전기방사 공정을 통한 헬리컬 구조의 은나노 섬유의 제조

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부(하이브리드 재료), 2020. 8. 유웅열.This study aimed to fabricate silver nanofibers using new process parameters of electrospinning and develop transparent and stretchable electrodes for stretchable electronics using them. A series of research was carried out to achieve goals as follows. A multi-physics model for the simulation of gas-assisted melt-electrospinning (GAME) process was developed to understand the roles of process parameters. By numerically calculating the stresses acting on the jet during a single-nozzle GAME process, the shear viscous stress was identified as the main factor of jet stretch. The jet stretch ratio increased sharply when shear viscous stress reached the level at which jet sharpening occurred, leading to stable jet formation. This stress was defined as the critical shear viscous stress to determine stable spinnability. In addition, a multi-nozzle GAME was simulated, proposing a spinnability diagram for stable spinning. A new process was designed to fabricate helical fibers. Here, the effect of solidification behavior of the jet on the formation of intrinsic curvature and on the final morphology of electrospun fibers was investigated. Fiber morphology during electrospinning was observed to dramatically change from straight to helical due to rapid solidification of the jet. Investigation of the resulting jet morphologies revealed that fiber structure changed from straight to helical as the vapor pressure increased. A similar effect was observed with conductive solutions prepared by adding large amounts of metal ion to the polymer solution. Simulations revealed that the jet near the nozzle tip was subject to a strong electrical field due to increased charge density. The thickness of the emerging fiber was rapidly reduced with fast and simultaneous solidification, resulting in helical nanofibers. A mechanism was suggested that can describe the formation of helical fibers. Transparent and stretchable electrodes (TSEs) was fabricated using electrospun silver nanofibers. Here, a composite comprising shape memory polymer–TSE (SMP–TSE) using crosslinked polycyclooctene as a substrate was fabricated, which showed wrinkle-free deformation and switchable optical transparency. Because of its considerable elongation without residual strain and the shape memory behavior of polycyclooctene, in-plane buckled nanofibers were formed effectively. Due to these in-plane buckled nanofibers, the electrode maintained its resistance during 3,000 cycles of a bending test and 900 cycles of a tensile test. Furthermore, SMP–TSE was able to electrically control its temperature, optical transparency, elastic modulus, and shape memory behavior. Finally, SMP–TSE was demonstrated for a smart electrode that could control its optical and mechanical properties. Keywords: Electrospinning, Numerical simulation, Process parameters, Silver nanofibers, Transparent and stretchable electrode Student number: 2014-22539본 연구의 목적은 전기방사 공정에서의 새로운 공정 변수들을 이용하여 은나노섬유를 제작하고 이를 활용하여 투명·신축 전극을 제작하는 것이다. 이를 위한 일련의 연구들이 다음의 순서로 진행되었다. Gas-assisted 용융 전기방사 공정에 대한 multi-physics 모델링을 개발하였으며 이를 통하여 공정 변수들의 역할을 파악하였다. 