Extent Of Gelation Research Articles

Scaling Properties of Gelling Systems in Nonlinear Shear Experiments.

We study model near-critical polymer gelling systems made of gluten protein dispersions stabilized at different distances from the gel point. We impose different shear rates and follow the time evolution of the stress. For sufficiently large shear rates, an intermediate stress overshoot is measured before reaching the steady state. We evidence self-similarity of the stress overshoot as a function of the applied shear rate for samples with various distances from the gel point, which is related to the elastic energy stored by the samples, as for dense systems close to the jamming transition. In concordance with the findings for glassy and jammed systems, we also measure that the stress after flow cessation decreases as a power law with time, with a characteristic relaxation time that depends on the shear rate previously imposed. These features revealed in nonlinear rheology could be the signature of a mesoscopic dynamics, which would depend on the extent of gelation.

Development and Evaluation of a Thermosensitive In Situ Gel Formulation for Intravaginal Delivery of Lactobacillus gasseri.

In situ administration of vaginal probiotics has been proposed as an effective prevention strategy against gynecological diseases caused by microecological disorders. In this study, a thermosensitive in situ gel formulation was prepared for intravaginal delivery of Lactobacillus gasseri (L. gasseri). The optimized formulation was characterized for the rheological properties, in vitro release properties, and microencapsulation efficiency. The mixtures of poloxamer 407 (26.0% w/w) and 188 (9.0% w/w) produced an increase in gelation extent at 37 °C after dilution in simulated vaginal fluid (SVF). The presence of a low concentration of hyaluronic acid (HA, 0.3% w/w) improved the mucoadhesive properties and the capability to gel at 37 °C. Additionally, the viability of L. gasseri encapsulated with alginate or via co-extrusion technique with fructooligosaccharide (FOS, 0.5% w/w) was maintained at 11 log CFU/mL for eight weeks at 4 °C. In conclusion, the evaluation of the in situ thermosensitive gel formulation was shown to be efficacious for intravaginal delivery of L. gasseri with suitable textural and rheological properties.

Freeze-Thaw-Induced Physically Cross-linked Superabsorbent Polyvinyl Alcohol/Soy Protein Isolate Hydrogels for Skin Wound Dressing: In Vitro and In Vivo Characterization.

In this work, polyvinyl alcohol (PVA)- and soy protein isolate (SPI)-based scaffolds were prepared by physical cross-linking using the freeze-thaw method. The PVA/SPI ratio was varied to examine the individual effects of the two constituents. The physicochemical properties of the fabricated scaffolds were analyzed through Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. The SPI concentration significantly affected the properties of scaffolds, such as the extent of gelation (%), pore size, porosity, degradation, swelling, and surface wettability. The in vitro degradation of fabricated hydrogels was evaluated in phosphate-buffered saline and lysozyme solution for a duration of 14 days. The in vitro compatibility of prepared hydrogels was evaluated by the MTT assay with NIH-3T3 cells (fibroblast). The water vapor transmission rate (WVTR) assays showed that all hydrogels possessed WVTR values in the range of 2000-2500 g m-2 day-1, which is generally recommended for ideal wound dressing. Overall, the obtained results reveal that the fabricated scaffolds have excellent biocompatibility, mechanical strength, porosity, stability, and degradation rate and thus carry enormous potential for tissue engineering applications. Furthermore, a full-thickness wound healing study performed in rats supported them as a promising wound dressing material.

How alginate properties influence in situ internal gelation in crosslinked alginate microcapsules (CLAMs) formed by spray drying.

Alginates gel rapidly under ambient conditions and have widely documented potential to form protective matrices for sensitive bioactive cargo. Most commonly, alginate gelation occurs via calcium mediated electrostatic crosslinks between the linear polyuronic acid polymers. A recent breakthrough to form crosslinked alginate microcapsules (CLAMs) by in situ gelation during spray drying ("CLAMs process") has demonstrated applications in protection and controlled delivery of bioactives in food, cosmetics, and agriculture. The extent of crosslinking of alginates in CLAMs impacts the effectiveness of its barrier properties. For example, higher crosslinking extents can improve oxidative stability and limit diffusion of the encapsulated cargo. Crosslinking in CLAMs can be controlled by varying the calcium to alginate ratio; however, the choice of alginates used in the process also influences the ultimate extent of crosslinking. To understand how to select alginates to target crosslinking in CLAMs, we examined the roles of alginate molecular properties. A surprise finding was the formation of alginic acid gelling in the CLAMs that is a consequence of simultaneous and rapid pH reduction and moisture removal that occurs during spray drying. Thus, spray dried CLAMs gelation is due to calcium crosslinking and alginic acid formation, and unlike external gelation methods, is insensitive to the molecular composition of the alginates. The 'extent of gelation' of spray dried CLAMs is influenced by the molecular weights of the alginates at saturating calcium concentrations. Alginate viscosity correlates with molecular weight; thus, viscosity is a convenient criterion for selecting commercial alginates to target gelation extent in CLAMs.

