Researchers use nanotexturing to create bacteria-killing stainless steel surfaces

When it comes to designing medical devices and food processing equipment, the risk of microbial contamination must be mitigated. Researchers at the Georgia Institute of Technology have created a nanotextured surface that kills bacteria while not harming mammalian cells by using an electrochemical etching process on a common stainless steel alloy.

Although the process requires further testing, 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.

Postdoctoral Fellow Yeongseon Jang, Associate Professor Julie Champion and Postdoctoral Fellow Won Tae Choi are shown in Champion’s laboratory at Georgia Tech. With Professor Dennis Hess (not shown), the researchers developed a new nanotextured surface for stainless steel that kills common bacteria. Photo Credit: Rob Felt, Georgia Tech

Beyond the anti-bacterial effects, the nano-texturing also appears to improve corrosion resistance.

“This surface treatment has potentially broad-ranging implications because stainless steel is so widely used and so many of the applications could benefit,” said Julie Champion, an associate professor in Georgia Tech’s School of Chemical and Biomolecular Engineering. Champion adds that many of the current approaches use a surface film that has a tendency to wear off. Their method modifies the steel itself, allowing for permanent change.

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.

Initially the team attempted to create a super-hydrophobic surface on the stainless steel. However, this proved ineffective so 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.

The research team experimented with varying levels of voltage and current flow in a standard electrochemical process. Typically, electrochemical processes are used to polish stainless steel; however, in this case the technique was used to roughen the surface at the nanometer scale.

“Under the right conditions, you can create a nanotexture on the grain surface structure,”  said Dennis Hess, professor and Thomas C. DeLoach, Jr. Chair in the School of Chemical and Biomolecular Engineering 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. Champion compares it to a mountain range with peaks and valleys.  And 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. The researchers used samples of a common stainless alloy known 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.

Close-up image shows an untreated stainless steel sample (left), and a sample that has been electrochemically treated to create a nanotextured surface. The sample was prepared by using a potentiostat in Professor Preet Singh’s laboratory at Georgia Tech. Credit: Rob Felt, Georgia Tech

Application of the current moves electrons from the metal surface into the electrolyte, altering the surface texture and concentrating the chromium and molybdenum content.

To more fully assess the antibacterial effects, the team experimented, allowing bacterial samples to grow on treated and untreated stainless steel samples for periods of up to 48 hours. At the end of that time, the treated metal had significantly fewer bacteria on it. Mouse fibroblast cells, however, did not seem to be bothered by the surface.

For the future, the researchers plan to conduct long-term studies to make sure the mammalian cells remain healthy. The researchers also want to determine how well their nanotexturing holds up when subjected to wear.

“In principle, this is very scalable,” said Hess. “Electrochemistry is routinely applied commercially to process materials at a large scale.”

www.gatech.edu