Piezologist: A Novel Wearable Piezoelectric-based Cardiorespiratory Monitoring System

Transcription

1 Piezologist: A Novel Wearable Piezoelectric-based Cardiorespiratory Monitoring System Mahmoud Al Ahmad Electrical Engineering Department United Arab Emirates University Al Ain, United Arab Emarites m.alahmad@uaeu.ac.ae Soha Ahmed Electrical Engineering Department United Arab Emirates University Al Ain, United Arab Emarites soha_ahmed@uaeu.ac.ae Abstract In this paper, the design, prototyping and software development of a novel wearable cardiorespiratory parameters monitoring sensor and software applications is illustrated. Piezologist is an unobtrusive chest worn device. It comprises a patch-type sensor and a mobile application. The sensor utilizes piezoelectric material as the cardiorespiratory signal sensing component and MetaWearC board as the signal acquisition unit. The board also comes with Bluetooth Low Energy (BLE) support which is utilized for the raw signal transmission. The novelty aspect of the system rests on the fact that not only using a single cheap piezoelectric sheet common cardiorespiratory parameters (such as heart rate,, and cycles) were obtained similar to previous studies but ECG waveform and blood pressure data were also extracted successfully using the same sensor. In addition, sensor packaging design and prototyping and their effect on the acquired signal strength on one hand and the package size (volume and weight) on the other hand were studied and reported. For performance validation purpose, the developed cardiorespiratory monitoring sensor system results were validated against two commercial sensor devices namely 3-lead ECG sensor from ehealth sensor kit and Zephyr belt-type BioHarness sensor, and the results were reported herein. The validation process outcomes confirmed that the cardiorespiratory signals extracted using Piezologist conform with a heartbeat, respiratory cycle and ECG waveform obtained using the commercial sensors. Furthermore, a usability study was conducted to compare the user experience offered by Piezologist against commercially available sensors for measuring cardiorespiratory parameters. The study highlighted the potential that Piezologist will take over the commercial available belt-type, watch-type and 3-lead ECG sensors. Keywords Home healthcare, biomedical signal processing, cardiorespiratory, heart rate extraction, mobile healthcare, wearable sensors, Sensors, Vital signs, ECG waveform, Respiration rate, Heartbeats. I. INTRODUCTION Healthcare is an imminent issue that dares all nations. The world s population of elderly (over 65) is expected to become 761 million by 2025, which is twice what it was in 1990[1]. In UAE, 15% of the population will be elderly by 2020 [2]. Furthermore, according to the World Health Organization (WHO) report, for Noncommunicable Diseases (NCDs) Country Profiles 2014, NCDs account for 65% of total deaths in UAE[3]. NSDs otherwise known as chronic diseases comprise cardiovascular diseases (such as heart attacks and stroke), chronic respiratory diseases (such as chronic obstructive pulmonary disease and asthma) diabetes and cancers[4]. In fact, cardiovascular diseases constitute 30% of mortality in UAE while chronic respiratory diseases represent 3% of total deaths according to the same report[4]. A viable solution to these issues might be realized by deviating towards mhealth (also referred to as "telehealth" or "e-health" or ehealth or Health 2.0 or Medicine 2.0 or telemedicine ) which can be defined as the synergy between information and communication technologies, medical devices and sensors to enable real-time and remote monitoring of patient s health status. This was made possible in recent years because of the advancement in information and communication technologies as well as the huge improvement made in body sensors technologies. This information collected via the mhealth devises, sensors, and technology can be sent directly to the patient to empower him in order to take the right decision regarding his health or it can be sent to the patient-healthcare provider represented in the hospital s physician and nurses or even to the patient s guardian to enable them to monitor the patient s health condition from long distance or even alarm them if the patient is going through a critical condition that requires immediate medical intervention. Standard vital signs include heart rate, respiratory rate, body temperature, and blood pressure. They are frequently measured by physician or nurses to evaluate individuals health status[5]. Cardiorespiratory analysis and monitoring are commonly exploited to assess human health condition in general or to diagnose specific disease in particular such as sleep disorders[6]. Sudden and unexpected alteration of cardiorespiratory parameters is usually associated with the evolution of diseases, or the declaration of medical emergencies such as sudden infant death syndrome (SIDS), sudden cardiac death, asphyxia, or obstructive sleep apnea (OSA) [5]. In this study, Piezologist system for extracting cardiorespiratory parameters such as heart rate, blood pressure, Electrocardiography (ECG) signal,, respiration period, exhalation period and inhalation period will be highlighted. The paper is structured as follows. In section 2, a review of previous similar work is briefly presented. Section 3 describes the system architecture, design, software development and sensor unit packaging and their consequences on performance /18/$ IEEE

