Nanostructured Glass–ceramic Coatings For Orthopaedic Applications-Part 5
3.6. Quantitative real-time polymerase chain reaction
Figure 11 shows the expression levels of bone-related genes (Runx-2, OPN, type I collagen and BSP) in relation to the house-keeping gene (GAPDH) after 1 and 7 days of culture. At 1 day, Runx-2 mRNA gene expression was higher in HOBs cultured on the SP coatings and Ti-6Al-4V discs, compared with that on HT coatings. At 7 days, Runx-2 expression level on HT coatings caught up with that on Ti-6Al-4V, while HOBs on SP coatings showed the highest Runx-2 expression level (figure 11a). No significant differences in BSP gene expression were found in HOBs cultured on HT and SP coatings, and Ti-6Al-4V discs at both time points (figure 11b). OPN gene expression level in HOBs on both types of coatings, especially on HT coatings, was higher than that on Ti-6Al-4V discs at both time points (figure 11c). Higher expression levels of type I collagen were observed in HOBs cultured on both coatings, compared with Ti-6Al-4V discs at day 7 (figure 11d). Collectively, these data indicate that both HT and SP coatings support the differentiation of HOBs.
Figure 6. Variation of the relative percentage of compositional elements of (a) HT and (b) SP coatings, (c) their Si/Ca molar ratio and (d) pH value changes of HCl –Tris-buffered solution after the immersion of coatings. Asterisk represents significant difference; p-value＜ 0.05. (a–c) Grey bars represent before immersion and black bars represent after immersion and (d) filled diamonds represent SP and filled squares represent HT.
Figure 7. Surface morphology of (a,b) the HT coatings after incubation in cell-free culture medium for 5 h and the EDS of (c) the HT coating before incubation and (d) the deposits on its surface after incubation. Scale bars, (a) 50 mm and (b) 10 mm. (Online version in colour.)
In this work, HT and SP coatings were fabricated using atmospheric plasma spray technique. Both types of coatings exhibited glass –ceramic structure and had nanostructured surfaces due to the high temperature and the super-high cooling rate of the plasma spray process [3,35,36]. The bonding strength of SP coating was superior to that of HT coating; and both were higher compared with the reported values of plasma-sprayed HAp coating [37–39]. Coefficient of thermal expansion is an important factor influencing the quality of the coatings including the formation of cracks, residual stress and bonding strength. The coefficients of HT and SP ceramics and Ti-6Al-4V alloy were reported to be 11.2× 10-6 K-1 , 6 × 10-6 K-1 and 8.4 – 8.8 × 10-6 K-1 , respectively, while that of HAp coatings is around 15.2× 10-6 K-1 . The close match of the coefficients of thermal expansion of the HT and SP coatings to that of Ti-6Al-4V alloy contributed to their higher bonding strength compared with HAp coatings. The superior bonding strength of the SP compared to HT coatings is possibly due to the presence of Ti in the SP coating which may enhance the chemical and diffusion bonding between SP coatings and the underlying Ti-6Al-4V . Zheng et al.  demonstrated that the bonding strength of the plasma-sprayed HAp coatings was greatly improved using mixed powder feedstock of Ti and HAp. Besides bonding strength, hardness is another important parameter for biomedical coatings as it affects their anti-wear properties. The hardness of our developed coatings was higher compared with those of thermal sprayed HAp despite that our coatings were tested under higher load, as can be seen in table 4. Actually, the hardness of these two coatings is also comparable to those of HAp ceramic blocks (also seen in table 4).