Epidermal systems and virtual reality: Emerging disruptive technology for military applications

This review study, presented at the 2nd World Conference on Advanced Materials for Defense (AuxDefense 2020), focuses on skin as sensory interface and explores the latest discoveries in bioelectronic science. The work analyzes at what extent invisibility is possible by emulating nature, and if military applications can really benefit from technology that combines epidermal systems and virtual reality — and from next generation of wearable textile computing technologies.info:eu-repo/semantics/acceptedVersio

Epidermal systems and virtual reality: Emerging disruptive technology for military applications

This review study, presented at the 2nd World Conference on Advanced Materials for Defense (AuxDefense 2020), focuses on skin as sensory interface and explores the latest discoveries in bioelectronic science. The work analyzes at what extent invisibility is possible by emulating nature, and if military applications can really benefit from technology that combines epidermal systems and virtual reality — and from next generation of wearable textile computing technologies.info:eu-repo/semantics/acceptedVersio

The role of printed electronics and related technologies in the development of smart connected products

The emergence of novel materials with flexible and stretchable characteristics, and the use of new processing technologies, have allowed for the development of new connected devices and applications. Using printed electronics, traditional electronic elements are being combined with flexible components and allowing for the development of new smart connected products. As a result, devices that are capable of sensing, actuating, and communicating remotely while being low-cost, lightweight, conformable, and easily customizable are already being developed. Combined with the expansion of the Internet of Things, artificial intelligence, and encryption algorithms, the overall attractiveness of these technologies has prompted new applications to appear in almost every sector. The exponential technological development is currently allowing for the ‘smartification’ of cities, manufacturing, healthcare, agriculture, logistics, among others. In this review article, the steps towards this transition are approached, starting from the conceptualization of smart connected products and their main markets. The manufacturing technologies are then presented, with focus on printing-based ones, compatible with organic materials. Finally, each one of the printable components is presented and some applications are discussed.This work has been supported by NORTE-06-3559- FSE-000018, integrated in the invitation NORTE59-2018-41, aiming the Hiring of Highly Qualified Human Resources, co-financed by the Regional Operational Programme of the North 2020, thematic area of Competitiveness and Employment, through the European Social Fund (ESF), and by the scope of projects with references UIDB/05256/2020 and UIDP/05256/2020, financed by FCT—Fundação para a Ciência e Tecnologia, Portugal