2 TABLE I. SUMMARY OF RESEARCH STUDIES CONTRIBUTIONS RELATING TO CARDIO AND RESPIRATORY PARAMETERS EXTRACTIONS Ref. Physiological signals [7] Heart rate [8] [5] [9] [6] [10] [11] heartbeat and respiration respiration, SCG and heart sounds The usability testing is presented in Section 4. In sections 5 we report the result attained and discuss their significance and implications. Section 6 concludes the study and outline future research direction. II. LITERATURE REVIEW In this section, measurement methods and material utilized in previous studies relating to cardiorespiratory analysis and extraction will be summarized. Physiological parameters extraction based on non-piezoelectric material and piezoelectric one is examined. Finally, problems with existing methods and devices will be highlighted A. Non-piezoelectric based methods Numerous techniques have been developed to measure heart rate and data in non-invasive and unconstrained manners[6]. The non-invasive methods include the following: electrocardiogram (ECG), which is widely utilized to analysis cardiac parameters[5]. For the respiration measurement, an NTC thermistor is commonly used. The thermistor is usually placed below the nose. It measures the variation in resistance resulting from temperature alteration between expiration and inspiration [11]. Besides detecting nasal air flow to measure respiration signal, thorax movement sensing was also analyzed to this end. It measures the transthoracic impedance variations due to the respiratory movements via electrodes[5]. As for the unconstrained methods, it was reported that in-sleep respiration monitoring can be achieved using visual information captured by cameras[6]. Components extraction CS a Tool sensors Main contribution Filtering bandpass: 10-30Hz for ECG Filtering bandpass: cardiac: 12-20Hz respiration: Hz Filtering bandpass: heart rate: Hz, respiratory signals: Hz Filtering bandpass: Hz for the respiratory signal; 1-50 Hz for the SCG signal and Hz for heart sounds. wavelet transformation: Empirical mode decomposition bandpass filter Butterworth filter Filtering: heart rate bandpass: 0.8 Hz to 2 Hz. Respiration rate: low pass filter with cutoff frequency of 0.5 Hz was used NFC Double face bed mattress doublesided tape Accelerometer, ECG electrodes PVDF polymer film piezoelectric polyvinylidene fluoride (PVDF) polymer-based patch belt PVDF piezo film Blueto oth Bed mattress and sheets Belt and hand band enclosed inside a textile pocket novel flexible AlN piezoelectric film sensor PVDF film PVDF and EMFi The robust (Instantaneous Heart Rate) monitor Prove the possibility of recording heart rate and respiration using a piezoelectric transducer design of curved structure of PVDF film in the sensor patch was capable of generating better signals simultaneously monitor respiration, seismocardiogram and heart sounds noninvasive and unconstrained measurement of respiration and heartbeat during sleep AIN designed double-sided arch and thin-shell structures of the PVDF Comparison between PVDF and EMFi a. CS= Communication Standard B. Piezoelectric based methods The utilization of piezoelectric materials in general and Polyvinylidene fluoride (PVDF) in particular to capture cardiorespiratory signal has received huge research interest in recent year[5][6][8][9][10][11]. PVDF characteristics such as flexibility, affordability, thin structure, and light weight made it an ideal solution for manufacturing portable and disposable physiological signal sensors [5]. Researchers have chosen various location and types to place these sensors (see Table 1). Some chose non-invasive methods such as wrist-band[10], belt [9][10] or chest patch [5][7] while others have moved one step further and made their technology unconstrained as well. Particularly, they placed the piezoelectric sensor on the bed mattress or the bed sheet to free the patient from any constraint[6][8] (see Table 1). C. Issues with existing measurement methods and material Each and every cardiorespiratory signal acquisition method and material has its own pros and cons. This section highlights some of the shortcomings that might obstruct their wide adaptation, especially in homes. First, non-invasive methods such as ECG, thorax movement sensing or NTC thermistor are cumbersome. Patient or caregiver has to deal with leads, wires or the placement of uncomfortable instrument below the nose[5]. Second, it is documented that commercial wearable ECG devices are susceptible to noise due to the close proximity of their electrodes in comparison to their medical counterpart[7]. This is due to the design constraint for wearable sensors which limit their size and weight [7]. Third, in these systems getting a good SNR necessitate the acquisition

