Preparation and characterization of polysaccharide - silica hybrid aerogels

Tuning bio-aerogel properties. Part 3: Exploring silica-pectin composite aerogels for drug delivery

The release of the model drug theophylline from silica-pectin aerogels was investigated. The composite aerogels were prepared via impregnation of pectin alcogels with silica sol, followed by in situ silica gelation and drying with supercritical CO2. The structural and physico-chemical properties of the aerogels were tuned via the preparation conditions (type of silica sol, calcium crosslinking of pectin or not). Theophylline was loaded via impregnation and its release into simulated gastric fluid was studied during 1 h followed by release into simulated intestinal fluid. The swelling, mass loss and theophylline release behavior of the composites were analyzed and correlated with material properties. It followed that only aerogels prepared with calcium-crosslinked pectin and polyethoxydisiloxane were stable in aqueous systems, exhibiting a slow theophylline release governed by near-Fickian diffusion.

Biopolymer-Polysiloxane Double Network Aerogels

A new series of transparent aerogels of biopolymer-polysiloxane double networks is reported. Biopolymer aerogels have attracted much attention from green and sustainable aspects but suffered from strong hydrophilicity and difficulty to make homogeneous structures in nanoscale; these drawbacks are overcome by compositing with a polysiloxane network. Alginate-polymethylsilsesquioxane aerogel has high optical transparency, water repellency, comparable superinsulation property and improved bending flexibility compared to pure polymethylsilsesquioxane aerogel. The nanoscale homogeneity is realized by separating the crosslinking steps for two networks in a sequential protocol: condensation of siloxane bonds and metal-crosslinking of biopolymer. The crosslinking order, biopolymer-first or siloxane-first, and universality/limitation of biopolymer-crosslinker pairs are discussed to construct fundamental chemistry of double network systems for their further application potentials.

Aerogel-Based Biomaterials for Biomedical Applications: From Fabrication Methods to Disease-Targeting Applications

Aerogel-based biomaterials are increasingly being considered for biomedical applications due to their unique properties such as high porosity, hierarchical porous network, and large specific pore surface area. Depending on the pore size of the aerogel, biological effects such as cell adhesion, fluid absorption, oxygen permeability, and metabolite exchange can be altered. Based on the diverse potential of aerogels in biomedical applications, this paper provides a comprehensive review of fabrication processes including sol-gel, aging, drying, and self-assembly along with the materials that can be used to form aerogels. In addition to the technology utilizing aerogel itself, it also provides insight into the applicability of aerogel based on additive manufacturing technology. To this end, how microfluidic-based technologies and 3D printing can be combined with aerogel-based materials for biomedical applications is discussed. Furthermore, previously reported examples of aerogels for regenerative medicine and biomedical applications are thoroughly reviewed. A wide range of applications with aerogels including wound healing, drug delivery, tissue engineering, and diagnostics are demonstrated. Finally, the prospects for aerogel-based biomedical applications are presented. The understanding of the fabrication, modification, and applicability of aerogels through this study is expected to shed light on the biomedical utilization of aerogels.

Structure-Related Mechanical Properties and Bioactivity of Silica-Gelatin Hybrid Aerogels for Bone Regeneration

We report the synthesis of mesoporous silica-gelatin hybrid aerogels with 15, 25, and 30 wt. % gelatin contents, using 3-glycidoxypropyl trimethoxysilane (GPTMS) as a coupling agent, for tissue-engineering applications. Aerogels were obtained using a one-step sol-gel process followed by CO₂ supercritical drying, resulting in crack-free monolith samples with bulk densities ranging from 0.41 g cm^-3 to 0.66 g cm^-3. Nitrogen adsorption measurements revealed an interconnected mesopore network and a general decrease in the textural parameters: specific surface areas (651-361 m² g^-1), pore volume (1.98-0.89 cm³ g^-1), and pore sizes (10.8-8.6 nm), by increasing gelatin content. Thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR) spectroscopy and uniaxial compression experiments confirmed that the structure, thermal properties and mechanical behavior of these aerogels changed significantly when the concentration of gelatin reached 25 wt.%, suggesting that this composition corresponds to the percolation threshold of the organic phase. In addition, the samples exhibited hydrophilic behavior and extremely fast swelling in phosphate-buffered saline (PBS), with swelling ratios from 2.32 to 3.32. Furthermore, in vitro bioactivity studies revealed a strong relationship between the kinetics of the nucleation and growth processes of hydroxyapatite in simulated body fluid (SBF) and the gelatin content. The live/dead assay revealed no cytotoxicity in HOB^® osteoblasts in vitro and a positive influence on cell growth, focal adhesion development, and cytoskeletal arrangement for cell adhesion. Mineralization assays confirmed the positive effects of the samples on osteoblast differentiation. The biomaterials described are versatile, can be easily sterilized and are suitable for a wide range of applications in bone tissue-engineering, either alone or in combination with bioactive-reinforced phases.

