IEEE Access
`TheJournal for rapid epen access publishing
`
`Received October 29, 2014, accepted November14, 2014, date of publication December 18, 2014, date of current version January 12,
`2015.
`
`Digital Object Identifier 10.1109/ACCESS.2014.23821 79
`
`A Neo-Reflective Wrist Pulse Oximeter
`
`GRANTHAM PANG, (Senior Member, IEEE), and CHAO MA
`Department of Electrical and Electronic Engineering, the University of Hong Kong, Hong Kong
`Corresponding author: G. Pang (gpang @eee.hku.hk)
`
`This work was supported by the CRGC through The University of Hong Kong, Hong Kong, under Grant 104002477.
`
`ABSTRACT This paperrelates to a genuine wrist pulse oximeter, which is a noninvasive medical device
`that can measure the pulse rate and oxygen saturation level in a person’s blood. The device is novel due to its
`innovative design.It is a new type of reflective oximeter, which has a concavestructure for housing the optical
`source and sensor. The neo-reflective sensor module of the device is designed to send the sensor data to a
`nearby intelligent mobile phone using wireless data transmission. The pulse oximeter has been developed and
`calibrated, and the calibration curve was analyzed. The innovative design of this pulse oximeter would enable
`the user to wear the low-cost device on one wrist continuously throughout the day, without the inconvenience
`of a conventional finger pulse oximeter.
`
`INDEX TERMS Biomedical sensor, pulse oximetry, blood oxygen saturation, wrist oximeter, reflective
`oximetry.
`
`I. INTRODUCTION
`
`A pulse oximeter is a non-invasive medical device that can
`measure the pulse rate and oxygen saturation level in a
`person’s blood. People with cardiac or pulmonary disease
`may check on their blood oxygenation continuously,
`especially while jogging or exercising. Pilots use them to
`determine if they need supplemental oxygen, especially when
`they fly a non-pressurized airplane. When climbing on high
`altitudes, alpinists may use pulse oximeters as well.
`In the blood vessels,
`the hemoglobin with oxygen
`are called oxygenated hemoglobin (oxy Hb), whereas
`the hemoglobin without oxygen are called deoxygenated
`hemoglobin (deoxy Hb). The basic principle of operation of a
`pulse oximeter depends on the use of LEDlight, which is used
`to determine the oxygen saturation. In particular, red LED
`light of around 630nm wavelength and infrared LED light of
`around 880nm wavelength are utilized. (Some pulse oxime-
`ters use LEDs of around 660nm and 940 nm wavelengths.)
`It must be noted that oxy Hb absorbs red and infrared light
`differently. One fundamental property is that oxy Hb absorbs
`more infrared light than red light, whereas deoxy Hb absorbs
`more red light than infrared light. A pulse oximeter calculates
`the oxygen saturation by comparing how muchred light and
`infrared light are absorbed by the blood.
`Currently,
`there are many different models of pulse
`oximeters in the market. They differ in size, quality and cost.
`The complex ones usedin hospitals are typically large and not
`portable. However, there are many hand held pulse oxime-
`ters for home use which are very compact and easy to use.
`
`The most common modelis the finger model, which displays
`the blood oxygenation level (SpO>) and pulse rate.
`A wrist-finger model is very common in the market[1].
`The device is actually a finger model as the sensoris placed
`on a finger which is then connected to a wrist display. Yet,
`the pulse oximeter on the finger is quite noticeable and the
`finger may not be too comfortable for long period and con-
`tinuous monitoring. Early work on reflectance type of pulse
`oximeter have been carried out by Mendelsonetal. [2], [3].
`Design to lower the power consumption has also been
`studied in [4] and [5].
`The developmentof a small-sized reflectance pulse oxime-
`ter was investigated by Santha etal. [6]. The distance between
`the light sources and the detector was examined, as well as the
`pressure of the reflective pulse oximeter sensor head onto the
`skin. In [7], a reflectance pulse oximeter with circuitry and
`method for obtaining the percentage of oxygen saturation is
`given. A microcomputer is used for the signal processing, as
`well as the calculation of the oxygen saturation based on the
`input light intensity signals.