3 TABLE II. COMPARISON BETWEEN PIEZOLOGIST AND COMMERCIALLY AVAILABLE BIOMETRIC MEASUREMENT WEARABLES Name Zephyr belt-type BioHarness sensor Dimensions LxWxH S: cm M:84 104cm L: to 129cm Type Weight Biometric Chest belt QardioCore 18.5 x 8.7 x 0.9 cm Chest belt Checkme Pro Motiv Kardia 8.5x5.5x1.3cm Diameter range: cm Thickness 0.254cm wristband circumference cm, sensor model: 8.2 x 3.2 x 0.35 cm, 3 cm stainlesssteel electrodes Chest belt, wristband, handheld 130g including batteries 64g HRV, RR, BM HRV, RR, BM HRV, SpO2, BM, SBP, PR, PI Monitoring mode Targeti ng Price continuous AT, HM $631.4 continuous AT, HM $449 Not optimized for continuous monitoring Ring < 1.55 g HR, BM continuous wristband, phone clip handheld 18 g or 30 grams Eko DUO 11.9 x4.7 x16 cm handheld 108 grams MC x 3.4 x 0.3 cm patch 7g Piezologist 2.7 x 2.7 x 0.8 cm patch 6g ECG, HR, BM ECG, heart sounds ECG, EMG, BM HRV, RR, BM, PP, SBP,DBP 30-second recording duration Specific recording interval AT, HM, SM, DD AT, HM,SM HM, DD HM $649 $199 $ Or $199 + membersh ip fees $299 continues HM, AT $2,500 continues AT, HM, SM, DD $190 of the signal while the user is at rest which makes it impractical for daily use[7]. Fourth, using non-piezoelectric method entangle the use of two different sensors one to acquire the cardio signal and the other to capture the respiratory signal. This, of course, will affect the size of the resulting system as well as the concurrency of the acquired data[5]. In other words, using two or more sensors to collect cardiorespiratory signal will require a powerful processing unit to coordinate the acquisition of different signals from different sources. Fifth, ECG electrodes were reported to cause skin allergies which makes them inconvenient for the extended wearing period[10]. Unconstrained methods such as the utilization of cameras for extracting cardiorespiratory signal is expensive in terms of the sensing units (ex. camera) and the processing unit required to perform image analysis which is usually powerful and power hungry. Furthermore, such systems are typically bulky, and non-portable which will hinder patient mobility and hence will be impractical for home use[6]. The majority of piezoelectric-based cardiorespiratory monitoring system utilizes PVDF[5][8][9] [10][11]. However, making an electric connection using PVDF is usually a cumbersome process[6]. Soldering is extremely hard using this material and as a result, conductive epoxy or spring clips are used as alternatives[6]. Those alternative solutions also cause a further problem such as stability and fatigue durability[6]. Although unconstrained piezoelectric-based sensors are perceived as user-friendly since they are freeing the user from being attached to wires, sensors, and electrodes, they have their own issues and limitations. First, the physical properties of the bed or the bed sheets, the sensor is embedded within, affect the flux of pressure fluctuation[6]. Second, the usability of such sensors will be limited to sleep disorder research or specific medical conditions studies such as sudden infant death syndrome or obstructive sleep apnea [5]. D. Comparison with exciting commercial biometric measurement sensors Our system features a small-sized, lightweight, low cost, usable, portable and reliable cardiorespiratory monitoring device. The high quality of our wearable medical sensor makes it convenient for remote health monitoring and disease detection or even prevention. Table 2 features a comparison between our system and commercially available cardiorespiratory systems. The comparison is concerned mainly with products that can measure two or more of the following biometrics concurrently: ECG, Electromyography (EMG), temperature (TEMP), heart rate (HR), heart rate variability (HRV), respiratory rate (RR), peripheral capillary oxygen saturation (SpO 2), systolic blood pressure (SBP), diastolic blood pressure (DBP), plus pressure (PP), perfusion index (PI), body movement (BM). Also, the domains we are interested in encompass Activity Tracking (AT), sleep monitoring (SM), health monitoring (HM), disease detection (DD). In terms of dimension and weight, Motiv is the smallest activity tracking and heart tracking sensor available in the market; Piezologist comes second. Nevertheless, the functionality provided by Piezologist by far exceeds the once provided by Motiv. Furthermore, Motiv (ring-type sensor) requires customers to buy sizing set to get the right fit. Besides, customers complained about the high number of false detection due to the sensor location. Specifically, they reported while they are sitting and tying the sensor mistakenly consider they are walking.

