[Image courtesy of John A. Rogers
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Of all our human organs, skin is arguably one of the most abused — yet it’s also arguably the most reliable. It protects everything inside us, helping us avoid harm by sensing obstacles in our way, making sure we stay hydrated, and ensuring we keep ourselves at the right temperature. It constantly replenishes itself, sloughing off former layers that we’ve either burned or dried out or scraped or ignored, while new ones grow in their places.

Click here for a photo gallery of future skin technology for humans and machines.

Many of skin’s properties would be useful in other applications — like helping people with artificial limbs regain some of what they’ve lost. And an electronic skin, or at least some tactile sensory ability, could help machines understand the delicate differences in force that are required to grip an apple, a hand or a piece of steel.

Researchers trying to duplicate its beneficial properties are building teeny stretchable electronics that can give artificial limbs a real sense of touch.

And scientists are making several changes to human skin itself, turning it into a 21st century interface capable of much more than feeling another person’s caress. From conductive tattoos that turn skin into a human-machine communications device, skin is getting plenty of upgrades.

Click through to the gallery for a look at some recent breakthroughs in skin technology.

Conductive Tattoos Made From Nano-Ink

Someday, patients with chronic diseases may not need to wear electronic monitors to keep track of glucose levels and heart rates. Their skin will do it for them — or rather, their tattoos will. Tattoos inked with conductive or fluorescent materials, perhaps nanoparticle fluids, could be a much less cumbersome medical monitoring method. Heather Clark, a professor of pharmaceutical sciences at Northeastern University, is leading efforts to create embeddable technology that can keep people constantly connected. This summer, she debuted an invisible nanoparticle tattoo that can track glucose and sodium levels in the blood by fluorescing in certain light. It comprises 120-nm-wide nanodroplets that contain a fluorescent dye and sensor molecules that bind to certain chemicals, as explained by Technology Review. An iPhone outfitted with a special filter is used as a light source; a greater number of molecules yields a stronger fluorescence. Currently, researchers take a photo of the fluorescence with the iPhone camera and analyze it with a computer to determine biomarker levels. Clark hopes to eventually build an iPhone app that can read the output itself. MIT engineers have also built nanoparticle tattoos, designed to help diabetics continuously monitor glucose levels. The particles also fluoresce in the presence of glucose and infrared light, according to MIT. A user would wear some type of electronic device to provide the IR beam and interpret the fluorescence. While a tattoo isn’t exactly painless, both systems would conceivably be less invasive than constant finger-pricking or blood draws.

Giving Robots a Sense of Touch

Today’s artificial limbs have improved greatly in recent years, but even the best replacements can’t substitute tactile sensation— what it feels like to touch something, to nearly touch and to be touched. New electronic skins could be a step toward touch-sensitive artificial limbs, and machinery like robots. In one example, German researchers are building skin that doesn’t directly feel, but infers the presence of an object through close proximity. Researchers at Technical University Munich built hexagonal circuit boards, about three-quarters of an inch square, each equipped with four infrared sensors and six temperature sensors. The sensors can detect an object from a distance of about 1 centimeter, which equates roughly to the distance at which the fine hairs on human skin can sense an object, the researchers said. “We thus simulate light touch,” Philip Mittendorfer, a TUM scientist who developed the skin-circuits, said in a statement earlier this summer. While this approach relies on rigid materials, other research groups in Europe and the US are working on tactile skin made of stretchable surfaces that can bend just like skin. Stéphanie Lacour at the University of Cambridge is building circuits on a substrate of clear elastic silicone, which can stretch and transform without breaking. The elastic materials can wrap around limbs or even fingers, an ideal option for future implants or even touchscreen interfaces (see the end of the gallery for more on this concept). And in the US, researchers at various institutions in California have been building stretchable electronic skin that could bring touch sensitivity to machines. One skin, built by Stanford University scientists, is 1,000 times more sensitive than human skin, according to research published earlier this year. It’s based on organic transistors, made of elastic material sandwiched between two electrodes. Another prototype developed at the University of California-Berkeley works in a similar manner, but is made of inorganic nanowire circuits.

Peelable Epidermal Electronics

Permanent tattoos not an option? Temporary ones could be a simple substitute. Last month, researchers in Illinois announced new peelable electronic circuits that can be rubbed onto the skin just like a temporary tattoo. The small circuits can bend and stretch with the skin, resisting breakage just like a well-moisturized epidermis. This research yielded several breakthroughs, from the adhering method — rubs on with water, just like a temporary tattoo — to the fabrication process, which enables bendy versions of semiconductor materials that are brittle when in bulk form. The semiconductor circuits could be used to monitor muscle activity, heart activity or even brain waves, researchers said.

Making New Human Skin

Burn victims and other patients in need of a second skin must either receive transplants or serve as their own tissue donors, in the painful and dangerous process of skin grafting. But sometimes that’s not sufficient; it would be better to have a viable, easily transplantable synthetic one instead. German researchers have apparently figured out how to make it in (relatively) large quantities. At the Fraunhofer Institute for Interfacial Engineering and Biotechnology, a new skin factory can produce 5,000 penny-sized discs of whitish translucent tissue every month. The designers say it can also come in shades of brown. Robots control the skin-growing process, monitoring the cell broth and carefully slicing swatches to prevent infection. It’s not being used to create skin grafts just yet, but the researchers involved say it will be a simple, streamlined way to make new tissue like skin, bladders and tracheas. For now, the skin is being used for cosmetics testing.

Growing Skin With Help From Other Creatures

If human tissue grown in petri dishes seems unappealing, those in need of a second skin could instead turn to other members of the animal kingdom. This summer, scientists in Germany proposed using spider silk as a biocompatible, biodegradable adhesive matrix for skin repair. Using dragline silk from golden orb web spiders, researchers wove matrices on steel frames and seeded them with fibroblasts, which provide the structural background for connective tissue. They added skin cell progenitors and were able to cultivate synthetic skin, consisting of the dermis and epidermis, according to a paper published in the journal PLoS One. The image at left shows living skin cells in green, with dead cells in red. Other animal products could conceivably be used as a tissue-growing matrix, but spider silk is a fairly simple and biodegradable starting point.

Skin As An Electronic Interface

Giving a new meaning to “wearing your heart on your sleeve,” scientists are developing new interfaces that turn skin into a display screen. They’re in the same vein as the nanoparticle tattoos, but devices like Skinput and Dattoo take body sensors to another level. Skinput, a project created at Carnegie Mellon University and Microsoft Research, turns your arm (or leg, or whatever) into a touchscreen. Using a small armband, a user would project an information display onto the skin, and would press the skin like you would an iPhone or any other touchscreen. The armband contains an acoustic sensor that uses tissue density and other biometric data to determine where you’ve tapped, and therefore what command you’ve executed. An earlier concept project from Frog Design, the Dattoo, is somewhat more involved — instead of a projected overlay, users would print an actual customizable electronics setup on his or her arm, consisting of tools like a loudspeaker, microphone or camera. The device would also include a user’s DNA, connecting each person to his or her device in the most intimate way possible. The Dattoo was conceived as a temporary tattoo, and could also be applied to clothing or other surfaces. But at this point, it’s still just an idea.

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