Effect of pH on polyelectrolyte multilayer formation and growth factor release

SALVI, Claire

Etd

Effect of pH on polyelectrolyte multilayer formation and growth factor release

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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 regularly leads to stress shielding, bone resorption and implant loosening. 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. Mass adsorption monitoring using quartz crystal microbalance with dissipation monitoring (QCM-D) showed that PLH was adsorbed in greater quantities than PMAA, and that more mass was adsorbed per bilayer as the number of bilayers grew. Moreover, the pH of the water used during the rinsing step significantly impacted the composition of the multilayer. Atomic force microscopy (AFM) and contact angle analysis (CAA) were used to determine the topography and surface energy of the PEMs. No visible change was observed in surface morphology as the assembly pH was varied, whereas the surface energy decreased for samples prepared at more basic pH. These variations indicate that the influence of the initial BMP-2 layer can be felt throughout the PEM and impact its surface structure.

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