IEEE Solid-States Circuits Magazine - Fall 2023 - 26

hundreds of mW of power to small
mm-size receivers with a penetration
depth of about 10-30 mm [93]. The
power conversion efficiency (PCE) of
the IPT systems for compact receivers
is significantly influenced by the operating
frequency, distance, and misalignment.
To enhance the PCE of the
IPT system, various techniques have
been proposed, including new circuit
topologies [94], impedance matching
techniques, and highly efficient PMU
circuits for both the TX and the RX
[95]. Since the basic two-coil configuration
exhibits limitations in terms
of distance coverage for small-sized
receivers, cascaded coil configurations
have been proposed to improve
the PCE for applications that require
larger distances. To alleviate the PCE
drop due to misalignment, several
methods, such as coil design and new
compensation topologies, have been
proposed. In addition, IPT systems
utilizing a TX coil array [93] or 3D TX
[96] coils are promising to solve the
misalignment problem for scenarios
of low coil coupling coefficient.
Since acoustic waves (from hundreds
of kHz to a few MHz) are less
attenuated by the human tissues
(about 0.5-1.0 dB∙cm-1∙MHz-1 [97])
compared with RF, UPT is a popular
WPT solution for implantable medical
devices. According to Food and
Drug Administration regulations, the
maximum power intensity for UPT is
720 mW/cm2 [98], while the maximum
power intensity for RF is 10 mW/cm2
[99], [100]. It is promising to deliver
more power by using UPT than IPT.
However, ultrasound requires a medium
to propagate, and it is hard to
transfer energy through air [92]. Research
has mainly focused on three
directions to improve the UPT system
PCE:
1) to explore new piezoelectric materials
and/or transducer structures
with higher electromechanical
energy conversion efficiency [101]
2) to optimize the structure to realize
acoustic impedance matching
[102]
3) to optimize the circuit to realize
electrical impedance matching
26
FALL 2023
and to lower electrical loss, by frequency
optimization, introducing
impedance matching strategies,
and efficient power transfer circuit
design [103], [104].
Recently, body-coupled energy and
optical techniques have also been investigated
for biomedical applications.
The body-coupled methods can transfer
energy to the full body area on the
skin surface, with limited power of a
few nW [105]. The harvested power of
optical techniques relies on the environmental
illumination intensity, varying
from 10 nW/cm2 to 10 mW/cm2,
and becomes ineffective in a dark environment
[106]. To apply the bodycoupled
or optical techniques to real
applications, improved effectiveness
and stably high energy density for
more scenarios are required.
Outlook
In the past several years, customized
wireless compact neural interface
shows obvious advantages to
cable-based neural interfaces. Frontier
research [13], [17], [21], [22], [23]
continuously improves key performances,
including
1) the low-noise multichannel data
acquisition solution [17], [22], [23]
2) efficiency of the on-chip processing
and closed loop control
strategy [13], [21]
3) the wireless data and power transmission
method [22], [23].
In the future, the emerging solutions
will enable more scenarios for complicated
neural circuit exploration.
Acknowledgment
This work was supported in part by
the National Science Foundation of
China (NSFC) under Grants 92164022
and 62227801, in part by the Tsinghua
University Initiative Scientific
Research Program,
in part by the
Beijing Innovation Center for Future
Chips, and in part by the Beijing National
Research Center for Information
Science and Technology. The
corresponding author is Milin Zhang.
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