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WIRELESS ENDOSCOPY

INTRODUCTION

The gastrointestinal diseases especially in the small bowel

are hard to detect because of its location. Traditionally,

long video endoscopic probes are used for the inspection of

the gastrointestinal (GI) tract. Although this technique

works well to obtain a detailed diagnosis, it is invasive and

may cause significant pain for patients. Therefore, wireless

capsule endoscopy has been developed to make standard

endoscopy less invasive and, in addition, to reach less

accessible areas of the GI tract (1).

Figure 1 shows a wireless capsule endoscopy system

for medical diagnostic applications. A miniature-sized

wireless endoscopy device reaches areas such as the

small intestine and delivers images and measurements

of physiological signals wirelessly to an external receiver

worn by the patient using one of the appropriate bands

such as industrial, scientific and medical (ISM), medical

implant communication service (MICS), and ultra wide-

band (UWB). The collected physiological signals and

image data are transferred to a computer for diagnosis,

review, and display of images. A high-resolution video-

based capsule endoscope produces a large amount of data

requiring a high-capacity wireless link. A wireless cap-

sule endoscopy system is designed such that the patient

will wear the external receiver (i.e., gateway) on the body

close to the capsule device (2). The gateway can be an

array of receivers placed around the human abdomen to

receive better signals from the capsule in the body. In

current commercial capsule systems, the images are

stored in the external receiver, and the patient returns

the external receiver to a physician who downloads video

images to a computer and uses special software to inspect

and detect abnormalities.

A remote receiver can be used to receive data from the

external receiver close to the patient’s body wirelessly.

This additional wireless link will provide the patient free-

dom of movement within a hospital room. With this sce-

nario, it is also possible for a health practitioner to view the

data online using the Internet independent of the patient

location (3).

Commercially available wireless capsules used for diag-

nosis of diseases within the GI tract are passive (4).

Actively controlled capsules are needed that can complete

the journey in a shorter time and be stopped or navigated

around a specific location to obtain a thorough diagnosis

and treatment. Some studies have proposed concepts for

magnetically propelled swimming robots (5). There are a

number of similar projects around the world developing

systems for control of capsule movements in the small and

large intestines. For example, magnetic forces are used in

Reference 6 to steer capsules in the GI tract to control and

navigate capsules. These approaches are expensive and

require many orientations of permanent magnets in the

external device as well as inside the capsule. Location

detection and position and movement control of capsules

are existing issues that have awaited solutions for more

than 50 years; in fact, these issues are described as “dream

features” in Reference 7.

Several commercial wireless capsules are already used

in clinical environments for visualization of abnormalities

in the GI tract. They offer almost similar design specifica-

tions. In addition to the camera-based wireless capsules,

these companies also manufacture capsule devices for pH

monitoring, temperature monitoring, and pressure mon-

itoring. The first camera-based capsule product was

the M2A capsule (8) manufactured by Given Imaging

(Yoqneam, Israel) at the time when significant advances

in integrated device technology enabled the design of

miniature cameras. The M2A capsule received approval

from the U.S. Food and Drug Administration (FDA) in

2001. Previously, wireless capsules measuring physiologi-

cal parameters such as pH, temperature, and pressure

existed since 1950 (2). The latest camera capsule PillCam

SB by Given Imaging (9) is now widely used around the

world in clinical settings for visualizing the small bowel to

detect and monitor many abnormalities, including bleed-

ing. With the help of camera-based capsules, the physi-

cians can detect, monitor, and diagnose abnormalities in

the entire small bowel without subjecting their patients to

an uncomfortable procedure or sedation.

The existing wireless capsules use narrowband wireless

links. To achieve a higher frame rate transmission of

images, they use some image compressing techniques. A

high-data-rate transmission will increase the resolution

and image quality of wireless capsules. Thus, a wideband

communication system such as UWB communication will

be an ideal wireless technology for future wireless capsule

endoscopy by achieving a data rate equal to or higher than

100Mbps. For such a high data-rate capacity, a wireless

capsule can transmit raw video data without any compres-

sion, resulting in a low-power transmitter, less delay in

real time, and increased picture resolution. With a high-

definition camera, such as 2 megapixels, UWB communi-

cation can send more than 30 frames per second (fps) (10,

11). At high frequencies, tissue absorbs significant signal

energy, thus affecting overall performance of a telemetry

link in a wireless endoscopy system. For high-frequency

designs, signals through different layers of tissue should

be optimized together with the wireless communication

device as shown in Figure 1b.

The existing wireless capsule devices use small batte-

ries to power the electronics in a capsule. New develop-

ments consider a wireless power mechanism to provide the

power supply or to recharge the battery of a wireless

endoscopy device (12). In addition, future capsules are

designed to be fully robotic with locomotion and motion-

control features (13). Robotic capsules are targeted for

drug-delivery applications in the human GI tract. The

navigable wireless capsule pill will carry the right amount

of drug and will go to the right location inside the body to

deliver the exact amount of the drug required (14). Real-

time wireless energy transfer via magnetic coupling is

necessary for these types of capsule endoscopes to provide

mechanical function, as they require a large amount of

power for continuous movement (15).

This article presents a detailed discussion of recent

studies onmotion control and localization that are targeted

J. Webster (ed.),Wiley Encyclopedia of Electrical and Electronics Engineering. Copyright# 2014 John Wiley & Sons, Inc.

features for greatermedical diagnosis, andwireless energy

transfer to increase the operating life of current wireless

capsules. These additional features will improve therapeu-

tic capabilities and enable a fully robotic wireless capsule

endoscopy system.

WIRELESS ENDOSCOPY DEVICES

A wireless capsule endoscope system consists of a tiny

camera, light-emitting diodes (LEDs), a wireless transmit-

ter, a battery, and an antenna. Recent research activities

in the development of wireless capsule devices include a

power management unit for receiving wireless energy

and regulation, and a motion control unit. Figure 2

shows the components required for a wireless capsule

device. It is important that integration of all these individ-

ual components will be low in complexity, small in form,

and light in weight in order to reach a very small section of

the GI tract.

History of Wireless Endoscopy Devices

The design of wireless capsules began in the 1950s. The

complete design of swallowable radio transmitters for use

in diagnosis of the organs within the digestive system first

appeared in the literature in 1957 by two different groups,

almost at the same time. These devices successfully oper-

ated in the GI tract and measurements such as pH, tem-

perature, and pressure were recorded wirelessly. Since

then, these devices have been called endoradiosondes,

capsules, smart pills, electronic pills, radio pills, wireless

capsules, wireless endoscopy, and so forth (16–18).

Because the integrated circuit (IC) technology was not

advanced, the early designs used simple electronic systems

with fewer components. The design by Mackay and Jacob-

son (16) used a circuit given in Figure 3a to transmit

pressure signals wirelessly using a core attached to a dia-

phragm moving inside the inductor. Temperature is meas-

ured through the transistor itself. The operation frequency

selected for this capsulewas 500KHz considering the better

signal penetration in the body and the effect of the skin

depth at low frequencies. This wireless capsule systemused

a several array of coils around the body to compensate the

changes resulted from the different orientations of the

passive capsule device. Mackay and Jacobson (16) also

described a wireless energy technique in their article by

using a large coil around the patient to induce energy into a

secondary winding at a frequency different than that of the

outgoing signal. However, a battery was used in actual

measurements because a better performance was obtained.

The device was shaped like a pill and measured 9mm in

diameter and 28mm in length (Figure 4a).

The capsule designed by Farrar and Zworykin (17, 18)

also contains an oscillator-based transmitter powered by a

battery having lifetime of 15h. It has a cylindrical pill

shape and measured 10mm in diameter and 30mm in

length. It uses a transistor oscillator circuit similar to

the one in Figure 3a. The components were packaged as

given in Figure 4b. The capsule had a flexible rubber at one

end that covers a pressure transducer. Similar to the

previous design, the pressure applied to the transducers

modulates the oscillation frequency of the transmitter

[known as frequency modulation (FM)], which is received

and demodulated by a frequency modulation receiver. The

Figure 1. (a) A wireless endoscopy system for medical monitoring. (b) Signal transmission through tissue layers inside the human body.

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Figure 2. A pill-sized wireless capsule and components.

2 Wireless Endoscopy

pressure signal was recorded using an oscilloscope. The

precise measurement of pressure activity in the intestine

was complicated for this design as the capsule would move

downstream. This indicates that position control was also

an important problem that time.

Figure 3b is a “echo capsule” that was designed in 1962

by a Japanese group (7) and was energized wirelessly. A

wireless energy link was investigated because miniature

batteries were not available in Japan at that time. The

circuit is aHartley oscillator in which a capacitorCc is used

as a storage element instead of a battery. The resonant

part of this circuit (L-C) receives a power signal from an

external coil, and the signal is rectified through the p-n-p

transistor. Cb is a blocking capacitor, and the electric

charge on this capacitor is discharged from base to collec-

tor. This circuit acts as a receiver to receive power and as a

transmitter to transmit information on various quantities

such as temperature, pressure, and pH. Temperature is

measured through the transistor, pressure is measured

through the movement of core inside the coil, and pH is

measured using the AgCl electrode. RD is a Zener diode

used for voltage regulation. The cylindrical pill-shaped

Figure 3. Circuits used in earlier capsules: (a) a pressure- and temperature-measuring capsule (16); (b) a pH, temperature- and pressure-measuring capsule (7); and (c) a pressure-measuring radio pill (19).

