merged document 12

WEARABLE PHOTOPLETHYSMOGRAPHIC SENSORS

Seminar Report

i

ABSTRACT

Photoplethysmography (PPG) technology has been used to develop

small, wearable, pulse rate sensors. These devices, consisting of infrared light-emitting

diodes (LEDs) and photodetectors, offer a simple, reliable, low-cost means of

monitoring the pulse rate noninvasively. Recent advances in optical technology have

facilitated the use of high-intensity green LEDs for PPG, increasing the adoption of this

measurement technique. In this review, we briefly present the history of PPG and recent

developments in wearable pulse rate sensors with green LEDs. The application of

wearable pulse rate monitors is discussed.

The principle behind PPG sensors is optical detection of blood volume

changes in the microvascular bed of the tissue. The sensor system consists of a light

source and a detector, with red and infrared (IR) light-emitting diodes (LEDs)

commonly used as the light source. The PPG sensor monitors changes in the light

intensity via reflection from or transmission through the tissue. The changes in light

intensity are associated with small variations in blood perfusion of the tissue and

provide information on the cardiovascular system, in particular, the pulse rate.

ii

CONTENTS

CHAPTER TITLE PAGE NO:

1. INTRODUCTION 1

2. PLETHYSMOGRAPHY 3

2.1 DEFINITION 3

2.2 TYPES OF PLETHYSMOGRAPHY 4

2.2.1 Air-Displacement plethysmography 4

2.2.2 Photo plethysmography 5

2.2.3 Strain gauge plethysmography 5

2.2.4 Impedance plethysmography 5

3. PHOTOPLETHYSMOGRAPHY 6

3.1 PRINCIPLE 6

3.2 LIGHT WAVELENGTH 6

3.2.1 The optical water window 7

3.2.2 Isobestic wavelength 7

3.2.3 Tissue penetration depth 8

3.3 DIFFERENT MODES OF PPG 9

3.3.1 Transmission Mode 9

3.3.2 Reflectance Mode 11

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3.4 PLETHYSMOGRAPHIC WAVEFORM 12

3.5 PHOTOPLETHYSMOGRAPHIC DEVICES 13

3.5.1 Earphone earbud PPG sensors 13

3.5.2 PPG ring sensor 14

3.5.3 Wristwatch-type sensors 15

3.5.4 Forehead sensors 16

3.6 ADVANTAGES 17

4. CONCLUSION 18

REFERENCE 19

iv

LIST OF FIGURES

FIGURE NO. TITLE PAGE NO:

3.1 TRANSMISSION MODE PPG 10

3.2 REFLECTIVE MODE PPG 11

3.3 PPG WAVEFORM 12

3.4 EAR PIECE PPG SENSOR 13

3.5 PPG RING SENSOR 15

3.6 WRIST WATCH TYPE SENSOR 16

3.7 FOREHEAD SENSORS 16


Dept. of ECE, LMCST 1

CHAPTER 1

INTRODUCTION

It is important to monitor the perfusion of the circulation. The most

important cardiopulmonary parameter is blood pressure, but monitoring it is

complicated. A second important parameter is blood flow, which is related to blood

pressure. We can monitor the blood perfusion in large vessels using ultrasound devices,

but it is not practical to use these routinely. Several devices for monitoring blood

perfusion have been developed but unfortunately, it is difficult to find a practical device.

However, the perfusion of blood flow and blood pressure can be determined easily

using a pulse rate monitor.

Wearable pulse rate sensors based on Photoplethysmography (PPG)

have become increasingly popular, with more than ten companies producing these

sensors commercially. The principle behind PPG sensors is optical detection of blood

volume changes in the microvascular bed of the tissue. The sensor system consists of a

light source and a detector, with red and infrared (IR) light-emitting diodes (LEDs)

commonly used as the light source. The PPG sensor monitors changes in the light

intensity via reflection from or transmission through the tissue. The changes in light

intensity are associated with small variations in blood perfusion of the tissue and

provide information on the cardiovascular system, in particular, the pulse rate. Due to

the simplicity of this device, wearable PPG pulse rate sensors have been developed.

This review describes the basic principles of PPG, previous and current developments

in wearable pulse rate monitors with a light source, and the elimination of motion

artifacts.

