ULTRASONIC WIRELESS POWER TRANSMISSION FOR MEDICAL IMPLANTABLE DEVICES

Ultrasonic wireless power transmission for implantable electronic devices Seminar 2015

JyothiEngg.College 1 EEE Dept.

CHAPTER I

INTRODUCTION

1.1 Back ground and Motivation

Advances in micromachining technologies and in low power electronics create

ever-increasing opportunities in personal healthcare devices. These opportunities include

new wearable and implantable medical systems for continuous and non-invasive

monitoring of physiological parameters of the human body (diagnostic devices) as well as

controlling and minimally-invasive treatment of its various diseases (therapeutic devices).

The application areas of diagnostic devices range from common measurements such

as ECG (electrocardiogram), blood pressure, sugar level, temperature, respiratory rate, and

oxygen saturation to others like gastrointestinal endoscopy, monitoring physical activity

and strain in orthopaedic devices, personal tracking, and fall detection. Unlike these,

therapeutic devices are designed to interfere with the human body by supporting or

restoring the functionality of diseased parts and organs (e.g. pacemakers, cardiac

defibrillators, drug and insulin pumps, neuro stimulators etc.) or replacing them (e.g.

cochlear and retinal implants, artificial hearts and joints, vascular grafts etc.).[2]

Generally, the in-vivo devices need the internal battery. The time that the internal

battery can operate is limited, the replacement of the battery is necessary. To replace the

battery, the surgical operation is demanded to the patient. It attends with the risk and the

physical pain of the patient. For overcoming of this problem, the wireless power

transmission technology is studied. In this study, the electric power is supplied to the in-

vivo device from outside of the human body by electromagnetic wave. However, the

human body is made of around 60% of water so, electromagnetic wave greatly attenuates in

the human body. And the influence of electromagnetic wave on the human body is feared.

Therefore, the wireless power transmission technology by ultrasonic is studied.

Ultrasonic is used in the observation of the baby in the womb; it is good at medical

viewpoint. And there is no influence of the electromagnetic radiation on biological

systems.[1]

Depending on the application, the requirements for implantable medical devices

vary in terms of the size, mode of operation (continuous or intermittent), lifetime, and, in



particular, energy supply. The latter is a key factor for most implantable systems. Table 1.1

illustrates some of the existing and emerging implantable medical devices along with their

power requirements.

Table 1.1: Power requirements for existing and emerging implantable medical devices

New implantable medical devices are used for monitoring biological parameters of

the human body, as well as devices aimed at improving human body treatment, reducing

discomfort and promoting better health and wellbeing. Biomedical implants have been

powered mainly by batteries so far. However, batteries frequently dominate the size and the

cost of the device and have to be replaced or recharged occasionally. As the devices get

smaller these facts make them less attractive as the primary power source. Therefore

alternative techniques of energizing implantable microdevices are needed. Energy

harvesting is an attractive alternative to batteries in low-power biomedical implants and has

received increasing research interest in recent years. Ambient motion is one of the main

sources of energy for harvesting, and a wide range of motion powered energy harvesters

have been proposed or demonstrated, particularly at the microscale.

Another alternative to batteries is wireless power delivery, where a receiver, instead

of harvesting ambient energy, is energized wirelessly by sending a signal of a particular



frequency. One of the most widespread and well established methods of wireless power

delivery is via inductively coupled coils. However this tends to have low efficiency at

larger distances, and as the size of the receiving device decreases this becomes an ever

greater problem. For inductive powering the coil size to separation ratio is the key issue. Its

efficiency can be improved up to a point by increasing the operating frequency, but this is

limited by higher cost, higher tissue attenuation and increased radiation for a given coil

size.

