Wireless Power Transfer Approaches for Medical Implants: A Review

: 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, e ﬃ ciency, 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 e ﬃ ciency, the maximum power transfer e ﬃ ciency 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.


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 d F >> λ are satisfied, the region is considered far-field.
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.

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][8][9][10][11][12][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.

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][8][9][10][11][12][13].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][16][17][18][19][20][21][22][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, 1 211 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][35][36][37][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][45][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.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.

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.The amount of induced voltage in implanted coils ( ) is given by the following equation 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.The amount of induced voltage in implanted coils ( V is ) is given by the following equation where N is 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. L 1 is the transmitter coil inductor that is located outside the body tissues, and L 2 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 (R s1 , R s2 ), and (C s1 , C s2 ).Capacitors C T and C R are added to the circuit to form an LC resonance with L 1 and L 2 , respectively.R L is the load resistance.

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.The amount of induced voltage in implanted coils ( ) is given by the following equation 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.The highest efficiency and voltage gain are achieved when both LC tanks are tuned at the operating frequency of the link

Secondary Coil Implanted receiver inside body tissues
where C 1 and C 2 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).M is related to the coupling coefficient (k) according to The quality factors for the transmitter (Q 1 ), receiver (Q 2 ), and load (Q L ) circuits are calculated as follows: The total efficiency, η ind , is calculated according to Equation ( 4), as derived in [3]: 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, 1 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].

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.
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].

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.Assuming far-field WPT, the budget power link as discussed in [71] can be described as follows: 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: where d is the distance between the transmitter and the receiver and is the wavelength.Considering the impedance mismatch losses,  Assuming far-field WPT, the budget power link as discussed in [71] can be described as follows: where P ta . is the transmitted power in dBW, G tg is the transmitting antenna gain in dBi, L f is the path loss in dB, G ra is the receiving antenna gain in dBi, N o is the noise power density in dB/Hz, B r is the bit rate in kb/s, G c is the coding gain in dB, and G d 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: where d is the distance between the transmitter and the receiver and λ is the wavelength.Considering the impedance mismatch losses, where r is the appropriate reflection coefficient.Both L f and L impedance are considered for more accurate evaluation.The received power by the receiver can be calculated as follows: where e p is the polarization mismatch loss between the transmitter and the receiver.Equation ( 8) can be also described as follows: In practice, the received power value for microwave design can be extracted from the value of 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 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, 1 216 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 mm 3 and 425.6 mm 3 , 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 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.
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.

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 ultrasonicbased 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

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.

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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.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: 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.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 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: where ρ o is the density of the medium, c o is the sound velocity of the medium, u a is the amplitude, a is the sound source radius of the circular plane A, k = ω c o is the wavenumber of a sound field, d is the distance along the z-axis, and J 1 ka 1 √ l 2 +d 2 is 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.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: 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.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  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].

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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.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 mm and were insensitive to misalignment.Recent works of ultrasonic-based WPT systems are shown in Table 4.  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 mm 3 and were insensitive to misalignment.Recent works of ultrasonic-based WPT systems are shown in Table 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.

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.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: 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): 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].V 1 and V 2 present the port voltages as follows: V 1 = jωL 1P I P + jωMI S (12a) 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 P CH is given by Equation ( 13): where I P and I S are the RMS currents in primary and secondary coils.L 1P and L 1S represent the self-inductance of the transmitting and receiving side coils, respectively.M is the mutual inductance.
The advantage of a hybrid system is that it occupies less space than separate units.In this case, there is a capability of having different operating bands, more alternatives, and backup.In [108], the authors proposed a coupled WPT-Power Line Communication (PLC) system that consists of a two-coil resonator system.A four-port system has been designed with two ports dedicated to power transfer and the other two to data transfer.Capacitors have been used to tune the power channel to the desired frequency and improve the matching of the filtering stage to the coupled inductors.
A hybrid inductive-based and a microwave-based WPT system are also presented in [109].One of the current challenges for wireless transfer for small sensors is to minimize the system size.Haerinia et al. [109] decreased the size of the compact system, at the same time implementing multi-functionality.This goal was obtained by designing an antenna with 14 mm × 15 mm dimensions and having 20 mm × 20 mm dimensions for the hybrid system including the antenna and coil.The coils operating frequency was 510 MHz and the antennas worked at 2.48 GHz and 4.66 GHz.Meng et al. [110] developed a hybrid inductive-ultrasonic WPT link to power biomedical implants over bone, air, and tissue.They optimized cascaded inductive and ultrasonic links for WPT applications.The hybrid link was designed for an air-tissue medium to operate at 1.1 MHz with a power transmission efficiency of 0.16%.Recent works related to the HWPT system are shown in Table 5.The receiver dimension should be as small as possible to make it more convenient for implant applications.In case the receiver dimension is larger than 40 mm, such design can be printed on a flexible material substrate such as Kapton to facilitate the surgery procedures [111].It is worth mentioning that rotational/lateral misalignment and bending are two conditions that may happen because of changes in the implanted antenna location or the person's movement.Therefore, the bending of a flexible substrate and the misalignment effect must be investigated precisely [112].

