Introduction: Calcium phosphate coating can potentially improve the bone compatibility of titanium (Ti). We previously reported an amorphous calcium phosphate (ACP) coating film fabricated using radiofrequency (RF) magnetron sputtering[1]. Evaluated in vivo, the ACP coating film showed high bioresorbability, and dissolution of this coating improved the bone biocompatibility of Ti[2].
Controlling the bioresorbability of the ACP coating film can extend its application to areas such as drug delivery systems (DDSs). It has been reported that niobium (Nb) stabilizes calcium phosphate glasses and decreases their resorption in simulated body fluids. Additionally, Nb ions enhance osteoblast differentiation and mineralization[3]. In this study, ACP coating films with various amounts of Nb were fabricated by RF magnetron sputtering. Subsequently, bonding strength between coating films and substrates, resorbability in simulated body fluid, and gene expression in cell culture were investigated.
Materials and Methods: Coating films were fabricated by RF magnetron sputtering on mirror-polished commercially pure (CP) Ti and blasted Ti-6Al-4V substrates (10×10×1 mm) using sintered target consisting of calcium phosphate and Nb2O5. The coating films were analyzed by ICP, XRD, Raman spectroscopy, and XPS. The bonding strength between the coating film and substrate was measured by the pulling test. Amount of ions eluted from the coating film in Tris buffer solution was determined. Quantitative analysis of gene expression from SaOS-2 cells was conducted by real-time PCR.
Results and Discussion: Nb content in coating films increased and Ca/P atomic ratio of coating films decreased with addition of Nb. It is speculated that Nb reached the substrate with higher probability compared with Ca and P during sputtering. Moreover, addition of Nb stabilized the ACP phase. Raman spectra of the Nb-containing ACP coating films showed that the addition of Nb caused an increase and decrease in NbO4 and (O-P-O)s(Q0) peak intensities, respectively. Thus, Nb probably acted as network former.
Bonding strength between the coating film and blasted substrate was >60 MPa, which is greater than that recommended in ISO13779-2 (>15 MPa). Figure 1 shows the effect of Nb content in the coating film on amounts of eluted ions after immersion of coating films in Tris buffer solution for 86.4 ks. Total amount of eluted ions from the coating films decreased with an increase in Nb content of the coating films. It was considered that the formation of NbO4 structure was one of the reasons for suppressing ion elution. The amount of eluted Nb ion was constant, independent of Nb content in the coating films.
Gene expressions such as ALP, COLI and TGF-β1 from Nb-containing ACP coating films were identical to that from non-coated substrate after a 7-day cell culture.
Conclusion: Nb ions were act as network former in the ACP coating film and controlled the bioresorbability of ACP coating films.
Grant-in-Aid for Young Scientists (A) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
References:
[1] T. Narushima; K. Ueda; T. Goto; H. Masumoto; T. Katsube; H. Kawamura; C. Ouchi; Y. Iguchi Mater. Trans. 2005, 46, 2246.
[2] K. Ueda; T. Narushima; T. Goto; M. Taira; T. Katsube Biomed. Mater. 2007, 2, S160.
[3] A. Obata; Y. Takahashi; T. Miyajima; K. Ueda; T. Narushima; T. Kasuga ACS Appl. Mater. Interfaces 2012, 4, 5684.