Aerogel Microspheres Research Articles

Chitosan–graphene oxide films and CO2-dried porous aerogel microspheres: Interfacial interplay and stability

The intimate interplay of chitosan (CS) and graphene oxide (GO) in aqueous acidic solution has been explored to design upon casting, nanostructured “brick-and-mortar” films (CS-GO-f) and by acidic-to-basic pH inversion, porous CO2-dried aerogel microspheres (CS-GO-m). Owing to the presence of oxygenated functional groups in GO, good-quality crack-free hybrid films were obtained. Mechanical properties were improved independently of the GO content and it was found that a 20wt% loading affords hybrid film characterized with a Young modulus three times superior to that reached with the same loading of layered clay. The presence of graphene oxide was found to be detrimental for the thermal stability of the polysaccharide at T <350°C, a fact attributed to the well-established decomposition of the oxygenated functional groups of the graphene sheets. Irrespective to the graphene oxide loading, chitosan-graphene oxide mixture preserves the gelation memory of the polysaccharide. Supercritical drying of the resulting soft hydrogels provides macroporous network with surface areas ranging from 226m2g−1 to 554m2g−1. XPS and RAMAN analyses evidenced the selective reduction of GO sheets inside of these microspheres, affording the hitherto unknown macroporous chitosan-entangled-reduced graphene oxide (CS-rGO-m) aerogels. Improvement in both hydrothermal stability (under water reflux) and chemical stability (under acidic conditions) have been noticed for chitosan-graphene oxide microspheres with respect to non-modified chitosan and chitosan-clay bio-hybrids, a result rooted in the substantial hydrophobic character imparted by the addition of graphenic material to the polysaccharide skeleton. In essence, this contribution demonstrates that graphene oxide loading do not disturb neither the filmogenicity of chitosan nor its gelation ability and constitutes a promising route for novel chitosan-based functional hybrid materials.

Supercritical processing of starch aerogels and aerogel-loaded poly(ε-caprolactone) scaffolds for sustained release of ketoprofen for bone regeneration

Bone regeneration requires scaffolds with a suitable nanostructured 3D-network and decorated with bioactive functionalities to promote cell ingrowth and induce the intended biological responses. In this work, highly porous poly(ε-caprolactone) (PCL) scaffolds containing biodegradable mesoporous microparticles (starch aerogel microspheres) and a bioactive compound (ketoprofen, a NSAID) were designed and produced using supercritical technologies (scCO2 impregnation/deposition and foaming) for bone regeneration purposes. One-micron-sized starch aerogel microspheres were processed for the first time. The effects of the incorporation of aerogel powders in the synthetic PCL-based scaffolds on the morphological, physico-chemical and mechanical properties of the construct were evaluated by N2 adsorption-desorption analysis, scanning electron microscopy, mercury intrusion porosimetry, 3D-modeling, dynamic mechanical analysis and differential scanning calorimetry methods. Scaffolds containing starch aerogels presented an increased porosity and pore interconnectivity to promote the bone tissue growth processes at the expense of a minor decrease in the mechanical properties. The scaffolds showed sustained release of ketoprofen (37°C, pH 7.4) in the timeframe of days with faster release rate in the case of scaffolds containing starch aerogels. Therefore, scaffolds containing starch aerogel microspheres and obtained by supercritical foaming are an attractive solution to obtain drug-loaded scaffolds with accurate pore structure (porosity, pore size distribution, interconnectivity) for cell in-growth and with sustained release profiles of bioactive compounds.

