Electronic skin or e-skin is a thin electronic material that mimics human skin in one or more ways. Specifically, human skin can sense pressure and temperature, stretch, and can heal itself. Electronic skin aims to apply these functions to robotic and health applications.
In September 2010, a team from the University of California, Berkeley announced preliminary results of their research into electronic skin. They developed a technique to attach nanowire transistors to sticky substrate. The resulting squares of superconductive material were imbedded in a thin pressure-sensitive rubber. The resulting product was able to sense a wide range of pressures and capable of withstanding being bent 2000 times. The researchers hoped the e-skin could be used to restore a sense of touch to patients with prosthetic limbs. The research was funded in part by the National Science Foundation and the Defense Advanced Research Projects Agency. The results were published in Nature Materials.
In the same issue of Nature Materials, a different electronic skin project by Stanford University headed by Zhenan Bao was also described. The Stanford project focused on stretching an electricity-conducting-rubber membrane between two electrodes. They developed a unique structure for the rubber that allowed it to regain its shape after pressure was applied; previously the material would become too entangled at the micro-level to fully bounce back. With up to 25 million sensors per square centimeter, the skin was sensitive enough to detect a fly landing on it.
In February 2011, the Stanford team developed a stretchable solar cell that could be used to power their electronic skin. An accordion-like micro-structure allowed the cells to stretch up to 30% without damage. The team also added biological and chemical sensors to the skin to supplement the pressure sensors. Bao said she imagined the artificial skin could one day be used on robot hands capable of detecting things such as disease or intoxication of humans via touch.
In August 2011, an international team announced an electronic patch for monitoring patient's vital signs which was described as "electric skin". The device was created by embedding sensors in a thin film and then placing the film on a polyester backing similar to those found on temporary tattoos. A small coil provided power through induction. In tests, the device stayed in place for 24 hours without adhesives, relying instead on the van der Waals force, and was flexible enough to move with the skin it was placed on. Project researcher John A. Rogers commented "What we are trying to do here is to really reshape and redefine electronics ... The goal is really to blur the distinction between electronics and biological tissue."
The team proposed that in addition to monitoring patient health, the electronic device could be used to monitor brain waves, detect speech by sensing vibrations in the larynx, and emit heat to help in healing. They suggested that it could perhaps even be made sensitive to touch and be used as artificial skin. A number of public and private enterprises provided funding for the project, including the Air Force Research Laboratory and the U.S. Department of Energy. The results of the research was published in Science. Rogers founded a company, MC10, to explore commercial uses for the skin.
In November 2012, Bao's Stanford team developed an electronic skin capable of healing itself by combining a self-healing plastic and nickel, a conductive metal. Unlike self-healing polymers developed by other researchers, their skin did not require high temperature or UV lights to activate. The individual plastic molecules of the skin break-apart relatively easily, but the bonds also easily reform. Cut pieces healed to 75% strength within a few seconds and fully in less than 30 minutes when pressed together at room temperature. Additionally, the process could be repeated many times – in experiments the material showed near perfect healing after 50 breaks. Other self-healing materials alter their structures in the process and thus can only heal once.
In addition to being self-healing, the electronic skin was pressure-sensitive and very flexible. It was the first material to exhibit all these properties at the same time. It was also the first conductive self-healing polymer. The e-skin could detect both downward pressure and pressure from bending; thus, in principle, it could detect both the pressure and angle of a normal human handshake. Bao's team suggested the material could be useful in prosthetics and to create self-healing wires for electronic devices. The research was published in Nature Nanotechnology.
In July 2013, a different UC Berkeley team announced they had created an electronic skin that lights up when touched. Pressure triggered a reaction in the skin that lights up blue, green, red, and yellow LEDs; as pressure increased the lights got brighter. The material was composed of synthetic rubber and plastic and was thinner than a piece of paper. Sandwiched between layers, organic LEDs were lit by semiconductor-enriched carbon nanotubes and a conductive silver ink. The skin was made up of hundreds of circuits, each of which contained a pressure sensor, a transistor, and a tiny LED. Pressure changed the resistance of the sensor thereby changing the amount of electricity flowing into the LED.
The Berkley team suggested the invention could be useful in artificial skin for prosthetic limbs, attached to human skin to monitor health, and used in robotics. The invention was announced in Nature Materials. Previously, flexible sensors and flexible displays had been demonstrated, but never at the same time.
Also in 2013, the University of Cincinnati reported the first ever artificial skin capable of sweating similar to natural sweat rates and with the surface texture and wetting properties of regular skin. Although not electronic, this microfluidic skin can enable electronic skin to have improved skin grip with slight sweating, similar to how human palm and finger tips expel small amounts of eccrine sweat during gripping.
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- Scott Jung (February 23, 2011). "Scientists Develop Stretchable Solar Cells for Electronic Skin". medGadget. Retrieved July 27, 2013.
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- Jon M. Chang (July 23, 2013). "Electronic ‘Skin’ Responds to Your Fingertips". ABC News. Retrieved July 23, 2013.
- Hou, Linlin (2013). "Artificial microfluidic skin for in vitro perspiration simulation and testing" (PDF). Novel Devices Lab. Lab-on-Chip.