Wolverine, one of the X-Men in the popular movie series, possesses the mutant power to heal himself. Scientists have long been working to create materials that can do the same. The applications reach far and wide, from batteries and electronics to cars and biosensors. Combining forces, research groups at the University of Riverside and University of Colorado have developed a material that is not only self-healing, but also transparent, stretchable and conductive. The study is published in Advanced Materials.
Christoph Keplinger, an assistant professor at the University of Colorado, Boulder and an author on the paper, had previously demonstrated that stretchable, transparent, ionic conductors can be used to power artificial muscles. However, he could not overcome the obstacle of wear and tear on the system. Enter Chao Wang, an adjunct assistant professor of chemistry at UC Riverside and collaborating author.
Wang studies self-healing materials that can repair damage caused by time and wear, extend the life of the material and lower the cost by negating the need for replacement. The challenge has always been in creating bonds within materials that are stable and reversible under varying electrochemical conditions.
Wang developed a solution by using ion-dipole interactions. These are forces between charged ions and polar molecules that are stable under electrochemical conditions. He combined a polar, stretchable polymer with a mobile, high-ionic-strength salt to create the self-healing property within a conductive material. The strength of the bond increases as ionic charge, or molecular polarity increases.
By combining a top and bottom layer of autonomous self-healing and conductive polymer with a stretchable, middle layer that is a transparent, non-conductive rubber-like membrane, the team developed the first transparent, stretchable, self-healing, conductive artificial muscle. The material can stretch to 50 times its original size, and when cut, can completely re-attach within 24 hours at room temperature, without any outside stimulus.
With the new material created, the team demonstrated that the artificial muscle could move when signaled. Artificial muscle is a general term used for materials that can contract, expand, or rotate when stimulated by voltage, current, pressure or temperature. When given an electronic stimulus, the new artificial muscle can contract and expand.
Applications for such a material are still being explored, and the team looks forward to cross-discipline collaborations. Current thoughts include giving robots the ability to self-heal after mechanical failure, extending the lifetime of lithium ion batteries used in electronics and electric cars, and improving durability of biosensors used in the medical field and environmental monitoring.
“Creating a material with all these properties has been a puzzle for years,” said Chao Wang, “We did that and now are just beginning to explore the applications.”