Alignment and Actuation of Liquid Crystals via 3D Confinement and Two-Photon Laser Printing

While three-dimensional (3D) printing of materials has become standard practice for many applications, the extension into the time domain (four-dimensional (4D) printing) poses new challenges, including the use of appropriate (stimuli)-responsive materials and a design that leads to their desired responses. Liquid crystalline (LC) materials have been demonstrated to be versatile for the preparation of active 3D microstructures using two-photon laser printing. To achieve the desired actuation, the alignment of the LC material has to be controlled during the printing process. In most case studied to date, the alignment relied on surface modifications and therefore complex alignment patterns and concomitant actuation were not possible. Here, we introduce a novel strategy for spatially aligning LC domains in three-dimensional space by utilizing 3D-printed polydimethylsiloxane (PDMS)-based microscaffolds as confinement barriers, which induce the desired director field. The director field resulting from the boundary conditions is calculated with Landau de Gennes theory and validated by comparing experimentally measured and theoretically predicted birefringence patterns. We demonstrate our procedures for structures of varying complexity, starting from planar and radial alignments, but then going into complex alignment patterns in 3D that vary also in z-direction. We then used two-photon laser printing to fabricate microstructures of desired shapes with the prescribed orientation patterns. The 4D printed microstructures show the desired actuation as predicted from the alignment pattern using a combination of liquid crystal and elasticity theories implemented with the finite element method. Overall, we obtain excellent agreement between theory and experiment. This opens the door for rational design of 4D (micro)printing of LCEs in the future.