Nanotech Surfaces Reduce Implant Bacteria

The engineers took nanoparticles of zinc oxide (ZnO), a well-known antimicrobial agent, and titanium oxide (TiO), which readily forms on titanium once implanted, and pressed them into dime-sized discs, with two types of surfaces: microstructured surfaces with surface irregularities that measured about five microns in diameter, and nanostructured surfaces that had surface irregularities that measured only 0.023 microns in diameter.

The engineers then put Staphylococcus epidermidas on the discs, waited an hour, and then counted bacterial growth. The microstructured ZnO discs were host to 1,000 times more bacteria than the nanostructured zinc oxide discs. Similar, but less striking, results were duplicated on the titanium oxide discs. The engineers then conducted similar experiments with bone-forming cells, and found that twice as many of these cells grew and formed bone on nanostructured discs. Other indicators of healthy bone growth, such as collagen synthesis, were also stronger with nanostructured discs. The results were published in the July 2, 2006, online issue of the Journal of Biomedical Materials Research.

The key factor, according to the researchers, seems to be surface area. The nanostructured surfaces increased the surface area by 25 to 35%. This additional area, along with the unique surface energetics of the nanomaterials, gave bone-forming cells more places to adhere. However, with bacteria, increased surface area may work the other way, exposing the cells to more of the germicidal properties of the zinc oxide.

"We've found a method of coating implants that discourages bacteria growth,” said lead author Thomas Webster, now an associate professor of engineering at Brown University (Providence, RI, USA), "and it does so significantly. The hope is that this technique will lead to safer, longer-lasting implants.”

Millions of people worldwide undergo hip, knee, and shoulder replacement procedures every year. In about 1-2% of cases, the implant gets infected, most commonly by S epidermidas, which enters a surgical wound and adheres to an implant. The bacteria multiply, forming a slimy, antibiotic resistant biofilm around the implant, usually resulting in additional surgery to clean the implant or replace it outright.



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