Tailoring the Dependency between Rigidity and Water
Uptake of a Microfabricated Hydrogel with the Conformational Rigidity
of a Polymer Cross-Linker
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Abstract
Many diverse applications utilize
hydrogels as carriers, sensors,
and actuators, and these applications rely on the refined control
of physical properties of the hydrogel, such as elastic modulus and
degree of swelling. Often, hydrogel properties are interdependent;
for example, when elastic modulus is increased, degree of swelling
is decreased. Controlling these inverse dependencies remains a major
barrier for broader hydrogel applications. We hypothesized that polymer
cross-linkers with varied chain flexibility would allow us to tune
the inverse dependency between the elastic modulus and the degree
of swelling of the hydrogels. We examined this hypothesis by using
alginate and poly(acrylic acid) (PAA) modified with a controlled number
of methacrylic groups as model inflexible and flexible cross-linkers,
respectively. Interestingly, the polyacrylamide hydrogel cross-linked
by the inflexible alginate methacrylates exhibited less dependency
between the degree of swelling and the elastic modulus than the hydrogel
cross-linked by flexible PAA methacrylates. This critical role of
the cross-linker’s inflexibility was related to the difference
of the degree of hydrophobic association between polymer cross-linkers,
as confirmed with pyrene probes added in pregel solutions. Furthermore,
hydrogels cross-linked with alginate methacrylates could tune the
projection area of adhered cells by solely altering elastic moduli.
In contrast, gels cross-linked with PAA methacrylates failed to modulate
the cellular adhesion morphology due to a lower, and smaller, elastic
modulus range to be controlled. Overall, the results of this study
will significantly advance the controllability of hydrogel properties
and greatly enhance the performance of hydrogels in various biological
applications