Arterial blood pressure (ABP) is one of the most important

hemodynamic characteristics of the cardiovascular system. It not only changes with the

heart pulsation, but also varies naturally throughout the day as part of the circadian

rhythm. In addition, ABP also changes in response to stress, drugs or diseases. Thus,



the development of an accurate and reliable method for continuous ABP measurement

has attracted much research effort. Although both invasive and non-invasive methods

have been developed, the latter are more desirable for replacing the invasive method

that is currently used in clinical practice.

Photoplethysmography (PPG) technology has been used to develop

small, wearable, pulse rate sensors. These devices, consisting of infrared light-emitting

diodes (LEDs) and photodetectors, offer a simple, reliable, low-cost means of

monitoring the pulse rate noninvasively. Recent advances in optical technology have

facilitated the use of high-intensity green LEDs for PPG, increasing the adoption of this

measurement technique. In this review, we briefly present the history of PPG and recent

developments in wearable pulse rate sensors with green LEDs. The application of

wearable pulse rate monitors is discussed.



CHAPTER 2

PLETHYSMOGRAPHY

2.1 DEFINITION

Plethysmography is a non-invasive diagnostic treatment used for

screening and patient follow-ups with various arterial and venous pathologies. This

treatment is concerned with the measurement of volume and volume displacement of

blood. The screening provides a circulatory assessment via a waveform representation

of pulsatile peripheral blood flow. Instrumentation providing blood volume parameters

exists but nothing to measure volume directly. An example of this instrumentation is

the use of an ultrasound.

While ultrasound provides hemodynamic (hemodynamic refers to the

forces generated by the heart and the motion of blood through the cardiovascular

system) data on vein segments, plethysmography provides information that is indirectly

related to venous volume changes. The data obtained is not specific to venous function

because limb volume changes may be caused by several factors.

Rapid changes are typically associated with changes in blood volume or

movement artifact. If movement is controlled, information specific to blood volume can

be obtained. Further separation of arterial and venous flow effects can be observed

through electronic filtration.

Venous flow changes typically involve long transient time constants

with duration of seconds or minutes. Venous displacement measurements are typically

associated with shifts in body position and limb compressions which allow

measurements of magnitude and duration.



2.2 TYPES OF PLETHYSMOGRAPHY

The different types of plethysmography is classified according to the

basis of the source used. Four main types of plethysmography exist. All of them are

used in clinical applications inorder to monitor the patients at different parts of the

body. They are:

1. Air-Displacement

2. Photo

3. Strain gauge

4. Impedance

2.2.1 Air-Displacement plethysmography

It is mainly done for the whole body. With air-displacement

plethysmography, the volume of an object is measured indirectly by determining the

volume of air it displaces inside an enclosed chamber (plethysmograph). Thus, human

body volume is measured when a subject sits inside the chamber and displaces a volume

of air equal to his or her body volume. Body volume is calculated indirectly by

subtracting the volume of air remaining inside the chamber when the subject is inside

from the volume of air in the chamber when it is empty. The volume of air inside the

chamber is calculated by slightly changing the size of the chamber (e.g. by moving a

diaphragm in one of the walls) and applying relevant physical gas laws to determine

the total volume from the changing air pressure within the chamber as its size is

altered. By subtracting the remaining volume of air inside the chamber when the patient

is inside from the volume of air in the chamber empty, you get the body volume.



2.2.2 Photo plethysmography

They are mainly used for monitoring blood perfusion in different parts

of the body. It is the simplest way of monitoring the blood perfusion non-invasively.

Photoelectric plethysmography is concerned with assessment based on cutaneous blood

volume. An electrode consisting of an infrared LED and a photosensor is attached to

the skin. Light transmitted into the skin is scattered and absorbed by tissue in the

illuminated field. Blood attenuates the reflected light and intensity of reflected light

changes with blood tissue density. The voltage signal generated by the photosensor is

amplified by a DC circuit. Low frequencies are passed which produces relatively stable

tracing. This corresponds to blood density in the underlying tissue.

