Freeze-dried Gel Research Articles

Role of Capping Ligands on the Nanoparticles in the Modulation of Properties of a Hybrid Matrix of Nanoparticles in a 2D Film and in a Supramolecular Organogel

We incorporate various gold nanoparticles (AuNPs) capped with different ligands in two-dimensional films and three-dimensional aggregates derived from N-stearoyl-L-alanine and N-lauroyl-L-alanine, respectively. The assemblies of N-stearoyl-L-alanine afforded stable films at the air-water interface. More compact assemblies were formed upon incorporation of AuNPs in the air-water interface of N-stearoyl-L-alanine. We then examined the effects of incorporation of various AuNPs functionalized with different capping ligands in three-dimensional assemblies of N-lauroyl-L-alanine, a compound that formed a gel in hydrocarbons. The profound influence of nanoparticle incorporation into physical gels was evident from evaluation of various microscopic and bulk properties. The interaction of AuNPs with the gelator assembly was found to depend critically on the capping ligands protecting the Au surface of the gold nanoparticles. Transmission electron microscopy (TEM) showed a long-range directional assembly of certain AuNPs along the gel fibers. Scanning electron microscopy (SEM) images of the freeze-dried gels and nanocomposites indicate that the morphological transformation in the composite microstructures depends significantly on the capping agent of the nanoparticles. Differential scanning calorimetry (DSC) showed that gel formation from sol occurred at a lower temperature upon incorporation of AuNPs having capping ligands that were able to align and noncovalently interact with the gel fibers. Rheological studies indicate that the gel-nanoparticle composites exhibit significantly greater viscoelasticity compared to the native gel alone when the capping ligands are able to interact through interdigitation into the gelator assembly. Thus, it was possible to define a clear relationship between the materials and the molecular-level properties by means of manipulation of the information inscribed on the NP surface.

Fast Dissolving Tablets of Aloe Vera Gel

Purpose: The objective of this work was to prepare and evaluate fast dissolving tablets of the nutraceutical, freeze dried Aloe vera gel. Methods: Fast dissolving tablets of the nutraceutical, freeze-dried Aloe vera gel, were prepared by dry granulation method. The tablets were evaluated for crushing strength, disintegration time, wetting time, friability, drug content and drug release. A 32 full factorial design was applied to investigate the combined effect of two formulation variables - amounts of microcrystalline cellulose and mannitol. Results: The results of multiple regression analysis revealed that in order to obtain a fast dissolving tablet of the Aloe vera gel, an optimum concentration of mannitol and a higher content of microcrystalline cellulose should be used. A response surface plot was also provided to graphically represent the effect of the independent variables on the disintegration time and wetting time. The validity of the generated mathematical model was tested by preparing a check point batch. Conclusion: This investigation has demonstrated that satisfactory fast dissolving Aloe vera gel tablets can be formulated. It also showed the potential of experimental design in understanding the effect of formulation variables on the quality of fast dissolving tablets. Keywords: Aloe vera, Fast dissolving tablet, Factorial design, Mathematical model, Mannitol, Microcrystalline celluloseTropical Journal of Pharmaceutical Research Vol. 8 (1) 2009: pp. 63-70

Self-organized structural hierarchy in mixed polysaccharide sponges

Hierarchically structured polysaccharide sponges have been prepared by freeze drying of anisotropically ordered alginate–dextran hydrogels produced by dissipative densification in the presence of a uniaxial Ca2+ ion diffusion gradient. The freeze-dried gels exhibit elastic/viscoelastic properties, and consist of aligned micrometre-sized capillaries with complex wall structures comprising interconnected networks of helically ordered micropores. Magnetite or gold nanoparticles can be readily incorporated into the gels with retention of the patterned microstructure. The ability to fabricate such complex microstructures by facile processes should significantly extend the utility of soft materials in diverse encapsulation-based applications.

