Reentrant polymer foam materials with negative Poisson's ratios, as described by Lakes (1), are a new class of materials with unique properties. The thermodynamic restriction on the compressibility, K, of an elastic material is that it must be positive for stability. The compressibility is related to the shear modulus, G, and Poisson's ratio, v, by K = 3/2[(1 2v)/G(1+ v)], so v is restricted to...