Phillips Hydrocolloids Research Centre

Hydrogels: Methods of Preparation, Characterisation and Applications in Molecular and Environmental Bioengineering

Syed K.H Gulrez et al. — Thu, 18 Aug 2011 02:59:58 PDT

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 associations in acacia gums

S Al-Assaf et al. — Tue, 09 Feb 2010 03:42:59 PST

The tendency of polysaccharides to associate in aqueous solution has long been recognised. Molecular associations can profoundly affect their performance in a given application due to its influence on the molecular weight, shape and size. This will ultimately determine how the molecules will interact with each other, with other molecules and with water. There are several factors, such as hydrogen bonding, hydrophobic association, ion mediated association, electrostatic interaction, concentration dependence and the presence of proteinaceous components, which affect this behaviour. Our objective is to highlight the role of the proteinaceous component, present in acacia gum, to promote associations when the gum is subjected to various processing treatments such as maturation, spray drying and irradiation. The results demonstrate the ability of the proteinaceous component to promote hydrophobic associations which influence the size and proportion of the arabinogalactan high molecular weight component (AGP). Heat treatment in solid state (maturation) increases the hydrophobic character of the gum and hence its emulsification performance. Spray drying also involves aggregation through hydrophobic association but changes the surface properties of peptide moieties to become more hydrophilic compared to the association promoted by the maturation treatment in the solid state. Irradiation induced cross-linking, in the presence of unsaturated gas, was used to introduce C-C bonds into the carbohydrate moiety and thus confirms the hydrophobic association prompted by the heat used in the maturation and spray drying. This association can be reversed by treatments such as filtration or high pressure homogenisation. The results reported here reconcile the contradiction about structure of gum arabic proposed by the wattle blossom and twisted hairy rope models and shows that the AGP fraction is basically an aggregated fraction made up of AG units stabilised by low molecular weight highly proteinaceous components. .

Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM™): Part 1—Controlled maturation of Acacia senegal var. senegal to increase viscoelasticity, produce a hydrogel form and convert a poor into a good emulsifier

S Al-Assaf et al. — Fri, 15 Jan 2010 05:37:51 PST

The Acacia gum (Acacia senegal var. senegal), which is a food additive approved by Codex Alimentarius, is defined within the Acacia subgenus (family Leguminosae). The structural characteristics associated with a “poor” and “good” emulsifying A. senegal var. senegal exudate gum are identified and it is shown that it is possible to maturate the “poor” emulsifier in a process comparable to that which occurs to the exudate gum as the age of the tree increases from 1 to 15 years and when the gum is stored naturally after collection. Thus, the molecular parameters of the “good” gum can be matched, and the emulsification effectiveness can attain the level of the “good” emulsifier. The process can be further continued to produce a series of gums of precisely structured molecular dimensions with improved properties and if the maturation is continued can yield a hydrogel form of Acacia. The new forms of A. senegal which are described here, and which have not previously been available are designated Acacia (sen) SUPER GUM™ which are constant in properties and with precisely structured molecular dimensions, unlike the naturally occurring gum. The viscosity can be increased up to 20 times compared to the starting material. The controlling factor is the agglomeration of the proteinaceous components within the molecularly disperse system that is A. senegal gum to increase the amount of arabinogalactan protein (AGP) emulsifying component up to more than double the amount present originally. This results in a dramatic increase in the interfacial surface properties and coverage of the oil droplet in oil in water emulsion. No other chemical change is initiated by the maturation process.

Chain scission of hyaluronan by carbonate and dichloride radical anions: Potential reactive oxidative species in inflammation?

S Al-Assaf et al. — Fri, 15 Jan 2010 05:16:09 PST

The reactions of the carbonate and dichloride radical anions, CO3− and Cl2−, with the extracellular matrix glycosaminoglycan hyaluronan (HA) have been studied using the kinetic technique of pulse radiolysis and also by steady-state irradiation combined with gel permeation chromatography/multiangle laser light scattering(gpc/MALLS) to measure the rates of reaction with HA and the yield of HA chain scission, respectively. For comparison, the same measurements were made for the reactions of the free radicals OH, Br2−, and N3. The carbonate and dichloride radical anions were found to react relatively quickly with HA (7.0 × 105 and 6.9 × 106 dm3 mol−1 s−1, respectively) although they are much less reactive than the hydroxyl radical, OH. Significant yields (20 and 38%, respectively) of chain scission of HA by these radical anions were also determined from the gpc/MALLS experiments, providing some support for their potential participation in the depolymerization of HA in vivo. These results are compared with data obtained for the other free radicals (hydroxyl, azide radicals, and dibromide radical anions) investigated in this study in order to gain an insight into their mechanism of reaction with HA. Earlier chain scission yields of HA by hydroxyl radicals determined by the authors have also been revised using the gpc/MALLS technique employed in the current study. The yields of 52% (absence of air) and 44% (in air) are much lower than the previous values. In the current study, the effect of oxygen on the yields of HA chain breaks is discussed in terms of the reactivity of HA peroxyl radicals in the presence of superoxide radical anions. The relevance of the results of this study to mechanisms of inflammation is discussed.

Hyaluronan and proximal tubular cell migration

T Ito et al. — Fri, 15 Jan 2010 04:53:18 PST

Background The ubiquitous polysaccharide hyaluronan has been associated with both acute renal injury and progressive renal disease. The aim of this study was to examine the effect of hyaluronan on proximal tubular cell migration.

Methods The proximal tubular cell line, HK-2 cells, were grown in monolayer culture, and cell migration following addition of hyaluronan characterized in an in vitro model of injury that we have previously developed and characterized.

Results Addition of well-defined preparations of exogenous hyaluronan increased cell migration; however, optimum enhancement of migration was seen with hyaluronan of high molecular weight. Activation of the mitogen-activated protein kinase (MAPK) signaling cascade, as assessed by increased expression of the dually phosphorylated active form of MAPK, could be demonstrated following addition of hyaluronan. This was blocked by the addition of a specific antibody to the hyaluronan receptor, CD44. Hyaluronan-dependent enhanced migration was also abrogated by addition the CD44 blocking antibody, and by inhibition of MAPK kinase (MEK) activity. Generation of a denuded area also led to increased synthesis of endogenous hyaluronan and activation of MAPK, and blockage of either CD44 or MAPK activation inhibited proximal tubule cell (PTC) migration and re-epithelialization under nonstimulated conditions.

Conclusion We have demonstrated that hyaluronan activation of the MAPK pathway through binding to its receptor CD44, enhances proximal tubule cell (PTC) migration. In addition, the results suggest that mechanical injury of PTC stimulated hyaluronan generation. These observations may have implications for both recovery from acute tubular injury and progressive renal fibrosis.

Comparative rheological behavior of hyaluronan from bacterial and animal sources with cross-linked hyaluronan (hylan) in aqueous solution

S Al-Assaf et al. — Mon, 21 Dec 2009 12:37:18 PST

Using a variety of rheological techniques, the behavior of hyaluronan (Mw 0.8-2.2 × 106), cross-linked 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*[ ] 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. We attribute this behavior to the semirigid character of the hyaluronan chain and to the predominance of entanglements over the cross-link points present in hylan in the semidilute domain. Due to the higher apparent molecular weights that 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. © 2001 John Wiley & Sons, Inc. Biopolymers 59: 191-204, 2001