Highly-anisotropic elements for acoustic pentamode applications
  Pentamode metamaterials are a class of acoustic metafluids that are characterized by a divergence free modified stress tensor. Such materials have an unconventional anisotropic stiffness and isotropic mass density, which allow themselves to mimic other fluid domains. Here we present a pentamode design formed by an oblique honeycomb lattice and producing customizable anisotropic properties. It is shown that anisotropy in the stiffness can exceed three orders of magnitude, and that it can be realistically tailored for transformation acoustic applications. 
Christopher N. Layman, Christina J. Naify, Theodore P. Martin, David   C. Calvo, Gregory J. Orris
cond-mat.mtrl-sci physics.flu-dyn