Utilization of retrograded waxy maize starch gels as tablet matrix for controlled release of theophylline

Theophylline tablets were prepared using waxy maize starch gels and the effect of retrogradation on the release of theophylline was investigated. Slurries of starch and theophylline were autoclaved in a polypropylene tablet mold and then stored at 4 °C or cycled at 4/30 °C for 8 days. By increasing the period of retrogradation at 4 °C the size of air cells in freeze-dried gels was decreased and the cell walls became thinner. Retrogradation reduced the pore size of the gels and hindered gel swelling in an amylase-containing dissolution medium. These effects became more significant with temperature cycles. The resistance to enzymatic erosion and decreased swelling by the retrogradation under 4/30 °C cycles resulted in a retarded release of theophylline. Thus, temperature-cycled retrogradation of a waxy maize starch gel provided a compact matrix structure that effectively retarded the drug release.

Topical application of a bioadhesive black raspberry gel modulates gene expression and reduces cyclooxygenase 2 protein in human premalignant oral lesions.

Reduced expression of proapoptotic and terminal differentiation genes in conjunction with increased levels of the proinflammatory and angiogenesis-inducing enzymes, cyclooxygenase 2 (COX-2) and inducible nitric oxide synthase (iNOS), correlate with malignant transformation of oral intraepithelial neoplasia (IEN). Accordingly, this study investigated the effects of a 10% (w/w) freeze-dried black raspberry gel on oral IEN histopathology, gene expression profiles, intraepithelial COX-2 and iNOS proteins, and microvascular densities. Our laboratories have shown that freeze-dried black raspberries possess antioxidant properties and also induce keratinocyte apoptosis and terminal differentiation. Oral IEN tissues were hemisected to provide samples for pretreatment diagnoses and establish baseline biochemical and molecular variables. Treatment of the remaining lesional tissue (0.5 g gel applied four times daily for 6 weeks) began 1 week after the initial biopsy. RNA was isolated from snap-frozen IEN lesions for microarray analyses, followed by quantitative reverse transcription-PCR validation. Additional epithelial gene-specific quantitative reverse transcription-PCR analyses facilitated the assessment of target tissue treatment effects. Surface epithelial COX-2 and iNOS protein levels and microvascular densities were determined by image analysis quantified immunohistochemistry. Topical berry gel application uniformly suppressed genes associated with RNA processing, growth factor recycling, and inhibition of apoptosis. Although the majority of participants showed posttreatment decreases in epithelial iNOS and COX-2 proteins, only COX-2 reductions were statistically significant. These data show that berry gel application modulated oral IEN gene expression profiles, ultimately reducing epithelial COX-2 protein. In a patient subset, berry gel application also reduced vascular densities in the superficial connective tissues and induced genes associated with keratinocyte terminal differentiation.

Rheological behavior of heat-induced wheat gliadin gel

Wheat gliadin gel is prepared for the first time through heating alkaline solution of propanol/water (50/50, v/v) at pH = 9.3 and 50 °C. Dynamic rheological tests were performed to characterize the gelation time and the number of elastically effective chains of the gliadin gel. Scanning electron microscope was used to observe the morphology of the freeze-dried gel.

Water sorption and physical properties of locust bean gum–pectin–starch composite gel dried using different drying methods

Locust bean gum–pectin–starch gel was dried with four different drying methods: air, vacuum, freeze and microwave vacuum drying to a moisture content of about 3–5% on wet basis. The effect of the drying method on water sorption properties of dried gel was evaluated at 25 °C. Three different equations proposed in the literature (GAB, Oswin, Hasley) were used to fit the sorption data. All gave the best fit to the experimental data with a correlation coefficient of 0.99. The porosity of the dried gels varied with different drying methods and ranged between 68% and 87%. Freeze-dried gel developed the highest porosity, whereas the lowest porosity was obtained from air drying method. Air and microwave vacuum-dried gels had narrow pore size distribution where as freeze and vacuum-dried gels had wider pore size distribution. Also, the freeze-dried gels had pore structural collapse and low bulk density. Isoteric heat of sorption, which is an indicator of state of water absorbed by the solid material, was calculated for each drying methods using the Clausius–Clapeyron equation.

