Researchers at Washington University in St. Louis (WUSTL) developed the material that in experiments showed the ability to lift a weight by simply shining a light on it. Tests are being conducted to determine the polymer’s best use, but the main goal has been to see whether the material can do work, a trait that could facilitate the development of an artificial muscle. A proprietary polymer that contracts and expands in response to light could someday be used to create artificial muscles implanted in the human body and controlled by illuminating the skin. Jonathan Barnes, Ph.D., of WUSTL, said the discovery was made when he and his team synthesized polymer chains with viologens in their backbones. When a blue LED light was shined on the molecules, they folded into pleats with the help of well-known photoredox catalysts that can transfer electrons to the viologens. The researchers next incorporated the polymers into a flexible, water-soluble 3D hydrogel. “We have developed a new polymer that has a novel mechanism for actuating materials — making materials shrink, expand, or hold a ‘memory’ of a particular shape — all with a simple stimulus,” Barnes said. Stimuli-responsive materials have been applied in a variety of industries. For example, some of them change color and are used as windshield coatings to instantly shade drivers in blinding sun. Other materials can be formed into vessels that respond to changes in nutrient concentrations and feed agricultural crops as needed. Still other applications are in the biomedical area. When the team shined light on the gel, the accordion effect that occurred within the molecule tugged the gel in on itself, causing the material to shrivel to one-tenth its original size. When the light was turned off, the material expanded. As the polymer-embedded hydrogel changed form, it also changed color. “The beauty of our system is that we can take a little bit of our polymer, called a polyviologen, and put it in any type of 3D network, turning it into a stimuli-responsive material,” Barnes said. The researchers have now made other improvements, including making the gels stronger, more elastic, and faster. They also developed polymers that respond to multiple stimuli at once and constructed gels that respond to light at different wavelengths. Materials that respond to red or near-infrared light, which can penetrate human tissue, could be used in biomedical applications, such as drug-delivery devices or, eventually, as artificial muscles. Barnes said his team has only begun to test the limits of these new materials. Currently, the researchers are studying the self-healing properties of polyviologen-embedded hydrogels, and they are exploring the possibility of 3D printing the polymers into different types of materials.
