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signals
Review
Wireless Power Transfer Approaches for Medical
Implants: A Review
Mohammad Haerinia 1, * and Reem Shadid 2
1School of Electrical Engineering and Computer Science, University of North Dakota,
Grand Forks, ND 58202, USA
2School of Electrical Engineering Department, Applied Science Private University, Amman 11931, Jordan;
re_shadid@asu.edu.jo
*Correspondence: mohammad.haerinia@und.edu
Received: 6 September 2020; Accepted: 10 November 2020; Published: 16 December 2020
Abstract:
Wireless power transmission (WPT) is a critical technology that provides an alternative for
wireless power and communication with implantable medical devices (IMDs). This article provides a
study concentrating on popular WPT techniques for IMDs including inductive coupling, microwave,
ultrasound, and hybrid wireless power transmission (HWPT) systems. Moreover, an overview of
the major works is analyzed with a comparison of the symmetric and asymmetric design elements,
operating frequency, distance, efficiency, and harvested power. In general, with respect to the
operating frequency, it is concluded that the ultrasound-based and inductive-based WPTs have a
low operating frequency of less than 50 MHz, whereas the microwave-based WPT works at a higher
frequency. Moreover, it can be seen that most of the implanted receiver’s dimension is less than
30 mm for all the WPT-based methods. Furthermore, the HWPT system has a larger receiver size
compared to the other methods used. In terms of efficiency, the maximum power transfer efficiency is
conducted via inductive-based WPT at 95%, compared to the achievable frequencies of 78%, 50%, and
17% for microwave-based, ultrasound-based, and hybrid WPT, respectively. In general, the inductive
coupling tactic is mostly employed for transmission of energy to neuro-stimulators, and the ultrasonic
method is used for deep-seated implants.
Keywords:
wireless power transfer; implanted device; inductive link; microwave link; ultrasound
link; hybrid link
1. Introduction
In recent years, medical progress has evolved with an increased interest in instruments for sensing
and controlling the specific functions of the brain. These medical instruments considerably decrease
morbidity and improve the standard of life for certain patients. Sensor systems are now quite advanced
but providing power to these devices is still a major challenge. The answer to this issue is using
wireless power transmission (WPT) technologies for a range of biomedical implants. WPT is a secure
and appropriate energy supply for recharging biosensors and electrical implanted devices as well as
for data communication in these specific applications.
WPT comprises two main methods: near-field and far-field transmission. The region is considered
near-field if it satisfies two conditions: First, the distance between the transmitter and receiver coil
(d) should be less than one wavelength (λ) at the operating frequency (d<λ), and second, the largest
dimension of the transmitter coil (D) should be less than
λ
/2. In contrast, for far-field D >
λ
/2. Moreover,
the near and far fields are defined in terms of the Fraunhofer distance (d
F
=2D
2
/
λ
); if the conditions d
F
>> D and dF>> λare satisfied, the region is considered far-field.
Signals 2020,1, 209–229; doi:10.3390/signals1020012 www.mdpi.com/journal/signals
Signals 2020,1210
There are three major ways to accomplish a near-field WPT: (1) capacitive coupling based on
electric fields; (2) inductive coupling based on magnetic fields; and (3) magnetic resonant inductive
coupling, which include a resonant circuit in transmitter and receiver coils. Far-field WPT is also
known as microwave coupling. Hybrid wireless power transmission (HWPT) includes both far-field
and near-field WPT.
The biomedical implants are intended to be used for biological studies, therapy, and medical
diagnostics. Novel biological materials also provide additional biocompatibility and efficiency, as well
as reduced expenses. Implantable medical devices (IMDs) can be classified into two primary categories
based on their methodologies for the transmission of power. Transfer mechanisms such as inductive
coupling, optical charging, and ultrasound are included in the first category. The second category
is split into two subsections: batteries, such as lithium; and natural harvesting, including biofuel
cell, thermoelectricity, piezoelectricity, electrostatic, and electromagnetic [
1
]. Various WPT techniques
are reviewed in the literature. For instance, the ultrasound and inductive coupling methods were
evaluated by Taalla et al. [
2
] and Shadid et al. [
3
], respectively. In this paper, common WPT approaches
for IMDs, including inductive coupling, microwave, and ultrasound, are studied. HWPT systems,
a mixture of two different methods, are also reviewed.
2. Different Approaches for a Wireless Power Transfer System
The lifespan of IMDs is limited to battery capabilities. Patient pain and the danger of infection are
the major development concerns in implantable medical systems because using implanted batteries
can cause diseases [
4
]. Therefore, the WPT link is a safer option to power biomedical implants [
4
].
Improving WPT techniques and efficiency will enable rechargeable batteries to be employed for IMDs
rather than non-rechargeable batteries, which usually have a greater weight and volume and a shorter
period of effectiveness compared to rechargeable batteries. Medical implants like implanted spinal
cord stimulators can use a rechargeable battery to improve their capability and reduce overall costs [
5
].
Lately, there has been a great interest in the usage of WPT for medical applications. The development
of implantable electronic devices in biological systems has made it easier to use this technology for
powering various IMDs, such as biological sensors, pacemakers, and neurostimulator, working in a
range of power from a few microwatts to a few watts. In Figure 1, the power ranges of common IMDs
are illustrated [
1
,
6
]. The WPT systems for the neurostimulator and the pacemaker are discussed in
detail in [7–13].
Signals 2020, 3 FOR PEER REVIEW 2
There are three major ways to accomplish a near-field WPT: (1) capacitive coupling based on
electric fields; (2) inductive coupling based on magnetic fields; and (3) magnetic resonant inductive
coupling, which include a resonant circuit in transmitter and receiver coils. Far-field WPT is also
known as microwave coupling. Hybrid wireless power transmission (HWPT) includes both far-field
and near-field WPT.
The biomedical implants are intended to be used for biological studies, therapy, and medical
diagnostics. Novel biological materials also provide additional biocompatibility and efficiency, as
well as reduced expenses. Implantable medical devices (IMDs) can be classified into two primary
categories based on their methodologies for the transmission of power. Transfer mechanisms such as
inductive coupling, optical charging, and ultrasound are included in the first category. The second
category is split into two subsections: batteries, such as lithium; and natural harvesting, including
biofuel cell, thermoelectricity, piezoelectricity, electrostatic, and electromagnetic [1]. Various WPT
techniques are reviewed in the literature. For instance, the ultrasound and inductive coupling
methods were evaluated by Taalla et al. [2] and Shadid et al. [3], respectively. In this paper, common
WPT approaches for IMDs, including inductive coupling, microwave, and ultrasound, are studied.
