Non-invasive blood pressure and oximetry sensor, measurement method and wearable device

Abstract

The application discloses a non-invasive blood pressure and blood oxygen measurement sensor, a measurement method and a wearable device, and the sensor comprises an ultrasonic array sensor, a PPG sensor, an AD conversion module and an MCU unit; the ultrasonic array sensor comprises a plurality of piezoelectric ceramics, and the piezoelectric ceramics are fixed around the periphery of the PPG sensor; the measurement method comprises the following steps: (1) collecting a PPG signal; (2) collecting a motion artifact signal; (3) judging whether the motion artifact signal exceeds a threshold value and processing the PPG signal to obtain blood pressure and blood oxygen values or directly outputting the PPG signal to obtain blood pressure and blood oxygen values. The application detects the motion artifact of the PPG signal by means of the high sensitivity of the ultrasonic array, takes the signal as a reference signal, removes the motion artifact, obtains a high-quality PPG signal, and achieves the purpose of improving the pulse wave signal quality and the blood pressure and blood oxygen measurement accuracy, so that the blood pressure and blood oxygen measurement under the non-stationary state of the human body is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of biological signal sensors, in particular to a non-invasive blood pressure and blood oxygen measurement sensor capable of improving pulse wave signal quality, a wearable device comprising the sensor and a non-invasive blood pressure and blood oxygen measurement method. BACKGROUND

[0002] Wearable health monitoring technology has always played an important role in the field of medical health, and is an important means of preventing various chronic diseases. Among them, wearable non-invasive blood pressure and blood oxygen measurement technology is an important part, and its theoretical research and practical application are relatively in-depth and extensive.

[0003] As for blood pressure measurement, existing wearable non-tubing blood pressure measurement technology mainly includes mechanism-driven and data-driven parts. Mechanism-driven refers to relying on the mapping relationship between pulse wave itself or its changes and blood pressure to measure blood pressure, which is mainly divided into photoplethysmography (PPG) signal blood pressure estimation method, skin surface pressure wave estimation blood pressure wave, oscillograph method and pulse wave transmission time and blood pressure mapping relationship method. Data-driven refers to a method that relies on a large amount of pulse wave data to establish a mapping relationship between pulse wave and blood pressure, which is divided into machine learning methods relying on artificial pulse wave feature markers and deep learning methods of automatic feature learning. However, whether it is based on mechanism-driven or data-driven blood pressure measurement method, it is necessary to measure the pulse wave signal, so obtaining accurate pulse wave signal is the key to improving the accuracy of blood pressure measurement. For blood oxygen saturation measurement, the currently popular and mature method is to measure the size of blood oxygen content through transmission or reflection PPG, based on the theory that the light absorption coefficients of hemoglobin and oxygenated hemoglobin are different in different wavelength ranges, and the size of blood oxygen can be obtained from the Lambert-Beer law. Although the application of the Lambert-Beer law in transmission and reflection PPG is different, but no matter which way to measure blood oxygen, the quality of the collected pulse wave still has a great influence on the accuracy of blood oxygen measurement.

[0004] There are many existing technologies to improve the quality of pulse wave, such as the wristband blood pressure measurement device proposed by Jian Li et al. in Thin, soft, wearable system for continuous wireless monitoring of artery blood pressure. The device uses a micro-pump to pressurize to obtain a better pulse wave signal; Yunghua Kao et al. in Design and Validation of a New PPG Module to Acquire High-Quality Physiological Signals for High-Accuracy Biomedical Sensing. The distance between the photoelectric receiving tube and the LED is optimized to maximize the ratio of pulsatile (AC) and non-pulsatile (DC) components in the PPG waveform to obtain high-quality PPG signals. Both represent general methods to improve the quality of pulse wave signals, with their own advantages, but also have certain use defects, the former is less comfortable, and the latter can only improve the quality of the collected PPG signal, but cannot eliminate the motion artifact signal in the non-quiet state. In addition, baseline drift and motion artifact removal are important means to improve the quality of pulse wave signals. The existing motion artifact removal methods mainly rely on accelerometers, but accelerometers reflect the overall motion state of the measured part and cannot well reflect the contact between the sensor and the skin, so the use accuracy is not high. SUMMARY

[0005] The present application provides an ultrasonic array and photoelectric fusion non-invasive blood pressure and blood oxygen measurement sensor, motion artifact removal method and wearable device to solve the problem of poor signal quality caused by motion artifact affecting PPG signal in non-quiet state, which cannot be used for blood pressure and blood oxygen measurement. To solve the above technical problems, the technical solution provided by the present application is:

