A silicone membrane for wearable devices is more comfortable and breathable thanks to better-sized pores made with the help of citric acid crystals. The new preparation technique fabricates thin, silicone-based patches that rapidly wick water away from the skin. The technique could reduce the redness and itching caused by wearable biosensors that trap sweat beneath them. The technique was developed by bioengineer and professor Young-Ho Cho and his colleagues at KAIST and reported in the journal Scientific Reports.

Smaller, more uniformly sized pores are made in the PDMS membrane by mixing PDMS, toluene, citric acid, and ethanol. Toluene dilutes PDMS so that it can easily mix with the other two constituents. Toluene and ethanol are then evaporated, which causes the citric acid to crystallize within the PDMS material. The mixture is placed in a mold where it solidifies into a thin film. The crystals are then removed using ethanol, leaving pores in their place. (Credit: Prof. Young-Ho Cho, KAIST)

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

Attachable biosensors often use a silicone-based compound called polydimethylsiloxane (PDMS), because it has a relatively high water vapor transmission rate compared with other 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 solutes, 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 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 less costly than using beads and leads to 93.2 percent smaller and 425 percent 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 did.

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

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

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

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