Watch the hydrogel walking.
(August 10, 2015) In
research published in Nature Materials, a team led by scientists from the RIKEN
Center for Emergent Matter Science in Japan has developed a new hydrogel that
works like an artificial muscle—quickly stretching and contracting in response
to changing temperature. They have also managed to use the polymer to build an
L-shaped object that slowly walks forward as the temperature is repeatedly raised
and lowered.
Hydrogels are polymers that can maintain large quantities of
water within their networks. Because of this, they can swell and shrink in
response to changes in the environment such as voltage, heat, and acidity. In
this sense they are actually similar to the plant cells, which are able to
change shape as the amount of water within them changes in response to
environmental conditions.
However, most hydrogels do this very slowly, and must absorb
and excrete water to either expand or shrink in volume. The unique property of
the hydrogel developed by the RIKEN team is that it acts like an artificial
muscle, which does not contract equally in all directions. Rather, they
contract in one dimension while expanding in another, meaning that they can
change shape repeatedly without absorbing or excreting water.
The secret to the new hydrogel's property is electrostatic
charge. Using a method that they published earlier this year, the team arranged
metal-oxide nanosheets into a single plane within a material by using a
magnetic field and then fixed them in place using a procedure called
light-triggered in-situ vinyl polymerization, which essentially uses light to
congeal a substance into a hydrogel. The nanosheets ended up stuck within the
polymer, aligned in a single plane. Due to electrostatic forces, the sheets
create electrostatic resistance in one direction but not in the other.