Self cleaning textiles could improve PPE

The lotus effect is achieved with an incredible number of microscopic bumps, called asperities, on the lotus’s skin. The result is that instead of spreading out and adhering to the surface, the water remains as a discrete droplet and simply rolls off the leaf with gravity or the wind. On their way the droplets sweep up dirt, microbes and detritus, effectively self-cleaning the leaf without any effort from the plant.

The lotus effect is a well-studied and understood phenomenon, and scientists have been investigating and producing artificial hydrophobic surfaces since the mid-1960s, and the actual structure of the lotus leaf was first seen by scanning electron microscope in the 1970s.

50 years on, and amid a viral pandemic, the topic of self-cleaning and hydrophobic materials has re-entered the mainstream with a vengeance.

Hydrophobic or water resistant?

Hydrophobicity is not a new concept, but its applications certainly are, and especially in the medical field.

Materials in medicine are often disinfected with harsh chemicals or microbicides. The problem here is that microbes evolve resistances to these substances, quickly reducing their effectiveness.

Consistent overapplication of these substances also leads to the evolution of so-called 'superbugs', such as MRSA, which are completely unaffected by common microbicides and therefore pose a serious threat of untreatable infection. All these factors have made self-cleaning, hydrophobic surfaces of particular interest.

Creating a hydrophobic material on a flat, solid surface is relatively simple, because the surface makes for a foundation for the regular asperities needed for hydrophobicity. Making hydrophobic textiles is another proposition entirely, as all the shifting peaks and troughs of the warps and wefts are almost the logical opposite of the ideal flat surface mentioned before.

While ostensibly hydrophobic textiles exist, it’s more reasonable to call these textiles water resistant than hydrophobic. The famous Gore-tex, for instance, resists water simply by having holes smaller than typical water drops.

However, these materials still wet like any others, and therefore require an additional chemical hydrophobic spray coating. This application wears off over time, further reducing the hydrophobicity.

Water off a duck’s back

The trick would be to create a textile treatment that forms all the hydrophobic asperities as it is applied, achieving something known as multilength scale roughness where the surface is covered in asperities that vary in size from nanoscopic filaments to microscopic cones. This is also often termed 'self-similarity', because the material appears similar as you zoom between nanoscopic and microscopic scales.

This is precisely the approach taken by Anthony J. Galante et al at the University of Pittsburgh in April 2020. The team has created a textile treatment from polytetrafluoroethylene (PTFE) nanoparticles that are thermally sintered to polypropylene microfibers. Textiles treated this way are up to 350 times more hydrophobic than the control samples, and they also resist attachment of virus particles to the surface by up to 96%. The treatment is also very resistant to abrasion, the biggest weakness of spray-on hydrophobic coatings.

These properties make textiles treated this way perfect for hospital gowns, medical scrubs and other Personal Protective Equipment (PPE). Blood, or any other substance one might come across in medicine, will simply roll straight off the material instead of soaking in and creating a harbour for pathogens to grow and spread.

Superhydrophobic materials like this join the already wide and diverse spread of antimicrobial materials available today. These range from copper-infused stainless steels to innovative biocidal plastics, among many more examples.

It pays to make sure that the materials you are working with are suitable, and the information you need is displayed in an easily accessible and understandable way. Nobody wants to be picking through complex specification sheets when lives are on the line, so using material property databases like Matmatch can make the difference.

So, while the sacred lotus might have had a head start on us, but material science is quickly catching up with nature. The science is promising, so we just might match the lotus before long, albeit a few million years behind.