[0006] A non-invasive blood pressure and blood oxygen measurement sensor, comprising an ultrasonic array sensor, a PPG sensor, an AD conversion module and an MCU unit; the ultrasonic array sensor comprises a plurality of piezoelectric ceramics, which are fixed around the periphery of the PPG sensor; the PPG signal collected by the PPG sensor is input to the MCU unit; the MCU unit transmits a voltage signal to the piezoelectric ceramic to make the piezoelectric ceramic output an ultrasonic signal, the ultrasonic signal is reflected to cause the piezoelectric ceramic to expand and contract and generate an electric signal, the electric signal is converted into a digital signal by the AD conversion module and input to the MCU unit.

[0007] The design idea of the technical scheme is that the piezoelectric crystal is a material with special properties, when the voltage acts on the piezoelectric crystal, it will expand, and when the voltage is removed, it will shrink back to the original thickness. If the voltage is applied and removed quickly and repeatedly, the piezoelectric crystal will also expand and shrink quickly, and when the frequency exceeds 20 KHZ, an ultrasonic signal will be generated. Moreover, the piezoelectric crystal can also receive ultrasonic waves, the principle is the reverse process of generating ultrasonic waves. In addition, if the interface causing the reflection of the ultrasonic wave is in motion, the reflected ultrasonic signal will change in frequency due to the Doppler effect, and the piezoelectric crystal will reflect this change in the frequency of the generated voltage. In this way, a single piezoelectric crystal is a sensor that can emit and receive dynamically changing ultrasonic waves. The ultrasonic array sensor of the present application is composed of several piezoelectric ceramics (PZT), and the ultrasonic wave emitted by each piezoelectric ceramic is controlled by the MCU. The emitted ultrasonic wave bounces back to the piezoelectric ceramic on the skin surface, causing the piezoelectric ceramic to deform and generate a corresponding electric signal. Then the electric signal is converted into a digital signal by an AD converter and transmitted to the MCU. The high sensitivity of the ultrasonic array is used to detect the motion artifacts of the PPG signal. The MCU determines whether the skin interface is shaking according to the difference in frequency between the transmitted and returned electromagnetic waves, and uses this signal as a reference signal to filter out the motion artifacts, obtaining a high-quality PPG signal. The purpose of improving the quality of the pulse wave signal and the measurement accuracy of blood pressure and blood oxygen is achieved, so that the non-invasive blood pressure and blood oxygen measurement sensor can measure blood pressure and blood oxygen in a non-stationary state of the human body, and has the possibility and prospect of application on wearable devices.

[0008] As a further preferred embodiment of the above technical solution, the piezoelectric ceramic is provided with 8, which is arranged in a circular ring around the periphery of the PPG sensor.

[0009] As a further preferred embodiment of the above technical solution, a micro shield is arranged outside each piezoelectric ceramic, and all the shields are grounded. The piezoelectric ceramics work simultaneously, and there is a crosstalk problem (noise) between them. Therefore, a micro shield is used to eliminate the interference signals between the piezoelectric ceramics, to ensure the accuracy of data acquisition.

[0010] As a further preferred embodiment of the above technical solution, the PPG sensor comprises a combined light source and a photoelectric receiver; the MCU unit inputs a voltage to the combined light source to control the emission of light signals, and the reflected light signals are received by the light receiver and input to the MCU unit.

[0011] As a further preferred embodiment of the above technical solution, the photoelectric receiver is a silicon photocell that can convert the received light signal into an electric signal; and the combined light source is composed of a green light LED, a red light LED and an infrared light LED. The PPG sensor of the reflective combined light source is used to measure the PPG signal. The combined light source is composed of green light (520 nm), red light (660 nm) and infrared light (940 nm) LEDs. The blood pressure is estimated by the PPG signal collected by the green light LED, and the algorithm principle is to use the mapping relationship between the pulse waveform feature parameters and the blood pressure. The blood oxygen is calibrated and calculated by the ratio of the direct current signals of hemoglobin and oxygenated hemoglobin obtained by the dual-color light of reflective red / infrared light, and the principle is that the content of hemoglobin and oxygenated hemoglobin reflected by the direct current signal directly reflects the size of the blood oxygen saturation. The blood pressure and blood oxygen share one photoelectric receiver, and the light source is controlled by the PWM wave provided by the MCU unit to obtain the corresponding PPG signal during actual measurement.