Figure 4. Structures and package styles of earlier capsules in 1950s and 1960s: (a) pressure-measuring capsule (16); (b) pressure-sensitiveradio transmitter (17); (c) a pH-, temperature- and pressure-measuring capsule (7); and (d) mechanical layout of pressure measuring radioin Reference 19.

Wireless Endoscopy 3

capsule had 8mm in diameter and 25mm in length

(Figure 4c).

Another FM oscillator-based design shown in Reference

19 measured pressure using the oscillator circuit shown in

Figure 3c. The design of inductor (L1) was done by winding

around a small ferrite core and placed parallel to a ferrite

disk. The inductance changes as the air gap between this

disk and the core changes, which in turn leads to a change

in the oscillation frequency. The carrier frequency is

selected between 300KHz and 500KHz. The mechanical

layout is shown in Figure 4d. The pill had a battery lifetime

of 80h for recording. The volume of the pill is approxi-

mately 2 cm3 (less than 1 cm� 3 cm).

Current wireless endoscopy devices integrate more

complex systems on the same platform when compared

with the systems in 1950s and 1960s (Figure 5). Recent

significant technology improvements have enabled the

design of small cameras and batteries. In the last 10 years,

research projects examining developing wireless capsules

have concentrated mostly on the visual sensor system. In

2000, First clinical trial of a camera-based wireless endos-

copy (8) has successfully been achieved.

An example of recent wireless capsule design is depicted

in Figure 5. In addition to image data, detection and

subsequent measurement of physiological signals such

as temperature, pH, and pressure, are usually necessary

to improve patient diagnosis, as listed in Table 1. The

image data are obtained from a complementary metal–

oxide–semiconductor (CMOS) camera and is in a digital

format. Physiological signals obtained from inside the

human body are initially analog and go through an ampli-

fication/filtering (A/F) process to increase the signal

strength and to remove the unwanted signals and noise.

A multiplexer is used to switch between each data. An

analog-to-digital conversion (ADC) stage in the microcon-

troller is used to convert the analog physiological signals

into a digital format for digital processing. The microcon-

troller will code the data before the data are sent to the

wireless transceiver. The multiaccess protocol to enable a

multiuser function is implemented in the microcontroller.

A multiuser function would be an important feature if

more than one patient is in the same care room in a

hospital. It contains a battery and power-management

circuitry. The power-management circuit is usually a volt-

age regulator chip used to distribute the power source to

the individual blocks.

Commercial Endoscopy Devices

Table 2 presents the details of commercially available

wireless capsule endoscopy technologies that are being

used in clinical environments. These devices are used

to provide diagnostic images for the esophagus, small

bowel, and the colon, and they can identify the source of

GI bleeding. For example, PillCam SB (Given Imaging,

Yoqneam, Israel), EndoCapsule (Olympus Corporation,

3500 Corporate Parkway, P.O. Box 610, Center Valley,

PA 18034–0610, U.S.A.), andMiroCam (IntroMedic, Seoul,

Korea) capsules are for small bowel. The current wireless

endoscope devices are efficiently being used to diagnose

disorders such as Crohn’s disease, Celiac disease, cancer-

ous tumors, iron-deficiency anemia (IDA), obscure GI

bleeding, and ulcerative colitis.

One state-of-the-art technology for wireless endoscope

devices is PillCamSB 3. PillCamSB 3 recently received the

FDA approval; it uses adaptive frame rate technology to

deliver more detailed images and coverage in the small

bowel (9). It has a physical dimensions of 11mm� 26mm

and weighs less than 4g, transmitting images at a rate

between two and six images per second.

Commonly usedwireless transmission frequencies have

been ultrahigh frequency (UHF) around 400MHz. Given

Imaging devices use the Zarlink’s radio frequency (RF)

chip (20) for wireless transmission in the MICS band

(402–405MHz). The allowable channel bandwidth (BW)

for this band is 300kHz. Because of the limited transmis-

sion bandwidth used for the commercial capsule devices,

the image transfer rate has been limited to 2–35 frames per

second. As high-definition cameras continue to be devel-

oped, they will become more attractive for use in wireless

capsule endoscopy devices. A higher pixel camera will

require higher image transfer rate. Thus, future wireless

capsule devices will target higher-bandwidth data trans-

mission that could facilitate a better diagnosis.

Table 1. List of Sensors Used Inside Wireless Endoscopy Devices

Devices Signal Types

Camera Camera images (data rates:

�10Mbps)

Temperature Physiological signal (30�C

to 40�C)

Oxygen Physiological signal

Pressure Physiological signal

pH value Physiological signal (1–13)

Figure 5. Sample design of recent wireless capsules.


Currently, all video-based commercial systems use LED

illumination. Sayaka, by RF System Lab (Traverse City,

MI), has bothwireless power transfer and localization capa-

bilities. This battery-free capsule contains three rotor coils

for posture control and fourLEDs for focusadjustment. This

capsule with posture and orientation control has the ability

to stay in a specific area of the intestine to obtain higher

quality images. Another endoscope, EndoCapsule, devel-

oped by Olympus Corporation (Tokyo, Japan), was mainly

used in Europe but in 2007 received marketing clearance

fromtheFDA.Thedevice contains sixLEDswithadjustable

illumination to maintain optimal imaging. The capsule by

SmartPill is designed to measure pressure, pH, and tem-

peratureas itpasses through theGI tract.A receivingdevice

worn by the patient collects data that are later examined by

a physician. Another commercially available capsule for

endoscopy isMiroCam. This system has a different wireless

transmission technique than other capsule technologies.

Instead of using RF signals to transmit images, MiroCam

uses natural electrical impulses in the human body as the

transport medium (21). Figure 6 presents the physical

shapes of commercial capsules.

Current video-basedwireless capsule developments will

continuously be improved to have better features. For

example, at the moment they have an average battery

life of 8 h and provide 2–35 frames/s transmission. A

high-capacity radio system is currently necessary for

this technology to examine visually the digestive tract

wirelessly with a better resolution. In most of capsules,

small batteries are used to supply the energy to the elec-

tronic boards. A wireless power mechanism either charg-

ing a battery externally or directly powering from an

external wireless source will be a significant design

improvement for wireless capsule endoscopy technology.

Wireless Power Link

A critical challenge for wireless capsule devices is the

limited energy source. Currently, the battery in wireless

capsules provides the energy to the active electronic com-

ponents in the device. Although miniature rechargeable

battery technologies are available, the lifetime they pro-

vide may not satisfy the desired operation time for detect-

ing and transmitting useful data. One way to enhance this

operation lifetime is to charge the battery wirelessly.

Inductive links have been used to power medical

implants to eliminate the use of batteries or to charge

batteries to extend the lifetime of the implants (22). Unlike

Table 2. Comparison Of Commercial Wireless Capsule Devicesa

Model Company

Camera

(Sensor)

Frequency

(MHz)

Data

Rate Power Source

Physical

Dimension

Image Rate

and

Resolution

Operation

Time

PillCam

(SB 1)

(SB 2)

(SB 3)

Given Imaging Micron, CMOS 402–405 &

433 ISM

(Zarlink)

800kbps

(FSK)

Battery 11mm�26mm,

<4 g

2–6 images/s 8 h

256� 256

pixels

50,000

images

PillCam

ESO 3

Given Imaging Two cameras,

CMOS,

256� 256

pixels

Battery 11mm�26mm,

3.7 g

35 images/s,

2,600

images

30min

PillCam

Colon

Given Imaging Two cameras,

CMOS,

256� 256

Battery 11.6�31.5mm,

2.9 g

4–35 images/s 10h

144,000

images

EndoCapsule Olympus

Optical

CCD camera,

1920� 1080

— — Battery 11mm�26mm,

3.8 g

2 images/s 8 h

Sayaka RF System Lab CCD Image

sensor

— — Wireless

power

9mm�22mm 30 images/s 8 h

(870,000

images)

MiroCam IntroMedic 320� 320

pixels

12MHz body

as trans.

Channel

(HBC)

— Battery 11mm� 24mm

3.3 gr

3 images/s 11þ h

(118,800

images)

OMOM

capsule

ChongQing

JinShan

Science &

Technology

640� 480 — — Rechargeable

battery

13mm� 27.9mm,

<6 g

2–15 frame/s 8h

SmartPill Smartpill Corp. Acidity (pH),

pressure,

and

temperature

— — Battery 13mm� 26mm Only sensor

discrete

data

—

aAll data within this table are obtained from the follownig websties: www.givenimaging.com, http://www.olympus-europa.com/endoscopy/,

www.rfsystemlab.com, and www.smartpillcorp.com.


http://www.givenimaging.com/

http://www.olympus-europa.com/endoscopy/

http://www.rfsystemlab.com

http://www.smartpillcorp.com

the short-range inductive implant systems where the wire-

less link has a distance of 1–2 cm (centimeters), wireless

capsule devices are placed deep inside the body with a

distance of up to 10 cm or 20 cm for some patients and thus

require a long range wireless transfer (2). The length and

width of commercially available wireless capsule devices is

usually 26mm� 11mm, which is barely smaller than the

average diameter of the adult upper esophagus. Wireless

power can substantially reduce the overall size and weight

because the need for batteries is eliminated.