웨어러블 센서 및 에너지 소자의 공간 신호 및 열 전달 증진을 위한 나노복합체를 이용한 기계적 순응성 향상

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2020. 8. 홍용택.Electronic skin (e-skin) that mimics mechanical and functional properties of human skin has a strong impact on the field of wearable electronics. Beyond being just wearable, e-skin seamlessly interfaces human, machine, and environment by perfectly adhering to soft and time-dynamic three-dimensional (3D) geometries of human skin and organs. Real-time and intimate access to the sources of physical and biological signals can be achieved by adopting soft or flexible electronic sensors that can detect pressure, strain, temperature, and chemical substances. Such extensions in accessible signals drastically accelerate the growth of the Internet of Things (IoT) and expand its application to health monitoring, medical implants, and novel human-machine interfaces. In wearable sensors and energy devices, which are essential building blocks for skin-like functionalities and self-power generation in e-skin, spatial signals and heat are transferred from time-dynamic 3D environments through numerous geometries and electrical devices. Therefore, the transfer of high-fidelity signals or a large amount of heat is of great importance in these devices. The mechanical conformability potentially enhances the signal/heat transfer by providing conformal geometries with the 3D sources. However, while the relation between system conformability and electrical signals has been widely investigated, studies on its effect on the transfer of other mechanical signals and heat remain in their early stages. Furthermore, because active materials and their designs for sensors and energy devices have been optimized to maximize their performances, it is challenging to develop ultrathin or soft forms of active layers without compromising their performances. Therefore, many devices in these fields suffer from poor spatial signal/heat transfer due to limited conformability. In this dissertation, to ultimately augment the functionalities of wearable sensors and energy devices, comprehensive studies on conformability enhancement via composite materials and its effect on signal/heat transfer, especially in pressure sensors and thermoelectric generators (TEGs), are conducted. A solution for each device is carefully optimized to reinforce its conformability, taking account of the structure, characteristics, and potential advantages of the device. As a result, the mechanical conformability of each device is significantly enhanced, improving signal/heat transfer and consequently augmenting its functionalities, which have been considered as tough challenges in each area. The effect of the superior conformability on signal/heat transfer is systematically analyzed via a series of experiments and finite element analyses. Demonstrations of practical wearable electronics show the feasibility of the proposed strategies. For wearable pressure sensors, ultrathin piezoresistive layers are developed using cellulose/nanowire nanocomposites (CNNs). The unique nanostructured surface enables unprecedentedly high sensor performances such as ultrahigh sensitivity, wide working range, and fast response time without microstructures in sensing layers. Because the ultrathin pressure sensor perfectly conforms to 3D contact objects, it transfers pressure distribution into conductivity distribution with high spatial fidelity. When integrated with a quantum dot-based electroluminescent film, the transferred high-resolution pressure distribution