Castings Replicate Si-Template Optical Properties
Paula M. Powell
Sensing and drug-delivery applications may someday benefit from a technique that replicates the optical properties of a porous Si template in polymer castings. The fabrication process, devised at the University of California, San Diego, derives from template-based synthesis techniques pioneered at the University of Florida in Gainesville. It enables tuning the porosity and average pore size by adjusting the electrochemical preparation conditions to develop photonic crystals, dielectric mirrors, microcavities and other optical structures from flexible materials that are compatible with biological systems or harsh environments.
Michael J. Sailor of the University of California, San Diego, says that imprinting polymers from porous Si photonic crystal templates enables the construction of elaborate optical nanostructures out of almost any plastic.
Researcher Yang Yang Li and colleagues constructed a multilayered porous Si template with nanometer-scale pores through an anodic electrochemical etch of crystalline Si wafers. This produced a sinusoidally varying porosity gradient with sharp features in the optical reflectivity spectrum that approximated a rugate filter. Thermal oxidation converted the structure to porous SiO2, ready for use as a template for solution-cast or injection-molded thermoplastic polymers, with the type of polymer used dependent on the application.
Consider the need to monitor drug delivery in vivo. According to professor Michael J. Sailor, a project co-investigator, it's possible to tune the spectral reflectance peaks of the porous Si filters and the polymer castings across a range of at least 400 to 10,000 nm. This allows placing peaks at wavelengths corresponding to a region of relatively low absorption in human tissue.
He cited the example of a porous Si photonic structure with a spectrum exhibiting two resonances that can be imaged through 1 mm of soft tissue in a human hand. Measurement of the decay in the intensity of a rugate peak would enable monitoring the release of a drug from an implanted medical device made of that material. The scientists tested this hypothesis by monitoring the release of caffeine from an impregnated poly(lactide) structure molded from a porous Si photonic crystal template (see figure). It remains to be tested in vivo.
Another example involves mechanically deformable filters, where application of moderate compressive stress would produce a measurable blue spectral shift. In both this and the biomedical sensing applications, Sailor noted that the casting technique provides a means to overcome potential limitations related to the chemical and mechanical instability of porous Si, while retaining the simplicity of fabricating optical structures through electrochemical synthesis.
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