Studies on the Electrospun Polymeric Materials for Piezocapacitive/Piezoelectric Pressure Sensor and Dry Electrode

다국어 초록 (Multilingual Abstract)

Chapter 1 explored and discussed the electrospun Spandex nanofiber web, having a very high amount of nano-sized open cell can be used as a piezo-capacitive sensor for monitoring both static and dynamic pressures due to excellent electrospinnability and good elastic properties. Compared to our previously reported SBS and TPU electrospun nanoweb, Spandex showed relatively linear increases of capacitance with applied pressure and restored its initial thickness when the pressure was released due to the improved resilience and elasticity. Moreover, a small amount of ionic liquid (IL) was added in the Spandex dope solution to increase the sensitivity of the sensor to pressure, which induced a substantial amount of capacitance change with pressure, as well as reducing the capacitance-pressure hysteresis. In this work, the hysteresis of the sensor was assessed by measuring the capacitance values during 20 cyclic loadings and unloading and was improved significantly from 7.5% to 1.8%. Creep and stress relaxation behaviors were also tested through measuring capacitance under the constant loading and loading under the constant capacitance as a function of time, respectively, using a dynamic pressure tester and an LCR meter.

Chapter 2 described a novel hybrid pressure-sensor. In this study, a mixture solution of Spandex and polyvinylidene fluoride (PVDF) was electrospun under controlled compositions to obtain Spandex-PVDF hybrid nanoweb. Their physico-mechanical and piezo-responsive behaviors were investigated. When the dynamically changing force was applied to this hybrid sensor, dynamic pressure could be measured through the electric current generated by the orientation change of C-F dipoles in the PVDF part of the composite nanoweb as a function of time. When the static force was applied to this hybrid sensor, static pressure could be measured through the increasing capacitance value caused by the decreasing thickness of the composite nanoweb rather than the piezoelectric current. The piezoelectric and piezocapacitive properties of this hybrid sensor could be measured simultaneously using a piezoelectric amplifier and an LCR meter, respectively. Overall, 25~50 wt.% addition of PVDF in Spandex greatly enhanced the piezoelectric output signal as well as reduced pressure-capacitance hysteresis and higher capacitance change with the applied pressure due to the rubbery Spandex of this hybrid sensor. Moreover, AgNO3, which acts as a precursor of silver nanoparticle (AgNP) formed in the PVDF nanoweb, was added into the PVDF/Spandex electrospun solution to get enhanced piezoelectric performance of the sensor caused by the preferential formation of β-phase of PVDF.

In the third chapter, the discussion was carried out on a piezocapacitive-piezoelectric tactile sensor fabricated by polylactic acid (PLA)-Spandex electrospun nanofiber-webs with various architectures. The spandex nanofiber web has been reported as a piezocapacitive sensor due to its excellent electrospinnability and excellent elastic properties mentioned in chapter 1. On the other hand, electrospun PLA is high shear-piezoelectricity, helix orientation through drawing effect and preferential aligning of C=O functional group through applied high DC voltage during the electrospinning for nanofiber web preparation. However, the piezoelectric signal of the nanoweb is generated by the change of preferential orientation of C=O dipoles in PLA under external pressure. Consequently, in this study, Spandex and PLA were electrospun under controlled conditions through the coaxial electrospinning to obtain shell-core structured nanofiber web considering Spandex and PLA as a shell and core part, respectively. Moreover, Spandex and PLA nanoweb-based hybrid sensor were also fabricated through, consecutive electrospinning of PLA and Spandex, and stacking of individual electrospun PLA and Spandex nanoweb. These hybrid sensors combinations of two different characteristic polymer nanoweb are capable of simultaneously measuring piezocapacitance as well as piezoelectricity by applying pressure. The results are reported in detail.

