Using a nanosensor "tattoo" and a modified iPhone, cyclists could closely monitor sodium levels to prevent dehydration, and anemic patients could track their blood oxygen levels.

Heather Clark, a professor in the Department of Pharmaceutical Sciences at Northeastern University, is leading a team working to make this possible. The team begins by injecting a solution containing carefully chosen nanoparticles into the skin. This leaves no visible mark, but the nanoparticles will fluoresce when exposed to a target molecule, such as sodium or glucose. A modified iPhone then tracks changes in the level of fluorescence, which indicates the amount of sodium or glucose present. Clark presented this work at the BioMethods Boston conference at Harvard Medical School last week.

The tattoos were originally designed as a way around the finger-prick bloodletting that is the standard technique for measuring glucose levels in those with diabetes. But Clark says they could be used to track many things besides glucose and sodium, offering a simpler, less painful, and more accurate way for many people to track many important biomarkers.

"I don't think there's any doubt that this sort of technology will catch on," says Jim Burns, head of drug and biomedical research and development at Genzyme.

The tattoo developed by Clark's team contains 120-nanometer-wide polymer nanodroplets consisting of a fluorescent dye, specialized sensor molecules designed to bind to specific chemicals, and a charge-neutralizing molecule.