  Elastic wave manipulation is important in a wide variety of scales in
applications including information processing in tiny elastic devices and noise
control in big solid structures. The recent emergence of topological materials
opens a new avenue toward modulating elastic waves in solids. However, because
of the full-vector feature, and the complicated couplings of the longitudinal
and transverse components of elastic waves, manipulating elastic waves is
generally difficult, compared with manipulating acoustic waves (scalar waves)
and electromagnetic waves (vectorial waves but transverse only). Up to date,
topological materials, including insulators and semimetals, have been realized
for acoustic and electromagnetic waves. Although topological materials of
elastic waves have also been reported, the topological edge modes observed all
lie on the domain wall. A natural question can be asked: whether there exists
an elastic metamaterial with the topological edge modes on its own boundary
only? Here, we report a 3D metal-printed bilayer metamaterial, insulating
topologically the elastic waves. By introducing the chiral interlayer
couplings, the spin-orbit couplings for elastic waves are induced, which give
rise to nontrivial topological properties. The helical edge states with the
vortex feature are demonstrated on the boundary of the single topological
phase. We further show a heterostructure of the metamaterial, which exhibits
tunable edge transport. Our work may have potential in devices based on elastic
waves in solids.