Universal method for synthesis of artificial gel antibodies by the imprinting approach combined with a unique electrophoresis technique for detection of minute structural differences of proteins, viruses and cells (bacteria). Ib. Gel antibodies against proteins (hemoglobins)

Using the molecular imprinting approach, we have shown that polyacrylamide-based artificial antibodies against human and bovine hemoglobin have a very high selectivity, as revealed by the free-zone electrophoresis in a revolving capillary. By the same technique we have previously synthesized gel antibodies not only against proteins but also against viruses and bacteria. The synthesis is thus universal, i.e., it has the great advantage of not requiring a modification - or only a slight one - for each particular antigen. The combination synthesis of artificial gel antibodies and electrophoretic analysis reveals small discrepancies in shape and chemical composition not only of proteins, as shown here and in paper Ia, but also of viruses and bacteria, to be illustrated in papers II and III in this series. Upon rehydration, the freeze-dried gel antibodies, selective for human hemoglobin, regain their selectivity. The gel antibodies can repeatedly be used following the removal of the antigen (protein in this study) from the complex gel antibody/antigen by an SDS washing or an enzymatic degradation.

The inner gel component of Aloe vera suppresses bacterial-induced pro-inflammatory cytokines from human immune cells

The present study was carried out to examine the anti-inflammatory activity of the inner leaf gel component of Aloe barbadensis Miller. A simple in vitro assay was designed to determine the effect of the inner gel on bacterial-induced pro-inflammatory cytokine production, namely TNF-α and IL-1β, from peripheral blood leukocytes stimulated with Shigella flexneri or LPS. This report describes the suppression of both cytokines with a freeze-dried inner gel powder and a commercial health drink from the same source. Comparison was made with a human monocytic cell-line (THP-1 cells) and a similar trend in responses was demonstrated.

Peripheral nerve regeneration through the space formed by a chitosan gel sponge

The clinical treatment of traumatized peripheral nerves often requires grafting of autologous cutaneous nerves. However, there are drawbacks in sacrificing healthy nerves and tissue scarring. In this study, an artificial material, freeze-dried chitosan gel sponge, was examined as a scaffold for nerve regeneration in rats. An 8-mm gap was made by removing a segment of the sciatic nerve, and the distal and proximal stumps were sandwiched by chitosan gel sponge. Rats were killed at 4, 7, 14, and 28 days, and 2 and 4 months after the operation and histological and morphometric evaluations were performed. Regenerating axons were observed at 4 days after the operation. Regenerating nerves extended the distal stump at 14 days after surgery. By electron microscopy, numerous macrophages appeared to phagocyte chitosan, and made a dense cell layer on the chitosan. Regenerating axons did not touch the chitosan, and extended through the space surrounded by macrophage-stacked chitosan. Regenerating nerves were well-myelinated 2 months after surgery. Regenerating nerves were on average 2.45 and 2.75 microm in diameter at 2 and 4 months, respectively, after surgery. These results indicate that the chitosan gel sponge sandwich might be suitable as a graft for peripheral nerve regeneration.

Preparation of Carbon Nanotube-alginate Nanocomposite Gel for Tissue Engineering

A novel scaffold material based on an alginate hydrogel which contained carbon nanotubes (CNTs) was prepared, and its mechanical property and biocompatibility evaluated. Soluble CNTs were prepared with acid treatment and dispersed in sodium alginate solution as a cross-linker. After which, the mechanical property (elastic deformation), saline sorption, histological reaction, and cell viability of the resultant nanocomposite gel (CNT-Alg gel) were evaluated. The CNT-Alg gel showed faster gelling and higher mechanical strength than the conventional alginate gel. Saline sorption amount of freeze-dried CNT-Alg gel was equal to that of the alginate gel. In terms of histological evaluation and cell viability assay, CNT-Alg gel exhibited a mild inflammatory response and non-cytotoxicity. These results thus suggested that CNT-Alg gel could be useful as a scaffold material in tissue engineering with the sidewalls of CNTs acting as active sites for chemical functionalization.

