Piezoelectric sensors and their applications in cardiac monitoring (1)

I. introduction

Heart disease is the cause of invalidity and death, in developed countries, common diseases, cardiovascular system diseases has become the most common diseases and an important cause of death, and as our population ages, the proportion of cardiovascular diseases each year.

Clinical cardiovascular diagnosis in addition, rely primarily on medical devices. ECG and heart is to detect cardiovascular disease by two different means, ECG mainly applies to arrhythmia and myocardial ischemia in qualitative and quantitative diagnosis, evaluation of cardiovascular drugs. PCG able to effectively compensate for the lack of cardiac auscultation, cardiac auscultation cannot record sounds or not easy to distinguish the signal with the graphic form of the record for the doctor of use [1]. Phonocardiogram in combination with the ECG, is able to significantly increase cardiovascular disease identification and diagnosis, understanding cardiovascular function, the treatment of pathological and judgment of certain disease mechanism provides valuable information on the application of increasingly widespread. On the human body weak physiological signal acquisition and processing of effective has been the medical areas of research focus. There are a variety of human weak signal acquisition of sensors, such as Piezoelectric ceramic sensor, Doppler effect sensor, etc, but at the cost structure and are present on certain issues. There is a new polymer piezoelectric PVDF Piezoelectric sensor developed, its simple structure, high sensitivity, accurately measuring weak signal of the human body. We apply them to the human heart sound signal acquisition, developed two channel integrated micro recorder, dynamic record sounds and ECG. Experiments showed that the film between the sensor and the whole structure, performance, match, the PCG ECG monitoring system capable of more accurate monitoring and analysis of the human heart sounds, ECG system later formed the basis of the product.

Second, Piezoelectric sensor design

PVDF films is a new polymer piezoelectric materials, in medical application is widespread in the sensor [2, 3].

It has the piezoelectricity and mechanical properties of thin film flexible, use it to make pressure sensor, has a delicate design, ease of use, high sensitivity, frequency bandwidth, and the human body can exposure to safe and comfortable, close to the body, as well as acoustic impedance and human tissues acoustic impedance is very close, and a series of features [4], can be used for human heart pulse signal detection, etc. Pulse sounds carry important human physiological parameters of information, through the efficient processing of signals, accurate waveform, heart rate frequency available for doctors to provide reliable diagnosis.

　　

Piezoelectric sensor design mainly consider the sensitivity of the sensor and signal to noise ratio, according to the measurement signal frequency and response rate, we design the structure of thin-film sensors are as shown in Figure 1.

In acquisition of human heart sound signals, due to heart of a wide frequency response range, while the output of physical signals value is very weak, with rigid substrates and hollow design. This can improve the sensor films receive sounds of form variables to improve signal strength. The disadvantage of this design is not firm and use the structure of time need correction. PVDF piezoelectric film of piezoelectric constant generally D33 = 15 × 10-12C/N, g value is relatively high, but has a high internal impedance, generally up to 1012 Ω, produce the output impedance of the transducer is large, is not conducive to subsequent signal acquisition and zoom. To prevent signal attenuation, we use the high output impedance FET as impedance converter, which is the predecessor to the measurement system. We use the type field effect tube of high input impedance characteristics, according to the quiescent point design impedance converter, as in Figure 2 (a) as shown in the human body, the sensor signal impedance converter, access to reliable, low-impedance output signal. The output impedance as in Figure 2 (b) the figure. You can see, the changes in the signal frequency, the sensor's output impedance guaranteed has remained virtually unchanged.