Embracing Compliance and Instabilities to Achieve Function in Mechanical Metamaterials and Devices

Abstract

The use of soft materials has led to the development of soft devices that have the potential to be more robust, adaptable, and safer for human interaction than traditional rigid systems. State-of-the-art developments push these robotic systems towards applications such as soft rehabilitation and diagnostic devices, exoskeletons for gait assistance, grippers that can handle diverse objects, and electronics that can be embedded in the human body. Furthermore, compliance has found its way into the design of materials that derive their properties from their structure, not from their chemical composition (i.e. metamaterials or architected materials). Applications of metamaterials range from tunable auxetic behavior, stiffness, optical properties and phononic and acoustic band-gap behavior to tunable surface properties such as the drag coefficient, and chemistry. These examples illustrate the potential of using compliance to create new and improved functionality in structural and robotic applications. While the geometrical non-linearities and instabilities that arise when using soft or flexible materials directly complicate the design process and fall outside the scope of traditional engineering, exactly these non-linearities make the systems inherently capable of rich behavior. As such, to bring these systems closer to application and to uncover their true potential, we need to gain a better understanding of the principles that govern their behavior. The main focus of this dissertation is on the design of non-linear structures and devices that exhibit a nontrivial relation between input and output (i.e. loading and response). I propose analytical, computational and relatively simple experimental techniques that allow us to effectively explore the design space, and that lead to an understanding of the relation between shape and function in compliant systems. More specifically, I explore the effect of the shape and boundary conditions in periodic porous structures and soft deformation sensors, propose a new type of soft inflatable actuators that harnesses snap-through instabilities to achieve instantaneous changes in length, force and pressure, and develop a design strategy that enables mechanical metamaterials for which the volume and shape can be dramatically reconfigured. Altogether, the proposed techniques and case studies can inform simplified routes for the design of tunable mechanical metamaterials and soft devices over a wide range of length scales.