Biomedical Engineering Reference
In-Depth Information
overpotentials at the electrode site, which can affect the behavior of neuronal
cultures. In order to study the electrode interface, it is essential that minimal
impedance is maintained. Existence of high impedance gives rise to high
capacitance build-ups and unwanted potentials across the interface.
There are a variety of mechanisms to optimize low impedance. Porous
platinum black electroplated while sonicating or sputtering iridium oxide or
titanium nitride are acceptable solutions to lower the electrode impedance.
Over the electrodes, biocompatible insulating layers are distributed such as
polyimide, silicon nitrate or oxide to prevent electrical shorting within the cell
growth solution. Cell adhesion is encouraged by depositing a laminin or
fibronectin cellular matrix. Alternatively, the fabrication can take place in
silicon, using the available microelectronic technologies with capacitive
recording and stimulation. To position neurons firmly in place, wells can be
used to hold them. The wells can be etched in silicon. However, the neurons
often tend to escape from wells, especially if glial cells are nearby to help them
attach and move over the margins of the well.
There are ways to optimize CMOS technologies to improve connections. The
ever increasing number of electrodes and the quantity of acquired data requires
faster processing and better multiplexing of data from arrays of electrodes. The
advantages of modern CMOS technologies are related to high connectivity. In
CMOS, chip multiplexing is used to enable electrodes with a high spatial and
temporal resolution to develop connections with ease through the on-chip logic
obtained from the computer systems through digital interfaces and user-
friendly software. Extreme miniaturization allows the fabrication of 16 384
electrodes on 6
6 mm chips.
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However, direct integration of these electrodes
with onboard processing is not trivial. Some of the state-of-the-art devices
incorporate 128-electrode (addressable) devices mounted on a processing chip
(CMOS gate-array mixed-mode technology) (Figure 3.6).
32,33
Recent advancements in planar microelectrode arrays (PEAs) in the form of
active CMOS-integrated electrode arrays
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are providing new perspectives for
the development of high-performance microelectrode arrays. This technology is
enhancing the spatial resolution and signal-to-noise ratio of MEAs by incor-
porating larger numbers of electrodes and the integration of an amplification
circuit. High-resolution active pixel-sensor based microelectrode arrays
(APS-MEA) exploit the APS technology, which is used in CMOS cameras for
fluorescence and light-sensitive measurements.
These interfacing platforms offer new tools for studying the dynamic features
of large assemblies of neurons and thus offer promising new perspectives for
both fundamental and applied neurophysiology. Neuron cells functioning as a
component of biosensing devices can be employed in a number of applications.
Hybrid robots, controlled by an array of neurons extracted from the motor
cortex of animal embryos and deposited on a glass substrate embedded with
electrodes, can direct unmanned aircraft or manage other dangerous situations
where the risk to an operator is great. These neural networks, termed live
computational devices, are formed by enzymatically extracting the neurons
from mature embryos and growing them until they interconnect and establish
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