Even for adults, one of the scariest aspects of a visit to the doctor, is the use of needles to withdraw blood samples for medical tests. Once the sample is collected, it is sent to a testing facility, that analyses the sample, and then sends back a report. By this time, there might be changes in the body of the patient. Wouldn't it be great if there was a non invasive way to continuously collect real-time information on the physiological condition of a patient? Turns out, the answer may lie in wearable sweat sensors.
Sweat can be more easily sampled than say tears, saliva or intestinal fluids. By measuring the concentration of biomarkers such as glucose, cortisol and sodium present in the sweat, sensors can derive information on diseases, dehydration, fatigue or stress in an individual. In fact, there are actually dedicated sweat test centers, but in this case, collecting the sweat and sending the samples for testing is an even more laborious process than a blood test. The patient has to sit still for as long as it takes for the sampling vial to fill up with sweat, which can take up to half an hour. The process is particularly wearisome for children.
The challenge is to miniaturise the testing apparatus. Some of the approaches being used include wristbands, armbands and stickers. The advancements needed to make viable sweat sensors that are sensitive enough while working with tiny power supplies, flexible electronics, and advancements in microfluidics, which involve precisely handling small amounts of liquids using the tineist of pipes. All of these are coming together in research institutions around the world, and viable sweat sensors are being realised. The sweat sensors can capture the sweat using absorbent pads. A variety of materials can be used to embed the flexible electronics, including plastics, fabrics and elastomers.
Between 2016 and 2017, a collaborative team of researchers from UC Berkeley and Stanford combined two existing technologies to create a non invasive sweat sensor. These were a flexible circuit board, and a flexible array of sensors. The two were put together in a neat little wristband, and could analyze the physiological condition of the wearer at a molecular level. Till that point of time, non invasive sensors could only measure the concentration of a single molecule at a time. By using an array of sensors, the team developed what they call a perspiration analysis system, that could be placed onto the wrist of any patient. The band was designed to stick to the skin. The circuit board provided the signal processing, and wirelessly transmitted the collected data to a paired smartphone.
In 2019, researchers at UC Berkeley have developed a process for manufacturing sweat sensors on rolls of flexible plastic substrate, similar to a filmstrip of an analog camera. The sensors are embedded onto the plastic in a manner similar to the way a printing press imprints text on a newspaper. Inside the sensors are spiraling microfluidic tubes, that can derive information on the rate at which the wearer is sweating. The tubes also have tiny chemical sensors that can measure the concentrations of potassium, sodium and metabolites, a class of molecules that interact with enzymes. The process allows for production of scale, which means large volumes at low cost. In initial tests, the trackers could understand when the liquid loss of the volunteer testers was too much during intense exercise sessions.
In November this year, a team at Caltech led by Professor Wei Gao published a paper describing an even more sensitive sweat sensor in Nature Biotechnology. The previous sensors could monitor concentrations of electrolytes, lactate and metabolites, all of which occur in high concentrations in sweat. The sensor developed by Gao's team can detect molecules in much lower concentrations, increasing the range of conditions that can be detected by sweat sensors. The sensor developed in Caltech can be used to track cardiovascular, liver or kidney diseases as well as neuropsychiatric conditions and eating disorders. This is accomplished by the additional monitoring of uric acid and an amino acid called tyrosine. Gao explains, “Such wearable sweat sensors have the potential to rapidly, continuously, and noninvasively capture changes in health at molecular levels. They could enable personalized monitoring, early diagnosis, and timely intervention. " Gao's team created the microfluidic channels in plastic film to capture the sweat using a carbon dioxide laser, and used the same device to craft the more sensitive graphene sensors. The performance of the sensors was checked by comparing the measurements with those derived from traditional blood tests. The tests showed that the sweat sensors were doing their jobs. The performance of the sensors was checked by comparing the measurements with those derived from traditional blood tests. The tests showed that the sweat sensors were doing their jobs. The performance of the sensors was checked by comparing the measurements with those derived from traditional blood tests. The tests showed that the sweat sensors were doing their jobs.
400 million people around the world suffer from diabetes, and hundreds of millions more are prediabetic. Sweat sensors hold the promise of helping them tremendously, as one of the metabolites that sweat sensors can measure is glucose. Traditionally, home tests for measuring glucose levels for diabetic patients involved devices that drew blood with a fingerprick. Sweat sensors can do away with such devices, and additionally provide real time tracking of blood glucose levels. Diabetic patients have to stay within a narrow healthy range by ensuring that their blood sugar levels do not spike by too much, or sudden fall, both of which can have serious consequences. By pairing such a sensor with a smartphone app, a person can take the countermeasures well in advance of a situation.
Apart from finding out if the patient has any problems, in particular cases, dedicated sweat sensors can make sure that the patient is following the recommended treatment, and track the progress of recovery. Students from Johns Hopkins University have developed such a sensor for patients suffering from cystic fibrosis, a genetic condition linked to respiratory problems. Patients suffering from the disease have a high level of chloride in their sweat. A new class of drugs to treat the problem by addressing the protein problems that cause symptoms of the disease. The treatment regime also reduces the concentration of chloride in the sweat. For patients suffering from cystic fibrosis, sweat sensors offers a way to track the progress of treatment and make the appropriate adjustments if necessary. Healthcare in general stands to benefit from continuous, real time monitoring of the patient's health. In the future, these sensors could find their way into smartclothes, fitness trackers or smartwatches, and not require a dedicated band or a sticker just for sweat monitoring. While a fitness tracker today can monitor activity levels and heart rate, a sweat sensor can provide so much more information for healthcare professionals. The technology promises to be particularly helpful to those living in remote and rural areas, away from easy and quick access to healthcare. a sweat sensor can provide so much more information for healthcare professionals. The technology promises to be particularly helpful to those living in remote and rural areas, away from easy and quick access to healthcare. a sweat sensor can provide so much more information for healthcare professionals. The technology promises to be particularly helpful to those living in remote and rural areas, away from easy and quick access to healthcare.