Cactus Spine-Inspired Image
Schematic illustration of cactus spine-inspired epidermal swear-collecting patch and the droplet transpotation through the wedge-patterned channel. (Credit: Postech)

A sweat-collecting patch has been developed using the principle based on how the cactus spines attract water. The skin-attachable patch quickly collects sweat by mimicking the principle behind cactus spines. The sweat sensor can reduce the hassle for diabetic patients who repeatedly have to draw blood and can also be used in wearable devices for daily healthcare monitoring.

However, the practical use of sweat sensors is impeded by irregular and low sweat secretion rates. There is a pressing need to effectively collect these sweat secretions.

Cacti, which grow in arid environments, move water droplets that form on the tip of their spines to their base in order to survive. During this process, the fine water droplets move due to the difference in pressure acting on the inside and outside of the curved surface of the water droplet. This phenomenon is called the Laplace pressure.

The patch applied this principle of how cactus spines collect water. The researchers mimicked the structure of the cactus spine by using the wedge-shaped wettability patterns with superhydrophobic/superhydrophilic surfaces. Through this, a sweat droplet on the wedge-patterned surface spontaneously moves to the wide end of the wedge pattern because the Laplace pressure difference between the front and back surfaces of the droplet is maximized.