  Magnetic microrobots that swim through liquid media are of interest for
minimally invasive medical procedures, bioengineering, and manufacturing. Many
of the envisaged applications, such as micromanipulation and targeted cargo
delivery, necessitate the use and adequate control of multiple microrobots,
which will increase the velocity, robustness, and efficacy of a procedure.
While various methods involving heterogeneous geometries, magnetic properties,
and surface chemistries have been proposed to enhance independent control, the
main challenge has been that the motion between all microwsimmers remains
coupled through the global control signal of the magnetic field. Katsamba and
Lauga proposed transchiral microrobots, a theoretical design with magnetized
spirals of opposite handedness. The competition between the spirals can be
tuned to give an intrinsic nonlinearity that each device can function only
within a given band of frequencies. This allows individual microrobots to be
selectively controlled by varying the frequency of the rotating magnetic field.
Here we present the experimental realization and characterization of
transchiral micromotors composed of independently driven magnetic helices. We
show a swimming micromotor that yields negligible net motion until a critical
frequency is reached and a micromotor that changes its translation direction as
a function of the frequency of the rotating magnetic field. This work
demonstrates a crucial step towards completely decoupled and addressable
swimming magnetic microrobots.
