Most “impossible” materials that one can imagine seem impossible because they have not yet been made, not because there is a sound theoretical reason that they cannot be made . For example, active, adaptive, matter that mimics some of the features of tissues provides many targets.
Our objective is to create a new class of adaptive materials, composites, and polymers. We are investigating the fundamental mechanical properties of multi-layered composites–where the layers are not covalently bonded together–and the changes in these properties that can be achieved using different applied forces. These adaptive composites respond to stimuli in terms of a specific function, such as a change in their Young’s modulus .
As part of our ongoing research, we are exploring different types of materials (e.g., foams, thin films), various types of applied forces and stimuli (e.g., pneumatics, magnetism), and diverse functions (e.g., compressive strength, tensile strength, optical properties). Our aim is to make new adaptive materials, composites, and polymers that one might consider “impossible”, possible.
Stepped Moduli in Layered Composites
We have created adaptive composites that respond to mechanical stimuli by changing their Young’s modulus. These composites are fabricated by combining a shorter layer of elastic material (e.g., latex) and a longer layer of stiffer material (e.g., polyethylene and Kevlar), and joining them at their ends. Tension along the layered composite increases its length, and as the strain increases, the composite changes the load-bearing layer from the elastic to the stiff material. The result is a step in the Young’s modulus of the composite (Figure 1). The characteristics of the step (or steps) can be engineered by changing the constituent materials, the number of layers, and their geometries (e.g., sinusoidal, hierarchical, two-dimensional web-like (Figure 2), rod-coil, embedded, and ring structures). For composites with two or more steps in modulus, the materials within the composites can be layered in a hierarchical structure to fit within a smaller volume, without sacrificing performance. These composites can also be used to make structures with tunable, stepped compressive moduli. An adaptation of these principles can generate an electronic sensor that can monitor the applied compressive strain. Increasing or decreasing the strain closes or opens a circuit and reversibly activates a light-emitting diode (Figure 3).
1. Whitesides, G.M., “Reinventing Chemistry”, Angew. Chemie, 2015, 54, 3196-3209
2. So, J., Tayi, A., Güder, F., and Whitesides, G.M, “Stepped Moduli in Layered Composites”, Advanced Functional Materials", 2014, 24, 7197-7204