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`11. HR Asada, et a]. “Mobile monitoring with wearable photoplethysmographic
`biosensors” was published in IEEE Engineering in Medicine and Biology Magazine,
`Vol. 22, Issue 3, IEEE Engineering in Medicine and Biology Magazine, Vol. 22,
`Issue 3 was published in May-June 2003. Copies of this publication were made
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`EXHIBIT A
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`10/25/2016
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`IEEE Xplore Document - Mobile monitoring with wearable photoplethysmographic biosensors
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`Volume: 22 Issue: 3
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`Mobile monitoring with wearable photoplethysmographic
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`Related Articles
`A Survey on
`Wearable
`Sensor-Based
`Systems for
`H53:
`
`AMON: a
`wearable
`multiparameter
`medical
`monim'i” -
`
`Awearable
`health care
`system based
`on knitted in..
`
`5
`Authoris]
`
`H H Asada '. P Shaltis 1 A Reisner; Sokwoo Rhee ', RC Hutchinson
`
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`Abstract:
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`Published in: IEEE Engineering in Medicine and Biology Magazine ( Volume: 22, Issue. 3, May-June 2003)
`
`Page(s): 28 - 40
`
`Date of Publication: 22 July 2003
`
`Print ISSN: 0739-5175
`
`INSPEC Accession Number: 7680816
`
`DOI: 10.1109/MEMB.2003.1213624
`
`Publisher: IEEE
`
`Sponsored by: IEEE Engineering in Medicine and Biology Society
`
`Contents
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`Download Citations
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`Wew References
`
`Wbs System Paradigm
`For novel healthcare applications to employ WBS technology, several system criteria must be met.
`The WBS hardware solution must be adequate to make reliable physiologic measurements during
`‘ H
`.
`_
`_
`_
`t
`I
`'
`I_
`actiVities ofdaily Iivmg or even more demanding Circumstances such as fitness training or military
`battle. There must exist data processing and decision—making algorithms forthe waveform data.
`These algorithms must prompt some action that improves health outcomes. Finally, the systems must
`be cost effective when compared with less expensive, lower technology alternatives.
`
`We address both technical and clinical issues ofwearable biosensors (WBS). First, design concepts ofa WBS are presented, with emphasis on the
`ring sensor developed by the author's group at MIT. The ring sensor is an ambulatory, telemetric, continuous health-monitoring device. This WBS
`combines miniaturized data acquisition features with advanced photoplethysmographic (PPG) techniques to acquire data related to the patient‘s
`cardiovascular state using a method that is far superior to existing fingertip PPG sensors. In particular, the ring sensor is capable of reliably monitoring
`a patient's heart rate. oxygen saturation. and heart rate variability. Technical issues, including motion artifact. interference with blood circulation, and
`battery power issues, are addressed, and effective engineering solutions to alleviate these problems are presented. Second, based on the ring sensor
`technology the clinical potentials of WBS monitoring are addressed.
`
`0004
`
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`Keywords
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`IEEE Keywords
`Biomedical monitoring, Biosensors, Wearable sensors, Patient monitoring, Telemetry, Data
`acquisition, Cardiology, Sensor phenomena and characterization, Heart rate measurement. Heart rate
`am
`http:/fieeexpl ore.i eee.org/docum entJiZi 3624/
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`Fu" Text
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`Abstract
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`Authors
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`IEEE Xpl ore Document - Mobile monitoring with wearable photoplethysm ographic biosensors
`
`Keywords
`
`INSPEC: Controlled Indexing
`electromyography. patient monitoring, biomedical transducers. biomedical telemetry. cardiovascular
`system. blood pressure measurement. plethysmography. electroencephalography
`
`INSPEC: Non-Controlled Indexing
`battery power. mobile monitoring, wearable photoplethysmographic biosensors, cardiovascular
`monitoring. ring sensor. ambulatory telemetric continuous health-monitoring device. miniaturized data
`acquisition features. cardiovascular state. oxygen saturation. heart rate variability. technical issues.
