  The recent advancements in nanoscale 3D printing and microfabrication
techniques have reinvigorated research on microrobots. However, precise motion
control of the microrobots on biological environments using compact actuation
setups remains challenging to date. This work presents a novel control
mechanism and contact design that enables bidirectional steering via biasing
the neutral position of the microrobot. Equipped with rockers to contact the
substrate, the microrobot, hence microrocker bot, is capable of well-controlled
forward and backward movement on flat and non-flat biological surfaces. The
100um by 113um by 36um robots were 3D printed via two-photon lithography and
subsequently deposited with nickel thin films. Under a relatively small static
magnetic field, the microrocker bot tilts either forward or backward to align
the thin film magnetization direction with the magnetic field lines. When
combined with an oscillating magnetic field, the robot undergoes stick-slip
motion in the predisposed direction, dictated by the neutral position tilt. The
microrocker bots are further equipped with sharp mechanical tips that can be
selectively engaged. When the frequency and offset of the actuation sawtooth
waveform are optimized, the robot travels up to 100um/s (1 body length per
second) forward and backward showing very linear trajectories. Finally, to
prove the functionality of the microrocker bots in direct contact with
biological surfaces, we demonstrate the robot's ability to traverse forward and
backward on the surface of a Dracaena Fragrans leaf, and upend/engage on its
mechanical tip.
