Biomedical Engineering Reference
In-Depth Information
technique. For motor control neural interfaces, the functionally representative modulation of activity
in cortical columns (i.e., hand, arm, etc., of the homunculus) is the signal of interest. In the rat, corti-
cal columns consist of a dense network of cells with estimates of 100 000 cells/mm3 with pyramidal
cells appearing 89% of the time [
26
]. The ability to resolve sufficient spatial and temporal neuronal
representation for the development of BMIs will likely require reliable neural probes that are capable
of recording large ensembles of single neurons for long periods of time. The process of extracting
signals from the motor, premotor, and parietal cortices of a behaving animal involves the implan-
tation of subdural microwire electrode arrays into the brain tissue (usually layer V) [
27
]. Current
chronic fixed probe technology requires at the surgical stage for the sensor to be placed very close
to the neurons of interest for obtaining the highest quality recordings. Moreover, once the probe
is placed, the relative distance to the neuron should remain fixed. Dealing with the experimental
variability of chronically implantable arrays has been a formidable task because the neural tissue can
shift over time with respect to the “fixed” electrode tips, and sensor sites can degrade from immune
response and encapsulation. Therefore, the viability of chronic electrodes and recording from single
neurons has been limited from months to years. This limitation presents a problem for long-term
animal model research and neuroprosthetic development for humans. A variety of approaches have
been proposed to improve the electrode tissue interface which include novel electrode material coat-
ings for engineering the glial response and tissue inflamation [
28
,
29
], multisite recording shanks for
targeting columnar cortical structure [
22
,
30
], microdrive electrodes for tuning into cellular activity
[
31
], and rapid injectable probes for minimizing implantation injury [
32
].
Two major types of fixed probes commonly used for neural recording are: 1) conventional wire
microelectrodes assembled (most often by hand) from insulated tungsten wires and 2) microma-
chined electrodes fabricated using integrated circuit microfabrication technologies. Conventional
wire microelectrodes are still the “workhorse” of neural recordings because of ease of fabrication
and good characteristics in terms of cell yields; however, because they are assembled from discrete
components, there is a limit to the electrode array density as well as their functionality. Wire micro-
electrodes have been extensively used for acute and chronic applications of neural recording [
33-35
]
and have provided the precise firing information of single neurons from cortical and subcortical
structures. Typical wire microelectrodes consist of bundles of 50-µm-diameter insulated tungsten
wire with sharp electropolished tips. However, the overall bundle size is large for subcutaneous
insertion, and the wire spacing is not accurately controlled. For BMIs, the ultimate application of
a fully implantable device warrants the need for integration between the amplifiers and electrode
arrays. Traditional microwire arrays may not be prepared for this functional integration.
The limitations of mechanically assembled wire electrode arrays can be overcome by using
microfabrication and micromachining techniques used in integrated circuits [
36
,
37
]. Moreover,
integration of signal processing circuitry onto the substrate of the probe is possible. The recording
