WO2018013656A1

WO2018013656A1 - Single wearable device system for measuring blood pressure

Info

Publication number: WO2018013656A1
Application number: PCT/US2017/041670
Authority: WO
Inventors: Milan RAJ; Bryan Mcgrane; Roozbeh Ghaffari
Original assignee: Mc10, Inc.
Priority date: 2016-07-12
Filing date: 2017-07-12
Publication date: 2018-01-18
Also published as: EP3484354A4; EP3484354A1; CN109688914A

Abstract

A system for sensing blood pressure in a user is disclosed. The system includes a wearable sensor device attached to a location on the body remote from the heart. The sensor device can include an accelerometer (or acoustic) sensor, an ECG sensor and/or a PPG sensor. A controller receives SCG, ECG and PPG waveforms from the sensors to determine a pulse arrival time, pulse transit time and pre-ejection period. The controller is operative to determine the blood pressure based on the determined pulse arrival time and the distance from the sensor device to the heart.

Description

SINGLE WEARABLE DEVICE SYSTEM FOR MEASURING BLOOD PRESSURE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of and priority to U.S. Provisional Application No. 62/361,330, filed July 12, 2016 and U.S. Provisional Application No. 62/414,892 filed October 31, 2016, entitled "SINGLE WEARABLE DEVICE SYSTEM FOR MEASURING BLOOD PRESSURE," which is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates generally to blood pressure monitoring. More particularly, aspects of this disclosure relate to using a single sensor attached to a body to measure blood pressure by determining pulse arrival time.

BACKGROUND

[0003] Integrated circuits (ICs) are the cornerstone of the information age and the foundation of today's information technology industries. The integrated circuit, a.k.a. "chip" or "microchip," is a set of interconnected electronic components, such as transistors, capacitors, and resistors, which are etched or imprinted onto a semiconducting material, such as silicon or germanium. Integrated circuits take on various forms including, as some non-limiting examples, microprocessors, amplifiers, Flash memories, application specific integrated circuits (ASICs), static random access memories (SRAMs), digital signal processors (DSPs), dynamic random access memories (DRAMs), erasable programmable read only memories (EPROMs), and programmable logic. Integrated circuits are used in innumerable products, including computers (e.g., personal, laptop and tablet computers), smartphones, flat-screen televisions, medical instruments, telecommunication and networking equipment, airplanes, watercraft and automobiles.

[0004] Advances in integrated circuit technology and microchip manufacturing have led to a steady decrease in chip size and an increase in circuit density and circuit performance. The scale of semiconductor integration has advanced to the point where a single semiconductor chip can hold tens of millions to over a billion devices in a space smaller than a U.S. penny. Moreover, the width of each conducting line in a modern microchip can be made as small as a fraction of a nanometer. The operating speed and overall performance of a semiconductor chip (e.g., clock speed and signal net switching speeds) has concomitantly increased with the level of integration. To keep pace with increases in on-chip circuit switching frequency and circuit density, semiconductor packages currently offer higher pin counts, greater power dissipation, more protection, and higher speeds than packages of just a few years ago.

[0005] The advances in integrated circuits have led to related advances within other fields. One such field is sensors. Advances in integrated circuits have allowed sensors to become smaller and more efficient, while simultaneously becoming more capable of performing complex operations. Other advances in the field of sensors and circuitry in general have led to wearable circuitry, a.k.a. "wearable devices" or "wearable systems." Within the medical field, as an example, wearable devices have given rise to new methods of acquiring, analyzing, and diagnosing medical issues with patients, by having the patient wear a sensor that monitors specific characteristics. Related to the medical field, other wearable devices have been created within the sports and recreational fields for the purpose of monitoring physical activity and fitness. For example, a user may don a wearable device, such as a wearable running coach, to measure the distance traveled during an activity (e.g., running, walking, etc.), and measure the kinematics of the user's motion during the activity.

SUMMARY

[0006] One example of a body function that is desirable to monitor is blood pressure and more specifically, to monitor resting and ambulatory blood pressure continuously over an extended period of time. An adequate blood pressure level is necessary for blood to travel from the heart around the body. A blood pressure that is too low (hypotension) may lead to inadequate blood flow, or hypoperfusion, of critical organs. A blood pressure level that is too high (hypertension) may, over time, have detrimental health effects on organs such as the heart (myocardial infarction), the brain (stroke, hemorrhage), and kidneys (renal failure). Unfortunately, traditional blood pressure measurement methods such as using an external cuff that constricts around a patient's arm cannot provide continuous monitoring of blood pressure. [0007] Another method for blood pressure measurement is the use of pulse arrival time (PAT). Pulse Arrival Time is the time it takes the pressure wave generated by the heart to travel a predefined point along a peripheral artery. The Pulse Arrival Time can be determined as the time difference between the time an electrical pulse signal (e.g., an ECG signal) of the heart is detected and the time that the resulting mechanical impulse of the pressure wave is detected along the peripheral artery. Knowing PAT, and the distance between the two points of measurement, one can determine the Pulse Wave Velocity (PWV). Knowing the basic properties of the blood vessel wall, and the PWV, one can calculate the mean pressure exerted on the wall along the arterial path, and thus obtain blood pressure. The PWV and the basic properties of the vessel wall can be pre-determined constants and incorporated in a function based on the PAT to determine blood pressure. The basic properties of the vessel wall can be determined empirically by measuring the blood pressure using a traditional blood pressure cuff to calibrate the sensor. Previous techniques to measure PAT required two sensors, one at the heart and another at a remote location. However, the use of two separate devices requires time synchronization with a high level of precision. Further additional sensors add to the expense of the blood pressure measurement.

[0008] Thus, there is a need for a reliable system for determining continuous blood pressure measurements using PAT. There is also a need for a system that uses a single sensor at a location on the body that may determine PAT. There is also a need for a system that may determine localized blood pressure in a specific region of a body.

[0009] According to one example, an attachable sensor device for sensing a pulse arrival time is disclosed. The sensor device includes an ECG sensor in contact with the skin of a user to measure an ECG waveform. A pulse sensor is in contact with the skin of the user to measure a pulse waveform. A controller receives the ECG waveform from the ECG sensor and the pulse waveform from the pulse sensor to determine a pulse arrival time based on a pulse detected from the ECG waveform and the pulse detected from the pulse waveform.

[0010] Another example is system for sensing blood pressure. The system includes a wearable sensor device attached to a location on the body remote from the heart. The sensor device includes an ECG sensor and a pulse sensor. A controller receives an ECG waveform from the ECG sensor and a pulse waveform from the pulse sensor to determine a pulse arrival time. The controller is operative to determine the blood pressure based on the determined pulse arrival time and the distance between the location and the heart.

