  Spin-momentum locking, a key property of the surface states of
three-dimensional topological insulators (3DTIs), provides a new avenue for
spintronics applications. One consequence of spin-momentum locking is the
induction of surface spin accumulations due to applied electric fields. In this
work, we investigate the extraction of such electrically-induced spins from
their host TI material into adjoining conventional, hence topologically
trivial, materials that are commonly used in electronics devices. We focus on
effective Hamiltonians for bismuth-based 3DTI materials in the ${\rm Bi}_2{\rm
Se}_3$ family, and numerically explore the geometries for extracting
current-induced spins from a TI surface. In particular, we consider a device
geometry in which a side pocket is attached to various faces of a 3DTI quantum
wire and show that it is possible to create current-induced spin accumulations
in these topologically trivial side pockets. We further study how such spin
extraction depends on geometry and material parameters, and find that
electron-hole degrees of freedom can be utilized to control the polarization of
the extracted spins by an applied gate voltage.
