Smart composite hydrogel with pH-, ionic strength- and temperature-induced actuation.

A facile and versatile photo-patterning method to fabricate "smart" hydrogels with defined lateral and vertical inhomogeneity of hydrogel composition and dimensions has been developed via generating programmable composite hydrogels and bilayer hydrogels based on thermal and ionic strength-responsive poly(N-isopropylacrylamide) and pH-sensitive poly(acrylic acid) hydrogels. These hydrogels are capable of responding to triple-stimuli and inducing reversible "on" and "off" states upon external stimulation due to abrupt volume changes of the responsive hydrogel networks. Moreover, the composite and bilayer hydrogels show a reversible and repeatable direction-controllable bending behavior upon variation of temperature, ionic strength and pH, which is the result of the structural inhomogeneity and the modulation of the hydrogel solvation state in response to these changes. Importantly, different bending behaviors can be structurally programmed by controlling the patterned components, which undergo different swelling or shrinkage and further generate asymmetric internal stresses within the composite hydrogels in a specific manner. Additionally, such asymmetric internal stresses drive the shape deformations of the composite hydrogels, which are promising for potential applications in soft robots and actuators.