Rheology of hydroxyethylated starch aqueous systems. Analysis of gel formation

Starch and its Derived Products: Biotechnological and Biomedical Applications

Starches are one of the most abundant renewable natural resources available to us, however their potential as a biomass feedstock for the production of a vast range of commercially viable chemicals/components for application in many areas of industrial, food and biomedical sciences is currently under-exploited. This review begins by presenting an overview of starch sources, composition and structure, and physicochemical characteristics. Specific topics discussed include amylose and amylopectin structure, their location in the amorphous and crystalline regions of starch granules, granule morphology, gelatinisation and pasting characteristics. The remainder of the review then focuses upon the biotechnological production of starch hydrolysis products, such as maltodextrins, glucose and fructose syrups, and cyclodextrins, and the chemical modification of starch, namely, oxidation, stabilisation (esterification and etherification), and cross-linking. Finally some specific examples of the development of starch-derived biomaterials for application in areas such as orthopaedics, bone cements, tissue engineering, and hydrogels are presented.

A comparison of the activation energy of viscous flow for hen egg-white lysozyme obtained on the basis of different models of viscosity for glass-forming liquids

The paper presents the results of viscosity determinations on aqueous solutions of hen egg-white lysozyme at a wide range of concentrations and at temperatures ranging from 5°C to 55°C. On the basis of these measurements and different models of viscosity for glass-forming liquids, the activation energy of viscous flow for solutions and the studied protein, at different temperatures, was calculated. The analysis of the results obtained shows that the activation energy monotonically decreases with increasing temperature both for solutions and the studied protein. The numerical values of the activation energy for lysozyme, calculated on the basis of discussed models, are very similar in the range of temperatures from 5°C to 35°C.

Characterization of the rheological, mucoadhesive, and drug release properties of highly structured gel platforms for intravaginal drug delivery

This investigation describes the formulation and characterization of rheologically structured vehicles (RSVs) designed for improved drug delivery to the vagina. Interactive, multicomponent, polymeric platforms were manufactured containing hydroxyethylcellulose (HEC, 5% w/w) polyvinylpyrrolidone (PVP, 4% w/w), Pluronic (PL, 0 or 10% w/w), and either polycarbophil (PC, 3% w/w) or poly(methylvinylether-co-maleic anhydride) (Gantrez S97, 3% w/w) as a mucoadhesive agent. The rheological (torsional and dynamic), mechanical (compressional), and mucoadhesive properties were characterized and shown to be dependent upon the mucoadhesive agent used and the inclusion/exclusion of PL. The dynamic rheological properties of the gel platforms were also assessed following dilution with simulated vaginal fluid (to mimic in vivo dilution). RSVs containing PC were more rheologically structured than comparator formulations containing GAN. This trend was also reflected in formulation hardness, compressibility, consistency, and syringeability. Moreover, formulations containing PL (10% w/w) were more rheologically structured than formulations devoid of PL. Dilution with simulated vaginal fluids significantly decreased rheological structure, although RSVs still retained a highly elastic structure (G' > G'' and tan delta < 1). Furthermore, RSVs exhibited sustained drug release properties that were shown to be dependent upon their rheological structure. It is considered that these semisolid drug delivery systems may be useful as site-retentive platforms for the sustained delivery of therapeutic agents to the vagina.

Physicochemical characterization of bioactive polyacrylic acid organogels as potential antimicrobial implants for the buccal cavity