[0011] Another example is a method of measuring blood pressure with an attachable sensor device. A sensor device including an ECG sensor and a pulse sensor is attached to a location on the skin of a user. Blood pressure is measured for a calibration value. A distance from the location to the heart is input. An ECG waveform and a pulse waveform is measured at the location. A pulse arrival time is determined based on the ECG waveform, pulse waveform and input distance from the location to the heart. Blood pressure is determined based on the pulse arrival time and calibration value.

[0012] The above summary is not intended to represent each embodiment or every aspect of the present disclosure. Rather, the foregoing summary merely provides an exemplification of some of the novel aspects and features set forth herein. The above features and advantages, and other features and advantages of the present disclosure, will be readily apparent from the following detailed description of representative embodiments and modes for carrying out the present invention when taken in connection with the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The disclosure will be better understood from the following description of exemplary embodiments together with reference to the accompanying drawings, in which:

[0014] FIG. 1 shows a system for a single wearable sensor for sensing blood pressure of a user;

[0015] FIG. 2 is a block diagram of the wearable sensor device in FIG. 1;

[0016] FIG. 3 is a graph showing the sampled ECG signal of the sensor device and the sampled PPG signal of the sensor device in FIG. 1;

[0017] FIG. 4A is a top view of an example of the single wearable sensor in FIG. 1 and highlights the specific features and components used to enable the system of FIG. 2;

[0018] FIG. 4B is a bottom view of an example of the single wearable sensor in FIG. 1 and highlights the skin-facing optical sensor used in FIG. 2;

[0019] FIG. 5 is a flow diagram showing the process of measuring blood pressure in the system in FIG. 1; [0020] FIG. 6 is a graph showing an ECG signal, a PPT signal and a seismocardiogram (SCG) signal;

[0021] FIG. 7 A is a graph showing a sampled ECG waveform and a SCG waveform; and

[0022] FIG. 7B is a close up view of the waveforms in FIG. 7A.

[0023] The present disclosure is susceptible to various modifications and alternative forms, and some representative embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMB ODEVIENT S

[0024] This disclosure is susceptible of embodiment in many different forms. There are shown in the drawings, and will herein be described in detail, representative embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the present disclosure and is not intended to limit the broad aspects of the disclosure to the embodiments illustrated. To that extent, elements and limitations that are disclosed, for example, in the Abstract, Summary, and Detailed Description sections, but not explicitly set forth in the claims, should not be incorporated into the claims, singly or collectively, by implication, inference, or otherwise. For purposes of the present detailed description, unless specifically disclaimed: the singular includes the plural and vice versa; and the word "including" means "including without limitation." Moreover, words of approximation, such as "about," "almost," "substantially," "approximately," and the like, can be used herein in the sense of "at, near, or nearly at," or "within 3-5% of," or "within acceptable manufacturing tolerances," or any logical combination thereof, for example.

[0025] FIG. 1 shows a user 100 that has attached at least one wearable sensor device such as a wearable sensor device 110 for the purpose of obtaining pulse arrival time to determine blood pressure. The wearable sensor device 110 only has to be attached at a location that is near a blood vessel such as an artery or a capillary bed. The wearable sensor device 110 can be remotely located from the heart of the user 100 but in sufficient proximity to measure heart signals and identify features such as the R-wave of the ECG waveform or the aortic opening feature of the SCG waveform from the heart beat. Only a single sensor device is needed to determine the blood pressure in this example.

[0026] Example locations may include the wearable sensor 110 on the mid left shoulder, a wearable sensor 112 on the lower left shoulder, a wearable sensor 114 on the left arm and a wearable sensor 116 on the left arm. The locations of the wearable sensors 110, 112, 114 and 116 shown in Fig. 1 are for illustration only. As explained above, blood pressure can be determined by sensing two or more signals as single location on the body, for example, using only a single wearable sensor. And the wearable sensors can be positioned in many other locations, such as, the right shoulder, arm, wrist, foot, neck, or thigh, etc. near a blood vessel and where the sensor can detect the PPG, ECG and/or SCG signals from the body.

[0027] The wearable sensor device such as one of the sensor devices 110, 112, 114, and 116 are in communication with a smart device such as a user device 130. The user device 130 can be a computing device such as a smart phone, a tablet, a laptop or desktop computer, a personal digital assistant or a network of computers (e.g., a cloud or a cluster). The user device 130 allows programming and control of the sensor devices 110, 112, 114 and 116. Although the sensor devices 110, 112, 114, and 116 are used for non-invasive blood pressure monitoring, they can have other measurement and sensing functions in relation to the user 100.

[0028] The time data from the sensor device such as one of the sensor devices 110, 112, 114 and 116 associated with the sensed pulses may be uploaded to a cloud storage server 160 periodically and analyzed by applications running on the cloud application server 162 using postprocessing techniques. The user may access these applications or the output of the applications by accessing the cloud server 162 such as through a website. As will be explained below, any of the sensors 110, 112, 114 and 116 may be used to capture blood pressure related data to determine pulse arrival time. The user device 130 determines blood pressure measurements based on the collected pulse arrival data. Alternatively, the determination of blood pressure measurements may be performed on one of the sensors 110, 112, 114, and 116 or on the cloud application server 162 based on pulse arrival data.

[0029] FIG. 2 shows an example of a wearable sensor device 200 such as the sensor device 110 in FIG. 1 in accord with aspects of the present disclosure. The wearable device 200 can provide conformal sensing capabilities, providing mechanically transparent close contact with a surface (such as the skin or other tissues or organs of the body) to provide measurement and/or analysis of physiological information from the user 100. According to some embodiments, the wearable device 200 senses, measures, or otherwise quantifies the motion of at least one body part of a user in proximity to the wearable device 200 (e.g., the sensors in the wearable device 200 can detect the motion of the body part, either directly or indirectly). Additionally, or in the alternative, according to some embodiments, the wearable device 200 senses, measures, or otherwise quantifies the temperature of the environment of the wearable device 200, including, for example, the skin and/or body temperature at or adjacent to the location that the wearable device 200 is coupled to the body of a user. Additionally, or in the alternative, according to some embodiments, the wearable device 200 senses, measures, or otherwise quantifies other characteristics and/or parameters of the body (e.g., human or animal body) and/or surface of the body, including, for example, electrical signals associated with cardiac activity (e.g., ECG), electrical signals associated with muscle activity (e.g., electromyography (EMG)), changes in electrical potential and impedance associated with changes to the skin (e.g., galvanic skin response), electrical signals of the brain (e.g., electroencephalogram (EEG)), bioimpedance monitoring (e.g., body-mass index, stress characterization, and sweat quantification), and optically modulated sensing (e.g., photoplethysmography (PPG) and pulse-wave velocity), and the like.

