Hydrophobic Modification of Pectin Aerogels via Chemical Vapor Deposition.

Pectin aerogels, with very low density (around 0.1 g cm-3) and high specific surface area (up to 600 m2 g-1), are excellent thermal insulation materials since their thermal conductivity is below that of air at ambient conditions (0.025 W m-1 K-1). However, due to their intrinsic hydrophilicity, pectin aerogels collapse when in contact with water vapor, losing superinsulating properties. In this work, first, pectin aerogels were made, and the influence of the different process parameters on the materials' structure and properties were studied. All neat pectin aerogels had a low density (0.04-0.11 g cm-1), high specific surface area (308-567 m2 g-1), and very low thermal conductivity (0.015-0.023 W m-1 K-1). Then, pectin aerogels were hydrophobized via the chemical vapor deposition of methyltrimethoxysilane using different reaction durations (2 to 24 h). The influence of hydrophobization on material properties, especially on thermal conductivity, was recorded by conditioning in a climate chamber (25 °C, 80% relative humidity). Hydrophobization resulted in the increase in thermal conductivity compared to that of neat pectin aerogels. MTMS deposition for 16 h was efficient for hydrophobizing pectin aerogels in moist environment (contact angle 115°) and stabilizing material properties with no fluctuation in thermal conductivity (0.030 W m-1 K-1) and density for the testing period of 8 months.

Creating and exploring carboxymethyl cellulose aerogels as drug delivery devices.

Carboxymethyl cellulose (CMC) is a well-known cellulose derivative used in biomedical applications due to its biocompatibility and biodegradability. In this work, novel porous CMC materials, aerogels, were prepared and tested as a drug delivery device. CMC aerogels were made from CMC solutions, followed by non-solvent induced phase separation and drying with supercritical CO2. The influence of CMC characteristics and of processing conditions on aerogels' density, specific surface area, morphology and drug release properties were investigated. Freeze-drying of CMC solutions was also used as an alternative process to compare the properties of the as-obtained "cryogels" with those of aerogels. Aerogels were nanostructured materials with bulk density below 0.25 g/cm3 and high specific surface area up to 143 m2/g. Freeze drying yields highly macroporous materials with low specific surface areas (around 5-18 m2/g) and very low density, 0.01 - 0.07g/cm3. Swelling and dissolution of aerogels and cryogels in water and in a simulated wound exudate (SWE) were evaluated. The drug was loaded in aerogels and cryogels, and release kinetics in SWE was investigated. Drug diffusion coefficients were correlated with material solubility, morphology, density, degree of substitution and drying methods, demonstrating tuneability of new materials' properties in view of their use as delivery matrices.

The European Polysaccharide Network of Excellence (EPNOE) research roadmap 2040: Advanced strategies for exploiting the vast potential of polysaccharides as renewable bioresources.

Polysaccharides are among the most abundant bioresources on earth and consequently need to play a pivotal role when addressing existential scientific challenges like climate change and the shift from fossil-based to sustainable biobased materials. The Research Roadmap 2040 of the European Polysaccharide Network of Excellence (EPNOE) provides an expert's view on how future research and development strategies need to evolve to fully exploit the vast potential of polysaccharides as renewable bioresources. It is addressed to academic researchers, companies, as well as policymakers and covers five strategic areas that are of great importance in the context of polysaccharide related research: (I) Materials & Engineering, (II) Food & Nutrition, (III) Biomedical Applications, (IV) Chemistry, Biology & Physics, and (V) Skills & Education. Each section summarizes the state of research, identifies challenges that are currently faced, project achievements and developments that are expected in the upcoming 20 years, and finally provides outlines on how future research activities need to evolve.

Hyaluronic Acid Aerogels Made Via Freeze-Thaw-Induced Gelation.

