Wearable sensor monitors sweat in real time

Available on-demand, in abundance and containing multiple biomarkers, sweat is an increasingly appealing medium for monitoring health, according to researchers at Penn State. But not everyone — especially critically ill patients — can build up enough sweat to provide a robust enough sample for current analysis techniques.

That may no longer be an issue, thanks to the team at Penn State that has developed a novel wearable sensor capable of continuously monitoring low rates of perspiration for the presence of a lactate — a molecule the body uses to break down sugars for energy. This biomarker can indicate oxygen starvation in the body’s tissues, which is a key performance indicator for athletes as well as a potential sign of serious conditions such as sepsis or organ failure.

The researchers published their approach in the journal Small. They reported that their device, for which they filed a patent, sits on the surface of the skin like a bandaid and can collect up to 10 times the amount of sweat from low-intensity activities like walking, or even lying down or answering emails, compared to other wearable sweat sensors.

“Sweat offers a source of biomarkers we can monitor via non-invasive systems in near-real time for how the body is performing during exercise or to monitor or manage various health conditions,” said co-first and co-corresponding author Farnaz Lorestani, assistant research professor of engineering science and mechanics at Penn State. “The challenge is figuring out how to collect sweat when the person isn’t sweating that much, and we solved that challenge by engineering a platform with granular hydrogels, developed by Professor Amir Sheikhi’s research group, capable of collecting sweat even in low-intensity conditions, like checking emails or laying down.”

Under low-intensity conditions, Lorestani said, most people sweat from 10 to 100 nanolitres per minute per square centimetre of skin — significantly smaller than the liquid in a tear drop. Conventional monitoring devices use a hydrogel — a matrix of molecules called polymers combined with water to absorb and filter samples — to uptake the sample and process it through a laser-induced graphene (LIG) sensor. LIG involves using a laser to convert carbon dioxide into specific patterns of atomically thin carbon layers called graphene, which is highly sensitive and can be outfitted with detectors to accurately identify biomolecules. The issue, according to Lorestani, is that this approach fails with small sweat samples since liquid is lost during the uptake process.

To solve this issue, the researchers made two key changes. First, instead of the typical hydrogel, they used a granular hydrogel scaffold, comprising jammed microscale hydrogel particles called microgels that are interlinked to each other. This technology builds on Sheikhi’s prior work on protein-based granular biomaterials for tissue engineering and regeneration. For the second change, the researchers situated the scaffold to feed into a microfluidic chamber made by patterning LIG in a compact spiral, the design of which increases surface area to improve fluid transport and minimise sweat loss. The scaffold sits on the skin, attached by a skin-safe adhesive, where it collects sweat from the skin’s surface. That sweat is absorbed by the granular hydrogel, which transports it to the microfluidic chamber, where it travels to the sensor that can identify lactate.

“We hypothesised that the porous medium in the granular hydrogel scaffold increases absorption capacity when compared with previously used hydrogel materials,” Lorestani said. This is likely due to the tiny void spaces between the granular spheres, which enable capillary-driven fluid uptake — the same phenomenon that wicks water from a plant’s roots up its stem. “The compact coil-shaped microfluidic channel design also contributes to accuracy and sensitivity.”

Critically, Lorestani said, they also designed the device — about the size of a standard bandaid and made with cost-effective materials — to be comfortable, with an almost ‘skin-like’ feel. The team tested their design by applying the flexible sensor to individuals and monitoring them in various conditions, from sedentary office work to daily activities to riding a stationary bike. Across conditions, the researchers found that the device could absorb enough sweat to accurately identify the presence of lactate within two hours.

“The proof-of-concept demonstration features a cost-effective, sensitive and versatile flexible sensing platform for early biomarker detection, where sweat production is minimal or sporadic, such as at rest or during mild physical activities,” Lorestani said, explaining that the platform could be adapted to detect other biomarkers by changing the sensor from one for lactate to another. “Overall, our goal is to build a healthier society by making non-invasive, continuous, personalised health monitoring more accessible to everyone — and this work is a step in that direction.”

Image caption: A team at Penn State has developed a novel wearable sensor capable of continuously monitoring low rates of perspiration for the presence of a lactate — a molecule the body uses to break down sugars for energy. Image credit: Farnaz Lorestani.