Development of a Mucoadhesive in Situ Gelling Formulation for the Delivery of Lactobacillus gasseri into Vaginal Cavity.

Local administration of vaginal probiotics, especially lactobacilli, has been recently proposed as an effective prevention strategy against candidosis recurrences, which affect 40–50% of women. In this context, the aim of the present work was the development of a mucoadhesive in situ gelling formulation for the vaginal administration of Lactobacillus gasseri. Mixtures of poloxamer 407 (P407) and methylcellulose (MC), two thermosensitive polymers, were prepared and subjected to rheological analyses for the assessment of their sol/gel transition temperature. The association of P407 (15% w/w) with MC (1.5% w/w) produced an increase in gelation extent at 37 °C even after dilution in simulated vaginal fluid (SVF). The presence of 0.5% w/w pectin (PEC) produced a reduction of vehicle pH and viscosity at 25 °C that is the vehicle resistance to flow during administration. The presence of a low concentration of xyloglucan (XYL) (0.25% w/w) increases the mucoadhesive properties and the capability to gelify at 37 °C of the formulation after dilution with SVF. A three-component (P407/MC/PEC; 3cM) and a four-component (P407/MC/PEC/XYL; 4cM) mixture were selected as promising candidates for the delivery of L. gasseri to the vaginal cavity. They were able to preserve L. gasseri viability and were cytocompatible towards the HeLa cell line.

Poly (vinyl alcohol)/nano-diamond composite films and hydrogels prepared by gamma ray

Abstract Poly (vinyl alcohol) and nano-diamond, PVA/ND, hydrogels were prepared and assessed as prosthetic material suitable for replacement of the nucleus pulposus. The hydrogels were prepared by gamma irradiation at various doses (15 kGy, 25 kGy, 35 kGy, 45 kGy) and at various ND concentrations ranging from 0.25 wt.% to 3 wt.%. Extent of gelation, equilibrium water content, and viscoelastic properties of swelled hydrogels at definite water contents were measured and examined as a function of ND concentration as well as gamma dose. According to viscoelastic measurements, the strength of hydrogels increased considerably over that of pure PVA at a low concentration of ND. By increasing irradiation dose, gel percent and strength of hydrogels increased. Hydrogel water content was in a range of 80 wt.% to 90 wt.% similar to that of natural nucleus pulposus. The G″ values of hydrogels were much smaller than G′ values indicating elastic behavior. Also PVA/ND nanocomposite films were prepared at various ND concentrations by solution casting. The ND particles were uniformly distributed within PVA films. Tensile modulus and strength of the films increased over pure PVA.

Nitroalcohol induced hydrogel formation in amine‐functionalized polymers

Certain β-nitroalcohols degrade under basic conditions or upon heating to form formaldehyde. This reaction provides an elegant approach to generate formaldehyde within a system at a desired time using the stimulus of pH or temperature. Using β-nitroalcohols as a delivery agent for formaldehyde, polymer crosslinking can be induced via stimulus. Such an approach is akin to those used to prepare "self-healing" polymers, which have received much attention recently. Herein, we describe the use of certain β-nitroalcohols as a masked formaldehyde delivery system and demonstrate its use as a crosslinking agent of amine functionalized polymers to form hydrogels. We examine the temperature and pH dependence of 2-nitro-1,3-propanediol and 2-(hydroxymethyl)-2-nitro-1,2-propanediol on the rate and extent of gelation and characterize the resulting gel by swelling and FTIR experiments.