`A wrist-worn pulse oximeter is described in [8] which has
`illustrated the feasibility of reflectance oximetry based on a
`sensor mounted onto a wrist band. The motivation is that the
`
`typical finger-tip model is a transmittance pulse oximeter,
`andis practical only when the patient is hospitalized or lying
`steadily. Usually, the sensor would press on a finger or the ear
`lobe which would sometimes cause discomfort or pain as the
`sensor would block the blood flow. In their prototype model,
`the reflectance sensor and LED light source are mounted onto
`
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`IEEE Access
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`G. Pang, C. Ma: Neo-Reflective Wrist Pulse Oximeter
`
`a wrist band of width 30mm. The wristband sensor design is
`novel and a wireless data transmission module would further
`
`enable the data to be sent to a remote receiver for processing
`and display. Yet, the calibration of the oximeter and further
`details on the implementation are not shown.
`In [9], a ring-shaped photodiode designed for use in a
`reflectance
`pulse
`oximetry
`sensor
`is
`shown.
`Li and Warren [10] have also developed a small and low-
`cost wireless reflectance oximeter suitable for wearable and
`
`surface-based applications including at the wrist location.
`High quality photoplethysmographic (PPG) data can be col-
`lected. Further signal processing is needed to obtain the actual
`value of oxygen saturation. In [11], another reflectance type
`PPG device is developed for the continuous monitoring of
`arterial oxygen saturation. The result obtained was in good
`agreement with the finger PPG.
`
`Il. THE INNOVATIVE FEATURES OF A NEO-REFLECTIVE
`WRIST PULSE OXIMETER
`
`geometric dimension is bigger than the scattering length, and
`then the photon will mainly scatter before it get sized by the
`photo-transistor. Therefore, light through wrist tissue can be
`described by photon diffusion equation [8]:
`
`aa, t) — DV7A(r, t) + ,M(, t) = SC, t)
`
`()
`
`where ((r,t) is the fluence rate, which represents the effective
`photon density at the position r and at time t. D denotes a
`diffusion coefficient, 1 represents absorption coefficient of
`the tissue and S(r, t) represents the light source.
`Hence, the density of the photon reflecting from the body
`tissue is determined by the flux of the illumination and the
`detected light intensity. The equation of the density of the
`photonreflected is [8]:
`
`R(p, 1, t) = (4xDe)9/2Zot? exp(—pact)exp(—
`
`
`
`p+ 9,
`
`4Det
`
`Q)
`
`In this paper, we present another novel reflective wrist
`pulse oximeter that has been developed in our laboratory.
`As mentioned in the previous section,
`recent mature
`technology and products of oximeter are mostly based on
`the principle of transmittance of light through a finger.
`However, oximetry based on the principle oflight reflectance
`from humantissue has a great potential for wide acceptance.
`In our design, the light source and sensor are placed in a
`concave structure using the principle of reflective oximetry.
`In addition, the design is for wrist wearing as it is a more
`convenient and comfortable location than the finger for long
`duration measurementand continuous monitoring. It must be
`noted that for pulse and blood pressure monitoring, some
`products located at the wrist already exist in the market
`(e.g. Casio BP-100 and Omron HEM-608).
`To summarize, our design has a novel sensor module
`design. A concave structure is designed to house the sensor
`module components so as to obtain improved operational
`efficiency when collecting the wrist pulse signals. In order
`to obtain desirable wrist pulse signals, the sensor module is
`(4)
`SpO, = a — BR
`designed to be placed at an inner part of the wrist. The special
`design of its concave shape helps the sensor module to stay in
`whereRis defined as follows:
`the chosen location of the wrist. The device has a very simple
`F/B
`hardware structure without even a display because all data
`~ Kke/Ibe
`further processing, calculations and display will not be carried
`out on the wearable wrist device. A wireless transmission
`and a@, 6 are parameters that need to be determined experi-
`mentally. Hence, when the values of the DC and AC compo-
`nent of the red and infrared light intensity are obtained, the
`blood oxygen saturation can be calculated.