2.2.3 Strain gauge plethysmography

Strain gauge plethysmography uses a transducer filled with mercury or

indium- gallium metal alloy conductor. Stretching the strain gauge causes a decrease is

diameter causing an increase in voltage. When wrapped around a limb segment, the

gauge provides a circumferential measurement that can be used to compute area. The

“slice volume” of the limb segment changes as the limb volume expands and contracts.

The mercury gauge is a very sensitive indicator of changes in the digital volume and

permits measurement of systolic blood pressure at any level of the extremity.

2.2.4 Impedance plethysmography

The final type of plethysmography is impedance plethysmography. A

weak current is passed through a limb and the electrical resistance to current flow is

measured. Four conductive bands are taped around the limb as outer and inner pairs of

electrodes. The inner pair is then used to measure electrical resistance.



CHAPTER 3

PHOTOPLETHYSMOGRAPHY

3.1 PRINCIPLE

The principle of PPG has been reviewed previously, and is explained

briefly here. Light travelling though biological tissue can be absorbed by different

substances, including pigments in the skin, bone, and arterial and venous blood. Most

changes in blood flow occur mainly in the arteries and arterioles (but not in the veins).

For example, arteries contain more blood volume during the systolic phase of the

cardiac cycle than during the diastolic phase. PPG sensors optically detect changes in

the blood flow volume (i.e., changes in the detected light intensity) in the microvascular

bed of tissue via reflection from or transmission through the tissue.

3.2 LIGHT WAVELENGTH

The interaction of light with biological tissue is complex and includes

the optical processes of (multiple) scattering, absorption, reflection, transmission and

fluorescence (Anderson and Parrish 1981). Several researchers have investigated the

optical processes in relation to PPG measurements. Researchers have highlighted the

key factors that can affect the amount of light received by the photodetector; the blood

volume, blood vessel wall movement and the orientation of red blood cells (RBC). The

orientation effect has been demonstrated by recording pulsatile waveforms from dental

pulp and in a glass tube where volumetric changes should not be possible, and more

recently by N¨aslund et al (2006) who detected pulsatile waveforms in bone. The

recorded pulses do bear a direct relationship with perfusion, and the greater the blood

volume the more the light source is attenuated. However, attempts at pulse amplitude

quantification (‘calibration’) have been largely unsuccessful.



The interaction of light with biological tissue can be quite complex and

may involve scattering, absorption and/or reflection. Anderson and Parrish examined

the optical characteristics and penetration depth of light in human skin. The wavelength

of optical radiation is also important in light–tissue interactions (Cui et al 1990), and

for three main reasons:

1. The optical water window

2. Isobestic wavelength

3. Tissue penetration depth

3.2.1. The optical water window

The main constituent of tissue is water that absorbs light very strongly

in the ultraviolet and the longer infrared wavelengths. The shorter wavelengths of light

are also strongly absorbed by melanin. There is, however, a window in the absorption

spectra of water that allows visible (red) and near infrared light to pass more easily,

thereby facilitating the measurement of blood flow or volume at these wavelengths.

Thus, the red or near infrared wavelengths are often chosen for the PPG light source

(Jones 1987).

3.2.2. Isobestic wavelength

Significant differences exist in absorption between oxyhaemoglobin

(HbO2) and reduced hemoglobin (HB) except at the isobestic wavelengths (Gordy and

Drabkin 1957). For measurements performed at an isobestic wavelength (i.e. close to

805 nm, for near infrared range) the signal should be largely unaffected by changes in

blood oxygen saturation.



3.2.3. Tissue penetration depth

The depth to which light penetrates the tissue for a given intensity of

optical radiation depends on the operating wavelength (Murray and Marjanovic 1997).

In PPG the catchment (study) volume, depending on the probe design, can be of the

order of 1 cm3 for transmission mode systems. PPG can provide information about

capillary nutritional blood flow and the thermoregulatory blood flow through arterio-

venous anastomosis shunt vessels.

Within the visible region, the dominant absorption peak corresponded

to the blue region of the spectrum, followed by the green-yellow region (between 500

and 600 nm) corresponding to red blood cells. The shorter wavelengths of light are

strongly absorbed by melanin. Water absorbs light in the ultraviolet and longer IR

regime; however, red and near-IR light pass easily. As a result, IR wavelengths have

been used as a light source in PPG sensors.