The biodegradability, biocompatibility, and bioactivity of hyaluronic acid (HA), a natural polysaccharide, combined with the low density, high porosity, and high specific surface area of aerogels attract interest for biomedical applications such as wound dressings. In this work, physically cross-linked HA aerogels were prepared via the freeze-thaw (FT) induced gelation method, solvent exchange, and drying with supercritical CO2. The morphology and properties of HA aerogels (volume shrinkage, density, and specific surface area) were investigated as a function of several process parameters: HA concentration, solution pH, number of FT cycles, and type of nonsolvent used during solvent exchange. We demonstrate that the HA solution pH plays a key role in the aerogel formation, as not all conditions result in materials with high specific surface area. HA aerogels were of low density (<0.2 g/cm3), high specific surface area (up to 600 m2/g), and high porosity (≥90%). Scanning electron microscopy pictures revealed that HA aerogels present a porous structure with meso- and small macropores. The results show that HA aerogels are promising biomaterials with tunable properties and internal structure that offer high potential as, e.g., wound dressings.

Release Kinetics of Dexamethasone Phosphate from Porous Chitosan: Comparison of Aerogels and Cryogels.

Porous chitosan materials as potential wound dressings were prepared via dissolution of chitosan, nonsolvent-induced phase separation in NaOH-water, formation of a hydrogel, and either freeze-drying or supercritical CO2 drying, leading to "cryogels" and "aerogels", respectively. The hydrophilic drug dexamethasone sodium phosphate was loaded by impregnation of chitosan hydrogel, and the release from cryogel or aerogel was monitored at two pH values relevant for wound healing. The goal was to compare the drug-loading efficiency and release behavior from aerogels and cryogels as a function of the drying method, the materials' physicochemical properties (density, morphology), and the pH of the release medium. Cryogels exhibited a higher loading efficiency and a faster release in comparison with aerogels. A higher sample density and lower pH value of the release medium resulted in a more sustained release in the case of aerogels. In contrast, for cryogels, the density and pH of the release medium did not noticeably influence release kinetics. The Korsmeyer-Peppas model showed the best fit to describe the release from the porous chitosan materials into the different media.

Tuning bio-aerogel properties for controlling drug delivery. Part 2: Cellulose-pectin composite aerogels.

The release of the model drug theophylline from cellulose-pectin composite aerogels was investigated. Cellulose and pectin formed an interpenetrated network, and the goal was to study and understand the influence of each component and its solubility in simulated gastric and intestinal fluids on the kinetics of release. Cellulose was dissolved, coagulated in water, followed by impregnation with pectin solution, crosslinking of pectin with calcium (in some cases this step was omitted), solvent exchange and supercritical CO2 drying. Theophylline was loaded via impregnation and its release into simulated gastric fluid was monitored for 1 h followed by release into simulated intestinal fluid. The properties of the composite aerogels were varied via the cellulose and pectin concentrations as well as the calcium content in the precursor solutions. The release kinetics was correlated with aerogel specific surface area, bulk density as well as network swelling and erosion. The Korsmeyer-Peppas model was employed to identify the dominant release mechanisms during the various stages of the release.

Crosslinker-Free Hyaluronic Acid Aerogels.

Aerogels based on hyaluronic acid (HA) were prepared without any chemical crosslinking by polymer dissolution, network formation via nonsolvent-induced phase separation, and supercritical CO2 drying. The influence of solution pH, concentration of HA, and type of nonsolvent on network volume shrinkage, aerogel density, morphology, and specific surface area was investigated. A marked dependence of aerogel properties on solution pH was observed: aerogels with the highest specific surface area, 510 m2/g, and the lowest density, 0.057 g/cm3, were obtained when the HA solution was at its isoelectric point (pH 2.5). This work reports the first results ever on neat HA aerogels and constitutes the background for their use as advanced materials for biomedical applications.

Tuning the properties of porous chitosan: Aerogels and cryogels.

Highly porous chitosan-based materials were prepared via dissolution, non-solvent induced phase separation and drying using different methods. The goal was to tune the morphology and properties of chitosan porous materials by varying process parameters. Chitosan concentration, concentration of sodium hydroxide in the coagulation bath and aging time were varied. Drying was performed via freeze-drying leading to "cryogels" or via drying with supercritical CO2 leading to "aerogels". Cryogels were of lower density than aerogels (0.03-0.12 g/cm3vs 0.07-0.26 g/cm3, respectively) and had a lower specific surface area (50-70 vs 200-270 m2/g, respectively). The absorption of simulated wound exudate by chitosan aerogels and cryogels was studied in view of their potential applications as wound dressing. Higher absorption was obtained for cryogels (530-1500%) as compared to aerogels (200-610%).

