Scientists have synthesised an almost weightless fabric which is nearly invisible and can trap less than one micrometre fine particulate matter - a material perfect for protection of respiratory organs from air pollutants.
The fabric made of nylon nanofibres with a diameter less than 15 nanometres beats other similar materials in terms of filtering and optical properties.
The scientists from the Russian Academy of Sciences characterise their material as lightweight (10-20 milligrammes per square metre), almost invisible (95 per cent light transmission - more than that of a window glass), showing low resistance to airflow and efficient interception of less than 1 micrometre fine particulate matter.
Previously, the same team had demonstrated that reducing fibre diameter from 200 nanometres down to 20 decreased filter resistance to airflow by two thirds, and this effect could no longer be explained by classical aerodynamics.
The scientists used the technique called electrospinning - a jet of a dissolved polymer is ejected through a special nozzle aiming at a target under action of an electric field.
From the other side, ethanol is electrosprayed. The polymer jet and the alcohol ions take the opposite electric charges. Colliding in the air, they form ultra-thin fibrous films.
Electrospinning technology as a way to produce nonwoven fibrous filters was developed back in the 1950s to purify air in atomic industry.
However, instead of obtaining nanomats on a solid conducting substrate like before, the researchers used the new technology to form a free filter covering a 55mm hole in a non-conductive polycarbonate screen.
The unique optical and filtering properties originate from a special mechanism of "healing" holes and defects in free standing filters.
Such holes literally attract fibres landing onto the filter surface. As a result, a good filter without big holes can be obtained from a minimum amount of nanofibres and, accordingly, with a minimum resistance to airflow.
The testing of nylon-4,6 electrospun films showed that these almost weightless and invisible fabrics trap no less than 98 per cent of airborne dust particles.
For testing, the scientists used particles from 0.2 to 0.3 micron in diameter. This roughly corresponds to the dust that is not caught by the nasal pharynx, and penetrates the lungs causing a number of dangerous medical conditions.
Submicron particles (less than 1 micrometre in diameter) are the ones also used to test industrial and medical filters. To assess performance, resistance to airflow was also tested.
Experiments to measure resistance have been made on singular samples so far. In real filters a multi-layer surface with a complex configuration is normally used.
The experiments showed that the nylon-4,6 filtering material had the best properties out of all types of fabric previously described.
The research was published in the Macromolecular Nanotechnology Journal.
