Analysis of LED screen technology attached to the skin

An elderly woman who lives alone in a small mountain village takes a break from lunch. She has a thin, square rubber patch on the back of her hand. Like a plaster, it stretches and flexes with the movement of her fingers. As she reaches for her teacup, the square patch lights up with a message: "Take your blood pressure medication." She smiles, recalling how she used to try very hard to remember to take her medications, even when her smartphone sent her such reminders. But now, thanks to the business card-sized patch on her hand, she never misses a dose. Indeed, she sees on the screen that her blood pressure is in a healthy range. In the coming years, thin, bright, waterproof screens that stick to the skin without adhesive will begin to appear. In addition to the elderly, they will also appear on the hands and arms of athletes, travelers, hipsters and early adopters. They will discreetly update runners and cyclists on their heart rate and hydration needs, UV exposure, and even display maps of the route ahead. They will be used to pass secret messages between friends and lovers. Fashion trendsetters will no doubt use them to pass information and important data to each other at parties and festivals. The screens could even be used to share emotions with viewers, indicating your interests, anxieties, excitement, and availability. Depending on the context, it could foster friendships, deepen conversations, or create great alone time. For the elderly or infirm, these screens could display electrocardiogram waveforms and collect data from wireless electrodes placed elsewhere on the body. They could also alert those with hearing impairments of incoming calls or knocks at the door. These thin, flexible screens aren't limited to skin; they could just as easily be applied to the curved surfaces of clothing and other objects. Their color, brightness, or patterns could change as you move or react to the world around you. With the advent of thin, flexible circuits that can twist, bend, and stretch, people could attach semiconductor circuits to the skin, wrapping them around the curves of a hand, arm, calf, or torso. The first generation of these flexible wearables were sensors used to measure vital signs in hospitals and elsewhere. Sports drink company Gatorade released a flexible sweat-monitoring patch earlier this year. But to get useful information from these sensors, users still have to pull out their smartphones or consult a nearby computer. Smartwatches are meant to make accessing this information less cumbersome, but many people find them bulky and their small screens difficult to read. Displays have long been a hurdle in the development of wearables, and many researchers have been working to fill this gap, including my group at the Organic Transistor Laboratory in the Faculty of Engineering at the University of Tokyo. Conventional display technology is difficult to make flexible. Although rollable TVs and foldable smartphones are finally available, they are really expensive. And they can only bend or fold in one direction, not twist or stretch. Now, truly bright flexible screens are finally on the horizon, giving us the information we need, all the time, without having to look through our phones. My team has developed and demonstrated several versions of this skin display. Dai Nippon Printing Co., Ltd. is working to bring our skin display technology to market, possibly within the next three years.

Not all types of screens can be stretched. In an LCD screen, for example, light is emitted from a set of electrodes at the back, with a layer of liquid crystal between them. Switching an electric current on and off changes the orientation of the liquid crystals, which in turn changes the polarization of the light, allowing it to pass through a polarizing filter into the viewer's eye, or be blocked by the filter. Stretching the LCD screen changes the thickness of the liquid crystal layer, altering the arrangement of the crystals. Screens based on organic light-emitting diodes (OLEDs) have no such limitations. These OLEDs can indeed be printed onto thin, flexible substrates. Today's rollable screens take advantage of this property. However, OLED screens that can be stretched and bent in multiple directions have not yet been commercialized, although Samsung is reportedly working on such screens. We have made a low-resolution prototype OLED screen in our lab. However, it will still take quite some time for researchers to develop a material that is both flexible and durable, and can also protect the device from oxygen and moisture. So my team focuses on micro-LED displays based on inorganic light-emitting diodes. We are not the only ones taking this approach. For example, Rogers’ group at Northwestern University, a team at Imec and TNO, the Netherlands Institute for Applied Scientific Research, and researchers at VTT, Finland, are also investigating the use of LED arrays for flexible displays. We recently produced our second-generation full-color skin displays using commercial micro-LEDs. In these displays, a picture element (pixel) consists of a 1.5-square-millimeter package; each package contains one red, one green, and one blue LED. Because the devices are made using standard semiconductor manufacturing techniques, the individual LEDs and their packages are rigid. But the LEDs are small, so we mounted them on a rubber sheet and connected them with stretchable wires to create a very flexible display.