4 or application. Yet, wearable patient monitoring sensors remain superior to traditional medical devices and apparatus. Physicians diagnose patients directly without getting a handle on minor tasks such as leads positioning and wires getting in the way. Similarly, patients are enjoy moving around and performing their daily tasks. Wearable sensors made it possible to acquire clinical valid data without restraining patient movement or sacrificing their comfort and freedom. Fig. 1. Sensor structure and dimensions With reference to sensor type, MC10 and Piezologist are both patch-type which is preferable in comparison to the other types of sensors. However, MC10 main problem is its extremely high cost which is due to two reasons. First, the sophisticated software front end provided as part of the service which includes mobile app and web portal. Second, the remote data access capability provided by the system. But unlike MC10, Piezologist will offer both capabilities as optional service. In respect of the number of biometric monitored, Zephyr belt-type BioHarness sensor, QardioCore, and Checkme Pro provide a comprehensive list of biometric data. Nevertheless, each sensor has some issues. For instance, Zephyr belt-type BioHarness sensor ECG signal is not correct and the temperature value is computed using algorithm rather than being measured. QardioCore, as reported on their own website, is not suitable for pectus excavatum (sunken chest). Checkme Pro provides a single-channel ECG, it is not optimized for continuous monitoring and to acquire an accurate blood pressure readings calibration is required. Not to mention, the discomfort resulting from wearing a belt-type sensor consciously as the case for both Zephyr belt-type BioHarness sensor and QardioCore. Furthermore, the comprehensive and impressive biometric data provided by the three sensors come with a very high price tag, unlike Piezologist. With regard to price, both Kardia and Eko DUO are listed for a reasonable price. Yet both sensors do not support continues monitoring. Furthermore, patients need clinicians to interrupt the results. This explains the membership cost for Kardia owners. On the other hand, Eko DUO is currently working on developing a smart analysis system that will help interrupt the data for the layperson. To conclude, overall these devices proved the feasibility of monitoring various biometric data concurrently but they are either expensive, uncomfortable or complex either in creation III. SYSTEM DESCRIPTION, ARCHITECTURE, AND DESIGN In this section, Piezologist hardware specification details and sensor case design prototypes will be presented. A. Sensor specification details Piezologist utilizes a piezoelectric ceramic material. Lead zirconate titanate (PZT) is a polycrystalline piezoelectric material. Its structure consists of various crystallites. Those crystals produce an electrical current as a result of mechanical stress. Ceramics exhibiting piezo-electric properties belong to the group of ferroelectric materials. The sensor specification can be found in Table 3. The sensor used in Piezologist is from murata TM model number 7BB-20-6L0. The sensor structure and dimensions can be seen in Fig. 1 B. Microcontroller specification details Piezologist utilizes a coin-sized commercial off the shelf (COTS) microcontroller named MetaWearC from mbientlabtm. Table 4 illustrate the microcontroller specification details. C. Mobile application prototype The mobile application prototype developed to allow the user views the status of various biometrics and we are presently working to incorporate more metrics. Currently, the application allows the user to view various heart associated metrics such as HR, SBP, DBP, and PP( See Fig. 2). Furthermore, a respiration-related matrix such as RR, respiration period, Inhalation period, and exhalation period. Also, ECG characteristics such as the duration in seconds of the PR interval, QT interval, QRS interval, and RR interval. Besides, the app displays the extracted waveform for respiration signal, a heart rate signal and the ECG signal. The blood pressure values were extracted following Taradeh et al work[12]. In addition, the ECG waveform was extracted using Al Ahmad method[13]. The HR and PP were extracted using Al Ahmad and Ahmed work[14]. TABLE III. SENSOR SPECIFICATION DETAILS Resonant Frequency (khz) Resonant Impedance (ohm) Capacitance (nf) Plate Size dia. D Element Size dia. A Electrode Size dia. B Thickness T Plate Thickness T Plate Material 6.3 ±0.6kHz 1000 max 10.0 ±30%[1kHz] Lead Wire:AWG32 Length (50mm)

