Fabrication, characterization and analytical modeling of gradient auxetic closed cell foams

Auxetic (negative Poisson’s ratio) and gradient open cell foams have shown promise for their conformability and high impact energy absorption—useful in applications like protective equipment, footwear and prosthetics. Recent methods fabricated auxetic closed cell foam by steaming conventional closed cell foam. Methods developed herein control the cell structure of auxetic closed cell foam, to produce novel intentionally anisotropic and gradient auxetic closed cell foam. Pins passed through the foam constrained or stretched regions during steaming to further modify cell structure, and impart gradients in cell structure, Young’s modulus and Poisson’s ratios. Fabricated foams had Poisson’s ratios between 1 and −1. Imparted Young’s moduli of 1–12 MPa were similar to foams used in footwear, prosthesis, helmets and other impact protection. The effect of changes to cell structure on Young’s moduli and Poisson’s ratio are explained by combining analytical models of auxetic open cell and conventional closed cell foam.