Cellulose Nanofibril Based-Aerogel Microreactors: A High Efficiency and Easy Recoverable W/O/W Membrane Separation System

Hereby we report a novel cellulose nanofirbril aerogel-based W/O/W microreactor system that can be used for fast and high efficient molecule or ions extraction and separation. The ultra-light cellulose nanofibril based aerogel microspheres with high porous structure and water storage capacity were prepared. The aerogel microspheres that were saturated with stripping solution were dispersed in an oil phase to form a stable water-in-oil (W/O) suspension. This suspension was then dispersed in large amount of external waste water to form W/O/W microreactor system. Similar to a conventional emulsion liquid membrane (ELM), the molecules or ions in external water can quickly transport to the internal water phase. However, the microreactor is also significantly different from traditional ELM: the water saturated nanocellulose cellulose aerogel microspheres can be easily removed by filtration or centrifugation after extraction reaction. The condensed materials in the filtrated aerogel particles can be squeezed and washed out and aerogel microspheres can be reused. This novel process overcomes the key barrier step of demulsification in traditional ELM process. Our experimental indicates the novel microreactor was able to extract 93% phenol and 82% Cu2+ from external water phase in a few minutes, suggesting its great potential for industrial applications.

Capacitive deionization of NaCl solutions with ambient pressure dried carbon aerogel microsphere electrodes

We first used carbon aerogel microspheres prepared via emulsion polymerization and ambient drying techniques as electrode material for capacitive deionization.

Robust Silica Aerogel Microspheres from Rice Husk Ash to Enhance the Dissolution Rate of Poorly Water-Soluble Drugs

The present paper demonstrates the capability of specially prepared robust silica aerogel microspheres (RSAMs) to enhance the dissolution rate of poorly water-soluble drugs. A sol-gel/mineral oil emulsion method has been developed for RSAMs from rice husk ash (RHA), a biogenic source. The particles were characterized for their Brunauer–Emmett–Teller (BET) specific surface area, Barrett, Joyner and Halenda (BJH) pore volume and pore diameter, and morphology by optical microscopy and scanning electron microscopy (SEM). The dissolution rate of ibuprofen, a poorly water-soluble drug, was investigated by adsorbing it onto RSAMs upon dissolving it in supercritical carbon dioxide (scCO2) at 150 bar and 40°C. This resulted in a loading of ∼0.13 g ibuprofen/g loaded RSAMs in 24 h. X-ray diffraction analysis was used to characterize the nature of the adsorbed ibuprofen onto RSAMs. It was observed that the loaded drug on the aerogels is in amorphous form. An in vitro drug-release kinetic studies confirmed a significant enhancement in the dissolution rate, namely ∼100% of the loaded ibuprofen released as compared to that of ∼11% of crystalline ibuprofen in 15 min.

Synthesis of polyvinyl alcohol/cellulose nanofibril hybrid aerogel microspheres and their use as oil/solvent superabsorbents

Superhydrophobic and crosslinked poly(vinyl alcohol) (PVA)/cellulose nanofibril (CNF) aerogel microspheres were prepared via a combination of the water-in-oil (W/O) emulsification process with the freeze-drying process, followed by thermal chemical vapor deposition of methyltrichlorosilane. The oil phase and the cooling agent were judiciously selected to ensure that the frozen ice microspheres can be easily separated from the emulsion system. The silanized microspheres were highly porous with a bulk density ranging from 4.66 to 16.54mg/cm3. The effects of the solution pH, stirring rate, and emulsifier concentration on the morphology and microstructure of the aerogel microspheres were studied. The highly porous structure of the ultralight aerogel microspheres demonstrated an ultrahigh crude oil absorption capacity (up to 116 times its own weight). This study provides a novel approach for the large-scale preparation of polymeric aerogel microspheres with well-controlled particle sizes that can be used for various applications including oil and chemical spill/leak clean-up.

Synthesis of silica aerogels microspheres prepared by ink jet printing and dried at ambient pressure without surface hydrophobization

Silica aerogel microspheres were synthesized by sol–gel process via ambient pressure drying using tetraethylorthosilicate as precursor, and ink jet printing to shape the microspheres. Aerogels were synthesized with and without the surface hydrophobization step and at different volumetric ratios of sol precursor and a continuous oil phase, which was used to stabilize the spheres ejected by printer. By the first time, SiO2 aerogel precursor was successfully ambient dried without modification step, keeping its integrity (cracks free) and with minimal size contraction, which was verified by comparison with the aerogel spheres obtained by the process of surface hydrophobization and drying.

