Paper Skin Mimics the Real Thing

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Photo: Aftab Hussain

Human skin’s natural ability to feel sensations such as touch and temperature difference is not easily replicated with artificial materials in the research lab. That challenge did not stop a Saudi Arabian research team from using cheap household items to make a “paper skin” that mimics many sensory functions of human skin.

The artificial skin may represent the first single sensing platform capable of simultaneously measuring pressure, touch, proximity, temperature, humidity, flow, and pH levels. Previously, researchers have tried using exotic materials such as carbon nanotubes or silver nanoparticles to create sensors capable of measuring just a few of those things. By comparison, the team at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia used common off-the-shelf materials such as paper sticky notes, sponges, napkins and aluminum foil. Total material cost for a paper skin patch 6.5 centimeters on each side came to just $1.67.

”Its impact is beyond low cost: simplicity,” says Muhammad Mustafa Hussain, an electrical engineer at KAUST. “My vision is to make electronics simple to understand and easy to assemble so that ordinary people can participate in innovation.”

The paper skin’s low cost and wide array of capabilities could have a huge impact on many technologies. Flexible and wearable electronics for monitoring human health and fitness could become both cheaper and more widely available. New human-computer interfacessimilar to today’s motion-sensing or touchpad devices—could emerge based on the paper skin’s ability to sense pressure, touch, heat, and motion. The paper skin could also become a cheap sensor for monitoring food quality or outdoor environments.

Last but not least, cheap artificial skin could give robots the capability to feel their environment in the same way that humans do, Hussain says. In a paper detailing the research—published in the 19 February issue of the journal Advanced Materials Technologiesthe researchers said:

The envisioned applications of such artificial skin takes a lot of surface area coverage (like robotic skins or skins for robots). There, lowering cost is crucial while not compromising performance. In that sense, if mechanical ruggedness can be proved, there is no scientific or technical reason for not accepting paper skin as a viable option.

The team’s low-cost approach often seems as approachable as a classroom experiment. As an example, researchers built a pressure sensor by sandwiching a napkin or sponge between two metal contacts made from aluminum foil. The same simple device could also detect touch and flow based on changes in pressure. Its aluminum foil even allowed it to act as a proximity sensor for electromagnetic fields with a detection range of 13 centimeters.

In another case, the researchers created a pH sensor. The simple solution came from drawing a film using a pencil. The pencil lead, made of graphite, undergoes chemical reactions in the presence of either acidic or basic (alkaline) substances. The reactions increase or decrease the film’s resistance.

The paper Post-it Notes provided the main material of the paper skin device and helped detect humidity based on the papers’ ability to absorb moisture. Double-sided adhesive tape helped keep everything together.

Lab testing with the paper skin showed that its results seemed equally as good as other artificial skin devices made of more exotic materials, says Joanna Nassar, an electrical engineer at KAUST and lead author on the study. In some cases, it outperformed many rival artificial skin devices.

“Compared with the sophisticated and complex artificial skin platforms found in the literature, Paper Skin not only provides the most functionalities on one platform, including 13-cm range proximity sensing, but also displays improved sensing performances over the highly expensive counterpart materials,” Nassar says.

One comparison suggests that the paper skin provides twice the temperature sensitivity of an earlier artificial skin platform based on carbon nanotubes. It also resets and becomes ready for a new temperature measurement four times as fast as the nanotube-based system.

Another comparison found that the paper skin had pressure-sensing capabilities twice as good as a pressure sensor based on carbon nanotubes. It was also five times as pressure-sensitive as a nanowire pressure sensor, and 40 times as sensitive as one made from silicon nanoribbons.

A low-cost approach to artificial skin that produces results comparable to more expensive approaches is likely to produce some skepticism among other researchers. It was not easy to get the research published in the first place, Hussain says. But he and his colleagues hope their unorthodox approach can spur faster development of artificial skin devices.

The KAUST team’s next step will involve testing the paper skin’s possible uses in medical monitoring systems. Researchers hope to test its capability to monitor real-time vital signs such as heart rate, blood pressure, breathing patterns and movement. Reliability tests will also see how long the cheap materials can hold together and whether the sensor’s performance is affected by mechanical stress like twisting and bending.

“Why should we be waiting for decades for an exotic material to be qualified to serve as the key for an application which we can benefit from today?” Hussain says. “Start with what we have and improve the status quo with the new material(s) when ready.”

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