`Equation (4) has assumed a linear relationship between
`oxygen saturation and the R value, which is commonly used
`in the field and in the market products. Yet, considering
`the differences in light intensities between red and infrared
`light, it is also reasonable to state a second-order relationship
`between R and the oxygen saturation:
`SpO,= a — BR — yR?
`
`Whentime is long enough, the change ratio of reflected light
`intensity will be close to —pact, as below:
`a)
`GB)
`fim, 5, NR, 1.0 = —Hact
`intensity
`the change ratio of reflected light
`As a result,
`W = Iac/Ipc is proportional to the absorption coefficient of
`the tissue. The AC part of the signal represents absorption
`of fluctuating wave of pulsing blood, while the DC part
`represents absorption of human tissue and vessel.
`The absorption of red and infrared light by oxygenated
`hemoglobin (HbO2) and reduced hemoglobin (Hb) is much
`bigger than that by other human tissue or water. Besides at
`different wavelengths, the light absorbance of HbO2 and Hb
`are different. By this principle, we can deduce the oxygen
`saturation from the different ratio of reflective light intensity
`rate when two light with different wavelengths go through
`vessel blood inside humantissue [7], [8].
`Wal
`Tio/lic
`SpO2 = a P =e Prag7p.
`
`(5)
`1563,
`
`module will transmit all sensor data to an application of a
`nearby smart phone, which would perform all those tasks.
`Hence,the device can communicate in a cordless manner with
`a remote monitoring system. The radio interface can use the
`Bluetooth protocol.
`
`Ill. PRINCIPLE OF OPERATION OF REFLECTIVE OXIMETRY
`The scattering coefficient is much bigger than the absorption
`coefficient for most human tissue. Therefore, it is possible
`to build a model where the photon movement in the body
`tissue can be considered as a diffusion process, when the
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`G. Pang, C. Ma: Neo-Reflective Wrist Pulse Oximeter
`
`The parameters a, B and y can be determined in an
`experimental study. It can be shown that the second order
`equation can potentially provide a more accurate calculation
`for the value of oxygen saturation.
`
`IV. SYSTEM DESIGN
`Fig.
`1 gives the overall system design of the reflective
`oximeter, which consists of Concave Sensor Module,
`Signal Processing Module, A/D Converter Module, Wireless
`Transmission Module and Smart Phone.
`
`stored inside the smart phone. As a result of using minimal
`hardware for the device and the use of a flexible software
`
`solution for handling the sensor data, the cost of device will
`be very low. In this paper, we focus on the description of the
`wrist concave sensor and the signal processing module.
`
`V. CONCAVE SENSOR MODULE
`
`As shownin Fig. 3 and Fig. 4, the concave oximeter sensor
`consists of a concave support structure, one red LED and one
`infrared LED and two photo-transistors.
`
`Module
`
`Wireless
`Transmission
`Module
`
`Signal
`Processing
`
`Smart
`Phone
`
`Wireless
`Transmission
`Module
`
`FIGURE 1. System design diagram.
`
`The Concave Sensor Moduleis used to acquire the original
`light signal reflected from fluctuating blood andtissue. In the
`Signal Processing Module, the original signal is separated
`into a DC and AC component.
`is
`the analog signal
`In the A/D converter Module,
`transferred to the digital signal by sampling and regulating.
`This is because the digital signal is easier to be transferred by
`wireless transmission module and more suitable for further
`
`processing and application.
`The Wireless Transmission Module is madeof a transmitter
`
`and a receiver, which would establish a communication link
`between the wrist band and the smart phone. Fig. 2 shows
`the wireless transmission of sensor data from the wrist worn
`
`pulse oximeter to the smart phone.
`
`
`
`FIGURE 2. Wireless transmission of sensor data from the wrist pulse
`oximeter.