As described previously, the study of wirelessly ener-

gizing wireless capsule devices started with the develop-

ment of the early capsule devices. The designs discussed in

References 7, 23, and 24 used an inductive link for wireless

power transfer. One or two large circle-shaped coils as

shown in Figure 7a are placed around the human body

Figure 6. Wireless video capsule endoscopes: (a) PillCam SB2 (Given Imaging), (b) MiroCam (IntroMedic, Seoul, Korea), (c) Endo Capsule(Olympus Corporation), and (d) OMOM (Jinshan Science and Technology, Chongqing, China).

Figure 7. Wireless power link using a class E driver for energy transfer in wireless capsule applications: (a) using a Helmotz coilconfiguration for wireless energy transmission; or (b) using an array of power transmitting coil for wireless energy. (c) Class E powertransmitter and receiver circuits.


to energize the capsule placed inside the body by an

external source. These earlier battery-less devices operate

based on passive telemetry. They use a resonant circuit

whose characteristic frequency is sensed from outside. It

operates exactly in the similar fashion with the radio-

frequency identification (RFID) technology. The capsules

with wireless power source are generally smaller in size

than a battery-powered capsule in addition to the

unlimited lifetime they provide.

Figure 7b shows another efficient and less complex

structure to energize wireless capsules inside the human

body.With the use of multiple arrays of coils, in which each

array transmits a power signal, the device inside the

human body will receive energy even if it moves in the

digestive system (25).

The autonomous robotics capsule device in (13) uses

wireless energy for power supply. It successfully provides

a wireless power source more than 400mW. The device’s

drivingsource isbasedonadirect current (dc)motorused for

the linear actuators of the microrobot. A very low (10KHz)

transmitting frequency is used for wireless power to reduce

humanbody absorption.Another study implements amulti-

coil technique for inductive powering of an endoscopic cap-

sule (12,15).ThesystemisshowninFigure8,whichconsists

of external two Helmholtz coils transmitting energy to a

9-mm three-dimensional (3D) coil power receiver (similar to

Figure 7a). Three orthogonal coils have been designed for

receiving energy to supply the power to the device. The

wireless power link is able to transmit a power around

300mW at 1MHz transmission frequency, which is suffi-

cient enough for use in locomotion-based systems.

A block diagram of an inductive link used for wireless

power transmission system is depicted in Figure 7c. The

power signal is sent from the primary site (i.e., external

unit) to the secondary site using two coils. A class-E

transmitter is preferred to achieve a highly efficient power

transfer (15, 22). The transistor device (i.e., T1 transistor)

acting as a switch is driven by a square wave at an

oscillation frequency fosc. The network’s resonant fre-

quency should be the power transmission frequency and

is given by equation 1. The capacitor C2 connected in series

with the transmitting coil (L2) is acting as a dc-blocking

capacitor, and its value should be small enough to be

nearly resonant with L2.

f osc � 1= 2pffiffiffiffiffiffiffiffiffiffiffi

L2C1

p

� �

¼ 1= 2pffiffiffiffiffiffiffiffiffiffiffi

L3C3

p

� �

(1)

It is clear from the battery-less systems that it is necessary to

bring the power transmitter very close to the skin. This way,

a wireless power system can transfer energy through the

10- to 20-cm thick skin to reach the device inside the body

(26).

As described previously, a wireless capsule device can

also be used as a method of drug-delivery systems. Such

capsules should be controlled mechanically to navigate the

device to the correct location. This will require a wireless

energy capability tomaintain sufficient energy in a capsule

device to achieve the required navigation and motion

control.

Wireless Communication Links

It is commonly known that propagation through the body

has higher losses at higher frequencies, and the character-

istics vary depending on characteristics of the antenna. It

is therefore important to optimize the through-body signal

propagation together with the antenna to obtain optimum

antenna radiation at the design frequency (27). Unlike the

wireless transmission in the air, the signal propagates

through a few layers of skin when a wireless capsule is

making its journey. Because of the different tissue layers,

the signal propagation will have different electrical prop-

erties in the intestine system with the dielectric constant

ranging from 40 to 70 and the conductivity in the range of

0.7 to 1.9S/m (28). The wireless capsule endoscopy com-

munication system should be optimized according to these

characteristics and should have following features:

� Compact and forming a pill shape

� Low power

� Wideband for high-quality image data transfer

A communication between the capsule and the external

unit should be formed using an appropriate frequency

band. Commonly used frequency bands are UHF-433

MHZ ISM, MICS, and UWB. International communica-

tions authorities allocate the 401–406MHz band with

300-kHz channel bandwidths for communication between

an implanted medical device and an external controller.

TheMICS band has been the most popular frequency band

choice and is used in Given Imaging’s wireless capsule

endoscopy devices. The narrowband-based transmitter

designs including 433 MHZ and MICS bands can support

only a few hundred kilobytes per second (2). It is thus

necessary to use higher frequencies with wider bandwidth

to achieve a similar video quality obtained with that of

wired endoscopy systems.

As high-definition cameras are continuously being

developed, they will be attractive for use in wireless

Figure 8. Nine-millimeter power-receiving coils and capsule pro-totype implementation. The capsule system uses a Helmholtz coilmethod and class E power transmitter for wireless energy trans-mission (12).


capsules. However, a higher pixel camera will require a

higher image transfer rate. As an example, if 1920� 1080

pixel (2 megapixels) charge-coupled device (CCD) sensors

to be integrated in capsule device, it will require a data rate

of 33.2Mbit/frame, considering 2 bytes are used per pixel.

Currently, such a high data rate transmission is not pos-

sible with any of the available frequency bands. If UHF

frequencies are used because of the restricted bandwidth,

then transmission of such a rate will result in a transmis-

sion time of 10 s per frame or more, which will result in

very small motion for a video streaming. Although com-

pression techniques could be used to some extent (29), it

reduces the image equality. Thus, a wireless capsule

endoscopy device should use a frequency band with larger

bandwidth for high-definition image transmission.

Wideband (Including UWB) Communication Link. Awire-

less endoscope is a biomedical system requiring a large

amount of data that will be delivered to outside the body to

achieve high-resolution pictures and images. For high-

data-rate and short-range applications, wideband commu-

nication (e.g., UWB) is an ideal physical layer solution by

achieving a data rate equal or higher than 100Mbps (30).

The UWB communication uses an operation frequency

higher than 3GHz (3.1–10.6GHz). A wideband wireless

capsule device can transmit raw video data without any

compressing, resulting in low-power, less delay in real

time, and increased picture resolution. With a high-defini-

tion camera such as 2 megapixels, UWB telemetry can

easily transmit up to 10 fps or more.

In addition to its high-capacity wireless link, a wideband

telemetry link can have some additional benefits because it

operates based on pulses rather than sinusoidal carrier

signal, enabling the low-power transmitter to increase the

battery life and decrease the interference effect on the other

wireless systems inmedical centers. A high-frequency com-

munication requires small electronic components such as

capacitors and inductors, which helps to design a compact

communication system. The radio signal propagation

through tissue layers at the GHz frequency presents signif-

icant signal loss, and thus designing UWB link for wireless

capsule is a challenging task. Recent developments have

shown that through careful arrangement of signal power

and optimizing channel model between the capsule and the

receiver, aUWBcommunication linkcanbeestablished that

can enable a high-capacity wireless link for high-quality

video data transmission (10, 30–33).

Ultra-wideband communication transmits data using

short pulses with 1nswidth or shorter. It is easier to obtain

a modulated transmit signal with pulses compared with

narrowband-based continuous signal transmission. Gen-

erating an ultra-wideband signal for data communication

is more energy efficient. There have been different meth-

ods of generating transmission pulses to obtain wideband

(34, 35). Figure 9 shows a very low-power circuit technique

to generate narrow pulses and hence a wideband spec-

trum. As illustrated in Figure 9, a reference oscillator

signal is digitized and its delayed replica is passed through

the Exclusive-OR (XOR) gate to obtain a narrow square

pulse (i.e., UWB pulse). After the signal is mixed with the

medical data, the modulated signal is then passed through

a band pass filter. The signal at the output of the filter is a

wideband signal that can carry a large amount of data

(Figure 9b).

In addition to its high-data-rate capability, UWB tech-

nology provides some desirable advantages for wireless

capsule technology including low-power transmitted

design, low radio frequency and electromagnetic interfer-

ence (EMI) effects in medical environment, and a size

antenna. Unlike the narrowband technologies, UWB can

support scalable data rates. For example, it can easily be

designed to transmit from 10Mbps to 100Mbps without

introducingmuch complexity in the hardware. In addition,

UWB frequency bands are less crowded; thus, it will

provide robust and reliable data communication from

the capsule to the external monitoring device.

Wireless capsule devices are located in the body with a

distance of up to 10 cm. Thus, accurate knowledge of the

propagation though inhomogeneous structure body is

required to optimize a UWB-based wireless communica-

tion link. The performance of the communication link is

affected by the dielectric properties of the human tissues,

x(t)

Delay

τ

a(t)

Band Pass

FilterXOR,

AND..etc

Inverter/

bufer

A

τ

Less than 1 ns

Narrow pulse

Digitised

signal

(a)

AntennaImage data

fosc.Tosc.

0

–50

–1000 2 4 6 8 10

(b)

dBm

Figure 9. Wideband signal-generation technique.


in addition to the distance between the transmitter

inside the capsule and the external receiver. Several stud-

ies in the literature have reportedUWB channel character-

istics that can be used to configure transmitter and the

receiver parameters such as transmit and receive power

levels and the sensitivity (32, 36). Figure 10 demonstrates

a simulation result for UWB signal propagation through

the different tissue layers of the digestive system (32).

