The Device: The next step in artificial skin technology could owe its inspiration to beetle wings. Researchers looking to create a flexible sensor that detects mechanical force similar to the way skin senses touch were intrigued by microhairs on beetle wings that interlock when the beetle is at rest. “As one can imagine, a large interfacial contact is made during the interlocking, which is an excellent characteristic for a sensitive sensor,” study lead Kahp-Yang Suh of Seoul National University wrote in an email. With so much surface area in contact, a sensor relying on connections between nanofibers would be able to detect even minute perturbations that changed the fibers’ relative positions.
So Suh and his colleagues decided to copy the beetle wing design, building two nanofiber arrays laid on flexible silicon polymers and coating them with a thin layer of platinum. The result was a sensing system of interlocking platinum-covered fibers in which mechanical distortion altered the fibers’ electrical resistance—changes that could be measured with a multimeter.
The researchers tested the sensitivity of their sensor by bouncing water droplets and detecting heartbeats. “It represents some new, clever ideas for a class of high performance tactile sensors that can be configured in thin, flexible formats, for direct integration with the surface of the skin,” John Rogers, a materials scientist at the University of Illinois at Urbana-Champagne, who was not involved in the research, told The Scientist in an email. With its skin-like structure, the new device has potential applications for health monitors that must be worn for long periods of time, and the development of skin for prosthetic and robotic limbs that can sense the environment like real skin.
What’s New: It’s not a one-sensation device: like the skin, the new sensor can detect multiple types of mechanical disturbance. “It is nice that their device is capable of sensing shear and torsion, which are difficult for most other sensors,” Zhenan Bao of Stanford University, who did not participate in the research, wrote in an email. The interlocking nanofiber sensor can also detect pressure, while exhibiting high sensitivity compared to other types of sensors, said Suh.
Additionally, “the mechanism is a bit different than other tactile sensors of which I am aware,” Rogers noted. Bao’s work, for example, has utilized only a single layer of nanofibers to transmit pressure into changes in electrical conductance. Other devices have used deformable membranes or elastomers that change in their ability to store electrical charge in response to mechanical force; stretchable rubber conductors whose resistance changes upon disturbance; or elements that create output voltages or currents from mechanical force, Rogers explained. “The present work offers an alternative that seems to enable remarkable sensitivity, but with a relatively simple design.”
The Importance: Flexible sensors that respond to stimuli much like real skin have a variety of potential applications, Suh explained, including biomedical sensors, artificial skin, and flexible displays like highly sensitive touch screens. The fact that Suh’s sensor produces different signals in response different types of force “is important because it is quite similar to real sensing mechanism of our skin, so that our sensor is potentially useful for future skin electronics,” he wrote.
“It’s a valuable contribution to an emerging field of research, in which materials and devices are developed in ‘soft’ forms that can interface naturally with the human body, for various applications in wellness monitoring, clinical health care, and others,” Rogers added. Suh and his colleagues were able to measure their heart beats, for example, while wearing the device on their wrists.
The technology is also one step closer to a sensor that could be integrated into prosthetic limb systems that provide feedback on real-world sensory information.
Needs Improvement: The electronics of the circuit need finessing, explained Suh. “We need a better electrical network to process and transmit what type of physical signal is exerted onto the surface,” Suh said, while the setup to measure, process, and display the signal is bulky and needs miniaturizing. In addition, said Bao, a resistance-based sensor may not be as easy to integrate into electronic circuits as other types of sensors, like the capacitive sensors Bao’s group designed, which transmit force into changes in electrical charge storage and are “readily integratable with the electronics used for capacitive touch sensors in displays.”
And while the technology is promising, other aspects of skin sensation have yet to be captured in the sensor, Suh acknowledged. Creating a sensor that exhibits fatigue, or decreasing sensitivity as a signal is continuously applied, would mimic how real skin works. Force sensing is just one function of wide variety of sensations, like temperature, chemical, or strain sensations “that one would ultimately need to integrate into a multifunctional system if the goal really is to reproduce or enhance all of the various operations of real skin,” added Rogers.
Pang et al., “A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibres,” Nature Materials, doi: 10.1038/NMAT3380, 2012.