A muscle-like substance that expands and contracts in response to light

Scientists have succeeded in developing an innovative material that expands and contracts in response to light projected onto it

Just like controlled-release drugs slowly release their charge after they "sensing" a change in the body's acidity level, implanted 'artificial muscles' will one day be able to expand and contract in response to light projected onto the skin.

"We were able to develop a polymer with an innovative mechanism for reactive materials - those materials capable of contracting, expanding or maintaining a defined structure - in response to a simple external stimulus," said the lead researcher. Materials that react to external stimuli are currently applied in a variety of industries. For example, some of them change their color and are used as coatings for car windshields in order to prevent drivers from being blinded. Other materials can be designed in the form of receptacles that respond to changes in nutrient concentrations and feed agricultural crops as required. Other applications are currently also embedded in the fields of biomedicine.

The goal of the researchers from the University of Washington was to test whether their new material is capable of doing work, a property that could aid in the development of an artificial muscle. During his studies, the lead researcher examined a group of molecules, known as viologens, which change their color following ionization processes (adding or subtracting electrons). The researcher hypothesized that if it were possible to connect several such molecules together, they would fold to form an accordion-like structure, since segments that receive a single electron recognize each other. He also wondered if the act of folding could give rise to a three-dimensional network that allows for movement, and if at all it is possible to create such a process that would also be reversible.

In order to test this, the researcher synthesized polymer chains whose ends were joined with viologens molecules. When a blue LED light was projected onto the molecules they folded into a pleated tissue with the help of a well-known photo-recycling catalyst capable of transferring electrons to the viologens molecules. In the next step, the researchers embedded the polymers in a water-soluble and flexible 30D hydrogel. When the team of researchers shone light on the gel, the system shrank to a tenth of its original size. When the projection was stopped, the system returned to its original size due to the expansion of the material. During its resizing, the gel also changed color. "The beauty of our system lies in the fact that we are able to take a small amount of our polymer, called polyviologen, and insert it into any three-dimensional network, while turning it into a material that responds to external stimuli," explains the lead researcher. The system requires less than one percent of hydrogel in order to have a responsive system. So the polymer, which is present in very little concentration, does not affect at all the other properties of the material in which it is found. To test if the innovative material is capable of doing work, the researchers connected the gel to a strip of electrical wire with a piece of wire at the end. The researchers took a small amount of the wire and placed the hydrogel in front of blue light. The gel was able to lift a small weight - an amount that weighs XNUMX times the weight of the active substance polyviologen - and after five hours it even rose by a few centimeters.

The researchers have since made more improvements to their system, for example - making the gels stronger and more flexible, and even making them faster in terms of their movement rate. In addition, the researchers also developed polymers that respond to several stimuli at the same time. They also developed gels that respond to different wavelengths. Materials that respond to red or near-infrared light, that is, rays capable of penetrating through human tissue, could be used in biomedical applications, such as drug delivery devices, or, eventually, as artificial muscles. The lead researcher says that his research group has only begun to test the limits of these new materials. Currently, researchers are examining the self-repair properties of polyviologen-containing hydrogels, as well as the possibility of XNUMXD printing the polymers into various types of other materials.

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