Enhanced loading and controlled release of rhBMP-2 in thin mineralized collagen coatings with the aid of chitosan nanospheres and its biological evaluations

Abstract

Loading of an appropriate amount of rhBMP-2 and avoiding its “burst-release” are key challenges for upgrading the biological performance of thin bioactive coatings on metal implants. In this study, we adopted incorporation of chitosan nanospheres into thin mineralized collagen coatings to enhance rhBMP-2 loading and improve releasing behavior based on the good affinity of chitosan for proteins and the large surface area of nanospheres. We realized the incorporation process by electrophoretic injection. Using chitosan nanospheres, we were able to increase the rhBMP-2 loading amount in the thin coating by 2.7-fold (from 446 ng cm⁻² to 1186 ng cm⁻²), and showed that the rhBMP-2 exhibited sustained release behavior. MC3T3-E1 cells cultured on the rhBMP-2/chitosan nanosphere-incorporated thin coatings (Col/Cs/BMP) showed good cell attachment and proliferative behavior, and high levels of differentiation and mineralization. In in vivo tests, spiral CT analysis and histological observations demonstrated that Col/Cs/BMP coatings on metal implants were able to increase bone density and accelerate new bone growth after implantation for 4 to 8 weeks, and the boundary between host bones and new bones disappeared after implantation for 8 weeks. The pull-out tests further confirmed that the Col/Cs/BMP coatings could significantly enhance osseointegration. The present results indicate that incorporation of chitosan nanospheres into thin coatings is an effective way to enhance the loading amount properly, improve the release behavior of rhBMP-2 and finally accelerate the osseointegration process.