[0012] As a further preferred embodiment of the above technical solution, the ultrasonic array sensor, the PPG sensor, the AD conversion module and the MCU unit are packaged in a polydimethylsiloxane outer package. The polydimethylsiloxane (PDMS) has the advantages of softness, non-toxicity, transparency and good biocompatibility, and as an outer package, the sensor of the present application is more suitable for making human wearable devices.

[0013] Based on the same technical concept, the present application also provides a wearable device containing the non-invasive blood pressure and blood oxygen measurement sensor of the above technical solution.

[0014] Based on the same technical concept, the present application also provides a non-invasive blood pressure and blood oxygen measurement method, which uses the non-invasive blood pressure and blood oxygen measurement sensor of the above technical solution or the wearable device of the above technical solution to measure blood pressure and blood oxygen.

[0015] As a further preferred embodiment of the above technical solution, the non-invasive blood pressure and blood oxygen measurement method comprises the following steps:

[0016] (1) collecting the PPG signal by the PPG sensor and inputting it to the MCU unit;

[0017] (2) transmitting a voltage signal to the piezoelectric ceramic by the MCU to make the piezoelectric ceramic output an ultrasonic signal, the piezoelectric ceramic receives the reflected ultrasonic signal and generates an electric signal, which is converted into a digital signal by the AD conversion module and then inputted to the MCU unit, i.e. the motion artifact signal is obtained;

[0018] (3) the MCU unit determines whether the motion artifact signal exceeds a threshold value: if the motion artifact signal exceeds the threshold value, adaptive filtering is started to process the PPG signal, and if the processed PPG signal meets the quality requirement, the PPG signal is outputted to obtain blood pressure and blood oxygen value; if the processed PPG signal does not meet the quality requirement, the measurement step is restarted; if the motion artifact signal does not exceed the threshold value, the PPG signal is directly outputted to obtain blood pressure and blood oxygen value.

[0019] As a further preferred embodiment of the above technical solution, after the PPG signal is collected in step (1), the PPG signal is pre-amplified, analog low-pass filtered, level-lifted and then inputted to the MCU unit after analog-to-digital conversion. In order to ensure the direct current component of the PPG signal for blood oxygen measurement, no high-pass filter is designed in the present application. The level-lifting is to lift the negative voltage signal to a positive voltage to facilitate subsequent AD conversion.

[0020] As a further preferred embodiment of the above technical solution, the cut-off frequency of the analog low-pass filter is 8 Hz, and the analog low-pass filter is completed by using a second-order Sallen-Key filter. The cut-off frequencies of the analog low-pass filter and the digital low-pass filter are both 8 Hz, and the digital filter is a FIR filter, the linear phase shift of which can facilitate the subsequent real-time signal processing, and the specific filter parameters are obtained by using a tool in MATLAB. The second-order Sallen-Key filter is suitable for processing small signals.

[0021] As a further preferred embodiment of the above technical solution, the AD conversion module uses IIC communication in step (2). The AD conversion module uses IIC communication, which facilitates the microcontroller (MCU) unit to control the ultrasonic array sensor and the reflective PPG sensor in an addressing manner.

[0022] As a further preferred embodiment of the above technical solution, the adaptive filtering process uses a recursive least square algorithm in step (3), and the specific content of the recursive least square algorithm is as follows:

[0023] Let the input signal be x(k), the filtered output signal be y(k), and the error amplitude average weighted sum be , wherein M is the filter length; the forgetting factor is , , it is assumed that there are optimal filter coefficients , a matrix and a vector , , the gain of which is:

[0024] ;

[0025] expected sequence and the estimation error between the two, i.e. is:

[0026]

[0027] weight coefficient is updated as:

[0028]

[0029] covariance matrix is updated as:

[0030]

[0031] wherein, represents the input discrete signal; represents the filtered output; is the estimated value of the recursive least square algorithm, and the initial value after, the least square recursive operation can be started.

[0032] Compared with the prior art, the present application has the beneficial effects that:

[0033] The non-invasive blood pressure and blood oxygen measurement sensor and wearable device of the present application rely on the high sensitivity of the ultrasonic array to detect the motion artifact of the PPG signal, and take this signal as a reference signal, combine the RLS adaptive filtering algorithm to filter out, and through the removal of the motion artifact of the PPG signal, a high-quality PPG signal is obtained, so as to improve the pulse wave signal quality and thus improve the blood pressure and blood oxygen measurement accuracy, so as to realize the blood pressure and blood oxygen measurement under the non-stationary state of the human body.

