Supercritical drying of aerogels using CO2: Effect of extraction time on the end material textural properties

Cellulose Aerogels for Thermal Insulation in Buildings: Trends and Challenges

Cellulose-based aerogels hold the potential to become a cost-effective bio-based solution for thermal insulation in buildings. Low thermal conductivities (<0.025 W·m−1·K−1) are achieved through a decrease in gaseous phase contribution, exploiting the Knudsen effect. However, several challenges need to be overcome: production energy demand and cost, moisture sensitivity, flammability, and thermal stability. Herein, a description and discussion of current trends and challenges in cellulose aerogel research for thermal insulation are presented, gathered from studies reported within the last five years. The text is divided into three main sections: (i) an overview of thermal performance of cellulose aerogels, (ii) an identification of challenges and possible solutions for cellulose aerogel thermal insulation, and (iii) a brief description of cellulose/silica aerogels.

Three Pillars of Novel Nonthermal Food Technologies: Food Safety, Quality, and Environment

This review gives an overview of the impact of novel nonthermal food technologies on food safety, on quality, and on the environment. It confirms that research in this field is mainly focused on analyzing microbial and/or chemical aspects of food safety. However, recent research shows that in spite of various food safety benefits, some negative (quality oriented) features occur. Finally, this paper shows the necessity of analyzing the environmental dimension of using these technologies.

Kinetics of Supercritical Drying of Gels

Supercritical drying of gels is considered as the most important step of aerogel production since it enables preservation of the three-dimensional pore structure which lead to unique material properties such as high porosity, low density, and large surface area. An understanding of the kinetics of supercritical drying is necessary to provide insight into material development, scale-up, and optimization of the aerogel manufacturing process. Thus, investigation of supercritical drying is gaining increased attention in recent years. This review paper covers the experimental considerations and techniques to study the kinetics of supercritical drying, fundamental mass transfer mechanisms during the drying process and modeling efforts to predict the drying kinetics for varying operating conditions and gel properties. Transport phenomena involving diffusion, convection, spillage by volume expansion, and axial dispersion are discussed by providing the fundamental equations and empirical correlations to predict transfer coefficients. A detailed review of literature covering experimental and theoretical studies on kinetics of supercritical drying is presented.

Multifunctional Carbon Aerogels Derived by Sol-Gel Process of Natural Polysaccharides of Different Botanical Origin

In this manuscript, we describe the results of our recent studies on carbon aerogels derived from natural starches. A facile method for the fabrication of carbon aerogels is presented. Moreover, the complete analysis of the carbonization process of different starch aerogels (potato, maize, and rice) was performed using thermogravimetric studies combined with a detailed analysis of evolved decomposition products. The prepared carbon aerogels were studied in terms of their morphology and electrical properties to relate the origin of starch precursor with final properties of carbon materials. The obtained results confirmed the differences in carbon aerogels' morphology, especially in materials' specific surface areas, depending on the botanical origin of precursors. The electrical conductivity measurements suggest that carbon aerogels with the best electrical properties can be obtained from potato starch.

Ambient Pressure Hybrid Silica Monoliths with Hexamethyldisilazane: From Vitreous Hydrophilic Xerogels to Superhydrophobic Aerogels

Hybrid silica-based monoliths were synthesized at ambient pressure, using minimum amounts of the silylating agent hexamethyldisilazane (HMDZ). Depending on the synthesis approach, the materials ranged from dense and vitreous xerogels to transparent and superhydrophobic aerogels. Emphasis was given to understanding the role of the silylating agent, its content and incorporation process on the final morphology, and properties of the xerogels/aerogels. It is proven that as a coprecursor, increasing HMDZ content contributes to increase the lipophilic/hydrophilic balance, induce high surface areas, and decrease densities, but there is a maximum usable content for producing monoliths. Conversely, as a postsynthesis modifier, there is an optimum HMDZ content that maximizes hydrophobicity (water contact angle of ∼144°) and induces high surface area (∼700 m²·g^-1), keeping the density low (∼300 kg·m^-3). It is proven that the aging period in the hydrophobizing solution is a crucial parameter. The most superhydrophobic xerogels were obtained using HMDZ as a postsynthesis modifier, achieving values of water contact angles as high as ∼173°, at the cost of density increase to ∼600 kg·m^-3 and decrease of the surface area to ∼300 m²·g^-1. The best compromise between low density, high surface area, and superhydrophobicity is obtained using HMDZ both as a coprecursor and as a postsynthesis modifier, in a low HMDZ/tetraethoxysilane total molar ratio (<0.2), with an aging period of 16-24 h. The use of subcritical drying, along with the minimization of the expensive organic modifier quantities, allows envisaging a safe and low-cost large-scale production of a variety of materials, including superhydrophobic aerogels with potential distinctive applications.

