MSE PhD Defense: Autonomous Stimuli-Responsive Metamaterials Based on Liquid Crystal Elastomers

Numerous responsive materials have been developed in recent decades and applied toward engineering challenges ranging from medicine to robotics. For example, polydimethylsiloxane (PDMS), hydrogels, shape memory polymers (SMPs), liquid crystal elastomers (LCEs), and many other materials can be engineered to respond to many environmental stimuli, such as non-polar solvents, humidity, heat, light, magnetic fields, and electric fields. However, there are some major limitations with most responsive materials: they typically respond slowly to the environment, and the deformation triggered by the environmental input is often small. In this dissertation, we propose to address these limitations by integrating responsive materials with mechanical metamaterials that are governed by scale-independent nonlinear mechanisms. These mechanisms can amplify and speed up the deformation of the responsive materials beyond what is possible for the responsive material
by itself.

In this dissertation, we specifically focus on the use of nonlinear mechanisms to improve the responsiveness of LCEs to changing heat or light in the environment, enabling autonomous, large-amplitude, rapid deformation in engineered systems. We demonstrate the potential utility of this strategy in several contexts. First, we engineer a metamaterial based on rotating squares with hinges consisting of LCE-PDMS bilayers. These bilayers, combined with geometric properties, such as the hinge thickness, determine how the metamaterial deforms in response to temperature variations in the environment, including the possibility of achieving either local or global deformation changes in response to localized temperature variations. Next, we build a kirigami-inspired robot that autonomously changes its trajectory in response to the environment, without any electronic controlsystem. The LCEs govern the behavior of modular “control units” that can be placed throughout the kirigami. These, in turn, locally impose mechanical constraints that control how the kirigami bends, and thereby where the robot moves. Finally, with the goal of attaining autonomous functional changes beyond what simple mechanical constraints can achieve, we explore the idea of using LCEs, in combination with hydrogels, to regulate pneumatic circuits. We integrate LCEs with modular mechanical valves. We achieve basic logic gates and construct fluidic networks that can regulate the output pressure based on the local environment. We use this strategy to autonomously control the trajectory and function soft pneumatic robots. These prototypes illustrate the potential for designing autonomous intelligent materials that rapidly and autonomously undergo large-amplitude shape and functional transformations in response to their environment. The principles developed in this dissertation can, in principle, be implemented at smaller length scales in the future, e.g., to develop responsive/intelligent metamaterial systems for micro- or medical robotics.