Nanomodified Surfaces Seal Leg Implants Against Infection

An innovative study demonstrates that nanometer-sized Titanium (Ti) topographies can be useful in increasing skin density while reducing bacteria adhesion, thus improving the efficacy of transcutaneous Ti-based orthopedic implants.

Researchers at Brown University (Providence, RI, USA;) have developed two ways in which to modify the surface of Ti leg implants to promote skin cell growth, thereby creating a natural skin layer and sealing the gap where the device has been implanted into the body. The first approach involves an electron beam of titanium coating fired at the implant abutment, creating a landscape of 20-nanometer mounds. These mounds imitate the contours of natural skin and trick skin cells into colonizing the surface and growing additional keratinocytes.

The second approach is an anodization process, which involves dipping the abutment into hydrofluoric (HF) acid and passing a jolt of electric current through it. This causes the Ti atoms on the abutment's surface to disperse and then regroup as hollow, tubular structures rising perpendicularly from the abutment's surface; as with the mounds, keratinocytes can then quickly colonize the nanotubular surface. In laboratory tests, the researchers reported nearly a doubling of skin cell density on the implant surface; within five days, the keratinocyte density reached the point at which an impermeable skin layer bridging the abutment and the body had been created.

To further promote skin cell growth around the implant, the researchers developed a synthetic molecular chain to bind fibroblast growth factor 2 (FGF-2)--a single-chain polypeptide that plays a significant role in the process of wound healing and angiogenesis--to the Ti surface, while maintaining the protein's keratotic ability. In vitro tests again showed the greatest density of skin cells on abutment surfaces using the nanomodified surfaces laced with FGF-2. Moreover, the nanomodified surfaces created more surface area for FGF-2 proteins than would be available on traditional implants. The study was published ahead of print on February 11, 2011, in the Journal of Biomedical Materials Research A.

"You need to close the area where the bacteria would enter the body, and that's where the skin is,” said lead author Thomas Webster, PhD, an associate professor of engineering and orthopedics at Brown University. "You definitely have a complete layer of skin; there's no more gap for the bacteria to go through.”

Since Ti is biocompatible, it is used extensively in medical applications, including surgical implements and implants. The Ti is often alloyed with about 4% aluminum (Al), or 6% Al and 4% vanadium. Titanium also has the inherent property to osseointegrate, enabling use in dental implants; this property is also useful for orthopedic implant applications, which benefit from titanium's lower modulus of elasticity to more closely match that of the bone. As a result, skeletal loads are more evenly shared between bone and implant, leading to a lower incidence of bone degradation due to stress shielding and periprosthetic bone fractures, which occur at the boundaries of orthopedic implants.

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