A new hydrogel, developed by a research team headed by scientists at the RIKEN Center for Emergent Matter Science in Japan, stretches and contracts swiftly like an artificial muscle in response to temperature change.
In addition, the research team have made a L-shaped object using the polymer. A changing temperature caused this object to walk forward slowly.
Hydrogels have the ability to maintain large amounts of water in their networks like plant cells. This causes them to swell or shrink as a result changes in environmental conditions such as voltage, heat, and acidity.
In order for most hydrogels to shrink they have to excrete water, and in order to expand they have to absorb water. This process takes place very slowly in most hydrogels. However, the hydrogel that was developed in this study works like an artificial muscle. Equal contraction does not occur in all directions. Furthermore, even without excreting or absorbing water, they can repeatedly modify their shape, and while expanding in one direction they could be contracting in the other direction.
Watch a video of the expansion and contraction of the hydrogel with changing temperature
This unique property is due to electrostatic charge. The research team had previously developed a method which involved using light for congealing a substance into a hydrogel. The team used a magnetic field to set metal-oxide nanosheets into a single plane in a material. Following this, they used light-triggered in-situ vinyl polymerization to fix the nanosheets in place. In this procedure, light was used for congealing a substance into the hydrogel. Finally, the nanosheets were aligned in a single plane and they were fixed within the polymer. Electrostatic resistance was created by the sheets because of the electrostatic forces. However, this resistance took place only in one direction and it did not take place in the other direction.
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.
Furthermore, the research team observed that the process took just one second, which was very fast. The deformation rate was about 70% per second, which was comparatively higher than that observed in other hydrogels.
The team designed an L-shaped polymer piece that could literally walk in a water environment. Changing temperature induced the legs to respond. As a result, the legs lengthened and contracted, which made the polymers to walk.
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.
Researchers from the University of Tokyo and the National Institute of Material Science had collaborated with the RIKEN researchers in this study. They fabricated the nanosheets utilized in this hydrogel.
The study paper entitled, "Thermoresponsive actuation enabled by permittivity switching in an electrostatically anisotropic hydrogel", has been published in Nature Materials.