  Recent theoretical work suggests that systematic pruning of disordered
networks consisting of nodes connected by springs can lead to materials that
exhibit a host of unusual mechanical properties. In particular, global
properties such as the Poisson's ratio or local responses related to
deformation can be precisely altered. Tunable mechanical responses would be
useful in areas ranging from impact mitigation to robotics and, more generally,
for creation of metamaterials with engineered properties. However, experimental
attempts to create auxetic materials based on pruning-based theoretical ideas
have not been successful. Here we introduce a new and more realistic model of
the networks, which incorporates angle-bending forces and the appropriate
experimental boundary conditions. A sequential pruning strategy of select bonds
in this model is then devised and implemented that enables engineering of
specific mechanical behaviors upon deformation, both in the linear and
non-linear regimes. In particular, it is shown that the Poisson's ratio can be
tuned to arbitrary values. The model and concepts discussed here are validated
by preparing physical realizations of the networks designed in this manner,
which are produced by laser cutting two-dimensional sheets and are found to
behave as predicted. Furthermore, by relying on optimization algorithms, we
exploit the networks' susceptibility to tuning to design networks that posses a
distribution of stiffer and more compliant bonds, and whose auxetic behavior is
even greater than that of homogeneous networks. Taken together, the findings
reported here serve to establish that pruned networks represent a promising
platform for the creation of novel mechanical metamaterials.
