**4. Conclusions**

We believe that the materials investigated in this work may be useful in constructing tissue engineered grafts of the future. This study provides a direct comparison between two materials that can be structured for laser two-photon polymerisation, assessing their support of primary rat myogenic cell growth. The differences between Zr and Al surfaces could potentially be attributed to a difference in adsorption of collagen, which in turn affects cellular adhesion strength, proliferation and the rate of extracellular matrix remodelling, but the results to support this are insignificant, suggesting a more thorough investigation into this process should be carried out.

An additional in vitro study should be performed in the future in order to evaluate how the adhesion-associated protein expression and activation changes over longer periods of time (several days or a couple of weeks) and whether that impacts cellular adhesion, viability and metabolism in a significant way.

Future studies in tissue engineering are likely to focus on integrating several materials with different properties in order to precisely guide cellular behaviour as well as investigating the role that artificial 3D niches with different geometries might play in affecting cellular differentiation and tissue maturation. As such, an extra option in the form of Al-containing hybrid organometallic polymer, presented in this work, is a choice, judging by its biocompatibility demonstrated in this study. In conjunction with its other advantages, like grea<sup>t</sup> 3D structuring capabilities, we believe the materials described in this work have potential to add value for the field of biomedical engineering.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/23/3932/s1, Figure S1: A curve of collagen I content vs Sirius Red absorbance at 550 nm. An average of three independent experiments with n = 3 in each is presented ± standard deviation. Regression analysis was performed, showing that the dependence is linear in the measurement range (0–100 μg) with R<sup>2</sup> = 0.994 and f(x) = 0.0199x−0.0322, Figure S2: Characterization of rat skeletal muscle-derived progenitor cells. Fluorescent microscope images of cells immunostained for CD45, CD34, c-kit and Myf5. The scale bars correspond to 100 microns, Figure S3: Representative images of cells grown on glass, Al and Zr surfaces after 24, 48, 72 and 96 h. The scale bars correspond to 100 microns, Figure S4: Flow cytometry data on cellular viability on Glass, Al and Zr surfaces after 24, 48, 72 and 96 h. The results are presented as average absorbance +/- standard deviation, derived from 3 independent experiments with n = 3 per experiment. No statistically significant differences were found between the surfaces, Figure S5: Images of the Western blot membranes, showing the expression levels of p-Akt (Ser473), p-Akt (Thr308), Akt and FAK proteins. Tubulin was used as a reference. From left to right: 1st membrane: ladder, Al(4 h), Al(24 h), Zr(4 h), Zr(24 h), Glass(4 h), Glass(24 h), Al(4 h), Al(24 h), Zr(4 h); 2nd membrane: ladder, Zr(24 h), Glass(4 h), Glass(24 h), Al(4 h), Al(24h), Zr(4 h), Zr(24 h), Glass(4 h), Glass(24 h).

**Author Contributions:** Conceptualization, E.B. and D.B.; methodology, E.B., S.J.B., J.G.H., D.B.; formal analysis, all authors; investigation, all authors; resources, J.G.H. and D.B.; data curation, D.B.; writing—original draft preparation, all authors; writing—review and editing, E.B., J.G.H. and D.B.; supervision, J.G.H. and D.B.; project administration, D.B.; funding acquisition, J.G.H. and D.B.

**Funding:** This research was funded by INFOBALT Lithuania and the World Federation of Scientists. The work was supported by the European Commission via the Marie Skłodowska-Curie research fellowship programme AngioMatTrain (Grant agreemen<sup>t</sup> 317304), and a Lancaster University Faculty of Science and Technology Early Career Internal Grant, a Royal Society Research Grant (RG160449), an EPSRC First Grant (EP/R003823/1), and the Research Council of Lithuania (Grant No. SEN-13/2015).

**Conflicts of Interest:** The authors declare no conflict of interest.