To date power delivery by ultrasound has been used mainly in the fields of non-

destructive testing and remote sensing. Several groups reported systems for delivering

power through a steel wall. Acoustic waves, due to their lower speed, have much smaller

wavelengths than radio waves for a given frequency, which means that more directional

transmitters and receivers can be achieved at reasonable frequencies. For powering

embedded sensors, in the body or in structures, acceptable attenuation levels can also be

reached.[3]

1.2 Literature review

Ultrasound in the human body is primarily used for medical imaging. Typical

frequencies for this application are in the range 3–6 MHz, and with acoustic velocities in

human tissues from 1500–2000 m/s, this results in wavelengths of 0.3–0.7 mm. With

transducers of overall dimensions of at least a few millimeters, in either single element or

array form, this allows reasonably directional transmission. This makes ultrasonic power

delivery an attractive method of energizing implanted microdevices wirelessly and it

receives increased attention today.

The idea of using acoustic waves to transmit energy was proposed by Cochran et al.

as early as 1985 [4]. The system they built is an internal fixation plate that contains a

piezoelectric element generating current when excited mechanically by external ultrasound.

This current is then delivered to the electrodes at a bone fracture site in order to stimulate

healing or prevent nonunion. Using an ultrasonic transducer (with input voltage of 10–20 V

at a frequency of 2.25 MHz) the authors performed external excitation of piezoceramic

samples (of 5 × 5 × 0.9 mm size) deeply implanted in living soft tissues (near femur site of

beagles) [5]. It was shown that the system is able to generate direct (rectified) currents of

20 μA providing power output of approximately 1.5 mW/cm2.



Suzuki et al. reported a combined system for delivering power and data to an

implantable medical device [6]. The optimal operating frequency of 1 MHz was found for

piezo oscillators of 30 mm diameter, giving a maximum efficiency of 20%. During the

experiment the distance between source and receiver varied in the range 7–100 mm. Philips

et al.designed a peripheral nerve stimulator powered by externally applied ultrasound [7].

The piezoceramic test chips of 3.5 mm diameter were implanted inside a living tissue (near

sciatic trunk of large American Bullfrogs) and excited externally by a 6 mm transducer at a

frequency of 2.25 MHz. With the output power of up to 1.5 mW/cm2 the ultrasonic system

was capable of providing enough energy to the peripheral nerve micro stimulator.

Shih et al. presented a subcutaneously implantable device which receives energy

though externally applied ultrasound [8]. It has a bulk piezoelectric resonator packaged by

biocompatible cohesive gel (in cubic, spherical and irregular shapes) and incorporating an

acoustic antenna which can receive refracted waves propagating inside the package.

During the experiment the spherical package of 7.8 mm radius implanted inside the soft

tissue (streaky pork) at 0.5–8 cm separations was excited externally by an ultrasonic

transducer at frequencies in the range of 5–100 kHz. The maximum power transmission

efficiency of -40 dB (0.01%) was obtained at a frequency of 75 kHz and 2.5 cm separation.

Arra et al. built and tested an ultrasonic powering system with the potential to be used in

implantable applications [9]. Their acoustic link operates in degassed water at a frequency

of 840 kHz and the transducer diameters are about 25– 30 mm, giving efficiencies of 21–35

% at transmitter receiver distances between 5 mm and 105 mm.

Recently Ozeri et al. investigated an ultrasonic transcutaneous energy transfer for

wireless power delivery to implanted micro devices [10]. The authors proposed a system

consisting of two piezoelectric transducers of 15 mm diameter and 3 mm thickness with a

1.3 mm thick acoustic matching layer (made of graphite). Experimental measurements were

carried out for soft tissues (slices of pork of 5–35 mm thickness) immersed in a test water

tank. Operating at a frequency of 673 kHz the system demonstrated the overall power

transmission efficiency of 27% (at 5 mm separation). The authors analysed in detail such

important design considerations for maximum power transfer as selection of operating

frequency, acoustic impedance matching, circuit design for excitation of transducers, and

output power conditioning. They also built a finite element model in order to study the

pressure intensity profile generated by a transducer and define the preferred receiver

location in its different zones (near/far field and the focus).



In the past reports, the transducer is designed for the acoustic matching with water.

