  Mechanical metamaterials are periodic lattice structures with complex unit
cell architectures that can achieve extraordinary mechanical properties beyond
the capability of bulk materials. A new class of metamaterials is proposed,
whose mechanical properties rely on deformation-induced transitions in
nodal-topology by formation of internal self-contact. The universal nature of
the principle presented, is demonstrated for tension, compression, shear and
torsion. In particular, it is shown that by frustration of soft deformation
modes, large highly non-linear stiffening effects can be generated. Tunable
non-linear elasticity can be exploited to design materials mimicking the
complex mechanical response of biological tissue.