Highly adsorptive graphene aerogel microspheres with center-diverging microchannel structures

Novel graphene oxide aerogel microspheres (GOAMs) with well-defined “center-diverging microchannel” structures are obtained by a novel approach, i.e. a combination of electrospraying and freeze-casting. A formation mechanism, i.e. radial-directional freezing–thawing, was proposed. The reduced GOAMs exhibit excellent adsorption ability for various organic liquids and oils due to their highly hierarchical hydrophobic structure and a random packing effect.

Mechanism for formation of Hollow and Granular Silica Aerogel Microspheres from rice husk ash for drug delivery

The present work describes a method for preparation of Hollow Silica Aerogel Microspheres (HSAMs) and Granular Silica Aerogel Microparticles (GSAMs) for drug delivery, from rice husk ash (RHA), an inexpensive source of biocompatible silica. This method involves a two-step sol–gel process for preparation of wet gel microspheres using an improved sol–gel/mineral-oil emulsion (containing dual surfactants), followed by solvent exchange and aging in aqueous ethanol, to replace water in the microspheres with ethanol. It is shown that the amount and concentration of the ammonium hydroxide (NH4OH) solution added during gelation process determine if hollow or granular wet gel particles form. A mechanism for the formation of HSAMs or GSAMs is proposed and validated by experiments. The wet gel particles are subsequently dried with supercritical carbon dioxide at 150bar and 50°C to obtain HSAMs or GSAMs. HSAMs afford high loading of drugs as illustrated by loading of 0.47g ibuprofen per g of HSAM. Further, a fast release of ibuprofen from loaded HSAMs compared to pure crystalline ibuprofen and other loaded forms, indicates that HSAMs produced by the present method are good drug delivery vehicles.

Synthesis of silica aerogel microspheres by a two-step acid–base sol–gel reaction with emulsification technique

Silica aerogel microspheres were synthesized by a two-step acid–base sol–gel reaction in water-in-oil emulsion systems, in which tetraethoxysilane was used as a precursor and ethanol as a solvent, and HCl and NH4OH as acid–base catalysts in two steps. The synthesis process and parameters of the emulsion process including viscosity, surfactant concentration and stirring rate have been investigated. In the emulsifying process, the viscosity of silica sol is vital to restrain the occurrence of flocculation phenomenon for forming monodisperse alcogel microspheres. The smooth silica aerogel microspheres can be formed from the silica sol with the viscosity of 107 mPa s. The resultant silica aerogel microspheres with similar surface areas above 650 m2/g, bulk densities in the range of 0.094–0.138 g/cm3, and mean diameters ranging from 40.3 to 126.1 μm can be formed by controlling these parameters of the emulsion process. The minimum of polydispersity and roundness of silica aerogel microspheres are 0.058 and 1.11, respectively. Furthermore, silica aerogel microspheres show a high capacity of uptaking bean oil, isopropanol, kerosene and n-hexane, highlighting the possibility to remove oils from water for oil spill cleanup.

High-performance all-solid state asymmetric supercapacitor based on Co3O4 nanowires and carbon aerogel

An all-solid state asymmetric supercapacitor has been fabricated using carbon aerogel (CA) microspheres as the negative electrode and Co3O4 nanowires on nickel foam (Co3O4–NF) as the positive electrode separated by PVA-KOH membrane as the electrolyte. For the desirable porous structure, high specific capacitance and rate capability of CA and Co3O4–NF, broader potential window of the two electrodes, no binder and conductive agent added, the asymmetric supercapacitor can be cycled reversibly in a wide potential window of 0–1.5 V with an energy density of 17.9 Whkg−1 at a power density of 750 W kg−1. The energy density of the asymmetric supercapacitor is significantly improved in comparison with those of the symmetric supercapacitors based on CA (6.28 Whkg−1) and Co3O4–NF (2.42 Whkg−1). The asymmetric supercapacitor can also deliver a high energy density of 10.44 Whkg−1, even at a high power density of 7.5 kW kg−1. In addition, the asymmetric device shows good stability with approximately 85% of its initial capacitance after 1000 cycles.