Lastly, chapter 4 discussed the rubber nanoweb-based dry electrode for biopotential vital signal monitoring. A great variety of dry electrodes have been developed over many decades in order to overcome the drawbacks of conventional Ag/AgCl gel electrodes, but their applications are still restricted due to the low accuracy of the dry electrodes. Hence, the aim of this study is to find out much proper base material to replace conventional PVDF without the reduction of dry electrode performance after electroless silver plating. For this purpose, chlorinated polyisoprene (CPI) and poly(styrene-b-butadiene-b-styrene) (SBS) rubber were selected as a promising candidate due to its excellent elastic properties, which may improve electrode durability and skin-contact. Electroless silver plating technique was employed to coat nanofiber web with silver, and AgNW based dry electrodes were fabricated. The key electrode properties (contact impedance, step response, and noise characteristic) for both the CPI-AgNW and SBS-AgNW dry electrode were investigated thoroughly using agar phantom systems. Human subject tests were also carried out to determine the real performance of AgNW dry electrodes on a human body in terms of biopotential recording and electrical impedance tomography (EIT) measurements. The experimental results revealed that CPI-AgNW and SBS-AgNW dry electrodes could exhibit performances comparable to Ag/AgCl gel electrodes, indicating the newly designed AgNW dry electrodes have superior accuracy to most existing dry electrodes.

목차 (Table of Contents)

• -Contents-
• List of Figures ………...…………………………...……………………….............. vi
• List of Tables …………………………………………………………………..…… xv
• Abstract …...………………..……………………….…………................................. xvi
• Chapter 1. Electrospun Spandex Nanofiber Webs with Ionic Liquid for Highly Sensitive, Low Hysteresis Piezocapacitive Sensor …………………... 1
• 1.1. Introduction ….………………………………...….............................................. 1
• 1.1.1. Polymer-Based Pressure Sensor …………………………........................ 1
• 1.1.2. Electrospun Nanofiber Webs as a Piezocapacitive Sensor ……...……. 2
• 1.1.3. Background of This Study ...………………………..………………........ 2
• 1.1.4. Electrospun Spandex Nanofiber Web as a Piezocapacitive Sensor ..... 3
• 1.1.5. Objectives ..……………...……………….………………........................... 6
• 1.2. Experimental ….………………………………………....................................... 7
• 1.2.1. Materials ...…………………………………………..…………………….. 7
• 1.2.2. Preparation of Nanofiber Webs ...………………………….………......... 7
• 1.2.3. Fabrication of Electrodes …..……………………………………..… 10
• 1.2.4. Piezocapacitive Measurement and Hysteresis ...………… ……….... 10
• 1.2.5. Creep and Stress Relaxation Tests ...……………….……………….. 11
• 1.3. Results and Discussion ….…………………………................................. 13
• 1.3.1. FE-SEM Micrographs of Spandex Nanofiber Webs ...………........... 13
• 1.3.2. Capacitance Variation ...……………………....………………….… 15
• 1.3.3. Hysteresis Percent Calculation ...…………………….…………....... 18
• 1.3.4. Effect of Physical Ageing and Ionic Liquid Contents in Spandex Nanoweb ………………………………………………………………….. 22
• 1.3.5. Creep and Stress Relaxation Properties ...……………….……….…. 28
• 1.4. Conclusion .….………….………...…....................................................... 31
• 1.5. References ….………………………….................................................... 32
• Chapter 2. Novel Hybrid Pressure Sensor Based on Electrospun Spandex-Polyvinylidene Fluoride Composite Nanofiber Webs …….…..... 36
• 2.1. Introduction ….…………………………………...................................... 36
• 2.1.1. Polymer-Based Piezoelectric Sensor ……………………………….. 36
