3D Printing of Liquid Crystal Elastomer Actuators

Abstract

Soft robotics offer advantages over their rigid counterparts due to the intrinsic softness of their consisting materials, soft robotic matter. When equipped with programmable shape morphing and controllable function, soft robotics are best qualified for interaction with delicate objects, exploration of unknown terrains, and large, impact-resistant deformations. Towards this goal, new materials and fabrication methods are needed to create actuators with programmable shape- morphing behavior akin to human muscles. Liquid crystal elastomers (LCEs) are soft materials comprised of anisotropic liquid crystal mesogen molecules, which when aligned, give rise to reversible contraction with high energy density when heated above their nematic-to-isotropic transition temperature (TNI). However, the ability to produce LCE actuators with programmed director alignment in arbitrary, bulk forms is a grand challenge. The focus of my Ph.D. thesis is to create programmable LCE actuators through the integration of design, synthesis, and multi-material 3D printing methods. Towards this goal, solvent-free, oligomeric LCE inks were synthesized that incorporate rigid mesogens along their backbone as well as photopolymerizable groups at the chain ends. By varying the molecular composition of these oligomeric species, LCE inks with the appropriate viscoelastic response were designed for high operating temperature-direct ink writing (HOT-DIW), an extrusion-based 3D printing method. By tailoring polymer backbone and crosslinking chemistries of our LCE inks, their TNI could be varied from 92°C to 127°C after printing and UV cross-linking, and enable custom thermal response. We further demonstrated that patterned LCEs with programmed director alignment along the print path were produced when printing in the nematic phase. These 3D LCEs exhibit large reversible contractility and high specific energy density. Our integrated approach allows for prescribed LCE alignment in arbitrary geometric motifs. Building on this seminal advance, we created untethered soft robotic matter that repeatedly shape-morphs and self-propels in response to thermal stimuli through passive control. Specifically, we designed and printed active LCE hinges with orthogonal director alignment that interconnect rigid polymeric tiles. These hinges can be programmed as mountain or valley folds to produce reversible active origami structures. Moreover, in a single structure, we programmed hinges made of LCEs with disparate TNI to enable sequential folding and demonstrated untethered, reversible sequential folding in soft, active origami for the first time. We further demonstrated a self- compacting prism with a modular geometric locking mechanism capable of sequential folding with three temperature-specific, stable configurations. To enable the informed design of untethered robotic matter, LCE hinge bending angle and torque can be prescribed by geometry and LCE chemistry. We then exploited their exemplary performance by programming LCE hinges into the “rollbot”, an exemplar self-propelling structure with passive control. Specifically, we designed a pentagonal prism with low TNI LCE hinges and propellers with high TNI LCE hinges, informed by our torque and bending angle characterization, enabling reversible reconfiguration and self- propulsion across a heated surface. To expand upon these capabilities, ewe developed a novel method of 3D printing aligned LCE filaments with embedded, coaxial liquid metal by co-extrusion of LCE and liquid metal through a core-shell nozzle. Our innervated LCE (iLCE) fibers are electrothermally heated well above TNI with programmable and predictable heat generation through the core of the filament, which resulted in large, prescriptible contractile strains akin to those of our neat 3D printed LCEs. The iLCE fibers enable self-sensing of actuation through the resulting change of resistance with respect to actuation strain, where a change of resistance is directly predictable from strain. Moreover, our iLCEs exhibited reliable reversible actuation and considerable work output, which combined with self-sensing capabilities allows for closed loop control. Specifically, our actuators automatically reach target resistance and strain values rapidly and repeatedly despite large bias load perturbations. As a final demonstration, we patterned iLCEs with a spiral printpath to demonstrate programmable 3D shape morphing. Analogous to iLCE fibers, these spiral iLCEs were electrothermally heated, exhibited self-sensing, and were regulated with closed loop control. In summary, we have developed a new platform for creating soft robotic matter through the design, synthesis, and assembly of LCE inks, which can be seamlessly integrated with structural, sensing, and functional materials. Our platform may be harnessed for applications including soft robotics, reconfigurable electronics, adaptable structures, and well beyond.