Sol–gel synthesis highly porous titanium dioxide microspheres with cellulose nanofibrils-based aerogel templates

Highly porous titanium dioxide microspheres had been prepared via a template-assisted sol–gel process with cellulose nanofibril aerogel microsphere template. The modified porous titanium dioxide microspheres showed a typical super-hydrophobic property. The method reported in this study may be applied to fabricate other inorganic materials with desired porous structure.

Polysaccharide-based aerogel microspheres for oral drug delivery.

Polysaccharide-based aerogels in the form of microspheres were investigated as carriers of poorly water soluble drugs for oral administration. These bio-based carriers may combine the biocompatibility of polysaccharides and the enhanced drug loading capacity of dry aerogels. Aerogel microspheres from starch, pectin and alginate were loaded with ketoprofen (anti-inflammatory drug) and benzoic acid (used in the management of urea cycle disorders) via supercritical CO2-assisted adsorption. Amount of drug loaded depended on the aerogel matrix structure and composition and reached values up to 1.0×10(-3) and 1.7×10(-3) g/m(2) for ketoprofen and benzoic acid in starch microspheres. After impregnation, drugs were in the amorphous state in the aerogel microspheres. Release behavior was evaluated in different pH media (pH 1.2 and 6.8). Controlled drug release from pectin and alginate aerogel microspheres fitted Gallagher-Corrigan release model (R(2)>0.99 in both cases), with different relative contribution of erosion and diffusion mechanisms depending on the matrix composition. Release from starch aerogel microspheres was driven by dissolution, fitting the first-order kinetics due to the rigid starch aerogel structure, and showed different release rate constant (k1) depending on the drug (0.075 and 0.160 min(-1) for ketoprofen and benzoic acid, respectively). Overall, the results point out the possibilities of tuning drug loading and release by carefully choosing the polysaccharide used to prepare the aerogels.

Mesochanneled hierarchically porous aluminosiloxane aerogel microspheres as a stable support for pH-responsive controlled drug release.

The molecular-scale self-assembly of a 3D aluminosiloxane (Al-O-Si) hybrid gel network was successfully performed via the cocondensation of hydrolyzed alumina (AlOOH) and (3-aminopropyl)trimethoxysilane (APS). It was transformed into a microspherical aerogel framework of Al-O-Si containing mesochannels with tunable hierarchically bimodal meso/macroporosities by a subcritical drying technique. Good homogeneity of AlOOH and APS brought during the synthesis guaranteed a uniform distribution of two metal oxides in a single body. A systematic characterization of the aerogel support was carried out using FTIR, SEM, TEM, nitrogen adsorption/desorption analysis, WAXS, SAXS, and ξ-potential measurement in order to explore the material for drug uptake and release. The drug loading and release capacity and chemical stability of an aluminosiloxane aerogel were studied using two nonsteroidal antiinflammatory drugs, ibuprofen and aspirin. A comprehensive evaluation of the aluminosiloxane aerogel with ordered mesoporous MCM-41 was also performed. Aerogel supports showed a high drug loading capacity and a pH-responsive controlled-release property compared to MCM-41. Meanwhile, kinetic modeling studies indicate that the drug releases with a zero-order profile following the Korsmeyer-Peppas model. The biocompatibility of aluminosiloxane aerogels was established via ex vivo and in vivo studies. We also outline the use of aluminosiloxane aerogel as a support for a possible 3D matrix for an osteoconductive structure for bone tissue engineering.