• 2.1.2. Piezoelectric Properties of Polyvinylidene Fluoride (PVDF) …......... 37
• 2.1.3. Electrospinning and Hybrid Pressure Sensor …..…………………… 42
• 2.1.4. Objectives …………………………………...……………………… 45
• 2.2. Experimental ….………………………………………............................ 46
• 2.2.1. Materials ...………………………....……………………………….. 46
• 2.2.2. Preparation of Spandex-PVDF Composite Nanofiber Webs ...……... 46
• 2.2.3. ATR-FTIR Spectrum Measurement ...…………………………........ 47
• 2.2.4. Fabrication of Sensors for Piezoelectric and Piezocapacitive Measurements .............................................................................................. 47
• 2.3. Results and Discussion ….…………………………................................. 53
• 2.3.1. Morphology of Electrospun Nanoweb .……………...……………… 53
• 2.3.2. Attenuated Total Reflectance Infrared (ATR-IR) Spectroscopy ……. 57
• 2.3.3. Piezoelectric Properties ...………………...………………………… 60
• 2.3.4. Piezocapacitive Properties and Hysteresis ...……………..…………. 64
• 2.3.5. Quantitative Effect of Spandex on the Piezoelectric and Piezocapacitive Properties of Spandex/PVDF Composite Nanoweb through Thickness Normalization ………………………………………… 69
• 2.3.6. Creep and Stress Relaxation Properties ……………………..…….... 73
• 2.4. Conclusion ….……………………………………………….................... 76
• 2.5. References ….……………………………................................................ 78
• Chapter 3. Piezocapacitive/Piezoelectric Tactile Sensor Based on Polylactic Acid/Spandex Electrospun Nanofiber Webs at Various Architecture ………………………………………………………………… 82
• 3.1. Introduction ….………………………………...…................................... 82
• 3.1.1. Polylactic Acid (PLA) as a Piezoelectric Material ...………..………. 82
• 3.1.2. Piezoelectric Characteristics of PLA Nanofiber Webs ...………....... 87
• 3.1.3. PLA-Spandex Nanofiber Web-Based Hybrid Pressure Sensor with Various Nanoweb Architecture ...………...……………………………….. 89
• 3.1.4. Objectives .....………………………….……………………………. 91
• 3.2. Experimental ….………………………………........................................ 92
• 3.2.1. Materials ...…………………………………….……………………. 92
• 3.2.2. Electrospinning Set-up and Fabrication of Sensor ..………………… 92
• 3.2.2.1. PLA-Spandex (Core-Shell) Nanoweb Sensor …………….…… 93
• 3.2.2.2. PLA-Spandex Electrospun Nanoweb-Stacking Sensor ...…....... 98
• 3.2.2.3. PLA-Spandex Consecutive-Electrospun Nanoweb Sensor ……. 101
• 3.2.3. Piezoelectric and Piezocapacitive Measurements …………………... 103
• 3.3. Results and Discussion ….………………………………...….................. 104
• 3.3.1. Morphology of Coaxial Electrospun Nanoweb …………………….. 104
• 3.3.2. Pressure-Capacitance Variation and Hysteresis of PLA-Spandex Hybrid Sensor with Various Sensor Architecture …………………….…… 107
• 3.3.3. Creep Properties of PLA-Spandex Hybrid Sensor with Various Sensor Architecture ...…………………………………………………....... 115
• 3.3.4. Piezoelectric Properties of PLA-Spandex Hybrid Sensor with Various Sensor Architecture ...………….…………………..…………….. 118
• 3.3.5. Quantitative Effect of Spandex on the Piezoelectric and Piezocapacitive Properties of Spandex/PLA Hybrid Sensor with Various Nanowebs Architecture through Thickness Normalization ...……..………. 123
• 3.4. Conclusion ….…………………………………........................................ 129
• 3.5. References ….………………………………...…..................................... 131
• Chapter 4. Rubber Electrospun Nanofiber Web-Based Dry-Electrode for Biopotential Monitoring ...…………………………………………………. 135
• 4.1. Introduction ….………………………………...…................................... 135
• 4.1.1. Conventional Ag/AgCl Gel Electrodes …………………………….. 136
• 4.1.2. Issues of Dry Electrodes ………………………………….………… 136