HWPT systems, a mixture of two different methods, are also reviewed.
2. Different Approaches for a Wireless Power Transfer System
The lifespan of IMDs is limited to battery capabilities. Patient pain and the danger of infection
are the major development concerns in implantable medical systems because using implanted
batteries can cause diseases [4]. Therefore, the WPT link is a safer option to power biomedical
implants [4]. Improving WPT techniques and efficiency will enable rechargeable batteries to be
employed for IMDs rather than non-rechargeable batteries, which usually have a greater weight and
volume and a shorter period of effectiveness compared to rechargeable batteries. Medical implants
like implanted spinal cord stimulators can use a rechargeable battery to improve their capability and
reduce overall costs [5]. Lately, there has been a great interest in the usage of WPT for medical
applications. The development of implantable electronic devices in biological systems has made it
easier to use this technology for powering various IMDs, such as biological sensors, pacemakers, and
neurostimulator, working in a range of power from a few microwatts to a few watts. In Figure 1, the
power ranges of common IMDs are illustrated [1,6]. The WPT systems for the neurostimulator and
the pacemaker are discussed in detail in [7–13].
Figure 1. Power ranges of implantable medical devices (IMDs).
Dru
g
Pump
Neurostimulato
r
Cardiac Defibrillato
r
Biosenso
r
Cochlear Implant
Pacemaker Retinal Implant VA D *
μW mW W
*Ventricular Assist Device
Figure 1. Power ranges of implantable medical devices (IMDs).
There are reliability problems with the classic wireless power links. An option that facilitates the
growth of a number of bio-implants is the use of CMOS processes. In this procedure, the standard
CMOS is included with the implanted receiver. This reduces the cost, improves productivity, and
provides compatibility and reliability of prototypes [
14
,
15
]. The usage of CMOS for WPT systems is
presented in [
15
–
23
]. A backward communication unit transmits the information to an external data
communicator using modulation. Typically, FSK [
24
], PSK [
25
], ASK [
26
], OOK [
27
], LSK [
28
], PPSK [
29
],
Signals 2020,1211
QMPM [
30
], QPSK [
31
], and impedance modulation [
32
] have been used for data communication
units in medical applications. Long-term RF and microwave exposure are dangerous. The device
layout must comply with the associated safety regulations to protect patients from electromagnetic
radiation damage. It is possible to evaluate maximum permissible exposure (MPE) in environments
for electromagnetic field intensity by assessing the specific absorption rate (SAR). IEEE Standard Basis
C95.1 expresses SAR limitation. According to IEEE 1992 standard, the maximum SAR value must be
below 1.6 W/kg for any 1 g of the body tissue and below 0.08 W/kg for the whole body. Nevertheless,
the maximum SAR limitation is 4 W/kg for every 10 g of tissue of body parts such as hands, feet, ankles,
and wrists, as per IEEE 2005 standard.
The authors proposed an arrangement to decrease the SAR and improve safety for inductive WPT
systems [
33
]. They designed a multi-transmitter configuration consisting of an array of symmetric
resonant elements. The array will significantly decrease the amount of electric field generated by the
fed loop and thus reduce the electromagnetic exposure of the biological tissues.
Mainly, determining SAR can be achieved by using numerical techniques and empirical models
using fabricated tissue phantoms [
4
,
34
–
38
]. In [
39
], the body tissue was used as the power transfer
channel. According to this technique, medical electrodes are attached on the body surface to
supply power to a miniaturized implant with a differential input. The maximum SAR value was
studied in [
40
,
41
]. The empirical results can be obtained
in vivo
[
8
,
42
,
43
], using a living organism,
or
in vitro
[
44
–
46
], outside of a living organism. To mimic the biological effects of human body tissue,
the phantom is very popular among researchers in this field. The material type and amount needed
for muscle phantom fabrication is summarized and measured in [
47
]. The tissue’s electromagnetic
properties play an important role in the design of implantable devices. An assessment of variation
in tissue electromagnetic properties was provided by Bocan et al. [
48
]. The recent reports on tissue
electromagnetic properties are depicted in Table 1.
Table 1. Summary of different approaches in analyzing tissue electromagnetic properties.
Reference Year Tissue Frequencies Models/Methods
[49] 2020 In vivo, ex vivo - FEM *
[50] 2019 Muscle, fat, skin 50 MHz, 300 MHz, 700 MHz,
and 900 MHz FDTD **
[51] 2019 Body (0.5–26.5) GHz Measured properties,
Cole–Cole
[52] 2018 Brain, liver 200–1600 Hz Measured properties
[53] 2018 Muscle, fat, skin 915 MHz and 2 GHz Measured properties
[54] 2017
Blood, liver, fat, brain
10 kHz–10 MHz
Bottcher–Bordewijk
model, measured
properties
[55] 2016 Muscle, bladder,
cervix 128 MHz Measured properties,
Cole–Cole
[56] 2016 Body/14 tissues 2.1 GHz, 2.6 GHz FDTD
[57] 2016 Head (0.75–2.55) GHz Phantom/FEM
[58] 2016 Muscle 500 MHz–20 GHz Fricke
[59] 2015 Eye/6 tissues (0.9–10) GHz FDTD
[60] 2015 Skin (0.8–1.2) THz FEM
[61] 2014 Eye, head/14 tissues (0.9–5.8) GHz FDTD
[62] 2010 Head - FEM
[63] 2009 Head/16 tissues 50 MHz–20 GHz Measured properties,
FDTD
[64] 2006 Eye, head/15 tissues 900 MHz, 1800 MHz,
2450 MHz FDTD
[65] 2004 Body
400 MHz, 900 MHz, 2400 MHz
Visible human, FDTD
[66] 2004 Body/51 tissues 30 MHz–3 GHz FDTD
[67] 2002 Head/10 tissues 900 MHz, 1800 MHz Visible human, FDTD
* Finite element method. ** Finite-difference time-domain.
