Smart Diapers Get Wiser with Hand-Drawn Electrode Sensors: Affordable and Accurate Humidity Detection

Babies naturally signal when they need a diaper change, but researchers at Penn State have created a sensor to aid workers in daycares, hospitals, and other facilities in providing quicker assistance to those under their care. According to Huanyu Cheng, James L. Henderson, Jr. Memorial Associate Professor of Engineering Science and Mechanics at Penn State, the team has been working on creating devices to gather important health data. The aim is to identify health issues and diseases before it is too late by making early predictions.

Huanyu Cheng, the lead author of a study published in the Nano Letters journal, has described the design and production of a hand-drawn electrode sensor. The sensor is made using a pencil on paper treated with sodium chloride solution and is highly sensitive to changes in humidity. It provides precise measurements in a broad range of relative humidity levels, from 5.6% to 90%.

According to Cheng, research into wearable sensors has gained significant attention due to their various applications in healthcare, disaster management, and military defense. Flexible humidity sensors have become essential in healthcare, but it's still challenging to create low-cost, high-sensitivity, and disposable sensors using simple fabrication methods.

'Wiser' Smart Diaper

Li Yang, a professor in the School of Artificial Intelligence at the Hebei University of Technology in China, mentioned that the aim was to create a low-cost, accessible solution that people could easily make and use. The sensor can be made quickly and simply without expensive equipment using a pencil and paper treated with sodium chloride solution. The device leverages the paper's natural reaction to changes in humidity and the interaction of graphite in pencil with water molecules and the sodium chloride solution.

The solution ionizes when the paper absorbs water, and electrons flow to the graphite in the pencil, triggering the sensor. This sensor detects changes in humidity and sends a signal to a smartphone, which then displays and records the data. Drawing on the pre-treated paper within specific lines creates a miniaturized paper circuit board. The paper can be connected to a computer using copper wires and conductive silver paste to function as a humidity detector.

For wireless applications, such as "smart diapers" and respiratory monitoring masks, the drawing is connected to a small lithium battery, which transmits data to a smartphone through Bluetooth. The electrode was drawn directly on a solution-treated face mask for respiratory monitoring. The sensor accurately distinguished between the mouth and nose breathing and classified three breathing states: deep, regular, and rapid. Cheng noted that the data collected could be used to detect respiratory problems and other diseases, leading to advancements in telemedicine and the Internet of Things.

A Penn State-led research team integrated four humidity sensors between the absorbent layers of a diaper to create a "smart diaper," capable of detecting wetness and alerting for a change. Huanyu "Larry" Cheng/Penn State

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Hand-Drawn Electrode Sensor

Furthermore, Cheng emphasized the significance of respiratory rate as a crucial vital sign. Studies have revealed it to be an early warning of various illnesses, including cardiac events, pneumonia, and clinical deterioration, as well as emotional stressors such as cognitive overload, temperature changes, physical exertion, and exercise-induced fatigue. Although human skin experiences minimal humidity changes, the researchers could still detect these changes with pencil and paper humidity sensors, even after the test subjects applied lotion or exercised. Cheng explained that since the skin is the largest organ in the body, any issues with moisture processing could indicate a health problem.

Cheng added that the water loss rate on the skin could indicate different disease conditions. This can be identified through the pencil-on-paper humidity sensor that can detect changes in the skin's humidity, even after applying lotion or physical activity. The skin is the largest organ in the body, and changes in its moisture level can signal other health issues. The researchers also developed a "smart diaper" that incorporates four humidity sensors to detect wetness and alert for a change. Cheng stated that the sensor could provide short-term notifications for diaper changes and long-term patterns for informing parents about their child's overall health. The idea for the smart diaper was inspired by Cheng's personal experience as a parent of young children.

Phys stated that the hand-drawn electrode sensor is not only limited to monitoring respiration and perspiration but has multiple other applications. Cheng and his team have used it as a non-contact switch that detects changes in air humidity from a finger's presence without actual contact. This feature was demonstrated by operating a small elevator, playing a keyboard, and lighting up an LED array. The sensor can be crucial in limiting contact with shared surfaces and reducing the potential for contamination.