• 4.1.3. Chlorinated polyisoprene (CPI) and Poly(styrene-butadiene-styrene) (SBS) Rubber Nanofiber Webs as a Dry Electrodes …………………….… 139
• 4.2. Experimental ….………………………………........................................ 142
• 4.2.1. Materials ...…………………………………………………………. 142
• 4.2.2. Preparation of Nanofiber Webs Containing Silver Nanoparticles ….. 142
• 4.2.3. Electroless Silver Plating of Electrospun Nanoweb ...………….…… 143
• 4.2.4. Fabrication of Nanoweb Dry Electrodes ...…………………….……. 147
• 4.2.5. Electrode Properties on Agar Phantom System ...………………....... 149
• 4.2.6. Biopotential Recording on Human Subjects .………………….……. 152
• 4.2.7. EIT System and Imaging Experiments …………………………....... 155
• 4.3. Results and Discussion ….…………………………………..................... 158
• 4.3.1. Morphology of Electrospun Nanoweb Before and After Silver Plating …………………………………………………………………….. 158
• 4.3.2. Electrode Properties of AgNW Dry Electrodes on Agar Phantom System …………………………………………………………………….. 161
• 4.3.2.1. Contact Impedance ……………………………...….……......... 161
• 4.3.2.2. Step Response ………………………………………………..... 164
• 4.3.2.3. Noise Characterization ………………………………………… 166
• 4.3.3. Biopotential Recording …………………………………………….. 168
• 4.3.3.1. ECG Measurements …………………………………………… 168
• 4.3.3.2. EMG Measurements (Hand and Leg Muscle Activities) ………. 171
• 4.3.4. EIT Test ………………………………………….……………......... 177
• 4.3.4.1. Static EIT Test with Agar Phantom …………………….…....... 177
• 4.3.4.2. Dynamic Lung EIT Test on Human Chest ……………….……. 180
• 4.3.5. Electrode Stability Test …………………………………………….. 183
• 4.4. Conclusion ….………………………………...…..................................... 186
• 4.5. References ….………………………………...…..................................... 188
• Summary and Conclusion ………………………………………………...... 192
• Acknowledgments…………………………………………………………... 194
• -List of Figures-
• Figure 1-1. A molecular structure of Spandex composed of polyether-polyurea with both urea and urethane in the backbone chain (a). A schematic illustration of soft rubbery segment and hard segment domain and the existence of ionic liquid in the backbone chain (b)..……………..…………… 5
• Figure 1-2. Schematic diagram of electrospinning experimental set-up used for preparing nanofiber web ...……………………………..……...…………. 9
• Figure 1-3. Schematic diagram of nanoweb sensor fabrication and an LCR meter (a) and photographs of a home-built dynamic pressure tester (b)…....... 12
• Figure 1-4. FE-SEM images of electrospun Spandex nanofiber webs (a) and i-Spandex nanofiber webs (b) ...…………………………………………….... 14
• Figure 1-5. Twenty cycle time vs. capacitance curves of as-spun TPU (R190A) (a), cross-linked TPU (R190A) (b), as-spun SBS (c), cross-linked SBS (d), as-spun Spandex (e), and as-spun i-Spandex (f) nanofiber web-based sensors .............................................................................................................. 17
• Figure 1-6. Hysteresis calculation for a single cycle load vs. capacitance curve ……………………………………………………...…………….……. 20
• Figure 1-7. 15th cycle load vs. capacitance curves of as-spun TPU (R190A) (a), cross-linked TPU (R190A) (b), as-spun SBS (c), cross-linked SBS (d), as-spun Spandex (e), and as-spun i-Spandex (f) nanofiber web-based sensors.. 21
• Figure 1-8. 15th cycle load vs. capacitance curves of as-spun Spandex (a), Spandex nanofiber webs after physical aging (b), as-spun Spandex with 0.1 phr IL (c), 0.4 phr IL (d), 1 phr IL (e), and 3 phr IL (f) nanofiber webs-based sensors ...……………………………….…………………………………….. 25
• Figure 1-9. Hysteresis and ΔC results of different type's nanoweb based sensors (a) and Spandex nanoweb based sensors with different conditions (b).. 26
