Temperature-controlled hydrogel can walk
Scientists have developed a new hydrogel that stretches and contracts just like an artificial muscle. The team created an L-shaped object made out of the hydrogel and immersed it in a water bath. When the water’s temperature was varied, it slowly "walked" forward.
The researchers at the Riken Center for Emergent Matter Science in Japan got the hydrogel to move by tweaking its properties. Polymers like hydrogels carry large amounts of water within their structure which gives them the capacity to respond to variations in environmental factors such as acidity, voltage and temperature. Typically, this response time is quite slow, as the hydrogel must excrete or absorb water to correspondingly shrink or expand.
The Japanese scientists altered the new hydrogel to enable it to contract only in one dimension while expanding in another. Since the hydrogel doesn’t contract equally in every direction, it’s able to alter its shape without absorbing or excreting any water. To test the hydrogel's properties, the team created an L-shaped polymer and changed the temperature repeatedly to observe its response.
Metal-oxide nanosheets were arranged on a single plane using a magnetic field. The nanosheets were then fixed in place using a process called light-triggered in-situ vinyl polymerization, where the light helped to stick them together within the polymer. The nanosheets create electrostatic resistance in one direction, but not the other.
The polymer’s "legs" not only lengthened and contracted at pace, allowing it to move forward, but its overall volume also remained the same. This breakthrough, the researchers claim, allows them to create fast-reacting hydrogels that can function in other environments.
"In principle, this hydrogel can walk not only in water, but also in various non-aqueous media including an open-air environment," Yasuhiro Ishida, one of the paper’s authors, tells Gizmag. The L-shaped polymer can walk indefinitely, Ishida says, as long as the heating and cooling cycle is repeated.
The resulting hydrogel is reported to change shape in around one second, with a deformation rate of about 70 percent per second.
Possible applications for the hydrogel include creating artificial muscles that can automatically open and close systems in response to temperature changes, to prevent them from overheating, for instance. Biological applications are also possible.
"Such water-rich, soft actuators would also find applications as components of artificial internal organs and artificial muscles," says Ishida.
The team is now planning to create hydrogel actuators that respond to stimuli other than temperature.
A paper on the research was recently published in the journal Nature.