Medical Design Briefs - August 2021 - 11

development of high-density microelectrode
arrays to stimulate or record neural
activity in various neurostimulation
devices, cochlear implants, retinal prostheses,
and multi-electrode electrophysiology
catheters. A higher density array of
electrodes allows for a greater number
of discrete neurons or neuron groups to
be activated, resulting in increased localization
and control of the desired biological
response. Research groups, startup
companies, and contract and medical
device manufacturers are developing
neurostimulation, cardiac, electrophysiology,
and cochlear devices as well
as retinal prostheses, with a focus on the
construction of high-density micro -
electrode arrays or single electrodes with
enhanced electrical and electrochemical
properties at their interface with the biological
environment.
High-charge storage/injection capacity,
low impedance, and high-capacitance
electrodes and microelectrode arrays
are of great interest to most device manufacturers.
Most sensing, recording, and
stimulating applications - including
deep brain stimulation and ultra-highdensity
heart mapping catheters used in
electrophysiology devices - aim for the
best signal-to-noise ratio with the highest
selectivity. The need to use implantable
microelectrode arrays with multiple
sensing, recording, and stimulation sites
close to the tissue is critical. Fabrication
of electrodes small enough for communication
with single neurons is feasible
considering the overall dimensions of
the implant. However, size reduction of
the actual conducting site is inevitably
accompanied by an increase in the
impedance of the electrode. Smaller
electrodes suffer from low signal-tonoise
ratios and reduced charge transfer
capacity. The size of an electrode for
clinical use is determined by a trade-off
between high selectivity (obtained by
small size) and optimized electrochemical
characteristics (obtained by electrochemically
available surface area.)
Larger electrodes inject more charge
before exceeding electrochemically safe
limits but consume more space. As a
result, the spatial selectivity or resolution
of a device produced from such electrodes
is limited.
A greater number of electrodes results
in more geometric surface area, increasing
charge injection capacity and capacitance,
and enabling the delivery of a
higher resolution signal. This is expected
to improve device performance.
Medical Design Briefs, August 2021
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Macro-structure
10-3-10-4m
Micro-structure
10-5-10-6m
Nano-structure
10-7-10-9m
Length Scale
Fig. 1 - Schematic of a hierarchically structured surface, best defined as topographic features comprised
of varying length scales. For most applications, these varying length scales are the coarse-scale
rough structures that are about several microns in size to a range of 10-100 μm, and a finer structure
subset on top of the coarse structures in the range of about a few nanometers to 1 μm in size.
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Hierarchical-structure
10-3-10-9m
1 mm
200 µm
250 µm
200 µm
250 µm
2 mm
500 µm
200 µm
Fig. 2 - Examples of various hierarchically restructured electrodes for implantable medical device
applications.
50 µm
50 µm
50 µm
50 µm
Fig. 3 - Scanning electron microscope micrographs of select hierarchically restructured Pt10Ir samples.
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Medical Design Briefs - August 2021

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