Increase In Gelation Research Articles

Effect of Fibrillization pH on Gelation Viscoelasticity and Properties of Biofabricated Dense Collagen Matrices via Gel Aspiration-Ejection.

Reconstituted hydrogels based on the self-assembly of acid-solubilized collagen molecules have been extensively used as in vitro models and precursors in biofabrication processes. This study investigated the effect of fibrillization pH-ranging from 4 to 11-on real-time rheological property changes during the gelation of collagen hydrogels and its interplay with the properties of subsequently biofabricated dense collagen matrices generated via automated gel aspiration-ejection (GAE). A contactless, nondestructive technique was used to characterize the temporal progression in shear storage modulus (G', or stiffness) during collagen gelation. There was a relative increase in G' of the hydrogels from 36 to 900 Pa with an increase in gelation pH. Automated GAE, which simultaneously imparts collagen fibrillar compaction and alignment, was then applied to these precursor collagen hydrogels to biofabricate native extracellular matrix-like densified gels. In line with viscoelastic properties, only hydrogels fibrillized in the 6.5 < pH ≤ 10 range could be densified via GAE. There was an increase in both fibrillar density and alignment in the GAE-derived matrices with an increase in gelation pH. These factors, combined with a higher G' in the alkaline precursor hydrogels, led to a significant increase in the micro-compressive modulus of GAE-densified gels of pH 9 and 10. Furthermore, NIH/3T3 fibroblast-seeded GAE-derived matrices densified from gels fibrillized in the pH range of 7 to 10 exhibited low cell mortality with >80% viability. It is anticipated that the results of this study can be potentially applicable to other hydrogel systems, as well as biofabrication techniques involving needles or nozzles, such as injection and bioprinting.

Microfluidic fabrication of imageable and resorbable polyethylene glycol microspheres for catheter embolization.

Radiopaque and degradable hydrogel microspheres have a range of potential uses in medicine including proper placement of embolic material during occlusion procedures, acting as inherently embolic materials, and serving as drug carriers that can be located after injection. Current methods for creating radiopaque microspheres are either unable to fully and homogeneously incorporate radiopaque material throughout the microspheres for optimal imaging capabilities, do not result in degradable or fully compressible microspheres, or require elaborate, time-consuming preparation. We used a simple one-step microfluidic method to fabricate imageable, degradable polyethylene glycol (PEG) microspheres of varying sizes with homogenous dispersion of barium sulfate-a biocompatible, high-radiopacity contrast agent. The imageability of the microspheres was characterized using optical microscopy and microcomputed tomography as a function of barium sulfate loading. Microspheres with 20%wt/vol barium sulfate had a mean CT attenuation value of 1,510 HU, similar to that of cortical bone, which should enable visualization with soft tissue. Compared with unloaded microspheres, barium sulfate-loaded ones saw an increase in gelation and degradation times and storage modulus and decrease in swelling. Imageable microspheres retained compressibility and were injectable via catheter. The developed radiopaque, degradable PEG microspheres have various potential uses for interventional radiologists and imaging laboratories.

Formulation and Characterization of a SIS-Based Photocrosslinkable Bioink.

