*3.7. Collagen Synthesis*

Finally, we wanted to investigate whether the surface had any e ffect on the synthesis of collagen type I. Collagen is one of the main components of the extracellular matrix. Healthy tissues constantly undergo remodelling of their protein structures with the creation and digestion of new proteins by parenchymal cells, like fibroblasts. In tissue engineering, it is essential to have cells be able to model their environments by secreting ECM proteins, like collagen I.

Even though the adsorption was weakest to the Al-based hybrid and so was the cellular adhesion and proliferation, the synthesis of collagen in cells grown on the Al hybrid was the highest. The results are presented in Figure 7. The di fference is already visible after one week, but it becomes significant after 2 weeks. This is to be expected as the process of extracellular matrix remodelling takes a substantial amount of time.

**Figure 7.** Collagen synthesis. Relative amount of collagen produced by cells grown on the di fferent surfaces after 1 day, 1 week and 2 weeks. The results are presented as averages +/- standard errors. Statistically significant di fferences are indicated using di fferent symbols. §—*p* < 0.05 between 1 day and 1 week glass; <sup>±</sup>—*p* < 0.05 between 1 day and 2 week glass; @@@—*p* <0.001 between 1 day and 1 week Al; ###—*p* < 0.001 between 1 day and 2 week Al; \$\$—*p* < 0.01 between 1 day and 1 week Zr; %—*p* < 0.05 between 1 day and 2 week Zr; &&&—*p* < 0.001 between 1 week and 2 week Al; !!!—*p* < 0.001 between 2 week glass and 2 week Al; ˆˆˆ—*p* < 0.001 between 2 week Al and 2 week Zr. Other statistically significant di fferences (those between both di fferent materials and di fferent timepoints) are not presented.

Other statistically significant differences were observed between Al surfaces after 1 day, 1 week and 2 weeks with *p* < 0.001 between all periods. As well as Zr surfaces after 1 day and 1 week with a *p* < 0.05.

The results show promise for tissue engineering applications, because they show a significant increase in the collagen produced in the cells grown on these surfaces. This means that the cells had not only attached to the surfaces, but they are comfortable and viable enough to start remodelling their environment via collagen synthesis.
