  Willis coupling in acoustic materials defines the cross-coupling between
strain and velocity, analogous to bianisotropic phenomena in electromagnetics.
While these effects have been garnering significant attention in recent years,
to date their effects have been considered mostly perturbative. Here, we derive
general bounds on the Willis response of acoustic scatterers, show that they
can become dominant in suitably designed scatterers, and outline a systematic
venue for the realistic implementation of maximally bianisotropic inclusions.
We then employ these inclusions to realize acoustic metasurfaces for sound
bending with unitary efficiency.
