Gelation and rheological characterization of Carbopol® in simulated gastrointestinal fluid of variable chemical properties

Carbopol® is a hydrophilic polymer commonly used in the preparation of oral controlled-release matrix tablets. These matrices are subjected to dissolution testing to investigate the rate and mechanism of drug release. The rate of drug release from these matrices is influenced by the viscoelastic properties of the gel layer formed upon hydration and surrounded tablet core. This study evaluates the gelation behavior and rheological characterization of Carbopol® in dispersion media, of varied chemical properties, commonly used in dissolution testing. The rheological properties of Carbopol® polymer underwent gelation were determined using a controlled-stress rheometer. Carbopol® gelation was not found in simulated gastric fluid of low pH (1.2-5.0) and simulated intestinal fluid of pH (5.0-6.5) during fasted (Fa) and fed (Fe) conditions. However, in water and at high pH (6.8-7.8), gelation occurred in phosphate buffers of high buffering capacity (?). Furthermore, no gelation was found in sodium chloride solutions of different ionic strengths (µ). These results highlight the importance of investigating the gelation behavior and rheological characterization of Carbopol® in dispersion media prior to dissolution testing. These preliminary studies can give an insight on the formation/absence of the gel layer around Carbopol® matrices which is responsible for controlling the release of drugs.