`motion artifact. blood circulation
`
`H.H. Asada
`
`Dept. of Mech. Eng.. MIT. Cambridge, MA. USA
`
`Related Articles
`
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`for Hea...
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`Paritelopoulos;..
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`
`Haruhiko Harry Asada is a Ford professor of mechanical engineering and director of the Brit and Alex
`d‘ArbeIoff Laboratory for Information Systems and Technology in the Department of Mechanical
`Engineering at Massachusetts Institute of Technology (MIT). He specializes in robotics. biomedical
`engineering. and system dynamics and control. His current research areas include wearable health
`monitoring. robotic aids for bedridden patients. vast DOF actuator systems. and multiphysics
`simulation. He received the 8.8.. M.S.. and Ph.D. degrees in precision engineering in 1973, 1975. and
`1979. respectively. all from Kyoto University. Japan. He was a visiting research scientist at the
`Robotics Institute of Carnegie—Mellon University from 1980 to 1981. Hejoined the Department of
`Mechanical Engineering at MIT as faculty in 1982 and became a full professor in 1939. He is a Fellow
`of ASME.
`
`Back to Top
`
`P. Shaltis
`
`Phillip Shaltis received the BA degree in physics from Albion College. Albion. MI, in 1999 and the
`BS. degree in mechanical engineering from the University of Michigan. Ann Arbor. MI in 2000. He will
`be finishing dual MS. degrees in mechanical and electrical engineering at MIT in 2003 and plans to
`continue work towards the Ph.D. degree in mechanical engineering at MIT. His research interests
`include biomedical instrumentation. biomedical signal processing. analog circuit design. and system
`analysis and control.
`
`| A. Reisner
`
`Andrew Reisner received the 3.5. in mechanical engineering and biological sciences at Stanford
`I Initlnruitu in 100’)
`the M n frnm l-lnnmrrl l/Indingl thnnl in 1007 and frninnd in amnrnnnmr mnflirinn
`http:/fieeexplorei eee.org/docum ant/1213624/
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`0005
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`10/25/2016
`
`Sokwoo Rhee
`
`Sokwoo Rhee received the 3.8. degree in mechanical engineering from Seoul National University,
`Seoul. South Korea, in 1995, and the MS. and Ph.D. degrees in mechanical engineering from MIT in
`1997 and 2000, respectively. He is currently the chieftechnology officer and vice president of
`technology at Millennial Net, Inc. in Cambridge, MA. He was also a postdoctoral research associate in
`the Department of Mechanical Engineering at MIT in 2000—2002. His research interests include
`biomedical instrumentation. ultra—low power wireless communication, system analysis, and control.
`
`R.C. Hutchinson
`
`Reginald C. Hutchinson received the 8.3. degree in mathematics from Morehouse College, Atlanta,
`GA; the 8.8. degree in mechanical engineering from the Georgia Institute of Technology, Atlanta,
`Georgia, in 1999; and dual M.S. degrees in mechanical engineering and electrical engineering from
`MIT, Cambridge, MA, in 2002. He is currently working as a consultant for Accenture in New York City.
`His research interests include design, control of electromechanical systems, controls and system
`dynamics, and analog and digital signal processing.
`
`IEEE Xplore Document - Mobile monitoring with wearable photoplethysmographic biosensors
`uriwvlany u.
`.WL, we WW. no... . .a. valu meuma. quIvUi u. Ivar, am. named n. mumgeucy “momma
`at the Harvard—affiliated emergency medicine residency program. He is presently an attending
`physician at the Massachusetts General Hospital in the Department of Emergency Medicine, an
`instructor at Harvard Medical School, and a visiting scientist at MIT. Dr. Reisner‘s research is oriented
`toward the intersection of diagnostic expert systems, medical sensor technology. and the clinical
`problem of circulatory shock.
