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Journal of Medical Devices - ASME

Numerical and Experimental Simulations of the Wireless Energy Transmission and

Harvesting by a Camera PillElizabeth ShumbayawondaCentre of Biomedical EngineeringUniversity of Surrey Guildford, Surrey GU2 7XH, UKe-mail: [email protected]

Ali A. SalifuDepartment of Mechanical Engineering SciencesUniversity of Surrey Guildford, Surrey GU2 7XH, UKe-mail: [email protected]

Constantina Lekakou1

Department of Mechanical Engineering SciencesUniversity of Surrey Guildford, Surrey GU2 7XH, UKe-mail: [email protected]

John P. CosmasDepartment of Electronic and Computer Engineering Brunel University Uxbridge UB8 3PH, UKe-mail: [email protected]

ABSTRACT

This paper investigates the energy transmitted to and harvested by a camera pill travelling along the

gastrointestinal tract. It focuses on the transmitted electromagnetic (EM) energy in the frequency range of

0.18 to 2450 MHz and compares it to the mechanical energy due to the motion of the pill and the force

exerted from the intestine in its peristalsis onto the pill, and the electrochemical energy due to the change

of pH along the path of the pill. A comprehensive multilayer EM power transmission model is constructed

and implemented in a numerical code, including power attenuation through each layer and multi-

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reflections at material interfaces. Computer simulations of EM power transmission through a multilayer

abdomen to a pill travelling in the intestine are presented for the human abdominal cavity as well as

phantom organs and phantom environments, coupled with corresponding experimental studies using

these phantom components and environments. Two types of phantom abdomen are investigated: a

ballistic gel and a multilayer duck breast. Phantom small intestine involves gelatin gel layers with

embedded phantom chyme. Due to limitations related to the energy safety limit of skin exposure and

energy losses in the transmission through the abdomen and intestines, inductive range frequencies are

recommended which may yield energy harvesting of 10-50 mWh during 8 hours of pill journey,

complemented by about 10 mWh of mechanical energy and 10 mWh of electrochemical energy

harvesting, in addition to about 330 mWh typically stored in the coin batteries of a camera pill.

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INTRODUCTION

A camera pill is the most common type of an electrofunctional pill which

combines the idea of “Fantastic Voyage”[1] with the great potential of disease diagnosis

and treatment in difficult-to-reach parts of the body, such as the inner folds of the

intestine. Intense efforts have been made to introduce this technology for the

gastrointestinal (GI) tract [2-4], which entails a pill journey that may last up to 12 hours.

There are currently several commercially available camera pills, for example the

PillCam® capsule by Given Imaging, the EndoCapsule® by Olympus, the CapsoCam® by

CapsoVision, the MiroCam® by IntroMedic, the OMOM® by Jianshan and the Norika® by

RF System [5-6]. These commercial pills are of a general cylindrical shape of 9-11 mm

diameter, 23-32 mm length and 2-4 g mass, and most have one video camera operating

at a frame rate of 2-6 fps [5-6]. An in-depth study of the specifications of each of the

above commercial camera pills reveals that their high-power consuming parts, operating

intermittently, include 4-6 LEDs operating at about 15 mW for each LED, the CMOS that

may consume 2 mW operating at 2 fps, and the transceiver that may operate at 15 mW

in data transmission mode. Current commercial capsules have coin-type silver oxide

batteries [7] of about 330 mWh in total that may last for about 8 hours, corresponding

to a time averaged power of 41 mW. The big challenge is to develop on-board energy

harvesting technology to prolong the battery life so that the camera pill conducts a

comprehensive investigation through the whole length of the GI tract. Continuously

harvested energy would also offer the opportunity to reduce the battery size or even

eliminate the whole battery or extend the pill functions. Different types of energies may

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be suitable for harvesting, some already in the environment such as electromagnetic

(EM) energy, others produced by the pill such as kinetic energy, while others may be

generated by the pill interactions with the walls and the fluid of the GI track.

Electromagnetic (EM) energy harvesting involves wireless energy in a wide range

of frequencies. At first thought, EM energy harvesting seems very attractive due to the

potential large amounts of EM energy in the atmosphere at megahertz level (radio,

GSM) and gigahertz level (Bluetooth, WiFi); however, the energy transmitted through

the human body should be constrained by the energy density safety limit, Elim (in mWh

cm-2) reaching the external body surface area, given by:

E lim=S lim t lim /60 (1)

where Slim and tlim are the power density and average time limit, respectively, depending

on the transmitted frequency as presented in Table 1 [8]. Table 1 shows that the

highest safety limit is for frequencies below 1.34 MHz.

Electromagnetic induction is used typically for long-term continuous

communication processes usually at the frequencies of 13.56 MHz or 28 MHz. It would

require a coil in the pill and an external energizing coil, which should maintain alignment

for maximum energy transfer; this has been tested successfully for distances up to 1 m

[9]. Pan et al [10] tested an inductive transmission system at 181 kHz to transmit 150

mW in the air for which they estimated a specific absorption rate (SAR) of less than

0.329 W kg-1 [11] that was lower than the basic short-term limit of 10 W kg-1 set by the

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International Commission on Non-Ionizing Radiation Protection (ICNIRP) or the

occupational exposure limit of 0.4 W kg-1 based on the whole-body average. However,

the long term environmental exposure of the general public has a whole-body average

limit of 0.08 W kg-1.

On the other hand, RF communication is much more effective with the Medical

Implant Communication System (MICS) band specified at 403-405 MHz and a power

limit of 25 W in the air, and the Industrial Scientific and Medical (ISM) bands at 433.92

MHz, 915 MHz, 2.4 GHz and 5.8 GHz [12, 13]. However, such frequencies may suffer

high absorption loss in their transmission through the different layers of the body to the

camera pill. At the same time, such RF frequencies are also used for the data

communication from the camera pill to the external receiver, hence, a comprehensive

analysis of the transmission losses would offer double benefits.