Assuming the distance from theGI tract to the skin surface

is about 8–9 cm, a receiver can easily be placed around the

human belt or up to 10–15 cm from the wireless capsule

accommodating a signal loss from �80dB to �90dB.

Assuming a UWB transmit signal has a power level

between �10dBm and 0dBm, a receiver with a sensitivity

of �100 dBm or �90dBm is required to detect UWB

signals.

Several path loss studies for in body applications are

demonstrated in Figure 11. The loss is plotted against

its actual depth inside the body to a body surface device

(11, 37, 38). Characteristics for commonly used frequencies

are presented for a reference to the in-body UWB commu-

nication link. Table 3 gives average loss values for various

transmission frequencies within the human body. It is

clear that the loss varies from 40 to 90 dB when a wireless

link is used for wireless capsule applications, depending up

the frequency selected. Therefore, the designers should

consider accommodating this type of loss when designing a

wireless link for a wireless capsule device.

A simulation shown in Figure 12 was conducted in

Reference 32 to obtain the corresponding temperature

increase for different UWB signal power levels absorbed

by the human body. The temperature increase is obtained

after the steady state is achieved. The simulation considers

the variation of basal metabolic rate and blood perfusion.

The initial body temperature is considered to be 37�C. The

results in Figure 12a illustrate that the temperature of the

whole body has increased to 37.173�C from the initial body

temperature of 37�C. In short, the selected signal power

Figure 10. Path loss in different tissue regions with the distance from the antenna toward the front of the human body (32).

Figure 11. Implant to body surface path loss studies: (a) a path loss study for in-body UWB communication link conducted in (11) and (b) apath loss study for implant to body surface at MICS frequencies (403.5MHz) (37).


levels of UWB pulses are not high enough to cause a

significant temperature increase.

It is necessary to analyze the electromagnetic effects of

UWB signals. Specific absorption rate (SAR) and specific

absorption (SA) are two indicators measured to define the

safety limits of absorbed electromagnetic power by the

human body. The internationally recommended SAR level

regulated by the International Council on Non-Ionizing

Radiation Protection (ICNIRP) standard and the Institute

of Electrical and Electronics Engineers (IEEE), standard is

2W/kg, which is 10 g averaged in the frequency range of

10 kHz to 10GHz (39, 40). Usually the ICNIRP regulated

SAR level of 2W/kg determines the maximum allowable

delivered signal power to the antenna for UWB communi-

cation (32). The exposure of electromagnetic fields produc-

ing higher than 2W/kg can cause an increase in body

temperature that could affect the biological mechanism

of the human body. The SAR variations for the power levels

that are shown in Figure 12 meet the limit of 2W/kg

defined by the safety standards.

Antenna Design

The antenna is a crucial element in a wireless capsule

endoscopy device. The design method used for antennas in

wireless capsules is the optimization of an antenna with

respect to a range of tissue properties surrounding the

device (27) (as shown in Figure 1b). A device that travels in

the GI system comes into contact with various tissue types

and fluids, which have variable permittivity and conduc-

tivity. In addition, it is important to consider the physical

shape of a wireless capsule package when designing an

antenna. The antenna can be inserted on top of the elec-

tronics boards. Alternatively, the capsule shape can also be

divided into two regions where the antenna can be placed

into the upper half whereas the remaining electronic units

are packed into the lower half. Placing the antenna at one

side of the electronic units in the capsule is another possi-

bility. Recent antenna designs in the literature that use

the shape of a capsule efficiently are shown in Figure 13.

One of the earlier antennas designed for capsule endos-

copywas reported inReference 41,with dimensions of 8mm

diameter and 5.6mm height. The antenna was matched in

the band 410–442MHz inside a homogeneous tissue simu-

lating phantomwith a dielectric constant 56 and conductiv-

ity 0.8S/m. The antenna has seven loops with 0.5 conductor

width (Figure 13b). The antenna demonstrates an omni-

directional radiation pattern to obtain a signal independent

of the transmitter positions.Thebandwidth enhancement is

achieved by connecting the end of the helical antenna to the

ground. A similar antenna (42) reported by the same

authors has a conical helix shape and can be placed at

the top or bottom site inside a cylindrical capsule. The

antenna has bandwidth of 101MHz from (418MHz–

519MHz) with an omnidirectional radiation pattern. It

has a diameter of 10mm and a height of 5mm. A modified

stacked spiral-shapedwideband antenna for capsule endos-

copy that operates from411MHz to 600MHzwas presented

in Reference 43. In this antenna, two spirals are connected

to a feeding line and it has a diameter of 10mm and a

height of 7mm. Two spirals have about five turns and are

separated with a gap of 4mm (Figure 13c).

In Reference 44, another spiral antenna for a wireless

capsule endoscope system was presented. The antenna has

successfully been used in wireless telemetry based on ON-

OFF keying (OOK) modulation with 340� 340 pixel image

transmission. This design is similar to those in References

42 and 43 operating at 500MHz with bandwidths of

104MHz. The spiral antenna is structured on a 1-mm-thick

ground plane based on a spiral armwithwidth of 4mm. The

antenna has a diameter of 10mm and a height of 5mm.

Some researchers have investigated antenna designs

printed directly on the surface of the capsule casing. For

example, the antenna shown in Figure 13f (45) was pro-

posed for wireless endoscope systems. The antenna oper-

ates in the 1.4GHz telemetry band and considers the

presence of the small intestine and other tissue materials.

The antenna has good polarization characteristics that

support various orientations of the capsule and has a

dipole-like radiation pattern with an omnidirectional pat-

tern in two planes. The antenna is designed taking into

consideration the average body permittivity of 58.8 and a

conductivity of 0.84S/m.

Figure 13a shows a UWB antenna designed for wireless

capsule applications (46). This antenna operates from 3 to

5GHz to enable a large bandwidth for a high data rate

transmission. Another UWB antenna (Figure 12e) for

capsule applications is described in Reference 47. It is

based on a planar loop antenna. It operates in the UWB

low band of 3.4GHz to 4.8GHz. A compact monopole spiral

antenna based on medical implant band (MICS) is

presented in Reference 48. It has dimensions of 7mm�14mm. The length of the wire used to form the spiral

Table 3. Average Path Loss Measurements of Different

Tranmission Frequencies (The External Receiver is on the

Surface of the Body)

Antennas Loss Frequency Penetration

UWB (32) �86dB 4GHz 10 cm

UWB (11) �80dB 4GHz 7 cm

MICS (37) �60dB 403.5MHz 10 cm

418MHz (38) �21dB 418MHz 3 cm

916MHz (38) �27.5 dB 916.5MHz 3 cm

Figure 12. Side view of the temperature variation in the humanfor infrared-UWB signals with peak spectral input power limit of(a) �41.3 dBm/MHz and (b) �1.33dBm/MHz, and a total signalpower of (a) 0.0024mW (SAR¼0.2mW/kg) and (b) 24mW (SAR¼ 2W/kg) (32).


antenna is 26mm, which is a quarter wavelengths for the

frequency of 402MHz for a permittivity er of 55 and a

conductivity of 0.7 S/m. The antenna is designed on a

substrate with a relative permittivity of 2.2 and a thick-

ness of 0.5mm.

The main differences between in-body antennas and

external antennas are the strict size constraints and

demand for high efficiency. Substrates with high dielectric

constants are helpful in miniaturization, but the increased

conductivity of the body tissues with frequency becomes an

important problem for antenna operation (27, 46).

Table 4 summarizes the existing antennas described in

the literature in terms of their operation frequency, size,

and S11 performance. The operating frequency bandwidth

(BW) of antennas is obtained by the �10-dB band. It is

important to use a wide BW antenna because a communi-

cation system for wireless capsule endoscopy technology

will work efficiently despite the frequency shift in the

antenna operation. The signal transmission will still fall

within the�10-dB frequency BW. Awide BWwill be useful

for video-based wireless capsules because of the high data

rate requirement.

LOCALIZATION AND CONTROL TECHNIQUES WITHINTHE GI TRACT

It is essential to have an accurate knowledge of the position

and orientation of the capsule when it moves along

the GI tract where the endoscopic images are captured

(Figure 14). The success of drug administration, follow-up

interventions and other therapeutic operations heavily

depend on the accuracy of this spatial information.

Therefore, having a precise and reliable localization sys-

tem plays an important role in enhancing the capabilities

of a capsule. In general, two localization methods exist: the

magnetic localization methods and the electromagnetic

localization methods (49).

Magnetic Methods

Magnetic localization methods are used because 1) low-

frequency and dcmagnetic signals can pass throughhuman

tissue without any attenuation (50) and 2) magnetic

Figure 13. Antenna designs for wireless capsules: (a) capsule-shaped UWB antenna, (b) helical antenna (41), (c) stacked spiral antenna(43), (d) wideband spiral antenna (44), (e) planar UWB loop antenna (47), and (f) conformal dipole antenna (45).