`
`http:/fieeexpl ore.ieee.orgldocum ent11213624/
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`
`WearableTechnology
`
`©EYEWIRE
`
`Mobile Monitoring
`with Wearable
`Photoplethysmographic
`Biosensors
`
`MAY/JUNE 2003
`
`earable biosensors (WBS) will permit contin-
`uous cardiovascular
`(CV) monitoring in a
`number of novel settings. Benefits may be real—
`ized in the diagnosis and treatment of a number
`of major diseases. WBS,
`in conjunction with appropriate
`alarm algorithms, can increase surveillance capabilities for
`CV catastrophe for high-risk subjects. WBS could also play a
`role in the treatment of chronic diseases, by providing infor-
`mation that enables precise titration of therapy or detecting
`lapses in patient compliance.
`WBS could play an important role in the wireless surveil—
`lance of people during hazardous operations
`(military,
`fire-fighting, etc.), or such sensors could be dispensed during
`a mass civilian casualty occurrence. Given that CV physio-
`logic parameters make up the “vital signs” that are the most
`important information in emergency medical situations, WBS
`might enable a wireless monitoring system for large numbers
`of at-risk subjects. This same approach may also have utility
`in monitoring the waiting room of today’s overcrowded emer-
`gency departments. For hospital inpatients who require CV
`monitoring, current biosensor technology typically tethers pa-
`tients in a tangle of cables, whereas wearable CV sensors
`could increase inpatient comfort and may even reduce the risk
`of tripping and falling, a perennial problem for hospital
`patients who are ill, medicated, and in an unfamiliar setting.
`On a daily basis, wearable CV sensors could detect a
`missed dose of medication by sensing untreated elevated
`blood pressure and could trigger an automated reminder for
`the patient to take the medication. Moreover, it is important
`for doctors to titrate the treatment of high blood pressure,
`since both insufficient therapy as well as excessive therapy
`(leading to abnormally low blood pressures) increase mortal-
`ity. However, healthcare providers have only intermittent val-
`ues of blood pressure on which to base therapy decisions; it is
`possible that continuous blood pressure monitoring would
`permit enhanced titration of therapy and reductions in mortal—
`ity. Similarly, WBS would be able to log the physiologic sig—
`nature of a patient's exercise efforts (manifested as changes in
`heart rate and blood pressure), permitting the patient and
`healthcare provider to assess compliance with a regimen
`proven to improve health outcomes. For patients with chronic
`cardiovascular disease, such as heart failure, home monitoring
`employing WBS may detect exacerbations in very early (and
`
`H. HARRY ASADA,
`PHILLIP SHALTIS.
`ANDREW REISNER,
`SOKWOO RHEE, AND
`REGINALD C. HUTCHINSON
`
`Technical and Clinical Aspects of a Ring Sensor for
`Ambulatory, Telemetric, Continuous Health Monitoring
`in the Field, in the Hospital, and in the Home
`
`often easily treated) stages, long before the patient progresses
`to more dangerous levels that necessitate an emergency room
`visit and costly hospital admission.
`In this article we will address both technical and clinical is-
`sues of WBS. First, design concepts of a WBS will be pre-
`sented, with emphasis on the ring sensor developed by the
`author’s group at MIT. The ring sensor is an ambulatory, tele-
`metric, continuous health-monitoring device. This WBS com-
`bines miniaturized data acquisition features with advanced
`photoplethysmographic (PPG) techniques to acquire data re—
`lated to the patient’s cardiovascular state using a method that is
`far superior to existing fingertip PPG sensors [1]. In particular,
`the ring sensor is capable of reliably monitoring a patient’s
`heart rate, oxygen saturation, and heart rate variability. Techni-
`cal issues, including motion artifact, interference with blood
`circulation, and battery power issues, will be addressed, and ef-
`fective engineering solutions to alleviate these problems will be
`presented. Second, based on the ring sensor technology the
`clinical potentials of WBS monitoring will be addressed.
