IEEE Solid-States Circuits Magazine - Fall 2021 - 96

From the U.S. president fist-bumping a braincontrolled
robotic arm to monkeys playing
brain-controlled Pong, the past few years have
had a surge of neural interfaces in the news.
while RTh is independent of thickness
and is only a decaying function
of the aspect ratio. With a known RTh
and VTh for a given geometry (the
thickness and aspect ratio), the maximum
available power per piezo volume
can be calculated. For a PZT-5H
piezo with an aspect ratio of one,
this is provided in Figure 9(c) at the
series resonance frequency.
To harvest the maximum power,
the input impedance of the chip Rin
should be matched to the piezo resistance
RTh, and any mismatch will
decrease the delivered power. The
following equation, derived from the
conservation of energy, can be used
to approximate the delivered power
to the load:
IL . 2 11 2 asincc cmmm.
For a known load voltage, 2.5 V, the
rr
-R
V
Th
Th
V
V
Th
L
V
V
Th
L
preceding
expression can be numerically
solved for IL and different combinations
of VTh and RTh that are
ultimately mapped to different geometries
and volumes using the look-up
tables. Any geometry that provides
IA
L 100$
caused by encapsulation, misalignment,
and tissue inhomogeneity.
Improved encapsulation and tissue
modeling and a lower aspect ratio
would enable the use of a significantly
smaller piezo. Moreover, scaling
the piezo size to that of a neuron
[Figure 9(c)] could potentially harvest
nonnegligible amounts of energy
(~ 1 nW), enough to power a sensor.
At these scales, other elements, such
as the IC and interconnect, would
likely dominate the volume, but this
demonstrates that there is significant
room for innovation and advancing
the state of the art for miniaturizing
wireless neural implants.
n is a solution. The same
process can be repeated for other
aspect ratios at both series and parallel
resonant frequencies. A summary of
acceptable geometries is in Figure 9(d)
for the neural stimulating implant constraints
outlined earlier. Interestingly,
we find that decreasing the aspect
ratio of the piezo roughly maintains
the delivered power while decreasing
the volume. The smallest piezo volume
that delivers 250 nW of power while the
rectifier maintains
V .25 L
$
V is found
to be 0.013 mm3 for a piezo with an
aspect ratio of 1/4. This is an order of
magnitude smaller than the piezo used
in our prior art [46].
In [46], a margin of error was
included to account for nonidealities
96
FALL 2021
Summary
This tutorial covered the background
of and major trends in neural interface
technologies that are used in
BMIs and neuromodulation devices.
We presented methodologies to ensure
safe and efficient neurostimulation
and optimize the volume of a wireless
ultrasonic neuromodulating implant.
So many different parts must come
together in the realization of a neural
interface
that no single tutorial
can cover all aspects; in fact, we have
only scratched the surface. There are
numerous other areas where circuit
designers can contribute and significantly
advance the state of the art,
including machine learning for closing
the loop; communication; networking
and security for implanted
devices; strategies, such as beamforming
techniques for coping with
misalignment and implant migration;
packaging; and more. There are also
emerging sensing and stimulation
techniques beyond voltage recording
and charge-based stimulation
and numerous highly impactful evolvin
applications that will guide the
design of future devices to improve
IEEE SOLID-STATE CIRCUITS MAGAZINE
our understanding of the brain, treat
neurological conditions, and improve
the human condition.
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IEEE Solid-States Circuits Magazine - Fall 2021

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