The researchers, led by John Rogers, made two types of device: the first consists of arrays of sensors that monitor temperature thanks to the change in resistance in slivers of gold just 20 nm thick and 20 microns long. This device is fabricated using standard microlithographic techniques. The second device is made of multiplexed arrays of sensors based on diodes formed by patterned doping of silicon nanomembranes and it measures changes in temperature through changes in the turn-on voltage of the diodes. In both cases, the devices are deposited on thin, low modulus elastic sheets made of the plastic polyimide.

"The integrated sensors have physical properties – stiffness, thickness and mass density – that are very similar to those of human skin itself," explains Rogers. "As a result, when placed on the skin (using water-soluble backing), the patient is hardly aware that he is wearing anything. The skin on which the patch is placed can also be pinched or twisted without any harm coming to the thermometers."

"The devices consist of arrays of sensor nodes, each of which can measure local temperatures to a precision that approaches the millikelvin level," he told nanotechweb.org. "Such precision could come in useful for monitoring how heat flows through the bloodstream and tracking how temperature changes around dilating and contracting blood vessels – something that can provide a wealth of information about cardiovascular health."

And that is not all: running electrical current through the sensors induces Joule heating in the devices, and this heat can then be applied to skin. By monitoring temperature at the same time, the researchers are able to measure the thermal conductivity of the skin, which in turn reveals how hydrated it is. The applied heat might also be used to help wounds heal in certain cases, says Rogers.

The sensors are impermeable to moisture thanks to the fact that they are sandwiched between micron-thick layers of polyimide, which also serve to electrically insulate the devices. This means that they can be used even if the patient is sweating a lot, as may be the case if he has a fever. They are as good (in terms of sensitivity and mapping skin temperature) as the best thermal digital cameras routinely employed in hospitals today, says Rogers, but they are much cheaper and far less bulky of course. The patient can also carry on with whatever he is doing and doesn’t have to be immobilized during the temperature measurements.

At present, the devices only work using an external power supply but if such a source could be integrated directly into the sensors – via stretchable batteries, supercapacitors or other energy-storage elements – the sensors might be ideal in a variety of applications, in both hospital and home settings, says Rogers.

The team says that it is now busy testing out its devices on patients at a hospital associated with the Northwestern Medical School in Chicago. "We are also working on making the devices completely wireless," adds Rogers.

The present research is detailed in Nature Materials doi:10.1038/nmat3755.

Further reading

Semiconductor nanocrystals make ideal optical thermometers (Aug 2010)
Nanodiamond thermometer takes temperature of biological cells (Aug 2013)
Colour changing nanofluid senses ammonia (May 2013)
Hairy solution to making sensitive artificial skin (Aug 2012)