`
`WBS System Paradigm
`For novel healthcare applications to employ WBS technology,
`several system criteria must be met. The WBS hardware solu-
`tion must be adequate to make reliable physiologic measure-
`ments during activities of daily living or even more
`demanding circumstances such as fitness training or military
`battle. There must exist data processing and decision-making
`algorithms for the waveform data. These algorithms must
`prompt some action that improves health outcomes. Finally,
`the systems must be cost effective when compared with less
`expensive, lower technology alternatives.
`
`WBS Design Paradigm
`The monitoring environments for out-of-hospital, wearable
`devices demand a new paradigm in noninvasive sensor design.
`There are several design requirements central to such devices.
`Compactness, stability of signal, motion and other distur-
`bance rejection, durability, data storage and transmission, and
`low power consumption comprise the major design consider-
`ations. Additionally, since WBS devices are to be worn with-
`out direct doctor supervision, it is imperative that they are
`simple to use and comfortable to wear for long periods Of
`time. A challenge unique to wearable sensor design is the
`
`28
`
`IEEE ENGINEERING IN MEDICINE AND BIOLOGV MAGAZINE
`
`0739-5175/03/517.00©2003IEEE
`
`0007
`
`FITBIT, Ex. 1018
`
`

`

`WBS solutions, in various stages of
`
`technologic maturity, exist for measuring
`
`established cardiopulmonary “vital signs":
`
`heart rate, arterial blood pressure, arterial
`
`oxygen saturation, respiratory rate,
`
`temperature, and even cardiac output.
`
`MAY/JUNE 2003
`
`there are numerous WBS modalities that can offer physiologic
`measurements not conventional in contemporary medical mon-
`itoring applications, including acoustic sensors, electrochemi-
`cal
`sensors,
`optical
`sensors,
`electromyography
`and
`electroencephalography, and other bioanalytic sensors (to be
`sure, some of these sensors have well-established medical util-
`ity, but not for automated surveillance). These less established
`WBS modalities are outside the scope of this review.
`Wearable electrocardiogram systems represent the most
`mature WBS technology. Holter and related ambulatory
`electrophysiologic monitoring solutions have established util—
`ity in the diagnosis of cardiac arrhythmias. There has been
`substantial examination of this technology in the medical lit-
`erature, with excellent reviews available [10]. Temperature is
`technically trivial to measure using WBS, but the continuous
`monitoring of body temperature is only a soft surrogate for
`perfusion, and it lacks established utility outside of traditional
`clinical settings [1 l], [12]. There is not a satisfactory ambula-
`tory solution for cardiac output measurement; it has been
`shown that cardiac output can be extracted from thoracic
`bioimpedance measurements, although speaking and irregu-
`lar breathing, as well as posture changes and ambulation, can
`corrupt this signal. In the future, bioimpedance is likely to
`prove a powerful WBS modality, since the signal carries in-
`formation about pulsatile blood volumes, respiratory vol—
`umes, intracellular and extracellular fluid balances, and has
`been shown to enable tomographic imaging. Respiration can
`be measured using bioimpedance, chest wall geometry, and
`acoustic means. While the basic sensor technology exists for
`monitoring respiratory rate, it requires the conversion of a
`continuous waveform into an integer (breaths per unit time),
`or the imprecise conversion of the measured parameter into an
`estimated volumetric rate (liters of gas per unit time).
`Ambulatory systems for arterial blood pressure measure—
`ment exist. The portapres, employing the volume clamp tech-
`nique for measuring ABP, offers a continuous waveform. The
`technology cucumbers a finger and the wrist of the subject, is
`somewhat uncomfortable, and requires some expertise to set
`up for a subject (for instance, the finger cuff size must be care-
`fully matched to the finger). A more common WBS solution
`for 24-hour monitoring of ABP involves a portable version of
`the common oscillometric cuff that fits around the upper arm.
