Effect of Assembly pH on Polyelectrolyte Multilayer Surface Properties and BMP-2 Release

The effect of solution pH during layer-by-layer assembly of polyelectrolyte multiayer (PEM) coatings on properties relevant to orthopedic implant success was investigated. Bone morphogenetic protein 2 (BMP-2), a potent osteoconductive growth factor, was adsorbed onto the surface of anodized titanium, and PEM coatings prepared from solutions of poly-L-histidirie and poly(methacrylic acid) were built on top of the BMP-2. High levels of BMP-2 released over several months were achieved. Approximately 2 yg/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. BMP-2 was showii,to maintain bioactivity after release from a PEM and the presence of a PEM was shown to preserve BMP-2 structure. No visible change:was observed in surface roughness as the assembly pH was varied, whereas the surface energy decreased for samples prepared at more -basic pH. These results indicate that the initial BMP-2 layer affects PEM surface structure, but notthe fiinctional groups exposed on the surface.