pH-controlled release of proteins from polyelectrolyte-modified anodized titanium surfaces for implant applications

Titanium is a popular choice of implant material given its strength, durability, and biocompatibility; however, strong interfaces with the surrounding tissue are not achieved, resulting in stress shielding and implant loosening. One option for improving adhesion is modification of the surface chemistry and topography through anodization, while another option is coating the titanium surface with a protein-eluting polyelectrolyte complex. Morphogenetic proteins such as BMP-2 have been shown to cause cell migration, expression of different genes, and development of different tissues. Anodization was used to form a porous oxide structure across the surface. A polyelectrolyte coating of poly-l-histidine and poly(methacrylic acid) was prepared and was shown to be effective for sustained release of negatively charged species under physiological conditions. This complex demonstrated pH-dependent release, with maximum release at pH = 5-6, but low levels of sustained release at pH = 7-8. Smaller initial burst release and higher amounts of sustained release were observed when lower molecular weight poly(methacrylic acid) was used. Different methods of loading the polyelectrolyte with the model species were compared. Immersion of the coating for loading provided greater release, albeit a larger initial burst release.