We may soon be able to detect hydration levels, respiratory changes or a too-wet diaper, all with a new type of sensor — a pencil drawing.

Researchers at Pennsylvania State University in the U.S. have developed a way to monitor humidity and respiration by drawing an image which functions like a circuit board onto pre-treated paper.

This drawing can send signals to your smartphone, and has already been used by researchers to create a “smart diaper” capable of alerting parents about their baby’s discomfort.

Researchers believe the sensor could potentially be used in the future to monitor not only for a full diaper, but also for health concerns such as cardiac arrest and pneumonia, or even for switches that don’t require contact.

“Our team has been focused on developing devices that can capture vital information for human health,” Huanyu “Larry” Cheng, the James L. Henderson, Jr. Memorial Associate Professor of Engineering Science and Mechanics at Penn State and lead author of a new study about the research, said in a press release. “The goal is early prediction for disease conditions and health situations, to spot problems before it is too late.”

A study published December in the peer-reviewed journal Nano Letters describes how the sensor works and some of the early applications tested by researchers.


Researchers explained that the sensor is created by using a pencil to draw on paper treated with a sodium chloride solution.

As water molecules in the environment around the drawing absorb into the paper and mix with the sodium chloride solution, the solution becomes ionized, creating electrons that flow through the graphite of the pencil markings.

Printed circuit boards, which are used in computers and other electronic devices, consist of copper lines laid out in specific angular patterns. The copper is used as a conduit for electrical information to travel through, and it is this process that researchers are replicating — except this circuit board is laid out in pencil instead of copper.

The paper sensor can be connected to a computer with copper wires and conductive silver paste, or can be hooked up to a tiny lithium battery and used to wirelessly communicate with a smartphone.

The sensor can send a signal to a smartphone that would display humidity data on that smartphone’s screen. It is able to provide accurate readings for humidity levels from 5.6 per cent to 90 per cent, researchers say.

The idea of wearable sensors isn’t new — current flexible humidity sensors are a big part of modern health care, used in areas such as respiratory monitoring and skin humidity detection. However, it’s hard to achieve high sensitivity at a lower price point, making these sensors less accessible.

“We wanted to develop something low-cost that people would understand how to make and use — and you can’t get more accessible than pencil and paper,” Li Yang, professor in the School of Artificial Intelligence at China’s Hebei University of Technology, said in a press release. “You don’t need to have some piece of multimillion-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.”


By layering four humidity sensors between the absorbent layers of a diaper, researchers created a prototype of a “smart diaper,” which could detect wetness and signal the need for a change.

Cheng said the idea of this use for the sensor came out of his own personal experience, as he has two young children.

“There’s no easy way to know how wet is wet, and that information could be really valuable for parents,” he said. “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.”

Researchers also tested out a version that worked as a respiration monitor, by drawing the sensor directly onto a solution-treated face mask.

They were able to differentiate between mouth- and nose-breathing through the sensor, and the sensor could detect three different types of breathing: deep, regular and rapid breathing.

This type of data could be used to help alert medical staff if a patient was experiencing laboured breathing suddenly, or could be used to detect the onset of a variety of respiratory conditions.

When the sensor was applied to the task of skin hydration, the sensor could detect the underlying humidity of human skin even if patients had recently exercised or applied lotion.

“Different types of disease conditions result in different rates of water loss on our skin,” Cheng said. “The skin will function differently based on those underlying conditions, which we will be able to flag and possibly characterize using the sensor.”

Researchers say the potential applications for this type of sensor are huge.

It can even be used as a non-contact switch, with the sensor capable of detecting the presence of a finger hovering above it simply by detecting the change in humidity in the air.

“The atoms on the finger don’t need to touch the button, they only need to be near the surface to diffuse the water molecules and trigger the signal,” Cheng said. “When we think about what we learned from the pandemic about the need to limit the body’s contact with shared surfaces, a sensor like this could be an important tool to stop potential contamination.”

To test this application, researchers used the sensor to operate a few experiments. By hovering their finger over the sensor, they found they could move a miniature elevator up, light up LEDs and play keyboard notes.

It’s unclear how far away we are from smart diapers and smart face masks hitting the shelves, but researchers are hoping that this could lead to new innovations in the medical world.