단일 노즐을 이용한 공정에서 젯의 표면에 인가되는 응력을 수치해석을 통하여 분석함으로써 젯의 인장에 있어 점성 전단 응력이 주요하게 작용함을 밝혀냈다. 젯은 점성 전단 응력이 특정 값에 도달하였을 때에 안정적으로 형성되었으며, 젯의 인장률 또한 급격히 증가하였다. 이때의 응력을 안정적인 방사성을 판단하는 임계 점성 전단 응력이라 정의 하였다. 단일 노즐 공정뿐만 아니라 멀티 노즐 공정을 모델링하였으며 이로부터 안정적인 방사성을 판단할 수 있는 방사성 다이어그램을 제시하였다. 다음으로는 섬유의 구조를 헬리컬 구조로 제작하는 공정을 디자인 하였다. 우선, 젯의 초기 곡률과 젯의 위치에 따른 고화가 섬유의 구조에 미치는 영향을 조사하였다. 전기 방사된 섬유의 구조는 용매의 증기압이 증가함에 따라 발생하는 빠른 고화로 인하여 직선형태에서 헬리컬 구조로 변화하였다. 이는 금속 이온이 과량으로 첨가된 전도성 용액에 대해서도 유사한 결과를 보였다. 이에 대한 시뮬레이션 결과는 전하 밀도의 증가가 강한 전기장을 발생시켰으며, 이로 인하여 젯의 급격한 인장 및 고화가 발생하였음을 보여주며, 그러한 이유로 헬리컬 구조가 형성되었음 나타냈다. 이를 이용하여 헬리컬 구조의 섬유가 형성되는 메커니즘을 제시하였다. 마지막으로 전기방사를 이용해 제작한 은나노섬유를 이용하여 투명·신축 전극을 제작하였다. 전극은 형상기억 고분자인 crosslinked polycyclooctene을 기판으로 활용하였다. 제작한 전극은 잔류 변형이 없고 투명도를 제어할 수 있는 특성을 보였다. 큰 인장에도 잔류 변형을 보이지 않는 형상기억고분자 기판의 특성으로 인해 면내 굽힘 구조의 은나노섬유가 효율적으로 제작되었다. 이러한 특성으로 제작한 전극은 3,000회의 굽힘 평가와 900회의 인장평가에도 전도성을 유지할 수 있었다. 또한 제작한 전극은 전기적 자극을 통하여 온도, 투명도, 강성 및 형상기억 특성을 제어할 수 있다는 특징을 보였다. 제작한 전극을 활용하여 광학적 그리고 기계적 특성을 제어할 수 있는 새로운 형태의 스마트 전극을 시연 하였다. 핵심어: 전기방사공정, 전산모사, 공정변수, 은나노섬유, 투명·신축 전극 학번: 2014-22539Chapter 1. Introduction 1 1.1. Electrospinning 1 1.1.1. Introduction of electrospinning 1 1.1.2. Types of electrospinning 3 1.1.3. Parameters in electrospinning 6 1.1.4. Structures of electrospun nanofibers 16 1.1.5. Application of electrospun nanofibers 26 1.1.6. Limitation and perspective of electrospinning 44 1.2. Research objectives 47 Chapter 2. Numerical simulation of gas-assisted melt electrospinning 50 2.1. Needs for modeling of gas-assisted melt electrospinning 50 2.2. Methods 53 2.2.1. Gas-assisted melt-electrospinning process 53 2.2.2. Numerical simulation of single-nozzle GAME process 56 2.2.3. Calculation of electric field in multi-nozzle configuration 60 2.2.4. Numerical simulation of multi-nozzle GAME process 61 2.3. Results and discussion 62 2.3.1. Simulation of single-nozzle GAME process 62 2.3.2. Simulation of multi-nozzle GAME process 73 2.4. Summary 81 Chapter 3. Fabrication of inherently helical structure nanofibers 83 3.1. Needs for fabrication of helical nanofibers 83 3.2. Experimental 85 3.2.1. Preparation of dielectric solution for helical nanofibers 84 3.2.2. Preparation of conductive solution for helical nanofibers 86 3.2.3. Electrospinning and spinneret geometry 86 3.2.4. Characterization of Electrospun Fibers 87 3.3. Results and discussion 88 3.3.1. Effect of solvent vapor pressure on structure 88 3.3.2. Effects of solidification on structure 94 3.3.3. Numerical simulations of jet near nozzle 99 3.3.4. Further enhanced helical structrues 105 3.4. Summary 111 Chapter 4. Fabrication of a stretchable, wrinkle-free electrode with switchable transparency 113 4.1. Transparent and stretchable electrode 113 4.2. Experimental 116 4.2.1. Materials 116 4.2.2. Preparation of shape memory polymer substrate 117 4.2.3. Fabrication of free-standing silver nanofiber 117 4.2.4. Characterization of SMP–TSE 118 4.3. Results and discussion 119 4.3.1. Fabrication of free-standing silver nanofibers 119 4.3.2. Optoelectrical properties of silver nanofibers 128 4.3.3. Shape memory substrate 129 4.3.4. Shape memory polymer–transparent and stretchable electrode 137 4.4. Summary 143 Chapter 5. Conclusions 145 Chapter 6. Appendix 147 Reference 161 Korean abstract 189Docto