Signals 2020,1212
2.1. Inductive-Based Wireless Power Transfer
Inductive coupling is the process of transferring power by connecting a source that is generating a
varying magnetic field to a primary coil which is usually located outside the body tissue. Then, based
on Faraday
0
s law, the voltage is induced across the receiver secondary coil, which is usually implanted
inside the body tissues. Figure 2below illustrates this principle.
Signals 2020, 3 FOR PEER REVIEW 4
2.1. Inductive-Based Wireless Power Transfer
Inductive coupling is the process of transferring power by connecting a source that is generating
a varying magnetic field to a primary coil which is usually located outside the body tissue. Then,
based on Faraday′s law, the voltage is induced across the receiver secondary coil, which is usually
implanted inside the body tissues. Figure 2 below illustrates this principle.
Figure 2. Inductive coupling principle.
The amount of induced voltage in implanted coils () is given by the following equation
=−
=
=
.
(1)
where N is the number of turns, is the operating angular frequency, is the magnetic flux linkage,
and is the permeability of transfer medium. According to Equation (1), coupling between coils
depends mainly on the amount of between the primary (transmitter) and secondary (implanted)
coils. Thus, when the distance between the transmitter and the implanted receiver is decreased the
amount of coupled magnetic flux will increase.
Furthermore, the amount of transferred power using inductive coupling could be increased by
adding a capacitor for resonance. The simplified diagram of the resonant inductive WPT circuit is
shown in Figure 3. is the transmitter coil inductor that is located outside the body tissues, and
is the implanted receiver coil inductor, often with the rest of the implant electronics. Coil windings
have parasitic capacitance and resistance associated with them, which are shown as symmetric
elements (, ), and (, ). Capacitors and are added to the circuit to form an LC
resonance with and , respectively. is the load resistance.
Figure 3. Circuit model of magnetic resonant inductive coupling.
Primary Coil
Transmitter coil and
located outside the
human body
Secondary Coil
Implanted receiver
inside body tissues
Figure 2. Inductive coupling principle.
The amount of induced voltage in implanted coils (Vis)is given by the following equation
Vis =−NdΦ
dt =jNωΦ=jNωµ Z→
H·→
ds (1)
where Nis the number of turns,
ω
is the operating angular frequency,
Φ
is the magnetic flux linkage,
and
µ
is the permeability of transfer medium. According to Equation (1), coupling between coils
depends mainly on the amount of
Φ
between the primary (transmitter) and secondary (implanted)
coils. Thus, when the distance between the transmitter and the implanted receiver is decreased the
amount of coupled magnetic flux will increase.
Furthermore, the amount of transferred power using inductive coupling could be increased by
adding a capacitor for resonance. The simplified diagram of the resonant inductive WPT circuit is
shown in Figure 3.
L1
is the transmitter coil inductor that is located outside the body tissues, and
L2
is
the implanted receiver coil inductor, often with the rest of the implant electronics. Coil windings have
parasitic capacitance and resistance associated with them, which are shown as symmetric elements
(
Rs1
,
Rs2
), and (
Cs1
,
Cs2
). Capacitors
CT
and
CR
are added to the circuit to form an LC resonance with
L1and L2, respectively. RLis the load resistance.
Signals 2020, 3 FOR PEER REVIEW 4
2.1. Inductive-Based Wireless Power Transfer
Inductive coupling is the process of transferring power by connecting a source that is generating
a varying magnetic field to a primary coil which is usually located outside the body tissue. Then,
based on Faraday′s law, the voltage is induced across the receiver secondary coil, which is usually
implanted inside the body tissues. Figure 2 below illustrates this principle.
Figure 2. Inductive coupling principle.
The amount of induced voltage in implanted coils () is given by the following equation
=−
=
=
.
(1)
where N is the number of turns, is the operating angular frequency, is the magnetic flux linkage,
and is the permeability of transfer medium. According to Equation (1), coupling between coils
depends mainly on the amount of between the primary (transmitter) and secondary (implanted)
coils. Thus, when the distance between the transmitter and the implanted receiver is decreased the
amount of coupled magnetic flux will increase.
Furthermore, the amount of transferred power using inductive coupling could be increased by
adding a capacitor for resonance. The simplified diagram of the resonant inductive WPT circuit is
shown in Figure 3. is the transmitter coil inductor that is located outside the body tissues, and
is the implanted receiver coil inductor, often with the rest of the implant electronics. Coil windings
have parasitic capacitance and resistance associated with them, which are shown as symmetric
elements (, ), and (, ). Capacitors and are added to the circuit to form an LC
resonance with and , respectively. is the load resistance.
Figure 3. Circuit model of magnetic resonant inductive coupling.
Primary Coil
Transmitter coil and
located outside the
human body
Secondary Coil
Implanted receiver
inside body tissues
Figure 3. Circuit model of magnetic resonant inductive coupling.
Signals 2020,1213
The highest efficiency and voltage gain are achieved when both LC tanks are tuned at the operating
frequency of the link
ωo=
1
/√L1C1=
1
/√L2C2
, where
C1
and
C2
are a combination of the lumped
capacitor and the parasitic capacitance of the transmitter and implanted coils, respectively.
The delivered power is transferred between the transmitter and the implanted coils through
mutual inductance (M). Mis related to the coupling coefficient (k) according to
k=M
√L1L2
(2)
The quality factors for the transmitter (
Q1
), receiver (
Q2
), and load (
QL
) circuits are calculated as
follows:
Q1=ωoL1
Rs1
,Q2=ωoL2
Rs2
,QL=RL
ωoL2
(3)
The total efficiency, ηind , is calculated according to Equation (4), as derived in [3]:
ηind =k2Q1Q2
1+k2Q1Q2+Q2
QL
×1
1+QL
Q2
(4)
More details on how to derive the efficiency of inductive links can be found in [
68
]. In general,
the efficiency increases at high delivered power. However, different efficiencies have been achieved
for the same transmitted power depending on the system design. Inductive coupling is a common
and efficient way to transfer data and power into implantable medical instruments, including cardiac
pacemakers, implantable cardioverter defibrillators, recording devices, neuromuscular stimulators,
and cochlear and retinal implants.
