Temperature dependence of the elastic modulus of alginate gels

The temperature dependence of the elastic modulus for alginate gels was studied using two different gel systems: covalently crosslinked Na-alginate gels and in-situ prepared Ca-alginate gels. The modulus of physically crosslinked gels showed a complex behaviour. The temperature coefficient of the modulus of covalently crosslink gels changed from positive for the lowest crosslinked gels to negative for the highest crosslinked gels. This suggests a change from rubberlike to enthalpy-driven elasticity with an increasing degree of crosslinking for these gel networks.     

Some properties of alginate gels derived from algal sodium alginate

Alginic acid in soluble sodium alginate turns to insoluble gel after contact with divalent metal ions, such as calcium ions. The sodium alginate character has an effect on the alginate gel properties. In order to prepare a suitable calcium alginate gel for use in seawater, the effects of sodium alginate viscosity and M/G ratio (the ratio of D-mannuronate to L-guluronate) on the gel strength were investigated. The wet tensile strengths of gel fibers derived from high viscosity sodium alginate were higher than those from low viscosity sodium alginate. The tensile strength increased with diminishing sodium alginate M/G ratio. Among the gel fibers tested, the gel fiber obtained from a sodium alginate I-5G (1% aqueous solution viscosity = 520 mPa·s, M/G ratio = 0.6) had the highest wet tensile strength. After 13 days treatment in seawater, the wet tensile strength of the gel fiber retained 36% of the original untreated gel strength. For sodium alginates with similar viscosities, the seawater tolerance of low M/G ratio alginate was greater than that of the high M/G ratio one. This study enables us to determine a suitable calcium alginate gel for use in seawater.     

Temperature dependence of dielectric constant at 10 GHz of Na-DNA gels

Permittivity ε′ and dielectric loss ε″ of aqueous Na-DNA gels have been measured at 10 GHz in the temperature interval ?15 to + 45°C. The experimental results are analyzed in terms of a three-component equation (Na-DNA, interfacial water, bulk water) and yield a value of 35 water molecules/nucleotide interacting with DNA. According to theoretical and experimental data the presence of strongly bonded and weakly bonded water is considered. The modified water exhibits a mean dielectric relaxation time two-or threefold greater than that of bulk water.     

Optical anisotropy of calcium-induced alginate gels

Alginate gels formed by diffusion of calcium ions into solutions of sodium alginate were found to exhibit optical anisotropy depending on preparation conditions. When observed under crossed nicols, the anisotropic alginate gels showed a birefringence pattern which is characteristic of radial orientation of polymer chains. Calcium alginate gels were prepared from different concentrations of sodium alginate and calcium ion, and the conditions for formation of the anisotropic gels were determined. The gel-formation process was measured by monitoring the development of the birefringent layer and was compared with the model in which the diffusion of calcium ions dominates gel formation.     

Temperature dependence of the dielectric properties of blood

The aim of the work was to investigate the temperature effect in physiological range on the dielectric properties of a fish (Cyprinus carpio) blood. By applying an empirical non-damaging method and statistical analysis of the dielectric data, it is observed that a heating of carp blood induces an almost linear value variation of the electrical conductivity and permittivity of the intracellular matter of carp red blood cells. The dielectric data, however, reveal anomalous behaviour for conductivity and permittivity of the interior of carp erythrocytes at 23 degrees C and 23 degrees, 37 degrees C, respectively and for both volume fraction and dielectric loss factor tg delta at 30 degrees, 35 degrees, 37 degrees C. For the presented dielectric parameters the temperature coefficient and the Arrhenius activation energy were estimated. Moreover, this work led to conclusion that the changes of the volume fraction of carp erythrocytes induce corresponding variations of the maximum of the dielectric loss factor tg delta determined for a whole blood.     

Effects of sterilization treatments on some properties of alginate solutions and gels.

Aqueous sodium alginate solutions were subjected to various heat sterilization treatments. Sodium alginate powder was also treated by both gamma-irradiation and ethylene oxide sterilization. The effects of these treatments on the viscosities of sodium alginate solutions and both the diameter and strength of the beads formed in 0.1 M CaCl2 solutions were determined quantitatively. The viscosity of sodium alginate solutions and the gel strength of the calcium alginate beads decreased with increasing sterilization temperature while the bead diameters were found to increase. All these effects can be attributable to a reduction in the degree of polymerization of the alginate molecules as a result of the heat treatments. Ethylene oxide and gamma-irradiation treatments caused similar effects. Standard conditions for sterilization are necessary for comparative studies with alginate beads.     

