THE DEVICE: The epidermal electronic system (EES) is flexible, thinner than a human hair, and applied to the skin like a fake tattoo. Depending on the embedded circuitry, it can monitor body temperature, brain activity, or the contraction of the heart and skeletal muscles, and power tiny LEDs, transistors, or antennae for communications. It draws energy from its micro-scale solar collectors or wireless inductive coils, making it lightweight with no external power cords.
WHAT’S NEW:Traditional devices for sleep or heart monitoring include battery packs wired to large, inflexible sensors. This makes the units heavy, cumbersome, and a source of skin irritation. To create a chip that is as pliant as skin, John Rogers, an engineering professor at the University of Illinois at Urbana-Champaign, and colleagues created squiggly-shaped conductive gallium arsenide wires and mounted them on silicon that is less than 7 micrometers thick—two orders of magnitude thinner than a typical 500 micrometer silicon chip.
The combination of the thin silicon, the serpentine shape of the wires, and the layers of silicone and polyester in which the EES is mounted allow the chip to bend and stretch like skin without damaging the circuitry, according to the study published in Science last week (August 12).
“The flexibility is a factor of a million better in terms of matching to the skin compared to anything reported earlier,” said Rogers.
Furthermore, because the EES sticks to the skin with Van der Waals forces alone—much like a gecko’s toes stick to a wall—it requires no adhesives or conductive gels. In combination with the chip’s flexibility, this prevents the skin irritation common to previous types of semi-flexible bandages and sensors, said Michael Neuman, professor of biomedical engineering at Michigan Technological University. With earlier devices, “the skin stretches and the pad doesn’t, so it develops irritation right at the interface.”
In addition, the EES’s small size makes it possible to fit many more sensors onto the skin than is possible with larger sensors. “You can have 1000 or even a million contact points now,” Rogers said.
IMPORTANCE: Apart from improved physiological monitoring, the EES could allow humans to control machines by transmitting muscle movement to a computer that interprets their patterns like a language. In the study, the chip monitored contractions of the throat muscles used during speech, though no vocal sounds were produced, and was able to decipher simple commands to control a videogame with 90 percent accuracy. The results suggest that chips may aide communication in people who have lost speech ability, or allow people to use their muscles to communicate signals that will control computers or devices such as prosthetic limbs. Similarly, when the chip was placed on the forehead, it accurately measured neural activity, suggesting that brainwaves might be translated into a command language in a similar fashion.
The device could also potentially be used as an electronic bandage to speed up wound healing, burns, and other skin conditions, and in physical rehabilitation to both monitor muscle strain and stimulate muscle contractions.
Rogers said that much of the EES technology is already licensed to a Cambridge, Massachusetts, company called mc10, which he co-founded with George Whitesides of Harvard University. The company is experimenting with advanced surgical tools such as balloon catheters fitted with a chip that carries a pressure-sensing strain gauge, allowing doctors to apply the appropriate amount of pressure to a blood vessel to clear a blockage. They are also pursuing efforts to laminate EES devices directly onto the spinal cord to potentially bridge gaps in patients with spinal cord damage.
NEEDS IMPROVEMENT: Now that they have demonstrated that a variety of different sensory, stimulatory, and communications devices can function successfully in a flexible EES environment, the researchers are working to integrate them to work together on a single chip. In addition, they want to add wireless communication capability to transmit the data it measures to an appropriate recording device. “Because of the higher quality of the…thin silicon, wireless communication directly from the electronic skin should be feasible,” Zhenqiang Ma, professor of electrical engineering at the University of Wisconsin-Madison, wrote in a perspective that accompanied the study in Science.
A fully wireless EES could be especially useful for neonatal monitoring and cases of sleep apnea where traditional devices restrict movement and disrupt sleep, said Neuman. Additionally, the lack of external wires could serve to reduce the signal interference caused by the dangling cords during cardiac stress tests, which traditionally use adhesive electrodes connected to an electrocardiograph.
Another hurdle is to adhere the chip to the skin for longer periods of time. When placed on the skin, the current EES stays in place for up to 24 hours on the arm, neck, forehead, cheek, or chin, but doesn’t stick as well when wet, and sloughing skin cells are constantly loosening the chip’s grip. The researchers are exploring adhesives, such as fake tattoos, which can both improve adhesion and, from an aesthetic standpoint, disguise the circuitry, while still avoiding the irritation associated with previous adhesive-based sensors.
D.H. Kim, et al., “Epidermal electronics,” Science, ID: 1206157, 2011.