5 TABLE IV. MICROCONTROLLER SPECIFICATION DETAILS Processor Transceiver Flash program memory RAM I/O Expansion ADC mode ARM Cortex -M0 32 bit 2.4 GHz 256 kb 16 kb Digital I2C Bus 4Analog/Digital Pins 1 Digital Pin 10-bits 9-bits 8-bits D. Sensor case design and prototype In the first model developed (Error! Reference source not found. (a)) the sensor was placed on the top of the case, a circular plate was designed to hold the double face adhesive tape. This plate was attached to the case via winding. In the second model (Fig. 3 (b)), we reduced the circular plate size and we changed the securing mechanism to the snap-fit enclosure. Then, we decided to distance the sensor from the skin, we also decided to remove the circular plate to reduce the size. Furthermore, we changed the back case into a small led that snap-fit into the top case as illustrated in Fig. 3 (c). Distancing the sensor from the skin resulted in a small SNR. Therefore, we modified the design to place the sensor in the top of the case illustrated in Fig 3 (d). The adhesive tape is then attached to the sensor surfaces. IV. USABILITY STUDY Fifteen participants (10 females and 5 males) were recruited for product testing and user experience (UX) study. The study was conducted to compare Piezologist against two wearables (a belt-type and watch-type sensors). We will abstain from mentioning the two wearables trademark names for a legal purpose. The three wearable devices functionality, price, supporting applications and usage know-how were explained to the participants. Furthermore, participants were given a chance to interact with the three products and clarify any misconception. Participants ages ranged from years(mean=, standard deviation=). All had bachelor s degree, 2 pursuing a master degree and 13 had master s while 11 are pursuing a doctorate degree. Four participants (3 males, 1 female) reported using personal wellness tools such as food journaling tools, activity tracking or sleep tracking tools. Fig. 3. Piezologist case design and prototype evolution: a) first model b) second model c) third model d) the fourth model The majority of participants agreed that the three wearable devices, applications and functionality to be useful, innovative and needed. Specifically, more than 70% of the participants had a good first impression for the three wearable devices. More than 80% of participants viewed the three wearable devices to be innovative and necessary. As for the cost or VFM, 43 % considered the wristband wearable sensor to be a poor value for money; 21% considered it below average. Similarly, the belttype sensor 60% of participants considered being worth average value for money while 20% considered it to have below average value. The cost for the wristband and the belttype wearable were $1,600 and $600, respectively. Summary of this result is shown in Fig. 4. Illustrate in Fig. 5 the hardship associated with wearing the three wearable sensors especially for continues monitoring purpose (24/7). The belt-type sensor performed poorly. Precisely, 47% said it is hard to wear it while 7% said it is a very hard task. Furthermore, 84% of participant expressed their unwillingness to wear the product for an extended period of time. On the other hand, the wristband and Piezologist had almost comparable results, in terms of conveniences to wear and the likelihood of wearing it for a long number of hours. Product adaptation results shown in Fig. 6 conveys promising results for Piezologist in comparison to the other two wearable products. In fact, the result suggests that participants will likely embrace Piezologist technology and promote it. Fig. 2. Mobile Application prototype V. RESULT AND DISCUSSION To evaluate Piezologist performance, we implemented cardiorespiratory parameters logging mobile application. An Android smartphone was used for the application installation and data logging. Piezologist sensor patch adhered to the chest wall of the subject via a double-sided tape. For validation purpose and to acquire reference signals to compare our device performance against, recordings of respiration and ECG were performed via Two COTS devices namely 3-lead ECG sensor from ehealth sensor kit and Zephyr belt-type BioHarness sensor. Zephyr belt-type respiration sensor was wrapped around subject s epigastric region, and the locations of electrodes for ECG measurement were right shoulder

