Pieces of synthetic skin are drawn together magnetically; electrical conductivity returns as they heal, and the LED lights. Credit: Bao Group, Stanford U.
Key points:
- Engineers have developed synthetic skin that mimic’s the layering ability of human skin.
- In experiments, each layer successfully healed itself to restore overall function.
- In the future, the technology could be used to create devices that can recover from extreme damage.
Stanford researchers have achieved a first—successful demonstration of a multi-layer, thin-film sensor that automatically realigns during healing. This is a critical step toward mimicking human skin, which has multiple layers that all re-assemble correctly during the healing process.
The secret is in the materials. The backbone of each layer is formed of long molecular chains connected periodically by dynamic hydrogen bonds that allow the material to stretch repeatedly without tearing. The key is to design polymer molecular structures and choose the right combination for each layer—first layer of one polymer, the second of another and so forth.
The research team used PPG (polypropylene glycol) and PDMS (polydimethylsiloxane, better known as silicone). Both have rubber-like electrical and mechanical properties and biocompatibility and can be mixed with nano- or microparticles to enable electric conductivity. Critically, the chosen polymers and their respective composites are immiscible—they do not mix with one another yet, due to the hydrogen bonding, they adhere to one another well to create a durable, multilayer material.
Both polymers have the advantage that when warmed they soften and flow, but solidify as they cool. Thus, by warming the synthetic skin, the researchers were able to speed the healing process. At room temperature, healing can take as long as a week, but when heated to 70°C, the self-alignment and healing happen in about 24 hours. The two materials were carefully designed to have similar viscous and elastic responses to external stress over an appropriate temperature range.
With a successful prototype in hand, the researchers then added magnetic materials to the polymer layers, allowing the synthetic skin to not only heal but also self-assemble from separate pieces.
Next, the team says they will work to make the layers as thin as possible—thin enough that a stack of 10 or more layers would be no thicker than a sheet of paper.
“One layer might sense pressure, another temperature, and yet another tension,” said postdoctoral researcher Sam Root, co-author of the study published in Science. “The material of different layers can be engineered to sense thermal, mechanical, or electrical changes.”
“Our long-term vision is to create devices that can recover from extreme damage,” added co-author Chris Cooper, a Ph.D. candidate at Stanford University. “For example, imagine a device that when torn into pieces and ripped apart, could reconstruct itself autonomously. Drawn together magnetically, the pieces inch toward one another, eventually reassembling. As they heal, their electrical conductivity returns, and an LED attached atop the material glows to prove it.”