When the development of an inductive link using a power amplifier is applied, the output
power depends on the operating frequency and the distance range. The bandwidth to support
data communication and reasonable efficacy for power transfer, insensitivity to misalignments, and
biocompatibility are needed for a robust inductive link for medical implants [
69
]. In general, hundreds
of kilohertz to a few megahertz is the operating frequency, and the size of the implanted coil is between
several millimeters and a few centimeters. As the frequency increases, the electromagnetic wavelength
becomes more commensurate with the coil dimension and the space between the coils. In this stance,
the radiative and non-radiative components are part of the electromagnetic waves. Biological tissue
also creates significant problems for the propagation of electromagnetic fields and dilutes the electrical
field, thus affecting the efficiency of the inductive link [
33
]. According to Faraday’s induction law,
increasing the size of coils and the number of turns boosts inductive link efficiency [
33
]. When the
transmitting coil and the receiving coil have the same size, the maximum coupling is achievable.
Although, in practice, the implanted coil is significantly smaller than the transmitting coil [
70
]. Mainly,
the inductive-based WPT system is used for medical devices such as brain and spinal cord stimulators.
Lyu et al. [
8
] have developed a stimulator that occupies an area of 5 mm
×
7.5 mm and operates
at the resonant frequency of 198 MHz while having a 14 cm distance from the transmitter, which
is located outside of the body. The stimulator gets the energy that has already been stored by a
switched capacitor and releases the energy as an output stimulus once the voltage reaches a threshold.
The control unit utilizes positive feedback to trigger the circuit, so no stimulation control circuit block
is needed. An
in vivo
experiment was performed to demonstrate the performance of the stimulator.
Two electromyography (EMG) recording electrodes were implanted into the gastrocnemius muscle of
a rat while the ground electrode was attached to the skin.
A free-floating neural implant, which is insensitive to the location, is provided as an inductive
link in [
10
] for wireless energy transmission. The authors have created prototypes of floating implants
for precise measurements. The system works with a power transfer efficiency of 2.4% at 60 MHz and
provides 1.3 mW power to the implant 14–18 mm away from the transmitter. Their coil link is stable
against the lateral and angular misalignments of the floating implants if the coils continue to have the
Signals 2020,1214
high-Q resonator. The extra heat produced by the resonator coil also does not exceed safety limits.
Recent works of the inductive WPT scheme are evaluated and presented in Table 2. The panel consists
of printed and 3D coils. Printed coils maintain acceptable performance under lateral malalignment
and are reliable for implants [4].
2.2. Microwave-Based Wireless Power Transfer
Another way to efficiently transmit power wirelessly over long distances in the order of meters to
kilometers is microwave power transmission. Figure 4illustrates the external and implanted antennas
0
behavior. It should be noted that up-link is defined when the implanted antenna acts as a transmitter
and the external antenna act as a receiver, whereas down-link is vice versa.
Signals 2020, 3 FOR PEER REVIEW 6
Recent works of the inductive WPT scheme are evaluated and presented in Table 2. The panel consists
of printed and 3D coils. Printed coils maintain acceptable performance under lateral malalignment
and are reliable for implants [4].
2.2. Microwave-Based Wireless Power Transfer
Another way to efficiently transmit power wirelessly over long distances in the order of meters
to kilometers is microwave power transmission. Figure 4 illustrates the external and implanted
antennas′ behavior. It should be noted that up-link is defined when the implanted antenna acts as a
transmitter and the external antenna act as a receiver, whereas down-link is vice versa.
Figure 4. Microwave wireless power transmission (WPT) principle.
Assuming far-field WPT, the budget power link as discussed in [71] can be described as follows:
(/)=
−
= + −+
−
−
−10+
−
(5)
where is the transmitted power in dBW, is the transmitting antenna gain in dBi, is the
path loss in dB, is the receiving antenna gain in dBi, is the noise power density in dB/Hz,
is the bit rate in kb/s, is the coding gain in dB, and is the fixing deterioration in dB.
The path loss can be calculated through the equation below, taking into consideration that the
free-space signal strength reduces with the increase in distance between the transmitter and receiver:
=20log
4
(6)
where d is the distance between the transmitter and the receiver and is the wavelength.
Considering the impedance mismatch losses,
Table 2. Existing inductive-based WPT approaches for implantable power applications.
Reference Year Frequency
Output
Power
(mW)
Efficiency
(%)
Active
Range
(mm)
Transmitter
Dimension
(mm)
Receiver
Dimension
(mm)
[72] 2020 915 MHz - 1.93 40–50 - 30 × 30
[73] 2020 5.8 GHz 0.01 1.2 × 10
1 - 0.116 × 0.116
[7] 2019 430 MHz 1000 - 45 - 4.5 × 3.6
[74] 2019 13.56 MHz 57–447 5.7–44.7 20–50 75 × 75 20 × 30
[34] 2019 434 MHz 31.62 0.68 10 20 × 20 1.6 × 1.6
[8] 2018 198 MHz 1000 - 140 d
= 30.5 d
= 4.9
[41] 2018
60,300,
330 MHz - 2.12, 3.88,
1.68 12 d
= 17.2, 24, 26 d
= 4
[75] 2018 2, 4 MHz 126 25 6 d
= 35 d
= 20
Figure 4. Microwave wireless power transmission (WPT) principle.
Assuming far-field WPT, the budget power link as discussed in [71] can be described as follows:
Link margin (dB/Hz)=C
NoLink −C
NoRequired
=Pta +Gtg −Lf+Gra −N0−Eb
No−10 log Br+Gc−Gd
(5)
where
Pta
. is the transmitted power in dBW,
Gtg
is the transmitting antenna gain in dBi,
Lf
is the path
loss in dB,
Gra
is the receiving antenna gain in dBi,
No
is the noise power density in dB/Hz,
Br
is the bit
rate in kb/s, Gcis the coding gain in dB, and Gdis the fixing deterioration in dB.
The path loss can be calculated through the equation below, taking into consideration that the
free-space signal strength reduces with the increase in distance between the transmitter and receiver:
Lf=20 log 4πd
λ!(6)
where dis the distance between the transmitter and the receiver and
λ
is the wavelength. Considering
the impedance mismatch losses,
Limpedance =−10 log1−r2(7)
where ris the appropriate reflection coefficient. Both
Lf
and
Limpedance
are considered for more accurate
evaluation. The received power by the receiver can be calculated as follows:
Pr=Pta +Gtg +Gra −Lf−Limpedance −ep(8)
Signals 2020,1215
where epis the polarization mismatch loss between the transmitter and the receiver. Equation (8) can
be also described as follows:
Pr=GtgGraλ2
(4πd)21−|S11|21−|S22 |2ep×Pt(9)
In practice, the received power value for microwave design can be extracted from the value of
|S21|2=Pr
Pt(10)
Table 2. Existing inductive-based WPT approaches for implantable power applications.