[0030] The wearable device 200 described herein can be formed as a patch. The patch can be flexible and/or stretchable, and can include stretchable and/or conformal electronics and conformal electrodes disposed in or on a flexible and/or stretchable substrate. Alternatively, the wearable device 200 can be rigid but otherwise attachable to a user. In accordance with some embodiments of the invention, the wearable device 200 can include portions that are stretchable and/or conformable and portions that are rigid. Thus, the wearable device 200 can be any device that is wearable on a user, such as coupled to the skin of the user, to provide measurement and/or analysis of physiological information of the user. For example, the wearable device can be adhered to the body by adhesive, held in place against the body by tape or straps, or held in place against the body by clothing. The more conformal the sensing device is more likely to stay in position on the skin and produce more reliable and accurate sensor data.

[0031] In general, the wearable device 200 of FIG 2 can include at least one processor 201 and one or more associated memory storage modules 203. The wearable device 200 can further include one or more sensors, such as an accelerometer 205 and/or a temperature sensor 213 and/or an optical sensor 217. The wearable device 200 can optionally include one or more wireless transceivers, such as transceiver 207, for communicating with other sensor devices such as the master sensor device 110 or other computing devices such as the user device 130 in FIG. 1. The wearable device 200 can also include a power source 209 that provides power for the components of the wearable device 200. In accordance with some embodiments, the wearable device 200 can be configured to draw power from a wireless connection or an electromagnetic field (e.g., an induction coil, an NFC reader device, microwaves, and light).

[0032] The processor 201 can be a controller that is configured to control the wearable device 200 and components thereof based on computer program code. Thus, the processor 201 can control the wearable device 200 to measure and quantify data indicative of temperature, motion and/or other physiological data, and/or analyze such data indicative of temperature, motion and/or other physiological data according to the principles described herein.

[0033] The memory storage module 203 can be configured to save the generated sensor data (e.g., the time when a pulse in blood flow is sensed, accelerometer 205 information, temperature sensor 213 information, or other physiological information, such as ECG, EMG, etc.) or information representative of acceleration and/or temperature and/or other physiological information derived from the sensor data. Further, according to some embodiments, the memory storage module 203 can be configured to store the computer program code that controls the processor 201. In some implementations, the memory storage module 203 can be volatile and/or non-volatile memory. For example, the memory storage module 203 can include flash memory, static memory, solid state memory, removable memory cards, or any combination thereof. In certain examples, the memory storage module 203 can be removable from the wearable device 200. In some implementations, the memory storage module 203 can be local to the wearable device 200, while in other examples the memory storage module 203 can be remote from the wearable device 200. For example, the memory storage module 203 can be internal memory of a smartphone such as the user device 130 in FIG. 1 that is in wired or wireless communication with the wearable device 200, such as through radio frequency communication protocols including, for example, WiFi, Zigbee, Bluetooth®, medical telemetry and near-field communication (NFC), and/or optically using, for example, infrared or non-infrared LEDs. In such an example, the wearable device 200 can optionally communicate (e.g., wirelessly) with a user device 130 such as a smartphone via an application (e.g., program) executing on the smartphone.

[0034] In some embodiments, the sensor data (e.g., including data generated by sensor and data derived from the sensor data), including the temperature information, the acceleration information, and/or the other physiological information (e.g., ECG, EMG, etc.), can be stored on the memory storage module 203 for processing at a later time. Thus, in some embodiments, the wearable device 200 can include more than one memory storage module 203, such as one volatile and one non-volatile memory storage module 203. In other examples, the memory storage module 203 can store the sensor generated information indicative of motion (e.g., acceleration information), temperature information, physiological data, or analysis of such information indicative of motion, temperature, physiological data according to the principles described herein, such as storing historical acceleration information, historical temperature information, historical extracted features, and/or historical locations. The memory storage module 203 can also store time and/or date information about when the information was received from the sensor.

[0035] Although described as the processor 201 being configured according to computer program code in the form of software and firmware, the functionality of the wearable device 200 can be implemented based on hardware, software, or firmware or a combination thereof. For example, the memory storage module 203 can include computer program code in the form of software or firmware that can be retrieved and executed by the processor 201. The processor 201 executes the computer program code that implements the functionality discussed below with respect to determining the on-body status of the wearable device 200, the location of the wearable device 200 on a user, and configuring functionality of the wearable device 200 (e.g., based on the on-body status and sensed location). Alternatively, one or more other components of the wearable device 200 can be hardwired to perform some or all of the functionality.

[0036] The power source 209 can be any type of rechargeable (or single use) power source for an electronic device, such as, but not limited to, one or more electrochemical cells or batteries, one or more photovoltaic cells, or a combination thereof. In the case of the photovoltaic cells, the cells can charge one or more electrochemical cells and/or batteries. In accordance with some embodiments, the power source 209 can be a small battery or capacitor that stores enough energy for the device to power up and execute a predefined program sequence before running out of energy, for example, an NFC or RFID based sensing device.

[0037] As discussed above, the wearable device 200 can include one or more sensors, such as the accelerometer 205, a temperature sensor 213, electrical contacts 215 (e.g., electrical contacts or electrodes), and/or an optical sensor 217. In accordance with some embodiments, one or more of the sensors, such as accelerometer 205, the optical sensor 217 and/or electrical contacts 215, can be separate components from the wearable device 200. That is, the wearable device 200 can be connected (by wire or wirelessly) to each sensor (e.g., accelerometer 205, temperature sensor 213, electrical contacts 215, and optical sensor 217). This enables the wearable device 200 to sense conditions at one or more locations that are remote from the wearable device 200. In accordance with some embodiments, the wearable device 200 can include one or more integral sensors in addition to one or more remote sensors.

[0038] The accelerometer 205 measures and/or generates acceleration information indicative of a motion and/or acceleration of the wearable device 200, including information indicative of a user wearing, and/or body parts of the user wearing, the wearable device 200. In accordance with one embodiment, the accelerometer 205 within the wearable device 200 can include a 3- axis accelerometer that generates acceleration information with respect to the x-axis, the y-axis, and the z-axis of the accelerometer based on the acceleration experienced by the wearable device 200. Alternatively, the wearable device 200 can include three independent accelerometers (not shown for illustrative convenience) that each generate acceleration information with respect to a single axis, such as the x-axis, the y-axis, or the z-axis of the wearable device 200. Alternatively, the wearable device 200 can include an inertial measurement unit (IMU) that measures the velocity, the orientation, and the acceleration using a combination of one or more accelerometers, gyroscopes, and magnetometers. Thus, although generally referred to herein as an accelerometer 205, the accelerometer 205 can be any motion sensing element or combination of elements that provides acceleration information.