Exploration of torsional actuation and twist to writhe transition in nanostructured hydrogels

Torsional artificial muscles are a branch of actuators that react to a stimulus by rotating. This rotation is driven by a change in volume and mechanical properties such as modulus and was shown to be extremely large in the case of twisted fibers due to their helical geometry. The following thesis introduces a new method of fabrication of nanofiber yarns and nanocomposites with the aim of making hydrogel torsional catch actuators that combine responsiveness to pH changes and a high torsional output as well as a systematic approach to the modeling of their behavior using the single helix theory

Manufacturing polyacrylonitrile nanowires and nanofibers for sensing and energy storage applications

A novel flow guided assembly approach is presented to well align and accurately position nanowire arrays in pre-defined locations with high throughput and large scale manufacturing capability. In this approach, polyacrylonitrile (PAN)/N, N-dimethylformamide (DMF) solution is first filled in an array of microfluidic channels. Then a gas flow is introduced to blow out most solutions while pushing a little leftover against the channel wall to assemble into polymer nanowires. In this way, highly-ordered nanowires are conveniently aligned in the flow direction and patterned along both sides of the microchannels. In this study, we demonstrated this flow-guided assembly process by producing millimeter-long nanowires across 5 mm x 12 mm area in the same orientation and with basic I-shape , T-shape , and cross patterns. The assembled polymer nanowires were further converted to conductive carbon nanowires through a standard carbonization process. After integrated into electronic sensors, high sensitivity was found in model protein sensing tests. This new nanowire manufacturing approach is anticipated to open new doors to the fabrication of nanowire-based sensing systems and serve as the Good Manufacturing Practices (GMP) (a system for ensuring that products are consistently produced and controlled according to quality standards) for its simplicity, low cost, alignment reliability, and high throughput. By using the same polymer solution (polyacrylonitrile (PAN)/N, N- dimethylformamide (DMF) solution), a new, simple, and low-cost method has been developed in the production of porous composite nanofibers via a one-step foaming and electrospinning process. Sublimable aluminum acetylacetonate (AACA) was dissolved into polyacrylonitrile (PAN)/N, N-dimethylformamide (DMF) solution as the foaming agent. Silicon nanoparticles were then added and the resulting suspension solution was further electrospun to produce PAN/silicon composite nanofibers. The PAN nanofibers were then foamed during a thermal stabilization treatment and further carbonized into carbon/silicon composite nanofibers. Such mesoporous composites nanofiber mats were explored as the anode material for lithium ion batteries. Within this composite of nanofiber electrode, carbon nanofibers serve as the conductive media, while silicon nanoparticles ensure high lithium ion capacity and electrical density. The inter-fiber macrovoids and intra-fiber mesopores provide the buffering space to accommodate the huge volume expansion and consequent stress in the composite anode during the alloying process to mitigate electrode pulverization. Its high surface-to-volume ratio helps facilitate lithium ion transport between electrolytes and the active materials. Our electrochemical tests demonstrated higher reversible capacity and better capacity retention with this porous carbon/silicon composite nanofiber anode when compared with that made of nonporous composite nanofibers and CNF alone with similar treatments

