Light-driven
microswimmers: the material of the swimming body, which measures
just under one
millimetre in length, is chosen so that it changes shape when exposed
to green light.
This causes wave-shaped protrusions to form along the swimmer and
drive it in the
opposite direction when light patterns move over its surface.
© Alejandro Posada
(February 25, 2016) A
swimming microrobot formed from liquid-crystal elastomers is driven by a
light-induced peristaltic motion
Ciliates can do amazing things: Being so tiny, the water in
which they live is like thick honey to these microorganisms. In spite of this,
however, they are able to self-propel through water by the synchronized
movement of thousands of extremely thin filaments on their outer skin, called
cilia. Researchers from the Max Planck Institute for Intelligent Systems in
Stuttgart are now moving robots that are barely perceptible to the human eye in
a similar manner through liquids. For these microswimmers, the scientists are
neither employing complex driving elements nor external forces such as magnetic
fields. The team of scientists headed by Peer Fischer have built a
ciliate-inspired model using a material that combines the properties of liquid
crystals and elastic rubbers, rendering the body capable of self-propelling
upon exposure to green light. Mini submarines navigating the human body and
detecting and curing diseases may still be the stuff of science fiction, but
applications for the new development in Stuttgart could see the light-powered
materials take the form of tiny medical assistants at the end of an endoscope.
Their tiny size makes life extremely difficult for swimming
microorganisms. As their movement has virtually no momentum, the friction
between the water and their outer skin slows them down considerably – much like
trying to swim through thick honey. The viscosity of the medium also prevents
the formation of turbulences, something that could transfer the force to the
water and thereby drive the swimmer. For this reason, the filaments beat in a
coordinated wave-like movement that runs along the entire body of the
single-celled organism, similar to the legs of a centipede. These waves move
the liquid along with them so that the ciliate – measuring roughly 100
micrometres, i.e. a tenth of a millimetre, as thick as a human hair – moves
through the liquid.
The soft,
light-sensitive microrobot is moved by a dynamic, structured light field.
The swimming body
consists of a mixture of liquid-crystal molecules (LC) and dye molecules that
heats up when
illuminated. This causes the liquid-crystal molecules to bend so that the
material
deforms and
protrusions form on the illuminated surface. In a moving light field, the
protrusions move
along the swimming
body via peristalsis, thereby driving the body along. © Stefano Palagi
“Our aim was to imitate this type of movement with a
microrobot,” says Stefano Palagi, first author of the study at the Max Planck
Institute for Intelligent Systems in Stuttgart, which also included
collaborating scientists from the Universities of Cambridge, Stuttgart and
Florence. Fischer, who is also a Professor for Physical Chemistry at the
University of Stuttgart, states that it would be virtually impossible to build
a mechanical machine at the length scale
of the ciliate that also replicates its
movement, as it would need to have hundreds of individual actuators, not to
mention their control and energy supply.