Biomedical Engineering Reference
In-Depth Information
highly creative solution to this problem; another solution is to transfer the PSi membrane to
a backing material that readily permits diffusion of analytes. The Rochester group has
recently begun exploring the latter concept, by transferring PSi microcavities to a commer-
cial hydrogel material.
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Microcavities tuned to operate in the visible range (725 nm) were
contact laminated to a commercial wound-care product (“Nu Gel,” Johnson & Johnson)
(Figure 11.11). As one might expect, transfer to the gel causes a substantial redshift in the
optical spectrum and degradation in microcavity Q-factor (Figure 11.12), but importantly the
optical response of the device produced remains constant for at least 1 year and through
repeated cycles of gel hydration and drying. Response of the sensor to changes in solution
environment is also maintained, as cycles of exposure to water and increasing concentrations
of sucrose yielded a linear [sucrose] or optical response plot, and return to original (control)
response value. While this study did not test the ability of the sensor to selectively detect a
target analyte (i.e., there was no immobilized molecular recognition element employed), it
nonetheless establishes that such devices have potential for applications in general biosens-
ing. More specifically, one can envision applications in which hydrogel-immobilized PSi
biosensors could be used to monitor the progress of wound healing, and given the interest in
PSi as a drug-delivery matrix, could also actively participate in the wound-healing process.
11.8
Beyond Mesoporous Silicon: Larger-Volume Structures
One obvious limitation of the PSi sensors discussed so far is that, by definition, meso-
porous-type silion has an upper limit on the size of molecules that can be targeted given the
10-50-nm pore diameter. The Rochester group has recently demonstrated that
n
-type sili-
con can yield pores with diameters in the 60-120 nm range through careful choice of doping
level (0.01-0.001 ohm-cm) (Figure 11.13).
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Importantly, as with mesoporous silicon fabri-
cation, porosity may be varied by changing current as a function of time during the etch
FIGURE 11.11
PSi microcavity sensor after contact-adhesion to NuGel hydrogel.
