CAP-BONE

The "Chemistry NAWAROS group" at BOKU University was amongst the first groups that started research on cellulosic aerogels as the "young, third generation" of aerogels. Basic studies aiming at the preparation of highly porous aerogels from different pulps revealed that the fragile, open-porous struc- ture of alcogels can be largely retained if supercritical carbon dioxide (scCO2) is used in the final drying step. This technique was later adapted for converting shaped bacterial cellulose (BC) aquogels into ultra-lightweight aerogels that quantitatively retained shape and porosity. Cellulose phosphate (CP) aerogels were prepared for the first time via the Lyocell approach and have been tested with regard to hemocompatibility, growth and differentiation of skeletal stem cells. Based on previous work, the proposed project is intended to a) study the intriguing surface effects that distinguish bacterial cellulose aerogels from those obtained by regenerating plant cellulose from solution, b) understand the distinct differences in retaining the fragile network structure during scCO2 dry- ing for the two types of aerogels, c) advance basic concepts (use of porogens, surfactants, templating, scCO2 antisolvent precipitation, chemical modification, cross-linking etc.) for tailoring the properties (porosity, aggregate microstructure, hemocompatibility, mechanical and chemical properties) of cellu- losic aerogels, d) to further develop analytical techniques for characterizing porous soft matter of such low densities (down to 5 mg cm-3), and e) to investigate the tailored cellulosic aerogels regarding their use in selected biomedical applications. The application of mechanically sufficiently stable, cellulose phosphate-based hemocompatible aerogels with spread porosity including a sufficient percentage of macropores with diameters in the range of 50 = x = 400 m as a novel cell scaffolding material for bone grafting is one main target of the proposed project. The envisaged work has been motivated by several recent findings: 1) cellulose phosphates can be safely processed to aerogels via the Lyocell route 2) cellulose phosphates are hemo- compatible and non-toxic in cultured human osteoblasts and fibroblasts, 3) cellulose phosphorylation (moderate DS only) is a pre-requisite to biomineralization, i.e. the formation of calcium deficient hydroxyapatite (cdHap), 4) moderate calcification activates blood platelets without inducing an inflammatory response, 5) calcified cellulose phosphates support robust growth and spontaneous os- teogenic differentiation of skeletal stem cells. Tailoring the properties of cellulosic aerogels for controlled release of bioactive compounds is a sec- ond main objective of the proposed work. Preliminary studies have shown that bioactive compounds can be homogenously loaded into cellulose aerogels by scCO2 antisolvent precipitation. Full retention of porosity and quantitative rewettability of BC aerogels render them promising matrices for con- trolled release application in wound treatment, skin care or drug dehabituation. Through a better understanding of the above-mentioned micro- structural differences and hitherto puzzling surface effects it is expected that aerogels from commercial pulps can also be used in a multitude of applications (catalysis, filters, separation techniques, etc.) beyond the controlled release of bioactive compounds.

