Breathable polymer membranes for wearable devices

Prof. Young-Ho Cho’s group at KAIST has taken a major step toward a wearable polymer membrane, being is more comfortable and breathable thanks to better-sized pores made with the help of citric acid crystals. The results could be revolutionary in efforts to apply to daily wearable devices. The study was published on January 13 in Scientific Reports (Wearable Porous PDMS Layer of High Moisture Permeability for Skin Trouble Reduction, Scientific Reports, 11.1 (2021): 1-11).

“Wearable bioelectronics are becoming more attractive for the day-to-day monitoring of biological signatures found in sweat, like hormones or glucose, as well as body temperature, heart rate, and energy expenditure,” Professor Cho explained. “But currently available materials can cause skin irritation, so scientists are looking for ways to improve them,” he added.

Attachable biosensors use a polymer compound called polydimethylsiloxane (PDMS), as it has a relatively high water vapor transmission rate compared to other flexible materials. Still, this rate is only two-thirds that of skin’s water evaporation rate, meaning sweat still gets trapped underneath it.

Current fabrication approaches mix PDMS with beads or particles, such as sugars or salts, and then remove them to leave pores in their place. Another technique uses gas to form pores in the material. Each technique has its disadvantages, from being expensive and complex to leaving pores of different sizes.

A team of researchers led by Professor Cho from the KAIST Department of Bio and Brain Engineering was able to form small, uniform pores by crystallizing citric acid in PDMS and then removing the crystals using ethanol. The approach is significantly cheaper than using beads, and leads to 93.2% smaller and 425% more uniformly-sized pores compared to using sugar. Importantly, the membrane transmits water vapor 2.2 times faster than human skin.

The team tested their membrane on human skin for seven days and found that it caused only minor redness and no itching, whereas a non-porous PDMS membrane results in skin troubles.

Professor Cho said, “Our method could be used to fabricate porous PDMS membranes for skin-attachable devices used for daily monitoring of physiological signals for human emotion cognition.”

“We next plan to modify our membrane so it can be more readily attached to and removed from skin,” he added.

This work was supported by the Ministry of Trade, Industry and Energy (MOTIE) of Korea under the Alchemist Project.