In past analyses of EM energy and power transmission, power reflection at

middle-layer interfaces is generally neglected [14] or homogeneous simulating liquids

have been used in experimental studies, although it has been recognized that standing-

wave effects due to impedance mismatch at interfaces increase the SAR level [15, 16].

The major aim of this study is to conduct a theoretical and experimental analysis

of the electromagnetic energy transmission losses though the abdominal cavity to an

electrofunctional pill. The theoretical research focuses on the construction of a

comprehensive numerical model implemented into a computer code that includes a

multilayer material and the transmission and reflection terms at all material interfaces.

Computer simulations of the wireless EM energy transmission through a multilayer

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abdominal cavity to the pill will be conducted across a broad spectrum of wave

frequencies and the harvested EM energy will be estimated. Furthermore, experimental

studies of power transmission through phantom organs and combined phantom organ

environments will be carried out to investigate the accuracy of their representation of

the abdominal cavity environment in EM power transmission applications. The results

of the RF energy harvesting will be compared to other possible types of energy

harvesting by a camera pill.

THEORETICAL MODELLING OF THE EM ENERGY TRANSMISSION AND HARVESTING

The analysis of the EM energy harvesting comprises the construction of a

numerical model of the EM energy transmission to the pill from an external EM

radiation point source. This is followed by computer simulations of EM energy

transmission through multilayers to the camera pill located in the small intestine, at

different depths, using literature data about the dimensions of the multilayers in the

human abdomen and their material properties. These simulations are then repeated to

assess phantom abdomens and up to four layers of phantom intestines where

experiments were carried out to determine the EM energy transmission properties of

these phantom abdomens and phantom intestine.

The theoretical model considers electromagnetic energy harvesting from an

external point source to a camera pill travelling in the GI tract. Fig. 1 illustrates a

sequence of six biomaterial layers for the radiated EM energy to reach the pill located in

the intestine.

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The total power, po, emitted from the point EM power source may suffer a first

loss as is transmitted through the air, resulting in a power density, P1,ent, entering the

skin layer at a distance, Rair, from the EM point source, given by the equation:

P1 , ent=poe

−2α air Rair

4 πRair2

(2)

where air is the attenuation constant of air. The power density, P1,ent, entering the skin is

specified by the safety constraints given in Table 1, depending on the harvested or

transmitted EM energy frequency, and equation (1).

In the EM energy transmission across the multiple layers, a general numerical

scheme has been established of energy balance at the interfaces between any layer i

and its neighboring layers i-1 and i+1, as well as energy attenuation in its transmission

through each layer i, as illustrated in Fig. 2. The energy balance at each layer interface i-

1/i considers that the total power density arriving at the exit of layer i-1, Pi-1,ex, is split

into reflected and transmitted power densities, Pi-1,ex rf and Pi-1,ex tf, respectively:

Pi−1 , ex=Pi−1 , ex r f+P i−1, ex t f (3)

The power density at the start of layer i, Pi,start, is equal to the sum of power density

transmitted from the i-1 layer and the attenuated reflected energy from the exit of layer

i:

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Pi , start=Pi−1 , ex t f+Pi , exr f e−2 αi Δri ( Ri +Δr i)2

R i2 (4)

where ri is the thickness of layer i. That Pi,start energy then is further attenuated in its

transmission to the exit of layer i, according to the equation:

Pi , ex=Pi , start e−2 αi Δri

R i2

( Ri +Δr i)2

(5)

Equations (3)-(5) are then applied to all layers in the EM energy transmission path,

comprising a system of highly interdependent equations. For this reason, a C++

computer algorithm was written with an iterative numerical scheme following the

Gauss-Seidel method for systems of algebraic equations.

For each layer i the attenuation constant, reflection and transmission coefficients

depend on the layer material or medium and the EM wave frequency, f. The attenuation

constant, i, for layer i is given by:

α i=2πf

c [ μr ,i εr , i'

2 (√1+( εr , i' '

εr , i' )

2

−1)]1/2

(6)

where c is the speed of light, r,i is the material relative magnetic permeability, and ε r ,i'

and ε r ,i' '

are the real and imaginary components of the material relative complex

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permittivity, respectively. The following relations have been used for the reflection and

transmission of the EM energy, respectively, at the interface between layers i-1 and i:

r f ,i−1/i=|ρi−1/ i2 |

t f , i−1/ i=1−|ρi−1/ i2 | (7)

The reflection coefficient, i-1/i, at the interface between layers i-1 and i can be

calculated using the following relations [17]:

ρi−1/i=ηi−ηi−1

ηi+ηi−1

ηi=( μ i

εi )1/2

ε i=εi'− j εi

' '

ε i'=ε∞ , i+

ε 1, i

1+( ff 1 , i )

2+

ε2, i

1+( ff 2 , i )

2

εi' '=

σ i

2 πf (8)

where i and i are the intrinsic impedance and conductivity of the material of layer i.

