Fake fingerprints might sound like just another ploy to fool the feds. But the world’s first artificial prints—reported today—have even cooler applications. The electronic material, which mimics the swirling designs imprinted on every finger, can sense pressure, temperature, and even sound. Though the technology has yet to be tested outside the lab, researchers say it could be key to adding sensation to artificial limbs or even enhancing the senses we already have.
“It’s an interesting piece of work,” says John Rogers, materials scientist at the University of Illinois, Urbana-Champaign, who was not involved in the study. “It really adds to the toolbox of sensor types that can be integrated with the skin.”
Electronic skins, known as e-skins, have been in development for years. There are several technologies used to mimic the sensations of real human skin, including sensors that can monitor health factors like pulse or temperature. But previous e-skins have been able to “feel” only two sensations: temperature and pressure. And there are additional challenges when it comes to replicating fingertips, especially when it comes to mimicking their ability to sense even miniscule changes in texture, says Hyunhyub Ko, a chemical engineer at Ulsan National Institute of Science and Technology in South Korea.
So in the new study, Ko and colleagues started with a thin, flexible material with ridges and grooves much like natural fingerprints. This allowed them to create what they call a “microstructured ferroelectric skin” (expanded in the figure below). The e-skin’s perception of pressure, texture, and temperature all come from a highly sensitive structure called an interlocked microdome array—the tiny domes sandwiched in the bottom two layers of the e-skin, also shown in the figure below.
Here’s how it works: When pressure from the outside squishes the two layers together, an electric current is created, running through the thickness of the material. The current is then monitored through electrodes and registered as pressure—the larger the current, the stronger the pressure. The skin can also sense temperature, the team reports today in Science Advances, though in ways similar to other technologies. When exposed to warmth, the e-skin material relaxes, but when cooled, it stiffens. In both cases, subtle changes in stiffness generate currents that scientists can record as temperature spikes or drops, much in the same way that they infer pressure from currents.
But perhaps most unexpectedly, the artificial fingerprint can pick up on audio cues, too. That’s right—the e-skin can hear. Ko explains that noise can make small vibrations in the e-skin, which induce slight perturbations in the microdome array that send electrical signals. To test this, he and his team placed a set of speakers next to the e-skin and played a recording of the letters in the word skin. Using a machine that measures frequency of sound waves, they recorded the frequency patterns that the artificial skin “heard” from each letter. Later he compared the frequency recordings with that of the original source and then to that of a smartphone. The most surprising finding? “Our e-skin picked up sound better than the mic on a smartphone.”
Ko says that exactly how the pressure, texture, temperature, and acoustic signals will be transmitted into the brain is his next challenge. Other researchers have used optogenetics to transmit artificial skin sensations into the brains of mice, but Ko plans to investigate other technologies to find a technique that’s best suited for his lab’s e-skin.
“This is an area that has begun to heat up at the research level over the last few years and people are figuring out how to build high-performance electronic sensors … that can intimately laminate onto the skin,” Rogers says. “In the context of skin-mounted sensors, it’s a clever set of ideas and materials [used to] design this device.”