[0034] The present application will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0035] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application, and are incorporated in and constitute a part of this application. The embodiments of the present application illustrated in the drawings and their descriptions are used to explain the present application and are not intended to limit the present application. In the drawings:

[0036] Figure 1 is a structural schematic view of the non-invasive blood pressure and blood oxygen measurement sensor of Example 1 in the side view direction;

[0037] Figure 2 is a structural schematic view of the non-invasive blood pressure and blood oxygen measurement sensor of Example 1 in the bottom view direction;

[0038] Figure 3 is a structural schematic view of the piezoelectric ceramic and its outer shielding cover of Example 1.​​​

[0039] Figure 4 Application of the non-invasive blood pressure and blood oxygen measurement sensor of Example 1 in a wearable device (health monitoring wristwatch);

[0040] Figure 5 Flowchart of the non-invasive blood pressure and blood oxygen measurement method of Example 1.

[0041] Legend:

[0042] 1, polydimethylsiloxane outer package; 2, circuit board; 3, silicon photocell; 4, black barrier structure; 5, combined LED light source; 6, outer package of ultrasonic array sensor; 7, ultrasonic array sensor; 8, skin and tissue; 9, blood; 13 is a green light LED; 14 is a red light LED; 15 is an infrared light LED; 20 is an outer shield; 21 is a piezoelectric ceramic; 22, wearable device; 23, mobile terminal real-time display interface. DETAILED DESCRIPTION

[0043] The embodiments of the present application will be described in detail below with reference to the accompanying drawings, but the present application can be implemented in various different ways limited and covered by the claims.

[0044] Example 1:

[0045] As shown in Figure 1 and Figure 2 , the non-invasive blood pressure and blood oxygen measurement sensor of the present embodiment includes an ultrasonic array sensor, a reflective combined light source PPG sensor, an AD conversion module and an MCU unit.

[0046] Among them, the reflective combined light source PPG sensor is used to measure the PPG signal, the combined light source is composed of green light (520 nm), red light (660 nm) and infrared light (940 nm) LEDs, and the photoelectric receiver is a silicon photocell which can convert the received light signal into an electrical signal. The collected PPG signal is input into the MCU after preamplification, analog low-pass filtering (in order to ensure the direct current component of the PPG signal for blood oxygen measurement, a high-pass filter is not designed in the present embodiment), level lifting and analog-digital conversion for subsequent digital signal processing and physiological parameter calculation. The cut-off frequency of the analog low-pass filter and the digital low-pass filter is 8HZ, the digital filter is a FIR filter, and the specific filter parameters are obtained from the tool in MATLAB. The analog filter is a second-order Sallen-Key filter.

[0047] Among them, the AD conversion module, the MCU unit, the filter and other components are integrated on the circuit board.

[0048] The ultrasonic array sensor comprises 8 piezoelectric ceramics, which are arranged in a circular ring outside the PPG sensor, each piezoelectric ceramic is provided with a micro shield outside, and all the shields are grounded; the MCU unit transmits a voltage signal to the piezoelectric ceramic through the input and output interface of the piezoelectric ceramic to make the piezoelectric ceramic output an ultrasonic signal, the ultrasonic signal is reflected to cause the piezoelectric ceramic to expand and contract and generate an electric signal, the electric signal is converted into a digital signal by an AD conversion module and input to the MCU unit.

[0049] As shown in Figure 4 , the non-invasive blood pressure and blood oxygen measurement sensor of the embodiment can be used in a smart watch wearable device, and the measured person needs to fix the application in the measured part before starting measurement, and measurement can be started in two ways: one is to directly press the keys on the wearable measurement platform, and the other is to control through the mobile phone connection platform Bluetooth, more operations can be performed through the mobile phone control measurement platform, and more information can be consulted.

