Novel Hydrogel Actuator Based on Biomimetic Chemistry

Abstract

Hydrogel actuators were prepared by combining ionoprinting technique with reversible metal ion coordination chemistry found in mussel adhesive proteins. Hydrogels were formulated with biomimetic dopamine moiety, which contains a catechol side chain that is capable of forming mono-, bis-, and tris-complexes with ferric (Fe³⁺) ions with increasing pH. Catechol-Fe³⁺ complexation increased local crosslinking density, which induced hydrogel bending at the site of Fe³⁺ ionoprinting. The effect of pH on the dynamic response of hydrogel actuation was tracked by following the radius of curvature at the ionoprinting site. Both the rate of change and the maximum radius of curvature increased when the pH with increasing pH (2.5–9.5), indicating that pH can be used to modulate hydrogel actuation. Additionally, hydrogels containing Fe³⁺ demonstrated higher extent of deswelling when equilibrated at a basic pH. Similarly, dynamic mechanical analysis in the compression mode revealed that both the storage and loss modulus values for Fe³⁺-containing hydrogels increased with increasing pH. These results indicated that bis- and tris-complexes formed at an elevated pH level contributed to a faster rate of actuation and a more condensed network architecture. Hydrogel actuation and deswelling were also observed at pH of 3.5 although to a lesser degree, potentially due to a stronger affinity between network-bound catechol and Fe³⁺ ions as compared to complexes formed in a dilute solution.