Using the first three equations of system (8) and also the assumption that for all

biological tissues, i = 1, the following relation has been derived for the reflection

coefficient i-1/i:

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ρi−1/i = A+ jB- 2(C+jD)√E-jFG

whereA=εi−1

¿ +εi−1¿ −εi

¿−εi¿

B=−(εi−1' −εi

' )( εi−1' ' +εi

' ' )+(εi−1' ' −εi

' ' )( εi−1' +εi

' )C+ jD=( εi−1

' −εi' )+ j( εi−1

' ' −εi' ' )

E− jF=(εi−1' εi

' −εi−1' ' εi

' ' )− j ( εi−1' εi

' '+εi−1' ' εi

' )

G=( εi−1' −εi

' )2+( εi−1' ' −εi

' ' )2(9)

EXPERIMENTAL DESCRIPTION: MATERIALS AND PROCEDURES

Experimental studies of EM power transmission through phantom abdomen

components and environments were conducted using a hp 8714C Spectrum Analyzer

(300 kHz – 3 GHz) for the RF frequencies of 0.3, 13.3, 28.3, 433.92, 915, 1800 and 2450

MHz. The phantom bio-components comprised: gelatin gel phantom intestines filled

with phantom chyme; phantom muscle from ballistic gel; duck breast multilayer part.

The phantom small intestines were flat gelatin layers of 7 mm thickness with

embedded phantom chyme. Collagen is a major material of the extracellular matrix of

the human intestine walls, especially in the muscular layer [18]. Gelatin has been

employed as low-cost collagen substitute in tissue engineering [19] and, hence, been

also adopted in the current investigations. Porcine gelatin type A [supplied by Sigma

Aldrich, UK] was used in this study. A 4 wt% gelatin solution in water was cast in a tray,

left in the fridge overnight, and crosslinked in a 2 %w/v glutaraldehyde solution for 2 hr

following the procedure in [20] to yield a gel case of about 30-35 mm thickness, which

was washed with deionized water. The case was filled with a layer of phantom chyme

representing the chyme of human intestines consisting of the semi-fluid mass of

digested food. The phantom chyme was made using crushed nuts soaked in lemon juice

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for 12 hr (to simulate the digestion process in the stomach) and then washed using

deionized water to pH = 7 (considered as a representative average pH value in the small

intestine [21]). Finally, the phantom chyme was covered with a top gel layer of 30-35

mm thickness, yielding composite layers of phantom intestine medium consisting of

crosslinked gelatin gel with embedded phantom chyme.

Two types of phantom abdomen were fabricated and tested. The first phantom

abdomen was based on ballistic gel as proposed by [22] and [23], using a 10 wt% gelatin

solution to prepare the ballistic gel, with starting material porcine gelatin type A

[supplied by Sigma Aldrich, UK]. A gel thickness of 23 mm thickness (excluding the

intestine part) was found the most suitable to match the predicted power losses of

power transmission through the human abdomen. The second phantom abdomen

comprised a duck breast part (supplied from a food supermarket) trimmed to a

thickness of 20 mm and consisting of visible muscle, fat and skin layers; two different

duck breast parts were tested, coming from different suppliers and food supermarkets.

Each phantom component was tested for the transmission of an inputted power

at different frequencies. Then combined abdomen environments were tested including

the phantom abdomen (ballistic gel or duck breast) and layers of phantom intestines as

presented in Fig. 3(a) and (b). A custom-made rig from Perspex® was used to house the

tested medium or environment in each test, consisting of a bottom and top Perspex®

plate of 2 mm each and Perspex® spacers to maintain homogeneous thickness for the

soft tested media and vertical orientation and alignment of the transmitting and

receiving probes, as presented in Fig. 3. Initial tests yielded the transmitted power losses

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experienced by each Perspex® plate (top and bottom) at each tested frequency. Hence,

these transmission losses due to the Perspex® housing were excluded from the

calculated experimental results of the transmitted power through the phantom media

to be presented in the experimental results section.

NUMERICAL PREDICTIONS AND EXPERIMENTAL RESULTS

Computer Simulations of EM Energy Transmission through Multilayers to the Camera

Pill in the Small Intestine

First of all, data was collected from the literature [15, 24-25] about the

dimensions and material properties of each layer of human body in the abdomen and

abdominal cavity in the EM energy transmission line from an external point source to a

camera pill travelling in the small intestine, as presented in Fig. 1. This data and the

attenuation constant and reflection coefficient of each layer, calculated according to

relations (6) and (9) respectively, are presented in Table 2.

Computer simulations of the EM power transmission were carried out using our

algorithm, which implemented an iterative solution of equations (3)-(5) using the Gauss-

Seidel method. A power source of Pstart = 1 mW cm-2 of different frequencies in the range

of 0.181-5800 MHz (as reported in Table 2) was used at the external skin surface. Power

transmission to the camera pill was predicted for pill locations at different depths from

the skin surface, i.e. at the end of the abdomen, and after 1, 2, 3, 4 and 5 layers of small

intestine. The results are presented in Fig. 4. The abdomen consisted of five layers

comprising the skin, fat, muscle, peritoneum and omentum layers of thickness and

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properties as presented in Table 2. The internal reflections at layer interfaces increased

significantly the efficiency of power transmission through the abdomen despite the

relatively large thickness of the fat and omentum layers. As a result, power transmitted

through the abdomen experienced no net losses in the EM induction frequency range of

0.181-28 MHz, was reduced to about 20% and 10% at the RF frequencies of 433.92 and

915 MHz, respectively, and to 1% and 0.1% at the RF frequencies of 2450 and 5800 MHz,

respectively. Deeper locations of the pill further reduced the transmitted power by 10 to

100 times approximately as the depth increased from 1 to 5 layers of the small intestine.

The energy safety limit is another important factor that would ultimately affect

the possible amount of energy to be harvested by an electrofunctional pill. Table 1

presents the formulae for the power density limit as a function of frequency, classified

into different frequency ranges by FCC. These power density limits at different

frequencies have been calculated in Table 2 and apply for a safety time limit of 30 min.

Considering an average pill journey of 8 hr, a power density, Pstart = Slim/16, has been used

at the external skin surface, which varies as a function of frequency as displayed in Fig.

