Skin Displays Will Give Wearables Their Independence

An elderly woman, living alone in a small mountain village, relaxes in the afternoon after finishing lunch. A square sheet of thin rubber clings to the back of her hand. Like a poultice, it stretches and then wrinkles as she flexes her fingers.

As she reaches for her teacup, the square lights up with a message: “TAKE YOUR BLOOD PRESSURE MEDICINE." She smiles, remembering how she used to struggle and often fail to remember, even though her smartphone had been programmed to send her such alerts. But now, thanks to that business-card-size patch on her hand, she hasn't missed a single dose. Indeed, her blood-pressure value, which she can now see on the display, is well within a healthy range.

Stretchy, thin, bright, water-resistant displays that stick to skin without adhesives are going to start appearing in coming years. They'll be on the hands and arms of not just the elderly but also on those of athletes, travelers, hipsters, and early adopters. They're going to unobtrusively update runners and cyclists on heart rate and hydration needs, ultraviolet exposure, and even show maps of the route ahead. They'll be used to send secret messages between friends and lovers. The fashion-forward will undoubtedly flash messages and vital stats at each other at parties and festivals. Such a display might even share emotional cues with observers, suggesting that you are interested, anxious, available, or excited. Depending on the setting, it might foster friendship, deeper communication, or splendid isolation.

For the elderly or infirm, these displays could show electrocardiogram waveforms, collecting the data from wireless electrodes placed elsewhere on the body. They could also alert someone who is hard of hearing to an incoming phone call or a knock on the door.

While the first applications of wearable displays will likely be to communicate health and wellness information, the possibilities are endless.Yoshiaki Tsutsui/University of Tokyo

The locations where we place these thin, flexible, stretchable displays won't be limited to skin; the displays will be equally easy to apply to the curved surfaces of clothes and other objects. Their color, brightness, or patterns could change in response to your activity or in reaction to the world around you.

Thanks to the emergence of thin and flexible circuitry capable of twisting, bending, and stretching, people are already affixing semiconductor circuits to their skin, wrapping them around the curved surfaces of a hand, an arm, a calf, or a torso. The first generation of these stretchy wearables are sensors that are measuring vital signs in hospitals and elsewhere. And Gatorade, the sports-drink company, released a flexible sweat-monitoring patch earlier this year.

But to get useful information from these sensors, users still have to take out their smartphones or consult a nearby computer. Smartwatches aim to make getting this information less cumbersome, but some people find them clunky, and the tiny displays can be hard to read.

Displays have long been a pothole on the road to wearable devices, one that many researchers have been trying to fill, including my group in the University of Tokyo's Organic Transistor Lab at the school of engineering. Conventional display technology is hard to make flexible. While televisions that roll up and smartphones that fold are finally available, they're really expensive. And they can roll or fold in only one direction; they can't twist or stretch.

Now, truly flexible, bright displays are finally close to fruition, poised to feed us the information we need, no phone required. My group has developed and demonstrated several versions of such a skin display. And Dai Nippon Printing Co. is working to bring our skin-display technology to market, likely getting there within the next three years.

Not all types of displays can be made stretchable. In a liquid-crystal display, for example, light shines from behind a set of electrodes with a layer of liquid crystals between them. Turning electric current on and off changes the orientation of the liquid crystals and therefore the light's polarization, allowing it to either pass through a polarizing filter to the viewer or be blocked by the filter. Stretching the LCD changes the thickness of the liquid-crystal layer, altering the alignment of the crystals.

Displays based on organic LEDs (OLEDs) have no such limitation. These OLED displays are indeed printable onto thin, flexible substrates. Today's rollable displays take advantage of that capability. To date, however, no one has yet commercialized an OLED display that can stretch and bend in multiple directions, although Samsung has reportedly been working on one. In our laboratory, we did produce a prototype low-resolution OLED display. Still, it will take researchers a considerable amount of time to develop a stretchy, long-lasting material that can also be used to protect devices against oxygen and water vapor.

So my group has been working mainly with inorganic LEDs in the form of micro-LED displays. We are not the only ones taking this approach. For example, the Rogers Research Group at Northwestern University, a team from Imec and TNO in the Netherlands, and researchers at the VTT Technical Research Centre of Finland are also looking at using arrays of LEDs as part of stretchable displays.

Mounting conventional micro-LEDs on a rubber sheet and connecting them with stretchable wiring creates a display that can bend, twist, and stretch to as much as 130 percent of its original length.Dai Nippon Printing Co.

We recently produced our second-generation full-color skin display using commercially available micro-LEDs. In these displays, a 1.5-square-millimeter package makes up a picture element, or pixel; each contains one red, one green, and one blue LED. Because these devices are constructed using standard semiconductor manufacturing techniques, the individual LEDs and the packaging that surrounds them are hard. But the LEDs are tiny, and we mount them on a rubber sheet and connect them with stretchable wiring, creating a very flexible display.

These micro-LED packages are arranged in a 12-by-12 array. Unstretched, the pixel packages are spaced 2.5 mm apart, so the entire display is about 46 mm square (about 1.8 inches square), and it's just 2 mm thick. We can bend and twist it freely and stretch it to as much as 130 percent of its original length, expanding the distance between the pixels from 2.5 to 3.25 mm. Stretching distorts the picture somewhat, but text is still legible, and the display has proved resistant to wear and tear from stretching.

To make this stretchable display, we start with a very thin plastic substrate. We then use screen printing to define the wiring that connects the pixels into a circuit. For this wiring, we use silver paste—a resin containing silver flakes. When dry, this silver paste is elastic, conducting electricity even as it expands and contracts.