• Figure 1-10. Creep behavior of capacitance for as-spun TPU (R190A) (a), cross-linked TPU (R190A) (b), as-spun SBS (c), cross-linked SBS (d), as-spun Spandex (e), and as-spun i-Spandex (1 phr IL) (f) nanofiber web-based sensors ..…………………………………………….…………………..….… 29
• Figure 1-11. Stress relaxation behavior under constant capacitance for as-spun TPU (R190A) (a), cross-linked TPU (R190A) (b), as-spun SBS (c), cross-linked SBS (d), as-spun Spandex (e), and as-spun i- Spandex (f) nanofiber web-based sensors ............................................................................ 30
• Figure 2-1. Schematic illustration of α, β, and γ phases in PVDF [23,28]……..... 39
• Figure 2-2. Direct poling and corona poling of stretched PVDF film [28] ……. 40
• Figure 2-3. Piezoelectric behavior of poled PVDF film under external force and current field [29] ...…………..…………………………..………………... 41
• Figure 2-4. Enlarged view of C-F dipoles in PVDF nanofiber webs on the electrospun jet and on the collector cylinder. The C-F dipoles on the collector are oriented along the electric field direction [8] ...………………………...….. 44
• Figure 2-5. The simple equivalent circuit diagram for detecting the piezoelectric signal [8] ……………………………………………………...… 50
• Figure 2-6. The schematic diagram of nanoweb sensor fabrication (a), photographs of a piezo film lab amplifier (b) and a home-made dynamic pressure tester (c) .............................................................................................. 51
• Figure 2-7. Schematic diagram of nanofiber web sensor fabrication and an LCR meter (a) and photographs of a home-made dynamic/static pressure tester (b) ...………………………………………………...………...…...…... 52
• Figure 2-8. High resolution FE-SEM image of electrospun neat PVDF (a), AgNPs-PVDF (b), Spandex/PVDF (50/50) (c), Spandex/PVDF (75/25) (d), Spandex/AgNPs-PVDF (75/25) (e), Spandex/PVDF (90/10) (f), neat-Spandex (g) nanofiber webs, and plot of average fiber diameter vs. kind of nanofiber web (h) ...…………………………………………..…………...….
• 56
• Figure 2-9. ATR-IR spectra of neat PVDF (i), AgNPs-PVDF (ii), Spandex/PVDF (50/50) (iii), Spandex/PVDF (75/25) (iv), Spandex/AgNPs-PVDF (75/25) (v), and neat Spandex nanofiber web (vi) …………………….. 59
• Figure 2-10. Piezoelectric signals from electrospun neat PVDF (a), Spandex/PVDF (50/50) (b), Spandex/PVDF (75/25) (c), Spandex/PVDF (80/20) (d), Spandex/PVDF (90/10) (e), Neat Spandex (f), AgNPs-PVDF (g), and Spandex/AgNPs-PVDF (75/25) (h) nanofiber web-based sensors ………. 63
• Figure 2-11. Plots of time vs. capacitance during 20 cycles and load vs. capacitance at the 15th single cycle for electrospun neat Spandex (a and b), Spandex/PVDF (50/50) (c and d), Spandex/PVDF (75/25) (e and f), Spandex/PVDF (80/20) (g and h), Spandex/PVDF (90/10) (i and j), Spandex/AgNPs-PVDF (75/25) (k and l), and neat PVDF (m and n) nanofiber web-based sensor ... ………………………………...………...……………… 68
• Figure 2-12. A plot of 100 µm thickness normalized peak-to-peak piezoelectric voltage (Vp-p, normalized) and capacitance change (ΔCnormalized) vs. kind of nanofiber web sensor (a) and plot of Vp-p, normalized and average hysteresis % vs. kind of nanofiber web sensor (b) …………………...………. 71
• Figure 2-13. Capacitance creep behavior under constant loading of 200 N for electrospun Spandex (a), Spandex/PVDF (50/50) (b), Spandex/PVDF (75/25) (c), Spandex/PVDF (80/20) (d), Spandex/PVDF (90/10) (e), and Spandex/AgNPs-PVDF (75/25) (f) nanofiber web-based sensors ...….……… 74
• Figure 2-14. Stress relaxation behavior under constant capacitance for electrospun Spandex (a), Spandex/PVDF (50/50) (b), Spandex/PVDF (75/25) (c), Spandex/PVDF (80/20) (d), Spandex/PVDF (90/10) (e), and Spandex/AgNPs-PVDF (75/25) (f) nanofiber web-based sensors ..........……..
