Biomedical Engineering Reference
In-Depth Information
photodetector array were converted to voltage signals (V
1
,V
2
), and amplified
with a gain of 10 000 before low-pass filtering at the cutoff frequency. fc,of
500 kHz. The SPR system was initially aligned such that those two signals were
equal in magnitude, and then the difference between the two signals was
monitored to remove common mode noise. With additional noise reduction,
the ambient noise was less than 10 mV
RMS
in magnitude. As mentioned above,
the system was built to record both optical and electrical signals simultaneously
in response to electrical stimulation. A Teflon-insulated Pt-Ir wire electrode
was placed on the proximal end of the nerve, while another wire was located on
the distal end of it. The laser beam for the SPR detection was located between
the two wires. Electrical responses were amplified a thousand times using a
differential AC amplifier, filtered between 1Hz and 10 kHz, with a 60Hz
notch filter and digitized at 20 kHz. The optical and electrical signals were
simultaneously recorded and time labeled by a single DAQ board. For neural
recording experiments, sciatic nerves from knee (distal) to spinal cord
(proximal) were dissected from male Sprague-Dawley rats. Changes in the
refractive index unit were detected when the nerve was stimulated with ethanol
or nerve-blockers were applied (lidocaine 2%).
20
An additional optical methodology is provided by light-addressable poten-
tiometric sensor technology (LAPS).
21
In this case cells are anchored on a
semiconductor surface and the local surface potential is measured when a small
area on the surface is illuminated by a laser beam. The system is depicted in
Figure 5.10.
Detection takes place in the electrolyte-insulator-semiconductor structure.
The illumination of the silicon substrate leads to energy band transitions and
the generation of electron-hole pairs. The modulation of light induces an AC
photocurrent, which depends on the surface potential. When drugs are pumped
in, the cells react—for example, in neurons, if a given stimulus generates a
depolarization greater than the threshold, an action potential is induced, which
in turn changes the bias voltage and can be detected by the sensor. Scanning or
multi-light LAPS microphysiometers based on this effect can monitor cellular
acidification or Na
1
,K
1
,Ca
21
and Mg
21
changes in concentration, or the
effect of applied drugs.
d
n
4
t
3
n
g
|
2
n
3
.
Figure 5.10
The principle of LAPS. (a) Schematic of the cell-based biosensor. (b) The
cell-semiconductor interface. (c) Equivalent circuit model.
21
(Reprinted by kind permission of Elsevier BV.)
