Injectability of biodegradable in situ forming microparticle systems (ISM)

In situ forming microparticle (ISM) systems offer a new encapsulation technique that provide a prolonged release of drug along with much greater ease of preparation and administration than conventional microparticles and surgically implanted systems. In this study, the forces required to inject biodegradable emulsions of the ISM systems via different sizes of syringe and needle into a newly developed chicken meat model, were determined using a compression mode of Texture Analyzer. The obtained injection forces were finally correlated with the injectability of the formulations into living rats. Results indicated that the flow of the ISM formulations through the needle could be described by the well-known Poiseuille equation. The injection forces were directly proportional to the formulations' viscosity, injection speed, the second power of the syringe plunger radius and inversely proportional to the forth power of the needle radius. This test method could differentiate very well the required forces from various formulations and injection conditions as well as different injection sites (muscle, subcutaneous tissues or air). In comparison to the polymer solutions that form implants in situ, the ISM systems were more easily injectable with smaller needle size thus expected to be less painful and give better patient comfort/compliance.