A method has been developed for the fabrication of gradient-structured actuators with cross-link variation by using two distinct photoinitiator systems to selectively trigger thiol-ene polymerization and acrylate homopolymerization. The gradient-structured actuator undergoes bending deformation in both air and solvent environments, and can be controlled by carefully tailoring the photopolymerization conditions.

Abstract

Stimuli-responsive materials have recently gained significant attention in the field of soft robotics, sensors, and biomimetic devices. The most facile way for the fabrication of such materials remains to endow bilayer structures which are fabricated with the combination of active and passive layers. Although, easily fabricated, these structures suffer from the generation of stress points between connection areas. In this work we develop a method to create a thin film with controlled cross-link variation across its thickness. The cross-link gradient is achieved through polymerization induced diffusion of dithiol molecules in thiol-ene network. As a result, the film exhibits bending deformation upon illumination with light or exposure to a chemical solvent, thereby demonstrating dual responsiveness. Light actuation of the film is achieved via photothermal effects due to the incorporation of dye into the system which can absorb UV light and heat the network. While solvent induced actuation is due to anisotropic swelling. Furthermore, the straightforward fabrication procedure allows for the creation of more complex deformations by patterning the film using a photomask during photopolymerization.