Biomedical Engineering Reference
In-Depth Information
components: the neuron, the microelectrode, and the medium in between,
which for selective stimulation requires close contact of the electrode with the
cell. In an in vitro system (a culture dish) a dissociated neuronal cell adheres to a
microelectrode array substrate, and in this occurrence, the gap between the cell
and the electrode may be very narrow at between 30 and 300 nm.
Electrodes can be designed to exhibit selectivity to discriminately monitor a
neuronal process or stimulate a specific element. Examples of this selective
stimulation are seen in nerves, fascicles, axons and somata. Thicker neuronal
fibers with larger node of Ranvier spacing yield faster conduction as the
resistance to the flow of electrical current is inversely proportional to the cross-
sectional area of a conductor. Therefore, these fibers with a larger node of
Ranvier spacing can be stimulated at a lower current than thinner fibers. For
this reason, electrodes with smaller than 10 mm can achieve fiber selectivity by
physical proximity to the fiber's node of Ranvier.
The cell-electrode interface dictates the shape, size and position of the
electrode with respect to a neuron. The exposure of the electrode to the cell,
specific part of the neuron or extracellular medium determines the response of
the electrode and even the localization of the electrode's sensing area. Smaller
electrodes can be completely covered by the cell soma in which case the
electrode interface will only be exposed to the protein membrane and the
chemical monolayer of the glycocalix (responsible for the adhesion of the cell to
the electrode). The response of the electrode when it completely covered by the
soma is simpler to model, as it consists of a parallel resistance and capacitance,
and is more localized, In comparison, larger or non-aligned electrodes, which
may cover the entire nerve or extend beyond the cell body thereby exposing
themselves to the extracellular medium, are more complicated to model.
To optimize microelectrode arrays, there is a correlation between spatial
resolution of effected neurons to the magnitude of current that flows through
the stimulating electrode. In this way using microelectrodes and passing low
currents through optimizes spatial resolution. This is another reason for
employing microelectrode arrays. Cultured neurons under low currents and
potentials are able to increase the spatial resolution of the interface. Ultrafine
electrodes placed close the cell fibers can control and monitor individual
neuronal fibers, and potentially empower complete control of the experimental
platforms of low density neuronal cultures. From a practical standpoint,
however, careful selection of electrode positions at statistically significant
locations and avoidance of overlap in the electrode's sensing area are the most
experimentally sound methods to achieve selectivity and resolution.
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The composition of the electrode material and the charge transfer kinetics at
the metal electrolyte interface are also important factors in the stimulation
process of neurons. The electrode surface involves flow of capacitive and
faradaic currents, as well charge diffusion and migration to the electrode, which
are inherently complex processes. To lessen the nonlinear effects of charge
transfer and recovery at the interface, it is important to allow enough time
between current and potential pulse stimulations to keep the direct current at a
constant magnitude, which can allow for full recovery and reestablishment of
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