Catalytic Activity of Rhodium Complexes Supported on Al2O3–ZrO2in Isomerization and Hydroformylation of 1-Hexene

Two new nanostructured Al2O3–ZrO2 mixed oxides supports, A and B, have been prepared by sol–gel process and characterized by TEM, XRD and XPS methods. The freeze-dried gel gave support A whereas the support B was obtained by drying of gel at ambient temperature. Both supports reacted with Rh(acac)(CO)2 complex, producing Rh(CO)2 + tethered species, characterized by ν(CO) at 2011 and 2083 cm−1. The Rh(CO)2 +/A applied as catalyst for reaction of 1-hexene at 80 °C with H2/CO = 1, at 10 atm was found as active for isomerization (ca. 90% of 2-hexene after 6 h) and relatively poor catalyst of hydroformylation (<10% of aldehydes). At the presence of stoichiometric amount of PPh3 the yield of aldehydes increased up to 65%. The selectivity of the reaction was strongly dependent on the catalyst pre-treatment with CO or CO/H2 mixture.

Change of pore structure in freeze-dried silica gel during calcination

Change of pore structure in freeze-dried silica gel during calcination

Effect of preservatives on the functional properties of tapioca starch: analysis of interactions

The effect of different levels of potassium sorbate or sodium benzoate addition on the water sorption behavior and on the mechanical properties of freeze-dried tapioca starch gel was studied. It was observed that the addition of potassium sorbate or sodium benzoate to starch before the gelatinization process increased the amount of water sorbed by the system, but experimental moisture content of the mixture was lower than the one predicted by a mass balance, revealing the existence of an interaction between starch and the preservatives. In general, in the range of water activity 0.755–0.976, most of the preservative was bounded to starch. Mechanical properties of the powder obtained after freeze drying of gels were also influenced by the addition of the preservatives: an increase in bulk density, higher cohesiveness and initial Young modulus was observed after antimicrobial addition. Results obtained showed that sorbate and benzoate interact with tapioca starch and that this interaction has a great influence on system behavior.

Nanostructured amorphous manganese oxide cryogel as a high-rate lithium intercalation host

Amorphous manganese oxides have received increasing attention in recent years as high-capacity intercalation cathodes for rechargeable lithium batteries. Nanostructured electrodes have been shown to exhibit enhanced rate capabilities. In this paper, a nanostructured amorphous manganese oxide cryogel is reported that combines the advantages of the amorphous structure and the nano-architecture, leading to high lithium intercalation capacity and high rate capability. The cryogel is prepared by freeze-drying a Mn(IV) oxide hydrogel formed in situ via reduction of sodium permanganate solution with solid fumaric acid. The cryogel exhibits a nanoporous structure and a specific surface area of 350 m 2/g. X-ray and electron diffraction reveals the amorphous nature of this freeze-dried gel. The material exhibits specific capacities of 289, 217 and 174 mAh/g at C/100, C/5 and 2C rates, respectively, demonstrating high capacity and excellent rate capability. This work shows the feasibility of the freeze-drying method for obtaining manganese oxides with nano-architecture and high specific surface area, and suggests the promise of the synthesis route to nanostructured amorphous manganese oxides via a hydrogel.

Structure and properties of silk fibroin–poly(vinyl alcohol) gel

A series of porous silk fibroin–poly(vinyl alcohol) blend gels were prepared from corresponding aqueous mixtures by freeze- or air-drying, and their structure and mechanical properties were examined. The air-dried gels had higher crystallinity and much greater strengths than the freeze-dried gels, and therefore are suitable for mechanical uses. The freeze-dried gels had characteristic porous structure potentially useful as cell culture substrate. Its structure could be systematically varied by changing freezing temperature and freeze–thaw pretreatments before drying.

