The hydrogel is able to walk by lengthening and contracting in response to changing temperature. Image: Kim et al. and Nature Materials.
The hydrogel is able to walk by lengthening and contracting in response to changing temperature. Image: Kim et al. and Nature Materials.

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. The scientists have used this polymer to build an L-shaped object that slowly walks forward as the temperature is varied.

Hydrogels are polymers that can maintain large quantities of water within their networks. Because of this, they can swell and shrink in response to environmental stimuli such as voltage, heat and acidity. In this sense, they are similar to plant cells, which are able to change shape as the amount of water within them changes in response to environmental conditions.

Most hydrogels only do this very slowly, absorbing and excreting water to either expand or shrink in volume. In contrast, the hydrogel developed by the RIKEN team does not contract equally in all directions. Instead, it contracts in one dimension while expanding in another, meaning it can change shape repeatedly without absorbing or excreting water, acting like an artificial muscle.

The secret to the new hydrogel’s ability is electrostatic charge. Following a method they published earlier this year, the team using a magnetic field to arrange metal-oxide nanosheets into a single plane within the liquid precursor to the hydrogel. They then fixed them in place by taking advantage of a procedure called light-triggered in-situ vinyl polymerization, which essentially uses light to congeal the precursor 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.

According to 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°C, 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.”

In addition, the team members were intrigued to find that the process was very fast, taking just one second. The rate of deformation, at around 70% per second, is also higher than seen in other hydrogels.

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

The group now plans to develop the hydrogel for other practical applications. 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.”

The work was done by RIKEN in collaboration with The University of Tokyo and the National Institute of Material Science, which prepared the nanosheets used in the material.

This story is adapted from material from the RIKEN Center for Emergent Matter Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.