation
timescale through adjustments in the molecular weight of the PLA
arms, and (iv) attachment of biomolecules such as signaling
compounds to the PVA backbone. The resulting networks will be three
dimensional, hydrophilic, porous structures that can be further
modified by the attachment of molecules of interest to the pendant
--OH groups of PVA to impart therapeutic or other properties to the
hydrogel. Thiol-ene polymers are alternating copolymers, but
because a monomer can be derivatized in numerous ways, the ability
to vary the composition of the copolymers exists.
[0031] A list of some properties of the thiol-ene hydrogel
illustrated in FIG. 2 that can be influenced by modifying
parameters of the individual monomers is shown in Table 1.
Degradation rate, mechanical properties, crosslink density and
swelling can each be controlled with systematic changes in the
amounts, molecular weights or functionality of the various
monomers. For example, in considering control of the degradation
rate of the polymer matrix, the simplest method for controlling
this feature is to change the molecular weight of the oligomeric
PLA branches. The higher the molecular weight of the branch, the
more rapidly the system will degrade. This phenomena, which is
different from what might be observed in linear PLA systems, arises
because the PLA segments may act as crosslinks in the system. As
the molecular weight of the PLA crosslink increases, the
probability that any one of the repeat units and hence the
crosslink will be degraded is higher, thus leading to more rapid
degradation of the PLA crosslinks. TABLE-US-00001 TABLE 1 Influence
of Monomer Amounts and Structural Features on the Polymer Matrix
Properties Monomer Parameters Primary Secondary to be Varied
Influence Influence Molecular Weight of Degradation Rate Swelling
(minor) PLA Branches Number of PLA Branches Crosslink Density
Swelling per PLA Monomer (i.e., Mechanical Properties the
functionality) Amount of PLA Crosslink Density Swelling (minor)
Monomer Mechanical Properties Degree of Substitution on Crosslink
Density Swelling - also PVA Backbone Mechanical Properties changes
because of consumption of hydrophilic --OH functional groups Amount
of PEG Swelling Crosslink Monomer Density (minor)
[0032] Another parameter that dictates the network properties is
monomer functionality. For example, in the example shown in FIG. 2,
as the number of reactive functional groups on the PLA branched
oligomer or PVA increases, the extent of crosslinking increases,
giving more rigid hydrogels. Increasing the functionality of the
PVA monomers requires consuming additional --OH functional groups
and converting them to thiols or vinyl substituents. The loss of
hydroxyl functional groups reduces the network hydrophilicity to a
minor degree, thus impacting the swelling. Additionally, for both
of these changes, the increase in crosslink density impacts the
initial equilibrium swelling; however, the swelling is more easily
controlled by the amount of PEG added to the matrix. The
functionality of the PLA will be adjusted by starting with di-,
tri-, and tetra-functional alcohols in the PLA synthesis to obtain
di-, tri- and tetra-functional oligomers (i.e. oligomers with two,
three and four branches). The size of the oligomer chains is
controlled during the synthesis by changing the ratio of hydroxyl
groups to lactides. The PVA functionality can be manipulated by
replacing between about 2% and about 10% of the hydroxyl functional
groups with vinyl or thiol groups.
[0033] As noted above, the thiol-ene hydrogels of the present
invention are prepared from biocompatible monomers. A biocompatible
material does not promote an immune, allergenic or inflammatory
response in the body. The resulting hydrogels are therefore
biocompatible as well and can be used internally for the purposes
of tissue engineering. Because the individual monomers are
biocompatible and the polymerization process itself can be free of
toxic chemical initiators, it is also possible to encapsulate
biologically active materials, such as cells, tissue and tissue
aggregates during the polymerization process thereby trapping such
materials within the biocompatible hydrogel matrix. These materials
are then supported within the matrix and can function within the
correct temperature, water and nutrient environment. Cells of
interest for encapsulation include chondrocytes, immortalized cell
lines, stem cells, hormone-producing cells, fibroblasts and the
like. To have an optimal cell environment, the hydrophilicity and
transport properties (e.g. diffusion) must be controlled. In
particular, the matrices must allow for the ready transport of
nutrients and oxygen to encapsulated cells, as well as the removal
of cellular waste products. Suitable matrices include
multi-branched PLA and PVA chains either linked to each other or to
PEG segments to form a three dimensional structure. PEG is
extremely hydrophilic. Therefore, the presence of large amounts of
PEG (as well as the remaining hydroxyls from the PVA) assure that
the degree of swelling of the hydrogel is high and that transport
is facile.
[0034] In a further embodiment of the present invention, monomers
are synthesized that contain chemical links to allow for
derivatization of the polymeric material with functional molecules
as well as the necessary thiol and olefin moieties for formation of
the hydrogel. For example, the monomers could be derivatized to
contain multiple thiol groups, some of which are derivatized to
link a protein while others are left free to participate in the
thiol-ene polymerization thereby forming a thiol-ene hydrogel
containing bound protein. Alternatively, a thiol-ene hydrogel can
be produced with monomers having an excess of thiol groups and
after formation, the hydrogel can be derivatized with a protein.