Table 4. Comparison of Antennas for Wireless Capsule Devices

Antennas

�10dB

frequency

band

Dimensions

(mm�mm)

S11 @ central

frequency (dB)

UWB (46) 3.1–5GHz 23� 9 �14

Planar loop

UWB (47)

3.4–4.8GHz 11mm

diameter

�20

Spiral dipole

(45)

1.4GHz 12.25�9.6 �14

Spiral (44) 446–550MHz 5�10 �27

104MHz BW

Stacked

spiral (43)

411–600MHZ 7�10 �20

189MHz BW

Conical

spiral (42)

418–519MHz 5�10 �18

101MHz BW

Helical (41) 410–442MHz 5.6�8 �12

32MHz BW

Spiral

antenna

(48)

401–406MHz 5�5 �13 (402MHz)


localization is a non–line-of-sight method, in which the

capsule does not need to be in the line of sightwithmagnetic

sensors in order to be detected (51). In addition to being used

for the localization, the negligible interaction between the

magnetic field and the human body is also employed to

establish locomotion systems that can guide endoscopic

capsules magnetically such as robotic magnetic steering

(52–54), helical propulsion by a rotational magnetic field

(55–58), magnetic levitation (59, 60), and remote magnetic

manipulation (61). Furthermore, a localization system can

be used to provide feedback control information for an

actuation system (62). In this regard, the actuation and

localization systems should work together during a diag-

nostic procedure. However, the interference between the

two magnetic fields is a challenging research problem (49,

52, 63). Depending on how a capsule is propelled in the GI

tract, the magnetic localizationmethods can be divided into

two distinct classes.

Magnetic Localization of Capsules Under Natural Peristal-sis. Amagnetic source and a sensor are themost important

elements of a magnetic localization system. A stable and

reliable source of the magnetic field is essential for any

real-time magnetic localization system. Depending on how

the magnetic source is created and whether the capsule

acts as a field generator or contains a sensor, the localiza-

tion systems in this group are divided into three

subgroups.

In the first subgroup, a permanent magnet, which is

placed in the capsule, creates a uniformmagnetic field. The

magnitude and direction of the magnetic field depends on

the magnet’s position and orientation. Magnetic sensors

placed on the exterior of a patient’s body measure the

magnetic field. An analytical mathematical model describ-

ing the relationship between the magnetic field strength

and location of the permanent magnet can be employed to

determine the position and orientation of the capsule, i.e.,

to localize it. For example, if the magnet is modeled as a

magnetic dipole, then themagnetic flux density around the

dipole source is given by (64)

~B ¼ Bx~i þ By

~j þ Bz~k ¼

m0

4p

3ð~m �~rÞ~r

~rj j5�

~m

~rj j3

!

(2)

where Bx, By, and Bz are the components of the magnetic

flux density; ~m is the magnetic dipole moment of the

magnet;~r is the position vector of the magnet; and m0 is

the air magnetic permeability (4p� 10�7 JA�2m�1). The

localization parameters of the magnet (i.e., the capsule)

can be determined from equation 2. Weitschiles et al.

(65, 66) have employed this technique with a 37-chan-

nel, superconducting quantum interference device sen-

sor system to monitor the position of the capsule in the

GI tract, without considering the capsule orientation.

However, one issue associated with this position mon-

itoring system was that it had to take place in a mag-

netically shielded room to reduce the effect of the

environmental magnetic noise on the measured mag-

netic flux density. Schlageter et al. (67, 68) employed a

two-dimensional (2D) array of sixteen Hall sensors to

determine both position and orientation of a pill-size

magnet coated with silicone. When the magnet with the

volume of 0.2 cm3 moved up to 20 cm from the sensor

plane, its position and orientation were determined at

the rate of at least 20Hz. One limitation of this approach

is that the system’s accuracy drops significantly with the

distance (>20 cm) between the magnet and the sensor

array. Chao et al. (50) employed 50 cm� 50 cm� 50 cm

cubic sensor arrays to deal with this limitation, as

shown in Figure 15. Each of the four sensor planes

contains 16 magnetic sensors. An average position error

of 1.8mm and orientation error of 1.6� were obtained.

Many attempts including using 3D sensor planes

(69–76) have been proposed to enhance the accuracy

and widen the sensing volume of this localization sys-

tem, but with an increased cost and complexity.

Figure 14. Position and orientation of the capsule when it is inside the GI tract.


To reduce the cost, Aziz et al. (77) proposed a tracking

system based on only three 3-axis magnetic sensors placed

orthogonally in 3D space and an extra sensor for cancelling

environmentalmagnetic noise. A position error of up to 3 cm

was obtained when aF5mm�L6.0mm cylindrical magnet

was tested in a volume of 10 cm� 10 cm� 10 cm. Wu et al.

(78) built a wearable magnetic localization system for the

volumeof40 cm� 25cm� 40 cmwithsixsensingmodulesat

the front frame and the other four at the back frame, as

shown in Figure 16. Each module is composed of six linear

magnetic sensors that form three pairs of back-to-back

sensors arranged perpendicularly to each other in three

dimensions. Each pair is responsible for measuring one

dimension of the magnetic field. In this arrangement, the

top twomodules at the front were employed to eliminate the

interference of the earth magnetic field. Because this local-

ization system is based on the mathematical model of a

magnetic dipole, when the capsule is close to the sensing

module, thismodel is no longer valid, resulting in a decrease

in the localization accuracy (78).

Moussakhani et al. (79, 80) have derived a path loss

model for the wireless channel inside the human body

to simulate the electromagnetic wave propagation in

human tissues. This model is then employed to determine

numerically the theoretical performance limit, which is

reported to be 1 cm. Assuming that the capsule endoscope

contains a cylindricalmagnetmagnetized in the axial direc-

tion, they have theoretically demonstrated that the local-

ization accuracy can be in the order of 2–3mm at best. No

experimental results are presented to substantiate these

claims.

In the second subgroup, a coil is placed in the capsule.

Plotkin and Paperno (81, 82) proposed the idea of tracking

a receiving coil by a large 2D array of transmitting coils.

The magnetic field seen by the receiving coil is given by

~Br ¼~Bt �

~M (3)

where ~M is the magnetic moment of the receiving coil and~Bt is the magnetic flux intensity generated by the trans-

mitting coil. Because~Bt can be expressed approximately by

equation 2, computing the localization data is similar to

that of a permanent magnet. In other words, an optimiza-

tion algorithm can be employed to solve the inverse prob-

lem based on equation 3 once the electromotive force

induced in the receiving coil has been measured. In an

attempt to apply this localization concept to a capsule,

Nagaoka and Uchiyama (83) designed a single-axis coil

with 160 turns of copper wire with the size of F6.5mm�L2.3mm to be inserted into a capsule. A magnetic field

generator placed outside the body produced five alternat-

ing magnetic fields, each with different frequencies. The

electromotive force created by mutual induction was sent

to the outer detector through transmitting 75-kHz signals

from an integrated FM circuit. Because the magnetic field

strength decreases proportionally with the inverse third

power of the distance between primary and secondary

coils, the current flow in the primary coils was controlled

automatically to keep the induced electromotive force

within a constant range. To implement this approach, a

power amplifier was connected to the generator.

The received data at the detector not only were used for

Figure 15. Scheme of the cubic magnetic sensor array and its setup (50).

Figure 16. Wearable sensing modules by Wu et al. (78).


estimating the capsule’s position but also were used for

providing feedback signals continuously with the power

amplifier. Because the capsule cannot move with a high

speed in the GI tract, such a feedback signal does not affect

the tracking rate of the system (83). Although it was

reported that the system demonstrated an accuracy of

5mm when the capsule was 50 cm away from the genera-

tor, the experiments failed at several locations.

As an example to the third subgroup, Guo et al. (84)

developed another solution for the localization problem by

placing a three-axis magnetoresistive sensor inside a cap-

sule to measure the intensity of the external magnetic field

generated by three energized coils placed on the patient’s

abdomen. The three coils are excited in turn by square

waves with the same period of 0.03 s. At the end of every

cycle, there is a break period of 0.1 s when the coils are not

activated to estimate the Earth’s magnetic field magni-

tude. To obtain the real data of the magnetic field gener-

ated by the coils, the Earth’s magnetic field is subtracted

from the total magnetic field measured by the sensor at the

capsule location. A neural-network algorithm is employed

to estimate the three positional and three angular coor-

dinates of the capsule. However, it must be noted that this

is not a real-time localization system because the estima-

tion procedure is done after the completion of the experi-

ments. Another important point to keep in mind is that

when the sensor is close to the coils (less than 50mm), the

magnetic dipole assumption fails, which causes significant

localization errors. This situation becomes even worse

when the coils’ diameters are increased for enlarging the

localization range (84). Therefore, an improved localization

model based on Biot-Savart law was proposed to replace

the magnetic dipole model (85). It was reported that the

position and orientation errors range from 6.25mm to

36.68mm and from 1.2� to 8.1�, respectively. Eight ener-

gized coils excited by sinusoidal signals instead of square

waves are employed to improve the accuracy of thismethod

(86). After implementing an adaptive particle swarm opti-

mization technique, the mean position and orientation

errors of 14mm and 6.9� were obtained, respectively.

SUMMARY

One disadvantage of the magnetic localization systems

is that all devices or equipment around them must be

nonferromagnetic. Another disadvantage is that their

coverage volume is limited (87). Additionally, how to elim-

inate or minimize the interference between the magnetic

localization and magnetic actuation (if applicable) is a

significant issue. Alternately switching ON or OFF

between the actuation and sensing (localization) has

been suggested to solve this issue (63). But, because of

the hysteresis characteristics of a magnetic field, the

external magnetic field in the actuation mechanism is still

ON for a certain period of time after it has been turned

OFF. As a result, it would be virtually impossible to do a

real-time localization until the external magnetic field has

completely diminished. Needless to say, during the off

period of the magnetic actuation, the capsule may move

to a new position and orientation. In that case, the tracking

system will not provide accurate feedback data for the

actuation system. Another solution for this problem, is

to use a high-frequency alternating magnetic field (88,

89), which is presented in the next section.