Tuning bio-aerogel properties for controlling theophylline delivery. Part 1: Pectin aerogels.

A comprehensive study of release kinetics of a hydrophilic drug from bio-aerogels based on pectin was performed. Pectin aerogels were made by polymer dissolution, gelation (in some cases this step was omitted), solvent exchange and drying with supercritical CO2. Theophylline was loaded and its release was studied in the simulated gastric fluid during 1 h followed by the release in the simulated intestinal fluid. Pectin concentration, initial solution pH and concentration of calcium were varied to tune the properties of aerogel. The kinetics of theophylline release was monitored and correlated with aerogel density, specific surface area, and aerogel swelling and erosion. Various kinetic models were tested to identify the main physical mechanisms governing the release.

Polysaccharide-based aerogels for thermal insulation and superinsulation: An overview.

To reduce energy losses due to the insufficient thermal insulation is one of the current "hot" topics. Various commercial porous materials are used with the best conductivity around 0.03-0.04 W/(m·K). Aerogels are the only known materials with "intrinsic" thermal superinsulating properties, i.e. with thermal conductivity below that of air in ambient conditions (0.025 W/(m·K)). The classical thermal superinsulating aerogels are based on silica and some synthetic polymers, with conductivity 0.014-0.018 W/(m·K). Aerogels based on natural polymers are new materials created at the beginning of the 21st century. Can bio-aerogels possess thermal superinsulating properties? What are the bottlenecks in the development of bio-aerogels as new high-performance thermal insulationing materials? We try to answer these questions by analyzing thermal conductivity of bio-aerogels reported in literature.

Tailoring the morphology and properties of starch aerogels and cryogels via starch source and process parameter.

Porous starch materials with various morphology and properties were made via starch dissolution, retrogradation and drying either with supercritical CO2 ("aerogels") or lyophilisation ("cryogels"). Their properties were correlated with the rheological response of retrograded starch gels and crystallinity of aerogels and cryogels. All starch cryogels possess very low density (0.07 - 0.16 g/cm3), very large macropores and low specific surface area (around 3-13 m2/g). Their morphology is mainly the replica of sublimated ice crystals. The properties of starch aerogels strongly depend on starch source: the lowest density (around 0.1 g/cm3) and highest specific surface area (170-250 m2/g) was recorded for pea starch aerogels and the highest density (0.3-0.6 g/cm3) and lowest specific surface area (7-90 m2/g) for waxy maize starch aerogels. The morphology and properties of starch aerogels are interpreted by amylose and amylopectin evolution during retrogradation.

Biorefinery Approach for Aerogels.

According to the International Energy Agency, biorefinery is "the sustainable processing of biomass into a spectrum of marketable bio-based products (chemicals, materials) and bioenergy (fuels, power, heat)". In this review, we survey how the biorefinery approach can be applied to highly porous and nanostructured materials, namely aerogels. Historically, aerogels were first developed using inorganic matter. Subsequently, synthetic polymers were also employed. At the beginning of the 21st century, new aerogels were created based on biomass. Which sources of biomass can be used to make aerogels and how? This review answers these questions, paying special attention to bio-aerogels' environmental and biomedical applications. The article is a result of fruitful exchanges in the frame of the European project COST Action "CA 18125 AERoGELS: Advanced Engineering and Research of aeroGels for Environment and Life Sciences".

Enzymically attaching oligosaccharide-linked 'cargoes' to cellulose and other commercial polysaccharides via stable covalent bonds.

