Soft and Flexible Bilayer Thermoplastic Polyurethane Foam for Development of Bioinspired Artificial Skin

Inspired by the epidermis-dermis composition of human skin, here we have simply developed a lightweight, robust, flexible, and biocompatible single-electrode triboelectric nanogenerator (S-TENG)-based prototype of bilayer artificial skin, by attaching one induction electrode with unfoamed skin layer of microcellular thermoplastic polyurethane (TPU) foam, which shows high-performance object manipulation [by responding differently toward different objects, viz., aluminum foil, balloon, cotton glove, human finger, glass, rubber glove, artificial leather, polyimide, poly(tetrafluoroethylene) (PTFE), paper, and wood], due to electrification and electrostatic induction during contact with the objects having different chemical functionalities. Comparative foaming behavior of ecofriendly supercritical fluids, viz., CO2 over N2 under variable temperatures (e.g., 130 and 150 °C) and constant pressure (15 MPa), have been examined here to pursue the soft and flexible triboelectric TPU foam. The foam derived by CO2 foaming at 150 °C has been prioritized for development of S-TENG. Foam derived by CO2 foaming at 130 °C did not respond as well due to the smaller cell size, higher hardness, and thicker skin. Inflexible N2-derived foam was not considered for S-TENG fabrication. Object manipulation performance has been visualized by principal component analysis (PCA), which shows good discrimination among responses to different objects.