`This solution requires that the patient keep the monitored arm
`immobile while the cuff inflates for measurements. By report,
`this solution has been known to interfere with the sleep and
`other activities of monitored subjects (and has been reported
`to cause bruising of the arm at the cuff site) [I 3].
`
`trade-off between patient comfort, or long-term wearability,
`and reliable sensor attachment. While it is nearly needless to
`say that WBS technology must be safe, it should be noted that
`there have been tragic reports of serious injury resulting from
`early home monitoring technology [2]. Evolving regulatory
`guidelines for hospital and home monitoring technology can
`be found in the US. National Fire Protection Association
`Health Care Facilities Handbook.
`
`At the same time, the physiologic information generated
`by WBS technology must trigger some appropriate system ac-
`tion to improve health outcomes. Abnormal states must be ef-
`ficiently recognized while false alarms are minimized. This
`requires carefully designed WBS devices, as well as innova-
`tive postprocessing and intelligent data interpretation. Post-
`processing of sensor data can improve usability, as illustrated
`by recent improvements in pulse oximetry technology [31-[5].
`Data interpretation can occur in real time (as is necessary for
`detecting cardiovascular-related catastrophes) or offline (as is
`the standard-of-care for arrhythmia surveillance using Holter
`and related monitoring). Real-time alarm “algorithms” using
`simple thresholds for measured parameters, like heart rate and
`oxygen saturation, have demonstrated high rates of false
`alarms [6], [7]. Algorithms for off-line, retrospective data
`analysis are also in a developmental stage. Studies of novel
`automated “triage” software used to interpret hours of contin-
`uous noninvasive ECG data of monitored outpatients suggest
`that the software’s diagnostic yield is not equal to a human’s
`when it comes to arrhythmia detection [8], [9]. It will presum-
`ably require further
`improvements
`in WBS hardware,
`middleware, and software in order to fully exploit the promise
`of wearable ambulatory monitoring systems.
`It is important to bear in mind the present limitations of the
`technology, such as reliability, system complexity, and cost,
`but there is a Wide scope of exciting healthcare applications
`available for this technology, as will be discussed later in this
`article. WBS technology is a platform upon which a new para-
`digm of enhanced healtheare can be established. Considering
`that hardware solutions will
`inevitably become smaller,
`cheaper, and more reliable, and diagnostic software more so—
`phisticated and effective, it seems more a matter of when cost
`effectiveness will be achieved for WBS solutions, not if.
`
`Available WBS Monitoring Modalities
`WBS solutions, in various stages of technologic maturity, exist
`for measuring established cardiopulmonary “vital signs”: heart
`rate, arterial blood pressure, arterial oxygen saturation, respira-
`tory rate, temperature, and even cardiac output. In addition,
`
`IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE
`
`FITBIT, Ex. 1018
`
`

`

`No fully satisfactory WBS solution exists for ABP monitor-
`ing. Because this physiologic parameter has been the comer-
`stone of many decades of clinical and physiology practice, it
`will be important to develop future WBS solutions for monitor—
`ing ABP. It is also worth investigating surrogate measures of
`ABP that prove easier to measure, such as pulse wave velocity
`(which correlates well with degree of hypertension [14], [15])
`and the second-derivative of the photoplethysmograph. This ar-
`ticle focuses on a wearable ring pulse-oximeter solution, which
`measures the PPG as well as the arterial oxygen saturation. The
`PPG contains information about the vascular pressure wave-
`forms and compliances. Efforts to extract unique circulatory in-
`formation, especially an ABP surrogate,
`from the PPG
`waveform are discussed later in this article. The PPG provides
`an effective heart rate (measuring heart beats that generate
`identifiable forward—flow), useful for circulatory consider-
`ations though less useful for strict electrophysiologic consider-
`ations. For instance,
`the PPG signal may reveal heart rate
`variability, provided ectopic heart beats, which corrupt the as-
`sociation with autonomic tone, can be excluded.