But it is not designed to have electrical matching with input source, amplifier circuit and

rectification circuit etc. and it causes the large loss. In our study, we propose the efficient

ultrasonic wireless power transmission system. For realizing the efficient system, first, the

impedance matching between the input source, amplifier circuit, and transducer is

considered. And, the optimum position of two transducers necessary for obtaining a high

transmission efficiency in water is searched. Moreover, to achieve the high DC output

voltage, the voltage booster circuit is designed.

In the transmission experiment of ultrasonic by proposed system, the availability of

our system is confirmed, and the high DC output voltage for the operation of the in-vivo

devices is obtained.



CHAPTER II

POWER SOURCES FOR IMPLANTABLE ELECTRONIC

DEVICES

2.1 Batteries

Nowadays, electrochemical batteries are the dominant source of energy for

implantable medical devices. They are commonly categorized as primary (single-use) and

secondary (rechargeable). Various battery technologies exist and more are emerging, but

the most commonly used are lithium-based. Primary lithium batteries are used in low-

power or intermittent applications. Due to their high energy density (up to 1.25 Wh/cm3),

safety and reliability these can power such implantable medical devices as cardiac

pacemakers for 10 years or more. In addition, they can also be used in drug delivery [9] and

bio-monitoring systems [2].

Secondary lithium batteries (e.g. Li-ion) have lower energy densities (up to 0.57

Wh/cm3) but are smaller and can be recharged. This makes them usable in high-power

medical devices with strict size limitations such as LVAD (left ventricular assist device),

TAH (total artificial heart) or hearing aids. In order to recharge the battery, these devices

have to employ additional mechanisms of on-board energy generation such as energy

harvesting or wireless power delivery that will be discussed in the next sections.

Despite significant progress in battery technologies in the last 40 years, their

miniaturization capabilities still lag behind those of current microelectronic and MEMS

devices. This means that the size and the cost of the implantable medical devices will likely

be dominated by batteries. However, one promising technology was developed

commercially as early as 1999, which is rechargeable thin lithium-ion battery cell with

hybrid polymer electrolyte, also called plastic Li-ion. Since then, several similar concepts

have emerged offering lightweight design, flexibility, and the footprint of less than 1 mm2.

In addition, these cells have low self-discharge and very long lifetime (>90000

charge/discharge cycles vs. 1000 in traditional Li-ion cells). However, their energy density

is still limited (>800µW.h/cm2 for a total thickness of 35-60µm) and has to be improved to

satisfy energy needs of therapeutic medical devices. Micro scale 3D battery structures have



great potential to accomplish this in the near future. As for now, depending on the

application, batteries still have to be replaced or recharged occasionally. This inevitably

reduces patient comfort and brings additional financial and clinical burdens. To overcome

these limitations, alternative technologies are being investigated such as energy harvesting

and wireless power delivery. These provide the opportunity of having battery-less operation

and essentially perpetual power.

As for now, depending on the application, batteries still have to be replaced or

recharged occasionally. This inevitably reduces patient comfort and brings additional

financial and clinical burdens. To overcome these limitations, alternative technologies are

being investigated such as energy harvesting and wireless power delivery. These provide

the opportunity of having battery-less operation and essentially perpetual power.

2.2 Energy harvesting

Energy harvesting (also called scavenging) is an attractive alternative to batteries in

medical implants and has received increasing research interest in the last ten years. It deals

with extracting electrical energy from different ambient sources such as solar radiation, air

flow, heat sources, RF fields, and various motion sources. Depending on these, different

conversion principles are utilized in energy harvesting generators, and they will be

discussed below in more detail. The application in implantable medical devices brings

additional limitations in terms of the size and the amount of energy available for harvesting.

It is important to note that systems powered by external dedicated sources such as

inductively coupled coils, ultrasonic transducers or RF/optical transmission are not

considered here as energy harvesters and will be described separately in the next paragraph.

Although the conversion principles in these systems are the same as in energy harvesters,

the way the power is generated (for the device specifically) contravenes to what is

considered “free" ambient energy.

2.2.1 Motion sources

There are several motion sources inside the human body that can be utilized to

harvest energy. These include expansion of the chest (breathing) and blood vessels

(heartbeat) as well as motion-related everyday activities (walking, gesturing) and active

sports. To harness this energy, all the generators have to employ vibration-to-electric

energy conversion.



