Why is this blue guy smiling? Maybe because he’s helping track the health of the person he’s attached to while also making medical research history. Developed by University of Toronto PhD candidate Vinci Hung, the small temporary tattoo is actually a solid-contact ion-selective electrode (ISE) which monitors wearers’ pH changes. The point of the tattoo is to monitor metabolic changes in a non-invasive way through the wearer’s sweat, detecting metabolic disorders or signaling when an athlete is dehydrated during training.

Hung created the tattoo using standard screen printing techniques and regular temporary tattoo transfer paper. It’s totally safe and creates no lasting after-effects for the user. The design itself holds some secrets: the smiley face’s eyes are the sensor’s working and reference electrodes, while the ears are the contacts that a measuring device connects to. The ISE technology is already in use for several applications, but this is the first time it has been embedded into a skin-mounted object that is not dislodged by movement or excessive sweating.