[0039] According to some embodiments, the accelerometer 205 includes a detection range of ±4 times the force of gravity (Gs). However, the range can vary, such as being ±10 Gs or ±2 Gs. Further, the accelerometer 205 can have a sampling rate of 50 hertz (Hz) such that each second the accelerometer 205 generates 150 points of acceleration information, or 50 points within each axis. However, the sampling rate can vary, such as being 10 Hz to 250 Hz. [0040] According to some embodiments, one or more sensors of the wearable device 200, such as the accelerometer 205, can include a built-in temperature sensor, such as the temperature sensor 211 within the accelerometer 205. For example, the temperature sensor 211 within the accelerometer 205 can be used to calibrate the accelerometer 205 over a wide temperature range and to measure the temperature of the area of the body that the accelerometer 205 is coupled to. Other temperature sensors included with other device components can also be used. Other than the accelerometer 205, and temperature sensor 211, other subcomponents or elements of the wearable device 200 can include one or more microelectromechanical system (MEMS) components within the wearable device 200 that is designed to measure motion or orientation (e.g., angular-rate gyroscope, etc.). Alternatively, or in addition, the wearable device 200 can include a discrete temperature sensor, such as the temperature sensor 213 which can be positioned in a different location from the wearable device 200. The wearable device 200 can use the temperature information detected by the temperature sensor 211 and/or the temperature sensor 213 according to various methods and processes. For purposes of convenience, reference is made below to the temperature sensor 211. However, such reference is not limited to apply only to the temperature sensor 211, but applies to any one or more temperature sensors within or connected to the wearable device 200.

[0041] The electrical contacts 215 can be formed of conductive material (e.g., copper, silver, gold, aluminum, a hydrogel, conductive polymer, etc.) and provide an interface between the wearable device 200 and the skin of the user 100, for receiving electrical signals (e.g., ECG, EMG, etc.) from the skin. The electrical contacts 215 can include one or more electrical contacts 215, such as two electrical contacts 215, electrically connecting the skin of the user 100 to an amplifier circuit that can be part of an analog front end circuit 216, to amplify and condition electrical signals (e.g., ECG, EMG, etc). With two electrical contacts 215, one contact can be electrically configured as a positive contact and the other contact can be electrically configured as a negative contact. However, in some aspects, there may be more than two electrical contacts, such as four electrical contacts 215 (e.g., two positive and two negative electrical contacts), six electrical contacts 215, etc.

[0042] The optical sensor 217 can measure the photoplethysmography (PPG) signal when placed on the skin's surface, allowing for the monitoring of various biometrics including, but not limited to, heart rate, respiration, and blood oxygen measurements. The optical sensor 217 can include one or more light emitters that can emit red, green, infrared light or a combination thereof and one or more optical transducers (e.g., photodiode, CCD sensors). Using the one or more optical transducers, the optical sensor 217 can sense the wavelength of the reflected light. In this example, the optical sensor 217 illuminates the skin and the reflected light changes intensity based on the concentration of oxygen in a blood vessel such as an artery or a capillary bed. Thus, a pulse can be detected as a change in the amount of the reflected light due to a change in the concentration of oxygen in a blood vessel and thus the reflected light detected by the optical sensor 217. Of course other sensors can be included on the wearable device 200 to detect the pulse such as the accelerometer 205, a pressure sensor, a strain gauge sensor or an acoustic sensor to measure the mechano-acoustic signatures of the pulse.

[0043] In addition to the above-described components, the wearable device 200 can include one or more additional components without departing from the spirit and scope of the present disclosure. Such components can include a display (e.g., one or more light-emitting diodes (LEDs), liquid crystal display (LCD), organic light-emitting diode (OLED)), a speaker, a microphone, a vibration motor, a barometer, a light sensor, a photoelectric sensor, or any other sensor for sensing, measuring, or otherwise quantifying parameters and/or characteristics of the body. In other embodiments of the invention, the wearable device 200 can include components for performing one or more additional sensor modalities, such as, but not limited to, hydration level measurements, conductance measurements, and/or pressure measurements. For example, the wearable device 200 can be configured to, or include one or more components that, perform any combination of these different types of sensor measurements, in addition to the accelerometer 205 and temperature sensor 211.

[0044] Referring back to the temperature sensor 211, according to some embodiments, the primary purpose of the temperature sensor 211 is for calibrating the accelerometer 205. Accordingly, the temperature sensor 211 does not rely on direct contact to an object to detect the temperature. By way of example, the temperature sensor 211 does not require direct contact to the skin of a user when coupled to the user to determine the skin temperature. For example, the skin temperature affects the temperature information generated by the wearable device 200 without direct contact between the temperature sensor 211 and the skin. Accordingly, the temperature sensor 211 can be fully encapsulated and, therefore, be waterproof for greater durability. The thermal conductivity of the encapsulating material can be selected to control the ability of the temperature sensor 211 to detect the temperature without direct contact.

[0045] In accordance with some embodiments, a user's blood pressure can be measured using the wearable sensing device 200.

[0046] The form factor of the wearable device 200 allows positioning and repositioning of the sensor devices at different locations on the body of the user 100 in order to achieve the highest quality of data. In this example, the sensor device 110 placed on the shoulder of the user 100 in FIG. 1 is set in an electrocardiogram (ECG) sensing mode in order to receive ECG signals and determine the time-point of a pulse signal feature (e.g., associated with the R-wave) in the ECG from the heart of the user 100. Coincident with the generation of the R-wave ECG signal, the blood is pumped through blood vessels by the heart sending the pulse throughout the blood vessels. The sensor device 110 can also capture the time-point of the pulse associated with the pressure-wave of the pulse using a photoplethysmography (PPG) sensor or accelerometer. The pulse arrival time can then be determined from the ECG waveform and the pulse arrival (e.g., using the PPG signal data) at the location of the sensor device 110. The wearable device 200 can provide high quality ECG and PPG data as it conforms to the body and is coupled to the skin of the user 100 without bands or other fastening devices placing pressure on the arterial wall which would alter the measurement. This tight coupling also reduces the motion artifacts. The accelerometer 205 can be used to cancel out noise seen by the PPG sensor.