`
`Further signal processing and computing are done in the
`application in the smart phonefor saving hardware. The result
`can also be displayed by the smart phone andthe data can be
`
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`
`FIGURE 3. Section plan of Concave Sensor Module
`
`Concave Structure
`
` Photo-transistor
` Photo-transistor
`
`FIGURE 4. Top view of Concave Sensor Module.
`
`The soft concave support structure matches with the
`convexity of the human wrist, with the two LEDsinside
`the pinhole. The two photo-transistors are placed on the
`other end of the structure at a suitable distance from the
`
`LEDs. The separation distance has been verified after many
`experiments.
`The peak wavelengths of the red LED and infrared LED
`are 630nm and 880nm respectively, which are sensitive to the
`changes in oxygensaturation.
`The photo-transistors are placed directly in contact with the
`skin andtheir flat surfaces would not cause any discomfort to
`the wrist. The photo-transistors have nearly the same spec-
`tral responsivity at wavelength of 630nm and 880nm,which
`would have the same the sensitivity for better accuracy.
`The black cover wrist band attaches the structure to the
`
`wrist tightly to make it wearable, stable and comfortable, and
`also to reduce noise effect.
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`IEEE Access
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`G. Pang, C. Ma: Neo-Reflective Wrist Pulse Oximeter
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`The implemented sensor after many testing experiments is
`shown in Fig. 5, and the sensor with further plan is shown
`in Fig. 6. At the moment, the wireless data transmission
`module has not been implemented yet.
`
`
`
`
`FIGURE 5. Photo of the implemented sensor.
`
`FIGURE 6. Photo of the sensor with further plan.
`
`VI. SIGNAL PROCESSING MODULE
`
`The Fig. 7 showsthe diagram ofthe signal processing module
`consisting of pass filters and amplifiers, providing signal
`processing of wave filtering and amplifying. The received
`signal reflected from blood contains AC signal and DC signal,
`which are separated by the pass filters and then amplified
`respectively.
`
`Concave
`Sensor
`
`Module
`
` DC Component
`
`.
`Amplified
`AC Component
`
`FIGURE 7. Diagram of signal processing module.
`
`The DC Componentis filtered by low pass filter (LPF).
`Circuit Diagram of low pass filter is shown in Fig. 8. The Cor-
`ner Frequency of low pass filter is 0.16 Hz. As a result, the
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`Original
`Signal
`
`BC Component
`
`FIGURE 8. Circuit diagram of low pass filter.
`
`LPF canfilter fluctuating pulse wave (AC Component) and
`high frequency noise such as interference from daylight lamp
`and electromagnetic interference from power source.
`The AC Componentis through a band passfilter and an
`amplifier. Circuit diagram of Band Pass filter (BPF) and
`amplifier (shown with LED and Photo-transistor) is given
`in Fig. 9. The Center Frequency of the BPFis 1 Hz, which is
`close to the normal pulse frequency of a person. Asa result,
`the BPF can filter the signal that is caused by light going
`through the body tissue and vein (DC Component) as well
`as high frequency noise such as interference from electro-
`magnetic interference from power source and daylight lamp.
`As the AC componentof the signal is too small (about 1 mV),
`an amplifier is used to amplify it for further processing.
`
`av
`
`3v
`
`Component i}
`
`I
`
`Amplified OC
`
`FIGURE 9. Circuit diagram of band pass filter and amplifier (shown with
`LED and photo-transistor).
`
`The AC and DC signals of red and infrared light are used
`together to compute the oxygen saturation by equation (4).
`The computation can be carried out by the application inside
`a smart phone.
`The ACsignal is also used to obtain the pulse waveform.
`Bycalculating the cycle of pulse waves, we can get the heart
`rate. The AC signal presenting the pulse wave has other values
`of further medical research, which can be developed by other
`applications in the smart phone.
`
`Vil. SYSTEM IMPLEMENTATION AND
`EXPERIMENTAL RESUALTS
`
`In Fig. 10, the overall system implementation is shown. The
`supply voltage to the prototype is 3V.
