IEEE Solid-States Circuits Magazine - Fall 2019 - 45

express) acquisition module interfaces
with a computer. The system has been
validated in vivo by several labs, and
the results are starting to yield important insights into brain processes and
structures [11]-[13]. Because the CMOS
probe enables the recording of neural
activity across different brain regions
simultaneously, something that is difficult to do with conventional passive
probes, it is possible to study how different brain regions interact during a
particular task.
The main drawback of the Neuropixels probe is that the number of
electrodes that can be recorded simultaneously is limited by the number of
metal lines that can be accommodated
in the shank. We refer to this as the
wiring bottleneck. To overcome the
limitation, a neural probe that uses
dynamic multiplexing in the shank was
proposed [10]. This probe enables the
readout of all electrodes in the shank
utilizing fewer signal lines, which is
achieved by integrating active pixels
that provide in situ voltage-to-current
conversion. The design includes a
record number (1,400) of low-noise
and low-power readout channels, but
it requires a significantly larger base.
The use of CMOS-based probes is
already having an impact on the neuroscience field. At the time of this
writing, the latest update to Figure 4
[6] included three points generated
from the experimental data collected
with the Neuropixels probes (see

Neural Probes:
Single-Cell
APs

µs
EEG
ECoG

Optical
s
MRI
cm

mm
Spatial Resolution

µm

FIGURE 3: The temporal resolution versus spatial resolution versus
tissue coverage for different brain monitoring techniques. Neural
probes achieve the highest resolutions but cover much less volume
than other techniques. MRI: magnetic resonance imaging.

Simultaneously Recorded Neurons

Temporal Resolution

The active neural probe Neuropixels [9], [11] is a CMOS chip that has an
implantable needle with almost 1,000
electrodes and a nonimplantable base
with 384 recording channels (see Figure 5). The signals captured by the
electrodes are amplified, filtered, and
digitized with the low-noise and lowpower programmable readout channels integrated in the probe base. That
makes it one of the most advanced
neuroscience tools available. The fabrication of the probe required not only
CMOS processing but also a series of
post-CMOS processing steps to create
the outline of the probe and low-impedance electrodes. Using silicon-oninsulator technology, it was possible
to thin the shank to fewer than 25 µm
while keeping a thick base. The electrodes were fabricated using titanium
nitride, a very porous bio- and CMOScompatible material that is able to
maintain a stable impedance for several months after implantation. A photograph of the fully fabricated probe,
with details of the probe neck, tip, and
electrodes, is presented in Figure 6.
The complete Neuropixels data-acquisition system (see Figure 7) includes
a flexible printed circuit board (PCB),
on which the probe is directly bonded,
and a very small, light headstage. The
flex PCB and headstage, together, weigh
only 1.3 g and can be easily mounted
and carried on the head of the subject.
A PXIe (peripheral-component-interconnect eXtensions for instrumentation

500

Figure 8). It is worth noticing that,
although the 60-year trend shows a
doubling in the number of simultaneously recorded neurons every 6.7
years (taking into account only the in
vivo data reported in [6]), the latest
data show a triplication in just one
year. That suggests a promising new
trend, where CMOS probes will make
a crucial contribution.

Prosthetics and BCIs
Medical neural-implant applications
can be classified in three categories:
sensory prosthetics, brain pacemakers, and BCIs. The first makes use of
sensors (for example, sound, image,
and tactile) and electrical stimulation
to restore the ability to perceive a certain defective sensory modality. Brain
pacemakers are medical devices for
electrical stimulation that are applied
to different brain areas. They are used
mainly for the treatment and prevention of epilepsy, Parkinson's disease,
major depression, and other conditions. BCIs involve creating interfaces
between neural systems and external
devices by making use of neural stimulation and recording, artificial prostheses, or other assistive apparatus.
Their main use is for the restoration
of a lost or damaged motor function
that is, under normal conditions, controlled by the brain.
The origins of neural prostheses
date back to the 1960s when the first
electrophysiological experiments in

Doubling Time: 7.4 ± 0.4 Years (n = 56)

100
50

10
5

1960

1970

1980
1990
Publication Date

2000

2010

FIGURE 4: Moore's law for neuroscience: the number of simultaneously recorded neurons doubled approximately every seven years
from the early 1960s to 2011. Modified from [7].

IEEE SOLID-STATE CIRCUITS MAGAZINE

FA L L 2 0 19

IEEE Solid-States Circuits Magazine - Fall 2019

Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Fall 2019