  Mechanical metamaterials are architected manmade materials that allow for
unique behaviors not observed in nature, making them promising candidates for a
wide range of applications. Existing metamaterials lack tunability as their
properties can only be changed to a limited extent after the fabrication. In
this paper, we present a new magneto-mechanical metamaterial that allows great
tunability through a novel concept of deformation mode branching. The
architecture of this new metamaterial employs an asymmetric joint design using
hard-magnetic soft active materials that permits two distinct actuation modes
(bending and folding) under opposite-direction magnetic fields. The subsequent
application of mechanical forces leads to the deformation mode branching where
the metamaterial architecture transforms into two distinctly different shapes,
which exhibit very different deformations and enable great tunability in
properties such as mechanical stiffness and acoustic bandgaps. Furthermore,
this metamaterial design can be incorporated with magnetic shape memory
polymers with global stiffness tunability, which further enables the global
shift of the acoustic behaviors. The combination of magnetic and mechanical
actuations, as well as shape memory effects, imbue unmatched tunable properties
to a new paradigm of metamaterials.