5. The computer simulations of the power transmission through the abdomen and at

different depths inside the abdomen were repeated with Pstart at the safety limit

according to the EM power frequency and the results are presented in Fig. 5. In this

case, it can be seen that at 0.181 MHz over 60 mWh and 8-12 mWh can be harvested by

the pill during 8 hr at a location just behind the abdomen and in the intestines,

respectively, which may add 0.2-1.2 hr of pill operation, considering that the current pill

batteries of 330 mWh in total last for 8 hr. Due to the imposed stricter power safety

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limit at frequencies above 1.34 MHz (Table 1), the energy that can be harvested at 13.5

and 28 MHz falls to 0.6 mWh and 0.015 mWh for 8 hr behind the abdomen,

respectively, and to 0.04-0.1 mWh and 0.007-0.02 mWh in the intestines, respectively,

despite the low transmission losses at these frequencies (as seen in Fig. 4). The

harvested energy becomes lower at 0.027 mWh for 8 hr just behind the abdomen (and

even less in deeper layers of the small intestine) at the RF frequencies of 433 MHz and

915 MHz and falls further to 0.006 mWh for 8 hr behind the abdomen at 2450 MHz.

Experimental and Simulation Studies of RF Power Transmission through Phantom

Abdomen Environments

The experimental results of the EM power transmission through a single layer

and multiple layers of phantom small intestine (made from gelatin with embedded

chyme) are presented in Fig. 6 for a probe transmitted power of 3.162 mW. Computer

simulations of this RF power transmission were carried out, starting with the single small

intestine layer to determine the physical parameters for this medium from the

numerical fit of the predictions with the experimental results. In this manner, the

attenuation constant of the phantom small intestine was fitted to the value of int = 1.7,

13, 20, 83, 90, 95 and 99 m-1 at the frequencies of 0.3, 13.5, 28, 433, 915, 1800 and 2450

MHz, respectively. These values then were used in the computer simulations of the EM

power transmission through multiple layers of phantom small intestine and the

predictions were in relatively good agreement with the corresponding experimental

results as shown in Fig. 6, given also the experimental error which reaches a maximum

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±14%. However, the attenuation constant of these phantom small intestines is higher

than that of the human intestines, as presented in Table 2, and changes less at high

frequencies than for the human intestines. Human intestines are composed mainly of

collagen and elastin composite gel and gelatin is a collagen derivative. However, it is

unclear what type of chyme, if any, is associated with the properties of the small

intestine included in Table 2, and different types of chyme could bring significant

differences in the electromagnetic properties of the composite material.

A ballistic gel mass of thickness of 23 mm was used to match best the total

transmitted power ratio (Pex/Pstart) of the real abdomen across the high frequencies (433-

2450 MHz) of the investigated frequency spectrum, where the largest transmission

losses occurred according to the predicted RF power transmission through the human

abdomen in Fig. 4. The experimental data for the ballistic gel phantom abdomen

presented in Fig. 6 seem to be in relative agreement with the predictions for the real

abdomen (Fig. 4) at the frequencies of 433-2450 MHz, within the experimental error of

±10%. However, they differ by about 30% at the low frequencies of 13 and 28 MHz,

with the ballistic gel phantom abdomen exhibiting higher transmission losses. Computer

simulations were used to fit the attenuation constant of this ballistic gel abdomen to bg

= 6, 8, 11, 39.4, 60, 86 and 100 m-1 at the frequencies of 0.3, 13.5, 28, 433.92, 915, 1800

and 2450 MHz, respectively, and the results of these predictions are also presented in

Fig. 6. Further computer simulations were conducted for RF transmission through

environments of ballistic gel abdomen and 1 or 2 layers of phantom small intestine, with

good agreement with the experimental data of the latter case after the numerical fit of

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the reflection coefficient from the ballistic gel to the phantom small intestine to r f,bg/int =

0.4 for all investigated frequencies in the range of 0.3 – 2450 MHz.

Fig. 7 presents the corresponding experimental data and predictions for a duck

breast phantom abdomen of a thickness of 20 mm. The duck breast is a multilayer

material incorporating visible layers of skin, fat and muscle and the experimental data

presented in Fig. 7 had a high experimental error of ±30%. The experimental data of the

total transmitted power ratio (Pex/Pstart) in Fig. 7 differed from the predictions of Pex/Pstart

through the human abdomen in Fig. 4 by +4% (higher in Fig.7) at 0.3 MHz, -48% at 13.5

MHz, -32% at 28 MHz, +244% at 433 MHz, -28% at 915 MHz and -21% at 2450 MHz.

Computer simulations were used to fit the attenuation constant of the duck breast to

the values of db = 3.4, 13, 15, 24, 63, 95 and 111 m-1 at the frequencies of 0.3, 13.5, 28,

433, 915, 1800 and 2450 MHz, respectively. EM power transmission was also measured

and simulated through environments of the duck breast phantom abdomen and 1, 2, 3

and 4 layers of phantom small intestine, with all results presented in Fig. 7. Generally

good agreement is observed between experimental results and predictions for the

combined duck breast-intestine phantom environments with the reflection coefficient

from the duck breast to the phantom small intestine fitted to rf,db/int = 0.4 in the range of

0.3-2450 MHz.

DISCUSSION

This discussion section will start with an analysis of the variability and

repeatability of the data of transmitted RF power through the human abdomen and

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abdominal cavity environment (predictions of numerical simulations) or the

environment of the phantom organs fabricated or investigated in this study

(experimental data and predictions). It will then proceed to the results of the RF energy

harvesting from this study and compare them to estimates of other forms of energy

harvesting for a camera pill.

