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Scientists design antibacterial surface that can poke bacteria to death

Photo: Yudhajit Bhattacharjee

Bacteria have been one of our dreaded enemies, causing a wide range of diseases and killing many in the process. A report by World Health Organization says nearly 50,000 men, women and children are dying every day from infectious diseases caused by viruses, bacteria, protozoans and others. Though scientists around the world are striving to develop the best antibiotics, some bacteria continue to outsmart us by showing resistance to these drugs and turning into superbugs.

But, nature does have an answer to this problem of fighting bacteria -- if we look close enough -- in the form of shark scales and insect wings. “In nature, a few animals and insects posses antibacterial properties either by having some chemical groups that kill the bacteria, or by having microscopic wrinkles and folds on the skin that make it bacteria hard for bacteria to grow”, says Prof. Kaushik Chatterjee, Associate Professor at the Department of Materials Science, Indian Institute of Science, Bangalore.

Inspired by this, Prof. Chatterjee and his team of researchers at IISc have proposed a different approach to kill bacteria – by literally poking them to death! And how? They have used tiny surface undulations on titanium, a widely used metal in orthopaedic and dental applications.

“In the case of shark fins for example, there are numerous such micro-scale features that resist the attachment of bacteria on the surface. Recent studies have shown that ‘nano pillars’ on wings of insects like the cicada and the dragonfly can rupture bacterial cells, thus killing them in the process. Our work is based on the latter”, says Prof. Chatterjee, explaining the inspiration behind the study.

The researchers have tried to produce a ‘forest’ of nano–sized, tall structures on titanium by chemically etching the surface. “We used a process called reactive ion etching that is designed for the microelectronics industry, for producing ‘black’ silicon. Black silicon is used in electronics and photovoltaic applications because it absorbs light”, explains Dr. Jafar Hasan, a Research Associate at IISc involved in this study. Since silicon cannot be used for biomedical applications, the researchers instead applied the same technique on titanium, creating ‘black titanium’.

But how does this kill the bacteria? “When bacterial cells come in contact with the etched surface, they get poked by these nano-pillars, rupturing their cells”, comments Dr. Hasan. The researchers suggest the use of black titanium in orthopaedic applications like hip joints, knee joints, bone plates and screws which are planted inside the body and are required to be free from contamination and bacterial infections. However, their results have shown that black titanium does not kill the cells in our body like the way bacterial cells are killed. “Perhaps it is because our cells change their morphology very well based on the properties of the surface they are growing on, and adjust well”, he says.

The results of the study look very promising. The researchers found that within four hours of contact with the black titanium surface, about 95% of Escherichia coli that causes severe stomach infections, 98% Pseudomonas aeruginosa that causes cystic fibrosis, 92% of Mycobacterium smegmatis and 22% Staphylococcus aureus that causes sinusitis and skin infections, were killed.

“Controlling infections is a clinical challenge and usually, a degradable antibacterial drug is coated on the surface of orthopaedic devices. But, these drugs do not work on drug resistance pathogens, which is a growing concern today. This is a completely alternative way of killing bacteria without the use of any chemicals”, remarks Prof. Chatterjee.

Interestingly, the applications of black titanium are not limited to medical devices. They can be used for other surfaces that need to be bacteria-free like doorknobs and walls of hospitals. In addition, since black titanium also absorbs light, it could also be used in antireflective surfaces. The researchers are now conducting the next level of pre-clinical tests to replicate the laboratory results before making these surfaces fit for clinical use.

“In a country like India where low cost healthcare settings are prominent, maintaining sterility is a problem. This research provides an efficient way to curtail infections with a solution that works without expensive chemicals and drugs”, signs off Prof. Chatterjee.