The Equisetum enzyme hetero-trans-ß-glucanase (HTG) covalently grafts native plant cellulose (donor-substrate) to xyloglucan (acceptor-substrate), potentially offering a novel 'green' method of cellulose functionalisation. However, the range of cellulosic and non-cellulosic donor substrates that can be utilised by HTG is unknown, limiting our insight into its biotechnological potential. Here we show that HTG binds all celluloses tested (papers, tissues, hydrogels, bacterial cellulose) to radioactively- or fluorescently-labelled xyloglucan-heptasaccharide (XXXGol; acceptor-substrate). Glycol-chitin, glycol-chitosan and chitosan also acted as donor substrates but less effectively than cellulose. Cellulose-XXXGol conjugates were formed throughout the volume of a block of hydrogel, demonstrating penetration. Plant-derived celluloses (cellulose Iß) became more effective donor-substrates after 'mercerisation' in ≥3 M NaOH; the opposite was true for bacterial cellulose Iα. Cellulose-XXXGol bonds resisted boiling 6 M NaOH, demonstrating strong glycosidic bonding. In conclusion, HTG stably grafts native and processed celluloses to xyloglucan-oligosaccharides, which may carry valuable 'cargoes', exemplified by sulphorhodamine. We thus demonstrate HTG's biotechnological potential to modify various cellulose-based substrates such as textiles, pulps, papers, packaging, sanitary products and hydrogels.

Exploring Large Ductility in Cellulose Nanopaper Combining High Toughness and Strength.

Cellulose nanopaper is a strong lightweight material made from renewable resources with a wide range of potential applications, from membranes to electronic displays. Most studies on nanopaper target high mechanical strength, which compromises ductility and toughness. Herein, we demonstrate the fabrication of highly ductile and tough cellulose nanopaper via mechanical fibrillation of hemicellulose-rich wood fibers and dispersion of the obtained cellulose nanofibrils (CNFs) in an ionic liquid (IL)-water mixture. This treatment allows hemicellulose swelling, which leads to dissociation of CNF bundles into highly disordered long flexible fibrils and the formation of a nanonetwork as supported by cryogenic transmission electron microscopy (cryo-TEM) imaging. Rheology of the suspensions shows a 300-fold increase in storage and loss moduli of CNF-IL-water suspensions, compared to their CNF-water counterparts. The nanopaper prepared by removing the IL-water shows a combination of large elongation (up to 35%), high strength (260 MPa), and toughness as high as 51 MJ/m3, because of efficient interfibrillar slippage and energy dissipation in the highly disordered isotropic structure. This work provides a nanostructure-engineered strategy of making ductile and tough cellulose nanopaper.

Unidirectional All-Cellulose Composites from Flax via Controlled Impregnation with Ionic Liquid.

Mechanically strong all-cellulose composites are very attractive in the terms of fully bio-based and bio-degradable materials. Unidirectional flax-based all-cellulose composites are prepared via facile room-temperature impregnation with an ionic liquid, 1-ethyl-3-methyl imidazolium acetate. To determine the optimal processing conditions, the kinetics of flax dissolution in this solvent is first studied using optical microscopy. Composite morphology, crystallinity, density, the volume fraction of cellulose II and tensile properties are investigated, indicating that flax dissolution should be within certain limits. On the one hand, the amount of cellulose II formed through dissolution and coagulation should be high enough to "fuse" flax fibers, resulting in a density increase. On the other hand, only the surface layer of the fibers should be dissolved to maintain the strength provided by the inner secondary layer and avoid a detrimental decrease in crystallinity. The highest Young's modulus and strength, 10.1 GPa and 151.3 MPa, respectively, are obtained with a crystallinity of 43% and 20 vol% of cellulose II.

Cellulose Aerogel Microparticles via Emulsion-Coagulation Technique.

Cellulose aerogel microparticles were made via emulsification/nonsolvent induced phase separation/drying with supercritical CO2. Cellulose was dissolved in NaOH-based solvent with and without additives in order to control solution gelation. Two emulsions, cellulose solution/oil and cellulose nonsolvent/oil, were mixed to start nonsolvent induced phase separation (or coagulation) of cellulose inside each cellulose droplet leading to the formation of so-called microgels. Different options of triggering coagulation were tested, by coalescence of droplets of cellulose solution and cellulose nonsolvent and by diffusion of nonsolvent partly soluble in the oil, accompanied by coalescence. The second option was found to be the most efficient for stabilization of the shape of coagulated cellulose microgels. The influence of gelation on particle formation and aerogel properties was investigated. The aerogel particles' diameter was around a few tens of microns, and the specific surface area was 250-350 m2/g.