In Situ Raman Analysis of CO₂-Assisted Drying of Fruit-Slices

This work explores the feasibility of applying in situ Raman spectroscopy for the online monitoring of the supercritical carbon dioxide (SC-CO₂) drying of fruits. Specifically, we investigate two types of fruits: mango and persimmon. The drying experiments were carried out inside an optical accessible vessel at 10 MPa and 313 K. The Raman spectra reveal: (i) the reduction of the water from the fruit slice and (ii) the change of the fruit matrix structure during the drying process. Two different Raman excitation wavelengths were compared: 532 nm and 785 nm. With respect to the quality of the obtained spectra, the 532 nm excitation wavelength was superior due to a higher signal-to-noise ratio and due to a resonant excitation scheme of the carotenoid molecules. It was found that the absorption of CO₂ into the fruit matrix enhances the extraction of water, which was expressed by the obtained drying kinetic curve.

Recent Advances in Research on the Synthetic Fiber Based Silica Aerogel Nanocomposites

The presented paper contains a brief review on the synthesis and characterization of silica aerogels and its nanocomposites with nanofibers and fibers based on a literature study over the past twenty years and my own research. Particular attention is focused on carbon fiber-based silica aerogel nanocomposites. Silica aerogel is brittle in nature, therefore, it is necessary to improve this drawback, e.g., by polymer modification or fiber additives. Nevertheless, there are very few articles in the literature devoted to the synthesis of silica aerogel/fiber nanocomposites, especially those focusing on carbon fibers and nanofibers. Carbon fibers are very interesting materials, namely due to their special properties: high conductivity, high mechanical properties in relation to very low bulk densities, high thermal stability, and chemical resistance in the silica aerogel matrix, which can help enhance silica aerogel applications in the future.

A Continuous Extraction and Pumpless Supercritical CO₂ Drying System for Laboratory-Scale Aerogel Production

Production of aerogels starts with solution chemistry and may end with supercritical carbon dioxide drying, which both require a specialized system. Here we present a complete aerogel production system that was developed and used in our laboratory over the last nine years. Our aim was to develop a supercritical dryer and a protocol, whereby the CO₂ pump can be left out, and the entire flow system is operated by the pressure of the CO₂ cylinder. Drying pressure and temperature are controlled by the combination of the filling and heating temperatures. A continuous-mode solvent exchange system has also been developed, in which the solvent consumption during the process can be reduced to one-third of the batch method. In the new medium temperature 1.5 L volume supercritical dryer, the temperature is set to a constant 80–82 °C, and the pressure can be in the 90–200 bar range, depending on the conditions. We have performed approximately 200 dryings thus far, and prepared a wide range of monolithic aerogels, from pristine silica aerogels to polysaccharides and collagen. In this paper, we have summarized not only the technical details, but also the work experiences, as well as advantages and disadvantages of the systems.

Dynamic spongy films to immobilize hydrophobic antimicrobial peptides for self-healing bactericidal coating

A constant increase of nosocomial infections that are caused by adhesion and colonization of pathogenic microorganisms, especially drug-resistant bacteria, on the surfaces of healthcare devices has received considerable attention worldwide. In this study, bioinspired by antimicrobial skins of natural living beings, we developed a self-healing bactericidal coating through the immobilization of hydrophobic antimicrobial peptides (AMPs) into a multilayer film, which was constructed through the enhanced exponential layer-by-layer assembly of polyethylenimine (PEI) and poly(acrylic acid) (PAA). The (PEI/PAA) film shows particular dynamic properties from the as-prepared thin solid film to a spongy microporous structure via acid solution treatment, and then back to the thin solid film by eliminating micropores via the treatment of saturated humidity. Consequently, the loading and integration of hydrophobic AMPs such as gramicidin A (GA) into the (PEI/PAA) film were achieved via simple wicking action with GA solution and subsequent humidity treatment, respectively. The GA loading densities can be precisely controlled by using different concentrations of GA solution. We demonstrated that the GA immobilized (PEI/PAA) film has rapid self-healing properties, and that Gram-positive bacteria S. aureus including the methicillin-resistant type were efficiently killed through the contact-killing mode. Collectively, this self-healing bactericidal coating shows practical potential in a variety of healthcare applications.

