Inulin hydrogels as carriers for colonic drug targeting. Rheological characterization of the hydrogel formation and the hydrogel network.

Free radical polymerization converts aqueous solutions of methacrylated inulin into cross-linked hydrogels. The purpose of this work was to study the hydrogel formation and to characterize the fully cured hydrogels. The gelation process of aqueous solutions of methacrylated inulin was monitored as a function of time by means of linear oscillatory shear measurements, at a fixed frequency and amplitude. The fully cured inulin hydrogels were characterized by measurement of the frequency-dependency of the linear elastic modulus G'. The effects of the degree of substitution and feed concentration of methacrylated inulin on both the gelation kinetics and the rigidity of the obtained hydrogels were determined. The effect of the concentration of the initiators of the radical polymerization reaction has been studied as well. The weight fraction of polymer which was not incorporated in the hydrogel networks was determined using the anthrone reaction, and physical chain entanglements were determined by solution viscosity measurements. The gelation kinetics and the elastic modulus were proportional to the degree of substitution and feed concentration of methacrylated inulin. Increasing concentrations of radical-forming compounds also accelerated the hydrogel formation, but lowered the elastic modulus of the obtained hydrogels. The amount of polymer chains incorporated in the hydrogel network seemed to be especially influenced by the degree of substitution of the derivatized inulin, and for a feed concentration of 27% w/w of methacrylated inulin, entanglements have to be accounted for. The gelation kinetics and the elastic modulus of inulin hydrogels are not only affected by the degree of substitution and the feed concentration of methacrylated inulin, but also by the concentration of the initiators of the free radical polymerization reaction.

Rheological characterization of xanthan gum and hydroxypropylmethyl cellulose with respect to controlled-release drug delivery.

It has been observed previously that xanthan gum (XG) and hydroxypropylmethyl cellulose (HPMC) show different drug release behavior. In order to clarify these findings, the rheological properties of both polymers have been determined by oscillatory as well as by steady shear measurements. Aqueous solutions of 4 and 7% (w/w) polymer have been used to simulate the outer surface of a hydrated tablet. The dynamic moduli, i.e., storage modulus (G') and loss modulus (G") of the two polymers have been determined in pure water and USP phosphate buffer pH 7.4 at different dilutions. In this concentration range XG solution exhibits "gel-like" behavior, while HPMC behaves as a typical polymer solution. These findings are quite consistent with the reported higher ability of XG matrices to retard drug release than HPMC matrices for controlled-release formulation. The effects of differences in drug solubility and acidity, as well as the addition of lactose, and of the ionic strength of the medium on the rheological properties of XG and HPMC solutions have been studied in detail. Among these parameters, only the salt concentration exerts an enhancing effect on both moduli of XG, while no detectable influence on HPMC solution could be observed.