Biomedical Engineering Reference
In-Depth Information
1.00
µ
m
FIGURE 8.10
Cross-sectional SEM micrograph of a double-layered porous silicon fi lm with a large pore
layer on top of a small pore layer. The difference of pore size was obtained by simply switching the etching
current density during anodization of silicon. Notice the smooth and uniform interface between the two layers.
The scale bar is 1 μm.
the porous silicon-based optical materials show excellent sensing properties in responding to the
interaction of their porous structures with the guest molecules,
78,99-104
which potentially adds smart
functionalities for their biomedical applications. The ease with which porous Si can be integrated
into well-established Si microelectronics fabrication techniques should lead to more sophisticated,
active medical devices.
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8.2.2.3.1 Photonic Crystals and Porous Silicon Photonic Crystals
Photonic crystals are periodic dielectric structures that are ideally infi nitely extended with respect
to the interacting wavelength.
107
Such structures can diffract light. Photons in photonic crystals
behave analogous to electrons in atomic crystals, in the way that forbidden energy gaps form for
photons in photonic crystals analogous to electronic band gaps in atomic crystals. Photonic crystals
exist in nature and are commonly found in opals abalone shells,
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beetle shells,
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butterfl y wings,
110
and the King Penguin beak horns.
111
Inorganic photonic crystals are an active area of research for
optical switching, optical computing, and other optoelectronic applications,
112
and the capabilities
of these materials to act as sensors for chemical or biological compounds have led to a series of
developments in the biomedical fi eld. Asher and coworkers demonstrated one of the early applica-
tions of the photonic crystals in medicine. By incorporating a photonic crystal into a biocompatible
hydrogel matrix, the Asher group demonstrated glucose-sensing contact lenses that change color
depending on the concentration of glucose in the user's blood.
113 -115
One-dimensional porous silicon-based photonic crystals are probably the most easily con-
structed elaborate photonic crystals. These crystals can be conveniently fabricated using a computer-
generated periodically varying etching current density.
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The resulting structure contains
multilayers of alternating indexes of refraction along the direction perpendicular to the surface
(
z
-direction), displaying interesting optical features in its refl ectivity spectra corresponding to its
energy forbidden gaps for photons. Sensors for proteins, DNA, and small molecules have also been
developed based on these and related photonic crystals.
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Recently, Sailor and coworkers
developed a label-free platform based on porous silicon photonic crystals to monitor cellular or
enzymatic activities in real-time. This platform can be potentially applied to drug screening and
environmental fi eld tests.
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