Blood absorbs more light than the surrounding tissue. Therefore, a

reduction in the amount of blood is detected as an increase in the intensity of the

detected light. The wavelength and distance between the light source and photodetector

(PD) determine the penetration depth of the light.

Green light is suitable for the measurement of superficial blood flow in

skin. Light with wavelengths between 500 and 600 nm (the green-yellow region of the

visible spectrum) exhibits the largest modulation depth with pulsatile blood absorption.

IR or near-IR wavelengths are better for measurement of deep-tissue blood flow (e.g.,

blood flow in muscles). Thus, IR light has been used in PPG devices for some time.

However, green-wavelength PPG devices are becoming increasingly popular due to the

large intensity variations in modulation observed during the cardiac cycle for these

wavelengths. A green LED has much greater absorptivity for both oxyhaemoglobin and

deoxyhaemoglobin compared to infrared light.



Therefore, the change in reflected green light is greater than that in

reflected infrared light when blood pulses through the skin, resulting in a better signal-

to-noise ratio for the green light source. Several green-light-based

photoplethysmographs are available commercially. For example, MIO Global has

developed the MIO Alpha in cooperation with Philips; this measures the

electrocardiogram (ECG) with 99% accuracy, even while cycling at speeds of up to 24

kmph. For daily use, Omron has developed a green light pulse rate monitor (HR-500U,

OMRON, Muko, Japan).

Furthermore, the use of video cameras using the signal based on the red

green blue (RGB) colour space has been considered, as shown in Section 3.3. The green

signal was found to provide the strongest plethysmographic signal among camera RGB

signals. Haemoglobin absorbs green light better than red and green light penetrates

tissue to a deeper level than blue light. Therefore, the green signal contains the strongest

plethysmographic signal.

3.3 DIFFERENT MODES OF PPG

The different modes in PPG is categorized according to the positions in

placement of the light source and the photodetector. According to this way there are

two modes of operation of PPG. They are:

1. Transmission

2. Reflectance

3.3.1 Transmission Mode

In transmission mode, the light transmitted through the medium is

detected by a PD opposite the LED source. The transmission mode is capable of

obtaining a relatively good signal, but the measurement site may be limited. To be



effective, the sensor must be located on the body at a site where transmitted light can

be readily detected, such as the fingertip, nasal septum, cheek, tongue, or earlobe.

Sensor placement on the nasal septum, cheek or tongue is only effective under

anesthesia.

In transmission mode, when the IR LED illuminates, the light is forced

to fall on the part of the body on to which the device is kept. The light start transmitting

through the body part. In this case some of the light is absorbed by the capillary bed,

some will be reflected back from the capillary bed, some transmits through and reach

the photodiode kept at the opposite end of the source and the rest reflect back at the

surface itself. The light transmitted through the body part and reaching the photodiode

is noted and the waveform is noted as the plethysmographic waveform.

Fig 3.1 Transmission mode PPG

The fingertip and earlobe are the preferred monitoring positions;

however, these sites have limited blood perfusion. In addition, the fingertip and earlobe

are more susceptible to environmental extremes, such as low ambient temperatures

(e.g., for military personnel or athletes in training). The greatest disadvantage is that the

fingertip sensor interferes with daily activates.



3.3.2 Reflectance Mode

Reflectance mode eliminates the problems associated with sensor

placement, and a variety of measurement sites can be used (as discussed in the

following section). However, reflection-mode PPG is affected by motion artifacts

and pressure disturbances. Any movement, such as physical activity, may lead to

motion artifacts that corrupt the PPG signal and limit the measurement accuracy

of physiological parameters. Pressure disturbances acting on the probe, such as the

contact force between the PPG sensor and measurement site, can deform the arterial

geometry by compression. Thus, in the reflected PPG signal, the AC amplitude may

be influenced by the pressure exerted on the skin.

Fig 3.2 Reflectance mode PPG

In this case also as the light is illuminated, some light will be transmitted

through, some will be reflected back at the surface itself, some are absorbed by the

capillary bed. Here the reflected signals are captured by the photodiode kept adjacent

the source and the waveform is noted according to the amount of light falling on the

photodiode after reflection.