Magnetic Localization of Capsules Under ActiveActuation. Magnetic Localization via Measuring High-Frequency Alternating Magnetic Field. The capsule propul-

sion is based on a spiral or spirals assembled on the surface of

acapsule thatcontainsapermanentmagnet (88).Anexternal

rotating magnetic field was generated around the patient’s

bodyusingthreepairs of coils (i.e.,Helmholtz’s coils) placed in

three perpendicular directions to rotate the capsule and

hence propel it forward or backward depending on the direc-

tion of the current applied. The frequency of the rotating

magnetic field should be less than 10Hz because the capsule

isnotallowedtomove too fast in theGI tract.Because the low-

frequency (several Hz) rotatingmagnetic field does not inter-

ferewith a high-frequency (from1kHz to 1MHz) alternating

magnetic field (88, 89), exciting coils were placed around the

patient’s body to produce the high-frequency magnetic field

for localization purposes. The operating frequencywithin the

range of 1kHz to 1MHz is sufficient to avoid the attenuation

of a magnetic field while passing through the human body.

Detecting coil arrays are also placed around the patient to

measure the magnetic field induced by a resonating coil

integrated inside the capsule and thus to determine the

position and orientation of the capsule (90, 91).

Magnetic Localization via Inertial Sensing. The capsule

can be propelled via magnetic steering, for example, using

a 6 degrees of freedom robotic manipulator carrying a

Figure 17. Permanent magnet mounted at the end effector of a 6 degrees of freedom robot and four cylindrical permanent magnets placedin the capsule (52).


permanent magnet at its end effector, as shown in

Figure 17 (52). A permanent magnet assembled in the

capsule will create a magnetic link between the capsule

and the external permanent magnet and, hence, drag and

steer the capsule in the body. A three-axis accelerometer is

assembled in the capsule to provide the approximate loca-

tion andorientation of the capsule in the digestive tract and,

if needed, to provide valuable feedback data to the actuation

system in order to verify the magnetic link between the

external permanent magnet and the capsule. It was

reported that this system could provide position and orien-

tation accuracies of 3 cmand6�, respectively. Theposition of

the capsule can be estimated from the position of the end

effector as long as the magnetic link is still maintained

during the steering process. This system does not have a

conflict between its actuation and localization modules.

However, the space available in a capsule is limited to

accommodate both the permanent magnet and the inertial

sensor. Perhaps the most significant disadvantage of this

method is that the capsule has to be dragged on the tissue,

which can cause significant damage to the contacting tissue.

Magnetic Localization Via Measuring a Rotational Mag-netic Field from a Permanent Magnet. Kim et al. (92)

assembled a helical structure on the surface of the capsule

and used an externally generated rotational magnetic field

to rotate the capsule containing two permanent magnets.

Instead of using six coils around the patient’s body, a big

parallelepiped permanent magnet consisting of seven

smaller rectangular magnets was rotated to generate

the rotationalmagnetic field. Thismagnetic field generator

was driven by an electric motor mounted on a manipulator

so that it could rotate and be moved while propelling the

capsule, as shown in Figure 18a.

One notable feature of this method is that the external

magnetic field generated for actuation can be used for local-

ization. When the external permanent magnet is spinning,

the magnetic field strength at the capsule location changes

periodically.ThreeHalleffectmagnetic sensorssetuporthog-

onally inside the capsule, as shown in Figure 18b, are used to

provide the capsulepositiondata.Once the capsuleposition is

determined, a rotationmatrix that represents the orientation

of the capsule in three dimensions is obtained by comparing

the three calculated orthogonal components and the three

measured orthogonal components of the magnetic flux

density at this position. This localization system generated

x, y, and z position errors within the ranges of (þ2mm,

þ15mm), (�9mm, þ12mm), and (�10mm, þ3mm),

respectively. The orientation errors were within the ranges

of (�2�, þ13�) in pitch direction and (�4�, þ11�) in yaw

direction.

The methods presented previously can be used for local-

ization and actuation simultaneously. However, they are

costly and bulky and far from being used in clinical appli-

cations yet. Therefore, there is still an increasing need to

establish an effective localization technique.

Electromagnetic Localization Methods

The electromagnetic waves-based localizationmethods can

be used together with the magnetic actuation, which

appears to be a feasible method to remotely propel, localize

and control the motion of the robotic capsules. While radio

waves, visible waves, X-ray, and gamma ray of the electro-

magnetic waves schematically shown in Figure 19 can be

used for capsule localization or tracking, microwaves,

infrared waves, and ultraviolet waves, which have very

low penetrability through the human tissue, are not suit-

able for localization or tracking.

Radio Waves. Although radio waves have been widely

used for locating an object in outdoor and indoor environ-

ments with the accuracy of hundreds of millimeters (93),

applying the radio waves in tracking a capsule within the

GI tract is not straightforward. This is because high-fre-

quency signals attenuate significantly at different levels

when they pass through different human tissues, whereas

low-frequency signals because of their long wavelengths

cannot deliver the desired precision of several millimeters

(76). Based on the radio frequency, received signal

strength indicator (RSSI), angle of arrival (AOA), time

of arrival (TOA), time difference of arrival (TDOA) and

RFID techniques can be employed for localization. Of

these, RSSI and RFID have a practical significance for

the capsule localization. In contrast, TOA is not feasible

because the radio waves travel with a very high speed

(3� 108m/s); thus, an extremely strict time synchroniza-

tion less than 1ns is required to obtain an acceptable

position resolution. AOA is not reliable even in well-struc-

tured environments (94, 95).

Figure 18. (a) Rotating a permanent magnet for generating a rotational magnetic field. (b) Sensor module scheme inside the capsule (92).DoF¼degrees of freedom.


Received Signal Strength Indicator. The wireless capsule

endoscope is equipped with a telemetry capability. Fisher

et al. (96, 97) used this advantage to measure the strength

of the received RF signals at eight sensors placed uni-

formly on the patient’s abdomen. Thecloser thereceiver is to

the transmitter, the stronger the signal. These signals are

used to determine the position of the capsule. When two

adjacentantennas receive equal strength signals, the capsule

is assumed tobe inbetween them.This localization technique

is being used in the Given Imaging M2A capsule to estimate

the 2D position of the capsule with the accuracy of 3.77 cm.

Although this technique does not require any additional

hardware in the capsule, its lowaccuracymakes it unsuitable

for providing feedback datawith a possible actuation system.

Arshak and Adepoju (95) have proposed an empirical

signal propagation model that describes a relationship

between the RSSI value and distance from the transmitter

to the receiver (98);

RSSIðdÞ ¼ PT � PLðd0Þ � 10n log10d

d0

þ Xs (4)

where d is the distance between transmitter and receiver,PT

is the transmit power, PL(d0) is the path loss for a reference

distance d0, n is the path loss exponent, andXs is a Gaussian

random variable. Equation 4 can be used to determine the

distances between the capsule and each of the sensors. A

trilateration method is used to calculate the capsule location

for these distances from the transmitter to the receivers. In

place of equation 4, Shah et al. (99) proposed an algorithm

basedona lookup table for thepositionestimation. In order to

reduce the estimation error of this method, it is necessary to

develop a more appropriate attenuation model when RF

signal travels within the human body.

Radio Frequency Identification. A cubic antenna array is

built surrounding a patient’s body to receive signals from

an RFID tag placed in a capsule (100–102) (Figure 20). The

center of gravity principle is applied to the collected data to

estimate the position of the tag, which is the position of the

capsule. However, this tracking algorithm produced large

errors because of the limited accuracy of the center of

gravity principle. To deal with this problem (101, 102), a

bidirectional antenna has been used to transmit RF signals

in two opposite directions. The position errors of 0.5 cm in x

and y directions and 2 cm in z direction were estimated

using simulation data. The frequency is limited below the

UHF band for the RF signals in order to pass through the

human body. In this band, it is impossible to generate

directional radiation by a compact antenna less than 1 cm

in length. Another significant drawback of this system is

that when the longitudinal axis of the tag’s antenna is in

the same direction with the main axis of the patient, the

radiation pattern does not intersect with the cubic array,

and thus the matching algorithm will not be able to deter-

mine the position of the capsule. To solve this problem, at

least one more tag in a perpendicular direction with the

first tag needs to be assembled in the capsule.

Another technique for the localization of an RFID tag

placed in a capsule is based on phase difference. In a

system with one transmitter and several receivers, the

Localization of WCE

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Ult

ra

vio

let

10

15

– 1

01

6 H

z

X-r

ay

10

16

- 1

01

9 H

z

Ga

mm

a-r

ay

10

19

– 1

020 H

z

Figure 19. Electromagnetic waves for wireless capsule endoscopy localization.

Figure 20. Antenna array and radiation pattern of RF signals transmitted from an RFID tag (101).


