Characterizing the yielding processes in pluronic-hyaluronic acid thermoreversible gelling systems using oscillatory rheology

Pluronic F-127 (Pl), a triblock copolymer that exhibits thermoreversible sol-gel transition, is being used with additives such as polysaccharides for enhanced gel stability and mechanical strength for controlled drug delivery applications. This study aims at elucidating the gelation behavior and structure-property relationship (at physiological temperature) of pluronic in the presence of hyaluronic acid (HA) using linear and nonlinear rheology. Small amplitude oscillatory shear of the gels indicated weak frequency dependence of G ′ and G ′ ′. Large amplitude oscillatory shear (LAOS) response indicated flow induced yielding with both the moduli following power law decay beyond the critical strain amplitude. Stress waveform analysis, Fourier analysis, and Chebyshev decomposition indicated similar qualitative features for gels with different HA content. Interestingly, there is a non-monotonic variation in the dynamic light scattering timescales, G ′ (linear regime), and the critical stress values (at corresponding critical strains) with HA content. The physical insights obtained were understood in terms of a cage picture of soft glassy materials. Relaxation times corresponding to yielding transition obtained from strain rate frequency superposition measurements showed similar qualitative variation as those obtained from elastoviscoplastic modeling of the yielding processes occurring during an LAOS cycle. Based on these analyses, we obtain quantitative description of nonlinear rheological response of the Pl-HA gels.