Design of partially covered bilayer thin film actuators

Abstract

One approach to create bending deformation using active materials is to fully cover a passive substrate with the active material. The active layer expands/contracts in response to an external stimuli, while, the passive layer does not deform, thereby, generating a strain gradient through the thickness, and hence, bending. However, a recent experimental study [Pozo, M et al. ACS Appl. Mater. Interfaces, 13, 59381–59391 (2021).] showed that a partially covered (40%) bilayer deformed as much as the “standard” fully covered bilayer. To elucidate such a non-standard response of bilayers, we have developed an analytical model using the strain energy minimization. The developed model showed that the deformation of the bilayer may actually either increase or decrease or saturate with the increase in the coverage depending on the moduli ratio and the thickness ratio of the active and passive layers. Design strategies for actuators considering their curvature and bending stiffness are proposed based on the implications of the developed model. The present study also shows the potential for designing patterned actuators with multiple coverings (may have multiple active layers that are responsive to different stimuli) to realize different actuations based on the choice of the trigger from a single actuator creating multi-modal, multi-stimuli-responsive actuators.