RF waveforms at the ith receiver can be described by the

following equations (103)

IiðtÞ ¼ Ai cosð2pðf r � f cÞtþ fiÞ þ sini1

QiðtÞ ¼ Ai sinð2pðf r � f cÞtþ fiÞ þ sini2(5)

where I(t) and Q(t) are the in-phase and quadrature com-

ponents of the signal; Ai, fr, fc, f, s, and n denote the

received signal magnitude, the frequency at the receivers,

the carrier frequency, the phase difference between the

carrier at the tag and the carrier at the receiver, the noise

level, and Gaussian noise, respectively. Hekimian-

Williams et al. (103) showed that although the exact phase

value (1i) of a single signal received at an antenna cannot

be used to calculate the distance that the signal has

traveled, the phase difference (1i �1j) between signals

within the same burst arrived at different antennas can be

employed for the location estimation. Wille et al. (104)

developed an RFID navigation system using the phase

difference to track medical instruments such as needles

or catheters. Support vector regression (SVR), a machine

learning algorithm, was applied to estimate the position of

the RFID tag by employing the phase difference data

collected at different RFID receivers. Although the exper-

imental results indicate the feasibility of the phase differ-

ence method for accurate localization of RFID tags, a lot of

improvements are needed before applying this method to

capsule tracking. The reason is that two important factors

that could affect the accuracy of the localization method

were ignored during the experiments. One of these factors

is the orientation of the tag, which was kept constant in all

of the tests. The second factor is that the attenuation of the

RF signals through the human body was not considered.

Visible Waves. Even though visible waves cannot pene-

trate the human body, it has still been considered for the

capsule localization through computer vision (105–107).

These localization methods provide only basic information

about the location of a capsule, which is not sufficient. There-

fore, the localization information can be considered only as

referenceorcomplementary information for theendoscopists.

X-Ray. X-rays can be employed to track an endoscopic

capsule. Fluoroscopy, which is a type of imaging tech-

nique based on the X-ray radiation, is used to display

continuous X-ray images in real-time, which shows the

location of the capsule (53). However, this method can

only supply visual information of the capsule location via

radiation images. It is impossible to obtain actual param-

eters of its position and orientation to serve as feedback

data for an actuation system. Aiming to solve this issue,

Kuth et al. (108) proposed a method that takes advantage

of both X-ray imaging and image processing for automat-

ically determining position and orientation of the wire-

less capsule endoscopy.

Gamma Ray. Gamma rays are used in gamma-

scintigraphy technique to visualize the position of anEnter-

ion capsule, a drug-delivery type capsule, in real time (109).

The capsule that is loaded with gamma-emitting radioiso-

topes can be detected by scintillation cameras. Because

gamma rays are partly absorbed by the human tissues

when they travel from the radioactive source to the camera,

both dorsal and ventral images are taken to enhance the

tracking accuracy (110). However, similar to an X-ray–

based localization system, this method can be harmful to

patients.

Recently, Than et al. (111) used gamma rays to estimate

the position and orientation of the capsule in a phantom

replicating the conditions in the human body. No battery

consumptionandzero spaceoccupation inside the capsuleare

the primary advantages of this localization method. This

method can provide less than 0.5mm position error, and

2.1� orientation error in a localization time interval of

50ms with an average computational time of 7ms per time

interval. These are the best localization performance data

reported in the literature.

Other Localization Methods

Magnetic resonance imaging (MRI) and ultrasound,

which are the widely used diagnostic imaging techniques,

can be used for localization (112–116). The need for

custom-programmed pulse sequences, which are differ-

ent from the standard pulse sequences of commercial

MRI scanners, would be a disadvantage for anMRI-based

method (117, 118). In contrast, although the bones and

gas shield ultrasonic signals (119), the localization

method based on ultrasound offer the features of high

speed, safety, and low cost (120).

Comparison and Discussion

As presented in Table 5, the approaches that have a

promising potential for an accurate localization are either

influenced by the magnetic field to be used for actuation or

too complex and still at their proof-of-concept stage. The

next generation wireless capsule endoscopy is expected to

have full robotic capabilities (121, 122) such that it will be

able to accomplish both diagnosis and disease treatment.

To deliver these functions, building a complete localization

system, which is reasonably accurate in real time, mini-

mally invasive, able to work with an active actuation

system, and easily implementable, is greatly desirable.

This is a significant challenge for researchers to address

within the next 5 years. Possible solutions include design-

ing novel approaches, improving the proposed methods, or

even developing hybrid strategies to exploit a combined

advantage of different techniques.

ROBOTIC WIRELESS CAPSULES AND DESIGNCONSIDERATIONS

Minimally invasive surgery uses cutting-edge technology

to diagnosis and implement therapy with the expected

advantages of fewer traumas to the body, a shorter recov-

ery time, and shorter hospital stay than traditional surgi-

cal methods. Wireless capsule endoscopy is one of the

successful medical technologies for the minimally invasive

surgery (123). Recently, substantial national and interna-

tional research efforts have been directed to actively propel

the capsule endoscopy and accurately localize it in order to


enhance its diagnosis and therapy features, and hence to

make it a truly robotic minimally invasivemedical device. To

this end, on-board and off-board activation propulsion con-

cepts have been proposed and demonstrated under overly

simplified operation conditions. The literature contains sev-

eral propulsion systems such as earthworm-like (124, 125),

paddling-type (126, 127), legged (128), electrically driven

(129), and propeller-driven (130) capsules. All of these sys-

tems require onboard batteries or an energy source to

provide power to the locomotion systems. Because the

size ofwireless capsules is of primary importance, it is better

to take the power supply out of the capsule so that more

space can be saved for other functional modules and the

operation time can no longer be limited. To achieve this, the

magnetic actuation is considered as the best choice to propel

a robotic capsule externally with an on-board magnetic

source (130–132). In addition to these efforts, there is a

need to incorporate biopsy, suturing/clipping, and on-site

drug-delivery and other similar medical interventional

capabilities into a robotic capsule to widen its versatility

and effectiveness in diagnosing, and treating abnormalities

related to the GI tract (133–136).

One effective and straightforward method to propel a

robotic capsule is to generatemagnetic gradient fields (via a

Maxwell coil system),which inducesadirect pulling force on

a robotic capsule containing a permanent magnet

(137–140). Anothermethod is to assemble a spiral structure

on the outer surface of a magnetic capsule and then use

magnetic torques to rotate the capsule, as shown in

Figure 21. When the helices interact with the working

Table 5. A Comparison of the Key Methods Presented in this Study.

Take Extra

Space of the

Capsule

Consume Extra

Power of the

Capsule Accuracy

Interference with

Magnetic Actuation

Real

Time

Adverse

Health

Effects

Magnetic

localization

(passive WCE)

Permanent magnet

(50, 78)

Yes No High Yes Yes No

Secondary coil (74) Yes Yes Moderate Yes — No

Magnetoresistive

sensor (84–86)

Yes Yes Moderate Yes No No

Magnetic

localization

(active WCE)

HF alternating

magnetic field (90,

91)

Yes No High No Yes —

Inertial sensing (52) Yes Yes Low No — No

Rotating external

permanent magnet

(92)

Yes Yes Moderate No — No

Electromagnetic

waves

Radio frequency (96,

97)

No No Low No Yes No

Visible waves (105,

107)

No No Low No — No

X-ray (108) No No — No Yes Yes

Gamma ray (109, 110) Yes No — No Yes Yes

Others MRI (112, 114) Yes Yes High — Yes Little

Ultrasound (115, 116) Yes Yes — No Yes Little

aThe accuracy may be “high” (position error <2mm), moderate (position error is from 2mm to 20mm), or low (position error >20mm).

The — symbol within a cell indicates that the information is unknown.

Figure 21. (a): Propulsion of a millisized robotic device with spirals. The cylindrical shape will be the initial shape for performanceoptimization studies. (b) The propulsion concept based on a rotational magnetic field.


environment (fluid or solid) inside the organ, themicrorobot

canbepropelledbyconverting the rotation toa translational

movement like a screw (141–145). To induce a magnetic

torque constantly on this kind of spiral-type microrobot for

medical use, a rotating magnetic field with uniform magni-

tudes is required. As far as generating external magnetic

fields is concerned,electromagnets showbetterperformance

than permanent magnets in terms of controlling the field

strength and direction (130). Therefore, an electromagnetic

system should be employed as the external magnetic source

when accuracy is at a premium.

The passive capsules are moving under the natural

peristalsis within the GI tract (that is, the contraction of

smooth muscles to propel contents through the digestive

tract). They cannot be stopped or turned around while

within the body, and they cannot be actively navigated in

the GI tract. These distinct disadvantages greatly diminish

the effectiveness of these capsules for accurate diagnosis

and limit therapeutic capabilities. Therefore, significant

research efforts and resources have recently been directed

toward making the wireless capsule as an actively control-

lable “capsule robot” to realize diagnostic, therapeutic, and

surgical functions, such as noninvasive GI surgery and

targeted drug delivery (4, 119, 138, 146–148).

Traditional Robotic Approaches. Despite the significant

research progress in the traditional macrosized robotic

systems, the progress in miniaturized robotics has been

limited because of the difficulties associated with powering

and actuating them using onsite means, such as an electric

motor and a battery with enough energy density (119). The

existing actuation and power supply cannot be scaled fur-

ther to realize functional in-body robotic systems.This isnot

a feasible and practical method for functional in-body

robotic systems because a battery with required energy

density will increase the physical dimension of the system.

Magnetic actuation is of particular interest forminiaturized

robotic systems operating in confined spaces as a magnetic

force acts without any physical contact over large distances.