Decellularized extracellular matrices (dECMs) represent a promising alternative as a source of materials to develop scaffolds that closely mimic the native environment of cells. As a result, dECMs have attracted significant attention for their applications in regenerative medicine and tissue engineering. One such application is 3D bioprinting, in which dECMs can be used to prepare bioinks with the biomimicry attributes required for regeneration purposes. Formulating bioinks is, however, challenging, due to difficulties in assuring that the printed materials match the mechanical properties of the tissue which is to be regenerated. To tackle this issue, a number of strategies have been devised, including crosslinking methods, the addition of synthetic materials as excipients, and the use of synthetic matrices for casting. We are particularly interested in extrusion-based 3D bioprinting, mainly due to the ease of rapidly conducting tests for adjusting operating conditions such that the required rheological and mechanical properties are met when using it. Here, we propose a novel bioink that consists of an acid-based precipitation of a small intestinal submucosa (SIS) dECM. The formulated bioink also relies on photocrosslinking reactions to attempt to control gelation and ultimately the mechanical properties of the extruded material. Photoinitiation was explored with the aid of varying concentrations of riboflavin (RF). Manual extrusion and rheological flow tests confirmed the printability and shear-thinning behavior of all formulations. Photocrosslinking reactions, however, failed to promote a substantial increase in gelation, which was attributed to considerable entanglement of undigested collagen molecules. As a result, pendant amine groups thought to be involved in the photo-mediated reactions remain largely inaccessible. In silico computational fluid dynamics (CFD) simulations were implemented to determine shear stress values on the bioink along the exit of the printing nozzle. Moreover, we calculated a stability parameter as a means to estimate changes in the bioink stability during the extrusion process. Future studies should be directed toward assessing the role of temperature-induced gelation in the rheological properties of the bioink and the development of strategies to improve the efficiency of photocrosslinking processes.

Alterations of molecular characteristics of polyethylene under the influence of heat treatment

The influence of exposure to heat treatment in the presence of air oxygen on structural transformations of low-density polyethylene have been investigated by methods of IR spectroscopy, viscometry, light scattering, turbidimetry and gelation measuring. It was established that the destruction and crosslinking processes of macromolecules proceed simultaneously with exposure to the heat treatment. In a range of exposures prior to gelation increase of mass average molecular mass and degree of branching of macromolecules, accompany the elevated temperatures. The results obtained are compared with the results of previously conducted similar studies of influence of UV and γ-irradiation on low density polyethylene where the same experimental approach was applied. It has been concluded that the mechanisms of influence of γ-, UV radiation and heat treatment on polyethylene are identical.

Interaction between a tertiary amine methacrylate based polyelectrolyte and a sodium montmorillonite dispersion and its rheological and colloidal properties

AbstractThe rheological and colloidal properties of sodium montmorillonite dispersions were investigated in the presence of a special type of cationic polymer [modified poly(ethylene glycol)]. 2‐(Dimethylamino) ethyl methacrylate was polymerized with monomethoxy‐capped oligo(ethylene glycol) via aqueous atom transfer radical polymerization. The tertiary amine residues of the resulting polymer were then quaternized with methyl iodide to obtain a cationic polyelectrolyte. The rheology and ζ‐potential experiments showed that the cationic polymer adsorbed onto the sodium montmorillonite surface strongly. The rheological parameters (plastic viscosity and yield value) were obtained with a rotational low‐shear rheometer. The results indicated a gradual increase in gelation with the addition of the cationic polymer, which reached a maximum at a cationic polymer concentration of 0.4–0.8 g/L. This gel‐like dispersion showed pronounced thixotropy. A further increase in the polymer concentration resulted in a reduction in this gelation. The adsorption of the cationic polymer onto the clay surface reduced the ζ potential to small values, but no isoelectric point was observed. The basal‐spacing measurements showed that the cationic polymer strongly adsorbed onto the sodium montmorillonite instead of entering the montmorillonite layers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 300–306, 2005

The Influence of Reprocessing on the Structure-Property Characteristics of a Plasticised Poly(vinyl chloride) (PVC-p) Compound