Viscoelasticity of gelled waxy crude oil

When the temperature of waxy crude oil is below the wax precipitation temperature, wax will precipitate out from oil solution. On further cooling, more and more wax crystals separate out and interlock to form a network that entraps liquid oil into its structure, resulting in gelation of the crude oil. The gelation extent and strength can be expressed with viscoelastic parameters. A controlled-stress rheometer RS150H was used to measure the viscoelastic parameters of Daqing crude under different history conditions by oscillatory shear experiment. The results indicate that the storage modulus and loss modulus increase in an exponential relation as the measuremental temperature is decreased. There exists a worst heating temperature for the rheological behavior. The strength of wax crystal structure and gelation temperature increase with decreasing cooling rate. Under the same shear temperature, the strength of wax crystal structure decreases rapidly and finally tends to a steady state as the energy dissipation due to viscous flow is decreased. Under the same energy dissipation due to viscous flow, the closer the shear temperature is to the measuring temperature, the lower the storage modulus, loss modulus and gelation temperature will emerge. When the shear temperature is close to or higher than abnormal point, the viscoelastic parameters vary little. The recovery of wax crystal structure after shear action exhibits an irreversible property.

A novel thermally-activated crosslinking agent for chitosan in aqueous solution: a rheological investigation

The use of 2,5-dimethoxy-2,5-dihydrofuran (DHF) as a temperature-controlled gelation agent for chitosan under acidic conditions has been examined by dynamic oscillatory and viscometry techniques. In particular, the rate and extent of gelation have been examined over a range of different temperatures (40–98 °C), DHF concentrations (10–100 mM) and pH conditions (0.9–2.1). The gelation time, tG, decreases, and the maximum gelation rate increases substantially as a function of rising temperature. When fit with a simple Arrhenius function, the tG data yield an activation energy for gelation of 55±8 kJ mol-1. Gelation is found to occur on the shortest time-scale, and the strongest gels result, at the highest DHF concentrations investigated. Similarly, the gelation rate and gel strength are highest for the most acidic solution conditions examined. Experimental findings are interpreted in terms of a competition between the crosslinking reaction (which drives gel formation, and is initially dominant) and protolytic decomposition of chitosan (which disrupts the gel structure, and becomes increasingly important as time progresses). Syneresis phenomena additionally impact results obtained at DHF concentrations ≥50 mM.

Rheology and gelation of water-insoluble dextran from Leuconostoc mesenteroides NRRL B-523

Dynamic oscillatory testing has been used to study the rheology of water-insoluble dextran. The rheological properties (storage and loss moduli) of dextran gel were measured and dextran was found to be neither a strong gel nor a weak gel, but an entanglement network at a concentration of 250 mg/ml. The extent of gelation, illustrated by the gel elastic modulus G′, is found to decrease with increasing concentration of calcium ions. This was confirmed by shift of crossover frequencies towards higher values on the dynamic spectra and lower yield stress τ values obtained from stress ramp experiments. Finally, a comparison between gelation of dextran and alginate (a similar biopolymer) was made for clear understanding of effect of calcium ions on the dextran gelation.

Gel point studies for chemically modified biopolymer networks using small amplitude oscillatory rheometry

Solutions of xanthan gum form a strong gel upon the addition of aluminum ions (Al(III)). Previous studies have reported that the addition of chromium and ferric ions induces gelation in xanthan solutions; this study presents a characterisation of the transition from solution to gel for xanthan in the presence of Al(III). Xanthan-Al(III) solutions are shown rheologically to undergo a sol–gel transition upon heating, the extent of gelation is controlled by the temperature at which the system equilibrates. The rapid thermal equilibration of xanthan-Al(III) provides an ideal system to test the applicability of two established gelation criteria [1,2]. At the gel point, congruency in G′ and G″ is observed over 2–3 decades of frequency demonstrating the applicability of the method of Winter and Chambon [1]. The difficulties associated with determining an equilibrium relaxation modulus (E) as a criterion for gelation [2] are discussed.The gelation behaviour of xanthan-Al(III) gels is characterised for a range of polymer crosslinker combinations (stoichiometric ratios) and a limiting stoichiometric ratio below which gelation does not occur is determined. The relaxation exponent (n) is determined at the limiting stoichiometric ratio to be n=0.22. A value of 0.22 for n is not in agreement with previous work for systems at a limiting stoichiometric ratio. The relaxation exponent is found to vary between 0.22 and 0.16 for a 0.5wt% xanthan and a range of Al(III) concentrations. The temperature at which the gel point is observed, Tg, follows an asymptotic relationship with stoichiometric ratio suggesting the importance of the order–disorder transition of xanthan molecules on the gelation process of xanthan-Al(III) gels.

