IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 1, NO. 4, DECEMBER 2007 235Multichannel Ref l ective PPG Earpiece Sensor WithPassive Motion CancellationLei Wang, Member, IEEE, Benny PL Lo, and Guang-Zhong YangAbstract—This paper addresses the design considerations of anovel earpiece photoplethymograph (PPG) sensor and its in-situevaluation results. The device is encapsulated with multiple LEDsand photodiodes based on a ref l ective PPG design. A compact andlow power circuitry was developed for signal control and condi-tioning. PPG signals with an averaged ac/dc ratio of 0.001–0.01and 10% relative strength (compared to f i nger-based approach)were recorded from the superior and posterior auricular skins.The integrity of PPG signal and accuracy of heart rate detectionwere evaluated and the results showed that with adequate opticalshielding and the proposed passive motion cancellation, the devicewas able to reliably detect heart rate both during rest and mod-erate exercise. The proposed sensor design is low power, easy towear compared to conventional earlobe PPG devices.Index Terms—Body sensor networks, earpiece, heart-rate mon-itoring, photoplethysmograph (PPG), wearable sensors.I. I NTRODUCTIONCONTINUOUS and nonintrusive monitoring of cardio-vascular function is essential for the future developmentof pervasive healthcare [1]. Although extensive measurementof biomechanical and biochemical information is availablein almost all clinical settings, the diagnostic and monitoringutility is generally limited to the brief time points and perhapsunrepresentative physiological states such as supine and se-dated. Transient abnormalities, in this case, cannot always becaptured. Many cardiac diseases are associated with episodicrather than continuous abnormalities. These abnormalities areimportant but their timing cannot be accurately predicted andmuch time and effort has been wasted in trying to capture an“episode” with controlled monitoring. Important and even-lifethreatening disorders can go undetected because they occuronly infrequently and may never be recorded objectively.Thus far, a range of ECG monitoring devices that permit thecontinuous recording of heart-rate variability have been pro-posed. These include digital Holter devices for capturing ar-rhythmogenic events and chest-strip type devices for profes-sional sports and exercise [2]. Photoplethysmograph (PPG) de-vices have received signif i cant attention in recent years due totheir ease of being integrated with wearable, pervasive sensingdevices. PPG is based on the detection of subcutaneous bloodManuscript received June 11, 2007; revised September 12, 2007. This workwas supported in part by the U.K. EPSRC Biosensornet project and the UKDTI SAPHE project. Preliminary results of this paper were f i rst presented atBSN2007, Aachen, Germany, March 2007. This paper was recommended byAssociate Editor S. Leonhardt.The authors are with the Department of Computing, Imperial CollegeLondon, London, SW7 2RH, U.K (e-mail: g.z.yang@imperial.ac.uk).Color versions of one or more of the f i gures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identif i er 10.1109/TBCAS.2007.910900perfusion by shining lights through a capillary bed. As arterialpulsations f i ll the capillary bed, the volumetric changes of theblood vessels modify the absorption, ref l ection or scattering ofthe incident lights, such that the resultant ref l ective/transmittallights indicate the timing of the cardiovascular events, such asheart rate. The PPG sensor requires at least one light source(usually infrared) and one photo detector in its close proximity[3]. PPG sensors are commonly worn on f i ngers because of thehighest signal strength that can be achieved [4]. This conf i gura-tion, however,is not suitable for pervasivesensing as most dailyactivities involve the use of f i ngers.In recent years, different positioning of the PPG sensors hasbeen explored extensively. This includes body locations such asring f i nger [5], wrist [6], brachia [7], belly [8] and esophageal[9]. For commercial clinical PPG sensors, it is also common touse earlobe and forehead [10] as the anatomical regions of in-terest. An ear-clip can cause pain if it is used over a long periodof time, and neither approach is suitable for pervasive sensingapplications.Inparallelwiththesedevelopments,varioussignalprocessing methods have been investigated for minimizing mo-tion artifacts in wearable PPG signals. These include the use ofan accelerometer device to capture motion rhythms [11], [12]and the use of post-processing methods such as spatio-temporalPCA [13], Laguerre expansion [14], discrete wavelet analysisand f i lter banks [15], [16], Wigner–Ville distribution [17] andnonlinear methods [18] for artifact removal. The purpose of thispaper is to explore a ref l ective PPG sensor design that can beintegrated with the ear-worn activity recognition (e-AR) plat-form. The device is small, discreet to wear, and thus is suitablefor long-term pervasive monitoring.II. M ATERIAL AND M ETHODThe basic structure of the PPG sensor is illustrated in Fig. 1.The optical components used include LEDs DLED-660/905,DLED 660/940 from UDT and PDI-E835 from API, and photo-diodes PIN-8.0 from UDT, BPW34 and BPW34FS (with day-light f i lter) from Siemens. The active areas of these photodi-odes were 8 mm and 7 mm , respectively. For the dual-lightLED components, only the infrared light channel was used inour experiments. There were two sensing planes for the e-ARPPG sensor, which are perpendicular to each other as shown inFig. 1(a). The distances between the LED 1 and the photodiode1,theLED2andthephotodiode2,andtheLED1andthephoto-diode 3, were 10, 8, and 12 mm, respectively [19]. To assess therelative signal strength from the proposed e-AR sensor, a stan-dardpatchsensorwasalsoconstructedbyusingthesameopticalcomponents. Both sensors were equipped with multiple LEDsand photodiodes, operating by ref l ective lights [20], [21]. Foroptical shielding, all components were recessed into the base at1 mm depth, and the adjacent components were bridged with1932-4545/$25.00 © 2007 IEEE
