Because of its high specific strength, durability, and biocompatibility, titanium is a widely used material for orthopedic implants. However, its insufficient binding with the surrounding bone tissue is one of the leading causes of implant revision. Insufficient binding is mostly due to poor adhesion and growth of bone cells and tissue on the scaffold and micro-motions inhibiting the biological integration process. These phenomena eventually lead to stress shielding, bone resorption and implant loosening. Significant advances have been achieved over the past few decades; however, the complex interactions taking place at the bone-implant interface are not yet fully understood nor controlled. A promising solution to improve adhesion is to modify the implant surface chemistry and topography by coating it with a protein-eluting polyelectrolyte complex.
Bone morphogenetic protein 2 (BMP-2), a potent osteoconductive growth factor, was adsorbed onto the surface of anodized titanium, and polyelectrolyte multilayer (PEM) coatings prepared from solutions of poly-L-histidine (PLH) and poly(methacrylic acid) (PMAA) were built on top of the BMP-2. The effect of solution pH during the deposition process was investigated. High levels of BMP-2 released over several months were achieved. Approximately 2 μg/cm² of BMP-2 were initially adsorbed on the anodized titanium and a pH-dependent release behavior was observed, with more stable coatings assembled at pH = 6-7. Three different diffusion regimes could be determined from the release profiles: an initial burst release, a sustained release regime and a depletion regime. When trying to fit these regimes to various models, it appeared that pure diffusion could not explain the release behavior of the PEM coating, and that other mass transport phenomena were at work.