Understanding the Shape Memory Behavior of Self-Bending Materials and Their Use as Sensors

By depositing layers composed of poly (N-isopropylacrylamide)-based microgels and the polyelectrolyte polydiallyldimethylammonium chloride on a flexible substrate, responsive materials that bend upon drying can be fabricated; the extent of the bending depends on atmospheric humidity. This study shows that the bending conformation/direction can be templated, and exhibits shape memory. Detailed examination of the bilayer system leads to an understanding of the phenomena leading to this behavior. By close examination of microscopy images and diffraction patterns, this study is able to determine that the dried polymer-based layer is composed of both amorphous and crystalline phases; the amorphous phase can readily absorb water, which results in actuation, while the crystalline phases template the bending characteristics of the device. With an understanding of the bending behavior of the devices, this study is able to generate humidity sensors by interfacing them with stretchable strain sensors, which are also developed specifically for the bendable materials.