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

Neutral and semi-synthetic hydrophilic polymers are widely used in pharmace utical technology to formulate controlled release drug delivery systems. Th e behaviour of the gel layer, formed around the hydrophilic matrices after water uptake, is of major importance for the drug release profiles. Rheolog ical properties are related to the gel structure, interchain interactions a nd entanglements, and erosion and drug diffusion processes. Oscillatory tes t parameters were observed for highly concentrated Na carboxymethyl cellulo se (NaCMC), hydroxypropylmethyl cellulose (HPMC), and thermally pre-gelatin ized starch hydrogels obtained by the swelling of matrix tablets in a 0.1 m ol/dm(3) HCl solution (pH 1.0) and a pH 6.8 phosphate buffer. The-gel conce ntration range was 18-30% (w/w). The storage (G') and the loss (G") moduli, as well as the tangent of the phase angle (delta), were obtained under dyn amic conditions of non-destructive oscillatory tests, in the frequence rang e 0.01-4.0 Hz. The pH and the ion type of the environment exert an influenc e 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 chan ges is insignificant. No considerable differences of the functional paramet ers G', G" and tan delta, depending on the ionic strength, have been found. The visco-elastic behaviour of the pre-gelatinized starch gels is very sim ilar to those of the neutralized NaCMC, regardless of the media used. This type of gel structure may determine a general kinetic pattern of drug relea se. (C) 1999 Elsevier Science B.V. All rights reserved.