Swimming Microrobots. Some studies have proposed

propulsion concepts for magnetically propelled swimming

robots for operation within veins and arteries (149–153).

Most of these studies are empirical studies characterizing

the behavior of microrobotic devices experimentally and

using simplified mathematical models to understand the

theory behind their operation. Abbott et al. (149) have

compared three possible biologically inspired propulsion

methods based on external magnetic actuation for swim-

ming microrobots. Martel et al. (153) has pioneered the

research and development of 1) magneto-taxis propelled in

the blood vessels by magnetic gradients generated by a

magnetic resonance imaging (MRI) system and 2) develop-

ment of new microelectromechanical and nanoelectrome-

chanical systems based on the integration of bacteria as

biological components. Sitti (150) reported that 1) there are

many fundamental and applied research issues associated

with the propulsion of microrobots before they find appli-

cations inmedicine and other areas, and 2) making, power-

ing, and steering miniaturized robots are equally crucial

research problems, which must be solved urgently.

Dreyfus et al. (151) built a flexible artificial flagellum,

which is amicroswimmer, consisting of a chain of magnetic

particles linked by the DNA and attached to a red blood

cell. They demonstrated experimentally that the micro-

swimmer was propelled with an external uniform mag-

netic field to control its velocity and direction of motion.

External Magnetic Actuation. As stated, space has been

the primary constraint preventing the use of onsite actua-

tors and power sources for propulsion. Some attempts have

been made to transfer energy wirelessly to the actuators to

navigate the robotic capsule, but these efforts failed

because of the space issue and the complex energy trans-

duction principle based on coils and storing harvested

energy before use. Therefore, almost all realistic propul-

sion concepts proposed in the literature for robotic capsules

are based on magnetic propulsion. Even so, there is still no

functional robotic capsule externally controlled to operate

safely within the GI tract to perform diagnostic and thera-

peutic functions.

Lien et al (154) reported on a hand-held magnetic

controller for a robotic capsule for 3D movements within

the stomach. A permanentmagnet is placed in a cylindrical

capsule to allow remote maneuverability through the

handheld device containing a permanent magnet and a

stepper motor. Yim and Sitti (155) have proposed a mag-

netically actuated robotic capsule based on rolling locomo-

tion for operation in stomach. In many previous studies

(4, 138, 148, 154–156) and in our recent study (2, 10, 131,

132, 137) to evaluate the feasibility of a robotic capsule

propulsion concept based on a rotary magnetic field, an

effective distance between the external magnetic source

and internal magnetic source has to be maintained to

create enough propulsion force. Furthermore, in an

unpublished study, we equipped a robot manipulator

(ABB IRB120) with a permanent magnet to navigate a

cylindrical robotic capsule containing a permanentmagnet

in a transparent model of the GI tract, as shown in

Figure 22. We found that the motion of the capsule could

be discontinuous and the friction between the capsule and

the GI tract played a critical role for smooth propulsion. If

this propulsion concept were tested in a real GI tract or

porcine intestine sample, the tissue squeezed between the

capsule and the external magnetic field would easily be

Figure 22. Robot manipulator propelling a robotic capsule insidethe transparent model of the GI tract.


damaged. The majority of propulsion concepts proposed in

the literature for robotic capsules for operation in the

digestive tract are made of rigid surfaces, which poten-

tially apply significant stress on the tissue. It follows that it

is difficult to control the magnetic forces propelling such

capsule robots with no damage to the contact tissue. More

importantly, these devices cannot stably resist the peri-

stalsis forces in the GI tract to anchor themselves for

detailed imaging of target lesions, and they carry out other

tasks such as delivering therapeutic agents (4, 119). There-

fore, there is an increasing demand for novel robotic cap-

sule propulsion concepts.

Design Considerations for a Robotic Capsule

Assuming that the capsule preserves its current features of

telemetry, imaging, swallowable size, minimum invasion,

and safe operation, the new features the next-generation

wireless capsules should have include active locomotion,

accurate localization, soft anchoring, on-board pH, tem-

perature, pressure, blood sensors, interventional capabili-

ties such as local drug delivery, biopsy, and suturing. In

addition, the capsule should allow parameter variations in

quick, easy, nondestructive, and straightforward manners

before using it for a patient.

As argued in the previous section, the propulsion of a

spiral-type robot based on a rotating magnetic field is

advantageous because the maximum torque available to

the robot is proportional to the magnetic field intensity,

which declines slower than the magnetic field gradient

over a long distance (131, 132). The most important ele-

ment of such a robot is its traction element, which is the

spiral converting the rotational movement into a recti-

linear motion. We have optimized the geometry of the

helical structure because it plays a significant role in

determining the propulsion efficiency. As a medical robot

traveling in a deflated, winding, and slippery lumen, the

complexity of its working environment makes this optimi-

zation problem even more critical. Therefore, the resistant

characteristics of the GI tract should be evaluated to

provide more accurate data with the optimization process.

The results in Figure 23 indicate that the frictional torque

increases as the rotation frequency increases, indicating

the rotational resistance has a relationship with the rate of

strain of the small intestine. This dependence reveals the

viscoelasticity of the GI tract to some extent. After the

introduction of the spiral, the cross-section of the capsule

in the lateral direction is raised, which increases the

deformation of the intestine. This increase becomes larger

when the helical angle gets smaller. Therefore, when the

number of spirals is the same, a capsule with a smaller

helical angle causes more strain in the intestine and,

consequently, confronts a higher frictional resistance. In

this case, from 0.5Hz to 3Hz, the capsule with the helical

angle of 10� causes a torque in the range of 0.6 to 1.8mNm,

whereas the capsule helical angle of 5� results in the torque

magnitude between 0.8 and 2.3mNm. At low frequencies,

the torque is almost proportional to the frequency and the

proportionality constant is larger when the helical angle is

smaller. We have proposed an analytical model to estimate

this fictional torque and experimentally verified it (131).

Also, we have established a conceptual framework for

designing and optimizing the topology of a spiral-type

robotic capsule propelled in a fluidic and tubular environ-

ment using electromagnetic actuation (132). For each cap-

sule, a segment of copper wire (w1mm) was wound around

the outer surface and acted as the spiral structure. The

winding area was within the cylindrical part of the capsule

so that every spiral structure had the same dimensions

(15mm) in the longitudinal axis, as shown in Figure 24.

Dummy Pillcam SB2 capsules (Given Imaging) are used

the base of the robotic capsule. We have found that adopt-

ing a relatively small lead and using at least two spirals

can make a spiral-type capsule more balanced. Regarding

the optimized number of spirals, the simulations and two

sets of experiments are in agreement, indicating that a

capsule wound with two spirals performs satisfactorily

from the propulsion efficiency point of view. The capsule

assembled with two spirals with a square cross section of

1mm� 1mm and a lead of 12mm exhibits the best per-

formance when magnetically propelled in the small intes-

tine of a pig under a rotating magnetic field with the

frequency of 4Hz (132).

Clearly, the current research efforts show (157) that

there is a need to provide solutions for many fundamental

and applied research issues associated with the next-

generation capsule robots with active navigation including

anchoring and localization. To this aim, breakthrough

methodologies and solutions that will enable the next

generation pill-sized capsule robots and establish novel

methodologies to predict and optimize the performance of

these remotely actuated robotic capsules are needed.

Figure 23. Variation of the frictional torque with the helicalangle of the spirals and the rotational frequency of the capsule. Figure 24. Capsules assembled with different spirals.


CONCLUSION

A wireless endoscopy device is a noninvasive biomedical

system designed to obtain images of the small intestine.

There is considerable demand in the development of minia-

turized, low-power wireless capsules systems for better

medical diagnosis. Wireless telemetry and wireless energy

transfer will continue to be essential parts of these devices

operating inside the human body.We have discussed imple-

mentation issues andpresented details of design techniques

as guidelines for futurewireless capsule systems. There is a

continuous effort in the literature and in the commercial

domain to eliminate existing problems such as limited

battery lifetime and uncertainty of the capsule’s position

by including wireless energy and their motion-control capa-

bilities while a capsule is in the GI tract. A highly efficient

wireless power transmission is a primary requirement of

wireless capsule systems to eliminate the use of batteries or

to provide a continuous energy source. In addition, current

research projects in the area of wireless capsules are target-

ing to include additional sensing mechanisms, localization,

and motion control. Once these studies are completed suc-

cessfully, wireless capsule endoscopy deviceswill findmany

new applications in clinical settings. One such application

would be controlled drug delivery.

Video-based wireless capsule devices have a battery life

of about 8h and provide only 2–35 frames/s transmission in

real time. Although this can provide some level of accuracy,

for some diseases detailed imagesmay be required. A high-

capacity radio system is currently necessary for wireless

capsule technology to examine the digestive tract with

clearer and more detailed images.

ACKNOWLEDGMENTS

Mehmet R. Yuce’s work is supported by Australian

Research Council Future Fellowships Grant

FT130100430. Gursel Alici and Trung D. Than’s work is

supported by the research funds in the Intelligent Nano-

Tera Research Systems Laboratory, University of Wollon-

gong, Australia.

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Gastroenterol. Hepatol. 2012, 9, pp 392–405.

MEHMET RASIT YUCE

Monash University, Melbourne,

Australia

GURSEL ALICI AND TRUNG DUC THAN

University of Wollongong,

Wollongong, Australia


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