Depending on the physical principle behind vibration-to-electric energy conversion

employed in motion-driven generators, these devices can be classified into electromagnetic,

electrostatic, and piezoelectric.

Electromagnetic generators utilize the principle based on Faraday's law of

induction: a change of magnetic flux penetrating the coil creates a voltage across it due to

the electromagnetic induction which then drives a current in the circuit. This flux change

can be induced by mechanical motion or rotation of the circuit relative to the coil.

Electrostatic generators employ capacitors with mechanically variable geometry

(plate position and separation). There are two different operating principles of these

devices: constant charge and constant voltage. In the former external mechanical forces

induce the voltage change across the capacitor, while in the latter the forces induce the

current through the capacitor. This type of generators requires initial precharging (priming)

for operation, which can be avoided by using electret materials.

Piezoelectric generators utilize the direct piezoelectric effect, which is the

capability of some materials to produce an electric field when subjected to mechanical

deformations.

2.2.2 Heat sources

To harness the energy from thermal gradients inside the human body,

thermoelectric generators are used. They are based on the Seebeck effect: when two

connected materials are subjected to a temperature difference, a voltage drop is generated

across them.

2.2.3 Biological sources

The abundant presence of oxygen and glucose within cells of nearly any biological

organism opens opportunities of creating a virtually unlimited energy source powered by

these natural reactants - a fuel cell. It generates electrical energy through catalytic

electrochemical reactions: a fuel (e.g. glucose) is converted at the anode, while an oxidant

is converted at the cathode (oxidation/reduction reaction). As a result, ions are generated at

electrodes and then conducted through an electrolyte membrane, creating free electrons and

hence the electrical current.

The main difference of fuel cells compared to traditional batteries is that the fuel is

not stored inside the cell, but provided externally. Depending on the type of the fuel, they

can be broadly categorized into two types: hydrocarbon (based on methanol, butane, iso-

octane etc. fuels) and glucose based (biofuel). Although liquid fuels in the former have



higher energy densities than in traditional battery materials , these cells cannot be used

inside the human body as they are not biocompatible. Instead, glucose based solutions are

promising in medical applications.

2.2.4 Solar radiation

Solar radiation is an abundant source of energy, and photovoltaic power generation

has been used successfully for several decades. With power densities up to 100 mW/cm2

(outdoors, direct sun illumination) and 100 µW/cm2 (indoors, office lighting) it can be a

viable solution to low-power medical applications. However, the limited availability of

light inside the human body restricts the application of solar powering mainly to wearable

devices. In terms of implantable operation, one could think of subcutaneous (not deep) or

intraocular locations, where these devices could still receive enough light or solar energy.

2.2.5 RF radiation

RF radiation is another abundant source of energy nowadays, and its harnessing

inside the human body would suffer less from the above mentioned problems. However, as

the microwave energy has adverse effects on biological organisms, its maximum levels are

legally regulated and limited. This brings power generation capabilities in a typical urban

environment (not in the vicinity of cellular base stations) to <1 µW/cm2.This results in a

very limited applicability of the RF radiation energy harvesting for driving implantable

medical devices, even with low-power biosensing functions.

2.3 Wireless power delivery

Another substitute for batteries is wireless power delivery, where a receiver instead

of harvesting ambient energy is driven by a dedicated remote source operating at one (or

several) specific frequencies. This normally allows having higher levels of generated

power, so that more energy-demanding (therapeutic) medical devices can benefit. Similar

to energy harvesting, wireless power delivery can be a convenient and safe way of

replacing (or supplementing) batteries, in particular for deeply implanted medical devices.

The most well developed method of wireless power delivery is via inductively coupled

coils. Ultrasonic energy transfer is less widespread, but it has received increasing research

interest nowadays.