• 75
• Figure 3-1. PLA chain conformation of α-crystal and after drawing β-crystal formation [9] ………………………………………………...……….…..…… 85
• Figure 3-2. Schematic illustration about the distortion of PLA helical structure under shear stress [11,18] ...………………………………...……...…. 86
• Figure 3-3. Schematic illustration for deformation of PLA 31 helical structure under external pressure ...……………..………………………………...……. 86
• Figure 3-4. Schematic illustration for drawing effect on PLA during electrospinning (a) [9], Preferential dipole orientation of C=O functional group under very high DC electric field during electrospinning (b) ............................ 88
• Figure 3-5. Schematic diagram of electrospinning experimental set-up and photographs of a coaxial needle (left side bottom) used for preparing nanofiber web with core-shell structure ........ ………………………………... 95
• Figure 3-6. Fabrication of piezocapacitance sensor (a), and piezoelectric sensor (b) using Spandex-PLA core-shell nanofiber web ……………............. 97
• Figure 3-7. Schematic illustration for electrospinning and stacking of Spandex and PLA nanofiber webs; 2-layer (Spandex-PLA) nanoweb stacking (a) and 3-layer (Spandex-PLA-Spandex) nanoweb stacking (b) sensor fabrication .…………………………………………………………...…...…. 100
• Figure 3-8. Schematic illustration for consecutive electrospinning of Spandex and PLA solution; 2-layer (Spandex-PLA ) consecutive electrospun (a) and 3-layer (Spandex-PLA-Spandex) consecutive electrospun (b) nanoweb based-sensor fabrication .…………………………………………...
• 102
• Figure 3-9. FE-SEM image for coaxial electrospun PLA-Spandex (45/55) (a), PLA-Spandex (25/75) (b), and PLA-Spandex (15/85) (c) nanofiber web. The average diameter of the nanofiber is represented by "d"...…………….…. 106
• Figure 3-10. Time vs. capacitance curves of 20 cycles for as-electrospun neat PLA (a), coaxial electrospun (core-shell) PLA-Spandex (45/55) (b), PLA-Spandex (25/75) (c), PLA-Spandex (15/85) (d), and neat Spandex (e) nanofiber web-based sensor ...…………………………………………...…...
• 111
• Figure 3-11. Load vs. capacitance (hysteresis) curves of the 15th cycle for as-electrospun neat PLA (a), coaxial electrospun (core-shell) PLA-Spandex (45/55) (b), PLA-Spandex (25/75) (c), PLA-Spandex (15/85) (d), and neat Spandex (e) nanofiber web-based sensor .………..…….…....…………….…. 112
• Figure 3-12. Time vs. capacitance curves of 20 cycles and load vs. capacitance (hysteresis) curves of the 15th cycle for electrospun nanofiber webs of Spandex-PLA 2-layer stacking (a and b), Spandex-PLA-Spandex 3-layer stacking (c and d) based sensor ……………………………………….... 113
• Figure 3-13. Time vs. capacitance curves of 20 cycles and load vs. capacitance (hysteresis) curves of the 15th cycle for consecutively electrospun i-Spandex-PLA, 2-layer with 1 phr ionic liquid (a and b), with 2 phr ionic liquid (c and d), and Spandex-PLA-Spandex 3-layer without ionic liquid (e and f) nanofiber web-based sensor ...…………………………….…. 114
• Figure 3-14. Creep behavior of capacitance for as-spun neat PLA (a), PLA-Spandex (core-shell 45/55) (b), PLA-Spandex (core-shell 25/75) (c), PLA-Spandex (core-shell 15/85) (d) and neat Spandex nanofiber web-based sensor. 116
• Figure 3-15. Creep behavior of capacitance for electrospun nanofiber webs of Spandex-PLA 2-layer stacking (a) and Spandex-PLA-Spandex 3-layer stacking (b) based-sensor ……………………………………………………. 117
• Figure 3-16. Creep behavior of capacitance for consecutively electrospun i-Spandex-PLA, 2-layer with 1 phr ionic liquid (a), with 2 phr ionic liquid (b), and Spandex-PLA-Spandex 3 layer without ionic liquid (c) nanofiber web-based sensor ...……………………………………………………………….. 117
• Figure 3-17. Peak-to-peak piezoelectric output voltage (Vp-p) signals from electrospun as-spun neat PLA (a), PLA-Spandex (core-shell 45/55) (b), PLA-Spandex (core-shell 25/75) (c), and PLA-Spandex (core-shell 15/85) (d) nanofiber web-based sensor …….....................................................................
