Deformation Driven Programmable Metamaterials and Soft Machines

Abstract

Mechanical metamaterials are becoming an emerging frontier in scientific research and engineering innovation due to its unique properties, arising from innovative geometrical designs rather than constituent materials. Reconfigurable metamaterials can change their shapes and structures dramatically under external forces or environmental stimuli. It offers an enhanced flexibility in performance by coupling dynamically changing structural configuration and tunable properties, which has found broad potential applications in 3D meso-structures assembly and programmable machines. Despite extensive studies on harnessing origami, the ancient paper folding art, for design of mechanical metamaterials, the study on utilizing its close cousin, kirigami (“kiri” means cut), for programmable reconfigurable mechanical metamaterials and machines remains largely unexplored. In this dissertation, I explore harnessing the uniqueness of cuts in kirigami for achieving extraordinary mechanical properties and multifunctionalities in krigami-based metamaterials, as well as its potential applications in programmable machines and soft robotics. I first exploit the design of hierarchical cuts for achieving high strength, high stretchability, and tunable mechanical properties in hierarchical rotation-based kirigami mechanical metamaterials. Hierarchical line cuts are introduced to a thin sheet composed of non-stretchable materials (copy paper), less stretchable materials (acrylics), and highly stretchable materials (silicone rubber, PDMS), to explore the role of constituent material properties. The cut unit in the shape of solid rectangles with the square shape as a special case was demonstrated for achieving the extreme stretchability via rigid rotation of cut units. It shows that a higher hierarchical level contributes to a higher expandability and lower stiffness to constituent material. However, when such kirigami structure is applied onto less-stretchable materials (e.g. acrylics), its stretchability is almost eliminated regardless of the hierarchical level, because severe stress concentration at rotation hinges leads to the structure failure at the very beginning stage of pattern transformation. To address this challenge, I propose a hinge design which can significantly reduce the stress concentration at cut tips and enable high stretchability for rotation-based kirigmai structure, even on acrylic thin sheet. I also study the tunable photonic behavior of proposed hierarchical kirigami metamaterial by simple strain-induced structural reconfiguration. I then explore the programmability of kiri-kirgami structures by introducing notches to the simplest kirigami structure patterned with parallel line cuts for breaking its deformation symmetry. Engraving the flat-cut kirigami structure enables programmable control of its out-of-plane tilting orientation, thus generating a variety of inhomogeneous structural configurations on demand. I find that compared to the its counterpart without engraving notches, the introduced notches have a negligible effect on both the stress-strain curve over the large strain range and the extreme stretchability, however, they reduce the critical buckling force largely. Furthermore, I demonstrate the adaptive kiri-kirigami structure through local actuation with its tilting directions to be programmed and switched in response to the change of environmental temperature. Lastly, I demonstrate the potential promising outcome of kiri-kirigami structures as adaptive building envelope in energy efficient buildings, especially in electric saving for lighting and cooling load saving through numerical simulation. In addition to kirigami based soft metamaterials, I also investigate the utilization of soft materials and soft structures for robotics functions. First, I explore the design of soft doming actuator upon pneumatic actuation and its implications in design of multifunctional soft machines. I propose a novel bilayer actuator, which is composed of patterned embedded pneumatic channel on top for radial expansion and a solid elastomeric layer on bottom for strain-limiting. I show that both the cavity volume and bending angle at the rim of the actuated dome can be controlled by tuning the height gradient of the pneumatic channel along the radial direction. I demonstrate its potential multifunctional applications in swimming, adhesion, and gripping. I further explore harnessing doming-based bilayer doming actuator for developing soft climbing robot. I characterize and optimize the maximum shear adhesion force of the proposed soft adhesion actuator for strong and rapid reversible adhesion on multiple types of smooth and semi-smooth surfaces. Based on the switchable adhesion actuator, I design and fabricate a novel load-carrying amphibious climbing soft robot (ACSR) by combining with a soft bending actuator. I demonstrate that it can operate on a wide range of foreign horizontal and vertical surfaces, including dry, wet, slippery, smooth, and semi-smooth ones on ground, as well as under water with certain load-carrying capability. I show that the vertical climbing speed can reach about 286 mm/min (1.6 body length/min) while carrying over 200g object (over 5 times the weight of ACSR itself) during climbing on ground and under water.