Programming Molecular Order of a Liquid Crystal Elastomer with Magnetic-Field-Assisted DLP Printing

Shape memory polymers (SMPs) and hydrogels are two major materials classes of stimuli-responsive materials that have been widely used in 4D printing. As such, additively manufactured responsive architected materials are made of either SMPs or hydrogels. SMP can memorize its original printed shape and restore it when needed, but its stimuli-responsive morphing is only one-way in that mechanical processing is always required for shape programming. Stimuli-responsive hydrogels can deform reversibly depending on various environmental changes, but they suffer from extremely slow time-scale for shape change due to a quadratic scaling law for the diffusive water molecule migration process and are not suitable for applications where surrounding water is not available. Recently, liquid crystal elastomers (LCEs), combining polymeric elasticity with liquid crystalline anisotropy, have received growing attention as a new class of a 4D printing material that can overcome the limitations of SMPs and hydrogels.<br/>LCEs can show reversible stimuli-responsive deformation driven by rearrangement of molecular orientation of liquid crystal (LC) molecules. However, alignment of LC molecules has been achieved primarily using mechanical extension or viscous shear, which significantly limits its applicability in additive manufacturing. Here, we report a DLP system capable of printing an LCE structure while selectively programming molecular orientation of LC molecules using a magnetic field. We report a LCE precursor solution that maintains a nematic phase at room temperature. Using a custom-built DLP printing system with an integrated magnetic field generator, LC molecules are aligned in the desired orientation and selectively cured by patterned digital light projection. Therefore, LC orientations can be freely encoded in a single structure. This allows for spatial patterning of LC orientations independent of the geometry of the printed structure. Considering the advantages of DLP printing over other additive manufacturing methods, this approach can offer unmatched opportunities for programming various modes of shape deformation of responsive architected materials.