Multiple shape transformations of composite hydrogel sheets.

Soft materials undergoing shape transformations in response to changes in ambient environment have potential applications in tissue engineering, robotics and biosensing. Generally, stimulus-responsive materials acquire two stable shapes corresponding to the "on" and "off" states of the external trigger. Here, we report a simple, yet versatile approach to induce multiple shape transformations of a planar hydrogel sheet, each triggered by a particular, well-defined external stimulus. The approach is based on the integration of small-scale multiple polymer components with distinct compositions in the composite gel sheet. In response to different stimuli, the structural components undergo differential swelling or shrinkage, which creates internal stresses within the composite hydrogel sheet and transforms its shape in a specific manner.