[0047] As explained above, a method of blood pressure measurement based on pulse arrival time using the sensor devices such as the wearable device 200 may be employed. The concept for the measurement of pulse arrival time relies on the use of both the analog front-end (AFE) amplifier 216 in conjunction with electrodes 215 and a PPG sensor such as the optical sensor 217 in a location that is distal from a subject's heart (e.g. upper shoulder, wrist, leg, etc.). Given these two signals, the systems according to the invention can determine a data endpoint known as Pulse Arrival Time (PAT). This data has correlation to a subject's blood pressure and requires only one device as the salient sensing modalities are completely integrated into the wearable device 200. In this example, the sensor device 110 in FIG. 1 measures electrocardiogram (ECG) data of the user 100 generated from the heart via the electrical contacts 215 in FIG. 2. The sensor device 110 in FIG. 1 also measures the pulse from photoplethysmography (PPG) taken from the optical 217 sensor. Of course other types of sensors on the sensor devices such as the accelerometer, a pressure sensor, a strain gauge sensor or an acoustic sensor may be used to detect the pulse.

[0048] FIG. 3 is a graph 300 showing a visualization of the data streams from the sensor device 110 attached on the shoulder of the user 100 in FIG. 1. A first trace 310 is the output of the ECG sensed from the electrodes 215 of the sensor device 110. In this example, the ECG of the sensor device 110 is sampled at 250 Hz and digitized to 16 bits. A second trace 320 is the PPG output of the optical sensor 217 of the sensor device 110. In this example, the optical sensor is sampled at 400 Hz at 18 bits. An electrical wave is detected at a first time point 322 on the first trace 310. A pulse pressure wave is detected at a second time point 324 (e.g., the foot of the pressure pulse - FPP) on the second trace 312. As shown in FIG. 3, the time difference between the two time points 322 and 324 constitute the pulse arrival time 326 between the heart and the shoulder of the user 100 in FIG. 1 through the artery.

[0049] This time difference is the PAT metric and, if the distance from the sensing location to the heart is known, the Pulse Wave Velocity (PWV) metric can be calculated as:

PWV = Distance from heart/PAT

PWV can be used with mathematical models or functions to compute blood pressure. For example, one model that can be used is the Moens-Korteweg and Hughes equations, which model the PWV dependence on the blood vessel's characteristics and a blood vessel's elasticity dependence on pressure, respectively.

Moens-Koreweg: PWV = jEh/2Rp

Hughes: E = E₀ exp (ζ P)

Where E is Young's modulus of elasticity of the vessel wall, h is the vessel wall thickness, R is vessel radius, p is the blood density, E₀ is the nominal Young's modulus of elasticity, ζ is a mathematical constant and P is the effective blood pressure.

[0050] In this example, the user 100 wears the sensor device 110 and enables the ECG/PPG recording feature via a smart device such as the user device 130 such as by a Bluetooth low energy (BLE) signal. The user may visualize the ECG and PPG waveforms in real time based on data received from the sensor device 110 using the display on the user device 130. In this example, the user device 130 then makes a determination regarding the quality of the PPG waveform and, if it determines the quality is low, the user device 130 can automatically adjust the parameters of the PPG sensor of the sensor device 110 to improve the signal quality. In this example, the user device 130 sends commands via a BLE signal to adjust the PPG sensor parameters such as the LED output power, duty cycle, color (wavelength frequency), sampling rate, and LED on off time to improve signal quality of the optical sensor for PAT measurement.

[0051] Once the waveforms are acceptable for use, the system can be calibrated. In calibration mode, the user device 130 instructs the user to be seated and wear a blood pressure cuff to measure blood pressure for calibration purposes. In accordance with some embodiments, the blood pressure cuff system can conform to ISO 81060-2 or similar to ensure accuracy of the calibration procedure. The goal of this procedure is to calibrate out the features of the Moens- Korteweg and Hughes equations relating to blood vascular characteristics (i.e., E, h, and R). This results in a persistent calibration as these parameters are constant on a time scale much longer than changes in blood pressure. After the blood pressure cuff calibration measurement is complete, the user stores this value in the memory of the user device 130 as variable, BPCA_L, S_ITTING_. The user device 130 sends a BLE command to the sensor device 110 to record ECG and PPG waveforms for 30 seconds. The data from the waveforms is sent to the user device 130 via a BLE signal and the user device 130 starts to calculate the PAT based on detected features of the waveforms shown in FIG. 3. The user is instructed to input the approximate distance between the sensor device 110 and the heart, allowing the user device 130 to calculate the value, PWVS_ITTING- This value serves as the initial calibration point for PWV against blood pressure. The distance between the sensor device 110 and the heart may also be determined from tables stored in the user device 130 reflecting the estimated distance between the location of the sensor and the heart based on the height, weight, age and other biometric measurements of a user that are input to the user device 130. The distance may also be determined automatically by use of the sensor device 110 or other sensors.

[0052] The user device 130 can also instruct the user to stand up to calculate another calibration point. Similar to the above procedure, the user must take a blood pressure calibration measurement using the cuff and input this value as BP_CA_L,S_TA_NDING- The smart device 130 then sends a BLE command to the sensor device 110 to record ECG and PPG waveforms for 30 seconds and, using the same distance input by the user, calculates PWVS_TAN_DING- Using the two cuff-based calibration points BPCAL,SITTING and BPCAL,STANDING and the associated PWV data, the user device 130 can calculate the constant parameters of the Moens-Korteweg and Hughes equations. This results in a fully defined mathematical model that can determine blood pressure for any PWV determined by the user device 130 from the pulse arrival time derived from the ECG and PPG waveforms.

[0053] FIG. 4A is a top view of the wearable sensor device 110 and FIG. 4B is a bottom view of the wearable sensor device 110 in FIG. 1. The bottom of the wearable sensor device 110 is in contact with the skin of the user. The wearable sensor device 110 includes a number of islands 410, 412, 414, 416, 418, and 420 as well as a battery 422. The islands 410, 412, 414, 416, 418, and 420, and the battery 422 are coupled together by flexible conductive interconnections 424. In this manner, the wearable sensor device 110 can be in conformal contact and flex with movements of a user's skin.

[0054] The islands 410, 412, 414, 416, 418, and 420 can be used to mount different components (e.g., integrated circuits) on the top surface of the wearable sensor device 110 as shown in FIG. 4A. In this example, a flash memory chip 430 is mounted on the island 410, a microcontroller 432 is mounted on the island 412 and a power management integrated circuit 434 is mounted on the island 414. The memory chip 430 in this example can be a 64 MB memory chip that is part of the memory storage module 203 in FIG. 2. In this example, a flexible tab 436 can be attached to another island 438. The island 438 can hold an optical sensor integrated circuit 440. As shown in FIG. 4B, the flexible tab 436 can be folded over to allow the optical sensor integrated circuit 440 to be positioned on the bottom of the island 414 in order to be in contact with the user's skin.

