Wearable sensor tracks the pH of sweat

15 Jan 2020

National University of Singapore device adds new capability to health monitors.

A project at the National University of Singapore (NUS) has developed a sensor to analyze the pH of sweat, intended to augment the health monitoring capabilities of wearable devices.

The sensor is conceived as an add-on component to existing fitness trackers, and can continuously measure and monitor the acidity or alkalinity level of a user's perspiration. NUS has initially developed a prototype with the form factor of a watch, because of the popularity of smartwatch wearable devices.

"Most present day fitness trackers and smartwatches can measure health indicators such as heart rate, step count and even sleep cycle, but they are not able to track health indicators at the molecular level," said Ananta Narayanan Balaji of NUS.

"We created the pH Watch because sweat is a readily accessible bodily fluid composed of a wide array of biochemical markers that can be used to monitor the well-being of individuals in a non-invasive manner."

The pH Watch invented by NUS specifically leverages the potential of fitness trackers and smartwatches to operate as pulse oximeters, able to measure the heart rate and oxygen saturation levels of users.

Pulse oximeters are an established technology in which the oxygen content of blood is monitored by comparing the absorption of two wavelengths of infrared light directed through the skin. As wearable professional-class fitness devices monitoring heart rate have become more ubiquitous, so the potential for using the same device to provide oximetry data too has become more feasible.

This trend is also potentially applicable in the consumer sector. Current iterations of the Apple Watch, for example, are able to carry out sophisticated analysis of heart rate and pulse rhythm, and do so using an on-board sensor known to be capable of pulse oximetry and oxygen measurements too. To date this latter functionality has not been activated on the Apple device, however, possibly due to FDA considerations.

The NUS team wanted to design a pH sweat sensor capable of working alongside conventional pulse oximeters, and created a material that changes color according to the different pH sweat it comes into contact with. These color changes have been designed to match the recording capabilities of pulse oximeter components in existing devices.

According to NUS, this integration of a custom-made pH sensor and the team's bespoke algorithm into a fitness tracker or smartwatch with an existing pulse oximeter allows the NUS pH Watch to simultaneously monitor the sweat pH, heart rate and blood oxygen saturation values in real-time, with 90 percent accuracy.

Ubiquitous sensing of biomarkers

Although a number of approaches to sweat analysis have been developed in the past, including the manufacture of colorimetric sensor materials, the era of wearable fitness trackers has allowed some persistent hurdles of sensitivity and accuracy to be tackled, as well as allowing potential correlation between analysis of sweat and the simultaneous analysis of blood.

"We designed and fabricated the pH sensor from scratch using polyaniline polymer, which is an inexpensive, durable yet very flexible material," commented project member Chen Yuan. "It conforms well to the skin and can be added to any device with a pulse oximeter, to enable continuous measurement of sweat pH. The sensor can be reused for more than a year even with daily use."

The sensor is also designed to be a passive component and not require a power source, with the accompanying algorithm making only modest additional power demands on the device's battery.

Beyond its use in monitoring strenuous activity and sports science, the measurement of sweat pH could also be of medical value, assisting in the treatment of skin conditions such as dermatitis or acne. Diabetic patients may also benefit, since a high sweat pH during excessive or night sweating may suggest that the patient is experiencing low blood glucose levels.

"To the best of our knowledge, the pH Watch is the first demonstration of a reusable sweat sensor that can be readily integrated into today's smartwatches with pulse oximeters," commented project co-supervisor Shao Huilin. "It paves the way for ubiquitous sensing of biomarkers."