• 121
• Figure 3-18. Peak-to-peak piezoelectric output voltage (Vp-p) signals from electrospun nanofiber webs of Spandex-PLA 2-layer stacking (a) and Spandex-PLA-Spandex 3-layer stacking (b) based sensor …………………... 122
• Figure 3-19. Peak-to-peak piezoelectric output voltage (Vp-p) signals from consecutively electrospun i-Spandex-PLA, 2-layer with 1 phr ionic liquid (a), with 2 phr ionic liquid (b), and Spandex-PLA-Spandex 3-layer without ionic liquid (c) nanofiber web-based sensor .. ……………….……………………..
• 122
• Figure 3-20. A Plot of 100 µm thickness normalized peak-to-peak piezoelectric voltage (Vp-p, normalized) and capacitance change (ΔCnormalized) vs. Spandex-PLA (core-shell) nanofiber web sensor (a) and Spandex-PLA sensor various architecture (b) ...………………………………………………......… 126
• Figure 3-21. A plot of average hysteresis % vs. Spandex-PLA (core-shell) nanofiber web sensor (a) and Spandex-PLA sensor various architecture (b) … 127
• Figure 4-1. General structure of Chlorinated polyisoprene (CPI) (a), and Poly(styrene-b-butadiene-b-styrene) (SBS) tri block co-polymer (b) .............. 141
• Figure 4-2. Schematic of the manufacturing process of AgNWs with images corresponding to the steps: SBS or CPI electrospun solution with silver nitrate (a), deep brown color solution due to the silver nanoparticles reduced by DMF (b), electrospinning to fabricate silver nanoparticles embedded nanofiber web (c), nanoweb dipped in silver plating solution (d), silver-plated nanoweb (e), SBS-AgNW and CPI-AgNW dry electrode (left) and an Ag/AgCl gel electrode (right) (f) ...………………………………………………………… 145
• Figure 4-3. Water-soluble diamine silver complex compound (a), silver plating box, contain nanoweb and diamine silver complex compound after adding dextrose (b), and custom-designed silver plating equipment (c) …....... 146
• Figure 4-4. Schematic of nanoweb based dry electrode fabrication, and Ag/AgCl gel electrode (a), Ag/AgCl gel electrode and SBS-AgNW dry electrode set-up on human subjects for biopotential recording (b) ...…….…... 148
• Figure 4-5. A schematic experimental set-up for three-electrode contact impedance (a), and four-electrode contact impedance (b) [9] ...……….……… 151
• Figure 4-6. Schematic and photographs of electrode set-up on human subjects for biopotential recording, red dots denote Ag/AgCl gel electrode; blue dots refer to AgNW dry electrode. ECG measurement (a), EMG measurement for hand muscle activities (b), and EMG measurement for leg muscle activities (c) ...……………………………………………………..…………………… 154
• Figure 4-7. Electrode set-up for EIT test: agar phantom wrapped with an elastic belt composed of 16 electrodes (a), elastic belt with attached 16 electrodes, and the band was placed on subject's chest (back and front view) (b), and KHU Mark2 mfEIT system (c) ……………………………………… 157
• Figure 4-8. FIB-SEM images of as-spun CPI (a), as-spun SBS (b), silver-plated CPI (c), and silver-plated SBS (d) nanofiber webs ...…………...……... 160
• Figure 4-9. Average contact resistances and capacitance of each electrode at the ten BIS frequency ranging from 10 Hz to 500 kHz ...…………………..…. 163
• Figure 4-10. Step response of SBS-AgNW dry electrode (a), CPI-AgNW dry electrode (b), and Ag/AgCl gel electrode to 100 ms period, 50% duty cycle square wave ...………………………………………………………………... 165
• Figure 4-11. Sum of noise power spectral density of each electrode (a), all values were normalized to correct for overall electrode impedances in each electrode (b) ..........................................................................................…...… 167
• Figure 4-12. ECG recording of CPI-AgNW dry electrode and Ag/AgCl gel electrode (a), SBS-AgNW dry electrode and Ag/AgCl gel electrode (b), a comparison of the P, Q, R, S, T, and U wave of ECG signal between the CPI-AgNW dry electrode and Ag/AgCl gel electrode (c), and SBS-AgNW dry electrode and Ag/AgCl gel electrode (d) ...…………………………................
