Influence of aqueous medium on viscoelastic properties of carboxymethylcellulose sodium, hydroxypropyimethyl cellulose, and thermally pre-gelatinized starch gels

Abstract

Neutral and semi-synthetic hydrophilic polymers are widely used in pharmaceutical technology to formulate controlled release drug delivery systems. The behaviour of the gel layer, formed around the hydrophilic matrices after water uptake, is of major importance for the drug release profiles. Rheological properties are related to the gel structure, interchain interactions and entanglements, and erosion and drug diffusion processes. Oscillatory test parameters were observed for highly concentrated Na carboxymethyl cellulose (NaCMC), hydroxypropylmethyl cellulose (HPMC), and thermally pre-gelatinized starch hydrogels obtained by the swelling of matrix tablets in a 0.1 mol/dm 3 HCl solution (pH 1.0) and a pH 6.8 phosphate buffer. The gel concentration range was 18–30% (w/w). The storage ( G′) and the loss ( G″) moduli, as well as the tangent of the phase angle (δ), were obtained under dynamic conditions of non-destructive oscillatory tests, in the frequence range 0.01–4.0 Hz. The pH and the ion type of the environment exert an influence on both the elastic and viscous behaviour of the NaCMC gels. The viscous character of the polyelectrolyte gel is pronounced in the phosphate buffer, while the elastic nature is dominating in the acid medium, as a result of the polymer neutralization. The sensitivity of neutral HPMC gels to pH changes is insignificant. No considerable differences of the functional parameters G′, G″ and tan δ, depending on the ionic strength, have been found. The visco-elastic behaviour of the pre-gelatinized starch gels is very similar to those of the neutralized NaCMC, regardless of the media used. This type of gel structure may determine a general kinetic pattern of drug release.