is directly visualized without the need for pixel structures. The electroluminescent skin enables real-time smart touch interfaces that can identify the user as well as touch force and location. For high-performance wearable TEGs, an intrinsically soft heat transfer and electrical interconnection platform (SHEP) is developed. The SHEP comprises AgNW random networks for intrinsically stretchable electrodes and magnetically self-assembled metal particles for soft thermal conductors (STCs). The stretchable electrodes lower the flexural rigidity, and the STCs enhance the heat exchange capability of the soft platform, maintaining its softness. As a result, a compliant TEG with SHEPs forms unprecedentedly conformal contact with 3D heat sources, thereby enhancing the heat transfer to the TE legs. This results in significant improvement in thermal energy harvesting on 3D surfaces. Self-powered wearable warning systems indicating an abrupt temperature increase with light-emitting alarms are demonstrated to show the feasibility of this strategy. This study provides a systematic and comprehensive framework for enhancing mechanical conformability of e-skin and consequently improving the transfer of spatial signals and energy from time-dynamic and complex 3D surfaces. The framework can be universally applied to other fields in wearable electronics that require improvement in signal/energy transfer through conformal contact with 3D surfaces. The materials, manufacturing methods, and devices introduced in this dissertation will be actively exploited in practical and futuristic applications of wearable electronics such as skin-attachable advanced user interfaces, implantable bio-imaging systems, nervous systems in soft robotics, and self-powered artificial tactile systems.인간 피부의 기계적 특성 및 기능을 모방하는 전자피부(electronic skin, e-skin)는 웨어러블 전자기기 분야의 트렌드를 바꾸고 있다. 기존의 웨어러블 전자기기가 단지 착용하는데 그쳤다면, 전자피부는 인간의 피부와 장기 표면에 완벽하게 붙어 동작함으로써 기존에는 접근 불가능 했던 다양한 생체 신호를 높은 신뢰도로 감지하고 처리할 수 있다. 실시간으로 감지 가능한 생체 신호의 확장은 사물인터넷(Internet of Things, IoT)의 성장을 획기적으로 가속화하고 헬스케어, 의료용 임플란트, 소프트 로봇 및 새로운 휴먼 머신 인터페이스로의 응용을 가능하게 한다. 전자피부의 필수요소인 센서와 에너지 소자에서는 삼차원 표면의 공간신호와 열에너지를 손실 없이 전달하는 것이 매우 중요하다. 이러한 공간 신호와 열에너지는 다양한 기하 구조와 전자소자를 거쳐 처리 가능한 신호로 전달된다. 이 과정에서 3차원 표면에 빈틈없이 붙는 기계적 순응성(mechanical conformability)은 공간신호와 열에너지를 왜곡 없이 전달하는 것을 가능하게 한다. 전자피부의 기계적 순응성을 증가시키는 방법은 크게 다음과 같이 두 가지로 나눌 수 있다. (1) 전자피부를 두께를 낮추는 전략과 (2) 전자피부의 영률(Youngs modulus)을 낮추어 고무와 같이 부드럽게 만드는 전략이다. 하지만, 기존 센서 및 에너지 소자를 위한 재료와 디자인이 각 장치의 성능을 향상시키는 것에 초점이 맞추어져 있기 때문에, 고성능을 유지하면서 매우 얇거나 연질 형태의 소자를 개발하는 것은 매우 도전적이었다. 따라서 고유연성을 확보하지 못한 기존 센서와 에너지 소자는 공간 신호 및 열 전달이 심각하게 저해되고, 이로 인해 공간 압력의 왜곡, 열전 효율의 저하와 같은 한계를 보여준다. 이 논문에서는 웨어러블 센서와 에너지 소자의 비약적인 기능 향상을 궁극적인 목표로, 각 소자에 최적화된 재료와 제작방식, 구조를 이용해 이들의 기계적 순응성을 획기적으로 높이고, 이를 통한 공간 신호 및 열 전달의 향상을 심도 있게 분석한다. 특히, 두께를 낮추거나 영률을 낮추는 두 가지 전략 중 각 소자에 가장 적합한 전략을 선택하고, 체계적인 방법론을 적용하여 이들의 기계적 순응성과 공간 신호 및 열 전달을 증진시킨다. 이 과정에서 나노융복합재료가 각 전략을 구현하는 핵심 요소로 작용한다. 각 소자에 따른 구체적인 연구 내용은 다음과 같다. 첫째, 압력 센서의 경우 초박막 셀룰로오스/나노와이어 복합체를 이용하여 고성능의 저항방식 압력 센서를 개발한다. 이러한 복합체는 표면에 형성된 고유한 나노구조 덕분에 마이크로구조체를 이용한 기존 압력 센서보다 월등한 성능을 보여준다. 특히, 1 마이크로 미터 수준의 매우 얇은 두께로 인해 접촉 물체의 복잡한 형상에 완벽하게 순응할 수 있고, 이로 인해 고해상도 압력 분포를 왜곡 없이 저항 분포로 전달할 수 있다. 이러한 압력 센서를 양자 점 발광소자와 결합하여 고해상도의 압력분포를 높은 정밀도로 이미징 가능한 발광 소자를 보고한다. 둘째, 열전 소자의 경우 기존의 금속 전극으로 인한 낮은 유연성과 탄성중합체의 낮은 열 전도도를 극복하기 위해 열 전달 능력이 획기적으로 향상된 낮은 영률의 소프트 전극 플랫폼을 개발한다. 소프트 플랫폼은 내부에 은 나노와이어 기반의 신축성 전극을 갖고 있으며, 자기장을 통해 자가 정렬된 금속 입자들이 효과적으로 외부 열을 열전 재료에 전달한다. 이를 기반으로 제작된 고유연성 열전 소자는 삼차원 열원에 빈틈없이 붙어 열 손실을 최소화 하며, 이로 인해 높은 열전 효율을 달성한다. 