Prilling and supercritical drying: A successful duo to produce core-shell polysaccharide aerogel beads for wound healing

Bacterial infections often affect the wound, delaying healing and causing areas of necrosis. In this work, an aerogel in form of core-shell particles, able to prolong drug activity on wounds and to be easily removed was developed. Aerogel microcapsules consisted of a core made by amidated pectin hosting doxycycline, an antibiotic drug with a broad spectrum of action, and a shell consisting of high mannuronic content alginate. Particles were obtained by prilling using a coaxial nozzle for drop production and an ethanolic solution of CaCl2 as gelling promoter. The alcogels where dried using supercritical CO2. The influence of polysaccharides and drug concentrations on aerogel properties was evaluated. Spherical particles with high drug encapsulation efficiency (87%) correlated to alginate concentration in the processed liquid feeds were obtained. The release of the drug, mainly concentrated into the pectin core, was prolonged till 48h, and dependent on both drug/pectin ratio and alginate concentration.

Self-Healing Spongy Coating for Drug "Cocktail" Delivery

Optimized ratio in the codelivery of therapeutics is of crucial importance to promote the synergism rather than the antagonistic effects. In this study, a self-healing spongy coating was described to facilitate the surface-mediated delivery of drug "cocktails" proportionally. The formation of spongy structures within the coating was achieved by acidic treatment and freeze-drying. Various drug combinations can be readily integrated through wicking method and subsequent micropore self-healing. The ratio of drug loading can be precisely regulated by the composition of loading solution and the embedded drugs were released in proportion according to the initial ratio of drug combination.

Chitosan microspheres with an extracellular matrix-mimicking nanofibrous structure as cell-carrier building blocks for bottom-up cartilage tissue engineering

Scaffolds for tissue engineering (TE) which closely mimic the physicochemical properties of the natural extracellular matrix (ECM) have been proven to advantageously favor cell attachment, proliferation, migration and new tissue formation. Recently, as a valuable alternative, a bottom-up TE approach utilizing cell-loaded micrometer-scale modular components as building blocks to reconstruct a new tissue in vitro or in vivo has been proved to demonstrate a number of desirable advantages compared with the traditional bulk scaffold based top-down TE approach. Nevertheless, micro-components with an ECM-mimicking nanofibrous structure are still very scarce and highly desirable. Chitosan (CS), an accessible natural polymer, has demonstrated appealing intrinsic properties and promising application potential for TE, especially the cartilage tissue regeneration. According to this background, we report here the fabrication of chitosan microspheres with an ECM-mimicking nanofibrous structure for the first time based on a physical gelation process. By combining this physical fabrication procedure with microfluidic technology, uniform CS microspheres (CMS) with controlled nanofibrous microstructure and tunable sizes can be facilely obtained. Especially, no potentially toxic or denaturizing chemical crosslinking agent was introduced into the products. Notably, in vitro chondrocyte culture tests revealed that enhanced cell attachment and proliferation were realized, and a macroscopic 3D geometrically shaped cartilage-like composite can be easily constructed with the nanofibrous CMS (NCMS) and chondrocytes, which demonstrate significant application potential of NCMS as the bottom-up cell-carrier components for cartilage tissue engineering.

On the Road to Biopolymer Aerogels-Dealing with the Solvent

Aerogels are three-dimensional ultra-light porous structures whose characteristics make them exciting candidates for research, development and commercialization leading to a broad scope of applications ranging from insulation and catalysis to regenerative medicine and pharmaceuticals. Biopolymers have recently entered the aerogel foray. In order to fully realize their potential, progressive strategies dealing with production times and costs reduction must be put in place to facilitate the scale up of aerogel production from lab to commercial scale. The necessity of studying solvent/matrix interactions during solvent exchange and supercritical CO₂ drying is presented in this study using calcium alginate as a model system. Four frameworks, namely (a) solvent selection methodology based on solvent/polymer interaction; (b) concentration gradient influence during solvent exchange; (c) solvent exchange kinetics based on pseudo second order model; and (d) minimum solvent concentration requirements for supercritical CO₂ drying, are suggested that could help assess the role of the solvent in biopolymer aerogel production.

A sustainable freeze-drying route to porous polysaccharides with tailored hierarchical meso- and macroporosity

Bio-derived polysaccharide aerogels are of interest for a broad range of applications. To date, these aerogels have been obtained through the time- and solvent-intensive procedure of hydrogel fomation, solvent exchange, and scCO2 drying, which offers little control over meso/macropore distribution. A simpler and more versatile route is developed, using freeze drying to produce highly mesoporous polysaccharide aerogels with various degrees of macroporosity. The hierarchical pore distribution is controlled by addition of different quantities of t-butanol (TBA) to hydrogels before drying. Through a systematic study an interesting relationship between the mesoporosity and t-butanol/water phase diagram is found, linking mesoporosity maxima with eutectic points for all polysaccharides studied (pectin, starch, and alginic acid). Moreover, direct gelation of polysaccharides in aqueous TBA offers additional time savings and the potential for solvent reuse. This finding is a doorway to more accessible polysaccharide aerogels for research and industrial scale production, due to the widespread accessibility of the freeze drying technology and the simplicity of the method.