Development and characterization of flexible physiological sensors based on piezoelectric materials for wearable system

Abstract

With the advent of the IoT era, various kinds of sensors are being used everywhere. Especially, researches on physiological sensors that can detect biological signals in real time in everyday life are drawing attention as people’s interest in health increases. Physiological sensors can measure various bio-signals including vital signs that are fundamental elements of life, such as pulse, respiration, body temperature, and blood pressure. In particular, pulse rate and blood pressure measurements are essential to monitor a person’s health condition regardless of the type of disease. Although various studies have been conducted to measure pulse or blood pressure in wearable form, there are still issues to implement wearable physiological sensors, such as bulky volume, inconvenience to wear, low accuracy, etc. Piezoelectric sensors can be considered as a promising candidate due to its high sensitivity and low power consumption. However, most of the conventional piezoelectric materials were used in bulk form, making them difficult to apply as wearable or implantable sensors. In this dissertation, physiological signal measurement was conducted with wearable or implantable piezoelectric sensors by making piezoelectric materials into thin film.

In chapter 2, a feedback system is developed using a heart rate sensor to enable ventricular assist device (VAD) used by heart failure patients to have automatic blood flow controllability. The heart rate sensor should be located inside the human body because the sensors located outside the human body may fail or malfunction due to the external environment. In order to use piezoelectric sensor for implantation to human body, lead-free piezoelectric materials must be used. However, lead-free piezoelectric materials have a significant decrease in piezoelectric properties. We compensated for the insufficient piezoelectric properties by using single crystal piezoelectric materials having perovskite structure. A flexible piezoelectric sensor was fabricated using single crystal barium titanate zirconate (BTZ), and a microcontroller unit (MCU) that can determine the heart rate was designed. In addition, the MCU plays a role of controlling the motor speed by changing the voltage applied to the motor according to the counted heart rate. When the input frequency applied to the sensor was changed, we confirmed that the voltage applied to the motor was successfully changed through MCU. Finally, it was confirmed that the blood flow was automatically controlled according to the heart rate by conducting an in vitro experiment with sheep blood.

In chapter 3, continuous and noninvasive blood pressure (CNBP) monitoring system is implemented using wearable piezoelectric sensors. With a conventional piezoelectric sensor, it was difficult to obtain a clear pulse waveform to estimate blood pressure. In this dissertation, it was possible to obtain precise pulse waveforms enough to estimate blood pressure by greatly increasing the signal-to-noise (SNR) ratio by shielding external noise, optimizing the substrate thickness, and improving the contact force with skin. Based on the characteristics that the piezoelectric sensor has high linearity in the range of human blood pressure, a transfer function that can covert the pulse wave into blood pressure was developed. The accuracy evaluation of the piezoelectric blood pressure sensor was conducted through a FDA-approved sphygmomanometer, and it was confirmed that it met the international standard for noninvasive blood pressure measurement for adult male subjects. Finally, we demonstrated a CNBP monitoring with wearable piezoelectric sphygmomanometer by designing a wireless communication circuit and prototype.

In chapter 4, an energy extraction enhancement circuit (EEEC) for a flexible piezoelectric device is proposed to improve output characteristics. Instead of using a conventional impedance-matching approach, energy extraction is significantly enhanced by minimizing the capacitive load seen by flexible PZT device. The proposed EEEC also maximizes output voltage with extremely low static power consumption. The flexible PZT device with total thickness of 170 µm is used, which gives enough flexibility for attachment on clothes or human skin. The result with the proposed EEEC increases energy extraction up to 495% than that of conventional FBR.