Reference Year Frequency
Output
Power
(mW)
Efficiency
(%)
Active
Range
(mm)
Transmitter
Dimension
(mm)
Receiver
Dimension
(mm)
[72] 2020 915 MHz - 1.93 40–50 - 30 ×30
[73] 2020 5.8 GHz 0.01 1.2 ×10−51 - 0.116 ×0.116
[7] 2019 430 MHz 1000 - 45 - 4.5 ×3.6
[74] 2019 13.56 MHz 57–447 5.7–44.7 20–50 75 ×75 20 ×30
[34] 2019 434 MHz 31.62 0.68 10 20 ×20 1.6 ×1.6
[8] 2018 198 MHz 1000 - 140 doutT =30.5 doutR =4.9
[41] 2018 60,300,
330 MHz -2.12, 3.88,
1.68 12 doutT =17.2,
24, 26 doutR =4
[75] 2018 2, 4 MHz 126 25 6 doutT =35 doutR =20
[9] 2018 1.3 GHz 3981 - 5 doutT =10 doutR =0.2
[35] 2018 39.86 MHz 115 47.2 - doutT =63.9 doutR =21.56
[76] 2018 432.5 MHz 1.05 13.9 10 - -
[77] 2018 430 MHz - - 60 30 ×30 10 ×10
[78] 2018 3 MHz 772.8 38.79 5–15 doutT =45.2 doutR =36.4
[11] 2017 13.56 MHz 18 7.7, 11.7 10 doutT ≈30 doutR =10
[40] 2016 50 MHz 0.0657 0.13 10 doutT =21 doutR =1
[19] 2014 8.1 MHz 29.8~93.3 47.6~65.4 12~20 doutT =30 doutR =20
[21] 2019 12.85 MHz - 75.8 - 30.0 ×29.6 30.0 ×29.6
[79] 2019 1–100 MHz - - 15 - doutR =1.75
dinR =0.50
[42] 2018 433 MHz 0.1, 1, 4, 10 0.87 600 - doutR =10
[29] 2017 13.56 MHz ≤100 - 5–15 doutT =25 doutR =16
[10] 2017 60 MHz 1.3 2.4 16 doutT =45 doutR =1.2
[37] 2016 20 MHz 2.2 1.4 10 doutT =20,28 doutR =1
[43] 2016 40 MHz - 2.56 70 doutT =100 doutR =18
[30] 2015 2 MHz 1450 27 80 doutT =140 doutR =65
[80] 2015 800 kHz 30 w 95 20 doutT =70 doutR =34
[81] 2012 742 kHz - 85 0–50 doutT =38 doutR =16.5
It should be noted that some amount of transmitted power will be dissipated in tissue due
to radiation and coupling into the body [
82
]. The implant placement depth plays a key role in
the amount of lossy power led by the body tissue. The present-day challenges for this technique
include the minimization of energy loss, protecting both humans and animals against exposure to
excessive microwave radiation, and the reconfiguring of a wireless transmission system resulting from
modifications such as a shifting in range between transmitter and receiver [
83
]. Microwave WPT
can transfer a high amount of power between the transmitter and the receiver circuits. However,
it is worth mentioning that human tissues cause problems for the propagation of electromagnetic
fields and dilute the electrical field. Therefore, the reflection caused by the lossy mediums reduces
the overall power transfer efficiency. Pacemaker implantation is a popular method of curing people
with cardiac insufficiency. However, the lifetime of the pacemaker is restricted to the lifespan of
the battery and the installation of a subcutaneous pocket [
13
]. Asif et al. [
13
] built a rectenna-based
leadless pacemaker prototype. For energy transmission to the implanted unit, a wearable transmitting
antenna range was fabricated to evaluate the system
0
s efficiency through Vivo electrocardiogram (ECG)
outcomes. The authors assert that the calculations of SAR are within the limits suggested by IEEE
and claim that the proposed leadless pacing method is safer, and eliminates the battery, lead, and
Signals 2020,1216
device pocket. Zada et al. [
84
] provided a miniaturized implantable antenna with three frequency
bands (902–928, 2400–2483.5, and 1824–1980 MHz) operating at the industrial, scientific, and medical
(ISM) band and at the midfield band. A capsule-shaped and a flat type antenna were fabricated with
a volume of 647
mm3
and 425.6
mm3
, respectively. This triple band antenna was complemented
with microelectronics, sensors, and batteries for stimulation in different applications. The system
was examined in different tissues, including the scalp, heart, colon, large intestine, and stomach.
Asif et al. [
85
] took advantage of a microwave-based WPT technique to charge deep medical implants
like cardiac pacemakers. Their novel wideband numerical model (WBNM) was to provide an RF
power source of a leadless pacemaker while using a metamaterial-based antenna operating at 2.4 GHz.
They used tissue simulating liquid (TSL) mimicking the human body to prove the performance of their
design for implantable applications. A wireless powering technique was introduced by Ho et al. [
82
],
which overcomes the difficulty of miniaturizing the power source via adaptive electromagnetic energy
transport. This method is designed for micro-implants like micro-electromechanical system sensors
and opto-elements. Figure 5shows the wireless electrostimulator inserted into the lower epicardium
of a rabbit. Recent works of microwave-based WPT systems are reviewed and shown in Table 3.
Signals 2020, 3 FOR PEER REVIEW 8
al. [85] took advantage of a microwave-based WPT technique to charge deep medical implants like
cardiac pacemakers. Their novel wideband numerical model (WBNM) was to provide an RF power
source of a leadless pacemaker while using a metamaterial-based antenna operating at 2.4 GHz. They
used tissue simulating liquid (TSL) mimicking the human body to prove the performance of their
design for implantable applications. A wireless powering technique was introduced by Ho et al. [82],
which overcomes the difficulty of miniaturizing the power source via adaptive electromagnetic
energy transport. This method is designed for micro-implants like micro-electromechanical system
sensors and opto-elements. Figure 5 shows the wireless electrostimulator inserted into the lower
epicardium of a rabbit. Recent works of microwave-based WPT systems are reviewed and shown in
Table 3.
