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 (230‐240 °C) 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 ∼ 0°). However the ultra‐hydrophilic TiO2‐layers produced by irradiation are unstable and revert to the original high contact angles of ∼ 70° 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 0‐8° unchanged for at least 50 days and support the immobilization of rhBMP‐2 in a biologically active form.