| Author: | Liu, Mingran |
| Title: | Novel flexible piezoelectric nanocomposite sensors for monitoring of human pulse |
| Advisors: | Liu, Yang (ME) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Detectors Biosensors -- Materials Nanocomposites (Materials) Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xv, 138 pages : color illustrations |
| Language: | English |
| Abstract: | To date, monitoring sensors and devices have been widely applied in collecting human body signals from daily life to academic research. Vital signals, such as heartrate, respiration rate, and palpation, have become irreplaceable indexes in the evaluation of people's health status, athletic ability and pathology. In addition to the commonly used signals, many weak or unusual signals, such as Skin Blood Flow Oscillation (SBO), generated in human bodies have strong potential to predict or evaluate human bodies' abnormality or pathogenesis. Although many clues and investigations exhibited a high correlation between SBOs and diseases, its physiological and pathophysiological roles are still not clarified till now. One of the limitations for investigations on SBOs is that current monitoring devices such as Laser Doppler Flowmetry (LDF) cannot fulfill the requirements on real-time monitoring and maintaining high accuracy at the same time. To explore more possibility and potential of human body monitoring sensors and to reduce the limitations of monitoring devices on the research and investigations of the weak but significant human body signals such as SBOs, we investigated different novel piezoelectric flexible, sensitive and biocompatible nanocomposite membranes, tested different prototype sensors based on these membranes and presented our work in this thesis. Firstly, we investigated and fabricated composite sensors with enhanced piezoelectricity and flexibility. The fabricated sensors were designed in a simple but efficient sandwich structure. The sensing layers of the fabricated sensors were made of polyvinylidene fluoride (PVDF) or polyvinylidenefluoride-trifluoroethylene (PVDF-TrFE) based composite with Zinc Oxide (ZnO) microstructures as fillers. The microstructures and morphologies of pristine PVDF (P), PVDF-TrFE (PT), PVDF/ZnO (P/Z) and PVDF-TrFE/ZnO (PT/Z) were characterized by scanning electron microscope (SEM). The degree of crystallinity for P, PT, P/Z and PT/Z was obtained by analyzing the X-ray diffraction (XRD) results. Through experiments and analysis, we found that as piezoelectric membrane materials, PT samples performed better than P samples in both mechanical test and characterization results. Compared to P with a porous structure, a membrane made of PT was more flexible and stretchable. In addition, β‐phase crystal obtained in PT was larger than that in P. Moreover, we concluded that ZnO microstructures, as semiconductor fillers, had substantial influence on enhancing the dielectric constant and piezoelectricity of the composite membranes. Secondly, based on the result obtained from the first stage, we fabricated, characterized and tested different poly (vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)-based composite membranes. To improve the β-phase crystallinity and piezoelectricity of the membranes, and for the purpose of comparison, we added nano ZnO particles with different concentrations into PVDF-TrFE. To facilitate the formation of β-phase crystal, the membranes were fabricated by electrospinning method. After the electrospinning, we conducted an annealing process to the fabricated membranes to increase the size of β-phase crystal. Then, the fabricated PVDF-TrFE membranes, acting as the core sensing layer, were respectively built into multiple prototype sensors in a sandwich structure. We tested the sensitivity of the prototype sensors by utilizing an auto-clicker continuously clicking onto them. As a result, combining the addition of ZnO nanofillers and the annealing process, we successfully fabricated a highly sensitive pressure sensor. The optimal peak-to-peak voltage response generated from the prototype sensor was 1.788 V which shows a 75% increase compared to that of the pristine PVDF-TrFE sensor. Furthermore, human pulse waveforms were captured by the prototype sensor. This exhibits tremendous prospects of applying the novel nanocomposite membrane in human body monitoring. Thirdly, to obtain polymeric nanocomposite membranes with higher piezoelectricity based on the previous experiments, we designed, fabricated and investigated new compositions of the novel nanocomposite membranes. In this stage, to investigate the influence of hydroxyapatite (HAP) nanofillers on the PVDF/ZnO and PVDF-TrFE/ZnO polymeric nanocomposite, we designed six comparison samples fabricated by electrospinning and post-treated by annealing. Because of the addition of HAP, the tensile strength of sample PZH and sample PTZH was increased. Then, we fabricated sample PZH-S and sample PTZH-S by stretching PZH and PTZH, respectively. All of the six samples were characterized by SEM and XRD for their morphology and crystalline structure. The piezoelectric response tests were conducted by collecting and analyzing electric responses of all six samples to the auto-clicking stimuli. With the addition of HAP, the tensile strength of the material was increased. Compared with the samples without HAP (PZ, PTZ) which are easily snapped under stretching, the samples with HAP (PZH, PTZH) can be stretched to a larger extent (around 10%). Moreover, the size of β-phase crystals and piezoelectric response under stimuli were increased significantly with the addition of HAP. Our investigation results demonstrated the feasibility and strong potential of applying piezoelectric composite material in human body monitoring. They may also provide inspiration for studies of weak but significant signals in the human body. |
| Rights: | All rights reserved |
| Access: | open access |
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