Dual light-responsive shape transformations of a nanocomposite hydrogel sheet enabled by in situ etching shaped...

We report here on dual shape transformations of the same thermo-responsive hybrid hydrogel sheet under irradiation of a laser with two different wavelengths (808 nm and 450 nm). By in situ etching the silver nanoprisms in the sheet to silver nanodiscs by using chloride ions (Cl^-), two areas with distinct light extinction properties are integrated in a single sheet. The conversion of photon energy to thermal energy in local areas by the silver nanoprisms or nanodiscs under laser irradiation with an appropriate wavelength heats up the sheet locally and causes a local volumetric shrinkage, and hence a volumetric mismatch in different areas in the sheet. The sheet then transforms in a specific way to accommodate this volumetric mismatch. Different patterns of silver nanoprisms and nanodiscs in the sheet are achieved by controlling the delivery patterns of the etchant Cl^-. We demonstrate that by designing the distribution pattern of silver nanoprisms and nanodiscs in the sheet, the same hybrid sheet transforms either to a saddle or to a helical twisted shape, while with another type of distribution pattern, it transforms either to a hoof-like or to a boat-like shape under the irradiation of a laser with a wavelength of either 808 nm or 450 nm. We point out that to program arbitrary curvature of the transformed shape of our sheet, the pattern size of silver nanoprisms and nanodiscs (i.e. actuation area) in the sheet needs to be largely reduced, which however limits the heat generated and hence the shape transformations. Factors that affect the sheet's shape transformations are discussed and solutions are suggested to enhance its performance. The hybrid sheet may find applications in soft robotics, for example, as a robotic finger.