The attraction of placing active electronic circuit components into in-vivo drug delivery materials led to the exploration of elemental silicon as a biomaterial. In particular a porous form of Si produced by an electrochemical corrosion reaction has been of interest. Since the pioneering work of Canham and others in the late 90's demonstrating the biocompatibility and biodegradability of porous Si in vitro and in vivo [25-35], this material has been under intensive investigation for controlled drug delivery applications. Like mesoporous silica of the MCM41 class, porous Si offers tuneable structural properties: a large specific surface area, large free volume, and pore sizes that can be controlled from a few nanometers to several hundreds of nanometers depending on the preparation conditions. The surface of freshly prepared porous Si is easily modified via convenient chemistry with a large range of organic or biological molecules (ex: antibody, proteins, etc.) . Recently Swaan and coworkers have performed in vitro experiments showing that porous Si particles can be used as efficient delivery vehicles of insulin across intestinal epithelial cells . The drug permeation rate through the membrane is dramatically enhanced when delivered via porous Si particles compared with conventional liquid formulations.
Like other Si-based materials, porous Si offers attractive morphological and chemical properties for biomedical applications but it has one supplementary dimension: its optical properties. Porous Si displays fluorescence deriving from Si quantum dot structures that are produced during the etch , and it can also display unique optical reflectivity spectra [39, 40]. Both of these features allow porous Si to exhibit a signal that is affected in a predictable way when exposed to environmental changes [32,41-46]. This presents new possibilities for the development of more advanced functional systems, referred to as "intelligent systems," that incorporate a sensor for either diagnostic or therapeutic functions. The ease with which porous Si can be integrated into well-established Si microelectronics fabrication techniques should lead to more sophisticated, active devices for medical applications [35, 47-48].
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