In recent decades, significant advances in drug-delivery systems have enabled more effective drug administration. To deliver drugs to specific organs, a range of organic systems (e.g., micelles, liposomes, and polymeric nanoparticles) have been designed. They suffer from limitations, including poor thermal and chemical stability, and rapid elimination by the immune system. In contrast, silica particles offer a biocompatible, stable, and “stealthy” alternative. Bioactive molecules can be easily encapsulated within silica particles by combining sol-gel polymerization with either spray-drying or emulsion chemistry. Spray-drying faces challenges, including low yield, surface segregation, and size limitations. In contrast, sol-gel emulsions enable the production of nanoparticles with homogeneous drug distribution, and permit ambient temperature processing, necessary for handling biologicals. Independent control of the size and release rate can be readily achieved. Preliminary in-vivo experiments reveal enhanced blood stability of the nanoparticles, which, coupled with sustained release of anti-tumor agents, show good potential for cancer treatment.

Silica Particles: A Novel Drug-Delivery System †

Properties and applications of proteins encapsulated within sol–gel derived materials

Fabrication of porous hollow silica nanoparticles and their applications in drug release control

The synthesis and structure of an azobenzene functionalized isoreticular metal–organic framework (azo-IRMOF-74-III) [Mg2(C26H16O6N2)] are described and the ability to controllably release a guest from its pores in response to an external stimulus has been demonstrated. Azo-IRMOF-74-III is an isoreticular expansion of MOF-74 with an etb topology and a 1-D hexagonal pore structure. The structure of azo-IRMOF-74-III is analogous to that of MOF-74, as demonstrated by powder X-ray diffraction, with a surface area of 2410 m2 g−1 BET. Each organic unit within azo-IRMOF-74-III is decorated with a photoswitchable azobenzene unit, which can be toggled between its cis and trans conformation by excitation at 408 nm. When propidium iodide dye was loaded into the MOF, spectroscopic studies showed that no release of the luminescent dye was observed under ambient conditions. Upon irradiation of the MOF at 408 nm, however, the rapid wagging motion inherent to the repetitive isomerization of the azobenzene functionality triggered the release of the dye from the pores. This light-induced release of cargo can be modulated between an on and an off state by controlling the conformation of the azobenzene with the appropriate wavelength of light. This report highlights the ability to capture and release small molecules and demonstrates the utility of self-contained photo-active switches located inside highly porous MOFs.

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Photophysical pore control in an azobenzene-containing metal–organic framework

Towards establishing structure–activity relationships for mesoporous silica in drug delivery applications

Samples of porous sol-gel glass, after being stabilized at 750°C, were impregnated with progesterone, estradiol, estrone, and hydrocortisone. Diffusion coefficients for those steroids inside the pores filled with pure ethanol, ethanol-water mixture or physiological solution were determined. It is shown that permeability is favored for steroids having fewer hydroxyl and carbonyl groups. Solvents that readily dissolve steroids increase the diffusion rate and thus the amount of delivered material. The flow of steroids in small pores is less restricted than in larger pores.

E. Couch

Papers

Diffusion of Steroids in Porous Sol-Gel Glass: Application in Slow Drug Delivery