Summer Months Usher In More Heatstrokes. A New pH-Measuring Sweat Sensor Detects Early Signs

May 27, 2020 by Gary Elinoff

Heat-related illnesses cause more deaths than any other weather incident. Now, researchers are designing biomonitors—some that measure pH in sweat—to detect early indicators of heatstroke.

According to federal statistics, heat-related illnesses—like heatstroke—are the number one weather-related cause of death in the United States.  If the human body heats to over 40℃ (104°F) during periods of intense physical exercise or in hot weather, the body can struggle to regulate temperature and experience a heat stroke. 

But in recent years, electrical engineers and researchers have developed wearables, especially sweat-measuring sensors, to monitor biometrics that may indicate the early symptoms of heatstroke.

We've recently discussed a few of these sweat-reliant sensors, including:

The most recent development in sweat sensors comes out of Tokai University, where researchers recently announced a low-cost sensor that can measure the pH of the skin


Wearable electronic sensor for heatstroke detection

Wearable electronic sensor for heatstroke detection. Image (modified) used courtesy of Tokai University


Because of the close correlation between pH (acidity) and heatstroke, the skin-like electronic sensor allows for early detection of this life-threatening condition. Since heatstroke is often asymptomatic, this novel device has the potential to be a lifesaver worldwide.


Present Methods for the Detection of Heatstroke

The present methods for measuring sweat pH are expensive, and because they aren’t portable, they aren’t practical outside laboratory or clinical settings. To be of practical use, a predictive method is needed to monitor skin pH levels on the go.


 A pH Sensing System—in an Artificial E-Skin

A team of Tokai University scientists led by Dr. Ganesh Kumar Mani published a study in ACS Sensors that described their invention of a skin pH sensor that sticks to human skin without an adhesive. Dr. Mani explains, “The key to our invention lies in its base material: it is an ultra-flexible polymer thin film made up of polydimethylsiloxane (PDMS), a flexible and durable material on which the other parts of the sensor electrodes can be placed using high-quality deposition processes.” 

The key components of the device are two metal electrodes, one made of silver and silver iodate and the other made of antimony and antimony oxide. The way it works is that the measured voltage difference between these two electrodes varies in proportion to the pH of the sweat.


The sweat pH sensor is built on a nanosheet of PDMS

The sweat pH sensor is built on a nanosheet of PDMS and readily sticks to human skin. Image used courtesy of Tokai University


Dr. Mani foresees this technology being of use in biomedical fields and specifically personalized skincare technology. He also anticipates that additional research on PDMS nanosheets with different materials can also open the door for a host of applications. 


Communicating Results via Bluetooth

What does this new sensor from Tokai University have in common with other biometric sweat-reliant sensors? The Journal of the Royal Society Interface recently published a study reviewing wearable flexible sweat sensors for healthcare monitoring, many of which gather similar physiological metrics as the Tokai study. 

This study also explains how many sweat sensors communicate results via Bluetooth.


Diagram of a transducer output signal processing and transmission to an external monitoring device

Diagram of a transducer output signal processing and transmission to an external monitoring device. Image used courtesy of the Journal of the Royal Society Interface


In this implementation, the sweat is analyzed for glucose, lactate, sodium, and potassium by the electrodes and processed via analog circuitry. It is then digitized and transmitted via Bluetooth to a smartphone.


How Do Sweat Sensors Compare to Other Wearables for Heat Stroke? 

Many other wearables are on the market that can detect heatstroke without a sweat sensor. How do these compare to some of the novel sweat sensors coming out of universities like Tokai?

A study by the National Institute of Health used a sensor module to gather information on heart rate, temperature, humidity, and skin resistance. It did not, however, compute the heatstroke risk in situ. It instead takes the raw data and transmits it to the back-end monitor.


Architecture of a wearable heatstroke detection device

Architecture of a wearable heatstroke detection device. Image used courtesy of Shih-Sung Lin et. al


The back-end monitor computes the heatstroke risk and transmits the results to the user being monitored. That individual can walk, bike, or run over a relatively long distance since LoRa is a wide area network with a range of 10 km or more.


Sweat Sensors for Summer Months

With summer just around the corner, it's likely we'll see more interest in sweat sensors to prevent heatstroke. It seems, too, that the nanosheets described in the Tokai University study will play an important role in diagnostics. And as we observed in sweat sensors and wearables across the industry, Bluetooth low energy (BLE) is likely to emerge as the preeminent mode of information transmission because of its low energy consumption.



Have you ever designed a wearable that was to be adhered to the human skin? What were some interesting dimensions of that project? Share your experiences in the comments below.