이 논문은 다양한 전자소자의 유연성을 증진시키고 이를 통한 공간 신호 및 열 전달의 향상을 도모하고 분석하는 체계적이고 종합적인 방법론을 제시했다는 데 큰 의의가 있다. 제안된 방법론은 분야에 국한되지 않고 다양한 소자의 개발에 적용할 수 있어 웨어러블 기기와 전자피부 분야의 기계적, 기능적 발전에 크게 기여할 것으로 기대된다. 뿐만 아니라 이 연구에서 최초로 개발한 소재 및 소자들은 다양한 웨어러블 어플리케이션과 산업에 곧바로 융합되고 응용될 수 있다. 이를 통해 신체 부착 및 삽입 가능한 생체 이미징 시스템, 소프트 로봇을 위한 신경 체계, 자가 발전이 가능한 인공 감각 기관, 가상 및 증강 현실을 위한 새로운 유저 인터페이스와 같은 미래 지향적 융합 어플리케이션의 실현을 앞당길 것으로 기대된다.Chapter 1. Introduction 1 1.1 Wearable Electronics and Electronic Skin 1 1.2 Mechanical Conformability of Electronic Skin 6 1.2.1 Definition and Advantages 6 1.2.2 Thickness-Based Conformability 11 1.2.3 Softness-Based Conformability 15 1.3 Conformability for Enhanced Signal/Heat Transfer in Wearable Sensors and Energy Devices 19 1.3.1 Conformability for Spatial Signal Transfer in Pressure Sensors 20 1.3.2 Conformability for Heat Transfer in Thermoelectric Generators 22 1.4 Motivation and Organization of This Dissertation 24 Chapter 2. Ultrathin Cellulose Nanocomposites for High-Performance Piezoresistive Pressure Sensors 28 2.1 Introduction 28 2.2 Experimental Section 31 2.2.1 Fabrication of the CNNs and Pressure Sensors 31 2.2.2 Measurements 34 2.3 Results and Discussion 38 2.3.1 Morphology of CNNs 38 2.3.2 Piezoresistive Characteristics of CNNs 41 2.3.3 Mechanism of High Sensitivity and Great Linearity 45 2.3.4 Fast Response Time of CNN-Based Pressure Sensors 49 2.3.5 Cyclic Reliability of CNN-Based Pressure Sensors 53 2.3.6 Mechanical Reliability and Conformability 57 2.3.7 Temperature and Humidity Tolerance 63 2.4 Conclusion 66 Chapter 3. Ultraflexible Electroluminescent Skin for High-Resolution Imaging of Pressure Distribution 67 3.1 Introduction 67 3.2 Main Concept 70 3.3 Experimental Section 72 3.3.1 Fabrication of Pressure-Sensitive Photonic Skin 72 3.3.2 Characterization of Photonic Skin 74 3.4 Results and Discussion 76 3.4.1 Structure and Morphology of Photonic Skin 76 3.4.2 Pressure Response of Photonic Skin 79 3.4.3 Effect of Conformability on Spatial Resolution 85 3.4.4 Demonstration of High-Resolution Pressure Imaging 99 3.4.5 Pressure Data Acquisition 104 3.4.6 Application to Smart Touch Interfaces 106 3.5 Conclusion 109 Chapter 4. Intrinsically Soft Heat Transfer and Electrical Interconnection Platforms Using Magnetic Nanocomposites 110 4.1 Introduction 110 4.2 Experimental Section 115 4.2.1 Fabrication of SHEPs 115 4.2.2 Measurements 117 4.3 Results and Discussion 119 4.3.1 Fabrication Scheme and Morphology of SHEPs 119 4.3.2 Calculation of Particle Concentration in STCs 124 4.3.3 Enhancement of Heat Transfer Ability via Magnetic Self-Assembly 127 4.3.4 Softness of STCs 131 4.3.5 Mechanical Reliability of Stretchable Electrodes 133 4.3.6 Optimization of Magnetic Self-Assembly Process 135 4.4 Conclusion 139 Chapter 5. Highly Conformable Thermoelectric Generators with Enhanced Heat Transfer Ability 140 5.1 Introduction 140 5.2 Experimental Section 142 5.2.1 Fabrication of Compliant TEGs 142 5.2.2 Measurements 144 5.2.3 Finite Element Analysis 147 5.3 Results and Discussion 149 5.3.1 Enhancement of TE Performance via STCs 149 5.3.2 Mechanical Reliability of Compliant TEGs 157 5.3.3 Enhanced TE Performance on 3D Surfaces via Conformability 162 5.3.4 Self-Powered Wearable Applications 167 5.4 Conclusion 171 Chapter 6. Summary, Limitations, and Recommendations for Future Researches 172 6.1 Summary and Conclusion 172 6.2 Limitations and Recommendations 176 6.2.1 Pressure Sensors and Photonic Skin 176 6.2.2 Compliant TEGs 177 Bibliography 178 Publication List 186 Abstract in Korean 192Docto

