Rheological Characterization of Topical Carbomer Gels Neutralized to Different pH

Purpose. The primary objective of this study is to perform detailed and extensive rheological characterization of rheology of carbomer (Carbopol) microgels formulated using a solvent system typically used in topical gel formulations. Solvents like glycerin and propylene glycol can alter rheology and drug delivery characteristics of topical gels owing to their different viscosities and due to the change in solvent-polymer and solvent-solvent interactions. Methods. Aqueous gels with different pH were prepared by dissolv-ing cross-linked Carbopol polymers in a co-solvent system comprising water, propylene glycol, and glycerol and subsequently neutralizing the carboxylic groups of the polymers with triethanolamine (TEA). Oscillatory, steady, and transient shear measurements were per-formed to measure viscoelastic properties, temperature dependency, yield strength, and thixotropy of carbomer pharmaceutical gels. Results. The topical pharmaceutical gels exhibit remarkable tempera-ture stability. Flow curves obtained at different temperatures indicate Carbopol microgels show much more pseudoplastic behavior (lower power law index) compared to Carbopol gels dissolved only in water. Substantial yield strength is required to break the microgel network of the topical gels. The gel samples exhibit modest thixotropy at higher deformation rates. Conclusions. The rheological behavior of the Carbopol microgels do not change appreciably in the pH range 5.0–8.0, and the gels can be used as effective dermatological base for topical applications. KEY WORDS: Carbopol; constitutive model; microgel; pharmaceu-tical gel formulation; rheology; thixotropy; topical gel; viscoelasticity; yield stress