Solvent and Ca2+ triggered robust and fast stress generation by ultrathin triple-network hydrogels

Solvent-triggered isometric stress generation of an ultrathin and tough triple-network (TN) hydrogel was systematically studied as functions of the pre-strain and ethanol/water mixture solvent. The obtained results along with the solvent-induced changes in the volume and modulus of the hydrogel were further analyzed with emphasis on clarifying the stress generation mechanism of the TN gel. Ca2+-triggered stress generation of the gel was also preliminarily explored. High stress generation of 0.4 MPa was obtained within tens of seconds at the low pre-strain of 25% by alternating water and ethanol. Both negative and positive stress generation of the gel were achieved just by changing the composition of the ethanol/water mixture solvent. Mechanism analysis indicates that, modulus change of the gel is the dominant reason for their solvent-triggered high and fast stress generation. In the case of the Ca2+-triggered stress generation of the gel, it is also attributed to the modulus change induced by the physical cross-linking of Ca2+ between the negatively charged first network chains of the TN hydrogel.