In general, mechanical properties and gelation kinetics exhibit a positive correlation with the amount of gelation reagents used. Similarly, for catechol-containing hydrogels, which have attracted significant attention, because of their unique dual properties of cohesion and adhesion, increased amounts of cross-linking agents, such as organic oxidants and/or transition metals (Fe3+), result in enhanced mechanical strength and more rapid gelation kinetics. Here, we report a new metal–ligand cross-linking chemistry, inspired by mussels and ascidians, that defies the aforementioned conventional stoichiometric concept. When a small amount of vanadium is present in the catechol-functionalized polymer solution (i.e., [V] ≪ [catechol]), organic radicals are rapidly generated that trigger the gelation reaction. However, when a large amount of the ion is added to the same solution (i.e., [V] ≫ [catechol]), the catechol remains chemically intact by coordination that inhibits gelation. Thus, a large amount of cross-linking agent is not necessary to prepare mechanically strong, biocompatible hydrogels using this system. This new chemistry may provide insight into the biological roles of vanadium and its interaction with catechol-containing molecules (i.e., determination of the liquid state versus the solid state). Excess amounts of vanadium ([V] ≫ [catechol]) coordinate with catechol, which may result in a liquid state for ascidian blood, whereas excess amounts of catechol ([V] ≪ [catechol]) generate an organic radical-mediated chemical reaction, which may result in solid-state conversion of the mussel byssal threads.

A hybrid actuator for haptic devices is proposed in this paper. The actuator is composed of a DC motor and a magneto-rheological (MR) brake to realize transparency and stable force control. Two piston cylinders are connected with a flexible tube to lighten the weight of the structures on the endpoint that interacts with an operator. Also, the MR brake is designed to be suitable for hydraulic transmission. For the proposed hybrid actuator, a cooperative force control method using a pressure sensor instead of a force sensor is proposed. To verify the proposed control algorithm, a virtual wall collision experiment was conducted using a developed prototype of the hybrid actuator.