CONTROLLABILITY OF ELECTROSPINNING AND ELECTROSPRAYING - ADVANCES AND APPLICATION

Master'sMASTER OF ENGINEERIN

Electrospinning of Conducting Polymer Fibers for Stretchable Electronics

L’électronique étirable est un domaine prometteur en ce qui concerne les applications au biomédical. En effet, les dispositifs étirables peuvent être utilisés pour remplir diverses fonctions, qui incluent l’électronique portable (ou les vêtements intelligents), la peau artificielle, et de façon plus générale l’ensemble des fonctions qui exigent d’avoir de l’électronique placée directement sur la peau apte à se conformer au style de vie du patient, par exemple pour de la surveillance quotidienne des constantes biologiques d’un patient. De nombreuses stratégies ont été mises en place jusqu’à présent pour produire de l’électronique étirable, cependant celles-ci peuvent être grossièrement séparées en deux catégories principales. Dans la première se retrouvent toutes les stratégies où les matériaux sont étirés grâce à l’utilisation d’une géométrie spécifique, tandis que la seconde catégorie comprend l’ensemble des matériaux qui sont intrinsèquement étirables. Ainsi, des formes spécifiques comme des fibres peuvent être utilisées pour améliorer la capacité à s’étirer d’un matériau autrement peu étirable, ce qui inclut des matériaux conducteurs comme les métaux ou certains polymères conducteurs et semi-conducteurs utilisés en électronique organique. Cependant, la mise en pratique de ces fibres requière l’utilisation d’une technique apte à aisément générer des fibres conductrices. Pour les applications en biomédical, les matériaux électroniques organiques présentent l’avantage sur l’électronique classique de posséder une bonne compatibilité avec les systèmes biologiques du fait de leur capacité à aisément faire l’interface avec le milieu biologique. Ils présentent aussi l’avantage pour ces applications de disposer d’une capacité à conduire à la fois les ions et les électrons. Le but de ce projet de recherche est de démontrer la faisabilité de la fabrication de tels films, faits de nanofibres en polymère conducteur, qui maintiennent leur capacité à conduire le courant même lorsque ceux-ci sont étirés. Bien que de nombreuses méthodes existent pour produire de telles fibres, l’électrofilage apparaît comme étant l’une des méthodes les plus simples pour réaliser des couches poreuses et non tissées de nanofibres, couches qui peuvent aisément se conformer à la surface de leur substrat. En combinant l’électrofilage avec une technique appelée la polymérisation en phase vapeur, nous avons fabriqué des nanofibres conductrices de poly(3,4-éthylènedioxythiophène) dopé avec de l’acide paratoluènesulfonique (tosylate, PEDOT:Tos) directement sur du polydiméthylsiloxane (PDMS), un élastomère organique siliconé. Cette méthode simple à deux étapes nous a permis de produire des nanofibres de poly(3,4-éthylènedioxythiophène) dopé avec du tosylate (PEDOT:Tos) sur du PDMS. Des couches fibreuses non tissées composées de nanofibres conductrices possédant un diamètre moyen d’un peu moins de 700 nm ont ainsi été obtenues directement sur le PDMS. Nous avons caractérisé ces fibres pour étudier leur comportement électrique lorsqu’une tension était appliquée à leurs extrémités. Ces tapis de fibres ont alors pu être étirés tandis qu’un voltage fixé appliqué directement dessus forçait l’écoulement d’un courant à l’intérieur des films, courant qui a été mesuré. Cela nous a permis de démontrer que ces films possédaient la capacité de s’étirer jusqu’à 140% de leur longueur initiale sans variation majeure de la quantité de courant s’écoulant dans les films.----------ABSTRACT Stretchable electronics is a promising field for biomedical applications. Stretchable devices can be used for various purposes, including wearable electronics (or smart clothes), artificial skin, and more generally for any purpose requiring to have on-skin electronics that conform to the lifestyle of the patient, for example day-by-day biomonitoring. Many strategies have been used so far to produce stretchable electronics, however these can be split between two main categories. In the first one are the materials that stretch due to a specific geometry, while in the second category are the materials that are intrinsically stretchable. Specific shapes such as fibers can thus be used to improve the stretchability of an otherwise poorly-stretchable material, including conductive materials such as metals or conducting and semi-conducting polymers used in organic electronics. However, the practical application of fibers in stretchable electronics requires the use of a technique that can easily yield conductive fibers. For biological applications, organic electronic materials present the advantage over conventional electronic materials to possess a good compatibility with biological systems due to their ability to easily interface with the biological milieu and their mixed ionic / electronic conduction. The objective of this research project is to demonstrate the fabrication of such films, made with conductive polymer nanofibers that can still conduct the current even when stretched. Although many methods exist to produce such fibers, electrospinning is one of the easiest ways to directly make non-woven porous nanofiber mats that can conform to the surface of their substrate. By combining electrospinning with vapor phase polymerization, we fabricated conductive nanofibers of poly-(3,4-ethylenedioxythiophene) doped with paratolenesulfonate (tosylate, PEDOT:Tos) directly on polydimethylsiloxane (PDMS), an organosilicon elastomer. Non-woven fiber mats composed of conductive nanofibers with an average diameter of around 700 nm were obtained directly on PDMS. We characterized these fibers to study their electrical behavior when a strain was applied to them. These mats were then stretched while the current flowing inside them was measured, at fixed voltage. This allowed us to demonstrate a stretchability up to 140% of the initial length without major variation of the current flowing in the mats