[0050] As shown in Figure 5 , the non-invasive blood pressure and blood oxygen measurement method of the embodiment comprises the following steps:

[0051] Among them, the specific content of the recursive least square algorithm is as follows:

[0052] Let the input signal be x(k), the filtered output signal be y(k), and the error amplitude average weighted sum be , wherein M is the filter length; the forgetting factor is , , assuming that there is an optimal filter coefficient , matrix and vector , then , the gain of which is:

[0053] ;

[0054] The difference between the expected sequence and the estimate , that is, the estimation error is:

[0055] ;

[0056] The weight coefficient is updated as:

[0057] ;

[0058] The covariance matrix is updated as:

[0059] ;

[0060] wherein, represents an input discrete signal; represents a filtered output; is an estimated value of a recursive least square algorithm, and an initial value is given After, The least square recursive operation can be started.

[0061] The above merely describes the preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application. The above is merely the preferred embodiment of the present application, and the protection scope of the present application is not limited to the above-mentioned embodiments. Any improvement and transformation obtained by those skilled in the art without departing from the technical concept of the present application shall be considered as the protection scope of the present application.

Claims

1. A non-invasive blood pressure and blood oxygen measurement sensor, characterized in that, The system includes an ultrasonic array sensor, a PPG sensor, an AD conversion module, and an MCU unit. The ultrasonic array sensor comprises eight piezoelectric ceramics, which are fixed in a circular arrangement around the PPG sensor. The PPG signal acquired by the PPG sensor is input to the MCU unit. The MCU unit emits a voltage signal to the piezoelectric ceramics, causing them to output ultrasonic signals. The ultrasonic signals are reflected, causing the piezoelectric ceramics to expand and contract, generating electrical signals. These electrical signals are converted into digital signals by the AD conversion module and input to the MCU unit. The MCU determines whether the skin interface is shaking based on the different frequencies of the emitted and returned ultrasonic waves, and uses the digital signals as reference signals to filter out motion artifacts.

2. The non-invasive blood pressure and blood oxygen measurement sensor according to claim 1, characterized in that, Each of the piezoelectric ceramics is provided with a miniature shield, and all shields are grounded.

3. The non-invasive blood pressure and blood oxygen measurement sensor according to claim 1, characterized in that, The PPG sensor includes a combined light source and a photodetector; the MCU unit inputs a voltage to the combined light source to control it to emit light signals, and the light signals are received by the photodetector after reflection and input to the MCU unit.

4. The non-invasive blood pressure and blood oxygen measurement sensor according to claim 3, characterized in that, The photodetector is a silicon photocell that can convert the received optical signal into an electrical signal; the combined light source consists of a green LED, a red LED, and an infrared LED.

5. The non-invasive blood pressure and blood oxygen measurement sensor according to any one of claims 1-4, characterized in that, The ultrasonic array sensor, PPG sensor, AD conversion module and MCU unit are packaged in a polydimethylsiloxane outer package.

6. A wearable device, characterized in that, The wearable device includes the non-invasive blood pressure and blood oxygen measurement sensor according to any one of claims 1-5.

7. A non-invasive method for measuring blood pressure and blood oxygen saturation, characterized in that, Blood pressure and blood oxygen can be measured using the non-invasive blood pressure and blood oxygen measurement sensor as described in any one of claims 1-5 or the wearable device as described in claim 6.

8. The non-invasive blood pressure and blood oxygen measurement method according to claim 7, characterized in that, Includes the following steps: (1) The PPG signal is acquired by the PPG sensor and input to the MCU unit; (2) The piezoelectric ceramic is made to output an ultrasonic signal by transmitting a voltage signal to the piezoelectric ceramic through the MCU. The piezoelectric ceramic receives the reflected ultrasonic signal and generates an electrical signal. The electrical signal is converted into a digital signal by the AD conversion module and then input to the MCU unit to obtain the motion artifact signal. (3) The MCU unit determines whether the motion artifact signal exceeds the threshold: if the motion artifact signal exceeds the threshold, then the adaptive filtering is enabled to process the PPG signal. If the PPG signal meets the quality requirements after processing, the PPG signal is output to obtain the blood pressure and blood oxygen value. If the PPG signal does not meet the quality requirements after processing, the measurement step is restarted. If the motion artifact signal does not exceed the threshold, the PPG signal is directly output to obtain the blood pressure and blood oxygen value.

9. The non-invasive blood pressure and blood oxygen measurement method according to claim 8, characterized in that, After the PPG signal is acquired in step (1), the PPG signal is pre-amplified, analog low-pass filtered, level-upped and analog-to-digital converted before being input to the MCU unit; the cutoff frequency of the analog low-pass filter is 8Hz, and the analog low-pass filter is completed by a second-order Sallen-Key filter; the AD conversion module in step (2) uses IIC communication.

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