Light-triggered robotic arm bends, stretches
TOKYO – As the world gets smaller and
smaller, microrobots and nanomachines
become more important for fabrication,
assembly and other tasks. Controlling such
tiny machines can be problematic – cable-based controls don’t work at such small
scales or even, sometimes, in liquids.
Piezoelectric crystals have been shown
to move when exposed to electricity, but
electricity also relies on cables and can
cause problems with liquids.
The solution? A new cable-free microrobotic arm that can be controlled with
The robotic arms from Masahiro Irie
and colleagues at Rikkyo University are
made of crystals shaped like micron- or
millimeter-size flat rods. When exposed to
ultraviolet light, the rods bend toward the
light source; when irradiated with visible
light, they stretch back to their original
The molecules in the crystals are an
organic ring system containing five rings.
The central structural unit is a diaryleth-
ene group. UV light induces rearrange-
ment of the chemical bonds and causes a
ring closure within the molecule, resulting
in a shape change of each molecule. This
leads to a geometry change of the crystal,
which then contracts, but only where it is
exposed to the UV light – that is, on the
outer layer of the irradiated side of the
rod. This causes bending similar to that
of a bimetallic strip. Visible light causes
the reaction to reverse. The newly formed
sixth ring opens, the original crystal
structure is restored, and the crystal
New microrobotic arms made of crystals show reversible curling to a hairpin shape upon irradiation
with ultraviolet light. The crystals return to the original straight shape upon irradiation with visible light.
Courtesy of Masahiro Irie, Rikkyo University.