Coating a titanium implant with a biologic material enhances tissue healing, improves bone growth around the implant, and strengthens the attachment. “We designed a coating that, in effect, tells cells to grow bone around the implant,” says Andrés Garcia, professor at Georgia Tech’s Woodruff School of Mechanical Engineering and the Petit Institute for Bioengineering and Bioscience.

Conventional clinical practice has been to roughen the surface of a titanium implant or coat it with a flaky, hard-to-apply ceramic that bonds directly to bone. Total knee and hip replacements typically last about 15 years when the components may wear down or loosen. For younger patients, this means expecting a second surgery to replace the first artificial joint. About 40% of the 712,000 total hip and knee replacements in the United States in 2004 are performed on patients 45 to 64 years old.

“The coating consists of high-density polymer strands, akin to bristles on a toothbrush that we can modify into a bioactive protein,” says Garcia. In this case, the polymer amounts to controlled amounts of an engineered protein that mimics fibronectin, a protein in the body that acts as a binding site for cell surface receptors called integrins.

Garcia says it is important to control integrins binding to a titanium implant because integrins provide signals that direct bone formation. A previous method for binding integrins to titanium required coating the metal with a small biological signal containing arginine-glycine-aspartic acid (RGD). However, a region on fibronectin alone binds many different integrin receptors but with much less affinity than the full fibronectin protein.

“It has been common to mimic small sections of fibronectin, but when you take a small section and ignore the rest of the molecule you lose activity, so the signal is impaired,” says Garcia. For that reason, he has engineered a longer region of a fibronectin molecule that included the RGD peptide sequence as well as sections known to have sites that participate in integrin binding.

To evaluate the performance of the coated titanium in bone healing, Garcia’s team members coated the surfaces of small titanium cylinders and modified them with peptide sequences. Holes were drilled into a rat’s tibia and the cylinders pressed in. The researchers tested three types of coatings: uncoated titanium, titanium coated with the RGD peptide, and titanium coated with different densities of the engineered fibronectin fragment. Analysis four weeks later showed extensive and contiguous bone matrix and a 70% enhancement in the amount of contact between the implant and bone for the titanium implants coated with the engineered fibronectin fragment versus uncoated or RGD-coated titanium.

Georgia Tech professors David Collard (left) and Andrés Garcia display a piece of titanium coated with a thin, dense polymer that enhances bone formation around the metal after implantation

PICTURE Research at Georgia Tech shows that coating a titanium implant with a new biologically inspired material enhances tissue healing and improves bone growth around and attachment to the implant.