Rheological and physicochemical characterization of crosslinked-high isoelectric point gelatin hydrogels

ANDREA PORCARO, MARIEL L. OTTONE Y JULIO A. DEIBER

Buenos Aires

Jornada; VI Jornadas Internacionales de Proteínas y Coloides Alimentarios JIPCA VI; 2011

    Crosslinked gelatin hydrogels are hydrophilic macromolecular networks typically used in various food and biotechnological applications. These hydrogels, composed of polypeptide chains held together by chemical and physical crosslinks, have the capability of swelling in the presence of formulated aqueous solutions. Polydisperse gelatin chains are obtained through acid, alkaline or mixed processes, from different collagen natural sources, like bovine hides and pig skins, yielding an average isoelectric point  in the range 5 < pI < 9 . In this regard, natural polypeptides are polyampholytic macromolecules containing both positive and negative charges of rather weak amino and carboxyl ionizing groups. Here we study crosslinked polypeptide hydrogels formulated with a commercial pig skin gelatin (87.5 kDa) as obtained via an acid process, having pI¡Ö8. This case-study is the counterpart of that reported in a previous work (Deiber et al., Polymer, 50: 6065¨C6075, 2009) where a bovine hide gelatin of low isoelectric point was used as obtained via the classical alkaline process. It is important to indicate that the excess and total electrical charge fractions of crosslinked gelatin chains in a hydrated hydrogel at a given pH and ionic strength, are controlling the ion complexation phenomenon associated with bioactive molecules, which are first loaded and then released in a given site at a controlled rate.    In the present work, pig skin gelatin hydrogels with a protein concentration of 5 %w/w, crosslinked with glutaraldehyde (GTA), were characterized. One target was to understand the effect of pI on the functional properties of gelatin hydrogels. In this regard, characteristic rheological and physicochemical properties were evaluated via uniaxial extension and equilibrium swelling tests. Estimates of relevant microstructural parameters of the hydrogel matrix, generated with GTA concentrations within the range 0.025 to 1 % w/w, were obtained. These parameters are typically: average network mesh size, swelling ratio, average molecular mass of strands comprised between two consecutive crosslinks, average effective charge number per chain before and after the crosslinking process and the elastic modulus, among others. Further, to visualize the viability of polyion complexation between the hydrogel and bioactive macromolecules, values of the hydrogel zeta potential are provided, which are obtained from the application of the modified rubber elasticity and network swelling theories, as already discussed in a previous work.     Experimental results show, for example, that at low concentrations of GTA, pigskin gelatin hydrogels have elastic modulus smaller than those of bovine hide gelatin hydrogels. On the other hand, at high concentrations of GTA, this situation is inverted. In addition, it was found that pigskin gelatin hydrogels are more deformable than bovine hide gelatin hydrogels yielding different toughness values. The effective electrical charges of these hydrogels in equilibrium with formulated aqueous solutions are found to be different. It is then clear that hydrogel functional properties may depend significantly on the average pI of the gelatin type used.