A prototype for 3D electrohydrodynamic printing

Electrohydrodynamic direct writing is a flexible cost effective alternative technique that is capable of producing a very fine jet of liquid in the presence of an external electric field. This jet can then be used to pattern surfaces in an ordered and controlled fashion and offers a robust route to low cost large area micro and nano-manufacturing. Unlike other types of direct writing techniques, the liquid in electrohydrodynamic printing is subjected to both pushing and pulling forces. The pushing force is brought about by the constant flow rate that is maintained via high precision mechanical pumps while a pulling force is applied through a potential difference that is applied between the nozzle and the ground electrode and as a result a fine jet can be generated to pattern surfaces. The impracticality of use and the cost of building micrometre and sub-micrometre sized nozzles to print narrow line widths warrant an investigation into alternative means of dispensing printing inks using nozzles that are cheap to produce, easy to handle and consistent in delivery. The enormous capillary pressures that would have to be overcome in order to print highly viscous materials with micrometre and sub-micrometre sized nozzles may also limit the types of feed that could be used in printing narrow line widths. Thus, the initial work described is focused on improving print head design in an attempt to electrohydrodynamic print pattern narrow line widths using silk fibroin. This is followed by work where we attempt to design and construct of a new electrohydrodynamic printing machine with the sole purpose of expediting research in electrohydrodynamic printing in a flexible, feasible and user friendly manner. To achieve this, replicating rapid prototype technology is merged with conventional electrohydrodynamic printing phenomena to produce a EHD printing machine capable of print depositing narrow line widths. In order to validate the device the work also describes an attempt to print a fully formed human ear out of polycaprolactone. Finally, we investigate an approach to the electohydrodynamic printing of nasal septal scaffolds using the microfabrication system that was developed and optimized in our laboratory. In these initial stages we were successful in showing the degree of control and flexibility we possess when manufacturing constructs out of a biodegradable polymer ( polycaprolactone) from the micro to macro scale through manipulation of just one process parameter (concentration). This work also features characterization of scaffold mechanical properties using a recently invented Atomic force microscopy technique called PeakForce QNM (Quantitative Nanomechanical Property Mapping)

A novel Electrospinning Procedure for the Production of Straight Aligned and Winded Fibers

An electrospinning procedure allowing the spinning of a straight jet of polymer solution was developed. By using proper collector devices, it enables to collect winded and aligned fibers and to prepare polymeric constructs developing along the Z axis. The reported results are expected to provide basic understandings on which parameters are controlling the stability/instability of the process and implement new applications of electrospinning with specific reference to the preparation of well defined three-dimensional structure

Adaptive fabrication of biofunctional decellularized extracellular matrix niche towards complex engineered tissues

Recreating organ-specific microenvironments of the extracellular matrix (ECM) in vitro has been an ongoing challenge in biofabrication. In this study, I present a biofunctional ECM-mimicking protein scaffold with tunable biochemical, mechanical and topographical properties. This scaffold, formed by microfibres, displays three favorable characteristics as a cell culture platform: high-loading of key ECM proteins, single-layered mesh membrane with controllable mesh size, and flexibility for supporting a range of cell culture configurations. Decellularized extracellular matrix (dECM) powder was used to fabricate this protein scaffold, as a close replicate of the chemical composition of physiological ECM. The highest dECM concentration in the solidified protein scaffold was 50 wt%, with gelatin consisting the rest. In practice, a high density of dECM-laden nano- to microfibres was directly patterned on a variety of substrates to form a single layer of mesh membrane, using the low-voltage electrospinning patterning (LEP) method. The smallest fibre diameter was measured at 450 nm, the smallest mesh size of the membrane was below 1 μm, and the thickness of the membrane was estimated to be less than 2 μm. This fabrication method demonstrated a good preservation of the key ECM proteins and growth factors, including collagen IV, laminin, fibronectin, VEGF and b-FGF. The integrated fibrous mesh exhibited robust mechanical properties, with tunable fibril Young’s modulus for over two orders of magnitude in the physiological range (depending on the dECM concentration). Combining this mesh membrane with 3D printing, a cell culture device was constructed. Co-culture of human glomerulus endothelial cells and podocytes was performed on this device, to simulate the blood-to-urine interface in vitro. Good cell attachment and viability were demonstrated, and specific cell differentiation and fibronectin secretion were observed. This dECM-laden protein scaffold sees the potential to be incorporated into a glomerulus-on-chip model, to further improve the physiological relevance of in vitro pathological models.The Engineering and Physical Sciences Research Council (EPSRC