2.3.1 Inductively coupled coils

Inductive coupling is the most widespread and well established method of wireless

power delivery to implantable medical devices. It has been successfully used for powering

medical devices such as cochlear implants, LVAD/TAH (left ventricular assist device /

total artificial heart), and neural stimulators. Due to relatively low (compared to radiative

transmission) operating frequency (below 20 MHz) and hence the lower adverse effects on

tissue (e.g. heating), inductive coupling provides a safe and efficient method of energy

transfer to devices not deeply implanted into the human body (up to 25 mm).

The basic principle is that the primary coil (windings) is attached to the skin and is

driven by sinusoidal current which creates alternating magnetic flux. This flux penetrates

the turns of the implanted secondary coil, creating a voltage across it due to

electromagnetic induction which is then rectified and provided to the load.

The power transfer efficiency depends on the coupling coefficient between coils

which varies in the range of 0-1 (0 for no coupling, 1 for the best coupling). This value is

strongly dependent on the source-receiver separation and dimensions as well as the material

properties of the propagation medium. For example, the estimated values for coupling

coefficient vary in the range of 0.08-0.24 for a retinal prosthesis and 0.11-0.32 for an

artificial heart. Higher system efficiency is achieved when both primary and secondary

circuits are tuned to the same resonant frequency (resonant coupling). However, this brings

a challenging task of coil geometry optimization and requires considerable design effort.

Inductively coupled coils can be implemented in air-core or ferrite-core

configurations. The former is lighter which makes it more favourable for implantable

medical applications. However, air-core coils have lower inductance and, therefore,

reduced transfer efficiency. This restricts their application to low-power devices only. For

more energy demanding systems (e.g. LVAD and TAH), ferrite cores are used instead.

Miura et al. developed a transcutaneous energy transfer system for a ventricular

assist device based on ferrite-core configuration. Tested in vitro, they delivered 23.5 W of

power (93.4% efficiency) to a secondary coil at 0 mm separation, reducing to 15.5 W

(79.4%) at 25 mm separation. The primary and secondary coils were 90 mm and 70 mm in

diameter respectively (Fig. 2.1). The authors claimed the same transfer efficiency for the in

vivo experiment (when implanted into a goat).



Figure 2.1:Inductively coupled energy transfer system for a ventricular assist device (a)

primary coil (90 mm in diameter); (b) secondary coil (70 mm in diameter)

Analysing the performance of this system, it becomes clear that with reducing size

of the receiving coil the level of delivered power drops significantly. This gets worse as the

distance between coils increases. Therefore, for inductively coupled system, the ratio of

coil size to source-receiver separation is the key issue. The efficiency can be improved up

to a point by increasing the operating frequency, but this is limited by tissue attenuation.

This limits its applicability in miniature medical devices (10 mm or less) deeply implanted

into the human body (25 mm or more).

2.3.2 Ultrasonic energy transfer

A promising alternative to inductively coupled wireless power delivery is via

ultrasound which has been used mainly in the fields of non-destructive testing, remote

sensing, and therapy. As the ultrasound readily propagates through structural materials such

as steel, it has received particular attention in aeronautics, aerospace, gas, and power

industry as a replacement for traditional wiring techniques. Acoustic waves, due to their

lower speed, have much smaller wavelengths than radio waves for a given frequency,

which means that more directional transmitters and receivers can be achieved at reasonable

sizes. For powering embedded sensors in the body or in structures, acceptable attenuation

levels can also be reached.

The basic principle of any ultrasonic energy transfer system is illustrated in Fig.

1.10. An electrical signal (e.g. a continuous sine wave) is applied to a transmitting

transducer (typically a PZT disk) which in turn generates acoustic waves (through reverse

piezoelectric effect). These then propagate through the medium (e.g. soft tissue, water,

steel, air or any other medium which can propagate compressional waves) toward a



receiving transducer (similar PZT disk). The latter performs inverse conversion: it

generates electrical energy from mechanical vibrations induced by incoming acoustic

waves. The receiver side contains all the electronic modules (e.g. AC/DC voltage

conversion) that are required to drive a load.