Figure 5. Photograph of the electrostimulator inserted in the lower epicardium of a rabbit via open-
chest surgery [82].
Table 3. Existing microwave-based WPT approaches for implantable power applications.
Reference Year Frequency
Output
Power
(mW)
Efficiency
(%)
Active
Range
(mm)
Transmitte
r
Dimensions
(mm)
Receive
r
Dimensions
(mm)
[86] 2020 1.47 GHz 6.7 0.67 50 6 × 6 -
[87] 2020
0.403 GHz,
2.44 GHz - - 30–350 - 9.5 × 9.5
[88] 2019
1.64 GHz, 3.56
GHz - 32, 1.1 - 14 × 15 14 × 15
[13] 2019 954 MHz 10 65 110 - 10 × 12
[84] 2018
0.915, 1.9,
2.45 GHz 0.398 - 4.5 - 7 × 6
[38] 2018 400 MHz 19, 82 - 1, 3, 6,
12, 15 d = 18 1 × 1
[89] 2018 280 MHz 44 - 3 30 × 80 -
[90] 2017 2.45 GHz
2280,
600,
240, 96
- 1000–
4000
-
d = 63.6
[91] 2014 2.4 GHz - 15–78 10–100 63 × 39 × 50 63 × 39 × 50
2.3. Ultrasonic-Based Wireless Power Transfer
The ultrasound imaging is a well-known tool for evaluating patients’ physiological and
pathological conditions. In the passive ultrasonic recorder, the backscattered echo is derived from the
reaction of biological tissue′s acoustic properties to sound waves. Additionally, the acoustic emission
can be used for supplying energy wirelessly in the active biological environment [45]. The ultrasonic-
based WPT system has a transmitter converting electrical energy to ultrasonic energy, and a receiver
converting back the ultrasonic energy to electrical energy.
The basic model of the implantable ultrasonic coupling WPT system is shown in Figure 6. The
transmitting transducer powered by the transmitting module sends the ultrasonic waves, and the
ultrasonic energy is transmitted to the receiving transducer through the human tissue. The receiving
transducer converts the collected ultrasonic energy into electrical power. Accordingly, power is
Figure 5.
Photograph of the electrostimulator inserted in the lower epicardium of a rabbit via open-
chest surgery [82].
Table 3. Existing microwave-based WPT approaches for implantable power applications.
Reference Year Frequency
Output
Power
(mW)
Efficiency
(%)
Active
Range
(mm)
Transmitter
Dimensions
(mm)
Receiver
Dimensions
(mm)
[86] 2020 1.47 GHz 6.7 0.67 50 6 ×6 -
[87] 2020 0.403 GHz,
2.44 GHz - - 30–350 - 9.5 ×9.5
[88] 2019 1.64 GHz,
3.56 GHz - 32, 1.1 - 14 ×15 14 ×15
[13] 2019 954 MHz 10 65 110 - 10 ×12
[84] 2018 0.915, 1.9,
2.45 GHz 0.398 - 4.5 - 7 ×6
[38] 2018 400 MHz 19, 82 - 1, 3, 6, 12, 15 doutT =18 1 ×1
[89] 2018 280 MHz 44 - 3 30 ×80 -
[90] 2017 2.45 GHz 2280, 600,
240, 96 - 1000–4000 - doutR =63.6
[91] 2014 2.4 GHz - 15–78 10–100 63 ×39 ×50 63 ×39 ×50
2.3. Ultrasonic-Based Wireless Power Transfer
The ultrasound imaging is a well-known tool for evaluating patients’ physiological and
pathological conditions. In the passive ultrasonic recorder, the backscattered echo is derived from
the reaction of biological tissue
0
s acoustic properties to sound waves. Additionally, the acoustic
emission can be used for supplying energy wirelessly in the active biological environment [
45
].
The ultrasonic-based WPT system has a transmitter converting electrical energy to ultrasonic energy,
and a receiver converting back the ultrasonic energy to electrical energy.
Signals 2020,1217
The basic model of the implantable ultrasonic coupling WPT system is shown in Figure 6.
The transmitting transducer powered by the transmitting module sends the ultrasonic waves, and the
ultrasonic energy is transmitted to the receiving transducer through the human tissue. The receiving
transducer converts the collected ultrasonic energy into electrical power. Accordingly, power is
delivered to the implantable device through the receiving power module. The receiving power
processing module mainly includes a voltage-stabilizing circuit and a rectifier circuit.
Signals 2020, 3 FOR PEER REVIEW 9
delivered to the implantable device through the receiving power module. The receiving power
processing module mainly includes a voltage-stabilizing circuit and a rectifier circuit.
Figure 6. The basic model of the implantable ultrasonic coupling WPT system.
A model of the ultrasonic principle of WPT in the transducer of the transmitter and receiver is
shown in Figure 7 [92]; the radiated sound power P is given by the below equation:
=
( 1
√
+)
(11)
where is the density of the medium, is the sound velocity of the medium, is the amplitude,
a is the sound source radius of the circular plane A, =
is the wavenumber of a sound field, is
the distance along the z-axis, and (
) is the first-order Bessel function.
Figure 7. The field model of the ultrasonic coupling wireless transmission system [92].
The ultrasonic-based WPT system is an effective method for medical applications such as cardiac
defibrillators and deep brain stimulators (DBSs) [93]. A mode of clinical therapy is a stimulation of
excitable tissue for different disorders, such as Parkinson’s disease, urinary incontinence, and heart
arrhythmia. The traditional stimulus techniques use percutaneous cables to transport electricity to
the electrodes. The classical techniques are dangerous because they can cause infection [12].
The ultrasound- or inductive-based WPT is an interesting solution for this application. The
advantage of ultrasound compared to magnetic resonance and induction coupling is that these
x
z
y
dS
A
B
h
r
d
θ
l
Q(r,
θ
)
Figure 6. The basic model of the implantable ultrasonic coupling WPT system.
