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The patch, designed to be worn on the neck, is the first wearable that can monitor cardiovascular signals and biochemical levels in the human body at the same time. "This type of wearable would be very helpful for people with underlying medical conditions to monitor their own health on a regular basis," said Lu Yin, a nano-engineering PhD student at UC San Diego and co-author of the study, which was published on February 15 in the journal Nature Biomedical Engineering. "It would also serve as a great tool for remote patient monitoring, especially during the COVID-19 pandemic when people are minimising in-person visits to the clinic."
It could be particularly useful for people with underlying medical conditions such as high blood pressure or diabetes during the pandemic, as it could be used to track the onset of sepsis, which is characterised by a sudden drop in blood pressure paired with a rise in lactate level.
It would also be useful in intensive care units, where continuous monitoring is important, but currently involves inserting catheters into arteries and tethering patients to multiple monitors. "The novelty here is that we take completely different sensors and merge them together on a single small platform as small as a stamp," said Joseph Wang, a professor of nanoengineering at UC San Diego and co-author of the study. "We can collect so much information with this one wearable and do so in a non-invasive way, without causing discomfort or interruptions to daily activity."
The patch combines research in two complementary areas of engineering. Wang’s lab has been working on wearables that can monitor multiple signals simultaneously. At the same time, nanoengineering professor Sheng Xu has been working on electronic skin patches that can monitor blood pressure. "Each sensor provides a separate picture of a physical or chemical change. Integrating them all in one wearable patch allows us to stitch those different pictures together to get a more comprehensive overview of what's going on in our bodies," said Xu, who is also a co-corresponding author of the study.
The patch is made from a thin sheet of stretchy polymers that can fit the skin. And a blood pressure sensor, made from a small set of ultrasound transducers. These send ultrasonic waves into the body which bounce off arteries, and the sensor detects the echoes and translates them into a blood pressure reading.
The chemical sensors are printed on the patch using conductive ink. One side releases a drug called pilocarpine into the skin to induce sweat and detects the chemical substances in it – lactate, caffeine, alcohol. The other side passes a mild electrical current through the skin to release interstitial fluid from between cells and measures the glucose in it.
One of the biggest engineering challenges was eliminating interference between the different sensors. To do this, researchers had to determine optimal spacing, and figure out how to shield the chemical sensors from the blood pressure sensor. "Finding the right materials, optimizing the overall layout, integrating the different electronics together in a seamless fashion – these challenges took a lot of time to overcome," said co-first author Muyang Lin, a nanoengineering PhD student in Xu's lab.
Now, the team is developing a new version of the patch with even more sensors – and working on shrinking the electronics, and making everything wireless – currently, the patch needs to be connected to a power supply. "We want to make a complete system that is fully wearable," Lin said.
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