Consideration for Design of Medical implants and Related Regulations
Developments in wireless technology for medical devices are elevating the provision of healthcare with lower expenses.Wireless telecommunications can be used for both wearable and implantable applications, such as DBSs, tracking of vital signs, measuring biological parameters, and cardiac rhythm control.The main advantage of wireless technology compared to landline networks is that the patient is not required to be linked to a certain location by cables [117].Despite advances in biomedical implants such as the pacemaker, cochlear implant, and nerve stimulator, these devices need to be improved in terms of miniaturization, the biocompatibility of materials, sources of electric charge, and wireless communication.To develop an effective IMD, the doctor, the patient, and the technician must collaborate in collecting coherent initial information about different aspects of the device.In particular, the user's satisfaction, the doctor's technical priorities, and the workability of the model are necessary to be considered in the design process [118].There are important factors for designing medical implants.Since an electric device is implanted inside the human body, the organisms around the device may react to it.To avoid such an issue, the device should be made up of or coated by biocompatible materials.Moreover, the medical implants should have appropriate packaging to isolate components of the device from body tissue.
Another factor is the structure of the design itself.Before the design, enough data should be collected from patients, engineers, and previous designs, along with their advantages and drawbacks [118].The United States medical devices market is regulated by three different organizations: the Federal Communications Commission (FCC), the Food and Drug Administration (FDA), and the Centers for Medicare and Medicaid Services (CMS).Wireless medical instruments can be classified into two categories: short-range, such as inductive implants and medical body area networks, and long-range, such as wireless medical telemetry (WMTS).According to the FCC, short-range technology sends data to local receivers and long-range technology sends user data to a remote spot [117].The FDA's mission is to check if the proposed medical devices guarantee the factors of safeness and effectiveness for patient usage.The FDA divides medical devices into three classifications based on the risk factor.Class I includes the lowest-risk devices, and without FDA prior authorization, medical devices in this class may be advertised.The medical devices using wireless technologies are usually considered in Class II.The highest-risk medical devices fall under Class III and clinical trials are mandatory to get FDA approval.The FCC and FDA must permit before wireless medical devices can be marketed in the United States.It is worth mentioning that the FDA and FCC have distinct criteria, and one agency s authorization does not simply ensure the other s consent [117].
The designers of medical implants are currently dealing with challenges in materials, output power, size miniaturization, the efficiency of the wireless link, and cybersecurity [118].There are different types of cyber-attacks, including theft of protected health information and execution of fraudulent device commands, which require appropriate cybersecurity mechanisms [119].It is crucial to have a broad perspective of different aspects of wireless techniques before choosing the tactic for any specific applications.Figure 10 shows a comparison between the collected research papers for different approaches: the maximum dimension of a receiver, power transfer efficiency, and frequency.
From Figure 10a,b, it can be seen that most of the ultrasound-based WPT systems work at a low operating frequency, ranging from a few hundred kHz up to few MHz.Most likely, inductive-based WPT systems mostly operate at a low operating frequency but range at a higher frequency compared to ultrasound-based and reach up to a few hundred MHz.Moreover, it can be concluded that operating frequency for most of the research based on inductive WPT lay below 50 MHz.Furthermore, 13.56 MHz is the interest frequency for the majority of the inductive WPT research field.On the contrary, most of the collected papers' research related to microwave-based WPT located at a higher operating frequency; compared to both ultrasound-based and inductive-based WPT, it ranged from hundreds of MHz to hundreds of GHz.On the other hand, it was noticed that the operating frequency of hybrid wireless power transmission (HWPT) varies from low to high frequency.This is due to the fact that HWPT comprises a combination of more than one method to transfer power wirelessly, such as inductive-based and capacitive-based.can be marketed in the United States.It is worth mentioning that the FDA and FCC have distinct criteria, and one agency′s authorization does not simply ensure the other′s consent [117].
The designers of medical implants are currently dealing with challenges in materials, output power, size miniaturization, the efficiency of the wireless link, and cybersecurity [118].There are different types of cyber-attacks, including theft of protected health information and execution of fraudulent device commands, which require appropriate cybersecurity mechanisms [119].It is crucial to have a broad perspective of different aspects of wireless techniques before choosing the tactic for any specific applications.Figure 10 shows a comparison between the collected research papers for different approaches: the maximum dimension of a receiver, power transfer efficiency, and frequency.From Figure 10a,b, it can be seen that most of the ultrasound-based WPT systems work at a low operating frequency, ranging from a few hundred kHz up to few MHz.Most likely, inductive-based WPT systems mostly operate at a low operating frequency but range at a higher frequency compared to ultrasound-based and reach up to a few hundred MHz.Moreover, it can be concluded that operating frequency for most of the research based on inductive WPT lay below 50 MHz.Furthermore, 13.56 MHz is the interest frequency for the majority of the inductive WPT research field.On the contrary, most of the collected papers' research related to microwave-based WPT located at a higher operating frequency; compared to both ultrasound-based and inductive-based WPT, it ranged from hundreds of MHz to hundreds of GHz.On the other hand, it was noticed that the operating Figure 10a presents the maximum dimensions of the receiver versus the operating frequency.In general, it can be noticed that most of the receivers' dimensions are less than 30 mm for all the WPT-based methods.This is due to the fact that miniaturization of the implanted device is highly recommended in the medical field to simplify the surgery procedures and to be more comfortable for the patients.Besides, it is shown in Figure 10a that the receiver dimensions of HWPT have a larger size compared to the other methods of WPT.Moreover, the receiver dimensions for most of the collected research on ultrasound-based and microwave-based WPT are below 20 mm.Additionally, a cluster of biomedical implant receivers using the inductive-based technique have a maximum dimension of less than 20 mm and operate at a considerably lower frequency compared to the microwave-based technique with almost the same size.
Figure 10b presents the efficiency percentage versus the operating frequency.In general, it can be interpreted that the maximum power transmission efficiency is achieved via inductive-based WPT, whereas the minimum power transmission efficiency is achieved via HWPT; on the other hand, the maximum efficiencies achieved are 78% and 50% for microwave-based and ultrasound-based WPT, respectively.In the same way, we conclude from Figure 10b that the operating frequency is much higher for microwave-based WPT compared to inductive-based WPT, which has almost the same efficiency.

