Mimicking tiny features seen on insect wings and shark skin that effectively keep their surfaces free of bacteria, a team from Bengaluru’s Indian Institute of Science (IISc) has found a way to prevent bacterial infection on orthopaedic implants without using any chemicals. The team led by Prof. Kaushik Chatterjee from the Department of Materials Engineering at IISc relied purely on surface nanostructure to impart high bactericidal (ability to kill bacteria) efficiency to titanium metal used in implants.
Encouraging results were achieved in laboratory studies by turning the shiny surface into a rough structure through etching. The etched titanium surface is marked by randomly spaced nanopillars of 1 micrometre height and this makes it capable of killing commonly seen pathogenic bacteria that adhere to the surface. The results were published in the journal Scientific Reports.
Within four hours of contact, the rough surface of titanium was able to mechanically kill nearly 95% of E. coli, 98% of Pseudomonas aeruginosa, and 92% of Mycobacterium smegmatis. Though only 22% of Staphylococcus aureus attached to the surface were killed within four hours, the efficiency shot up to 76% at the end of 24 hours.
Hospital-based bacterial infections from orthopaedic implants caused by these bacteria that are often resistant to antibiotics leads to medical complications. According to a 2015 paper in The Lancet, infection rates in patients undergoing primary arthroplasty (surgical reconstruction or replacement of a joint) is greater than 2%; infections accounted for nearly 15% of revisions after hip arthroplasty and 25% after knee arthroplasty.
“We don’t know the precise mechanism by which the bacteria get killed. But we think the nanopillar architecture formed by dry etching mechanically ruptures the bacterial cells. Like in the case of wing surface of cicadas, the bacterial cell membrane might be getting stretched by the nanopillars,” says Dr. Jafar Hasan from the Department of Materials Engineering at IISc and the first author of the paper. Dr. Hasan is an Early Fellow of the Wellcome Trust DBT India Alliance and this paper was part of the fellowship work.
Bacteria are highly motile and have a tendency to adhere to the surface to form a biofilm. Since the titanium surface is marked by sharp tips, the cell membrane gets mechanically damaged when in contact with the nanostructures.
While disease-causing bacteria get killed, mesenchymal stem cells that form bone were unaffected by the etched surface. Unlike bacteria that have rigid membranes, the stem cells are bigger, softer and are able to easily conform and attach to the rough surface.
Bacterial cell viability reduced and reached a saturation limit when the surface was etched for 10 minutes. “Etching for 20 minutes did not increase the killing efficiency” says Dr. Hasan. The etched titanium was found to exhibit good corrosion resistance and retained its microstructure even when subjected to high-pressure autoclaving.
“We want to etch actual implants and carry out trials on rats and rabbits to test for bactericidal activity and to understand how the rough implant behaves inside the body and study how the bone attaches itself to the implant and grows (osseointegration),” says Prof. Chatterjee, who is the corresponding author of the paper.