Phase separation in calcium alginate gels

Alginates are polysaccharides consisting of beta-D-mannuronate and alpha-L-guluronate units. In the presence of bivalent cations like calcium the guluronate blocks form physically cross-linked gels. The gelation properties of alginates play an important role in the stability of extracellular polymer substances and in the food industry. When stock solutions of Ca2+ ions and alginate are mixed, the gelation starts before the Ca2+ ions are evenly distributed, which leads to non-uniform gels. In this contribution, Ca alginate gels were prepared by in situ gelation using glucono-delta-lactone and CaCO3. In this way, uniform gels could be prepared directly in the measuring cell. Below a critical concentration, highly viscous solutions were obtained, which were below the critical point of gel formation. In these solutions at low rotational speeds a Schlieren peak arose, which became smaller and steeper with increasing time until a new meniscus could be detected. This behaviour is in contrast to the peak broadening due to diffusion after a synthetic boundary was formed. Evaluation of the data leads to negative diffusion coefficients. It has been shown by others that the mutual diffusion coefficient must be negative in the spinodal region. This phenomena is known as uphill diffusion and leads to phase separation of a binary system. The formation of the gel phase in this case is therefore discussed as uphill diffusion.     

Diffusion characteristics of calcium alginate gels.

The diffusivity of a protein solute (bovine serum albumin) within calcium alginate gels made from sodium alginate of different guluronic acid content was determined. It was found that protein diffusion within alginate gels, prepared to be isotropic in structure, was greatest for gels prepared from sodium alginate of low guluronic acid content as opposed to those prepared from sodium alginate of high guluronic acid content. This finding was explained in terms of the difference in flexibility of the polymer backbone of the two alginates. The greater the polymer backbone flexibility, the greater the solute diffusivity within the gel.     

Structural regime identification in ionotropic alginate gels: influence of the cation nature and alginate structure

The morphologies of several ionotropic alginate hydrogels and aerogels were investigated by SAXS according to the nature of the divalent metal cation (Mn(2+), Co(2+), Zn(2+), Cu(2+)) and the guluronic fraction of the alginate. All alginate hydrogel and aerogel samples show isotropic small-angle X-ray scattering. Gelation results from cooperative associations of cations and chain segments and yields different nanostructures, that is, nanofibrillar morphology or multiple junction morphology, according to cation type and eventually mannuronic/guluronic ratio. Therefore, Mn and Cu gels present the same morphology whatever the guluronic ratio, whereas Co and Zn gels yield different nanostructures. In the size range investigated by SAXS (~10-200 ?), the structure of aerogels obtained by CO(2) supercritical drying is found to be inherited from the morphology of the parent hydrogel whatever the initial structural regime.     

Entrapment of concanavalin A-glycoenzyme complexes in calcium alginate gels

Glucose oxidase, invertase, and amyloglucosidase were entrapped in calcium alginate gels as concanavalin A complexes in order to prevent the leaching out of the enzymes from the porous matrix. The free as well as the gel-entrapped concanavalin A-glycoenzyme complexes exhibited a relatively high effectiveness factor, eta, indicating good accessibility to the substrates. Concanavalin A-invertase complex exhibited marked broadening of pH-activity and temperature-activity profiles and was highly resistant to temperature inactivation even after entrapment in the alginate beads. It was possible to entrap considerable quantities of invertase as concanavalin A complex in the beads without a marked decrease in eta. A column containing crosslinked concanavalin A-invertase complex entrapped in alginate beads retained the ability to completely hydrolyze 1M sucrose even after continuous operation for over four months.     

Gel strength of Ca-limited alginate gels made in situ

The gel strengths of Ca-alginate gels made in situ with different degree of cross-linking were determined by adapting three different methods: FIRA Jelly Tester, initial deformation (Youngs modulus, E) with a Stevens LFRA Texture Analyzer, and dynamic measurements with a Bohlin VOR Rheometer (dynamic storage modulus, G). The results showed that there were relatively large differences in absolute values, but that the deviations diminished when the results were expressed as relative gel strengths. The deviations of the Youngs modulus (E) from G increased with decreasing gel strength. Only dynamic measurements were suitable for quantifying low gel strengths.The gel strength and the breaking point were also measured as a function of the molecular weight of alginates isolated from stipes of Laminaria hyperborea. In the present Ca limited system, both the gel strength and the breaking point showed a marked increase with increasing molecular weight (Mw) up to 320–340 kD. This is considerably higher than with gels made by dialysis (Ca saturated), where the gel strength becomes independent on molecular weight around 100 kD. These results may have an impact on applications of alginate gels where the source of Ca crosslinking ions is limited.     

