Microstimulation with Chronically Implanted Intracortical Electrodes - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 312

Figure 16.3. The Utah Intracortical array. It contains 100 microelectrodes, and is an
example of a technology for efficiently fabricating high-density microelectrode arrays.
shows the footprint in a cat's cerebral cortex left by the 18 iridium shafts of an
array of the type depicted in Figure 16.2a. The array had been implanted for 30
days. The histologic section was cut perpendicular to the axis of the electrode
shafts at a depth of approximately 0.5mm below the array's superstructure and
below the surface of the brain, and shows the tracks of the 16 working electrodes
(T) and those of the 3 longer stabilizing shafts (S).
The state of the neural tissue shown in Figure 16.4a is typical of our findings
at the implant sites of these arrays. In spite of the numerous small blood vessels
permeating the tissue (some of which appear similar to the electrode tracks),
we have rarely seen evidence of vascular injury within the array's footprint. We
presume that as the electrodes are being inserted into the brain, their blunt tips
push the vessels aside rather than severing them. Figure 16.4b shows a histologic
section through the tip site of one of the microelectrodes from the same array
that had been subjected to 8 hours of electrical stimulation. Figure 16.4c shows
the tip site of an unpulsed microelectrode. The particulars of the stimulation
regimen are described below. A conspicuous feature of the stimulated site is the
aggregate of inflammatory cells (seen as irregular elongated profiles) around the
tip site [19].
In Figures 16.4b, c, there are normal-appearing neurons surrounding all of
the pulsed and the unpulsed tip sites. However, prolonged electrical stimulation
does convey a risk of injury to nearby neurons. There are several mechanisms by
which electrical stimulation might inflict tissue injury. A detailed discussion of
this topic is beyond the scope of this chapter, but the interested reader is referred
to the review by the author [20]. The propensity for neural damage is affected
strongly by the interaction of two variables, the stimulus charge per phase and the
stimulus charge density. In most cases, the stimulus waveform is configured so
that the positive and negative phases inject equal amounts of charge. Charge per
phase is the charge injected by the electrode during each presentation of either
the positive or the negative (anodic or cathodic) phases of the stimulus current.