6 Fig. 4. UX comparison results between two wearables and Piezologist (positive), left waist (negative), and left shoulder (ground). Then, measurements from the three devices were carried out simultaneously to eliminate the round-to-round bias. The acquired signals and data were transmitted to the logging mobile application via Bluetooth Low Energy (BLE) and to the computer via a network socket connection. In the computer, a Matlab server application was developed to receive and store the collected data. The voltage signals obtained from Piezologist were acquired with a 10-bit A/D converter, and the sampling frequency was 100 Hz. The heart and s were determined from the obtained signals using our developed algorithm Table V summarizes six subjects corresponding measured vital signs using conventional equipment and methods along with weight, height, age and subject s gender. RR: in breath per minute; HB: heartbeat in beat per minute. RR and HB represent the measured value with conventional meters. ERR and EHB represent the extracted value using the Piezologist. RR% and HB% are the errors in RR rate and HB, respectively. TABLE V. PERFORMANCE COMAPRISON Case RR HB ERR EHB RR% HB% Fig. 5 Comparison between the three wearables daily use

7 Fig. 6 Comparison between the three products customers adaptability VI. CONCLUSION Piezologist is a wearable medical monitoring system intended to monitor various biometrics and specific cardiorespiratory diseases. The system consists of piezoelectric sensor utilized to measure various vital sign parameters, a desktop application for remote medical data analysis and risk detection, and mobile-based data presentation and activity analysis. A user experience survey, conducted on 15 participants, provided a clear indication of the usability of the device and the adaptability of the technology presented by the system. Furthermore, conventional meters were utilized to validate the medical data acquired using the sensor. In the future research, additional experiments are desirable to assess the suggested algorithm over other subjects, and comparison between healthy subjects and patients with cardio or/and respiratory diseases will help improve the proposed system. ACKNOWLEDGMENT The authors gratefully acknowledge the participation of the volunteers in this study. REFERENCES [1] P. E. Ross, Managing care through the air, IEEE Spectr., vol. 41, no. 12, pp , [2] R. Fernandez-Millan, J.-A. Medina-Merodio, R. Plata, J.-J. Martinez- Herraiz, and J.-M. Gutierrez-Martinez, A Laboratory Test Expert System for Clinical Diagnosis Support in Primary Health Care, Appl. Sci., vol. 5, no. 3, pp , [3] WHO Global status report on noncommunicable diseases 2014, WHO, [4] WHO Noncommunicable diseases, WHO, [Online]. Available: [Accessed: 25-Jan-2018]. [5] Y.-Y. Chiu, W.-Y. Lin, H.-Y. Wang, S.-B. Huang, and M.-H. Wu, Development of a piezoelectric polyvinylidene fluoride (PVDF) polymer-based sensor patch for simultaneous heartbeat and respiration monitoring, Sensors Actuators A Phys., vol. 189, pp , Jan [6] N. Bu, N. Ueno, and O. Fukuda, Monitoring of respiration and heartbeat during sleep using a flexible piezoelectric film sensor and empirical mode decomposition, Annu. Int. Conf. IEEE Eng. Med. Biol. - Proc., pp , [7] S. Izumi et al., A Wearable Healthcare System with a 13.7 μ A Noise Tolerant ECG Processor, IEEE Trans. Biomed. Circuits Syst., vol. 9, no. 5, pp , [8] J. Siivola, New noninvasive piezoelectric transducer for recording of respiration, heart rate and body movements, Med. Biol. Eng. Comput., vol. 27, no. 4, pp , [9] P. Bifulco et al., Monitoring of respiration, seismocardiogram and heart sounds by a PVDF piezo film sensor, Proc. Int. Work. ADC Model. Test., pp , [10] Y. Xin, X. Qi, C. Qian, H. Tian, Z. Ling, and Z. Jiang, A wearable respiration and pulse monitoring system based on PVDF piezoelectric film, Integr. Ferroelectr., vol. 158, no. 1, pp , [11] S. Kärki and J. Lekkala, Film-type transducer materials PVDF and EMFi in the measurement of heart and s., Conf. Proc. IEEE Eng. Med. Biol. Soc., vol. 2008, pp , [12] I. Saadat, N. Al Taradeh, M. Al Ahmad, and N. Bastaki, Noninvasive piezoelectric detection of heartbeat rate and blood pressure, Electron. Lett., vol. 51, no. 6, pp , [13] M. Al Ahmad, Piezoelectric extraction of ECG signal, Sci. Rep., vol. 6, no. 1, p , Dec [14] M. Al Ahmad and S. Ahmed, Heart-rate and pressure-rate determination using piezoelectric sensor from the neck, in th IEEE International Conference on Engineering Technologies and Applied Sciences (ICETAS), 2017, pp. 1 5.