The process could potentially be used to attack microbial contamination on implantable medical devices and on food processing equipment made with the metal. While the specific mechanism by which the nanotextured material kills bacteria requires further study, the researchers believe tiny spikes and other nano-protrusions created on the surface puncture bacterial membranes to kill the bugs. The surface structures don't appear to have a similar effect on mammalian cells, which are an order of magnitude larger than the bacteria.
Beyond the anti-bacterial effects, the nano-texturing also appears to improve corrosion resistance. The research was reported December 12 in the journal ACS Biomaterials
Science & Engineering by researchers at the Georgia Institute of Technology.
Julie Champion, an associate professor in Georgia Tech's School of Chemical and Biomolecular Engineering stated: "A lot of the antimicrobial approaches currently being used add some sort of surface film, which can wear off. Because we are actually modifying the steel itself, that should be a permanent change to the material."
Champion and her Georgia Tech collaborators found that the surface modification killed both Gram-negative and Gram-positive bacteria, testing it on Escherichia coli and Staphylococcus aureus. But the modification did not appear to be toxic to mouse cells – an important issue because cells must adhere to medical implants as part of their incorporation into the body.
The research began with a goal of creating a super-hydrophobic surface on the stainless steel in an effort to repel liquids – and with them, bacteria. But it soon became clear that creating such a surface would require the use of a chemical coating, which the researchers didn't want to do. Postdoctoral Fellows Yeongseon Jang and Won Tae Choi then proposed an alternative idea of using a nanotextured surface on stainless steel to control bacterial adhesion, and they initiated a collaboration to demonstrate this effect.
"Under the right conditions, you can create a nanotexture on the grain surface structure," Hess explained. "This texturing process increases the surface segregation of chromium and molybdenum and thus enhances corrosion resistance, which is what differentiates stainless steel from conventional steel."
Microscopic examination showed protrusions 20 to 25 nanometers above the surface. "It's like a mountain range with both sharp peaks and valleys," said Champion. "We think the bacteria-killing effect is related to the size scale of these features, allowing them to interact with the membranes of the bacterial cells."
The researchers were surprised that the treated surface killed bacteria. And because the process appears to rely on a biophysical rather than chemical process, the bugs shouldn't be able to develop resistance to it, she added.
A second major potential application for the surface modification technique is food processing equipment. There, the surface treatment should prevent bacteria from adhering, enhancing existing sterilization techniques.
The researchers used samples of stainless as 316L, treating the surface with an electrochemical process in which current was applied to the metal surfaces while they were submerged in a nitric acid etching solution.