IEEE Solid-States Circuits Magazine - Fall 2023 - 31

S
[3],
ince the development
of the first cardiac
pacemaker implant
in 1958 [1], implantable
medical devices (IMDs)
have assumed a critical role across a
broad spectrum of clinical applications
[2],
[4],
[5]. These innovative
devices facilitate diagnosis,
deliver therapeutic interventions, and
promote regenerative processes.
Examples abound, from the widely
recognized deep brain stimulator to
drug delivery systems, vital sign monitoring
devices, and cochlear implants.
In addition, the significance
of IMDs extends into the realm of
research, where they have revolutionized
scientific approaches and methodologies,
particularly in behavioral
neuroscience. Cutting-edge IMDs designed
for neural interfaces can support
advanced neural stimulation and
neural recording. Such neural interface
IMDs have made significant contributions
to deepening our fundamental
understanding of brain functions and
advancing the development of therapies
for various brain injuries and disorders,
such as dystonia, depression,
and Parkinson's disease [6], [7], [8].
Stimulation serves as a means
to influence neuronal activities. Traditional
neuromodulation methods
have relied on dc stimulation through
tiny electrodes, and the effectiveness
of these techniques is undermined
by the limited spatial and temporal
precision with which individual cells
can be selectively targeted. The recent
emergence of optical stimulation
has revolutionized the study of
how neurons operate as members of
larger networks. Optical stimulation
enables precise and controlled manipulation
of neural activity by selectively
activating or inhibiting specific
neurons that have been genetically
targeted [9]. This approach offers advantages
over traditional electrical
stimulation methods, including high
spatiotemporal resolution, cell-type
specificity, and rapid reversibility
[10], [11]. Recording acts as a means
to capture neural activities. Within a
neuromodulation system, it is critical
to simultaneously record neural
activities across various superficial
and deep structures in response to
various stimulation patterns. Integrating
simultaneous neural recording
with neural stimulation offers an
enhanced capability to investigate the
dynamic interplay between neural activities
and the effects of controlled
stimulation. Through this integration,
researchers can uncover new insights
into the functioning of the nervous
system and may ultimately develop
therapeutic strategies for individuals
suffering from neurological disorders.
Wireless power and data transmission
are important features that
ease the usage of IMDs. Most current
experimental settings rely on bulky
and rack-mounted neural interface
systems. The long and complex wires
between the stationary controller and
signal acquisition interfaces impose
several limitations for studies involving
small, freely moving animals, including
biased behavioral outcomes,
tangling or breakage of wires, and
potential discomfort and stress of the
subject [12], [13], [14], [15]. Batterypowered
IMDs have sparked a significant
transformation by leveraging
the recent advances in wireless telemetry
technologies. However, their
utility is restricted by the battery
lifetime, particularly when optical
stimulation or other power-hungry
intervention is involved. If large batteries
are used to extend the duration
of the experiment, the added weight
and size of the battery will impose a
physical burden and bias on the subject's
behavior. Thus, it is significant
to implement a wirelessly powered
experimental arena to eliminate tethers,
batteries, and limitations on the
duration of studies for small, freely
moving animals.
In this review, we present the recent
technological innovations in
IMDs, specifically concentrating on
optical stimulation circuits, neural recording
front-end designs, and wireless
power and data transmission. We
highlight the latest progress made to
augment the energy efficiency of optical
stimulation circuits. In addition,
we discuss recent advancements in
analog front-end (AFE) designs for
neural recording, emphasizing the
designs refined for superior power
efficiency, improved noise performance,
and enhanced dynamic range
(DR). Beyond this, we assess an array
of methodologies aimed at enhancing
the efficiency and reliability of wireless
power transmission (WPT) toward
IMDs, alongside strategies intended to
enhance data telemetry in IMDs. Together,
these topics paint a vivid picture
of the ongoing developments in
creating next-generation IMDs.
Optical Neuromodulation
Traditional electrical neuromodulation
usually applies biphasic current
pulses directly through electrodes
to neurons, altering their membrane
potential and affecting the neuronal
firing activity. By contrast, optical
neuromodulation is an emerging
technique that employs light to control
genetically targeted neurons
that express opsins. When exposed
to light, these opsins, which are
light-sensitive ion channels, induce
changes in the neurons' membrane
potential, enabling precise control
of their activity. As it can selectively
excite or inhibit the genetically
modified neurons that express lightsensitive
opsins, optical neuromodulation
realizes cell-type selectivity,
which is its most distinct advantage
over electrical neuromodulation.
However, to effectively activate the
opsins, the light intensity needs to
be above a certain threshold. For
instance, the commonly used opsin
channelrhodopsin-2 can be effectively
activated at a light intensity
above the threshold of 1 mW/mm2
[10]. Hence, meeting this light intensity
requirement poses new challenges
in designing an efficient optical
neuromodulation system.
Lasers and LEDs are common light
sources in optical stimulators. Laserbased
optical stimulators offer several
advantages, such as high light
intensity, low beam divergence, and
narrow spectral bandwidth [16]. However,
their high power consumption
IEEE SOLID-STATE CIRCUITS MAGAZINE
FALL 2023
31
IEEE Solid-States Circuits Magazine - Fall 2023

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