Surface scanning electron micrographs of (a) not peened (NP), (b) conventional shot peened (CSP), and (c) severe shot peened (SSP) 316L stainless steel samples; atomic force microscope topographical images of (d) NP, (e) CSP, and (f) SSP samples.A simple process that roughens the surface and alters the grain size of metallic biomedical implants could deter the bacteria that cause infections and complications after surgery, according to researchers from Politecnico di Milano, Massachusetts Institute of Technology, Northeastern University, University of Cambridge, and King Abdulaziz University [S. Bagherifard et al., Biomaterials (2015), DOI: 10.1016/j.biomaterials.2015.09.019].
Stainless steel is widely used for medical devices and weight-bearing bone implants where its surface roughness and grain structure are known to have a profound effect on cell function. In fact, mechanical cues like these can have a greater effect than chemical ones on bacterial adhesion and the formation of undesirable bacterial colonies known as biofilms.
“The growing resistance of bacteria to conventional antibiotics, the need to develop advanced orthopedic implants with improved biocompatibility, along with the necessity of using a mechanically strong material able to withstand physiological strains and stresses, gave us the impetus for the development of advanced materials for bone implants,” explains Sara Bagherifard of Politecnico di Milano.
She and her colleagues transformed 316L stainless steel using a plastic deformation-based treatment called severe shot peening (SSP) in which the surface is bombarded with high-energy shots made of stainless steel, ceramic, or glass using compressed air. SSP increases surface roughness by creating overlapping indentations without restricting surface nanocrystallization or inducing chemical changes. The approach reduces grain size (from 63 µm to 25 nm) while increasing wettability, work hardening, and compressive residual stresses.
When shot-peened surfaces are exposed to bone-forming cells (or osteoblasts) and common strains of bacteria, the results are surprising. While the increase in surface roughness has little effect on the adhesion and proliferation of osteoblasts, the bacteria that cause most post-operative infections, Staphylococcus aureus and Staphylococcus epidermidis, show a remarkable decrease in adhesion and growth.
“The antibacterial effect of surface roughness and its potential ability to reduce the risk of biofilm formation without the use of antibiotics is of the utmost importance,” says Bagherifard.
The researchers believe the antibacterial effect can be put down to the scale of surface irregularities, which are comparable to the size of bacteria. The roughness seems to limit anchoring points for bacteria and reduces the area in contact with their membrane. This could also explain why other types of bacteria that have an extra outer membrane, such as Pseudomonas aeruginosa and Escherichia coli, appear largely unaffected. The reduction in grain size also influences cell morphology and enhances the spread of osteoblasts, while improving the performance and durability of load-bearing orthopedic implants.
“We believe this implant surface modification process is quite a breakthrough,” says Bagherifard, “because it is easy and can be readily adopted by industry, providing immediate solutions for patients.”