Release rates from microparticles

It may be necessary to consider the effect of a boundary layer on the release rate. A boundary layer of appreciable drug concentration on the surface of the device would hinder drug release by diffusion. The effect of the layer is more marked with drugs of low solubility and with microparticles having irregular surfaces. From this simple example, it can be seen that various factors affect the release rate from reservoir microcapsules. 

In general, compared to irregular xerogel matrix particles, microparticles encapsulate a far higher load of active ingredient (up to 90% in weight of the final materials), and afford a wide control over the release rate (from hours to months and up to unlimited retention of the entrapped ingredient), thanks to control of the microstructure and thus by the initial sol-gel chemistry. 

Figure 2.14 Influence of the synthesis pH on the release rate of Rhodamine 6G from microparticles. (Reproduced from ref. 7, with permission.)...

Results. The cumulative release of HPTS from the microparticles is shown in Figure 6. During the first hour, an initial fast-release rate is observed, followed by a slower sustained-release rate. The initial rate is attributed to surface diffusion of HPTS from the microparticles. The sustained-release rate results from diffusion of HPTS through an interconnected pore network. 

In 2004, Barbe and coworkers reviewed sol-gel miCToencapsulation of bioactive molecules for drug-delivery. Four years later, the same team published a first summary of their studies on silica-based microparticles doped with hydrophilic molecules obtained from water-in-oil (W/O) microemulsions. The Australian scientists showed how both the particle size and the release rate of silica-based microparticles can be finely and easily tailored in a wide range by controlling the conditions affecting the sol-gel process. In 2006, van Driessche and Hoste published the first account on the topic in a book addressing microencapsulation techniques, mainly covering the findings of Barbe and cowoikers. 

The type of polymer also affects the release rate of drugs from microspheres. For example, a study on the release of Taxol from PLGA and PCL microparticles indicated that drug-loaded PLGA microparticles released the drug faster than the PCL miaoparticles and that the biconcave particles released the drug faster than the spherical particles. ... 

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