Artificial skin: One more time, with feelingArtificial skin: One more time, with feeling

To the outside observer, it might have looked like a wack tug of war. Wack, because the ersatz rope being tugged was actually a very stretchy elastomer, and the setting was a lab at Stanford University (Stanford, CA). There is a scientific rationale for all of this: The two researchers needed to determine the elongation at break of an elastomer synthesized by Stanford chemical engineering professor Zhenan Bao, and the in-house testing device proved inadequate to the challenge. “The clamping machine typically used to measure elasticity could only stretch about 45 inches,” write Carrie Kirby and Tom Abate of the Stanford News Service. So, Cheng-Hui Li, a visiting scholar from China, and another lab member gripped the piece of plastic on opposite ends and started pulling. Eventually, they stretched the 1-inch polymer film to more than 100 inches. The material could form the foundation of a self-healing polymer that may restore the sense of touch to amputees wearing prosthetic devices.

Bao has been working on polymer-based skin for several years. PlasticsToday reported on her research into self-healing compounds back in 2012, and she has made significant progress since then. The new material, which Bao describes in a paper published this week in Nature Chemistry, not only self heals but expands and contracts when it is exposed to an electric field.

The stretching and self-healing properties are attributed to special organic molecules that crosslink with short polymer strands to create ligands, which join together to form longer polymer chains with spring-like coils and inherent stretchiness, explain Kirby and Abate. Metal ions were then added. When the material is strained, the knots loosen and allow the ligands to separate. At a relaxed state, the ligands tighten.

“Basically the polymers become linked together like a big net through the metal ions and the ligands,” Bao explained. “Each metal ion binds to at least two ligands, so if one ligand breaks away on one side, the metal ion may still be connected to a ligand on the other side. And when the stress is released, the ion can readily reconnect with another ligand if it is close enough.”

This research dovetails with Bao’s efforts to create artificial skin that can restore feeling to people with prosthetic limbs, which was profiled recently in the "All Tech Considered" segment on NPR’s All Things Considered.

Bao has developed a plastic that mimics the electrical properties of silicone in which she has embedded a pressure sensor. The electrical properties change depending on how hard one presses on the device. “But it’s not enough to just make a plastic with a built-in pressure sensor,” reports NPR’s Joe Palca. “You need to build an electrical circuit into the plastic that can relay what the pressure sensor is sensing.” This is achieved through electrodes embedded in the plastic that convert the touch signal into electrical pulses. Those signals ultimately will be sent to nerve bundles that transmit them to the brain.

To power the artificial skin, Bao’s team has embedded electronics in the polymer that absorb light and convert it into electricity.

Near-term applications include low-cost solar panels and touch screens—Bao has launched a startup to sell a version of the plastic for the latter application—but the ultimate and infinitely more exciting goal is to bring back the sense of touch to patients who have lost limbs.