**5. Conclusions**

Mixtures of tetra-*n*-butylammonium fluoride, DMSO, and small quantities of water can be used as an efficient, non-derivatizing solvent to prepare sufficiently low viscous dopes of cellulose phosphates (DSP ca. 0.2) for further processing into lyogels and aerogels. At moderate dissolution conditions (60 ◦C, 4 h), this solvent system fully preserves the chemical integrity of the processed cellulosic materials. Care should be taken during work-up of the phosphorylated materials to ensure quantitative removal of non-reacted reagen<sup>t</sup> and solvent components. Compared to aerogels from non-derivatized cellulose, cellulose phosphate aerogels suffer considerably less from shrinkage. This is presumably due to repulsive forces being effective throughout the entire solvent exchange procedure except for the last scCO2 drying step. With respect to the latter, it was interesting to observe that cellulose phosphate aerogels require shorter drying time to ge<sup>t</sup> rid of adhering ethanol. On the other hand, slightly increased surface polarity of the phosphorylated aerogels gives rise to a somewhat more pronounced sensitivity towards moisture sorption upon long-term storage. However, this drawback would be irrelevant if the materials would be used as lyogels instead. This would bear the advantage of circumventing that fraction of shrinkage that occurs during the scCO2 drying step. On the other hand, this would be at the expense of the better sterilization opportunities for aerogels. It can be summarized that phosphorylation targeting a low degree of substitution has no negative impact on key aerogel properties. This includes bulk density, pore size distribution, specific surface, or pore volume. Skin formation upon cellulose coagulation, on the contrary, is evidently less pronounced for cellulose phosphate aerogels. This would be beneficial for the design of dual-porous cell scaffolding materials, since their nanoporous struts separating interconnected large micron-size voids are supposed to maintain rapid transport of gases, nutrients and metabolic byproducts.

**Author Contributions:** Conceptualization, F.L.; data curation, C.B.S. and M.-A.N.; funding acquisition, T.R.; investigation, C.B.S., P.S.P., J.-M.N., and M.W.; methodology, C.B.S., M.-A.N. and F.L.; project administration, F.L.; resources, M.-A.N., M.W., and T.R.; supervision, F.L.; visualization, C.B.S. and H.H.; writing—original draft, C.B.S.; writing—review and editing, H.H., T.R., and F.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The financial support by the University of Natural Resources and Life Sciences, Vienna (BOKU), through the BOKU DOC Grant 2008 (Funder Id: 10.13039/501100006380) to C.B.S. is gratefully acknowledged.

**Acknowledgments:** Part of this work was carried out in the frame of the COST-Action "Advanced Engineering of Aerogels for Environment and Life Sciences" (AERoGELS, ref. CA18125) funded by the European Commission. The authors would also like to thank Dr. Sonja Schiehser (Institute for Chemistry of Renewable Resources, BOKU), Johannes Amberger (Institute for Chemical and Energy Engineering, BOKU), and Adeline Hardy-Dessources (Université Clermont Auvergne, CNRS, SIGMA Clermont, ICCF; formerly ENSCCF, France) and Christian Jäger (BAM Bundesanstalt für Materialforschung und -testung) for SEC analyses, lab assistance, thermoporosimetry, and 31P NMR measurements, respectively.

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