Design for Structural Complexity

Kirigami Is a Robotic Matter

We adapt kirigami - the art of paper cutting - to flexible mechanical metamaterials to realize a variety of highly stretchable and morphable structures with programmable deformation. The common characteristic of our kirigami metamaterials is an elastic sheet perforated with an array of periodic cuts. This configuration leaves a network of repeating facets connected by small ligaments that transform into a 3D textured surface upon stretching. In this talk, I show how to imbue elastic sheets with emergent functionalities by controlling the cut geometry and harnessing mechanical instabilities. I demonstrate several kirigami metamaterials constructed from thick or thin elastic sheets exhibiting complex deformation responses with emergent mechanical properties. Finally, I show that kirigami metamaterials can be used as a robotic matter to program the response of soft robots and review a few open questions in the design of kirigami metamaterials that highlight the necessity of adopting an inverse design approach.

Topological Defects Produce Exotic Mechanics in Complex Metamaterials

While topological defects play a crucial role in condensed matter, they are not widely explored in mechanical metamaterials. Here we introduce a systematic strategy to design frustrated metamaterials with local or topological defects. We uncover their distinct mechanical signatures, and show how defects can be harnessed to engineer a desired response. Our work presents a new avenue to systematically introduce frustration and defects with a topological signature in mechanical metamaterials.

Structured Fabrics with Tunable Mechanical Properties

Structured fabrics, like woven sheets or chain mail armors, derive their properties both from the constitutive materials and their geometry. Their design can target desirable characteristics, such as high impact resistance, thermal regulation, or electrical conductivity. Once realized, however, the fabrics’ properties are usually fixed. Here we demonstrate structured fabrics with tunable bending modulus, consisting of 3D particles arranged into layered chain mails. The chain mails conform to complex shapes, but when pressure is exerted at their boundaries, the particles interlock and the chain mails jam. We show that, with small external pressure (~93 kPa), the sheets become >25 times stiffer than in their relaxed configuration. This dramatic increase in bending resistance arises because the interlocking particles have high tensile resistance, unlike what is found for loose granular media. We use discrete-element simulations to relate the chain mail’s micro-structure to macroscale properties and to interpret experimental measurements. We find that chain mails, consisting of different non-convex granular particles, undergo a jamming phase transition that is described by a characteristic power-law function akin to the behavior of conventional convex media. Our work provides routes towards light-weight, tunable, and adaptive fabrics, with potential applications in wearable exoskeletons, haptic architectures, and reconfigurable medical supports.

Oligomodal Mechanical Metamaterials

Mechanical metamaterials are artificial composites that exhibit a wide range of advanced functionalities such as negative Poisson's ratio, shape-shifting, topological protection, multistability, extreme strength-to-density ratio and enhanced energy dissipation. To date, most metamaterials have a single property, e.g. a single shape change, or are pluripotent, i.e. they can have many different responses, but require complex actuation protocols. Here, we introduce a novel class of oligomodal metamaterials that encode a few but fixed number of distinct properties that can be selectively controlled under uniaxial compression. In particular, we realise a metamaterial that has a negative (positive) Poisson's ratio for low (high) compression rate [1]. Furthermore, we show that can inverse the design process using a fully nonlinear combinatorial solver. The ability of our oligomodal metamaterials to host multiple mechanical responses within a single structure makes them an early example of multi-functional matter and paves the way towards robust and adaptable devices.

[1] Bossart, A. & Dykstra, D. M. J., Van der Laan, J., & Coulais, C. (2021). Oligomodal metamaterials with multifunctional mechanics. Proceedings of the National Academy of Sciences, 118(21).