`The experimental
`results of the AC pulse signals
`obtained by red LED and infrared LED are shown
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`The Journal for rapid epen access publishing
`G. Pang, C. Ma: Neo-Reflective Wrist Pulse Oximeter
`
`
`
`
`
`FIGURE 12. The pulse signal obtained by the infrared LED.
`
`
`
`110
`
`linear equation
`5
`el
`drati
`Mey es
`quadratic equation
`SO-
`
`|
`4
`
`a
`
`=
`
`4
`
`4
`
`2
`
`Bor
`
`fOr
`
`60+
`
`SOP
`
`40
`a6
`
`aa
`
`FIGURE 13. The R curve modeled by two relationships.
`
`In the calibration process, the linear equation becomes
`
`SpO, = 130.4—44.4R
`
`Tf modeled by second-order, the quadratic equation is
`
`SpO, = 106.9 — 3.9R — 15.6 R”
`
`It can be seen that the quadratic (second-order), nonlinear
`relationship can provide a better fit with the data.
`
`Vill. CONCLUSIONS
`
`This paper describes the developmentof a new kind of wrist
`pulse oximeter. It is different from the conventional finger
`pulse oximeter due to the following reasons:
`1. The device has a novel design of the sensor module. The
`optical source has two LEDs(red andinfrared). The receiver
`is consisted of two photodiodes. A new optical structure is
`designed to house the sensor module components so as to
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`FIGURE 10. The photo of the overall implementation.
`
`in Fig. 11 and Fig. 12 respectively. A section of the recorded
`data verses time are shown. The two recorded photoplethys-
`
`
`mogramsboth display the cardiovascular cycle clearly.
`
`FIGURE 11. The pulse signal obtained by the red LED.
`
`Figure 11 showsthe initial pulse wave signalafter filtering.
`The system has detected the pulse wave signal efficiently,
`which indicates that the incident light can indeed visit the
`arterial vessel. The DC components and the AC components
`of the red and infrared light intensity can be obtained with
`proper processing of the signals. The oxygen saturation can
`then be calculated by the ratio of the light intensity change
`rates under the two different wavelengths.
`The calibration of an oxygen oximeter can be a
`complicated task. Experimental procedures are used to
`obtain the parameters required in equations (4) and (5).
`An oxygen saturation simulator is used and by doing some
`experiments, data relating the oxygen saturation and the
`R values are obtained. The Index 2XL SpO2 simulator by
`Fluke Biomedical [12] has been used. Figure 13 shows the
`empirical R to oxygen saturation curve under the linear and
`quadratic relationships.
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`G. Pang, C. Ma: Neo-Reflective Wrist Pulse Oximeter
`
`obtain improved operational efficiency when collecting the
`wrist pulse signals.
`2. The location in the wrist for placing the sensor module of
`the pulse oximeteris also unique. In order to obtain desirable
`wrist pulse signals, the sensor module is designed to be placed
`in an inner part of the wrist. The special design of its concave
`shape helps the sensor module to stay in the chosen location
`of the wrist.
`
`[8] Q. Cai, J. Sun, L. Xia, and X. Zhao, “Implementation of a wireless pulse
`oximeter based on wrist band sensor,” in Proc. 3rd Int. Biomed. Eng.
`Inform. (BMEI), Oct. 2010, pp. 1897-1900.
`[9] S. B. Duun, R. G. Haahr, K. Birkelund, and E. V. Thomsen, “A ring-
`shaped photodiode designed for use in a reflectance pulse oximetry sensor
`in wireless health monitoring applications,” FEEE SensorsJ., vol. 10, no. 2,
`pp. 261-268, Feb. 2010.
`[10] K. Li and S. Warren, “‘A wireless reflectance pulse oximeter with dig-
`ital baseline control for unfiltered photoplethysmograms,” [EEE Trans.
`Biomed. Circuits Syst., vol. 6, no. 3, pp. 269-278, Jun. 2012.
`{11] T. Zaman, P. A. Kyriacou,and S.K. Pal, “Free flappulseoximetry utilizing
`3. The device is a genuine wrist pulse oximeter as opposed
`reflectance photoplethysmography,” in Proc. 35th Annu. Int. Conf. IEEE
`Eng. Med. Biol. Soc. (EMBC), Jul. 2013, pp. 4046-4049.
