We've gotten one step closer to a world where tattoos are made from LEDs and glowing watch faces embedded in your wrist can broadcast the time. It's the next phase in the development of e-skin, and circuits and sensors contained in flexible plastic sheets are thinner than human skin. Just stick the sheets on your body, stand in sunlight to power them up, and glow all night long.

E-skin, or flexible, stretchable circuits that can be stuck to skin, has been around for several years. But now a group of engineers have made a leap forward by integrating polymer LEDs into it. They explain their creation in Science Advances, showing how the new e-skin can display red, green, and blue light, which can be used for displaying biosigns like blood oxygen content and heart rate. Their e-skin also measures heart rate the same way many smart watches and fitness trackers do, by measuring the absorption of green and red light into the blood in a process called photoplethysmography.

The skin itself is only 3 micrometers thick, which the engineers proudly declare "one order of magnitude thinner than the epidermal layer of human skin." It's also stretchier than previous e-skin formulas, making it ideal for wearing during exercise. Though some kinds of e-skin are powered by induction, this new type is solar powered. Walking around in the sun should be enough to keep it going.

Of course, the lights aren't just for measuring vital signs. They can also be used for decoration. Clubbers of the future might program the lights to change color or patterns as their heart rate changes. Or the lights could simply change algorithmically, to produce different patterns and color combinations over time.

Science Advances, 2016. DOI: 10.1126/sciadv.1501856