A sensor to be injected into the skin just like a tattoo that measures sodium concentrations in the blood has been developed by US scientists. The sensor could be used to monitor diseases or warn against dangerously low sodium levels during exercise.

Heather Clark from Northeastern University, Boston, and coworkers made plastic nanobeads that fluoresce to different extents with changes in sodium levels. The beads are coated with a biocompatible polymer and are injected just under the skin to allow the fluorescence to be monitored easily.

Technology for determining the amount of sodium in the bloodstream has been available for some time. However, it requires a blood sample from the patient, which limits the measurements to isolated time points. This is a problem for understanding a condition called hyponatremia, where sodium concentrations in blood serum are lower than normal. Hyponatremia can occur after certain types of surgery, with brain trauma or tumours, and has been found in endurance athletes.

The team tested the sensor by injecting it into the skin of mice and analysing the fluorescent images produced. Clark says she hopes that the final product will be a safe and minimally invasive sensor. 'We are currently developing a more biodegradable sensor,' she explains, 'to inject into the epidermis (the top layer of the skin) that falls off after seven days, so it would be like a non-permanent tattoo.' Clark envisions the sensor beads could be placed into the fingertip and monitored in much the same way that blood oxygen concentrations are in hospitals - with a pulse oximeter clipped to the finger.

'The technique has potential, however there is still some way to go,' says Ibtisam Tothill, an expert in analytical biochemistry at Cranfield University, UK, who adds that it must be proven to be quantitative before it can be applied to humans. Clark agrees that this is the next step and the group hope that it will be relatively straightforward to demonstrate the applicability to real situations. In the future, Clark also hopes to develop a nanoclinical analyser capable of measuring more than one analyte.
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