  Conceptually, all conceivable three-dimensional mechanical materials can be
built from pentamode materials. Pentamodes also enable to implement
three-dimensional transformation acoustics - the analogue of transformation
optics. However, pentamodes have not been realized experimentally to the best
of our knowledge. Here, we investigate inasmuch the pentamode theoretical ideal
suggested by Milton and Cherkaev in 1995 can be approximated by a metamaterial
with current state-of-the-art lithography. Using numerical calculations
calibrated by our fabricated three-dimensional microstructures, we find that
the figure of merit, i.e., the ratio of bulk modulus to shear modulus, can
realistically be made as large as about 1,000.