Abstract Synopsis: We have conducted a detailed investigation of the change in deformational, structural, molecular and rheological characteristics that are attendant on a PVC-p compound experiencing higher processing temperatures. Our study reflects an experience that occurs in the PVC industry and for which a loss of processing performance accompanies the excursions to higher temperatures. It is apparent that as this process continues the material will eventually become unprocessable, unless changes are made. The change that accompanies the material as it passes from an experience of low processing temperatures to high processing temperatures has been thoroughly mapped. However, it has been difficult to make precise observation of the molecular and structural changes occurring. This has not been without application of a number of likely candidate methodologies, including DSC, TGA, WAXS, GPC and fluorescence spectroscopy. It would seem that some additional crystallization is accompanying the increase in gelation that occurs at higher processing temperatures. At the same time, the observations on the influence of annealing hint that this is not the only consideration and that diffusion of the plasticiser into the PVC particles might also be playing a role or even residual stresses in the materials might be important. This study has not only been concerned with an in depth materials characterisation; it has also explored ways of alleviating the loss of processability. Two particular lines of study involving pre-shear history and blending with higher plasticised compounds have provided interest albeit inconclusive results. On the other hand, there are strong implications from the DSC study that reversibility of the "process aging" is not likely. In conclusion, it is interesting to observe that by conducting this study through an IUPAC Working Party it has been helpful to pool the resources and experience of an international community of scientists which has likely given us an opportunity of conducting this study at much lower costs than might have otherwise been the case.

Some aspects of the crosslinking of gelatin by dextran dialdehydes

The present paper describes the formation of hydrogels by reaction of gelatins with periodate oxidized dextrans. The effect of a number of reaction parameters on the rate of gelation is investigated. It is demonstrated that the time to onset of gelation depends on the nature of the gelatin, the nature of the dextran dialdehyde, the type of buffer, the ionic strength and the presence of added salts. The gelation time decreases with increasing concentration of aldehydes, increasing pH and increasing molecular weight of the dextran dialdehyde. In general, the gelation occurs more rapidly with decreasing concentration of indifferent electrolytes, except for the reaction in phosphate buffer where an increase in gelation is observed with increasing buffer strength. Ammonium and calcium salts are found to retard significantly the crosslinking reaction.

Influence of pH, electrolyte, and poly(vinyl alcohol) addition on the rheological characteristics of aqueous dispersions of sodium montmorillonite

The rheological characteristics of sodium montmorillonite dispersions were investigated as a function of pH and electrolyte (NaCl) concentration. Steady state and low shear rheology and instantaneous modulus measurements were used to obtain the yield value τ β , the plastic viscosity η PL, the residual viscosity η 0, and the modulus G 0. With the exception of the plastic viscosity, the other rheological parameters showed a minimum at pH = ∼7 increasing rapidly as the pH was increased above or decreased below this value. Moreover, τ β , η 0, and G 0 increased with increase of NaCl concentration (within the range 10 −3 to 10 −1 mole dm −3). These results indicate increase in gelation of aqueous montmorillonite dispersions in acid and alkaline conditions. This gelation becomes more pronounced the higher the NaCl concentration within the range investigated. The various possible interactions between the clay platelets and their dependence on pH and electrolyte concentration were considered assuming that the isoelectric point (iep) of the particle edges was around pH = 7. At pH's below the iep, edge-to-face attraction was considered to dominate the interaction. On the other hand, at pH's above the iep of the edge, the increase in gelation with increase in pH was considered to be due to an increase in double-layer repulsion as the net charges on the edges increase. Preliminary results on the effect of addition of PVA at pH = 7 showed a gradual increase in structure on the addition of polymer, reaching a maximum at an optimum adsorption value (corresponding to ∼1 4 coverage) above which further increase in polymer concentration results in reduction of this structure. This gelation at an optimum PVA concentration was considered to be due to polymer bridging.

Effect of association on the gelation of aqueous poly (Nvinyl2pyrrolidone) solutions with y-rays

In this study gelation of dilute aqueous poly(Nvinyl2pyrrolidone), PVP, solutions with y-rays is investigated. The gelation dose is found to increase with the addition of guanidine salts to polymer solutions. The mechanism responsible for the increase in gelation in guanidine sulfate is shown to be mainly due to the changes in the configurational behaviour of PVP. Since there was no change in viscosity-concentration relation of PVP in guanidine chloride solutions, the increase in gelation dose in the presence of this substance is explained as due to changes brought about in the indirect action of radiation by chloride ions.