Gel strength and solution viscosity of temperature-sensitive, in-situ-gelling polymers for endovascular embolization

The goal of this work was to investigate the relationship of the gel strength and stiffness (at 37°C) to solution viscosity (at 25°C) in poly(N-isopropylacrylamide-co-acrylic acid) solutions with regard to acid content, molecular weight and solution concentration. It was hypothesized that the gel strength could be maximized while minimizing the increase in solution viscosity. If so, there would be motivation to investigate these materials for arteriovenous malformation embolization. The copolymers were synthesized with 0-2 mol% content of acrylic acid (AAc) in benzene, dioxane, THF, 50:50 benzene/dioxane, or 50:50 dioxane/THF to obtain polymers of different molecular weight. The polymers were characterized for molecular weight by GPC/light scattering, for acrylic acid content by acid titration, for lower critical solution temperature by differential scanning calorimetry, and for solution viscosity (at 25°C) and gel strength (at 37°C) by rheometry. Solutions of lower-molecular-weight polymers were shown to have lower viscosities while possessing higher strengths as gels than the highest manageable concentrations of higher-molecular-weight polymers. This work demonstrates that the mechanical properties of poly(N-isopropylacrylamide-co-acrylic acid) can be increased while minimizing the increase in solution viscosity.