[0055] A motion sensor 6-axis internal measurement (IMU) integrated circuit 446 can be mounted on the island 416 that may be used for the accelerometer 205 shown in FIG. 2, a heart rate sensor integrated circuit 448 can be mounted on the island 418, and various electronic support components 450 can be mounted on the island 420.

[0056] As shown in FIG. 4B, the bottom of the islands 410 and 416 and the battery 422 can include four or more electrodes 460 to be in contact with the skin. The electrodes 460 can be electrically connected (e.g., either directly or through an amplifier) to the heart rate sensor integrated circuit 448. Of course, the electrodes 460 may be included as parts of other islands or in other locations on the islands other than those shown in FIG. 4B. The electrodes 460 constitute the electrical contacts 215 in FIG. 2. In this example, the battery 422 and the power management integrated circuit 434 constitute the power source 209 in FIG. 2. [0057] In this example, the microcontroller 432 is an onboard nRF51822 system on chip manufactured by Nordic Semiconductor that performs the functions of the processor 201 and transceiver 207 in FIG. 2. In this example, the heart rate sensor integrated circuit 448 is an ADS 1191 manufactured by Texas Instruments and can be an integrated part of the processor 201 in FIG. 2. The optical sensor integrated circuit 440 is a MAX30101 manufactured by Maxim Integrated to record the PPG waveform from the user's skin and serves as the optical sensor 217 in FIG. 2. As the optical sensor 440 needs to face the skin for proper signal acquisition, the component can be populated on the island 438 that is attached to the island 416 by the flexible tab 436. The flexible tab 436 is folded once at manufacturing, resulting in the skin-facing optical sensor shown in FIG. 4B. Alternatively, the optical sensor 440 can be mounted on the skin- facing side of one of the islands. The heart rate sensor integrated circuit 448 makes electrical contact with the subject's skin via electrodes 460 on the skin-facing side of the device as shown in FIG. 4B. With both these sensors facing the skin, the sensor device 110 is able to capture both ECG and PPG waveforms using a common system clock. The timing precision is a function of the digital bus speed set by the microcontroller 432 and supporting hardware. This level of timing precision is more than enough for blood pressure applications, where the limit can be as high as 5 milliseconds.

[0058] As explained above, once the ECG and PPG waveforms from the ECG and optical sensors can be recorded, the data can be transmitted to a smart device or equivalent such as the user device 130 in FIG. 1 via the BlueTooth® Low-Energy (BLE) radio for additional processing. The algorithm used to process the data by the user device 130 will conform to the diagram outlined in FIG. 3. The algorithm will identify the salient points of the respective waveforms and compare the time difference between them.

[0059] FIG. 5 is a flow diagram of the process of measuring blood pressure in the system shown in FIG. 1. Handshaking is performed between the user device 130 and the sensor device 110 (500). The handshaking involves sending identification information for the sensor device 110 and a MAC address to the user device 130. The user device 130 sets initial configuration data such as the location of the sensor device 110 on the body, the sampling rate and applicable storage parameters (502).

[0060] In this example, the sensor device 110 can continuously (or periodically) send the output of the ECG signal received from the electrical contacts 215 in FIG. 2 to the user device 130 (504). The output of the ECG signal can include one or more samples (e.g., 2, 3, 4, 5, 10, 20, or more samples) associated with a particular timestamp. The optical sensor on the sensor device 110 can also continuously (or periodically) send an output PPG signal to the user device 130 that can include one or more samples (e.g., 2, 3, 4, 5, 10, 20, or more samples) associated with a particular timestamp (506).

[0061] The user device 130 receives the ECG output waveform signal and the PPG output waveform signal from the sensor device 110. The user device 130 determines whether the quality of the PPG waveform is sufficient (508). If the quality of the PPG waveform is low (e.g., features, such as the foot, of the PPG waveform are not discernible), the user device 130 adjusts the parameters of the optical sensor to improve the signal quality (510). The PPG output waveform is then checked again to determine if the PPG waveform is sufficient (508). If the quality of the PPG waveform is still not above a predefined threshold, the user can be instructed to reposition the sensor device 110 on the body. If the quality of the PPG waveform is sufficient, calibration data is input in the form of a blood pressure measurement from a cuff device while standing and sitting (512). Based on the calibration data, the user device 130 determines the constant parameters for blood pressure determination (514).

[0062] The user device 130 senses the pulse based on the PQRS waves detected in the ECG waveform (516). The user device records the timestamp associated with the ECG waveform when the pulse is sensed. The user device 130 senses the pulse based on sensing a foot of the PPG signal from the optical sensor output waveform when the pressure wave reaches the location of the sensor device (518). The user device 130 records the timestamp associated with the PPG signal when the pulse is sensed. The pulse arrival time can be determined by the user device 130 based on the timestamps from the R-wave of the ECG waveform and the foot of the PPG signal from the PPG waveform (520). As shown in FIG. 3, the foot of the PPG waveform can be determined by determining a maximum first derivative of the waveform and determining the point of the tangent to the maximum first derivative. The blood pressure can be determined from the pulse arrival time by the user device 130 (522). The determined blood pressure can be stored in the memory of the user device 130 (524).

[0063] The process described in FIG. 5 is a real time determination of blood pressure by the user device 130. Some or all of the operations described above may be performed by the sensor device 110. Alternatively, the timestamp data and respective signals may be transmitted to the cloud server 162 and some or all of the above operations may be performed by the cloud server 162. Alternatively, the sensor device 110 can store the waveform data and transmit the stored data periodically to the user device 130 for measurement of the blood pressure at a delayed time.

[0064] The above described examples allow the use of a single sensor device to continuously, and non-invasively measure blood pressure from a subject without the persistent use of a blood pressure cuff. The use of a single patch mitigates the need for advanced network techniques, such as time synchronization across multiple devices, to generate accurate measurements. In addition, the sensor device decribed in FIGs. 1, 2 and 4A-4B has flexibility and mechanical compliance to the human body to allow for tight adhesion to the skin, giving high-fidelity ECG and PPG waveforms. This feature of the system allows for unobtrusive, comfortable use to the end user, enabling more frequent measurements that can help physicians better characterize their patients' vascular health through daily life.