Conclusions
This research has evaluated and discussed a survey of the following popular methods for wirelessly transferring power into IMDs: (1) inductive-based WPT, (2) microwave-based WPT, (3) ultrasound-based WPT, and (4) hybrid wireless power transmission (HWPT).In this research, the power delivered in the reviewed works to medical implants varied from a few µW to 5.4 W, with distance ranges from 1 mm to 4 m and maximum efficiency of up to 95%.Based on collected papers' research, it was concluded that ultrasound-based and inductive-based WPT works at low operating frequency (less than 50 MHz), whereas the microwave-based WPT typically works at a higher frequency.On the other hand, the HWPT could be found at a low or high operating frequency, depending on the combination used.It can be seen that the receiver dimension was less than 30 mm for all the WPT-based methods.Furthermore, HWPT had a bigger receiver size.The maximum power transfer efficiency was conducted via inductive-based WPT.Based on collected papers, the value of achievable maximum efficiencies were 95%, 78%, 50%, and 17% for inductive-based, microwave-based, ultrasound-based, and HWPT, respectively.This paper provides a perspective on different WPT approaches for biomedical applications by investigating the significant works in this field.Additionally, some points for designing effective IMDs and related commercial rules and regulations are presented.

Figure 3 .
Figure 3. Circuit model of magnetic resonant inductive coupling.

Figure 3 .
Figure 3. Circuit model of magnetic resonant inductive coupling.

Figure 3 .
Figure 3. Circuit model of magnetic resonant inductive coupling.

Figure 5 .
Figure 5. Photograph of the electrostimulator inserted in the lower epicardium of a rabbit via openchest surgery [82].

Figure 5 .
Figure 5. Photograph of the electrostimulator inserted in the lower epicardium of a rabbit via openchest surgery [82].

Figure 6 .
Figure 6.The basic model of the implantable ultrasonic coupling WPT system.

Figure 7 .
Figure 7.The field model of the ultrasonic coupling wireless transmission system [92].

Figure 6 .
Figure 6.The basic model of the implantable ultrasonic coupling WPT system.

Figure 6 .
Figure 6.The basic model of the implantable ultrasonic coupling WPT system.

Figure 7 .
Figure 7.The field model of the ultrasonic coupling wireless transmission system [92].

Figure 7 .
Figure 7.The field model of the ultrasonic coupling wireless transmission system [92].

Figure 10 .
Figure 10.A comparison of different approaches.(a) Maximum dimension of receiver versus frequency; (b) Efficiency versus frequency.

Figure 10 .
Figure 10.A comparison of different approaches.(a) Maximum dimension of receiver versus frequency; (b) Efficiency versus frequency.

Table 1 .
Summary of different approaches in analyzing tissue electromagnetic properties.

Table 2 .
Existing inductive-based WPT approaches for implantable power applications.

Table 2 .
Existing inductive-based WPT approaches for implantable power applications.

Table 3 .
Existing microwave-based WPT approaches for implantable power applications.

Table 3 .
Existing microwave-based WPT approaches for implantable power applications.

Table 4 .
Existing ultrasonic-based WPT approaches for implantable power applications.

Table 4 .
Existing ultrasonic-based WPT approaches for implantable power applications.

Table 4 .
Cont.Width and total length of 48 symmetric elements of the spherical transducer array.** Active area of a single element of the flat transducer array. * Width and total length of 48 symmetric elements of the spherical transducer array.** Active area of a single element of the flat transducer array. *

Table 5 .
Existing HWPT approaches for implantable power applications.