Modulating the movement of hydrogel actuator based on catechol–iron ion coordination chemistry

• Locally introduced catechol–Fe3+ ion coordination bonds induced hydrogel bending.
• The rate and extent of hydrogel movement can be modulated by multiple factors.
• Physical and viscoelastic properties of hydrogel corroborated with hydrogel movement.
• The difference in crosslinking density in the ionoprinted region and the bulk hydrogel created the driving force for hydrogel actuation.

Hydrogel actuators were prepared by combining ionoprinting technique with reversible metal ion coordination chemistry found in mussel adhesive proteins. Hydrogels were formulated with dopamine methacrylamide (DMA), which contains a catechol side chain that forms strong complexes with ferric (Fe3+) ions. Catechol–Fe3+ ion complexation increased local crosslinking density, which induced hydrogel bending at the site of ionoprinting. The effect of multiple factors on the dynamic response of hydrogel actuation was tracked by following the bending curvature at the ionoprinting site. In general, the extent and rate of hydrogel actuation increased with increasing pH, deposited Fe3+ ion content, and DMA content but was inversely proportional to hydrogel thickness. The ability to modulate hydrogel actuation using multiple parameters is potentially useful in controlling the actuator movements. Additionally, Fe3+ ion-containing bulk hydrogels demonstrated significant reduction in molecular weight between crosslinks as well as elevated storage and loss modulus values based on oscillatory rheometry when compared to those of Fe3+-free control. These differences in physical and viscoelastic properties contributed to the actuation of ionoprinted samples. Specifically, conditions that promoted a large crosslinking differential between the ionoprinted region and the bulk hydrogel (i.e., outside of the ionoprinted region) contributed to increased rate and extent of hydrogel folding. Faster actuation at elevated pH levels was attributed to the formation of complexes with higher catechol:Fe3+ ion stoichiometric ratios. 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.