A novel, simple method to simulate gelling process of injectable biodegradable in situ forming drug delivery system based on determination of electrical conductivity.

The purpose of the present study was to develop a novel, simple and determination-of-electrical-conductivity-based method to trace the gelling process of injectable biodegradable in situ forming organogels after administration. The electrical conductivity of pH 7.4 PBS solution with different amount of N-methyl-2-pyrrolodone (NMP) and drug-free organogel formulation contained 0.6mL NMP were determined at 37 °C, respectively. The electrical conductivity of PBS solution was linearly proportional to the amount of NMP. Organogel contained 0.6 mL NMP in PBS solution showed a descending of electrical conductivity as time runs, while the value of electrical conductivity was almost a constant at 7.58 ms/cm after 110 min, which was nearly equaled to the electrical conductivity of 0.6 mL NMP in PBS solution (7.59 ms/cm). This data indicated that the diffusion of NMP caused the descending of system electrical conductivity and NMP completely diffused from organogel after 110 min, which led to the constant electrical conductivity. Meanwhile, photographs of organogel showed that the gel formed from periphery to center gradually, and totally formed after 110 min. The diffusion terminal point of NMP from organogel could be perfectly anticipated and controlled by this method. Consequently, this electrochemical method had visually simulated the gelling process and located gelling time of organogel in medium solution by measuring variation of electrical conductivity.