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.

Once in the skin, the sensor molecules attract their target because they have the opposite charge. Once the target chemical is taken up, the sensor is forced to release ions in order to maintain an overall neutral charge, and this changes the fluorescence of the tattoo when it is hit by light. The more target molecules there are in the patient’s body, the more the molecules will bind to the sensors, and the more the fluorescence changes.

The original reader was a large boxlike device. One of Clark’s graduate students, Matt Dubach, improved upon that by making a modified iPhone case that allows any iPhone to read the tattoos.

Here’s how it works: a case that slips over the iPhone contains a nine-volt battery, a filter that fits over the iPhone’s camera, and an array of three LEDs that produce light in the visible part of the spectrum. This light causes the tattoos to fluoresce. A light-filtering lens is then placed over the iPhone’s camera. This filters out the light released by the LEDs, but not the light emitted by the tattoo. The device is pressed to the skin to prevent outside light from interfering.

Dubach and Clark hope to create an iPhone app that would easily measure and record sodium levels. At the moment, the iPhone simply takes images of the fluorescence, which the researchers then export to a computer for analysis. They also hope to get the reader to draw power from the iPhone itself, rather than from a battery.

Clark is working to expand her technology from glucose and sodium to include a wide range of potential targets. “Let’s say you have medication with a very narrow therapeutic range,” she says. Today, “you have to try it [a dosage] and see what happens.” She says her nanosensors, in contrast, could let people monitor the level of a given drug in their blood in real time, allowing for much more accurate dosing.

The researchers hope to soon be able to measure dissolved gases, such as nitrogen and oxygen, in the blood as a way of checking respiration and lung function. The more things they can track, the more applications will emerge, says Clark.