Fig. 2.2: Basic principle of an ultrasonic energy transfer system (T: transmitter, R: receiver)



CHAPTER III

ULTRASONIC WIRELESS POWER TRANSMISSION

SYSTEM

3.1 SYSTEM OVERVIEW

Figure 3 shows the ultrasonic wireless power transmission system. The input signal

is generated at the input source (signal generator), and the input signal is amplified by the

amplifier circuit. The amplified signal is inputted to the transmitting transducer. At the

transmitting transducer, the electric energy is converted to the vibrational energy, and it

transmits in water. The received power by the receiving transducer is converted the

vibrational energy into the electric energy, and it is rectified and boosted by the Cockcroft-

Walton circuit. Finally, the output DC power is obtained at the load resistance.

Table I shows the specifications of two transducers used in the system. In this case,

the transmitting transducer has the focus point, and the receiving transducer is flat-

transducer, it has no focus point. The resonance frequencies are 3.15MHz and 4.20MHz,

respectively. The transmitted ultrasonic frequency is 4.20MHz according to the

transmitting transducer.

Fig. 3.1. Wireless power transmission system by ultrasonic



By circumstances, two transducers are mutually different (focus, beam diameter,

and resonance frequency etc.), and are for the medical applications. It seems that these

cause the decrease of the transmission efficiency.

The Japan Society of Ultrasonics in Medicine shows as follows: the ultrasonic

intensity for the biological safety of diagnostic ultrasonic is under 1W/cm2. According this

rule, the input power of the transmitting transducer is 229mW (10V peak-to-peak) in this

experiment.

Table 3.1 Specifications of transducers

3.2 FEATURES INTRODUCED IN THE SYSTEM

3.2.1 Matching of transducer

The transducers used in this study are designed for the acoustic matching with the

human body and water. However, they are not designed for the electrical matching with the

input source, amplifier circuit etc. This causes the large propagation loss of ultrasonic.

Therefore, it is necessary to match the input impedance of the transmitting transducer to the

output impedance of the amplifier circuit by using the matching circuit.

Fig. 3.2. Return loss of the transmitting transducer



Fig.3.3 Impedance of the transmitting transducer

Figure 2 shows the return loss of the transmitting transducer, and fig.3 shows the

input impedance corresponding to the frequency. By the matching circuit, the input

impedance of the transmitting transducer is matched to the output impedance of the

amplifier circuit, and it is confirmed that there is no reflection between the amplifier circuit

and the transmitting transducer.

Figure 4 shows the output voltage waveform of the receiving transducer. In this

result, the output terminal of the receiving transducer is open-ended terminal. From fig.4,

the voltage amplitude of the matched transducer is 2 times of no-matched transducer, and it

is confirmed the availability of the matching between the transmitting transducer and the

amplifier circuit.

3.2.2 Matching of distance between two transducers

In the past report, it is known that the ultrasonic transmission efficiency is strongly

dependent on the distance between two transducers. From this report, the characteristic of

the output AC voltage corresponding to the distance between two transducers should be

measured.

Figure 5 shows the measurement result of the output AC voltage at close-range, and

fig.6 shows the measurement result at long-range. In fig.5, the output voltage amplitude is

vibrated, and the vibration interval is 0.18mm. This interval is the same to the half-

wavelength at 4.2MHz. The standingwaves generated between two transducers is the cause



of this vibration. In fig.6, the output AC amplitude changes according to the distance

between two transducers, and the distance that output voltage is maximum is about 70mm.

70mm is equal to the distance that the beam diameter of transmitting transducer agrees with

the beam diameter of receiving transducer. At over 70mm, the ultrasonic beam diffuses,

and the received power by the receiving transducer decreases. Therefore, the output voltage

amplitude also decreases.

Fig. 3.4. Output voltage waveform of the receiving transducer

Fig. 3.5. Short distance property

Fig. 3.6. Large distance property



3.2.3 Wireless power transmission system with Voltage boost rectifier

Generally, for driving a cardiac pacemaker, 50μW input power and 2.8V DC source

voltage are necessary. In previous section, the output voltage of the receiving transducer is

AC, and it is confirmed that the achieved output voltage is lower than 2.8V. Therefore, the

rectifier circuit and the voltage booster circuit are necessary.