`
`Development of a Wearable Biosensor—
`The Ring Sensor
`Technical Issues of PPG Ring Sensors
`Central
`to the ring sensor design is the importance of
`long-term wearability and reliable sensor attachment. Since
`continuous monitoring requires a device that must be
`noninvasive and worn at all times, a ring configuration for the
`sensor unit is a natural choice. Because of the low weight and
`
`
`
`MAY/JUNE 2003
`
`small size, rings are generally worn without removal more of-
`ten than watches. Additionally, recent studies have indicated
`that the finger is one ofthe best places for WES sensor attach-
`ment [16]. The primary vasculature of the finger is located
`near the surface and therefore makes it optimal for monitoring
`arterial blood flow using noninvasive optoelectronic sensors.
`Thus, a ring is ideal for long-term measurements. As will be
`illustrated in the following sections, the development of the
`ring sensor has stressed first an understanding of and then the
`subsequent elimination of front-end signal artifacts. By im-
`plementing a mechanical design that is sensitive to the true
`causes of signal corruption, significant improvements in over-
`all signal quality can be achieved and sensor effectiveness for
`various environments can be improved.
`Figure 1 shows the typical waveform of a photoplethys-
`mograph signal obtained from a human subject at rest. The
`signal comprises a large segment of dc signal and a small-am-
`plitude ac signal. The dc component of photon absorption re-
`sults from light passing through various nonpulsatile media,
`including tissue, bones, venous blood, and nonpulsatile arte-
`rial blood. Assuming that these are kept constant, a bandpass
`filter can eliminate the dc component. However, wearable
`PPG sensors do not meet this premise since, as the wearer
`moves, the amount of absorption attributed to the nonpulsatile
`components fluctuates. Power spectrum analysis reveals that
`this motion artifact often overlaps with the true pulse signal at
`a frequency of approximately 1 Hz. Therefore, a simple noise
`filter based on frequency separation does not work for PPG
`ring sensors to eliminate motion artifact.
`Furthermore, wearable PPG sensors are exposed to diverse
`ambient lighting conditions, ranging from direct sunlight to
`flickering room light. In addition, wearable PPG sensors must
`be designed for reduced power consumption. Carrying a large
`battery pack is not acceptable for long-term applications. The
`whole sensor system must run continually using a small bat-
`tery. Several ways to cope with these difficulties are:
`> secure the LEDs and the photodetector (PI) for short) at a
`location along the finger skin such that the dc component
`may be influenced less by the finger motion
`modulate the LEDs
`to attenuate the influence of
`uncorrelated ambient light as well as to reduce power
`consumption
`increase the amplitude of the ac component so that the
`signal-to-noise ratio may increase
`measure the finger motion with another sensor or a sec-
`ond PD and use it as a noise reference for verifying the
`signal as well as for canceling the disturbance and noise.
`In the following sections these methods will briefly be dis-
`cussed, followed by speeific sensor designs and performance
`tests. There are other techniques for reducing motion artifact
`for general-purpose PPG. These, however, are mostly signal
`processing techniques applicable to PPG intended for short—
`term use. The motion artifact problem we are facing in wear-
`able PPG design is different in nature; the source signal qual-
`ity must be improved before applying signal processing.
`Therefore, the focus must be placed on basic sensor design.
`
`>O
`
`
`
`LightAbsorption U0
`
`Pulsatile Arterial Blood
`Nonpulsatile Arterial Blood
`Venous Blood
`
`Tissue. Bone, Etc...
`
`Fig. 1. Illustrative representation of the relative photon
`absorbance for various sections of the finger. The dc compo,
`ment is significantly larger than the ac component.
`
`Control Volume 2
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`Digital Artery
`
`Fig.2. (a) For the reflective illumination method. movement of
`the photodiode relative to the LED (position 1 to position 2)
`leads to a photon path that no longer contains the digital ar-
`tery. (b) For the transmittal illumination method. movement of
`the photodetector relative to the LED still contains photon
`paths that pass through the digital artery.
