Artificial Skin Heals Itself Over and Over Again
by Prachi Patel
Materials Research Society | Published: 07 December 2012
(1)The artificial skin in an electrical
circuit containing an LED; (2) sample cut in two; (3) When the pieces
were gently pressed together about 90% of the material’s electrical
conductivity was restored in 15 seconds; (4) In about 30 minutes, sample
conductivity and mechanical strength were completely restored and it
could be flexed Image credit: Stanford University. Click image to
enlarge.
Researchers at Stanford University have made a material that is flexible, pressure-sensitive, and can repeatedly repair itself within minutes when cut. The material, an organic polymer containing tiny nickel particles, could find use as artificial skin for robots and prosthetics. It could also lead to self-healing laptops and cell phones, the researchers say.
In recent years, researchers have made electronic skin that looks and acts more and more like human skin. Various groups have developed soft, flexible, and stretchable materials that are electrically conductive or embedded with sensors so that they can sense pressure, or touch, which can be measured as electrical signals.
Stanford chemical engineering professor Zhenan Bao and her colleagues wanted to make synthetic skin that has important property of human skin: the ability to heal over and over again. While various self-healing polymeric materials have been reported, most require external stimuli such as heat or light to repair themselves or they can only heal once, says Chao Wang, a post-doctoral researcher in Bao’s group.
So Wang and graduate student Benjamin Chee-Keong Tee turned to a self-healing thermoplastic elastomer developed by Ludwik Leibler and his colleagues at ESPCI ParisTech in France. The plastic contains long polymer chains held together by weak hydrogen bonds. These weak bonds break when the material is damaged, but have the ability to rejoin at room temperature. “The hydrogen bonds are like Velcro,” Tee says. “You can pull them apart and when you bring them back together, they stick to each other.”
The researchers added nickel microparticles to the normally insulating plastic to make it conductive. They made different polymer samples containing varying concentrations of nickel particles, ranging from 5% to 31%. Higher concentrations made the material less flexible and interfered with its healing because the nickel particles prevented the hydrogen bonds from reconnecting as easily. Samples containing 31% nickel by volume displayed the most conductivity of 40 Siemens per centimeter.
To test the material’s healing ability, the researchers connected it to an electrical circuit and then cut it into two pieces. When they gently pressed the pieces together, about 90% of the material’s electrical conductivity was restored in 15 seconds. In about 30 minutes, its conductivity and mechanical strength were completely restored and it could be flexed. The material could heal itself completely after being cut 50 times at the same place.
Finally, the researchers tested the material’s sensitivity to pressure. They connected a material with 15% nickel particle concentration to a light-emitting device (LED). The more they bent or pressed the material, the brighter the LED shone. That’s because the force brings the nickel particles together, increasing the composite’s conductivity, Wang says.
The team reported their results in the November 11 issue of Nature Nanotechnology. They are now trying to make the composite more stretchable and transparent so that it could be used in touch screens.
ESPCI ParisTech’s Leibler says the new material is promising for artificial skin. The Stanford team has done “very nice work,” he says. “They achieve healing of both conductivity and mechanical strength. This is a tour-de-force.”
Read the abstract from Nature Nanotechnology here.
