  Machine-matter, of which mechanical metamaterials and meta-devices are
important sub-categories, is emerging as a major paradigm for designing
advanced functional materials. Various exciting applications of these concepts
have been recently demonstrated, ranging from exotic mechanical properties to
device-like and adaptive functionalities. The vast majority of the studies
published to date have, however, focused on the quasi-static behavior of such
devices, neglecting their rich dynamic behavior. Recently, we proposed a new
class of strain rate-dependent mechanical metamaterials that are made from
bi-beams (i.e., viscoelastic bilayer beams). The buckling direction of such
bi-beams can be controlled with the applied strain rate. The proposed approach,
however, suffers from a major limitation: 3D printing of such bi-beams with
such a 'strong' differential strain rate-dependent response is very
challenging. Here, we propose an alternative approach that only requires a
'weak' differential response and a rationally designed geometric artifact to
control the buckling direction of bi-beams. We present an analytical model that
describes the landscape of all possible combinations of geometric designs and
hyperelastic as well as viscoelastic properties that lead to the desired strain
rate-dependent switching of the buckling direction. We also demonstrate how
multi- and single-material 3D printing techniques can be used to fabricate the
proposed bi-beams with microscale and submicron resolutions. More importantly,
we show how the requirement for a weak differential response eliminates the
need for multi-material 3D printing, as the change in the laser processing
parameters is sufficient to achieve effective differential responses. Finally,
we use the same 3D printing techniques to produce strain rate-dependent gripper
mechanisms as showcases of potential applications.