`
`|EEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE
`
`Techniques for Reduced Motion Artifact
`Sensor Arrangement
`The location of the LEDs and a PD relative to the finger is an
`important design issue determining signal quality and robust—
`ness against motion artifact. Figure 2 shows a cross-sectional
`
`FITBIT, Ex. 1018
`
`

`

`31
`
`view of the finger with the ring sensor. The LEDs and PD are
`placed on the flanks of the finger rather than the dorsal and
`palmar sides. These locations are desirable for two reasons:
`> both flanks of fingers have a thin epidermal tissue layer
`through which photons can reach the target blood vessels
`with less attenuation
`the digital arteries are located near the skin surface paral—
`lel to the length of the finger.
`It should be noted that an arterial pulsation is not only
`greaterin magnitude than cutaneous pulsations but is also less
`susceptive to motion due to the naturally higher internal pres-
`sure. While the capillary collapses with a small external pres-
`sure on the order of 10~30 mmHg, the artery can sustain an
`external pressure up to 70~80 mmI-lg [17], [18]. Therefore,
`light static loads, such as contact with the environment, may
`not disturb the arterial pulsation.
`For these reasons, at least one optical device, either the PD
`or the LED, should be placed on one lateral face of the finger
`near the digital artery. The question is where to place the other
`device. Figure 2 shows two distinct cases. One case places both
`the PD and the LED on the same side of the fin-
`ger-base, and the other places them on opposite
`sides of the finger. Placing both the PD and the
`LED on the same side creates a type of reflective
`PPG, while placing each of them on opposite
`sides makes a type of transmittal PPG. In the fig-
`ure the average pathway of photons is shown for
`the two sensor arrangements. Although the exact
`photon path is difficult to obtain, due to the heter-
`ogeneous nature of the finger tissue and blood, a
`banana—shaped are connecting the LED and PD,
`as shown in the figure, can approximate its aver-
`age path [19]. Although these two arrangements
`have no fundamental difference from the optics
`point of view, their practical properties and per-
`formance differ significantly with respect to mo-
`tion artifact, signal-to-noise ratio, and power
`requirements [20H22].
`Reflective PPG needs more secure attach-
`ments of the LED and PD to the skin surface,
`when compared to transmittal PPG. Once an air
`gap is created between the skin surface and the
`optical components due to some disturbance, a
`direct optical path from the LED to the PD may
`be created. This direct path exposes the PD di-
`rectly to the light source and consequently
`leads to saturation. To avoid this short circuit,
`the LED light beam must be focused only in the
`normal direction, and the PD must also have a
`strong directional property (i.e., polarity), so
`that it is sensitive to only the incoming light
`normal to the device surface. Such strong di-
`rectional properties, however, work adversely
`when a disturbance pressure acts on the sensor
`bodies, since it deflects the direction of the
`LED and PD leading to fluctuations in the out-
`put signal. As a result, reflective PPG configu-
`rations are more susceptive to disturbances.
`In contrast, transmittal PPG configurations
`do not have the short circuit problems, since the
`LED and PD are placed on the opposite sides of
`the finger; no direct path through the air can be
`
`Reflective
`
`Start of
`Applied Motion
`
`
`
`Voltage(V)
`
`Fig. 3. Corruption of a continuous PPG waveform during the application of a
`simple chopping motion. Note that while the motion corrupts the reflective
`sensor signal. the transmittal sensor signal remains unaffected.
`
`LED Brightness
`
`Photodetector Output
`
`Data Sampling
`
`Short, Bright
`LED Pulse
`
`High-Speed
`Photo-
`detector
`
`Fig. 4. (a) The slow response time of the photodetector meant that the LED
`had to be modulated atlower frequencies for data sampling. to) A faster
`photodetector response time makes it possible to increase the modulation
`frequency of the LED.
`
`IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE
`
`MAY/JU NE 2003
`
`created. Additionally, this design allows us to use devices hav-
`ing a weak polarity, which is, in general, m