Biomimetic Hydroxyapatite Coating on Metal Implants

TLDR

Hydroxyapatite coatings on titanium implants combine metal strength with osteoconductivity, yet plasma spraying cannot coat porous implants, causes delamination, and releases particles. The study aims to develop a dense, strong, thick calcium‑phosphate coating on titanium and porous‑tantalum implants via a two‑step biomimetic process. The coating is produced by first soaking implants in a 5× concentrated SBF‑A solution, then immersing them in an SBF‑B solution with reduced Mg²⁺ and HCO₃⁻, while monitoring pH and coating thickness over time. The process yields a thin, uniform amorphous layer followed by rapid precipitation of a 30 µm crystalline coating that closely resembles bone mineral, promoting rapid bone formation and potentially shortening postoperative recovery.

Abstract

The combination of the high mechanical strength of metals with the osteoconductive properties of calcium phosphates make hydroxyapatite coatings on titanium implants widely used in orthopedic surgery. However, the most popular coating method, plasma spraying, exhibits some important drawbacks: the inability to cover porous implants and to incorporate biologically active agents, delamination, and particle release. The aim of this study was to elaborate a dense, strong, and thick calcium‐phosphate coating on titanium and porous‐tantalum implants using a two‐step biomimetic procedure. In the first step, the implants were soaked in a solution that was 5 times more concentrated than regular simulated body fluid (SBF‐A solution). A thin but uniform amorphous calcium‐phosphate coating was deposited on the metal. Then, the implants were immersed in the SBF‐B solution, which had a similar composition as the SBF‐A solution, but with decreased contents of crystal growth inhibitors (i.e., Mg 2+ and HCO 3 − ). This resulted in the fast precipitation of a 30 μm thick crystalline calcium‐phosphate coating. The pH of the SBF‐B solution and the thickness of the crystalline coating layer were studied as a function of time. The Fourier transform infrared spectra and X‐ray diffraction patterns showed that this new coating closely resembles bone mineral. Our biomimetic coating should facilitate rapid bone formation around the implant, reducing therewith the patient's recovery time after surgery.