Enhanced second order non-linear optical properties of silica sol-gel thin films through photo-induced poling

Sol-gel derived inorganic materials are of interest as hosts for non-linear optically active guest molecules and they offer particular advantages in the field of non-linear optics. Orientationally ordered glasses have been prepared using a sol-gel system based on tetramethoxysilane, methyltrimethoxysilane and a non-linear optical chromophore Disperse Red 1. The novel technique of photo-induced poling was used to generate enhanced levels of polar order. The level of enhancement is strongly dependent on the extent of gelation and an optimum preparation time of ∼100 h led to an enhancement factor of ∼5. Films prepared in this manner exhibited a high stability of the polar order.

The contractile basis of ameboid movement. VI. The solation-contraction coupling hypothesis.

The contracted pellets derived from a high-speed supernate of Dictyostelium discoideum (S3) were investigated to determine the functional activity associated with this specific subset of the cellular motile apparatus. A partially purified model system of gelation and contraction (S6) was prepared from the contracted pellets, and the presence of calcium- and pH-sensitive gelation and contraction in this model demonstrated that a functional cytoskeletal-contratile complex remained at least partially associated with the actin and myosin during contraction. Semi-quantitative assays of gelation and solation in the myosin-free preparation S6 included measurements of turbidity, relative viscosity, and strain birefringence. The extent of gelation was optimal at pH 6.8 and a free calcium ion concentration of approximately 3.0 x 10(-8) M. Solation was favored when the free calcium ion concentration was greater than 7.6 x 10(-7) M or when the pH was increased or decreased from pH 6.8. Gelation was reversibly inhibited by increasing the free calcium ion concentration to approxomately 4.6 x 10(-6) M at pH 6.8. The solation-gelation process of this model has been interpreted to involve the reversible cross-linking of actin filaments. The addition of purified D. discoideum myosin to S6 served to reconstitute calcium- and pH-regulated contraction. The results from this study indicate that contraction is coupled functionally to the local breakdown (solation) of the gel. Therefore, solation has been identified as a structural requirement for extensive shortening during contraction. We have called this concept the solation-contraction coupling hypothesis. Fractionation of a preparation derived from the contracted pellets yielded a fraction consisting of actin and a 95,000-dalton polypeptide that exhibited calcium-sensitive gelation at 28 degrees C and a fraction composed of actin and 30,000- and 18,000-dalton polypeptides that demonstrated calcium-sensitive genlation at 0 degrees C.

Effects of amino acids on gelation kinetics and solubility of sickle hemoglobin

Ten amino acids have been studied for their effects on the gelation of sickle hemoglobin using the recently developed assay of Hofrichter, Ross and Eaton. By monitoring kinetics and using high speed sedimentation, the rate and extent of gelation are directly measured. Of the amino acids tested, only phenylalanine significantly inhibited the gelation of sickle hemoglobin. The systematic study of the effects of additives, such as amino acids, on gelation serves as a basis for the study of potential non-covalent inhibitors of sickling.

Supersaturation in sickle cell hemoglobin solutions.

The kinetic inhibition of the gelation of hemoglobin S is compared to the change in hemoglobin S soulbility, when the solubility is altered by carbon monoxide, pH, or urea. By means of a new technique, the delay time and the extent of gelation are measured on the same sample. They delay time, td, is found to be proportional to a high power (30-40) of the hemoglobin S solubility. Togehter with the previously reported concentration dependence, this result demonstrates that the rate is proportional to a high power of the supersaturation, S, defined as the ratio of the total hemoglobin S concentration to the equilibrium solubility. The results obey the supersaturation equation td-1 = gammaSn, where gamma is an empirical constant (about 10(-7) sec-1) and n is about 35. The supersaturation equation can successfully account for observations on the kinetics of cell sickling and is therefore used to estimate the increase in the delay time for sickling necessary to produce significant clinical benefit to patients with sickle cell disease.

Mastication of Elastomers I. Natural Rubber in Nitrogen

Abstract In accordance with theoretical expectation, the mastication of acetone extracted natural rubber in the absence of radical acceptors leads to gel formation. Unextracted rubbers undergo no gelation. The extent of gelation is determined almost entirely by the initial molecular weight of the rubber: under the conditions studied here rubbers of initial molecular weight less than ca. 3×105 do not gel on mastication. Only very small quantities of radical acceptors are required to repress gelation suggesting that acetone extracted rubber still contains appreciable quantities of such materials.