Imagine having a fitness tracker that doubles as a temporary tattoo. Or a monitoring device that doctors can paste straight onto an ailing organ. Or a watch so unobtrusive that it looks like it's built right into your wrist. These are all long-term possibilities for electronic skin (or e-skin) — ultra-thin and ultra-stretchy material that can mimic the flexibility of human skin. New e-skin research is happening all the time, but today, researchers at the University of Tokyo are introducing a new method of turning it into an electronic display.

In a study published in Science Advances, researchers created organic polymer light-emitting diodes (PLEDs) — small sheets of energy-efficient lights — in three colors: red, green, and blue. When hit with electrical pulses, these lights can turn on and off, mimicking pixels on a normal screen. In one photo, for example, you can see blue PLEDs in a University of Tokyo logo that light up and dim at different power levels.

Arrangements of PLEDs can also display more complex information. Below, you can see seven red bars that form letters and numbers, like a calculator screen on your hand.

The light-up film can be crumpled into a ball and still work

Many researchers are working on e-skin and similar flexible electronics. They've made progress toward material that can detect pressure, blend in with its surroundings, measure body temperature, and do much more. And as futuristic as it still sounds, making light-up displays with incredibly thin film isn't a new development. In 2013, for example, researchers made flexible PLEDs thinner than spider silk. One of the major problems with these kinds of light films in the past, though, is the fact that they only lasted for a matter of hours when exposed to normal air. That's what the University of Tokyo study aimed to address.

Making any piece of ultra-stretchy electronics is often a matter of sandwiching materials together to produce something with the right properties, whether that's a red light or a method of sensing pressure. In this case, the researchers added a new protective coating, called a passivation layer, to various kinds of e-skin. The coating kept out oxygen and water vapor well enough to keep a light working for "several days." As you can see below, the protective coating didn't stop the e-skin from being flexible enough to crumple up.

There may be other benefits too. Researchers report that this PLED film also produced less heat and consumed less power than previous efforts. And the coating they used can also work on e-skin that does more than just light up. The experiment tested an e-skin that combined red and green PLEDs with a flexible sensor that measures oxygen levels through (real, human) skin. When laminated to a finger, the sensor kept delivering measurements for four days in ordinary air.

The study's findings don't apply to the whole, broad category of electronic skin. In fact, it's already possible to make super-thin tattoo-like monitoring devices that last for weeks, if you use non-organic conductive material. Organic and inorganic e-skins, says paper co-author Takao Someya, should be considered complementary materials with different strengths and weaknesses. Organic e-skin in particular is useful because it allows manufacturers to make larger displays at lower costs. "Inorganic semiconductor devices exhibit high electronic performance," he says. "However, these are expensive and unsuitable for large-area."

Today's study is only one contribution to the larger quest for consumer-ready e-skin. But until can actually use super-thin electronics that light up our skin, watching someone else try them will never get old.