Stretchable 3D Printed Thermoelectric Generators for Liquid Crystal Elastomer Actuation, Control and Energy Recovery

Liquid crystal elastomers (LCE) are a class of shape memory polymers that exhibit muscle like work energy densities and contraction strains. By transitioning between a liquid and crystalline phase the mesogens, induced by thermal stimuli, reorient themselves inducing macroscopic contractions and expansions upwards of 40%. By precisely controlling heating and cooling, LCE responses can be tailored for actuation and locomotion in soft systems. Recently, this material architecture has been explored for soft robotic actuators and systems. The main limitations of LCE actuators are their slow response times, stemming from high transition temperatures and passive convective cooling. LCEs also suffer from poor energy efficiencies with energy partially lost through convective heat transfer. As a potential solution, in this talk, I focus on recent work in bendable and stretchable thermoelectric devices (TED) that convert temperature differentials across oppositely doped semiconductors into a potential difference through the Seebeck effect along with acting as active heaters and coolers through the Peltier effect. Through the Seebeck effect, TEDs are introduced to address energy efficiency issues by introducing passive environmental energy harvesting along with internal regenerative energy recovery in soft robotic systems. Through the Peltier effect, TEDs are introduced to actively heat and cool LCE layers for actuation. This is enabled by developing 3D printed high density TEDs with semiconductors wired together with Eutectic Gallium Indium (EGaIn) liquid metal (LM) interconnects to actively heat and cool and recover energy from LCEs while staying mechanically compliant. These LCE-TED transducers, allow for single input multidirectional actuation enabling integration into a PI-controller with <0.5 degree error. As energy harvesters these LCE-TEDs are integrated into a robotic walker recycling environmental waste heat along with recovering small amounts of energy through regenerative energy harvesting. Energy harvesting and actuation characterization is given highlighting device power densities, deflection, blocking force, and robust mechanical characterization.