Smart nappy with embedded sensor detects when baby needs a change

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 disposal with simple, low-cost fabrication processes.

Li Yang, a professor at China’s Hebei University of Technology, explained: “You don’t need to have some piece of multi-million-dollar equipment for fabrication. You just need to be able to draw within the lines of a pre-drawn electrode on a treated piece of paper. It can be done simply and quickly.”

The device takes advantage of the way paper naturally reacts to changes in humidity and uses the graphite in the pencil to interact with water molecules and the sodium chloride solution.

As water molecules are absorbed by the paper, the solution becomes ionised and electrons begin to flow to the graphite in the pencil, setting off the sensor, which detects those changes in humidity in the environment and sends a signal to a smartphone, which displays and records the data.

Essentially, drawing on the pre-treated paper within pre-treated lines creates a miniaturised paper circuit board. It can then be connected to a computer with copper wires and conductive silver paste to act as an environmental humidity detector.

For wireless applications, such as smart nappies and mask-based respiration monitoring, the drawing is connected to a tiny lithium battery which powers data transmission to a smartphone via Bluetooth.

For the respiration monitor, the team drew the electrode directly on a solution-treated face mask.

The sensor could differentiate between mouth breathing and nose breathing and was able to classify three breathing states: deep, regular and rapid. The data collected could be used to detect the onset of various disease conditions, such as respiratory arrest and shortness of breath and provide opportunities in the smart internet of things and telemedicine.

Compared with breath, the human skin exhibits a smaller change in humidity, but the researchers were still able to detect changes using their pencil-on-paper humidity sensor, even after test subjects applied lotion or exercised.

The team also integrated four humidity sensors between the absorbent layers of a nappy to create a 'smart diaper', capable of detecting wetness and alerting for a change.

“That application was actually born out of personal experience,” said Cheng, who is the father of two young children. “There’s no easy way to know how wet is wet, and that information could be really valuable for parents. The sensor can provide data in the short term, to alert for diaper changes, but also in the long term, to show patterns that can inform parents about the overall health of their child.”