Exploitation of Super(de)wettability via Scalable Hierarchical Surface Texturing

The field of wettability is an age-old topic that has been revitalized in the last two decades. Historically, the diverse physical phenomena of wetting has influenced the development of inventions that dates back to the paleolithic era (2,600,000 to 10,000 BC) in the form of charcoal and ochre -based cave paintings, or the mesolithic (10,000 to 5,000 BC) and neolithic (5,000 to 2,000 BC) periods as pottery and soaps. Since the end of the Stone Age, human civilizations and scientific discoveries have progressed by leaps and bounds. Despite the advances in metallurgy, optics, chemistry, mechanics, mathematics and electricity, our understanding of fluid-surface interactions remained stagnant until 1804. Between 1804 and 1805, Thomas Young described the concept of a wetting contact angle, which controls the equilibrium shape of a fluid droplet on a surface, thus making wettability a quantified branch of physics. The late entry of this scientific field is astounding, considering the ubiquitousness of water on Earth. Despite Young’s discoveries, the area remained largely unexplored. Work on wettability was intermittent, with Edward Washburn on capillary effects in 1921 and later on, Robert Wenzel and Cassie-Baxter in 1936 and 1944 on the wetting of rough interfaces. In 1997, almost exactly 20 years ago, the field was rejuvenated by the corresponding discoveries of superhydrophilicity (water droplets spread into a sheet) and superhydrophobicity (water droplets ball up), by Wang et al. and Neinhuis et al. respectively. Since their work into these distinct super(de)wetting states, the field has grown exponentially. Today, its revival can be attributed to biomimetics (engineering mimicry / imitation of life) and a revolutionized understanding behind super(de)wetting mechanisms that are found in nature. The precise combination of hierarchical (multi-scale) texturing with select surface chemical composition is vital towards fabricating interfaces with specialized wetting properties. Knowledge behind the careful control of surface texturing holds immense potential for enabling a plethora of user-defined functional interfaces. As of the time of writing, the field of wettability encompasses multiple domains, such as superhydrophilicity (water-loving),[8] slippery superhydrophobicity (water-fearing), adhesive superhydrophobicity (an unintuitive love-fear relationship with water), superoleophobicity (oil-fearing), superamphiphobicity (water- and oil-fearing),[11] superomniphobicity (all-fearing) as well as a range of other important intermediary, cross-environment wetting states. Methods employed for achieving super(de)wettability can be broadly classified under 2 sub-classes. The first relies on intricate top-down photolithography (-drawing with light) or templating-based designs while the other uses the realms of chaotic, but deterministic and scalable bottom-up self-assembly. Both routes are promising for the development of unique super(de)wetting states, albeit with considerable drawbacks on both fronts. For instance, while lithography and templating have demonstrated exemplary surface texturing precision and super(de)wetting performance, these methods remain limited by poor scalability, complexity and costs in instrumentation and operation. Alternatively, scalable and cheap bottom-up self-assembly methods can exist within complex electro-, hydro-, aero-, thermal- or thermo-dynamically varied regimes. Consequently, each system requires intense cross-optimization research efforts in determining niche operating parameters. In this work, we explore a series of highly promising hierarchically structured material interfaces that were enabled by understanding, taming and controlling scalable but chaotic bottom-up methods. To this end, we demonstrate their potential within the entire super(de)wetting spectrum, showcased through a series of coatings and further exemplified by functional micro(fluid)mechanical systems (M-F-MS)