4D printing of mechanically programmable shape-shifting liquid crystal elastomers

4D printing is a promising method to develop actuators for applications in soft-robotics and biomedical devices where complex structures are required that might be difficult to create using traditional fabrication methods. However, most 4D printing relies on shear alignment upon printing to align liquid crystal or composite fibers that undergo anisotropic expansions/contraction when actuated. This limits the types of shape changes available to researchers as determining the print path required to induce a desired shape change is not trivial. Here, we use a new reactive printing method that enables the printing of a dual network liquid crystal elastomer (LCE) which can be mechanically programmed into the desired shape change. First, a thiol-acrylate Michael addition is completed upon printing an LCE oligomer solution into a catalyst bath. Next, the printed structure is dried, deformed to a desired shape change, and UV cured to crosslink excess acrylates in the network. The resulting LCE transforms between the printed and mechanically deformed shape when heated and cooled, respectively, and is capable reversible strains up to 100%. We demonstrate the versatility of this method by printing a variety of LCE actuators which could not be printed using conventional 4D printing methods.