• 170
• Figure 4-13. A comparison of EMG raw data recorded of hand muscle activities using a CPI-AgNW dry electrode and Ag/AgCl gel electrode (a), SBS-AgNW dry electrode and Ag/AgCl gel electrode (b), Rectified and linear envelope of raw EMG signal collected for the CPI-AgNW dry electrode (c), SBS-AgNW dry electrode (d), and a comparison of the linear envelope of EMG signals for CPI-AgNW dry electrode and Ag/AgCl gel electrode (e), for SBS-AgNW dry electrode and Ag/AgCl gel electrode (f) …………………… 174
• Figure 4-14. A comparison of EMG raw data recorded of leg muscles various activities, such as without walking but clench and unclench of leg foot/finger (a), walking at 3 km/h (b), and walking at 6 km/h (c) speed using CPI-AgNW dry electrode and Ag/AgCl gel electrode. Rectified and linear envelope of raw EMG signal collected at without walking (d), walking at 3 km/h (e), and walking at 6 km/h (f) speed for the CPI-AgNW dry electrode. A comparison of the linear envelope of EMG signals for CPI-AgNW dry electrode and Ag/AgCl gel electrode collected at without walking (g), walking at 3 km/h (h), and walking at 6 km/h (i) speed on treadmill ……………………………..
• 175
• Figure 4-15. A comparison of EMG raw data recorded of leg muscles various activities, such as without walking but clench and unclench of leg foot/finger (a), walking at 3 km/h (b), and walking at 6 km/h (c) speed using SBS-AgNW dry electrode and Ag/AgCl gel electrode. Rectified and linear envelope of raw EMG signal collected at without walking (d), walking at 3 km/h (e), and walking at 6 km/h (f) speed for the SBS-AgNW dry electrode. A comparison of the linear envelope of EMG signals for SBS-AgNW dry electrode and Ag/AgCl gel electrode collected at without walking (g), walking at 3 km/h (h), and walking at 6 km/h (i) speed on treadmill …………………………….
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• Figure 4-16. Static agar phantom images reconstructed at different frequencies by Ag/AgCl gel electrodes (left) and SBS-AgNW dry electrodes (right). The reference image where the blue dot refers to the lowest electrical conductivity (0.02 wt% NaCl), the red dot denotes the highest electrical conductivity (0.5 wt% NaCl), and the yellow circle represents medium conductivity (0.1 wt%) (a): at 1 kHz (b), at 10 kHz (c), and at 100 kHz (d) …. 179
• Figure 4-17. Series of time-difference pulmonary images representing air filling during inspiration recorded through EIT system at an injection current of 1mA and frequency of 50 kHz by Ag/AgCl gel electrodes (a), and SBS–AgNW dry electrodes (b) ...……………………………………………….…. 182
• Figure 4-18. ECG recording of SBS-AgNW dry electrode denoted by blue color signals, and Ag/AgCl gel electrode indicated by pink color signals for electrode stability test at the varying time. Both the electrodes were continuously attached to the subjects' chest for long 24 hs and obtained ECG signal at 0 h, at after 1 h, 2 hs, 3hs, 4hs, 5hs, 6hs, 8hs, 12hs, 16hs, and 24hs ..... 185
• -List of Tables-
• Table 1-1. Hysteresis and capacitance change (ΔC) of various type electrospun nanofiber web-based sensors ...…………………….…………… 27
• Table 2-1. 100 µm thickness normalized piezoelectric peak-to-peak output voltages (Vp-p, normalized), 100 µm thickness normalized capacitance change (ΔCnormalized), expected Vp-p, normalized by only PVDF part, and avg. hysteresis (%) of various types of electrospun nanofiber web-based sensors …………… 72
• Table 3-1. Summary of PLA-Spandex coaxial electrospinning set-up……… 96
• Table 3-2. 100 µm thickness normalized piezoelectric peak-to-peak output voltages (Vp-p, normalized), 100 µm thickness normalized capacitance change (ΔCnormalized), and average hysteresis (%) of PLA-Spandex various architecture nanoweb sensor …………………………………………….…... 128