The human abdomen and abdominal cavity is a complex environment and while

it is well represented by the layer diagram in Fig. 1, the thickness and thickness

distribution, composition and density of each layer vary enormously between people,

also depending on age and various conditions such as obesity and diabetes. The range

of waist circumference values may be used as an indication of the cumulative variability

and is considered to be between 60 and 120 cm [26]. A major factor for this variability is

the amount of visceral fat (also known as visceral adipose tissue) that is accumulated

between the organs in the abdominal cavity (in addition to the omentum layer in Fig. 1)

and in this case between the intestine length folds as is evidenced in MRI scans [27]. The

amount of visceral fat between the intestine folds varies enormously between people

and is not included in the human abdominal cavity environment model in this study as is

depicted in Fig.1. The intestine has a total length of 8±1.3 m, as reported by Hounnou et

al [28], and folds in its length so the power transmission to the pill might be through up

to 3 intestine parts in line for most of the length of the intestine [27]; up to 5 intestine

parts can be seen in an abdominal MRI image cross-section [27] from the skin to the

spine but are usually staggered and only in some parts of the abdominal cavity may be

found all in-line. Hence, the predicted power transmission and harvesting by a camera

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pill in the human intestine in Fig. 4 and Fig. 5 may be considered through the abdomen

and 1, 2, or 3 intestines. In fact, the pill is located inside an intestine part rather than at

its external wall, so the power is transmitted through the abdomen and 0.5, 1.5 or 2.5

intestines to reach the pill. Furthermore, it is unclear whether the properties of the

small intestine compiled from the literature [24] were measured for specimens of

intestine full with chyme (digested food) and from what type of food this chyme

originated. Given these uncertainties, on the assumption of the pill been a quarter of

the time in each part of the small intestine, i.e. after the abdomen or after the abdomen

and 1 or 2 or 3 intestines, and a starting power density at the outer skin surface equal to

the safety limit for the transmitted RF frequency, the following values of harvested

energy can be estimated after 8 hours of pill journey for different RF frequencies: 23,

0.21, 0.04, 0.01, 0.01, 0.002, 0.002 mWh for 0.18, 13.5, 28, 433, 915, 2450 and 5800

MHz, respectively.

With regards to the ballistic gel phantom abdomen, this is a single-type of

material with relatively low experimental error (±10%) in the transmitted power

measurements (Fig. 6) with regards to variability across the specimen surface (10

measurements) or between specimens (3 different specimens). The power losses

through the ballistic gel phantom abdomen are similar to those through the human

abdomen at the RF frequencies of 433-2450 MHz, whereas the ballistic gel experiences

lower transmitted power than the human abdomen at low frequencies, below 30 MHz

possibly due to the lack of reflected power at interfaces given that it has no different

layers.

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The duck breast phantom abdomen, on the other hand, is a complex material

system with visible skin, fat and muscle layers. As with the human abdomen and

abdominal cavity, the thickness and thickness distribution, composition and density of

each layer may vary across the same breast part and also between individual ducks: the

experimental data of the transmitted power through the duck breast in Fig. 7 exhibited

an experimental error of ±30% across the surface of the same breast part, due not only

to material layer variability but also to the shape of the original breast piece which

although it was compressed to a homogeneous thickness in the Perspex® housing, its

thicker part in the middle was compressed more and, hence, there was density

variation. The variability of the experimental data between two different duck breasts

from different suppliers rose to ±70%. However, the data of power transmitted through

the duck breast agreed with those through the human abdomen at the low frequency of

0.181 MHz where losses were low and the transmitted power was reinforced by

reflections at the interfaces between layers, given that both human abdomen and duck

breast phantom abdomen are multilayer materials.

It has been thought to compare the value of RF energy harvesting by a camera

pill to other energies possible to be harvested by a camera pill, including mechanical and

electrochemical energy.

Mechanical energy can be generated from the motion of the pill, the intestine

peristalsis and vibrations generated by the pill locomotion. The small intestine

undergoes peristalsis at a rate f = 11 contractions per minute to move chyme at an

average speed of 1 cm s-1 while the large intestine undergoes peristalsis at a rate f = 1-3

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contractions per minute to move chyme at an average speed of 0.083 cm s -1 [29].

Following the force analysis and external area function of a capsule pill proposed by

Woo et al [30], a force F = 1.22-1.42 N was estimated as exerted by the intestinal walls

on the pill [30] which is assumed for a collision duration t = 1 s while two collisions are

assumed per pulse. Assuming a pill velocity, v, equal to that of the interstitial fluid, an

energy conversion factor = 0.5 and a transducer conversion efficiency t = 0.5, the

following power may be harvested from the peristalsis of the small intestine:

p=2 fΔtFv ηηt=1 .1 mW (10)

Considering a total pill journey of 8 hours through the small intestine, the maximum

total harvested energy from intestinal peristalsis would be about 9 mWh which is 2.7%

of the 330 mWh usually stored by the coin batteries of the camera pill. Adding to this

energy the kinetic energy from the pill motion and pill rotation at each collision with the

intestine wall for a pill of a mass m = 4 g, radius of 5.5 mm and length of 23 mm, a

gyroscope efficiency factor of 0.5, and also = 0.5 and t = 0.5 as previously, a total

energy of 0.7x10-4 mWh was estimated as harvested from the kinetic energy of the

moving pill, which is negligible.

Energy may also be harvested by the change of pH as the pill travels through the

GI tract [31], which may bring a measurable voltage change in a pH sensor (about 33 mV

per unit of pH change [32]), which several types of commercial diagnostic pills actually

carry. Considering a charge current of 110 mA for charging the battery and an efficiency

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factor = 0.5, estimates of the present study for the mean power and harvested energy

are presented in Table 3 for the duration of the pill journey in different parts of the GI

tract with changing pH [33]. The total electrochemical energy harvesting due to the

change of pH in the GI tract is estimated to be 9.5 mWh which is about 2.9 % of the 330

mWh usually stored by the coin batteries of the camera pill.

