Effect of metal ion type on the movement of hydrogel actuator based on catechol-metal ion coordination chemistry

Catechol-containing hydrogels were ionoprinted with various metal ions (copper, zinc, aluminum, and titanium) to investigate the effect of the metal ion type on the movements of hydrogel actuators based on the coordination chemistry found in mussel adhesive proteins. The movement of the ionoprinted hydrogels was characterized by monitoring the bending curvature at the site of ionoprinting and compared with previously published results for iron ions. The rate of curvature change (R′) was highly dependent on the strength of catechol-metal ion interaction, which varied in the following order: Ti4+ \u3e \u3e Fe3+ ≈ Al3+ ≈ Cu2+ \u3e Zn2+. The extremely fast rate of actuation for Ti4+ ions was due to its ability to displace borate protecting group, which eliminated the need for pH change to initiate hydrogel movement. The maximum degree of bending curvature (Rmax) followed the order of Ti4+ \u3e Fe3+ \u3e Al3+ ≈ Cu2+ ≈ Zn2+, and is dependent on the stoichiometry of the catechol-metal ion complex (i.e., bis- vs. tris-complex) as well as the strength of the interaction. Depending on the metal and the applied voltage, hydrogels exhibited a wide range of R′ (0.1–2.5 mm−1 s−1) and Rmax (0.1–0.5 mm−1) values. When a single piece of hydrogel was ionoprinted with different metal ions, different sections of the hydrogel actuated independently at different rates and to different bending curvatures. Additionally, these actuators can be reprogramed to transform into different three-dimensional shapes and the movement of the hydrogel can be initiated on demand. These are unique features not observed in other hydrogel-based actuators