High-performance flexible piezoelectric acoustic sensors and energy harvesters for self-powered flexible electronic system

Abstract

Chapter 1. Energy harvesting form ambient vibrational energy for the goal of operating low-power consumption electronic devices has widely researched during the last decade years for enabling wireless applications. The purpose of this effort is to provide remote and sustainable source of electricity to recharge storage components such as capacitors and batteries. One of the most researched areas is the utilization of piezoelectric materials which have piezoelectric effect to convert mechanical and vibrational energy such as bending, stretching, and pressing into useful electric energy. The area of piezoelectric energy harvesting embraces material science, mechanics, and electric circuitry. The ultimate goal of these research is making high-performance piezoelectric energy harvesters to power the low-power consuming electronics by using the ambient vibrational and mechanical energy available in our environment. If this dream can be achieved, the maintenance price for periodic battery replacements as well as the requirement of an outer power sources could be noticeably decreased. This part introduces the principle of piezoelectricity, piezoelectric materials, and electric energy generation in piezoelectric materials.

Chapter 2. In this chapter, a self-powered flexible piezoelectric acoustic sensor (f-PAS) inspired by basilar membrane in human cochlea was demonstrated. The f-PAS covered the voice frequency spectrum via the combination of its low quality (Q) factor and multi-resonant frequency tuning, exhibiting four to eight times higher sensitivity than the conventional condenser sensor. Our piezoelectric acoustic sensor with a thin membrane design produced sufficient output voltages by the distinct resonant movement of the Pb[Zr0.52Ti0.48]O3(PZT) membrane under the minute acoustic sound stimuli. Multiple sensor channels were integrated in a single f-PAS chip with a size of 1.5 × 3 cm2, which acquire multi-tunable piezoelectric signals without any external power. A linear response of the resonance frequency of the curved piezoelectric membrane was theoretically investigated by a finite element method (FEM) calculation. Low Q factors from corresponding channels were achieved by optimal membrane thickness and channel length.

Chapter 3. In this chapter, a new platform of machine learning-based speaker recognition was demonstrated, via the flexible piezoelectric acoustic sensor (f-PAS) with a highly sensitive multi-resonant frequency band. The resonant self-powered f-PAS was fabricated by mimicking the operating mechanism of the basilar membrane in the human cochlear. The f-PAS acquired abundant voice information from the multi-channel sound inputs. The standard TIDIGITS dataset were recorded by the f-PAS and converted to frequency components by using a Fast Fourier Transform (FFT) and a Short-Time Fourier Transform (STFT). The machine learning based Gaussian Mixture Model (GMM) was designed by utilizing the most highest and second highest sensitivity data among multi-channel outputs, exhibiting outstanding speaker recognition rate of 97.5 % with error rate reduction of 75 % compared to that of the reference MEMS microphone.

Chapter 4. Flexible piezoelectric energy harvesters (f-PEHs) have been regarded as an overarching candidate for achieving self-powered electronic systems for environmental sensors and biomedical devices using the self-sufficient electrical energy. In this research, we realize a flexible high-output and lead-free piezoelectric energy harvester by using the aerosol deposition (AD) method and the laser lift-off (LLO) process. We also investigated the comprehensive biocompatibility of the lead-free piezoceramic device using ex-vivo ionic elusion and in-vivo bioimplantation, as well as in-vitro cell proliferation and histologic inspection. The fabricated LiNbO3-doped (K,Na)NbO3(KNN) thin film-based flexible energy harvester exhibited an outstanding piezoresponse, and output performance up to an open-circuit voltage of ~140V and a short-circuit current of ~1.8μA under normal bending and release deformation, which is the best record among previously reported flexible lead-free piezoelectric energy harvesters. Although both the KNN and PZT devices showed short-term biocompatibility in cellular and histological studies, excessive Pb toxic ions were eluted from the PZT in human serum and tap water. Moreover, the KNN-based flexible energy harvester was implanted into a porcine chest, and generated up to ~5V and 700nA from the heartbeat motion, comparable to the output of previously reported lead-based flexible energy harvesters. This work can compellingly serve to advance the development of piezoelectric energy harvesting for actual and practical biocompatible self-powered biomedical applications beyond restrictions of lead-based materials in long-term physiological and clinical aspects.

Chapter 5. In this chapter, the structure of the basilar membrane inside the human cochlea was inspired to produce a voice sensor. In a flexible electronic device such as an flexible energy harvester, flexible acoustic sensor, flexible pressure sensor, substrate having a uniform film thickness is used. However, in this study, the thickness of the substrate has a structure with a constant gradient thickness. In particular, the high frequency is sensed in the narrow part and the low frequency is sensed in the wide part, and has a structure for maximizing it. In the basilar membrane inside the cochlea, the narrow part has a thick thickness, and the wide part has a thin thickness, so it has an advantage of having a wider frequency coverage. This is because the thickness and the resonance frequency become proportional by the resonance frequency formula. In this experiment, polyimide (PI) liquid was poured into an aluminum mold of an inclined shape to harden it, and it was removed and used as a gradual substrate. The post-process including the transfer process, is based on the previously used method.