Osmotically activated agent release from electrostatically generated polymer microbeads

Release of lysozyme from poly(ethylene-co-vinyl acetate) microbeads into solution was examined The lysozyme exists as solid particles dispersed randomly throughout the polymer matrix. Microbeads were prepared in a novel fashion using electrostatics to reduce bead sizes, which ranged from 0.6-0.43 to 1.0-0.85 mm. The release into distilled water was diffusionally controlled, followed by a period of osmotically controlled transport, which lasted for up to 400 hours for 1.0-0.85-mm microbeads. The osmotically controlled release rate increased as the microbead radius increased, and was time- and weakly particle-size-dependent, decreasing with time and being initially greater for larger particles, but eventually becoming greater for smaller particles as time progressed. For example, for 0.60-0.43-mm microbeads, the initial mass fraction release rate from microbeads containing 106-75-mu m particles was 0.0074 h(-1), while that of microbeads containing less than or equal to 53-mu m particles was 0.0068 h(-1). At 100 h, however, the release rate from microbeads containing 106-75-mu m particles was 0.0024 h(-1), while that of microbeads containing less than or equal to 53-mu m particles was 0.0029 h(-1). The total fraction of agent released from the polymer matrix increased, as the agent particle size increased and as the microbead radius decreased. A mathematical model of the osmotic release mechanism developed was consistent with the experimental observations. This model can be used in the design of controlled release products for pharmaceutical, agricultural and veterinarial applications.