CONCLUSIONS

A comprehensive multilayer EM power transmission model was developed and

implemented in this study, taking into account multi-reflections at material interfaces.

This model approach led to accurate efficiency predictions of wireless power

transmission through the abdominal cavity to an electrofunctional pill. The predicted

EM energy that can be harvested by the pill in the small intestine is below 1 mW cm -2 in

the EM frequency range above 10 MHz, due to the energy safety limit of skin exposure.

For low frequencies below 1.34 MHz the high allowed safe energy value of 100 mW cm -2

for 30 min results in transmitted and harvested EM energy of 10-50 mWh depending on

pill location during its journey of 8 hours in the GI tract in the abdominal cavity. Such

harvested energy could be added to a mechanical energy of about 10 mWh harvested

by the pill colliding with the intestine in its peristalsis and an energy of about 10 mWh

harvested by the pill from pH changes in the GI tract, reaching a total harvested energy

of 30-80 mWh, which may supplement the energy of 330 mWh stored in typical camera

pill batteries and prolong the active operation of a camera pill by 44 to 116 min.

21


Both a duck breast and a ballistic gel of 23 mm thickness may represent a human

multilayer abdominal wall in EM power transmission, excluding the intestinal part or

other internal organs. The ballistic gel is a single-type material with low variability and

high repeatability in terms of its RF transmission properties. It matches a standard

human abdomen (Table 2) well at the RF frequencies of 433-2450 MHz but experiences

lower overall transmitted power at frequencies below 30 MHz, most possibly due to the

fact that it is not a multilayer material and lacks reflected power at interfaces. For this

reason, the duck breast, being a multilayer material, matched the human abdomen at

low frequencies. On the other hand, there was substantial variability between different

specimens of duck breast originating from different duck sources, as in fact is also the

case with human specimens. Gelatin gel layers of 7 mm thickness with embedded

phantom chyme exhibited higher attenuation constant than a standard human small

intestine (Table 2) although it is not known whether the literature data for the human

intestine was for intestines containing chyme and what type of chyme.

In conclusion, the real value of a comprehensive numerical model, including

multi-reflections at material interfaces, has been demonstrated in both the predictions

of the computational simulations and determining appropriate experimental phantom

models representing the human abdomen environment at different RF frequencies.

22


NOMENCLATURE

CMOS Complementary metal-oxide semiconductor

EM Electromagnetic

GI Gastrointestinal

GSM Global System for Mobile Communications

ISM Industrial Scientific and Medical (band)

LED Light emitting diode

MICS Medical Implant Communication System

RF Radio frequency

SAR Specific absorption rate

c Speed of light

Elim Energy density safety limit

f Frequency

F Force

i-1, i, i+1 Layers of abdomen

m Mass

p Power

po Power emitted from a point EM power source

P1,ent Power density entering the skin layer

23


Pi,ex Power density arriving at the exit of layer i

Pex Power density at the exit of the tested phantom organ assembly

Pi,start Power density at the start of layer i

Pstart Power density at the start of the skin layer

rf Reflection fraction of power

rf,bg/int Power reflection fraction at the interface between ballistic gel and

intestine

rf,db/int Power reflection fraction at the interface between duck breast and

intestine

rf,i-1/i Power reflection fraction at the interface between layers i-1 and i

Rair Distance between the EM point source and the skin

Ri Distance between the EM point source and layer i

Slim Power density limit

tf Transmission fraction of power

tf,i-1/i Power transmission fraction at the interface between layers i-1 and i

tlim Average time limit

v Velocity

VpH Open circuit voltage generated by a pH difference

Greek

symbols

24


air Attenuation constant of air

bg Attenuation constant of ballistic gel

db Attenuation constant of duck breast

i Attenuation constant of layer i

int Attenuation constant of intestine

ri Thickness of layer i

t Time duration

ε r ,i' Real component of the material relative complex permittivity of layer i

ε r ,i' ' Imaginary component of the material relative complex permittivity of

layer i

Energy conversion factor

i Intrinsic impedance of the material of layer i

t Transducer conversion efficiency

r,i Material relative magnetic permeability of layer i

i Conductivity of the material of layer i

25


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[33] Zarate, N., Mohammed, S. D., O'Shaughnessy, E., Newell, M., Yazaki, E., Williams, N. S., Lunniss, P. J., Semler, J. R., and Scott, S. M., 2010, “Accurate localization of a fall in pH within the ileocecal region: validation using a dual-scintigraphic technique”, American J. Physiology - Gastrointestinal and Liver Physiology, 299(6), pp. G1276–G1286

28


Figure Captions List

Fig. 1 Electromagnetic (EM) energy transmission from an external point source

to an electrofunctional pill travelling in the intestine via the sequence of

multiple material layers of the abdominal cavity

Fig. 2 General EM energy transmission scheme across the general layer i and its

neighboring layers i-1 and i+1, indicating transmitted and reflected

power densities at layer interfaces, and attenuated power densities

across the layer i

Fig. 3 Phantom abdomen environments in a custom-made housing rig from

Perspex® used in the experimental studies of RF power transmission: (a)

ballistic gel phantom abdomen and phantom small intestines with

embedded chyme; (b) phantom abdomen of duck breast with skin and

phantom small intestines with embedded chyme.