This study describes the formulation and physicochemical characterization of poly(acrylic acid) (PAA) organogels, designed as bioactive implants for improved treatment of infectious diseases of the oral cavity. Organogels were formulated containing a range of concentrations of PAA (3-10% w/w) and metronidazole (2 or 5% w/w, representing a model antimicrobial agent) in different nonaqueous solvents, namely, glycerol (Gly), polyethylene glycol (PEG 400), or propylene glycol (PG). Characterization of the organogels was performed using flow rheometry, compressional analysis, oscillatory rheometry, in vitro mucoadhesion, moisture uptake, and drug release, methods that provide information pertaining to the nonclinical and clinical use of these systems. Increasing the concentration of PAA significantly increased the consistency, compressibility, storage modulus, loss modulus, dynamic viscosity, mucoadhesion, and the rate of drug release. These observations may be accredited to enhanced molecular polymer entanglement. In addition, the choice of solvent directly affected the physicochemical parameters of the organogels, with noticeable differences observed between the three solvents examined. These differences were accredited to the nature of the interaction of PAA with each solvent and, importantly, the density of the resultant physical cross-links. Good correlation was observed between the viscoelastic properties and drug release, with the exception of glycerol-based formulations containing 5 and 10% w/w PAA. This disparity was due to excessive swelling during the dissolution analysis. Ideally, formulations should exhibit controlled drug release, high viscoelasticity, and mucoadhesion, but should flow under minimal stresses. Based on these criteria, PEG 400-based organogels composed of 5% or 10% w/w PAA exhibited suitable physicochemical properties and are suggested to be a potentially interesting strategy for use as bioactive implants designed for use in the oral cavity.

Dynamic viscoelastic and tensile properties of gluten and glutenin gels of common wheats of different strength

Dynamic viscoelastic properties at 25 degrees C of gluten and glutenin gels were obtained from Canadian common wheats of different strengths. The relaxation spectra showed a maximum intensity at a characteristic relaxation time (tau). The relaxation modulus associated with this maximum was taken as the strength of the glutenin or gluten gel transient network (G(tau)). The ratio of G(tau) for glutenin and gluten gels from the same cultivar ranged from 5.6 for an extra strong cultivar to 51.1 for a soft wheat. This gives indirect evidence that the gliadin fraction weakens the glutenin gel network more in weaker cultivars. In addition, the fact that both glutenin and gluten gels showed extensive stress relaxation coupled with the fact that addition of l-cysteine to a gluten gel eliminated the network structure at 25 degrees C and resulted in a power law stress relaxation spectrum suggests that the transient network in gluten is a reversible network. This power law relaxation pattern was not seen here for an entangled polymer melt (poly(dimethylsiloxane)). It was also found here that the viscosity of the gluten gel (G(tau) x tau) trended best with the tensile stress build-up in a uniaxial tensile test of gluten gels. Together, these results indicate that both network strength and relaxation times should be considered in characterizing the linear viscoelastic properties of hydrated cereal proteins.

Synthesis of hydroxypropyl methacrylate/polysaccharide graft copolymers as matrices for controlled release tablets

Hydrophilic matrices are an interesting option when developing drug delivery systems. With this aim, hydroxypropyl methacrylate was grafted onto hydroxypropyl starch and hydroxypropyl cellulose substrates by following the Ce(IV) redox initiation method. Different amounts of ethyleneglycol' dimethacrylate, 7 and 34 mol%, as the crosslinking monomer, were also added. The drying of grafted products was carried out by lyophilization, obtaining white powders. Reaction yields (percent grafting, grafting efficiency, etc.) and some physical characteristics of the powders (particle size, moisture uptake, density, morphology, etc.) were determined. These parameters indicate how useful these products may be as potential matrices for direct compressed tablets. In this light, the powder flowability and the binding properties of each copolymer were determined. The graft copolymers can be considered of great interest as direct compression excipients. Due to their different chemical structure and composition, they showed differences in viscoelastic properties that revealed an interesting range of possibilities for use in drug delivery formulations. Tablets formulated with conventional excipients were also tested. Dissolution tests of various tablets were carried out. In 12 hr, 60-80% of the model drugs was released.

Synthetic PMMA-grafted polysaccharides as hydrophilic matrix for controlled-release forms

This paper describes the rheological behavior of starch and cellulose acrylic graft copolymers synthesized with the aim of obtaining controlled-release excipients. The rheological characteristics determine the final release properties of matrix tablets. The study of the storage and loss moduli (G' and G", respectively) and the viscosity allowed us to know if the polymer behavior was that of a gel and, hence, if it could act as a barrier to drug diffusion. Since dynamic measurements showed a storage modulus higher than the loss modulus, we assessed that all the polymers were gels. Thus, knowing that all the graft copolymers had acceptable properties for compression, the release of theophylline as a model drug at different pH was studied. Polymers with higher absorption capacity, viscosity, and compactibility allowed formulations with slower release rates.