28 November 2013

Bactericidal coatings imitating the surface of insect wings

Black silicon turned out to be an excellent bactericide

Alexander Berezin, CompulentaElena Ivanova from the Swinburne Institute of Technology (Australia) and her colleagues found out that black silicon is a bactericidal material – so brilliant that it kills both gram-positive and gram-negative bacteria.

(In their previous work, the same group of researchers showed that the surface imitating the wings of cicadas covered with microneedles showed a bactericidal effect only in relation to gram–negative microorganisms - VM.)

Black silicon was discovered in the 1980s as an unexpected by-product of the reactive ion etching process. In fact, it is a monocrystalline silicon covered with a continuous coating of nanoheads about 10 microns high and less than 1 microns in diameter.

On the left – the surface of black silicon, on the right – the wings of Diplacodes bipunctata.
Although they differ, their bactericidal qualities are very close.
(Here and below are illustrations by Elena P. Ivanova et al.)

Ms. Ivanova reports that by placing various groups of bacteria on the surface of this material, she observed their death at an impressive rate of 450 thousand bacteria per minute per square centimeter. At the same time, the process had no chemical background, because everything happened due only to the mechanical properties of silicon "studded" with microneedles.

To check for sure whether there is a connection between the chemical composition of the surface and its ability to destroy microbes, scientists compared the effectiveness of black silicon with the surface of the wings of dragonflies of the species Diplacodes bipunctata. As it turned out, they have a similar microstructure, although, of course, they are not "made" of silicon. Tests have shown that both types of surfaces are almost equally dangerous for bacteria; this allows us to confidently speak about the mechanical nature of their bactericidal activity. And even the control coating of both surfaces with a thin layer of gold, which dramatically transformed their chemical composition, did not lead to a change in the situation with the mass death of bacteria.

Although only Staphylococcus aureus (very resistant to most antibiotics, we recall), as well as hay and Pseudomonas aeruginosa were grown in experiments on black silicon, the authors of the work are confident that the bactericidal properties of such a surface will be steadily reproduced in relation to other types of microbes. The fact is that the shape of microorganisms on such surfaces was noticeably different from their shape on glass and smooth silicon, and, according to the researchers, it was this mechanical deformation caused by microneedles of black silicon and the surface of dragonfly wings that ultimately destroyed the cell walls. Moreover, they were equally fatal for gram-negative and gram-positive bacteria (despite the fact that the cell walls of the latter are usually 4-5 times thicker). Both surfaces were lethal for the spores of the hay wand, although usually the spores are much less vulnerable than the microorganisms themselves.

All three groups of bacteria and spores of hay bacillus on flat surfaces (above, a–h) look normal,
while on black silicon and dragonfly wings (below) they are deformed by contact with many microneedles.In addition to the purely theoretical value of the study, which for the first time discovered the bactericidal properties of both black silicon and hydrophilic abiogenic surfaces in general, the authors declare its significant practical significance: black silicon is quite easy to obtain in large quantities today, since its production methods are widely used in the electronics industry.

More importantly, the killing effect of surfaces does not depend on their composition, and, in addition to monocrystalline silicon, coatings of almost any material can be structured in this way, using them in hospitals and medical institutions for sterile rooms and the prevention of nosocomial infections. No one needs to explain how important this is, because in the same USA or Russia, several tens of thousands of people die from such infections every year. Simply because they were in the hospital at the wrong time and in the wrong place, and not because of their illness.

Separately, it is worth noting that the widespread introduction of such surfaces is unlikely to be accompanied by the appearance of microorganisms resistant to them, as, unfortunately, happens with antibiotics. In addition to the non-chemical nature of such an impact, which is sharply different from the mechanisms of antibiotics, it is also convinced that the wings of dragonflies that have existed for hundreds of millions of years still remain quite deadly for bacteria.

The research report is published in the journal Nature Communications:
Ivanova et al., Bacterial activity of black silicon (full text available).

Portal "Eternal youth" http://vechnayamolodost.ru28.11.2013

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