Bacterial collagen-templated synthesis and assembly of inorganic particles

Collagen has been used as a common template for mineralization and assembly of inorganic particles, because of the special arrangement of its fibrils and the presence of charged residues. Streptococcal bacterial collagen, which is inherently secreted on the surface ofStreptococcus pyogenes, has been progressively used as an alternative for type I animal collagen. Bacterial collagen is rich in charged amino acids, which can act as a substrate for the nucleation and growth of inorganic particles. Here, we show that bacterial collagen can be used to nucleate three different inorganic materials: hydroxyapatite crystals, silver nanoparticles, and silica nanoparticles. Collagen/mineral composites show an even distribution of inorganic particles along the collagen fibers, and the particles have a more homogenous size compared with minerals that are formed in the absence of the collagen scaffold. Furthermore, the gelation of silica occurring during mineralization represents a means to produce processable self-standing collagen composites, which is challenging to achieve with bacterial collagen alone. Overall, we highlight the advantage of simply combining bacterial collagen with minerals to expand their applications in the fields of biomaterials and tissue engineering, especially for bone regenerative scaffolds.

Supercritical Fluid Technologies for the Incorporation of Synthetic and Natural Active Compounds into Materials for Drug Formulation and Delivery

Various active compounds isolated from natural sources exhibit remarkable benefits, making them attractive for pharmaceutical and biomedical applications, such as antioxidant, antimicrobial, and anti-inflammatory activities, which contribute to the treatment of cardiovascular diseases, neurodegenerative disorders, various types of cancer, diabetes, and obesity. However, their major drawbacks are their reactivity, instability, relatively poor water solubility, and consequently low bioavailability. Synthetic drugs often face similar challenges associated with inadequate solubility or burst release in gastrointestinal media, despite being otherwise a safe and effective option for the treatment of numerous diseases. Therefore, drug-eluting pharmaceutical formulations have been of great importance over the years in efforts to improve the bioavailability of active compounds by increasing their solubility and achieving their controlled release in body media. This review highlights the success of the fabrication of micro- and nanoformulations using environmentally friendly supercritical fluid technologies for the processing and incorporation of active compounds. Several novel approaches, namely micronization to produce micro- and nano-sized particles, supercritical drying to produce aerogels, supercritical foaming, and supercritical solvent impregnation, are described in detail, along with the currently available drug delivery data for these formulations.

A Brief Evaluation of Pore Structure Determination for Bioaerogels

This review discusses the most commonly employed methods for determining pore size and pore size distribution in bioaerogels. Aerogels are materials with high porosity and large surface areas. Most of their pores are in the range of mesopores, between 2 and 50 nm. They often have smaller or larger pores, which presents a significant challenge in determining the exact mean pore size and pore size distribution in such materials. The precision and actual value of the pore size are of considerable importance since pore size and pore size distribution are among the main properties of aerogels and are often directly connected with the final application of those materials. However, many recently published papers discuss or present pore size as one of the essential achievements despite the misinterpretation or the wrong assignments of pore size determination. This review will help future research and publications evaluate the pore size of aerogels more precisely and discuss it correctly. The study covers methods such as gas adsorption, from which BJH and DFT models are often used, SEM, mercury porosimetry, and thermoporometry. The methods are described, and the results obtained are discussed. The following paper shows that there is still no precise method for determining pore size distribution or mean pore size in aerogels until now. Knowing that, it is expected that this field will evolve in the future.

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.

