| With the development of modern society,people’s demand for a healthy life is also increasing.How to achieve convenient and real-time health monitoring and early diagnosis of diseases has become an urgent problem.Heart rate and blood pressure,as important vital signs of human body,can dynamically and directly reflect the physiological state of cardiovascular and cerebrovascular,and is also an important basis for disease prevention and treatment.Therefore,it is of great significance to develop a device that is comfortable to wear and can continuously monitor physiological signals such as heart rate and blood pressure,which is conducive to the prevention,diagnosis and treatment of hidden diseases such as hypertension and cardiovascular disease.At present,most health monitoring devices on the market are based on rigid devices,which cannot fit to the skin,which not only reduces the detection accuracy,but also affects the comfort of wearing,and cannot meet the wearable requirements in the real sense.As the core device of wearable electronic devices,flexible sensors with the characteristics of softness,good extensibility,and comfortable wearing can be conformal attached to the surface of human skin,which provides an effective solution to the above problems.Therefore,this thesis develops a high-performance pulse detection system based on polyvinylidene fluoride(PVDF)flexible pressure sensors,achieving accurate detection of human heart rate and blood pressure.The main research contents of this thesis are as follows:Firstly,the high performance PVDF flexible pressure sensor is fabricated by printing silver electrode on PVDF piezoelectric film using microelectronic printing technology,and using polyimide(PI)film as packaging material.Based on COMSOL finite element simulation analysis,the influence of PI thickness on thermal insulation performance and sensitivity of the PVDF flexible pressure sensor is studied.It is found that thermal insulation performance is positively correlated with PI thickness,and the sensor sensitivity is negatively correlated with PI thickness.Considering the simulation results and the flexibility of the sensor,the PI film with a thickness of 30 μm is finally selected for encapsulation,and the thickness of the fabricated PVDF flexible pressure sensor is 0.35 mm.Secondly,the signal conditioning circuit composed of charge amplifying circuit and low pass filtering circuit is designed by using charge measurement mode.The stretchable snake-like Cu/PI flexible circuit is fabricated by magnetron sputtering and laser direct writing process,and it is fitted to the polydimethylsiloxane(PDMS)flexible substrate with good tensile performance,and the flexibility of the signal conditioning circuit of the PVDF flexible pressure sensor is successfully realized.The PVDF flexible pressure sensor has the advantages of fast response speed(38 μs),high resolution,low detection limit,stable electrical performance,and good linearity in a wide range(0.2～1.5 MPa),with a sensitivity of 330 m V/MPa.Thirdly,combined with the pulse signal acquisition software developed based on Lab VIEW,the PVDF flexible pressure sensor and its flexible signal conditioning circuit,the complete pulse detection system is established,and the detection of human pulse and breathing signals is realized.The effect of thermal airflow,balloon bursting shock and long distance transmission on pulse detection is also studied.It is found that ceramic fiber thermal protection mats can be used for thermal insulation and shock protection,effectively reducing signal interference and still detecting good pulse signals with long distance shielded wire transmission.Accurate heart rate detection is achieved by preprocessing the collected raw pulse signal with wavelet denoising,removing baseline drift,and extracting feature points,with an average error of only 2.02%.Finally,the pulse data of several volunteers are collected by the built pulse detection system and three blood pressure calculation models are constructed.The mean absolute error(MAE)±standard deviation(STD)is used to evaluate the accuracy of blood pressure detection.The calculation error of systolic blood pressure(SBP)based on the univariate linear regression model of pulse wave propagation time(PTT)is 3.491 ± 1.966 mm Hg,and the diastolic blood pressure(DBP)is 2.836 ± 1.514 mm Hg.The calculation errors of SBP and DBP based on the binary linear regression model of PTT and heart rate are 3.431 ± 2.048 mm Hg and 2.745 ± 1.548 mm Hg,respectively.The third model introduces Kalman filter algorithm on the basis of binary linear regression model,which can adaptively calibrate the model parameters,thus reducing the calculation error of blood pressure.The SBP and DBP calculation errors are 2.293 ± 2.346 mm Hg and 1.884 ± 1.879 mm Hg respectively.The detection accuracy of the three blood pressure calculation models all meet the blood pressure detection standards of the American Association for the Advancement of Medical Devices(AAMI)(average error<± 5 mm Hg,standard deviation<8mm Hg),indicating that the pulse detection system based on PVDF flexible pressure sensor developed in this article can be used for accurate detection of pulse and blood pressure. |
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