Biomedical Engineering Reference
In-Depth Information
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Important factors to consider in the experimental setup are a
full understanding of electrode-electrolyte processes and mechanisms, and the
mitigation or elimination of currents and mechanisms that are independent of
the neuronal interface.
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3.4 Field Effect Transistors as Neurotransducers
Decades ago, neuroscientists had limited choices in experimental setup, and
typically incorporated a glass micropipette for intracellular recording and a
metal wire for extracellular recording. Since then a multitude of sophisticated
electrode systems have been developed, incorporating available micro-
fabricating technologies which were initially developed for microelectronic
manufacturing. Now the needle or the glass pipette is equipped with onboard
electronics (processing chip embedded in the miniaturized device) turning it
into an active neural device. In addition, recent technologies are using
photolithographic patterning and deposition of thin-film layers on glass, silicon
or plastic flexible substrates.
Semiconductor technology employs high density 3D lithography for
informing transistors and performing high speed processing. In recent research,
it has been shown that this very same technology can be employed to create
microelectrodes. When the two technologies are integrated, an improvement in
spatial resolution and sensitivity for signal detection from neuronal cultures is
illustrated. For example there are dimensional structures that can be built over
processing chips using CMOS gate-array technology, as illustrated in
Figure 3.4. Silicon microstructures, developed using the same modern micro-
electronic technology that yielded integrated circuits used in digital logic
circuits and microprocessors, can also be employed in the direct interfacing of
neuron cells in vitro systems.
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Neurons can be grown on miniaturized
field-effect transistors (FETs) or complementary metal-oxide-semiconductor.
Figure 3.4 illustrates the electrical measurement of extracellular activity with a
metal electrode and integration of CMOS devices for signal conditioning.
CMOS structures have been designed to continually monitor the changes in
the electrical potentials of the cell.
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The coupling to the cell membrane is
achieved capacitively through the electrical polarization of the silicon dioxide
layer covering the silicon device.
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This layer provides a good electrochemical
barrier for ionic species, but there is also a lowering of the quality of the
recorded electrical signal. At the quantum level, the electrical activity of the
neurons affects the electronic band structure of the silicon and is thus
transduced into a measurable electrical signal. When the neuron cells are
stimulated with an exterior stimulus, the transistor structure provides the
electric field, which polarizes the cell membrane resulting in changes to
the membrane potential, and the conformation of membrane proteins and the
voltage-gated ion channels. The interface also contains the protein molecules
(integrins, glicocalix) that are the first to be deposited on the substrate when cell
adhesion is initiated. This barrier shields the electric field and suppresses the
direct polarization of the membrane. However, the capacitive and ionic
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