Temperature-sensitive Hydrogel Can Walk

A novel hydrogel that stretches and contracts like an artificial muscle has been developed by Scientists from the RIKEN Center for Emergent Matter Science in Japan. The hydrogel, which can rapidly stretch and contract in response to changing temperature, was used to build an L-shaped object that slowly walks forward as the temperature is repeatedly raised and lowered.

Hydrogels are a form of polymer 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. But most of them do this very slowly, and must absorb and excrete water to either expand or shrink in volume.

The RIKEN research team changed the hydrogel’s properties to enable it to contract only in one dimension, while expanding in another, as do human muscle fibers. This make the new hydrogel able to change shape repeatedly without absorbing or excreting water.

Light-triggered In-situ Vinyl Polymerization

The scientists used a magnetic field to arrange metal-oxide nanosheets into a single plane within a material. They next fixed them in place using a technique called light-triggered in-situ vinyl polymerization, which essentially uses light to congeal a substance into a hydrogel.

The result is nanosheets captured 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.

Explained author Yasuhiro Ishida:

“We originally designed this material to be stretchable in one direction, but we also found that at a temperature called the lower critical solution temperature, which we calculated to be 32 degrees Celsius, the polymer rapidly changed shape, stretching in length. Intriguingly, the gel did not change in volume. The substance underwent the change in shape in air and in a liquid environment, showing that it doesn’t require the uptake of water. So in other words, it will work even in a normal air environment.”

Walking In Water

To demonstrate how the polymer might be put to practical use, the group designed an L-shaped piece of polymer that can actually walk, in a water environment, as the legs lengthen and contract in response to changing temperature.

According to Ishida,

“We are now planning further work to improve the properties of the substance. One idea we have is to use a hydrogel like this to make artificial muscles that could automatically open and close radiator systems as temperatures rise and fall. This could be used, for example, to prevent a device from overheating.”

Youn Soo Kim, Mingjie Liu, Yasuhiro Ishida, Yasuo Ebina, Minoru Osada, Takayoshi Sasaki, Takaaki Hikima, Masaki Takata, and Takuzo Aida
Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel
Nature Materials, doi: 10.1038/nmat4363

Photo: RIKEN Center for Emergent Matter Science