Small-angle X-ray scattering and rheological characterization of alginate gels. 3. Alginic acid gels

Alginic acid gels were studied by small-angle X-ray scattering and rheology to elucidate the influence of alginate chemical composition and molecular weight on the gel elasticity and molecular structure. The alginic acid gels were prepared by homogeneous pH reduction throughout the sample. Three alginates with different chemical composition and sequence, and two to three different molecular weights of each sample were examined. Three alginate samples with fractions of guluronic acid residues of 0.39 (LoG), 0.50 (InG), and 0.68 (HiG), covering the range of commercially available alginates, were employed. The excess scattering intensity I of the alginic acid gels was about 1 order of magnitude larger and exhibited a stronger curvature toward low q compared to ionically cross-linked alginate. The I(q) were decomposed into two components by assuming that the alginic acid gel is composed of aggregated multiple junctions and single chains. Time-resolved experiments showed a large increase in the average size of aggregates and their weight fraction within the first 2 h after onset of gelling, which also coincides with the most pronounced rheological changes. At equilibrium, little or no effect of molecular weight was observed, whereas at comparable molecular weights, an increased scattering intensity with increasing content of guluronic acid residues was recorded, probably because of a larger apparent molecular mass of domains. The results suggest a quasi-ordered junction zone is formed in the initial stage, followed by subsequent assembling of such zones, forming domains in the order of 50 A. The average length of the initial junction zones, being governed by the relative fraction of stabilizing G-blocks and destabilizing alternating (MG) blocks, determines the density of the final random aggregates. Hence, high-G alginates give alginic acid gels of a higher aggregate density compared to domains composed of loosely packed shorter junction zones in InG or LoG system.     

Temperature dependence of optical properties of aqueous dispersions of phosphatidylcholine

Aqueous dispersions of dipalmitoyl phosphatidyl choline exhibit a sharp decrease in turbidity at the crystal-liquid phase transition temperature of 41°C. The intensity of light scattered at 45°, 90°, and 135° also undergoes a sharp drop at the same temperature. Similarly, the refractive index of such dispersions decreases abruptly with the phase transition. Employing the relationship between light scattering intensity and specific refractive increment, it can be shown that about one half of the change in absorbancy and scattering are accounted for by the change in refractive index. The change in refractive index can be entirely accounted for by the known expansion and corresponding decrease in density of the bilayer. That part of the observed change in scattering and turbidity which is not accounted for by the observed change in average refractive index is apparently due to a decrease in the anisotropy of the bilayer during the melting process. Calculations based on a model which, although oversimplified, is consistent with the known thinning of the bilayer during the melting process, give quantitative agreement with experimental results. Below the phase transition temperature other changes in optical properties are observed; near 32°C, the light scattering envelope changes and the turbidity of dispersions drops markedly. The average refractive index remains constant in this region. For this and other reasons, it is postulated that these pre-transition changes indicate an aggregation-disaggregation phenomenon.     

Enzyme-catalyzed phase transition of alginate gels and gelatin-alginate interpenetrated networks

The enzyme-catalyzed gel-sol transition of calcium-alginate obtained by internal gelling strategy with the help of an entrapped alginate lyase is described. We show that alginate molecules and enzyme-produced oligoalginates shorten the gel time of physical gelatin gels (5% and 1.5%), probably due to local protein concentration increase. Interpenetrated networks composed of calcium-alginate and of gelatin were obtained only if elongation of gelatin helices inside a pre-existing calcium-alginate network could occur and only for low gelatin concentration (1.5%). The physical gelatin network is almost reversible inside the alginate one. Both networks can be obtained in the presence of alginate lyase, but gel-sol transition of calcium-alginate cannot be obtained in the presence of gelatin.     

