Despite their widespread use in a variety of medical device applications, many sensors have been limited by their inability to stretch along with the body’s movements without compromising functionality. Seeking to address this need in the pursuit of an artificial “super skin,” researchers at Stanford University (Stanford, CA) have developed a transparent pressure sensor capable of stretching to more than twice its original size without experiencing permanent deformation. If commercialized, this stretchy, skin-like sensor could potentially enhance artificial limbs and other medical applications.
|Nanotube springs in a transparent sensor developed at Stanford University can be stretched in all directions while maintaining electrical conductivity.|
“The technology is in its very early stages, but stretchable pressure and strain sensors have many of the right characteristics for integration with the curved and moving surfaces of the body, skin, internal organs, and prostheses,” explains Darren Lipomi, a postdoctoral fellow at Stanford. He envisions the sensors’ eventual use in such medical applications as skin for artificial limbs or burn victims, internal pressure sensors for hypertension or traumatic brain injury swelling, and a variety of health-monitoring applications.
“Our technology, which can sense a range of pressures from a pinch (10 kPa) to a few MPa, can sense a much greater range of interaction between the sensor and a body part than can a sensor that can only sense touch,” Lipomi adds.
The result of several years of intense research and development, the skin-like sensor features a transparent film made from single-walled carbon nanotubes. To create this film, the researchers first airbrush the nanotubes in a liquid suspension onto flexible silicone. The film is then elongated to align randomly oriented nanotube clumps—which can form as a result of the airbrushing process—in the direction that the material is being stretched.
Upon release of the material, however, the film rebounds to its original shape. Acting as tiny springs, the nanostructures serve as electrodes that can be compressed or extended without affecting the sensor’s performance. The elongation process is then repeated in a perpendicular direction to align the nanostructures so that they can remain electrically conductive while stretched in any direction.
The nanotube films initially presented a challenge because they resisted firmly adhering to the silicone when the device was being assembled, Lipomi notes. “While they stay in place after the whole thing is glued together, manipulating the transparent silicone membranes bearing the spray-painted carbon nanotube lines was difficult because if the active surface touched anything, the nanotubes could rub off,” he says.
Achieving success in device assembly, the sensor is engineered so that two of these nanotube-coated silicone layers face each other with a thin layer of silicone sandwiched between them. This middle silicone layer, which stores electrical charge like a battery, compresses when pressure is exerted on the sensor. In turn, the amount of electrical charge that the middle layer of silicone can store changes as well. This change in storage capability is then detected by the nanotube layers, which act like positive and negative battery terminals by measuring and transmitting the force type of deformation.
Although the researchers focused on the transparency and stretching capabilities of this particular sensor, they have previously created sensors sensitive enough to detect pressure exerted by a fly carcass. “We have only recently started experimenting with ways of increasing [this sensor’s] sensitivity, but we believe replacing a few of the materials, or even the smallest amount of analytical modeling, could improve the sensitivity significantly,” Lipomi adds.