Shape programmable chitosan films as soft microrobots toward the development of chemo-mechanical sensors

Shape-morphing materials showed tremendous potential in soft robotics; however, such mechanodynamic behavior has never been applied to develop a sensing platform. Achieving true reversibility in these materials is the goal that so far remains a significant challenge. Our work presents a monocomponent chitosan film fabricated using a solvent-casting method. Beyond exhibiting instantly reversible shape-morphing, a rare feat in itself, the film demonstrates a remarkable bidirectional capability in response to alkaline solution. The film responds rapidly to a specific stimulus – water –capability undergoing dramatic shape changes with a high bending angle in minimal time, owing to the spontaneous diffusive gradient. This responsiveness is further enhanced by the ability to program the shape simply by tailoring the aspect ratio of the film. Further, precise control over locomotion was achieved by arresting the deformed structure in a transient state via a solvent exchange (water-ethanol-water) process. We utilized this solvent-responsive behavior to demonstrate a proof of concept for detecting alcohol by creating a 'strip-on-a-drop’ based chemo-mechanical sensor. Programmed deformation combined with MATLAB-based digital image processing algorithms provided bending angles that consistently quantify alcohol content in a droplet. We believe our film offers a versatile platform for developing on-demand shape control with exciting possibilities.