Stimuli-Responsive Iron-Cross-Linked Hydrogels That
Undergo Redox-Driven Switching between Hard and Soft States
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Abstract
A unique class of stimuli-responsive
hydrogels, termed electroplastic
elastomers (EPEs), whose mechanical properties can be reversibly tuned
between hard and soft states with the application of an electric potential,
is described. Electrochemically reversible cross-links formed within
a permanent, covalently cross-linked polymeric hydrogel network are
switched between strongly binding Fe<sup>3+</sup> and weak to nonbinding
Fe<sup>2+</sup>, as determined by potentiometric titration. With the
incorporation of graphene oxide (GO) into the EPE, a significant enhancement
in modulus and toughness was observed, allowing for the preparation
of thinner EPE samples, 80–100 μm in thickness, which
could be reversibly cycled between soft (Young’s modulus: ∼0.38
MPa) and hard (∼2.3 MPa) states over 30 min. Further characterization
of EPE samples by magnetic susceptibility measurements suggests the
formation of multinuclear iron clusters within the gel