A model of the ultrasonic principle of WPT in the transducer of the transmitter and receiver is
shown in Figure 7[92]; the radiated sound power Pis given by the below equation:
P=πρocou2
aa2Za
0
J1ka 1
√l2+d22
ldl (11)
where
ρo
is the density of the medium,
co
is the sound velocity of the medium,
ua
is the amplitude, ais
the sound source radius of the circular plane A,
k=ω
co
is the wavenumber of a sound field,
d
is the
distance along the z-axis, and J1ka 1
√l2+d2is the first-order Bessel function.
Signals 2020, 3 FOR PEER REVIEW 9
delivered to the implantable device through the receiving power module. The receiving power
processing module mainly includes a voltage-stabilizing circuit and a rectifier circuit.
Figure 6. The basic model of the implantable ultrasonic coupling WPT system.
A model of the ultrasonic principle of WPT in the transducer of the transmitter and receiver is
shown in Figure 7 [92]; the radiated sound power P is given by the below equation:
=
( 1
√
+)
(11)
where is the density of the medium, is the sound velocity of the medium, is the amplitude,
a is the sound source radius of the circular plane A, =
is the wavenumber of a sound field, is
the distance along the z-axis, and (
) is the first-order Bessel function.
Figure 7. The field model of the ultrasonic coupling wireless transmission system [92].
The ultrasonic-based WPT system is an effective method for medical applications such as cardiac
defibrillators and deep brain stimulators (DBSs) [93]. A mode of clinical therapy is a stimulation of
excitable tissue for different disorders, such as Parkinson’s disease, urinary incontinence, and heart
arrhythmia. The traditional stimulus techniques use percutaneous cables to transport electricity to
the electrodes. The classical techniques are dangerous because they can cause infection [12].
The ultrasound- or inductive-based WPT is an interesting solution for this application. The
advantage of ultrasound compared to magnetic resonance and induction coupling is that these
x
z
y
dS
A
B
h
r
d
θ
l
Q(r,
θ
)
Figure 7. The field model of the ultrasonic coupling wireless transmission system [92].
The ultrasonic-based WPT system is an effective method for medical applications such as cardiac
defibrillators and deep brain stimulators (DBSs) [
93
]. A mode of clinical therapy is a stimulation of
excitable tissue for different disorders, such as Parkinson’s disease, urinary incontinence, and heart
arrhythmia. The traditional stimulus techniques use percutaneous cables to transport electricity to the
electrodes. The classical techniques are dangerous because they can cause infection [12].
Signals 2020,1218
The ultrasound- or inductive-based WPT is an interesting solution for this application.
The advantage of ultrasound compared to magnetic resonance and induction coupling is that these
methods are restricted to a short transfer distance, misalignment issues may occur [
94
], and the
magnetic field intensity must be under specified limitations for the safety of the body exposure. In the
ultrasonic method, the operating frequency needs to be changed according to sound radiation and
pressure distribution to obtain the optimum energy transition situation [
93
]. In the range of frequencies
individuals hear, Kim et al. [
94
] have developed an implantable pressure-sensing system driven by
mechanical vibration. The pressure inductor has a planar coil with a center of ferrite in which their
distance differs from the involved stress. An implantable pressure sensor prototype was designed,
as shown in Figure 8, and examined
in vitro
and
in vivo
. The acoustic receiver is a piezoelectric
cantilever and charges a capacitor by converting sound vibration harmonics into electrical energy. The
stored electric charge is discharged across an LC tank with an inductor sensitive to pressure during the
period that the cantilever is not shaking.
Signals 2020, 3 FOR PEER REVIEW 10
methods are restricted to a short transfer distance, misalignment issues may occur [94], and the
magnetic field intensity must be under specified limitations for the safety of the body exposure. In
the ultrasonic method, the operating frequency needs to be changed according to sound radiation
and pressure distribution to obtain the optimum energy transition situation [93]. In the range of
frequencies individuals hear, Kim et al. [94] have developed an implantable pressure-sensing system
driven by mechanical vibration. The pressure inductor has a planar coil with a center of ferrite in
which their distance differs from the involved stress. An implantable pressure sensor prototype was
designed, as shown in Figure 8, and examined in vitro and in vivo. The acoustic receiver is a
piezoelectric cantilever and charges a capacitor by converting sound vibration harmonics into
electrical energy. The stored electric charge is discharged across an LC tank with an inductor sensitive
to pressure during the period that the cantilever is not shaking.
Figure 8. An implantable pressure-sensing system.
Song et al. [95] investigated omnidirectional ultrasonic powering for deep implantable microdevices.
When testing the omnidirectionality and outcome of the power transmission under the acoustic Food
and Drug Administration (FDA) regulations, the piezoelectric devices with distinct geometries were
examined. The receivers were able to produce power in a range of milliwatts with a matched load
located 200 mm away from them. The receivers had symmetric geometry of 2 × 2 × 2 mmand were
insensitive to misalignment. Recent works of ultrasonic-based WPT systems are shown in Table 4.
Table 4. Existing ultrasonic-based WPT approaches for implantable power applications.
Reference Year Frequency
Output
Power
(mW)
Efficiency
(%)
Active
Range
(mm)
Transmitte
r
Dimension
(mm)
Receive
r
Dimension
(mm)
[96] 2020 700 kHz - - 200 - d
= 10
[97] 2017 1 MHz 0.1 - 85 d
= 0.55 -
[98] 2017 1.8 MHz - 2.11 30 d
= 10.8, 15.9 d
= 1.1, 1.2
[16] 2016 1 MHz 0.184 - - - -
[99] 2016 1 MHz - 25 3–7 d
= 8 -
[100] 2015 280 kHz 2.6 18 18 d
= 20 d
= 20
[101] 2015 3.4 MHz 0.001 - 100 - -
[17] 2015 30 MHz 0.1 - <100 - d
= 0.7,1
[18] 2014 1 MHz 28 1.6 105 29.6 × 72 * 1 × 5 **
[102] 2013 1.07 MHz - 45 - - -
[103] 2011 1.2 MHz 100 50 - d
= 44 -
[46] 2011 2.3 MHz 0.3 - 30–400 d
= 8 -
[12] 2011 1 MHz 23 - 120 - d
= 8
[104] 2010 35 kHz 1.23 - 70 - d
= 7
Figure 8. An implantable pressure-sensing system.
