Feeling healthy? Have a tattoo tell you whether it’s all going well on the inside. That is the idea behind UC San Diego’s Biosensor Tattoo: the tiny temporary tattoo monitors the pH levels and lactate content in a wearer’s sweat to assess the metabolic health of a patient. It’s supposed to help athletes know if they’re overdoing it, and whether their stamina is improving with exercise. But will it come in sailing anchor and “I (heart) mum” versions?