The COVID-19 pandemic increased public awareness of the importance of mask use for personal protection. However, when the mesh size of mask fabrics is small enough to capture viruses, which are usually around one hundred nanometers in size, the fabric typically also restricts air flow, resulting in user discomfort. Researchers from Japan have now developed a new filter material that effectively captures nanoparticles, although further improvements are needed to make it suitable for comfortable mask use.

In a study published this month in Materials Advances, researchers from the Institute of Industrial Science at the University of Tokyo have developed a filter capable of capturing nanoparticles such as viruses. While the filter demonstrates high filtration efficiency, its airflow resistance is currently higher than the standards required for face masks, indicating that additional development is necessary before it can be used for personal protective equipment.

The filter is constructed from nanosheets consisting of an ordered mesh composed of porphyrins, which are flat, ring-shaped molecules with a central hole. The tiny holes in the porphyrin molecules are suitably sized to allow the easy passage of the small gas molecules in air while blocking the movement of larger particles, such as viruses. The nanosheets are then supported on a fabric modified with nanofibers containing pores of several hundred nanometers to form the filter.

"The porphyrin-based nanosheets are constructed through interfacial reactions that are driven by the movement of reactants caused by the gradient of surface tension at the air鈥搒olvent interface, known as the Marangoni effect," says senior author Kazuyuki Ishii. "The nanosheets are then compressed and coated on nanofiber-modified fabric using a stamp method."

The team tested their filter using the standard procedure used to test N95 face masks. The results of the particle filtration tests revealed that the filter effectively trapped particles that were as small as viruses. The filter achieved a particle filtration efficiency of 96%, which exceeds the requirement of 95% for an N95 face mask.

"Our porphyrin-based filter collected nanoparticles with a diameter as small as one hundred nanometers," explains senior author Kazuyuki Ishii. "Importantly, the filter also showed only a moderate decrease of differential pressure in gas flow measurements. While this indicates that the filter is capable of trapping particles as small as viruses, the pressure drop measured is higher than current standards for face mask breathability."

The team's approach involving coating porous nanosheets on nanofibers is promising for creating materials capable of effectively filtering small particles like viruses. However, to ensure both user comfort and protection in face masks, further optimization is needed to reduce airflow resistance to acceptable levels.

Editor's note: While the new filter material demonstrates excellent particle capture, its current airflow resistance exceeds the maximum allowed for N95 respirators. Further research and development are needed before it can be used in face masks or similar personal protective equipment.