Actuation in simple gel and elastomer systems

Bending can be realized in polymer bilayer sheets when there is a mismatch in properties between the 2 layers. For gelatin hydrogels stimulated by swelling in water, the sheet curvature can be described by the mismatch in gel crosslink density, which directly influences swelling and modulus. For thermoplastic elastomer bilayer sheets stimulated by deformation, the curvature can be described by the elastic recovery difference between the 2 layers. The elastic recovery difference arises from the size and number of polymer crystal “crosslinks” in each layer to realize a “mechanically-programmable” material. In this manner, the curvature can be predicted from molecular level and small scale structural information and can aid in the design of polymer actuators and soft machines that can do work upon stimulation.