Technology Advances
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Microencapsulation and Controlled Release from Spherical Ceramic Particles Developed for Drug Delivery and Industrial Processes The Pitch An increasing range of applications, from the targeting of cancer tumors to protecting material surfaces against bacterial attack, require the production of delivery systems in particulate form. Although a range of organic materials are used to manufacture capsules and particles, only a limited number of inorganic controlled-release systems have been developed for industrial products. This is the case for ceramics, which despite a number of intrinsic advantages such as high mechanical strength, resistance to corrosion, thermal and electrical stability, bio-compatibility, and an environmentally benign nature have remained an untapped resource for the manufacture of controlled release systems. The relative difficulty in manipulating the internal microstructure of ceramics (as compared to polymers) using traditional routes as well as high processing temperatures, which are incompatible with the encapsulation of organic molecules, have limited their use as a controlled release matrix. The Australian company CeramiSphere has overcome both of these limitations by using sol-gel technology (i.e., the suspension of colloidal ceramic particles chemically converted to a gel) to create encapsulating ceramic microspheres. The Technology Sol-gel chemistry has revolutionized ceramic production by enabling the ambient temperature, solution-based synthesis of metal oxides with the ability to “tailor” porosity. By combining sol-gel with emulsion chemistry, it is possible to produce spherical particles with a designed microstructure resulting from a judicious choice of solvent/surfactant and sol-gel reaction parameters. By changing the solvent/surfactant combination, the particle size can be varied from 10 nm to 100 µm (see Figure 1). The size of the particles is controlled by the size of the emulsion droplet, which acts as a nanoreactor for the sol-gel reaction. When an active molecule is located in the aqueous droplet, encapsulation occurs as silicon precursors polymerize to build an oxide cage around the active species. Encapsulation efficiencies for hydrophilic molecules are typically >85%, with doping levels typically in the range 5–20 wt%. The release profiles can be tailored, independently of the particle size, by controlling the internal structure of the particles: pore volume, pore size, tortuosity, and surface chemistry. This can be
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Figure 1. (a) Microparticles (scale bar: 10 µm) and (b) nanoparticles (scale bar: 200 nm) demonstrating the size ranges that can be obtained using CeramiSphere technology.
readily achieved by controlling sol-gel processing parameters such as the water to alkoxide ratio, pH, alkoxide concentration, aging, drying time, and temperature. Hence, the release rate of the encapsulated species is controlled by adapting the structure of the internal pore network to the physicochemical properties of the active molecule. Although the CeramiSphe
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