Effect of starch, sucrose and their combinations on the mechanical and acoustic properties of freeze-dried alginate gels

Alginate gels (2%) with and without incorporated starch and/or sucrose were freeze-dehydrated to produce cellular sponges. The stress–strain relationships of these dried gels were all irregular and jagged, typical of brittle cellular solids. The stiffness was assessed in terms of the fitted (‘smoothed’) apparent engineering stress at two pre-selected engineering strain levels (40 and 60%) using a polynomial model. Their brittleness was assessed in terms of the apparent fractal dimension of their stress-strain curves using the compass and box counting algorithms which yield the Richardson's and Kolmogorov's dimensions, respectively. The presence of starch in the dried gels solid matrix invariably increased their stiffness, but not in a manner that could be predicted on the basis of a stochiometric relationship or density increase. Sucrose, in contrast, could have the opposite effect suggesting that its presence in the solid matrix might interfere with its mechanical integrity. All the dried gels had a ‘rich’ acoustic signature as judged by the magnitude of their apparent fractal dimension, which was determined by the ‘blanket’ algorithm. But if there were differences between the different dried gels' acoustic signatures, they were too subtle to be detected by the apparent fractal dimension alone.

ChemInform Abstract: Relationships Between Structural Features, Molecular Weight and Rheological Properties of Cereal β-D-Glucans

Publisher Summary Although cereal β-D-glucans exhibit similar C13-NMR spectra, their structural features are different from each other as evident by the ratios of tri- and tetrasaccharides released from lichenase hydrolysis (4, 3, and 2 for wheat, barley and oat p-D-glucans, respectively). This chapter demonstrates the correlation between the structural features, molecular weight, and some physical properties including solubility of freeze-dried samples and gel forming aptitude. Rheologically, cereal β-D-glucans behave similarly to random coil polysaccharides; however, some β-D-glucans can form gels under certain conditions, which is not observed for other random-coil polysaccharides including galactomannans. The gelling ability and insolubility of freeze-dried cereal β-D-glucans followed the order of wheat > barley > oat; this trend corresponds with the ratio of tri- and tetrasaccharides of cereal β-D-glucans. In addition to the structural features, molecular weight also plays an important role in the conformation of cereal β-D-glucans in aqueous systems. The results indicate that lower molecular weight β-D-glucan is easier to form a gel than larger molecules.

Reconstruction of rat peripheral nerve gap without sutures using freeze-dried alginate gel

Many materials have been used for artificial tubular prostheses to assist peripheral nerve gap reconstruction. However, the clinical use of these devices has been restricted because a microsurgical procedure requires specialized techniques and expensive equipment, such as operating microscope systems. Therefore the authors developed a new gluing method, without sutures, that uses freeze-dried alginate gel. A 7-mm gap in the sciatic nerve of rats was bridged with freeze-dried alginate gel. Regeneration was evaluated by electrophysiologic testing and histologic study. Eighteen weeks after surgery, functional reinnervation of motor and sensory nerves had occurred, as demonstrated by recovery of compound muscle action potentials (CMAP), compound nerve action potentials (CNAP), and somatosensory-evoked potentials (SEP). Histologically, many regenerated nerve fasciculi, including myelinated and unmyelinated fibers, were observed and the implanted alginate gel had disappeared. In conclusion, a gluing technique using alginate gel is a potential alternative to the conventional nerve autograft technique. Advantages include simple application and rapid repair. Freeze-dried alginate gel is a promising material for artificial nerve guides for peripheral nerves and also could be used for repair of disrupted pathways in central nervous tissue that is amorphous and cannot be sutured.

Regeneration of transected spinal cord in young adult rats using freeze-dried alginate gel.

We have recently reported that freeze-dried alginate gel, which was developed in our laboratory, enhanced peripheral nerve regeneration. The purpose of this study was to examine whether alginate gel is capable of promoting nerve regeneration in the severed spinal cord of adult mammals. Using Wistar rats at 30 days of age (P30), the T9-T10 spinal cord was totally resected and alginate gel was implanted across the gap. Forty-five days after surgery myelinated and unmyelinated axons regenerated throughout the gap with remaining alginate gel. The elongated axons established electrophysiologically functional projections across the gap. In conclusion, freeze-dried alginate gel could be a promising material as an artificial nerve guide for repair of injured central nervous system.