In this time, Cockcroft-Walton circuit is chosen as the circuit. Cockcroft-Walton

circuit is the circuit that rectifies and boosts the input voltage at the same time, and it is

used for CRT display etc. Figure 7 shows the designed Cockcroft- Walton circuit. By this

circuit, the input voltage is rectified, and output DC voltage becomes 4 times of the input

voltage amplitude.

Fig. 3.7. Cockcroft-Walton circuit

Fig. 3.8. Output voltage waveform in the Cock Croft-Walton circuit



Fig. 3.9. Output DC voltage vs. load resistance at 229mW input power

Figure 8 shows the measurement result of output DC voltage with the designed

Cockcroft-Walton circuit. In fig.8, dotted line shows the input AC voltage waveform at the

Cockcroft-Walton circuit (Vin), and solid line shows the output DC voltage. The obtained

output DC voltage is 9.1V, and 9.1V output DC voltage is enough to drive the cardiac

pacemaker. For investigation of the power transmission efficiency, the output DC power is

measured with the Cockcroft-Walton circuit and the load resistance. Figure 9 shows the

output DC power corresponding to the load resistance. (In this case, input AC power is

229mW.) From this result, it is confirmed that the power transmission efficiency is low.

This cause is that the conversion efficiency of the electric energy to the vibrational energy

is low. And, the input impedance of the Cockcroft-Walton circuit is very high, so, it is

impossible to match the output impedance of the receiving transducer to the input

impedance of the Cockcroft-Walton circuit. The mismatch raises the reflection of the

received power by receiving transducer.

However, the obtained output DC voltage is enough high, and the output power or

the output voltage satisfy the parameters for driving a cardiac pacemaker.



CHAPTER IV

ADVANTAGES AND LIMITATIONS

4.1 Advantages

1. Ultrasonic is good at medical viewpoint. And there is no influence of the

electromagnetic radiation on biological systems.

2. Ultrasonic waves, due to their lower speed, have much smaller wavelengths than

radio waves for a given frequency, which means that more directional transmitters

and receivers can be achieved at reasonable frequencies.

3. Direct wire connection significantly increases the risk of having infections that can

result in serious disabilities or even worse. A burden associated with the need of

penetrating the skin causes the patient discomfort. In addition, providing wired

energy to the sites deeply inside the human body and near vital organs (e.g. heart)

can affect the reliability of the implanted device and raise the safety issues. But here

we use wireless power transmission.

4. Improved energy density is obtained.

5. Ultrasonic wireless power delivery can be a convenient and safe way of replacing

(or supplementing) batteries, in particular for deeply implanted medical devices.

4.2 Limitations

1. Power transmission efficiency is low because here the transducers used are those of

imaging technologies.

2. Input impedance of the Cockcroft-Walton circuit is high- impossible to match the

output impedance of the receiving transducer to the input impedance of the

Cockcroft-Walton circuit. This mismatch raises the reflection of the received power

by receiving transducer.



CHAPTER V

CONCLUSION AND FUTURE SCOPE

Most suitable method for providing energy supply for medical electronic devices is

wireless energy transfer due to its benefits. The efficient method is considered to be

ultrasonic energy transfer.

In this work an efficient ultrasonic wireless power transmission system is proposed.

To realize the efficient system the impedance matching between the input source, amplifier

circuit, and transducer is considered. And, the optimum position of two transducers

necessary for obtaining high transmission efficiency is searched. Moreover, to achieve a

high DC output voltage, the voltage booster circuit is designed. In the transmission

experiment of ultrasonic by proposed system, the high DC output voltage necessary for the

operation of the in-vivo devices is obtained, and it is confirmed the availability of proposed

system.

However, the transducers used in the experiment are designed for the ultrasonic

imaging applications, and the high transmission efficiency of the electric power is not

achieved.

In the future works, we search the optimum transducer and the ultrasonic frequency

for wireless power transmission and we realize the more efficient ultrasonic wireless power

transmission system.



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ULTRASONIC WIRELESS POWER TRANSMISSION FOR MEDICAL IMPLANTABLE DEVICES

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