Fig. 4 Computer simulation predictions of the harvested power from a power

source of 1 mW cm-2 at skin surface to the camera pill located at different

depths (at the end of the abdomen or in different intestine layers) as a

function of the radiation wave frequency

Fig. 5 Computer simulation predictions of the harvested power from a power

source of safety limit power density at skin surface to the camera pill

located at different depths (at the end of the abdomen or in different

intestine layers) as a function of the radiation wave frequency

Fig. 6 Results of transmitted power at different frequencies in the range of 433-

29


2450 MHz from corresponding experiments of wireless EM energy

transmission through different phantom environment components (1-4

phantom small intestines with embedded chyme; phantom abdomen

from ballistic gel) and phantom environments (ballistic gel phantom

abdomen and 1-2 phantom small intestines with embedded chyme). (All

data points include % error bars derived from the maximum error

between three repeat experiments using new materials). Lines of

predictions with the following parameter fitted values: phantom

intestines (ri = 7 mm): int = 1.7 m-1 (at 0.3 MHz), int = 13 m-1 (at 13.5

MHz), int = 20 m-1 (at 28 MHz), int = 83 m-1 (at 433 MHz), 90 m-1 (at 915

MHz), 95 m-1 (at 1800 MHz), 99 m-1 (at 2450 MHz); rf,int = 0; ballistic gel

phantom abdomen (ri = 23 mm): bg = 6 m-1 (at 0.3 MHz), bg = 8 m-1 (at

13.5 MHz), bg = 11 m-1 (at 28 MHz), bg = 39.4 m-1 (at 433 MHz), 60 m-1

(at 915 MHz), 86 m-1 (at 1800 MHz), 100 m-1 (at 2450 MHz); rf,bg = 0.40

(0.3-2450 MHz).

Fig. 7 Results of transmitted power at different frequencies in the range of 0.3-

2450 MHz from corresponding experiments of wireless EM energy

transmission through phantom abdomen from duck breast and phantom

environments (phantom abdomen from duck breast and 1-4 phantom

small intestines with embedded chyme). (All data points include % error

bars derived from the maximum error between 3 repeat experiments

using new materials and 10 different points in the area of each

30


specimen). Lines of predictions with the following parameter fitted

values: phantom intestines (ri = 7 mm): int = 1.7 m-1 (at 0.3 MHz), int =

13 m-1 (13.5 MHz), int = 20 m-1 (28 MHz), int = 83 m-1 (433 MHz), 90 m-1

(915 MHz), 95 m-1 (1800 MHz), 99 m-1 (2450 MHz); rf,int = 0; duck breast

phantom abdomen (ri = 20 mm): db = 3.4 m-1 (at 0.3 MHz), db = 13 m-1

(13.5 MHz), db = 15 m-1 (28 MHz), db = 24 m-1 (433 MHz), 63 m-1 (915

MHz), 95 m-1 (1800 MHz), 111 m-1 (2450 MHz); rf,db = 0.40 (0.3-2450

MHz).

31


Table Caption List

Table 1 FCC Electromagnetic Energy Safety Limits for Uncontrolled Exposure for

an Average Time Limit tlim of 30 minutes

Table 2 Data and Parameters Calculated for Each Layer in the Wireless Energy

Transmission Line to an Electrofunctional Pill from an External EM Energy

Source, for Different EM Wave Frequencies

Table 3 Parameters and Estimated Energy Harvested from the pH Change in

Different Regions of the GI Tract

32


Fig. 1. Electromagnetic (EM) energy transmission from an external point source to an

electrofunctional pill travelling in the intestine via the sequence of multiple material

layers of the abdominal cavity

33


Fig. 2. General EM energy transmission scheme across the general layer i and its

neighboring layers i-1 and i+1, indicating transmitted and reflected power densities at

layer interfaces, and attenuated power densities across the layer i

34


Fig. 3. Phantom abdomen environments in a custom-made housing rig from Perspex®

used in the experimental studies of RF power transmission: (a) ballistic gel phantom

abdomen and phantom small intestines with embedded chyme; (b) phantom abdomen

of duck breast with skin and phantom small intestines with embedded chyme.

35


Fig. 4. Computer simulation predictions of the harvested power from a power source of

1 mW cm-2 at skin surface to the camera pill located at different depths (at the end of

the abdomen or in different intestine layers) as a function of the radiation wave

frequency

36


Fig. 5. Computer simulation predictions of the harvested power from a power source of

safety limit power density at skin surface to the camera pill located at different depths

(at the end of the abdomen or in different intestine layers) as a function of the radiation

wave frequency

37


Fig. 6. Results of transmitted power at different frequencies in the range of 433-2450

MHz from corresponding experiments of wireless EM energy transmission through

different phantom environment components (1-4 phantom small intestines with

embedded chyme; phantom abdomen from ballistic gel) and phantom environments

(ballistic gel phantom abdomen and 1-2 phantom small intestines with embedded

chyme). (All data points include % error bars derived from the maximum error between

three repeat experiments using new materials). Lines of predictions with the following

parameter fitted values: phantom intestines (ri = 7 mm): int = 1.7 m-1 (at 0.3 MHz), int

= 13 m-1 (at 13.5 MHz), int = 20 m-1 (at 28 MHz), int = 83 m-1 (at 433 MHz), 90 m-1 (at 915

MHz), 95 m-1 (at 1800 MHz), 99 m-1 (at 2450 MHz); rf,int = 0; ballistic gel phantom

abdomen (ri = 23 mm): bg = 6 m-1 (at 0.3 MHz), bg = 8 m-1 (at 13.5 MHz), bg = 11 m-1

(at 28 MHz), bg = 39.4 m-1 (at 433 MHz), 60 m-1 (at 915 MHz), 86 m-1 (at 1800 MHz), 100

m-1 (at 2450 MHz); rf,bg = 0.40 (0.3-2450 MHz).