[0065] The integration of ECG and PPG sensing modalities on a single wearable sensor device, such as a BioStamp TM device available from MCIO, allows for the ability to measure PAT almost anywhere on the body. This has advantages over prior art devices which are constrained to a single location (e.g. wrist, finger, chest, etc.). Using a wearable sensor device that may be attached at various locations around the body, one may characterize local blood pressure and gain insight into such diseases as Peripheral Arterial Disease. In addition, the use of a single device is an improvement over other systems that require at least two separate devices whose data needs to be time synchronized with a high level of precision. In using a single device for the measurement, the system described above does not require time synchronized data.

[0066] In accordance with some embodiments of the invention, an accelerometer (or an acoustic sensor) such as the accelerometer 205 of the wearable sensor device 200 shown in FIG. 2 can be used to detect and measure a biometric signal known as a seismocardiogram (SCG). The SCG signal can be detected and recorded by the accelerometer 205 of the wearable sensor device 200, for example, due to the tight mechano-acoustic coupling of the wearable sensor device 200 to the skin (or other organ) that enables the device to sense mechano-acoustic waveforms that propagate from the internal organs of the body to the surface of the skin. These waveforms are transduced by the onboard accelerometer 205 of the sensor device 200 into electrical signals that the device can measure, record and store or transmit to other devices such as the user device 130 in FIG. 1. In accordance with some embodiments, the SCG waveform can be more reliable than measurement of the ECG for sensors that are attached at points in the body that are relatively far from the heart or chest of the patient.

[0067] FIG. 6 is a diagram showing a ECG waveform 600, a SCG waveform 602 and a blood pressure waveform 604 during a time period of a heartbeat. As shown in FIG. 6, the salient features of the SCG waveform 602 correlate well to the characteristics of the heart and its pumping action as displayed with the ECG waveform 600. FIG. 6 shows a Q point 610, an R point 612 and a S point 614 on the ECG waveform 600. The SCG waveform 602 demonstrates an aortic opening point 620 that shows when blood from the left ventricle is ejected from the heart through the aortic valve. The BP waveform 604 includes a foot of the pressure pulse (FPP) point 624 that defines the onset of systole in the pressure pulse waveform. At a measurement site that is distal from the heart, the FPP 624, or "foot," of the BP waveform 604 marks when the pressure wave that pushes blood through the vasculature arrives at the distal location. The ECG waveform 600 may define a Pre-ejection Period (PEP) time 630 between the Q-point 610 and the AO point 620. A Pulse Arrival Time (PAT) 632 is shown between the R point 612 and the FPP point 624. A Pulse Transit Time (PTT) 634 is shown between the AO point 620 and the FPP point 624

[0068] In identifying these features shown in FIG. 6, the important aspects of the cardiac cycle can be determined. In particular, the Aortic Opening (AO) feature is the point when blood from the left ventricle ejects from the heart through the aortic valve, signifying the initial onset of blood propagating through the body. Knowing this point in time gives a more complete picture of the relationship between the mechanical and electrical characteristics of the heart, leading to insights about the difference between Pulse Arrival Time (PAT), Pulse Transit Time (PTT), and PEP (Pre-ejection Period).

[0069] The system 100 in FIG. 1 can use accelerometer data to determine the blood pressure using a sensor device 110 located remote from the heart. FIG. 7A is a graph of the amplitude of output waveforms from the sensor device 110, attached to the left shoulder of the user 100 as shown in FIG. 1, plotted against time. Specifically, FIG. 7A shows an ECG waveform 700 from the output of the analog front end (AFE) circuitry connected to the electrodes 215 of the sensor device 110. In this example, the ECG data from the sensor in the sensor device 110 can be sampled at 250 Hz and digitized to 16 bits and the ECG waveform 700 is derived from the ECG data. FIG. 7A also shows a SCG waveform 702 taken from the accelerometer of the sensor device 110. FIG. 7B is a close up view of the ECG waveform 700 and the SCG waveform 702. The ECG waveform 700 includes various valleys 710 that correspond to R-wave of the ECG signal. The SCG waveform 702 includes various peaks 712 that correspond to AO feature points.

[0070] The integration of the SCG waveforms with accompanying ECG and PPG waveforms gives a holistic picture of the dynamics of the vascular system. In particular, the inclusion of a motion-based sensor such as the accelerometer in combination with bio-potential (e.g., ECG) and/or optical (e.g., PPG) recording capabilities in a remote sensor such as the sensor device 110 in FIG. 1 provides significant advantages over other systems that cannot record all three sensing modalities from a common location. The main advantage is that it can be used to negate the PEP time from an overall PAT calculation. Whereas the PAT measurement may take into account mechanical processes of the heart that do not relate to blood propagation through the vasculature (e.g., left ventricular isovolumic contraction), a true PTT measurement ignores these processes and highlights only blood flow through the vasculature. A measured PEP time can be used by the system to quantify (and eliminate) the time associated with the mechanical process of the heart moving blood around through its chambers (i.e., atria and ventricles), leading to a more precise PTT value. For example, the Q-R interval time can be determined from the peaks of the Q-wave and the R-wave and the Q-R interval time can be subtracted from the PEP time to identify the portion of the PEP time that is included in the PAT measurement. The portion of the PEP time can be subtracted from the PAT measurement to more accurately determine the PTT measurement. The improved PAT calculation will be solely based on the dynamics of blood flow through the vasculature and not from the electro-mechanical dynamics of the heart.

[0071] In some embodiments, the aforementioned methods include at least those steps enumerated above. It is also within the scope and spirit of the present disclosure to omit steps, include additional steps, and/or modify the order of steps presented herein. It should be further noted that each of the foregoing methods can be representative of a single sequence of related steps; however, it is expected that each of these methods will be practiced in a systematic and repetitive manner.

[0072] While particular embodiments and applications of the present disclosure have been illustrated and described, it is to be understood that the present disclosure is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

CLAIMS What is claimed is:

1. An attachable sensor device for sensing a pulse arrival time, the sensor device comprising:

an ECG sensor in contact with the skin of a user to measure an ECG waveform;

a pulse sensor in contact with the skin of the user to measure a pulse waveform;

a controller receiving the ECG waveform from the ECG sensor and the pulse waveform from the pulse sensor to determine a pulse arrival time as a function of the ECG waveform and the pulse waveform.

2. The sensor of claim 1, wherein the controller is operative to determine a blood pressure measurement based on the pulse arrival time the distance between the location and the heart.

3. The sensor of claim 1, further comprising a memory for storing the ECG waveform and the pulse waveform.

4. The sensor of claim 1, further comprising a transceiver to transmit the ECG waveform and the pulse waveform to a user device.