After printing our circuitry, we solder the micro-LED chips to it, using a standard surface mounter used commercially to attach chips to circuit boards. We then laminate the plastic film onto a silicone-rubber substrate that has been prestretched on a frame. When we remove the completed device from the frame—now a sheet with multiple layers that include the LED packages, the silver wires, the thin plastic film, and the silicone substrate—it buckles. It's in this crinkled, contracted form that it is applied to someone's skin. It adheres without adhesive, thanks to natural characteristics of the silicone material.

University of Tokyo professor Takao Someya hopes skin displays will allow family members to quietly communicate their feelings to one another.Yoshiaki Tsutsui/University of Tokyo

So that's our display: micro-LEDs connected by printed silver wiring attached to a prestretched silicone substrate. Right now, we put the electronics—the controller, the wireless radio, and the batteries—in a separate hard package that is connected by wires to the display. For testing, we put the flexible display on the user's hand and strap the other electronics to the wrist, like a watch. Obviously, we will have to reduce the size of these external components and bring them into our flexible package before our device can be commercialized. This will present some challenges.

Challenge No. 1 is how to power these displays for a week or more at a time without bulky batteries. Researchers are working hard to improve power sources for wearables. Stretchable solar cells exist and are already reaching efficiencies of more than 12 percent, generating about 10 milliwatts per square centimeter outdoors. Still, getting sufficient power out of them to power a display presents a big challenge, one that will certainly involve developing display controllers and wireless radios that use much less power than those available today.

At the same time, we'll want to drive more pixels. We know that an array of 144 pixels, while usable for displaying text, isn't optimal. But for now, we are limited by the size of commercially available LEDs. Fortunately, micro-LEDs have uses beyond skin displays, and their manufacturers are pushing hard to make them smaller every year. Skin displays will undoubtedly benefit from that progress.

We also need to improve durability. Right now, our displays can withstand 10,000 stretch cycles in mechanical tests. But we imagine that for many applications, people will wear our displays much of the day, and day after day. So we need to do much better—say, a million stretch cycles. How did we get that number? Consider that a year has 525,600 minutes, and then consider how often someone extends or flexes his hand, and you see how durable a skin display must be.

However, there is a trade-off between durability and the degree of burden the display places on the skin. When we use a harder, more durable material, the display becomes less comfortable to wear. We need to do more research to pinpoint the sweet spot between durability and comfort.

And, of course, resolving broad issues common to many wearables—including ethics, privacy, and regulations specific to medical instruments—will also be very important to the future of skin displays, particularly those that display biometric information.

Based on our work so far, we don't expect any of these difficulties to be a showstopper. On the contrary, we expect to solve many of these challenges in the very near future.

A skin display isn't much use unless it has interesting data to communicate to its wearer. To gather this data, we turn to skin-conforming sensors, capable of detecting signals from the heart, brain, skin, muscles, and other organs.

The key part of these sensors is the electrode. To make flexible electrodes, we start with a mesh of nanofibers made of a water-soluble polyvinyl alcohol, a substance commonly used in adhesives and contact lenses. We then use vapor deposition to add a conductive layer of gold, 70- to 100-nm thick, to this mesh. To attach this electrode to a person's skin, we position it and then spray the sensor with water. The water dissolves some of the nanofibers, making them sticky. The electrode then easily adheres to the skin, conforming to curvilinear surfaces as small as sweat pores or the ridges of a fingerprint pattern. It operates even when stretched to 130 percent of its length—about the stretch of the skin on a knuckle when it bends.

Because the nanomesh allows water vapor to pass, these sensors are hypoallergenic, and can be worn on the skin continuously for a week without discomfort. Users usually forget they even have them on. Wearable electrodes are not completely new; wearable EKG monitors have for some time been marketed to athletes. But these are bulky devices and they're anything but breathable, making them impractical for long-term use.

So far, we've used our stretchy electrode to monitor muscle activity for electromyogram recording, and it has performed as well as conventional electrodes do. We can similarly create sensors that monitor the activity of the heart or brain from a position on the chest or head.

When we attach a skin-conforming sensor to a display, we can create a continuous, easily accessible flow of biometric information. But the applications of skin displays go well beyond health and wellness. Skin displays improve accessibility to information, making them particularly useful when you've got both hands busy and are on the move. For example, you could read a recipe while cooking without worrying about drying your hands to consult a smartphone. Or you could consult an instruction manual—at home or in a factory—without putting down a tool you're using.

And the applications in sports seem limitless. I already mentioned runners and cyclists, and the utility there is obvious. Other outdoors enthusiasts will also find uses. Consider skydivers, white-water kayakers, and skiers, who could check the output of a helmet-mounted action camera by stealing glances at their arms or hands. A fisherman could consult a fish finder without putting down his pole or risking his smartphone in a chest-deep stream.

And because skin displays will flex when they are struck instead of breaking, they will be useful even in contact sports. Tragically, roughly a dozen young American football players perish every year from heatstroke or heart problems. Either condition could be detected in many cases before it became fatal.

Finally, I believe skin displays can get us to a future of technologies that are gentle, kind, and spread warmth, not just information.

Let's go back to that elderly woman I described earlier. She's now resting in her living room after dinner. Because she lives alone, there is no one to talk to. Only the sound coming from the TV breaks the silence. Then she notices something flashing unexpectedly on the display attached to the back of her hand. It's the image of a heart, and it's from a grandson who lives in a city far away. It's just a little red heart, but she feels as though she's actually hearing her grandson's voice saying, “I love you, Grandma!" She puts her other hand over the display, holding that heart for a moment. This is what I wish for the future of electronics—devices that not only transmit data but also feelings.

This article appears in the June 2021 print issue as “Put Down That Smartphone: The Display Is on Your Skin."