3.4 PLETHYSMOGRAPHIC WAVEFORM

PPG sensors optically detect changes in the blood flow volume (i.e.,

changes in the detected light intensity) in the microvascular bed of tissue via reflection

from or transmission through the tissue. Photoplethysmographic waveform, consisting

of direct current (DC) and alternating current (AC) components. The DC component

of the PPG waveform corresponds to the detected transmitted or reflected optical

signal from the tissue, venous blood, non-pulsatile component of artery blood and

depends on the structure of the tissue and the average blood volume of both arterial

and venous blood. Note that the DC component changes slowly with respiration.

Fig 3.3 PPG waveform

The AC component shows changes in the blood volume that occurs

between the systolic and diastolic phases of the cardiac cycle. The fundamental

frequency of the AC component depends on the heart rate and is superimposed onto

the DC component. The AC current shows the pulsatile component of the artery blood.

The variation of blood flow represented by AC component is the flow during the

systolic phase of the cardiac cycle.



3.5 PHOTOPLETHYSMOGRAPHIC DEVICES

Wearable pulse rate sensors based on photoplethysmography (PPG)

have become increasingly popular in recent years with the technology advancing day

by day. Nowadays more than ten companies producing these sensors commercially.

The principle of PPG have become more popular as it is non-invasive. Due to the

simplicity of this device, wearable PPG pulse rate sensors have been developed. Some

of the mostly used devices are:

3.5.1 Earphone earbud PPG sensors

Fig 3.4 Earpiece PPG sensor



Earphone/earbud PPG sensors are also available and provide greater

comfort for the user. In this design, a reflective photosensor is embedded into each

earbud, as shown in Figure 3. The sensor earbuds are inserted into the ear and

positioned against the inner side of the tragus to detect the amount of light reflected

from the subcutaneous blood vessels in the region. The PPG sensor earbuds look and

work like a regular pair of earphones, requiring no special training for use.

A headset with an ear-clip, transmission-type PPG sensor allows

continuous, real-time monitoring of heart rate while listening to music during daily

activities. In addition, the proposed headset is equipped with a triaxial accelerometer,

which enables the measurement of calorie consumption and step-counting. However,

over the course of a variety of daily activities (e.g., walking, jogging, and sleeping),

the PPG sensor signal may become contaminated with motion artifacts.

3.5.2 PPG ring sensor

The most common commercially available PPG sensor is based on

finger measurement sites. The transmission mode PPG sensors are commonly used for

this operation. Finger sites are easily accessed and provide good signal for PPG sensor

probes. For example, a ring sensor can be attached to the base of the finger for

monitoring beat-to-beat pulsations. Data from the ring sensor are sent to a computer

via a radiofrequency transmitter, as shown in Figure.

To minimize motion artifacts, a double ring design was developed to

reduce the influence of external forces, acceleration and ambient light, and to hold the

sensor gently and securely to the skin, so that the blood circulation in the finger

remained unobstructed. Experiments have verified the resistance of the ring sensor to

interfering forces and acceleration acting on the ring body. Benchmark testing with

FDA-approved PPG and ECG sensors revealed that the ring sensor is comparable in

the detection of beat-to-beat pulsations, despite disturbances.



Fig 3.5 PPG ring sensor

3.5.3 Wristwatch-type sensors

Wristwatch-type sensors have been developed and commercialized by

several companies. These devices, although much easier to wear, are not usually used

in clinical settings, due to several technical issues. However, a novel PPG array sensor

module with a wristwatch-type design has been developed. The proposed module

measures the PPG signal from the radial artery and the ulnar artery of the wrist, whereas

previous methods obtained signals from the capillaries in the skin. Phototransistors and

IR-emitting diodes were placed in an array format to improve the PPG signal sensitivity

and level of accuracy.

Various arrays were considered for optimization. A conductive fiber

wristband was used to reduce external noise. In the experiments, the proposed module

was assessed and compared with the commercially available product produced by

BIOPAC. A reflective brachial PPG sensor has also been examined. Although the pulse

amplitude is lower than those from the finger and earlobe, the PPG pulse waveforms

from regions in the vicinity of a human artery could be detected and measured easily.



