Reprogrammable 4D Printed Liquid Crystal Elastomer Photoactuators by Means of Light-Reversible Perylene Diimide Radicals

Abstract

Reconfigurable soft actuators can be programmed to morph into different 3D shapes under the same stimulus exhibiting great potential for adaptive robotic functionalities. Liquid crystalline crosslinked materials programmed and controlled by light have demonstrated great potential in this area, however, their implementation is mainly based on azobenzene chromophores, using ultraviolet light that can potentially damage the device and its surroundings, especially if living cells are present. Here, an ink is presented, containing a green-absorbing perylene diimide chromophore, to prepare light active liquid crystalline elastomer (LCE) actuators via direct ink writing. Green light irradiation of the LCE elements leads to photothermal actuation, but also to new absorption bands in the far-red and near-infrared, ascribed to the formation of radical species. Far-red irradiation results in mechanical actuation and, advantageously, a recovery of the original absorption spectrum. This reversible transformation enables spatial reconfigurability of the actuator's response to far-red light. The reconfigurable system gives access to complex deformation modes by simply exciting the element with homogeneous far-red light, without the need for any structural modification of the actuator. This material strategy, using green and far-red light, less harmful than ultraviolet, shows significant promise for future development of reconfigurable actuators for biomedical applications.