Scientists at Harvard and MIT have developed smart tattoos as a way to potentially replace health trackers and fitness wearables one day. The tattoos wouldn't be a transmitter for a wearable or smartphone app to receive either, but they would be able to show you various health information by changing colour.

The ink in the tattoo reacts to your interstitial fluid, the fluid that surrounds the cells in your body, to show the current state of your blood. A green ink becoming increasingly more intense if you're dehydrated, while another green ink can turn brown to let people with diabetes know when their glucose levels increase.

It doesn't necessarily have to be a permanent tattoo either, as scientists believe they can develop them so they only last for as long as you need them.

And in case you didn't want other people to see you've got one of the smart tattoos - their design could potentially be anything, as tattoo artists will still be required - they could be made invisible and only detectable in a certain type of light.

A smartphone app could be used to analyse the data presented by the tattoo as well, in case you weren't sure exactly what it meant.

It will be some time before we see smart tattoos become a reality, but the idea alone is certainly intriguing, as they could provide far more accurate data than the current crop of health and fitness trackers.