Bimorph Silk Microsheets with Programmable Actuating Behavior: Experimental Analysis and Computer Simulations

Microscaled self-rolling construct sheets from silk protein material have been fabricated, containing a silk bimorph composed of silk ionomers as an active layer and cross-linked silk β-sheet as the passive layer. The programmable morphology was experimentally explored along with a computational simulation to understand the mechanism of shape reconfiguration. The neutron reflectivity shows that the active silk ionomers layer undergoes remarkable swelling (eight times increase in thickness) after deprotonation while the passive silk β-sheet retains constant volume under the same conditions and supports the bimorph construct. This selective swelling within the silk-on-silk bimorph microsheets generates strong interfacial stress between layers and out-of-plane forces, which trigger autonomous self-rolling into various 3D constructs such as cylindrical and helical tubules. The experimental observations and computational modeling confirmed the role of interfacial stresses and allow programming the morphology of the 3D constructs with particular design. We demonstrated that the biaxial stress distribution over the 2D planar films depends upon the lateral dimensions, thickness and the aspect ratio of the microsheets. The results allow the fine-tuning of autonomous shape transformations for the further design of complex micro-origami constructs and the silk based rolling/unrolling structures provide a promising platform for polymer-based biomimetic devices for implant applications.