Humidity- and Sunlight-Driven Motion of a Chemically Bonded Polymer Bilayer with Programmable Surface Patterns

We report a bilayer of sodium alginate/polyvinylidene fluoride (SA/PVDF) that is chemically bonded through a series of interfacial coupling reactions. The SA layer is hydrophilic in structure and is capable of strong interaction with water molecules, thus presenting high sensitivity to humidity, whereas the PVDF layer is hydrophobic, inert to humidity. This structural feature results in the bilayer having asymmetric humidity-responsive performances that can thus make its shape change with directionality, which cannot be achieved in an SA single layer. The responsive process to humidity can be adjusted by exposure of the bilayer to sunlight by means of a photothermal effect that accelerates dehydration of the bilayer to cause more rapid shape deformations. When the sunlight is removed, the bilayer adsorbs humidity again and returns to its original shape, indicating good reversibility. To exactly regulate the shape deformations of the bilayer with external stimuli, we employ Ca<sup>2+</sup>-treated filter paper to customize crosslinking reactions in the SA layer as desired patterns which are capable of causing different mechanical tensors and swellabilities in the bilayer so as to regulate and control the actuations for self-folding, curling, twisting, and coiling in response to sunlight and humidity.On the other hand, the chemically bonded bilayer has stronger interfacial toughness and is capable of reaching 300 J m<sup>–2</sup>, which is around 12 times the interfacial toughness of the physically combined bilayer; as a result, the chemically bonded bilayer is capable of sustaining continuous shape deformations without interfacial failure. The directionally mechanical actuations can be utilized in designing an indicator to roughly indicate the range of intensity of sunlight by coupling the chemically bonded bilayer into a typical electric circuit, in which the range of intensity of sunlight can be easily estimated by visual observation of the light-emitting diodes.