Knowledge of the biological events that occur at the cell-scaffold
interface plays a key role in tissue engineering. Vital
interactions occur on the molecular scale, and the proteins and
factors that are responsible for these interactions may be
incorporated into suitable scaffolding materials by derivatization
of the polymeric material of the present invention. For example,
signaling molecules, hormones, and growth factors each can be
integrated into the hydrogel through derivatization of the
monomers, macromers or polymers, thereby mimicking the native
environment (i.e., in the body) of those cells, resulting in more
efficient production of extracellular matrix and improved
tissue-like properties of the final material. One of the
significant advantages of the thiol-ene approach is the simplicity
with which the resulting polymeric networks can be derivatized. The
thiols can be easily modified either before or after polymerization
(if a slight excess of thiol is added to the polymerization, a
significant number of thiols will remain unreacted and
derivatizable).
[0035] A wide variety of molecules can be incorporated into the
polymeric material through --OH groups or --SH groups including,
but not limited to, proteins, pharmacologically active agents, and
agricultural chemicals. Alternatively, such molecules can be
encapsulated in the polymeric material in the event such molecules
would lose functionality if chemically bound to the polymeric
material. For example, types of proteins that can be incorporated
into the polymeric material include adhesion peptides (such as RGD
adhesion sequence), growth factors, hormones, antihormones,
signaling compounds, enzymes, serum proteins, albumins,
macroglobulins, globulins, agglutinins, lectins, extracellular
matrix proteins, antibodies, and antigens. Types of
pharmacologically active agents that can be incorporated into the
polymeric material include analgesics, antipyretics, nonsteriodal
antiinflammatory drugs, antiallergics, antibacterial drugs,
antianemia drugs, cytotoxic drugs, antihypertensive drugs,
dermatological drugs, psychotherapeutic drugs, vitamins, minerals,
anorexiants, dietetics, antiadiposity drugs, carbohydrate
metabolism drugs, protein metabolism drugs, thyroid drugs,
antithyroid drugs, and coenzymes. Types of agricultural chemicals
that can be incorporated into the polymeric material include
fungicides, herbicides, fertilizers, pesticides, carbohydrates,
nucleic acids, organic molecules, and inorganic biologically active
molecules. In a further embodiment, the polymeric material of the
present invention can be formed as commodity plastic products that
are typically considered to be disposable products. Because the
polymeric material of the present invention is degradable, such
products, when disposed of, will more rapidly degrade in the
environment. Such products include, for example, eating utensils,
plates, bowls, cups, food and beverage containers and
packaging.
[0036] The following experimental results are provided for purposes
of illustration and are not intended to limit the scope of the
invention.
EXAMPLES
Example 1
[0037] This example shows two possible derivatizations of
poly(vinyl alcohol).
[0038] Synthesis of a thiol macromer can proceed via several
approaches, two of which are presented in the scheme shown in FIG.
3. Both progress via a tosylate intermediate. Poly(vinyl alcohol)
was tosylated in anhydrous pyridine at 85.degree. C. overnight. The
insoluble PVA was pulled into solution as the reaction proceeded.
This tosylated PVA was then be reacted with dithiothreitol (DTT),
in one instance, and potassium thioacetate, in another instance, at
room temperature. Nucleophilic attack of the thiolate anion
displaced the tosyl group, covalently linking these molecules to
the PVA backbone through a thioether bond. PVA-thioacetate was
hydrolyzed via simple methanolysis, yielding the thiol macromer
(PVA-SH) in which the thiol groups have replaced some of the
hydroxyl groups. Using an excess of DTT in the other mechanism
guarantees that there will be free thiol groups in the resulting
molecule (PVA-DTT).
[0039] Once formed, the thiolated PVA can then be photopolymerized
in the presence of a multi-ene in an aqueous solution to provide a
crosslinked hydrogel network. Polymerizations of the thiolated PVA
with the PLA triacrylate or PLA triallyl yield a degradable,
hydrogel network in which the swelling and degradation time are
controlled by the amount and molecular weight of the trifunctional
PLA, respectively.
Example 2
[0040] This example demonstrates the feasibility of synthesizing
PLA multi-ene and PLA multithiol monomers for use in the present
invention.
[0041] A major advantage of thiol-ene hydrogels is the ability to
use a wide range of precursor molecules with varying structures and
chemistries. In particular, the technique affords the possibility
of having largely poly(lactic acid) polymers in which the
degradation rate is controlled by the PLA segment molecular weight.
To obtain this control, it is necessary to synthesize PLA
multi-enes and PLA multithiols. FIG. 4 shows the scheme for the
synthesis of PLA trithiol and PLA triacrylate.
[0042] Lactic acid oligomers with three branches were prepared with
glycerol used as an initiator to polymerize the lactide using
stannous octoate as the catalyst. Oligomers with different chain
lengths were obtained by adjusting the initiator/lactide ratio.
This oligomer was used to derivatize the hydroxyl end groups of the
three branches either with acrylates or with thiols. None of the
PLA thiol derivatives have previously been synthesized. Synthesized
macromers were characterized by FTIR and NMR. PLA triacrylate
showed all the reported IR and .sup.1H-NMR bands.
[0043] While various embodiments of the present invention have been
described in detail, it is apparent that modifications and
adaptations of those embodiments will occur to those skilled in the
art. It is to be expressly understood, however, that such
modifications and adaptations are within the scope of the present
invention, as set forth in the claims below.
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