Pentamode metamaterials with independently tailored bulk modulus and   mass density
  We propose a class of linear elastic three-dimensional metamaterials for which the effective parameters bulk modulus and mass density can be adjusted independently over a large range|which is not possible for ordinary materials. First, we systematically evaluate the static mechanical properties and the phonon dispersion relations. We show that the two are quantitatively consistent in the long-wavelength limit. To demonstrate the feasibility, corresponding fabricated polymer microstructures are presented. Finally, we discuss calculations for laminates composed of alternating layers of two different metamaterials with equal bulk modulus yet different mass density. This leads to metamaterials with effectively anisotropic uniaxial dynamic mass density tensors. 
Muamer Kadic, Tiemo Buckmann, Robert Schittny, Peter Gumbsch, and   Martin Wegener
cond-mat.mtrl-sci