Cross-sectional transmission electron microscope image of the functionally graded smart coating with nano-silver particles distributed throughout implant coating. Credit: NCSU

Cross-sectional transmission electron microscope image of the functionally graded "smart" coating of hydroxyapatite with nano-silver particles distributed throughout. Credit: NCSU.

A team of North Carolina State University and Oak Ridge National Lab researchers have published a new paper that reports on the possibility of using hydroxyapatite layers seeded with silver particles – customized for each patient – as a coating on joint and bone replacements to help ward off infection.

The interesting idea the group is promoting is to combine two already-known concepts (the benefits of hydroxyapatite to counter rejection while promoting healing and the antimicrobial property of silver) with ion beam-assisted deposition technology to apply varying layers of hydroxyapatite–silver mix.

They tested coatings of functionally graded hydroxyapatite impregnated with nano silver particles (10–50 nm). They report that the amount of Ag (wt.%) on the outer surface of the FGHA ranged from 1.09 to 6.59, which was about half of the average Ag wt.% incorporated in the entire coating.

The group, which published their findings in Acta Biomaterialia, considers their innovation a “smart” material for two reasons. Afsaneh Rabiei, an NCSU associate professor of mechanical and aerospace engineering and one of the paper’s authors explains the first reason saying, ““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,” she says. Rabiei, who is also an associate faculty member of biomedical engineering, notes that this is important because people have very different rates of bone growth (e.g., young people’s bones tend to grow far faster than the bones of older adults).

The second reason they consider it to be a smart material is the variable rate of silver release. According to an NCSU news release, Rabiai says the hydroxyapatite coating allows the silver to be released rapidly after surgery, when there is more risk of infection, due to the faster dissolution of this amorphous layer of the coating. Conversely, the release of silver will continue for the life of the implant but will slow down as the patient heals.

The group also reports adhesion strengths comparable to FGHA without silver. The dominant failure mechanism was epoxy failure, and they report no observations of coating delamination.