Song et al. [
95
] investigated omnidirectional ultrasonic powering for deep implantable
microdevices. When testing the omnidirectionality and outcome of the power transmission under
the acoustic Food and Drug Administration (FDA) regulations, the piezoelectric devices with distinct
geometries were examined. The receivers were able to produce power in a range of milliwatts with a
matched load located 200 mm away from them. The receivers had symmetric geometry of 2
×
2
×
2
mm3
and were insensitive to misalignment. Recent works of ultrasonic-based WPT systems are shown
in Table 4.
Table 4. Existing ultrasonic-based WPT approaches for implantable power applications.
Reference Year Frequency
Output
Power
(mW)
Efficiency
(%)
Active
Range
(mm)
Transmitter
Dimension
(mm)
Receiver
Dimension
(mm)
[96] 2020 700 kHz - - 200 - doutR =10
[97] 2017 1 MHz 0.1 - 85 doutT =0.55 -
[98] 2017 1.8 MHz - 2.11 30 doutT =10.8,
15.9
doutR =1.1,
1.2
[16] 2016 1 MHz 0.184 - - - -
[99] 2016 1 MHz - 25 3–7 doutT =8 -
[100] 2015 280 kHz 2.6 18 18 doutT =20 doutR =20
[101] 2015 3.4 MHz 0.001 - 100 - -
[17] 2015 30 MHz 0.1 - <100 - doutR =0.7,1
Signals 2020,1219
Table 4. Cont.
Reference Year Frequency
Output
Power
(mW)
Efficiency
(%)
Active
Range
(mm)
Transmitter
Dimension
(mm)
Receiver
Dimension
(mm)
[18] 2014 1 MHz 28 1.6 105 29.6 ×72 * 1 ×5 **
[102] 2013 1.07 MHz - 45 - - -
[103] 2011 1.2 MHz 100 50 - doutT =44 -
[46] 2011 2.3 MHz ≈0.3 - 30–400 doutT =8 -
[12] 2011 1 MHz 23 - 120 - doutR =8
[104] 2010 35 kHz 1.23 - 70 - doutR =7
[105] 2010 673 kHz 1000 27 40 - -
[106] 2003 100 kHz 5400 36 40 - -
[25] 2002 1 MHz 2100 20 40 - -
[26] 2001 1 MHz - 20 30 - -
* Width and total length of 48 symmetric elements of the spherical transducer array. ** Active area of a single
element of the flat transducer array.
2.4. Hybrid Wireless Power Transfer
A hybrid wireless power transmission (HWPT) system is a combination of two common methods
working as a unit system. The inductive WPT system uses magnate fields to transfer power, whereas the
capacitive WPT system uses electric fields. The capacitive WPT approach has two advantages compared
with the inductive one. First, there is no eddy current loss and, second, it uses a lightweight and
low-cost coupler. However, the capacitive method is limited to small power transfer and short distance
because of the small coupling capacitor. When the transfer distance is in several hundred millimeters,
the coupling capacitor is usually in the picofarad range. The voltages across the coupling plates of the
coupler, which could be improved with double-sided transformers or various compensation topologies
(such as double-sided LC and Z-source), are usually hundreds of times the input voltages to enhance
the system power level. Considering that the high voltage stressed in the coils of the inductive unit
could be fully used as a driving voltage for the capacitive coupler, combining both systems can be
done as a hybrid system. Therefore, it is important to take advantage of the inductive and capacitive
hybrid system to achieve higher power for HWPT.
A hybrid method includes inductive power transfer and capacitive power transfer, as shown in
Figure 9.
Signals 2020, 3 FOR PEER REVIEW 11
[105] 2010 673 kHz 1000 27 40 - -
[106] 2003 100 kHz 5400 36 40 - -
[25] 2002 1 MHz 2100 20 40 - -
[26] 2001 1 MHz - 20 30 - -
*
Width and total length of 48 symmetric elements of the spherical transducer array. **
Active area of
a single element of the flat transducer array.
2.4. Hybrid Wireless Power Transfer
A hybrid wireless power transmission (HWPT) system is a combination of two common
methods working as a unit system. The inductive WPT system uses magnate fields to transfer power,
whereas the capacitive WPT system uses electric fields. The capacitive WPT approach has two
advantages compared with the inductive one. First, there is no eddy current loss and, second, it uses
a lightweight and low-cost coupler. However, the capacitive method is limited to small power
transfer and short distance because of the small coupling capacitor. When the transfer distance is in
several hundred millimeters, the coupling capacitor is usually in the picofarad range. The voltages
across the coupling plates of the coupler, which could be improved with double-sided transformers
or various compensation topologies (such as double-sided LC and Z-source), are usually hundreds
of times the input voltages to enhance the system power level. Considering that the high voltage
stressed in the coils of the inductive unit could be fully used as a driving voltage for the capacitive
coupler, combining both systems can be done as a hybrid system. Therefore, it is important to take
advantage of the inductive and capacitive hybrid system to achieve higher power for HWPT.
A hybrid method includes inductive power transfer and capacitive power transfer, as shown in
Figure 9.
Figure 9. Drawing and diagram of hybrid wireless power transmission (HWPT).
As shown above, the inductive link will generate the alternating magnetic field through coils,
which provides the transmission medium. Furthermore, those currents will produce high voltages
on the transfer coil because of the self-inductance, whereas the capacitive link system requires high
voltages to produce the electric field for the capacitive coupler. Accordingly, the produced voltage
on coils of the inductive link system can be used for the capacitive system. A combined system to
construct a hybrid electric power transfer is established in [107]. and present the port voltages
as follows:
=
+
(12a)
=
+
(12b)
The efficiency of the inductive link is given by the same equation as Equation (4). Whereas the
power transfer from the capacitive link of the hybrid system is given by Equation (13):
=−(−) (13)
PrimaryCapacitor
PrimaryCoil
SecondaryCoil
SecondaryCapacitor
V1
V2
Vin
Vout
I
P
I
S
L
1P
L
1S
C
M1
C
M2
Figure 9. Drawing and diagram of hybrid wireless power transmission (HWPT).
As shown above, the inductive link will generate the alternating magnetic field through coils,
which provides the transmission medium. Furthermore, those currents will produce high voltages
on the transfer coil because of the self-inductance, whereas the capacitive link system requires high
voltages to produce the electric field for the capacitive coupler. Accordingly, the produced voltage
on coils of the inductive link system can be used for the capacitive system. A combined system to
Signals 2020,<