Standardization of a method for characterizing low-concentration biogels: elastic properties of low-concentration agarose gels

Low-concentration biogels, which provide an extracellular matrix for cells in vitro, are involved in a number of important cell biological phenomena, such as cell motility and cell differentiation. In order to characterize soft tissues, which collapse under their own weight, we developed and standardized a new experimental device that enabled us to analyze the mechanical properties of floating biogels with low concentrations, i.e., with values ranging from 2 g/L to 5 g/L. In order to validate this approach, the mechanical responses of free floating agarose gel samples submitted to compression as well as stretching tests were quantified. The values of the Young's moduli, measured in the range of 1000 to 10,000 Pa, are compared to the values obtained from other experimental techniques. Our results showed indeed that the values we obtained with our device closely match those obtained independently by performing compression tests on an Instron device. Thus, the floating gel technique is a useful tool first to characterize and then to model soft tissues that are used in biological science to study the interaction between cell and extracellular matrix.     

Density distribution of calcium-induced alginate gels. A numerical study

Charged polysaccharides often form hydrogels in the presence of cations. In many applications the polymer network density distribution and associated physical properties are of major practical importance. Depending on the detailed conditions, the resulting gel density may vary from fully homogeneous to strongly inhomogeneous. We have established a simple set of coupled chemical reaction–diffusion equations to model the gelling process of calcium-induced alginate gels. The necessary algorithms for numerical solution of the resulting simultaneous parabolic differential equations have been developed both for one-dimensional models and three-ldimensional models with cylindrical or spherical symmetry. The algorithms make use of the Crank–Nicolson implicit finite difference method. The results of the numerical analyses of the gel formation can be divided into several different regimes depending on the physical and chemical parameters of the alginates and the cations. The numerical results are in good agreements with reported experimental results. © 1995 John Wiley & Sons, Inc.     

The dependence of the elastic properties of osteoporotic cancellous bone on volume fraction and fabric

Osteoporosis is a progressive systemic skeletal condition characterized by low bone mass and microarchitectural deterioration, with a consequent increase in susceptibility to fracture. Hence, osteoporosis would be best diagnosed by in vivo measurements of bone strength. As this is not clinically feasible, our goal is to estimate bone strength through the assessment of elastic properties, which are highly correlated to strength. Previously established relations between morphological parameters (volume fraction and fabric) and elastic constants could be applied to estimate cancellous bone stiffness in vivo. However, these relations were determined for normal cancellous bone. Cancellous bone from osteoporotic patients may require different relations. In this study we set out to answer two questions. First, can the elastic properties of osteoporotic cancellous bone be estimated from morphological parameters? Second, do the relations between morphological parameters and elastic constants, determined for normal bone, apply to osteoporotic bone as well? To answer these questions we used cancellous bone cubes from femoral heads of patients with (n=26) and without (n=32) hip fractures. The elastic properties of the cubes were determined using micro-finite element analysis, assuming equal tissue moduli for all specimens. The morphological parameters were determined using microcomputed tomography. Our results showed that, for equal tissue properties, the elastic properties of cancellous bone from fracture patients could indeed be estimated from morphological parameters. The morphology-based relations used to estimate the elastic properties of cancellous bone are not different for women with or without fractures.     

Solution properties of sodium alginate

Sodium alginate fractions derived from three different sources—Laminaria hyperboria (75% guluronate), Fucus vesicularus (95% mannuronate), and Azotobacter vinelandii (85% mannuronate)—were investigated in aqueous solution over a wide range of ionic strength and pH using the techniques of light scattering, viscometry, and osmometry. Light-scattering data extrapolated to infinite ionic strength yielded b₀ = 4.7 ± 0.3 and 3.0 ± 0.2 nm for the unperturbed effective bond lengths of the guluronate- and mannuronate-rich samples, respectively. These values are in the same ratio as predicted by conformational analysis, although lower by a factor of 0.7, probably due, in part at least, to the fact that measurements cannot be made on pure homopolymers. A comparison of the light-scattering and the viscosity data indicated that Φ in the Flory-Fox equation is lower than for more flexible polymers and increases with molecular weight, probably due to decreasing hydrodynamic permeability. Mark-Houwink exponents obtained from data extrapolated to infinite ionic strength were found to be considerably greater than 0.5, and we attribute this entirely to a variation in Φ. Comparison of the results obtained for the two mannuronate-rich samples indicated that the value of Φ and its variation with molecular weight can, in the case of alginates, differ markedly for chains, which, although having chemical differences, have similar chain statistics.