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Cactus-Spine-Inspired wearable sweat sensor

Scientists at Pohang University of Science and Technology in South Korea (POSTECH) have developed a sweat-collecting patch based on how cactus spines attract water. The patch, which responds quickly to biochemicals in sweat and enables the continuous monitoring of changes in sweat biochemicals according to their changes in the wearer's blood, could help diabetes patients who have to repeatedly draw blood, and could also find uses in wearable devices for daily healthcare monitoring.

Sweat sensors are expected to be an effective wearable device for future non-invasive healthcare monitoring. Being able to capture sweat secretions is useful for analyzing bioanalytes in the body without the be need to draw blood, but can hampered by irregular and low sweat secretion rates. However, in this new study, which was reported in the journal Advanced Materials [Son et al. Adv. Mater. (2021) DOI: 10.1002/adma.202102740], a patch was developed that can be attached to the skin and quickly collects sweat by mimicking the principle behind cactus spines.

As cacti grow in dry environments, they have to transport water droplets that form on the tip of their spines to their base to help them survive. In this process, the water droplets move because of the difference in pressure acting on the inside and outside of the curved surface of the water droplet, a phenomenon called Laplace pressure.

This principle was used to mimic the structure of the cactus spine with wedge-shaped wettability patterns with superhydrophobic/superhydrophilic surfaces, which allowed sweat droplets on the wedge-patterned surface to spontaneously move to the wide end of the wedge pattern. This was due to the Laplace pressure difference between the front and back surfaces of the droplet being maximized. These wedge-patterned channels collect sweat quickly and spontaneously regardless of the slope of the microfluidic channels and do not require additional force.

The wedge-patterned channel also shows useful sweat-collecting efficiency as it transports almost all sweat droplets to the sensing area without leaving much inside the channel. This means it can collect sweat much faster than the conventional microfluidic channels to continuously monitor the bioanalytes, and the patch, therefore, offers good sweat-collecting efficiency and reduces how long is needed to fill the sensing area by transporting sweat.

As team leader Kilwon Cho said, “Difficulties in collecting sweat have hindered its use in wearable healthcare devices. This newly developed patch solves that issue by quickly collecting sweat and facilitating its use in various wearable healthcare devices, including blood sugar monitoring.”

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