The sensor is so cheap and simple to produce that its developers said it can be hand-drawn with a pencil onto paper treated with sodium chloride.

It also paves the way for wearable, self-powered health monitors that could detect other health concerns like cardiac arrests and pneumonia.

“Our team has been focused on developing devices that can capture vital information for human health,” said Professor Huanyu Cheng at Penn State university, lead author on the study. “The goal is early prediction for disease conditions and health situations, to spot problems before it is too late.”

The hydration sensor is highly sensitive to changes in humidity and provides accurate readings over a wide range of relative humidity levels, from 5.6 to 90 per cent.

Flexible humidity sensors have become increasingly necessary in health care, for uses such as respiratory monitoring and skin humidity detection, but it is still challenging to achieve high sensitivity and easy...