These micro-LED packages are arranged in a 12×12 array. When not stretched, the pixel package spacing is 2.5 mm, so the entire display is about 46 square millimeters and only 2 mm thick. We can bend and twist it freely, stretching it up to 130% of its original length, expanding the distance between pixels from 2.5 mm to 3.25 mm. Stretching will distort the image to some extent, but the text is still clear and legible, and the display has been proven to withstand wear and tear.

The fabrication of this stretchable display starts with a very thin plastic substrate. We then use screen printing to define the wiring that connects the pixels into a circuit, which is made from silver paste (a resin containing silver flakes). When dry, this silver paste is elastic and can conduct electricity even as it expands and contracts.

Once the printed circuit is complete, we solder the micro-LED chips to the plastic film circuit board using a standard commercial surface mount machine. The plastic film is then laminated to a silicone rubber substrate that has been pre-stretched on a frame. Once the finished device is removed from the frame, it can be bent after being removed from the frame. It is applied to human skin in this wrinkled, shrunken form. Due to the natural properties of silicon, it conforms to the skin without the need for adhesives. This is our display: printed silver wires connecting the micro-LEDs are glued to the pre-stretched silicon substrate. Now, we put the electronics such as the controller, wireless communication device and battery in a separate hard package, connected to the display by wires. For testing, we put the flexible display on the user's hand and strapped the other electronics to the wrist like a watch. Obviously, before the device can be commercialized, we need to reduce the size of these external components and put them into the flexible package. This will bring some challenges.

The first challenge is how to power a display for a week or more without bulky batteries. Researchers are working to improve power sources for wearable devices. Stretchable solar cells are already available with efficiencies of more than 12%, producing about 10 milliwatts per square centimeter outdoors. But getting enough power from them to drive a display is still a huge challenge, and will certainly involve developing display controllers and wireless communications that use much less power than today. At the same time, we want to drive more pixels. We know that an array of 144 pixels can be used to display text, but this is not the best pixel. At present, we are limited by the size of commercial LEDs. Fortunately, the uses of micro-LEDs are not limited to skin displays, and manufacturers are working to make their size smaller every year. Skin displays will undoubtedly benefit from this development.

We also need to improve durability. Currently, our displays can withstand 10,000 stretches in mechanical testing. But you can imagine that for many applications, people will wear our displays for most of the day, day after day. So we need to do better, like 1 million stretches. How did we come up with that number? There are 525,600 minutes in a year, so think about how often a person's hand stretches or bends, and you can get an idea of ​​how durable the skin display must be. However, there is a trade-off between durability and how well the skin tolerates the display. If we use harder, more durable materials, the display will be less comfortable to wear. We need to do more research to find the sweet spot between durability and comfort. Of course, addressing the broader issues common to many wearable devices, such as ethics, privacy, and medical device-specific regulations, will also be important for the future of skin displays, especially those that display biometric information. Based on the progress we have made so far, we believe that our work will not stop because of any one difficulty. Instead, we expect to solve many of these challenges in the near future.

Skin displays wouldn’t be of much use if they didn’t convey interesting data to the wearer. To collect that data, we turned to sensors that fit snugly against the skin, detecting signals from the heart, brain, skin, muscles, and other organs. The key part of these sensors is the electrodes. To make the flexible electrodes, we started with a mesh of nanofibers made from a water-soluble polyvinyl alcohol, a substance commonly used in adhesives and contact lenses. We then added a 70- to 100-nanometer-thick conductive layer of gold to this mesh using vapor deposition. To attach the electrode to a person’s skin, we’d lay it down and then spray the sensor with some water. The water dissolves some of the nanofibers, making them sticky. This way the electrode sticks easily to the skin, conforming to small curved surfaces like sweat gland pores or fingerprint ridges. It works even when it’s stretched to 130 percent of its length, which is about how much skin stretches when a knuckle is bent.