Mechanical properties of cellulose aerogels and cryogels.

Highly porous and lightweight cellulose materials were prepared via dissolution-coagulation and different drying routes. Cellulose of three different molecular weights was dissolved in an ionic liquid/dimethyl sulfoxide mixture. Drying was performed either with supercritical CO2 resulting in "aerogels", or via freeze-drying resulting in "cryogels". The influence of cellulose molecular weight, concentration and drying method on the morphology, density, porosity and specific surface area was determined. The mechanical properties of cellulose cryogels and aerogels under uniaxial compression were studied in detail and analyzed in the view of existing models developed for porous materials. It was demonstrated that the Poisson's ratio of cellulose aerogels is not equal to zero, contrary to what is usually reported in the literature, but decreases with an increase in density. Compressive modulus and yield stress of cryogels turned out to be higher than those of aerogels taken at the same density. This was interpreted by the different morphology of the porous materials studied.

Gelation of cellulose-NaOH solutions in the presence of cellulose fibers.

It is well known that when cellulose is dissolved in aqueous NaOH-based solvent, solutions are gelling with increasing time and temperature. The goal of this work was to understand if the presence of non-dissolved cellulose fibers influences gelation behavior of the whole system. One of the motivations is to control gelation when making all-cellulose composites with short fibers dispersed in cellulose-NaOH-water solutions. Gelation kinetics of cellulose(dissolving pulp)-NaOH-water solutions with added softwood kraft fibers were investigated using dynamic rheology. Fiber concentration, dissolving pulp degree of polymerization and solution temperature were varied. In all cases the addition of kraft fibers accelerates gelation and increases modulus at gel point while the presence of "inert" carbon fibers does not influence solution gelation kinetics. It was suggested that acceleration of gelation and reinforcement of cellulose gels is due to the interactions between dissolved and non-dissolved cellulose.

An Opinion Paper on Aerogels for Biomedical and Environmental Applications.

Aerogels are a special class of nanostructured materials with very high porosity and tunable physicochemical properties. Although a few types of aerogels have already reached the market in construction materials, textiles and aerospace engineering, the full potential of aerogels is still to be assessed for other technology sectors. Based on current efforts to address the material supply chain by a circular economy approach and longevity as well as quality of life with biotechnological methods, environmental and life science applications are two emerging market opportunities where the use of aerogels needs to be further explored and evaluated in a multidisciplinary approach. In this opinion paper, the relevance of the topic is put into context and the corresponding current research efforts on aerogel technology are outlined. Furthermore, key challenges to be solved in order to create materials by design, reproducible process technology and society-centered solutions specifically for the two abovementioned technology sectors are analyzed. Overall, advances in aerogel technology can yield innovative and integrated solutions for environmental and life sciences which in turn can help improve both the welfare of population and to move towards cleaner and smarter supply chain solutions.

Interactions between callose and cellulose revealed through the analysis of biopolymer mixtures.

The properties of (1,3)-ß-glucans (i.e., callose) remain largely unknown despite their importance in plant development and defence. Here we use mixtures of (1,3)-ß-glucan and cellulose, in ionic liquid solution and hydrogels, as proxies to understand the physico-mechanical properties of callose. We show that after callose addition the stiffness of cellulose hydrogels is reduced at a greater extent than predicted from the ideal mixing rule (i.e., the weighted average of the individual components' properties). In contrast, yield behaviour after the elastic limit is more ductile in cellulose-callose hydrogels compared with sudden failure in 100% cellulose hydrogels. The viscoelastic behaviour and the diffusion of the ions in mixed ionic liquid solutions strongly indicate interactions between the polymers. Fourier-transform infrared analysis suggests that these interactions impact cellulose organisation in hydrogels and cell walls. We conclude that polymer interactions alter the properties of callose-cellulose mixtures beyond what it is expected by ideal mixing.