  The current understanding of motility through body shape deformation of
microorganisms and the knowledge of fluid flows at the microscale provides
ample examples for mimicry and design of soft microrobots. In this work, a
two-dimensional spiral is presented that is capable of rotating by
non-reciprocal curling deformations. The body of the microswimmer is a ribbon
consisting of a thermo-responsive hydrogel bilayer with embedded plasmonic gold
nanorods. Such a system allows fast local photothermal heating and
non-reciprocal bending deformation of the hydrogel bilayer under
non-equilibrium conditions. We show that the spiral acts as a spring capable of
large deformations thanks to its low stiffness, which is tunable by the
swelling degree of the hydrogel and the temperature. Tethering the ribbon to a
freely rotating microsphere enables rotational motion of the spiral by
stroboscopic irradiation. The efficiency of the rotor is estimated using
resistive force theory for Stokes flow. The present research demonstrates
microscopic locomotion by the shape change of a spiral and may find
applications in the field of microfluidics, or soft micro-robotics.
