Stability of surface-enhanced ultrahydrophilic metals as a basis for bioactive rhBMP-2 surfaces

For several years the treatment of metals like cp titanium and 316L stainless steel with concentrated chromosulfuric acid at high temperatures (230-240degreesC) has formed the basis for preparing ultra-hydrophilic priming coats on these metals (Jennissen et al. Materialwiss. Werkstofftech. 30, 838-845, 1999). Metals treated in this way have been called surface-enhanced, displaying a characteristic ultrastructure, and can be easily modified to carry a biocoat of recombinant human bone morphogenetic protein 2 (rhBMP-2). The major oxide on surface enhanced titanium is TiO2. Thus this TiO2-layer could be responsible for the ultra-hydrophilic properties of the priming coat. Irradiation of TiO2 layers by ultraviolet light (Wang et al., Nature 388, 431-432, 1997) has been shown to endow these layers with ultra-hydrophilic properties (i.e. contact angles of similar to 0degrees). However the ultra-hydrophilic TiO2-layers produced by irradiation are unstable and revert to the original high contact angles of similar to 70degrees within several days. The question of whether the ultra-hydrophilic surfaces prepared by the chromosulfuric acid method show long-term stability was therefore important to answer. In addition the question if rhBMP-2 immobilized on such a surface will retain its biological activity was of great interest. In this paper it will be shown that ultrahydrophilic titanium mini-plates retain their ultra-hydrophilicity with contact angles of 0-8degrees unchanged for at least 50 days and support the immobilization of rhBMP-2 in a biologically active form.