Saphwan Al-Assaf

Hydrogels: Methods of Preparation, Characterisation and Applications

The terms gels and hydrogels are used interchangeably by food and biomaterials scientists to describe polymeric cross-linked network structures. Gels are defined as a substantially dilute cross-linked system, and are categorised principally as weak or strong depending on their flow behaviour in steady-state (Ferry, 1980). Edible gels are used widely in the food industry and mainly refer to gelling polysaccharides (i.e. hydrocolloids) (Phillips & Williams, 2000). The term hydrogel describes three-dimensional network structures obtained from a class of synthetic and/or natural polymers which can absorb and retain significant amount of water (Rosiak & Yoshii, 1999). The hydrogel structure is created by the hydrophilic groups or domains present in a polymeric network upon the hydration in an aqueous environment. This chapter reviews the preparation methods of hydrogels from hydrophilic polymers of synthetic and natural origin with emphasis on water soluble natural biopolymers (hydrocolloids). Recent advances in radiation cross-linking methods for the preparation of hydrogel are particularly addressed. Additionally, methods to characterise these hydrogels and their proposed applications are also reviewed.

Molecular weight, tertiary structure, water binding and colon behaviour of ispaghula husk fibre

Molecular variables, using aqueous and alkaline extracts, of the polysaccharide from ispaghula husk (IH) were examined using gel-permeation chromatography linked to multi-angle laser light scattering. Progressive extraction can yield a component with a molecular weight (MW)value up to about 7 x 106 Da, and gels, which accompany the extraction, have MW ranging from 10-20 x 106 Da. To mimic the polysaccharide degradation, particularly in the colon, the solid IH was degraded progressively using ionising radiation. A chain break occurs every 7.5 kGy in NaOH and every 15 kGy in water. The solid-state matrix is opened by the radiation to yield increased visco-elasticity of the aqueous extracts at critical radiation doses, before further degradation occurs after about 12 kGy. Differential scanning calorimetry is used to study the mechanism of interaction of water with IH. The first water to be taken up is non-freezing water and represents about twelve water molecules/disaccharide unit of the polysaccharide. As the water content is increased, the water becomes bound to the polysaccharide and freezes and melts at a temperature different from free water. This water is thermodynamically distinguishable from free water. It forms amorphous ice on cooling which crystallises exothermically and subsequently melts endothermically. Saturation occurs at a water content of 2-3 g water/g polymer, showing that about 60% of the water in the system is bound. The most surprising conclusion is that despite the fact that the IH swells in water to form a solid and stiff gel, the greater part of that water in the gel is still free and behaves like liquid water.

RHEOLOGICAL BEHAVIOUR OF HYALURONAN, HEALON AND HYLAN IN AQUEOUS SOLUTIONS

ABSTRACT Rheological properties of hyaluronan solutions are related not only to the molecular weight or concentration, but also to the origin of the samples. Here we present a comparative rheological study of hyaluronan, in aqueous solutions, from bacterial and animal sources with a cross-linked hyaluronan (hylan). Using a variety of rheological techniques, the behaviour of hyaluronan (Mw 0.8 − 2.2 × 106), crosslinked hyaluronan (hylan) (Mw 1.8 − 12.5 × 106) and Healon (Mw ˜ 5 × 106) (a proprietary hyaluronan) was studied over a large range of molecular weights. The object was to study the effect of the cross-links in hylan on the various rheological parameters, in comparison with linear hyaluronan. There are significant differences. The Huggins constant and the critical overlap parameter C*[η] are considerably lower for hylan and an increase in moduli at low frequencies was observed for hylan compared with the hyaluronan samples at all molecular weights studied. The results point to a difference in structure in dilute solution for hylan due to the ability to form networks, which can be removed by pressure filtration. In contrast, we do not find an increase of the steady shear viscosity and elastic modulus at higher concentrations when a homogeneous entangled network is reached. In the semi-dilute regime, the Newtonian viscosity scales as C4.1 to C3.6 by increasing the polymer concentration. The critical shear rate and the cross-over frequency (ωρ) vary as C-2 M-3. The plateau modulus G ∼ G0 scales as C2.15. We attribute this behaviour, which agrees well with the de Gennes prediction in a good solvent, to the semi-rigid character of the hyaluronan chain and to the predominance of entanglements over the cross-link points present in hylan in the semi-dilute domain. Due to the higher apparent molecular weights which are possible with hylan structures but not with the hyaluronans currently available, a wider range of applications can be achieved with hylans when viscoelasticity is required, particularly for the viscosupplementation of synovial fluid damaged by osteoarthritis.

Emulsification Properties and Molecular Weight Distribution of Combretum Glutinosum Gum

The determination of weight average molecular weight (Mw) and radius of gyration (Rg) of Habeil gum was performed on a gel permeation chromatography-multi angle laser light scattering (GPC-MALLS) system. The results showed three main components of Combretum glutinosum gum designated arabinogalactan protein (AGP), arabinogalactan (AG) and glycoprotein (GP). The Mw of the freeze dried soluble part, freeze dried deacetylated gum, and freeze dried calcium-free gum were 2.06×106, 8.03×105, and 1.40×106 g /mol, respectively. The Mw of hydrophilic and hydrophobic components collected from soluble part gum are 3.16×106 and 6.08×105 g /mol, respectively, while the Mw of hydrophilic and hydrophobic components collected from deacetylated gum are 1.39×106 and 8.36×105 g /mol, respectively. Emulsification studies of the gum fractions showed lowest degree of span value than the starting material indicating good uniformity and increasing in adsorption of protein on oil droplet surface. The results of emulsion studies indicated that a blend of 50% w/w deacetylated Combretum glutinosum gum and 50% w/w Acacia Senegal produced better emulsification properties than the pure mother gum.