Effect of mold assemblies-induced interfaces in the mechanical actuation of electro-responsive ionic liquid-based polycationic hydrogels

Abstract This work aims the development of electro-active polycationic soft-actuators based on copolymers of 2-hydroxyethyl methacrylate (HEMA) and 1-butyl-3-vinylimidazolium chloride (BVImCl), cross-linked with N,N’-methylene bis(acrylamide), and reinforced with mesoporous silica nanoparticles. Polycationic hydrogels were obtained by free radical polymerization in aqueous solution and using mold assemblies presenting different surface properties (e.g. Teflon® and glass). Obtained results demonstrated that a mold assembly-induced effect, at the mold/polymerization solution interface, originates “surface-to-bulk” heterogeneous copolymer concentration gradients when using Teflon®-Teflon® and Teflon®-glass mold assemblies, while homogeneous hydrogel networks were obtained when using only glass mold assemblies. Results were compared with those obtained from cross-linked poly(2-hydroxyethyl methacrylate) hydrogels to prove that the observed surface-induced polymerization effect occurs only in the presence of the ionic liquid co-monomer. Hydrogels synthesized in Teflon®-Teflon® molds presented the highest mechanical response under external electric stimulus and in different aqueous media of different pH and ionic strength. On the contrary, hydrogels synthesized in glass-glass mold assemblies presented limited mechanical actuation and only when immersed in saline aqueous media. The mechanical actuation of hydrogels synthesized in Teflon®-glass molds was shown to depend on the orientation of the applied electric current. The electro-actuation response of the synthesized electro-active polycationic hydrogels was discussed based on the ion enrichment/depletion mechanism. All prepared cationic hydrogels were also able to generate small electrical currents. Overall, these results show that mold assemblies presenting different surface properties can be employed to tune the electro-actuation responsiveness of ionic liquid-based soft actuators and lead to the development of engineering devices with a broad range of applications, including artificial muscles and tissue engineering scaffolds.