There's a common problem with health-tracking devices like smartwatches: they're not really attached to you, which leaves you relying on a short-lived battery and a wireless connection. Even a self-powered patch has its limits. That's where Harvard and MIT think they can help: they've developed smart tattoos that effectively place health sensors in your skin, no power or wireless link required. The ink in the tattoos reacts to the chemical composition of your interstitial fluid, which reflects the state of your blood. A green ink grows more intense to let athletes know when they're dehydrated, while another green ink turns brown to warn diabetics when their glucose levels go up.