Enhancement of osteoblast adhesion to UV-photofunctionalized titanium via an electrostatic mechanism

The mechanism underlying the recently found photofunctionalization of titanium is unknown. We focused on how the initial interaction between the cells and photofunctionalized titanium is enhanced at a molecular-level and the role played by the electrostatic status of the titanium surfaces in the possible regulatory mechanism for determining their bioactivity. Rat bone marrow-derived osteoblasts were cultured on untreated and ultraviolet (UV)-treated titanium surfaces. UV treatment converted the titanium surfaces from hydrophobic to superhydrophilic. The number of osteoblasts attached to UV-treated titanium surfaces was substantially greater than that attached to untreated surfaces (5-fold and 2-fold after 3 and 24 h of incubation, respectively). Osteoblasts cultured for 3 and 24 h on these titanium surfaces were detached mechanically by vibrational force and enzymatically by trypsin treatment. Cell adhesion evaluated by the percentage of remaining cells after these detachments was substantially greater for cells on UV-treated titanium surfaces compared to untreated titanium surfaces (110–120% greater for cells incubated for 3 h and 50–60% greater for cells incubated for 24 h). Osteoblasts on UV-treated surfaces expressed more vinculin. UV-enhancing effect in cell adhesion was also demonstrated under a serum-free condition. UV-enhanced cell adhesion was abrogated when the UV-treated titanium surfaces were electrostatically neutralized by either removing the electric charge or masking with monovalent anions, while the surfaces maintained superhydrophilicity. In conclusion, the establishment of osteoblast adhesion is accelerated and augmented remarkably on UV-treated titanium surfaces, associated with upregulated expression of vinculin. This study has identified an electrostatic property of UV-treated titanium surfaces playing a regulatory role in determining their bioactivity, superseding the effect of the hydrophilic nature of these surfaces. A mechanism underlying the UV-induced conversion of titanium from bioinert to bioactive, in which direct cell–titanium interaction is exclusively enabled, is proposed.