5. The sensor of claim 4, wherein the user device is in communication with the transceiver to receive the ECG waveform and the pulse waveform, the user device operative to determine the blood pressure based on the distance of the sensor device from the heart of a user and the pulse arrival time.

6. The sensor of claim 1, wherein the pulse sensor is a PPG sensor that generates a pulse waveform.

7. The sensor of claim 1, wherein the pulse sensor is an accelerometer that generates an SCG waveform for the pulse waveform.

8. The sensor of claim 1, further comprising a plurality of islands each coupled to each other via stretchable interconnections, the plurality of islands including a first island attached to the controller, a second island including a surface facing the skin including the pulse sensor, and third and fourth islands each including an electrode coupled to the ECG sensor.

9. The sensor of claim 1, wherein the sensor device is attached to a shoulder of the body.

10. The sensor of claim 6, wherein the controller is operative to adjust the output power of the light source of the optical sensor based on the PPG waveform.

11. The sensor of claim 1, wherein the ECG waveform and the pulse waveform are monitored over a predetermined time period.

12. The sensor of claim 1, wherein an R-wave generated from the pulse is detected from the ECG waveform and a foot generated from the pulse is detected from the pulse waveform.

13. A system for sensing blood pressure, the system comprising:

a wearable sensor device attached to a location on the body remote from the heart, the sensor device including an ECG sensor and a pulse sensor;

a controller receiving an ECG waveform from the ECG sensor and a pulse waveform from the pulse sensor to determine a pulse arrival time; and

wherein the controller is operative to determine the blood pressure based on the determined pulse arrival time and the distance between the location and the heart.

14. The system of claim 13, further comprising a memory for storing the ECG waveform and the pulse waveform.

15. The system of claim 13, wherein the sensor device includes a transceiver to transmit the ECG waveform and the pulse waveform.

16. The system of claim 15, further comprising a user device is in communication with the transceiver to receive the ECG waveform and the pulse waveform.

17. The system of claim 13, wherein the pulse sensor is a PPG sensor that generates the pulse waveform.

18. The system of claim 13, wherein the pulse sensor is an accelerometer that generates an SCG waveform for the pulse waveform.

19. The system of claim 13, wherein the sensor device includes a plurality of islands each coupled to each other via stretchable interconnections, the plurality of islands including a first island attached to the controller, a second island including a surface facing the skin including the pulse sensor, and third and fourth islands each including an electrode coupled to the ECG sensor.

20. The system of claim 13, wherein the sensor device is worn on a shoulder of the body.

21. The system of claim 17, wherein the controller is operative to adjust the output power of the light source of the optical sensor based on the pulse waveform.

22. The system of claim 13, wherein the ECG waveform and the pulse waveform are monitored over a predetermined time period.

23. The system of claim 13, wherein an R-wave generated from the pulse is detected from the ECG waveform and a foot generated from the pulse is detected from the pulse waveform.

24. A method of measuring blood pressure with an attachable sensor device, comprising: attaching the sensor device including an ECG sensor and a pulse sensor to a location on the skin of a user;

measuring blood pressure at an initial time with a cuff device for a calibration value; inputting a distance from the location to the heart;

measuring an ECG waveform and a pulse waveform at the location; determining pulse arrival time based on the ECG waveform, pulse waveform and input distance from the location to the heart; and

determining blood pressure based on the pulse arrival time and calibration value.

25. The method of claim 24, further comprising storing the ECG waveform and the pulse waveform in a memory.

26. The method of claim 24, wherein the sensor device includes a transceiver to transmit the ECG waveform and the pulse waveform.

27. The method of claim 24, wherein the pulse waveform is measured by an optical PPG sensor including a light source and an optical detector.

28. The method of claim 24, wherein the pulse waveform is an SCG waveform measured by an accelerometer.

29. The method of claim 24, wherein the sensor device includes a plurality of islands each coupled to each other via stretchable interconnections, the plurality of islands including a first island attached to a controller, a second island including a surface facing the skin including a pulse sensor, and third and fourth islands each including an electrode coupled to an ECG sensor.

30. The method of claim 24, wherein the sensor device is worn on a shoulder of the body.

31. The method of claim 27, further comprising adjusting the output power of the light source of the optical sensor based on the PPG waveform.

32. The method of claim 24, wherein the ECG waveform and the pulse waveform are monitored over a predetermined time period.

33. The method of claim 24, wherein an R-wave generated from the pulse is detected from the ECG waveform and a foot generated from the pulse is detected from the pulse waveform.

34. An attachable sensor device for sensing a pulse arrival time, the sensor device comprising:

an ECG sensor in contact with the skin of a user to measure an ECG waveform;

a PPG sensor in contact with the skin of the user to measure a pulse waveform;

an accelerometer sensor in contact with the skin of the user to measure an SCG waveform:

a controller receiving the ECG waveform from the ECG sensor, the pulse waveform from the PPG sensor and the SCG waveform from the accelerometer sensor to determine a pulse arrival time as a function of the ECG waveform, the pulse waveform, and the SCG waveform.

35. The sensor of claim 34, wherein the controller is operative to determine a blood pressure measurement based on the pulse arrival time the distance between the location and the heart.

36. The sensor of claim 34, further comprising a memory for storing the ECG waveform, the pulse waveform, and the SCG waveform.

37. The sensor of claim 34, further comprising a transceiver to transmit the ECG waveform, the pulse waveform, and the SCG waveform to a user device.

38. The sensor of claim 37, wherein the user device is in communication with the transceiver to receive the ECG waveform, the pulse waveform, and the SCG waveform and the user device is operative to determine the blood pressure based on the distance of the sensor device from the heart of a user and the pulse arrival time.

39. The sensor of claim 34, further comprising a plurality of islands each coupled to each other via stretchable interconnections, the plurality of islands including a first island attached to the controller, a second island having a surface facing the skin including the PPG sensor, a third island including the accelerometer sensor, and fourth and fifth islands each including an electrode coupled to the ECG sensor.

40. The sensor of claim 34, wherein the sensor device is attached a shoulder of the body.

41. The sensor of claim 34, wherein the controller is operative to adjust the output power of the light source of the optical sensor based on the PPG waveform.

42. The sensor of claim 34, wherein the ECG waveform, the pulse waveform, and the SCG waveform are monitored over a predetermined time period.

43. The sensor of claim 34, wherein an R-wave is detected from the ECG waveform, a foot of the pressure pulse is detected from the pulse waveform, and aortic opening feature is detected from the SCG waveform.