`to the usual finger pulse oximeter. It has a very simple
`(2007). Fluke Biomedical, Cleveland Calibration Lab., Cleveland,
`[12]
`hardware structure without even a display because all data
`
`OH, USA. [Online]. Available:—http://assets.fluke.com/manuals/
`processing, calculations and display will not be carried out
`index2xlumeng0000.pdf, accessed Dec. 12, 2014.
`on the wearable wrist device.
`
`is found that a
`it
`4. During the calibration process,
`second-order relationship between the R value and the oxygen
`saturation can provideabetter fit with the data.
`In the future, a Bluetooth module can be developed to
`transmit all sensor data to an application of a nearby smart-
`phone, which would perform all those tasks. As a result
`of using minimal hardware for the device and the use of a
`flexible software solution for handling the sensor data, the
`cost of device will be very low. It is expected that the final
`cost can be only around a fraction of the cost of a typical
`finger oximeter in the market.
`
`
`
`GRANTHAM PANGreceived the Ph.D. degree
`from the University of Cambridge, Cambridge,
`U.K., in 1986. He was with the Department of
`Electrical and Computer Engineering, University
`of Waterloo, Waterloo, ON, Canada, from 1986
`to 1996, and then joined the University of Hong
`Kong, Hong Kong. Since 1988, he has authored
`over 70 journal papers and 120 international
`conference papers. He holds five U.S. patents,
`one European patent, and one Chinese patent.
`His research interests include biomedical informatics, visual surveillance,
`machine vision for surface defect detection, optical communications,
`logistics, intelligent control, and expert systems.
`
`reflective pulse oximetry,”
`
`REFERENCES
`for use with
`“Device
`[1]
`I. Sarussi,
`U.S.Patent 20070038 050, Feb. 15, 2007.
`[2] Y. Mendelson and B. D. Ochs, “Noninvasive pulse oximetry utilizing skin
`reflectance photoplethysmography,” JEEE Trans. Biomed. Eng., vol. 35,
`no. 10, pp. 798-805, Oct. 1988.
`[3] ¥. Mendelson, J. C. Kent, B. L. Yocum, and M.J. Birle, “Design and
`evaluation of a new reflectance pulse oximeter sensor,” Med. Instrum.,
`vol. 22, no. 4, pp. 167-173, Aug. 1988.
`[4] Y. Mendelson and C. Pujary, “Measurement site and photodetector size
`considerations in optimizing power consumption of a wearable reflectance
`pulse oximeter,” in Proc. 25th Annu. Int. Conf. IEEE Eng. Med. Biol. Soc.,
`Sep. 2003, pp. 3016-3019.
`[5] Y. Mendelson, C. Pujary, and M. Savage, “Minimization of LED power
`consumption in the design of a wearable pulse oximeter,” in Proc. IASTED
`Conf. Biomed. Eng., 2003, pp. 249-254.
`[6] H. Santha, N. Stuban, and G. Harsanyi, “Design considerations of small
`size reflective type pulse oximeter heads in special applications,” in Proc.
`ist Conf: Electron. Syst. Integr. Technol., Sep. 2006, pp. 404-408.
`[7] G. Di, X. Tang, and W. Liu, “A reflectance pulse oximeter design using
`the MSP430F149," in Proc. IEEE/ICME Int. Conf. Complex Med. Eng.,
`May 2007, pp. 1081-1084.
`
`
`
`CHAO MA received the M.Sc. degree in elec-
`trical engineering from the University of Hong
`Kong, Hong Kong, in 2014. Heis currently with
`Valeo Inc., Troy, MI, USA. He was with Dr. Pang
`in the design of the neo-reflective wrist pulse
`oximeter in 2011, and then with Dr. Hofmann
`in the design of maximum power point
`track-
`ing circuit for solar panels and energy harvesting
`backpacks in 2013 and 2014. His current research
`interests are in lidar, active safety, auto-parking,
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