Superhydrophobic Silica Aerogels and Their Layer-by-Layer Structure for Thermal Management in Harsh Cold and Hot Environments

Personal thermal management (PTM) materials have recently received considerable attention to improve human body thermal comfort with potentially reduced energy consumption. Strategies include passive radiative cooling and warming. However, challenges remain for passive thermal regulation of one material or structure in both harsh hot and cold environments. In this work, silica aerogels derived from sodium silicate were prepared through a solvent-boiling strategy, where hydrophobization, solvent exchange, sodium purification, and ambient pressure drying (HSSA) proceeded successively and spontaneously in a one-pot process. This strategy leads to the synthesis of superhydrophobic silica aerogels with extremely low energy consumption without out the use of an ion-exchange resin or low surface tension solvents. Silica aerogels possess a high specific surface area (635 m²/g), high contact angle (153°), and low thermal conductivity (0.049 W/m K). A layer-by-layer (LBL) structure including the silica aerogel layer and an extra phase change material layer was designed. The structure demonstrates dual-functional thermal regulation performance in both harsh cold (-30 °C) and hot (70 °C) environments, where the time to reach equilibrium is postponed, and the inner temperature of the LBL structure can be kept above 20 °C in harsh cold environments (-30 °C) and below 31 °C in harsh hot environments (70 °C). A proof-of-concept experimental setup to simulate the illumination of sunlight also proved that the inside temperature of a model car protected by the LBL structure was below 28 °C, while the outside temperature was 70 °C. In addition, these results are well supported by theoretical COMSOL simulation results. The findings of this work not only provide an eco-friendly approach to synthesize silica aerogels but also demonstrate that the LBL structure is a robust dual-functional PTM system for thermal regulation in both harsh hot and cold environments.

Magnetic aerogel: an advanced material of high importance

Magnetic materials have brought innovations in the field of advanced materials. Their incorporation in aerogels has certainly broadened their application area. Magnetic aerogels can be used for various purposes from adsorbents to developing electromagnetic interference shielding and microwave absorbing materials, high-level diagnostic tools, therapeutic systems, and so on. Considering the final use and cost, these can be fabricated from a variety of materials using different approaches. To date, several studies have been published reporting the fabrication and uses of magnetic aerogels. However, to our knowledge, there is no review that specifically focuses only on magnetic aerogels, so we attempted to overview the main developments in this field and ended our study with the conclusion that magnetic aerogels are one of the emerging and futuristic advanced materials with the potential to offer multiple applications of high value.

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".

Chitosan-GPTMS-Silica Hybrid Mesoporous Aerogels for Bone Tissue Engineering

This study introduces a new synthesis route for obtaining homogeneous chitosan (CS)-silica hybrid aerogels with CS contents up to 10 wt%, using 3-glycidoxypropyl trimethoxysilane (GPTMS) as coupling agent, for tissue engineering applications. Aerogels were obtained using the sol-gel process followed by CO₂ supercritical drying, resulting in samples with bulk densities ranging from 0.17 g/cm³ to 0.38 g/cm³. The textural analysis by N₂-physisorption revealed an interconnected mesopore network with decreasing specific surface areas (1230-700 m²/g) and pore sizes (11.1-8.7 nm) by increasing GPTMS content (2-4 molar ratio GPTMS:CS monomer). In addition, samples exhibited extremely fast swelling by spontaneous capillary imbibition in PBS solution, presenting swelling capacities from 1.75 to 3.75. The formation of a covalent crosslinked hybrid structure was suggested by FTIR and confirmed by an increase of four hundred fold or more in the compressive strength up to 96 MPa. Instead, samples synthesized without GPTMS fractured at only 0.10-0.26 MPa, revealing a week structure consisted in interpenetrated polymer networks. The aerogels presented bioactivity in simulated body fluid (SBF), as confirmed by the in vitro formation of hydroxyapatite (HAp) layer with crystal size of approximately 2 µm size in diameter. In vitro studies revealed also non cytotoxic effect on HOB^® osteoblasts and also a mechanosensitive response. Additionally, control cells grown on glass developed scarce or no stress fibers, while cells grown on hybrid samples showed a significant (p < 0.05) increase in well-developed stress fibers and mature focal adhesion complexes.

Microbial Exopolysaccharides as Drug Carriers

Microbial exopolysaccharides are peculiar polymers that are produced by living organisms and protect them against environmental factors. These polymers are industrially recovered from the medium culture after performing a fermentative process. These materials are biocompatible and biodegradable, possessing specific and beneficial properties for biomedical drug delivery systems. They can have antitumor activity, they can produce hydrogels with different characteristics due to their molecular structure and functional groups, and they can even produce nanoparticles via a self-assembly phenomenon. This review studies the potential use of exopolysaccharides as carriers for drug delivery systems, covering their versatility and their vast possibilities to produce particles, fibers, scaffolds, hydrogels, and aerogels with different strategies and methodologies. Moreover, the main properties of exopolysaccharides are explained, providing information to achieve an adequate carrier selection depending on the final application.