(701f) Tuning the Mechanical Properties of Chondroitin Sulfate Hydrogels Independently of Polymer Composition Using Oligo(ethylene glycol) Diacrylates

Chondroitin
sulfate (CS) is a glycosaminoglycan that is a major component of the mammalian
extracellular matrix.  Therefore CS has been studied by a number of groups
as a component of tissue engineering scaffolds, both as a bioactive component
and as a structural component.  A major limitation of hydrogel scaffolds,
including those of crosslinked CS gels, is that they
typically lack required load bearing, stiffness and fracture properties. For
particular cell lines, there may be a specific composition of CS in the gel
that leads to optimal cell viability, proliferation, differentiation
and ECM production. In fact, we have demonstrated this for chondrocytes
encapsulated in agarose gels interpenetrated by a
copolymer network of poly(ethylene glycol diacrylate) and methacrylated chondroitin
sulfate (MCS).  Thus it is important
to be able to tune the modulus of a multicomponent hydrogel scaffold
independently of the level of its bioactive component. In this work, we show
how to tune the modulus of methacrylated CS (MCS)
gels of a specified polymer composition using low levels of oligo(ethylene glycol) diacrylates (OEGDAs) as crosslinkers,
and to modulate the hydrogel composition by copolymerization of MCS with
OEGDAs.

A
systematic study on hydrogels of chondroitin sulfate crosslinked
with ethylene glycol acrylate and methacrylate conjugated molecules was carried
out. Methacrylated chondroitin sulfate was prepared
by grafting photocrosslinkable methacrylate groups on
chondroitin sulfate backbone (49% and 63% degrees of methacrylation).
Methacrylated chondroitin sulfate (MCS) gels were
prepared through photoinitiated free radical
polymerization of precursor solution. A variety of difunctional
ethylene glycol-based crosslinkers were tested,
ranging from one to 13 ethylene glycol repeat groups between terminal acrylate
or methacrylate groups. The swelling degree and the compressive modulus and
fracture properties of the network were evaluated. In the first study, the
ratio of MCS to the OEGDAs or OEGMAs was kept relatively high to determine the
effectiveness of these molecules as crosslinkers for
MCS and therefore to tune the swelling and mechanical performance of
chondroitin sulfate hydrogels as desired. In the second study, copolymers of
MCS and OEGDAs were made to learn how to tune the mechanical properties while
varying the MCS composition to values that could be optimal for tissue
development. Homopolymer gels of MCS (13 wt%) were highly swollen (swelling degree of 230 g/g for
49% methacrylated MCS) and quite soft (compressive
Young's modulus of 6 kPa). However, addition of a low
percentage (on the order of 0.5 wt%) of cross-linker
reduced the swelling to as low as 13 g/g while increasing the compressive
moduli of the networks significantly (as high as 790 kPa),
with the modulus increasing as the number of EG repeats in the crosslinker increased. Diacrylates
were much more effective crosslinkers than dimethacrylates.  Fracture strain was insensitive to
the degree of OEGDA crosslinking, suggesting that fracture strain is inherently
limited by the highly extended nature of the CS molecule itself. In MCS/EG₁₃DA copolymers, changing total
polymer content and macromer/crosslinker
ratios decreased the swelling degree (2.6 for 20wt%-20wt% formulation) and
significantly increased the modulus. Furthermore, as the EG₁₃DA weight percent in the hydrogel
increased, fracture strain of the network increased between ~12% for 100% MCS
gels to ~30% for 1:1 MCS/EG₁₃DA gels of varying total polymer composition. Both of these synthesis
approaches- OEGDA crosslinking and OEGDA copolymerization- results in formation
of biodegradable networks where the modulus can be tuned independently of the
MCS composition. Thus we have demonstrated strategies for tuning the mechanical
properties of CS gels independently of the CS composition by a method that we
believe can be extended to other multicomponent hydrogel scaffolds.