Nanoassembling of thixotropically reversible alumino-siloxane hybrid gels to hierarchically porous aerogel framework

This work describes a versatile strategy for the formation of thixotropically reversible alumino-siloxane hybrid gel suitable for the fabrication of open-porous aerogel like framework. Aminated alumino-siloxane gel was synthesized by pH controlled sol–gel assisted co-condensation of hydrolyzed aluminum isopropoxide with 3-aminopropyltrimethoxy silane. A rare reversible thixotropic flow behavior was noticed for the first time in aminated alumino-siloxane hybrid gel. The hybrid gel was used for preparation of hydrogel microspheres via gel granulation technique. The hydrogel microspheres derived from thixotropic gel showed an open porous network with intercommunicated structure. It was then converted to aerogel microspheres, without the collapse of the transient hybrid porous network by ambient pressure drying technique. The porous framework was tailored to hierarchical domains containing meso/macro porosity, with a dense array of functional amine on internal porous framework. The textural properties of the hybrid alumino-siloxane aerogel microspheres have been characterized by SEM, TEM, FTIR and N2 gas adsorption/desorption analysis and the results are discussed. Hybrid aerogel microspheres exhibit 2–3 times superior properties in terms of surface area and pore features compared to their xerogel counterparts. Thus, open porous aerogel microsphere derived from thixotropic gel could be a new media for controlling host–guest interactions especially when it is intended to be used in adsorption, separation and catalysis/catalyst support.

Aerogel microspheres from natural cellulose nanofibrils and their application as cell culture scaffold.

We demonstrated that ultralight pure natural aerogel microspheres can be fabricated using cellulose nanofibrials (CNF) directly. Experimentally, the CNF aqueous gel droplets, produced by spraying and atomizing through a steel nozzle, were collected into liquid nitrogen for instant freezing followed by freeze-drying. The aerogel microspheres are highly porous with bulk density as low as 0.0018 g cm(-3). The pore size of the cellulose aeogel microspheres ranges from nano- to macrometers. The unique ultralight and high porous structure ensured high moisture (~90 g g(-1)) and water uptake capacity (~100 g g(-1)) of the aerogel microspheres. Covalent cross-linking between the native nanofibrils and cross-linkers made the aerogel microspheres very stable even in a harsh environment. The present study also confirmed this kind of aerogel microspheres from native cellulose fibers can be used as cell culture scaffold.

Superhydrophobic silica aerogel microspheres from methyltrimethoxysilane: rapid synthesis via ambient pressure drying and excellent absorption properties

Silica aerogels are nanostructured porous solids with an open pore structure, high surface area, high porosity, low bulk density, and low thermal/electrical conductivity, which have drawn great interest in both science and technology. Generally, silica aerogels are prepared via a sol–gel process with a subsequent drying step, in which a supercritical condition is usually employed to dry wet silica gels. However, the supercritical drying is a limiting factor for synthesis efficiency and cost. Ambient pressure drying (APD) is considered an alternative and more practical approach. However, the tedious surface modification and solvent exchange steps involved in conventional APD for the synthesis of silica aerogels (bead- or bulk-type) are still challenges. In this study, a novel APD method was successfully developed for the drying of precursor silica microspheres that were obtained via a water-in-oil (W/O) emulsion method using methyltrimethoxysilane (MTMS). The resultant silica aerogel microspheres were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and the Brunauer–Emmett–Teller (BET) method. The results indicate that silica aerogel microspheres possess similar characteristics to those aerogels prepared by supercritical drying. The silica aerogel microspheres have a mean diameter of approximately 300 μm, a bulk density as low as 0.08 g cm−3, a specific surface area as high as 853 m2 g−1, a pore size distribution between 2 and 45 nm, and an average pore size of 16 nm. In addition, the aerogels are superhydrophobic with a contact angle as high as 172°, and they exhibit excellent absorption capability and recyclability for organic liquids and oils. A new production route for silica aerogel microspheres will greatly expand the commercial utilization of these materials.