38


Fig. 7. Results of transmitted power at different frequencies in the range of 0.3-2450

MHz from corresponding experiments of wireless EM energy transmission through

phantom abdomen from duck breast and phantom environments (phantom abdomen

from duck breast and 1-4 phantom small intestines with embedded chyme). (All data

points include % error bars derived from the maximum error between 3 repeat

experiments using new materials and 10 different points in the area of each specimen).

Lines of predictions with the following parameter fitted values: phantom intestines (ri =

7 mm): int = 1.7 m-1 (at 0.3 MHz), int = 13 m-1 (13.5 MHz), int = 20 m-1 (28 MHz), int =

83 m-1 (433 MHz), 90 m-1 (915 MHz), 95 m-1 (1800 MHz), 99 m-1 (2450 MHz); rf,int = 0; duck

breast phantom abdomen (ri = 20 mm): db = 3.4 m-1 (at 0.3 MHz), db = 13 m-1 (13.5

MHz), db = 15 m-1 (28 MHz), db = 24 m-1 (433 MHz), 63 m-1 (915 MHz), 95 m-1 (1800

MHz), 111 m-1 (2450 MHz); rf,db = 0.40 (0.3-2450 MHz).

39


Table 1. FCC Electromagnetic Energy Safety Limits for Uncontrolled Exposure for an

Average Time Limit tlim of 30 minutes

Frequency Range (MHz)

Power Density* Safety Limit, Slim (mW cm-2)

(*Plane-wave equivalent)

0.3 – 1.34 1001.34 – 30. 180/f2

30 - 300 0.2300 - 1500 f/15001500 –1.0x105 1.0

40


41

TABLE 2. Data and Parameters Calculated for Each Layer in the Wireless Energy Transmission Line to an

Electrofunctional Pill from an External EM Energy Source, for Different EM Wave Frequencies

Dimension or Property of Layer i

Layer 1Skin

Layer 2Fat

Layer 3Muscle

Layer 4Peritoneum

Layer 5Omentum

Layer 6Small Intestine

ri (mm) 3.3 30 8.7 0.0001 15 70.181 MHzSlim (mW cm-2)ε r ,i

'

ε r ,i' '

i (m-1)

|ρi−1/i2 |

1001107.50391.0980.005

59.7992439.0440.131

|ρ skin / fat2 |=0.

6638.26737708.3270.477

|ρ fat /mus2 |=0.957

108.999198620.0031.195

|ρmus/ per2 |=0.

59.7992439.0440.131

|ρ per/om2 |=

0.969

11848.95461126.0840.602

|ρ fat /int2 |=0.783

13.56 MHzSlim (mW cm-2)ε r ,i

'

ε r ,i' '

i (m-1)

|ρi−1/i2 |

0.979285.253315.5292.379

11.82540.2361.103

|ρ skin / fat2 |=0.355

138.434832.7095.338

|ρ fat /mus2 |=

0.809

108.2632656.61310.149

|ρmus/ per2 |=0

11.82540.2361.103

|ρ per/om2 |=

0.784

362.6521836.9347.808

|ρ fat /int2 |=0.799

28 MHzSlim (mW cm-2)ε r ,i

'

ε r ,i' '

i (m-1)

|ρi−1/i2 |

0.230161.083213.7404.284

8.33921.2011.577

|ρ skin / fat2 |=0.403

94.466420.7377.615

|ρ fat /mus2 |=0.736

106.0631294.49714.331

|ρmus/ per2 |=0.

8.33921.2011.577

|ρ per/om2 |=

0.737

197.723950.38611.537

|ρ fat /int2 |=0.781

433.92 MHzSlim (mW cm-2)ε r ,i

'

ε r ,i' '

i (m-1)

|ρi−1/i2 |

0.28946.06129.09318.658

5.5661.7263.288

|ρ skin / fat2 |=0.467

56.86633.35019.354

|ρ fat /mus2 |=0.390

70.63193.62243.920

|ρmus/ per2 |

=0.533

5.5661.7263.288

|ρ per/om2 |=0.356

65.26479.59439.467

|ρ fat /int2 |=0.401


'

ε r ,i' '

i (m-1)

|ρi−1/i2 |

0.6141.32917.12525.031

5.4601.0104.126

|ρ skin / fat2 |=0.712

54.99718.62523.752

|ρ fat /mus2 |=0.584

68.60747.51652.251

|ρmus/ per2 |

=0.431

5.4601.0104.126

|ρ per/om2 |=0.520

59.38842.68250.277

|ρ fat /int2 |=0.370


'

ε r ,i' '

i (m-1)

|ρi−1/i2 |

1.038.00710.74244.302

5.2800.7678.345

|ρ skin / fat2 |=0.893

52.72912.75744.782

|ρ fat /mus2 |=0.715

66.24325.36978.646

|ρmus/ per2 |=0.

5.2800.7678.345

|ρ per/om2 |=0.728

54.42423.27879.292

|ρ fat /int2 |=0.932


'1.035.11411.520

4.9550.908

48.48515.377

60.46924.296

4.9550.908

48.67220.905


42

Table 3. Parameters and Estimated Energy Harvested from the pH Change in Different

Regions of the GI Tract

Location pH Time (min)

VpH

(mV)Power(mW)

Energy (mWh)

Stomach-to-duodenum

5 32 165 9 4.85

Duodenum-to-distal small intestine

0.5 113 16.5 0.9 1.70

Distal small intestine-to-cecum

1.5 39 49.5 2.7 1.76

Cecum-to-descending colon

1 39 33 1.8 1.17

TOTAL ENERGY (mWh)

9.47

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