Researchers at North Carolina State University have developed what they call a “smart coating” for helping surgical implants bond more closely with bone and ward off infection.

When patients undergo hip, knee, or other replacement surgery, their bodies can reject the implants. But the smart coating mitigates that risk by fostering bone growth into the implant. The coating creates a crystalline layer next to the implant and a mostly amorphous outer layer that touches the surrounding bone. This amorphous layer dissolves over time, releasing calcium and phosphate and thus encouraging bone growth.

“The bone grows into the coating as the amorphous layer dissolves, resulting in improved bonding, or osseointegration,” remarks Afsaneh Rabiei, an NC State associate professor of mechanical and aerospace engineering, associate faculty member of biomedical engineering, and coauthor of a paper describing this research. The bonding also makes the implant more functional because it helps ensure that the bone and the implant do a better job of sharing the load.

“We call it a smart coating because we can tailor the rate at which the amorphous layer dissolves to match the bone growth rate of each patient,” Rabiei says. This is important because people have very different rates of bone growth. For example, young people’s bones tend to grow far faster than the bones of older adults.

Currently, implant patients are subjected to an intense regimen of antibiotics to prevent infection immediately following surgery. However, the site of the implant will always remain vulnerable to infection. But by incorporating silver nanoparticles into the coating, the researchers harness silver particles’ capability to act as antimicrobial agents as the amorphous layer dissolves, according to Rabiei.

This feature will not only limit the amount of antibiotics patients require following surgery but will also provide protection from infection at the implant site for the life of the implant. Moreover, the silver is released more quickly right after surgery, when there is more risk of infection, because of the faster dissolution of the amorphous layer of the coating. Silver release slows down while the patient is healing. “That is another reason why we call it smart coating,” Rabiei explains.

Funded by the National Science Foundation (Arlington, VA) with assistance from the Center for Nanophase Materials Sciences and Shared Research Equipment User Facilities at Oak Ridge National Laboratory (Oak Ridge, TN), the research, “Functionally Graded Hydroxyapatite Coatings Doped with Antibacterial Components,” appears online in Acta BioMaterialia.