Preparation of carbon aerogel microspheres by a simple-injection emulsification method

Carbon aerogel microspheres were successfully prepared using a simple-injection emulsification method, employing sol–gel polycondensation of a resorcinol–formaldehyde solution containing sodium carbonate as a catalyst. This process was followed by solvent exchange using acetone, supercritical drying with carbon dioxide and carbonization in a nitrogen atmosphere. The effect of curing time before starting injection, injection rate and agitation rate of continuous phase on the particle size and the porous properties of the carbon aerogel microspheres was investigated. Adsorption of phenol by using the prepared carbon aerogel microspheres was also examined. The diameter of carbon aerogel microspheres was controlled in the range of 20–55μm by varying injection rate and agitation rate. The mean diameter of carbon aerogel microspheres decreased with increasing the injection rate and the agitation rate, whereas their mean diameter was independent of the curing time. The BET surface area and total pore volume of carbon aerogel microspheres increased with increasing the curing time. In contrast, their BET surface area and total pore volume decreased with increasing the injection rate and the agitation rate. The BET surface area, total pore volume, mesopore volume and micropore volume of the carbon aerogel microspheres with a mean diameter of 45μm were 903m2/g, 0.60cm3/g, 0.31cm3/g and 0.27cm3/g, respectively. The phenol-adsorption capacity of these carbon aerogel microspheres was 29.3mg phenol/g adsorbent.

Chitosan Bio‐Based Organic–Inorganic Hybrid Aerogel Microspheres

Recently, organic-inorganic hybrid materials have attracted tremendous attention thanks to their outstanding properties, their efficiency, versatility and their promising applications in a broad range of areas at the interface of chemistry and biology. This article deals with a new family of surface-reactive organic-inorganic hybrid materials built from chitosan microspheres. The gelation of chitosan (a renewable amino carbohydrate obtained by deacetylation of chitin) by pH inversion affords highly dispersed fibrillar networks shaped as self-standing microspheres. Nanocasting of sol-gel processable monomeric alkoxides inside these natural hydrocolloids and their subsequent CO(2) supercritical drying provide high-surface-area organic-inorganic hybrid materials. Examples including chitosan-SiO(2), chitosan-TiO(2), chitosan-redox-clusters and chitosan-clay-aerogel microspheres are described and discussed on the basis of their textural and structural properties, thermal and chemical stability and their performance in catalysis and adsorption.

Preparation of tailor-made starch-based aerogel microspheres by the emulsion-gelation method

The inherent biocompatibility and biodegradability of starch, a natural polysaccharide-based product, allows its use in the form of microspheres as a chemical carrier for life science applications. However, current methods of preparation of starch microspheres utilize chemical crosslinkers and drying methods (air drying, freeze drying) that leads to problems of degradability of the matrix and low specific surface areas and chemical loading capacities. In this work, corn starch aerogel microspheres, a special class of nanoporous materials, were prepared by the combination of an emulsion-gelation method and supercritical drying without the use of chemical crosslinkers. Effects of gelation temperature (368, 393 and 413K), oil-to-aqueous starch solution ratio (1:1, 2:1, 3:1) and surfactant content (3, 6 and 10% (w/w)) on the textural and morphological properties of the aerogel material were studied. The obtained starch aerogels were characterized using nitrogen adsorption–desorption measurements, helium pycnometry, CHN elemental analyses, thermogravimetry and scanning electron microscopy. Spherical starch aerogel microspheres with tailor-made specific surface areas (34–120m2g−1 range) and particle sizes (215–1226μm diameter range) were obtained. Aerogel textural properties were mainly influenced by the gelation temperature used, whereas the particle morphology depended on the three processing parameters studied. High specific chemical loading capacity (1.1×10−3gm−2) of the aerogel microspheres was obtained using ketoprofen as a model compound.