Aerogels are cellular solids that feature very low density, high specific surface area and consist of a coherent open porous network of loosely packed, bonded particles or fibers whose voids are filled with gas. Their intriguing properties in terms of high interconnected nano porosity, accessible internal surface and stiffness-to-weight ratio render them interesting to many applications. Cellulosic aerogels are of particular interest in this respect, last but not least due to the renewability and low cost aspects of cellulose as most abundant biopolymer on earth. However, tailoring of cellulosic aerogels for high-end applications is still in its infancy, as systematic research in this field started not much earlier than about one decade ago. Cellulose phosphate aerogels have been recently demonstrated to be promising cell scaffolding materials for in vitro generation of bone tissue as their interconnected, spread porosity, particular nano morphology and surface chemistry along with the low immunogenic potential and good hemocompatibility creates a suitable environment for attachment, spreading, proliferation and differentiation of osteochondroprogenitor cells. Beyond bone repair, cellulose aerogels are promising materials for many other applications, such as controlled release of bioactive compounds. However drawbacks compared to competing materials like poor dimensional stability during conversion of the respective lyogels to aerogels or during long-term storage on air, insufficient network homogeneity, poor transparency, lack of micron-size porosity or insufficiently narrow pore size distribution hitherto prevented cellulose aerogels from various large-scale applications. The CABONE project therefore investigate fundamental correlations between process variables (cellulose type, cellulose solvent, dissolution conditions, cellulose anti-solvent, solvent exchange protocol and supercritical CO2 drying conditions) and morphological features (cellulose aggregate structure, pore size distribution and interconnectivity, etc.) of the obtained aerogels. X-ray scattering (SAXS, WAXS) and solid-state NMR experiments revealed a significant impact of the above process variables on the cellulose network structure in terms of crystallinity, skeletal density, fibril diameter and fractal dimension which, in turn, had a great influence on mechanical properties, dimensional stability during scCO2 drying and storage, and morphological features, such as porosity, pore size distribution and specific surface. In an attempt to improve cell attachment, spreading and proliferation for tissue engineering applications, a technology has been developed using packed beds of fused microspheres as temporary templates which allows for the preparation of dual-porous cellulose II aerogels. By variation of the type of porogen material (paraffin wax, PMMA), porogen particle size (various fractions between 100 and 500 m have been tested) and choice of cellulose solvent (ionic liquids, salt hydrate melts), the properties of the obtained dual-porous aerogels consisting of nanoporous cellulose struts embedding micron-size pores can be greatly tuned. As the introduction of macroporosity inevitably results in a considerably loss of mechanical stability and in an attempt to reduce their sensitivity towards humid atmosphere and shrinkage, different approaches for simultaneous reinforcement and hydrophobization by interpenetrating networks of secondary polymers (PLA, PCL, CA and PMMA) have been studied using bacterial cellulose as model substrate and a combined scCO2 anti-solvent precipitation (secondary polymer) / drying (hybrid aerogel) approach. The obtained results confirmed significant reinforcement of the respective cellulosic aerogels under full preservation of their open porous aerogel structure. Dual-porous aerogels reinforced by percolating networks of PMMA have been demonstrated to feature good biocompatibility as evident from cell viability, proliferation, attachment and morphology of cultivated fibroblast cells. The experiments on reinforcement of cellulosic aerogels have shown that cellulose aerogels can be used as temporary scaffolds to obtain aerogels entirely composed of a secondary polymer whose morphology largely resemble the structure of the cellulose aerogel which is eventually selectively extracted using an ionic liquid, for example. In an attempt to introduce additional anchor groups for reinforcement via covalent cross-linking, to develop an alternative material for surface-phosphorylated cellulose and to improve the bioresorbability and biomineralization of cellulose, aerogels have been also prepared from oxidatively modified cellulose. It turned out that the obtained 2,3-dialdehyde (DAC), 2,3-dicarboxyl (DCC) and 6-carboxyl cellulose (TOC) aerogels are very promising materials which could find use as matrix materials for novel static true 3D displays of unparalleled color gamut as they feature good transparency in the visible and near-infrared light range and allow for homogeneous grafting of up-converting quantum dots. DAC has been furthermore found to afford mechanically super-strong films of excellent oxygen barrier properties interesting for packaging applications. The CAPBONE project has also contributed to the development of analytical tools (e.g., solid-state NMR, SAXS, WAXS, micro-CT, thermoporimetry, spin-echo magnetic resonance imaging), allowing for a more reliable analysis of the interior of ultra-lightweight cellulosic aerogels. In this respect, the collaboration with the French partner institution was of enormous importance. The outcome of the CAPBONE project communicated in 10 peer-reviewed SCI journal papers (6 already published, 4 in preparation), 5 book chapters, 30 published contributions to international conferences as well as habilitation, doctoral, master, diploma and bachelor theses, was also a source of a series of new collaborations with partner institutions in Japan, France, Brazil, Germany, Finland, and the USA.