Fig 3.6 Wristwatch-type sensors

3.5.4 Forehead sensors

Fig 3.7 Forehead sensors



Forehead sensors have shown greater sensitivity to pulsatile signal

changes under low perfusion conditions, compared with other peripheral body

locations [31]. The thin-skin layer of the forehead, coupled with a prominent bone

structure, helps to direct light back to the PD. Sensor placement on the forehead has

been shown to result in decreased motion artifacts during certain types of physical

activity.

3.6 ADVANTAGES

PPG sensors has provided many advantages over conventional

techniques. Some of the major advantages of this technology is:

1. PPG is inexpensive and cheap.

2. Since it consumes very less power, it is an ideal ambulatory device.

3. Does not need special training or guidance.

4. A range of clinically relevant parameters can be obtained from PPG signal.

5. They offer a simple, reliable, low-cost means of monitoring pulse rate non-

invasively.



CHAPTER 4

CONCLUSION

Wearable PPG sensors have become very popular. Although a great

deal of progress has been made in the hardware and signal processing, an

acceptable wearable PPG sensor device has yet to be developed. Green light

sources in PPG sensors minimize motion artifacts. Several filters and algorithms have

been examined to mimic daily activities on limited time scales. However, better

accuracy and reproducibility of real environments are required to eliminate

motion artifacts. Further research is needed for the development of practical

wearable PPG pulse rate monitors and pulse oximeters.

The calculation of blood pressure and pulse rate become very common

in clinical applications inorder to check the patients’ condition. Several devices for

monitoring blood perfusion have been developed but unfortunately, it is difficult to

find a practical device. However, the perfusion of blood flow and blood pressure can

be determined easily using a pulse rate monitor. PPG technology has provided an

easy method in analyzing the technical measurement within the blood and blood

volume changes associated with it. They are especially used to develop small

wearable pulse rate sensors which can be easily used by patients itself. No further

knowledge of using the device is required in using these devices. They can be used

without much knowledge.

Wireless wearable sensors have become more familiar with the new

incoming technologies where the doctor doesn’t need much guidance. The

technology has become popular much due to its non-invasive nature.



REFERENCE

1. Togawa, T.; Tamura, T.; Öberg, P.Å. Biomedical Sensors and Instruments,

2nd ed.; CRC Press: New York, NY, USA, 2011; pp. 19–190.

2. Challoner, A.V.J . Photoelectric plethysmography for estimating cutaneous

blood flow. In Non-invasive Physiological Measurement; Rolfe, P., Ed.;

Academic Press: Oxford, UK, 1979; Volume 1, pp. 127–151.

3. Kamal, A.A.R.; Harness, J.B.; Irving, G.; Mearns, A.J. Skin

photoplethysmography—A review.

Comput. Methods Programs Biomed. 1989, 28, 257–269.

4. Alen.J . P h o t o p l e t h y s m o g r a p h y a n d i t s a p p l i c a t i o n i n c l i n i c a l

p h y s i o l o g i c a l m e a s u r e m e n t .

Physiol. Meas. 2007, 28, R1–R39.

5. Anderson, R.R.; Parris, E.D. The optics of human skin. J. Invest. Dermatol. 1981,

77, 13–19.

6. Giltvedt, J.; Sita, A.; Helme, P. Pulsed multifrequency photoplethysmograph.

Med. Biol. Eng. Comput. 1984, 22, 212–215.

7. Cui, W.; Ostrander, L.E.; Lee, B.Y. In vivo reflectance of blood and tissue as

a function of light wavelength. IEEE Trans. Biomed. Eng. 1990, 37, 632–639.

8. Z i j l s t r a , W . G . ; B u u r s m a , A . ; M e e u w s e n - v a n d e r R o e s t ,

W . P . A b s o r p t i o n s p e c t r a o f h u m a n f e t a l a n d a d u l t

o x y h e m o g l o b i n , d e - o x y h e m o g l o b i n , c a r b o x y h e m o g l o b i n ,

a n d M e t h e m o g l o b i n . C l i n . C h e m . 1 9 9 1 , 3 7 , 1 6 3 3 – 1 6 3 8 .

9. Meada, Y.; Sekine, M.; Tamura, T. The advantage of green reflected

J. Med. Syst. 2011, 35, 829–834.

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Engineering