It's a health monitor only five centimeters square in size. It's almost invisible on the skin.

It's the first device of its kind that opens the door to electronic devices attached to a human body that can monitor a person 24/7. While currently a dedicated medical monitor, its implications for non-medical surveillance are huge.

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This new wearable medical device can be placed directly on the skin and worn 24/7 for health monitoring. It was developed by researchers from a Northwestern University and University of Illinois at Urbana-Champaign.

The wireless technology senses body heat through thousands of tiny liquid crystals embedded on a flexible substrate. When the device changes color, the wearer knows something is wrong with him.

Since it has 3,600 liquid crystals, the photonic device has 3,600 temperature points. Researchers said it provides "sub-millimeter spatial resolution that is comparable to the infrared technology currently used in hospitals."

The device has a wireless heating system that can be powered by electromagnetic waves present in the air. The heating system is used to determine the thermal properties of the skin.

The new device offers low cost and portability compared to existing infrared technologies that are expensive and limited to clinical and laboratory settings.

Researchers said the device is very practical. When a person's skin is stretched, compressed or twisted, the device stretches, compresses or twists along with it. The research team tested the device on people's wrists.

"Our device is mechanically invisible -- it is ultrathin and comfortable -- much like skin itself," said  Yonggang Huang, one of the senior researchers.

"This is the first device of its kind."

More testing is needed before the device can be put to use, however. Details of the device are reported online in the journal, Nature Communications.