Self-folding bilayers: Mechanistic design principles for programmable and on-demand shape reconfiguration

Despite lacking muscle cells, plants demonstrate shape-transforming movements in response to environmental conditions for accomplishing tasks such as growth, defense, nutrition, and reproduction. Motivated by the fascinating behavior of plants, this dissertation investigates the programmable, on-demand, and autonomous reconfiguration of stimuli-responsive bilayer systems (one layer expanding more than the other in response to the stimulus) into complex three-dimensional architectures. The mechanistic design principles responsible for bilayer morphing have been established and validated through a combination of nonlinear finite element modeling and experiments with cross-linked polymeric systems. Several classes of structures such as hemispherical domes, convex regular polygons, star-shaped planar bilayers and geometries with spatial cuts have been considered and the relationships between their initial shapes, applied stimuli, and final configurations have been proposed. The hemispherical domes demonstrated the snap-through behavior at critical mismatch strains and thus mimicked the fast leaf closure of Venus flytrap. Understanding the snap-through behavior of engineered materials opens up the possibilities to realize fast actuation with systems that are inherently soft. The convex regular polygons demonstrated a bifurcation type of structural instability where they transitioned from doubly curved axisymmetric shapes to singly curved asymmetric ones at critical strains. Bifurcation occurs solely due to nonlinear geometric effects and hence the results of this research contribute toward a rational understanding of bilayer self-folding behavior. The hinge-less bilayer stars morphed into axisymmetric gripper-like configurations at high mismatch strains. Self-folded grippers that are widely used in micro/ nano-manipulation have previously been fabricated from patterned multi-layer architectures with spatially separated hinges. As this work proposes an unpatterned, hinge-less, and simple bilayer design to realize the grippers, the findings would not only simplify the current fabrication procedures but also enable novel functionalities. Finally, the introduction of cuts within the bilayer geometry provided novel design frameworks to generate biaxially curved complex architectures. The computational models developed in this research are independent of length scales and material properties. Hence, the proposed mechanistic guidelines would be applicable to a variety of material systems and external stimuli to engineer sensors, actuators, artificial muscles, energy harvesting devices, deployable structures, flexible electronics, reconfigurable biomedical, and chemo-mechanical devices across length scales.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste