Photoresponsive Movement in 3D Printed Cellulose Nanocomposites

Photoresponsive soft liquid crystalline elastomers (LCEs) transform light's energy into dynamic shape changes and are considered promising candidates for production of soft robotic or muscle-like devices. 3D printing allows access to elaborated geometries as well as control of the photoactuated movements; however, this development is still in its infancy and only a limited choice of LCE is yet available. Herein, we propose to introduce biocompatible and sustainable cellulose nanocrystals (CNC) into an LCE in order to facilitate the printing process by direct ink writing (DIW) and to benefit from the anisotropic mechanical properties resulting from the extrusion-induced alignment of such nanoparticles. After a first printing step where the rheological influence of CNC allows the production of self-standing structures, a doping process introduces the azobenzene photoswitches in the composite, conferring photomechanical behaviors to the printed material. This approach results in soft composites, with an elastic modulus around 20-30 MPa, that present fully reversible photosoftening of 35% and photomechanical actuation occurring less than 3 s after illumination. The presence of CNC as reinforcement particles allows precise tailoring of mechanical properties, rendering such phototriggered materials suitable candidates for the production of actuators and 3D structures with particular and dynamic load cases.