A kirigami approach for controlling mechanical and sensing properties of films

Tuning the layout of elasticity in materials opens new opportunities to add various functionalities into a system, ranging from load-enduring capacity and shape-morphing capability in aeronautics to self-foldability and controlled diffusion rates in drug delivery applications. Recently, the Japanese art of paper cutting technique called kirigami has positioned itself as a simple yet powerful strategy to program unique functionalities into intrinsically inextensible, stiff materials without adjusting chemical compositions, including elastic softening, creation of complex 3D structures, and extreme stretchability. Thus, various applications have been realized by utilizing the kirigami principle. These applications include wearable electronics, sensors, stretchable lithium batteries, solar trackers, and reconfigurable structures. However, coupling the primary geometric deformation modes (i.e., bending and rotation) in kirigami films to control mechanical response as well as electronic properties (e.g., shift in resonant frequency) have been limited. In this thesis, we present a strategy where the inclusion of carefully designed cuts allows for fine tuning of mechanical and electronic properties of materials. Starting from fundamentals of kirigami mechanics, we show that stiffness tunability and deformability of kirigami structures are signicantly infuenced by the addition of minor cuts adjacent to major cuts. The dimension and position of minor cuts relative to major cuts determines geometric deformation modes between bending of beams and hinge rotations, which results in high tunability of mechanical properties. The experimental results are validated by beam mechanics with different boundary conditions (Chapter 2). The sensors for human activity monitoring and soft robotic systems require considerable extents of deformation. Furthermore, reducing or eliminating wiring components allows for more compliant and less complex systems by excluding semirigid wiring or connection points. We create a kirigami-inspired passive resonant sensor where the deformation normal to the planar surface changes the capacitance, inductance, and resonant frequency. This study demonstrates that the device allows for accurate measurements of large deformations (\u3e 10X sensor thickness) in both air and water media (Chapter 3)

Bioinspired Designs and Biomimetic Applications of Triboelectric Nanogenerators

The emerging novel power generation technology of triboelectric nanogenerators (TENGs) is attracting increasing attention due to its unlimited prospects in energy harvesting and self-powered sensing applications. The most important factors that determine TENGs’ electrical and mechanical performance include the device structure, surface morphology and the type of triboelectric material employed, all of which have been investigated in the past to optimize and enhance the performance of TENG devices. Amongst them, bioinspired designs, which mimic structures, surface morphologies, material properties and sensing/power generation mechanisms from nature, have largely benefited in terms of enhanced performance of TENGs. In addition, a variety of biomimetic applications based on TENGs have been explored due to the simple structure, self-powered property and tunable output of TENGs. In this review article, we present a comprehensive review of various researches within the specific focus of bioinspired TENGs and TENG enabled biomimetic applications. The review begins with a summary of the various bioinspired TENGs developed in the past with a comparative analysis of the various device structures, surface morphologies and materials inspired from nature and the resultant improvement in the TENG performance. Various ubiquitous sensing principles and power generation mechanisms in use in nature and their analogous artificial TENG designs are corroborated. TENG-enabled biomimetic applications in artificial electronic skins and neuromorphic devices are discussed. The paper concludes by providing a perspective towards promising directions for future research in this burgeoning field of study

Review of Printed Electrodes for Flexible Devices

Printed electronic technologies draw tremendous attention worldwide due to their ability to surpass the limitations of traditional high-cost electronics, based on rigid silicon, and the manufacturing of various devices on flexible substrates. As a critical component of flexible electronics, electrodes fabricated on soft, bendable, and stretchable substrates are of great importance. Based on the fabrication process, this paper classifies the mainstream technologies into two categories: top-down and bottom-up. Top-down technologies include physical evaporation methods, printing technologies and soft lithography, while bottom-up technologies involve polymer-assisted-metal-deposition methods and ion-exchange methods, respectively. In contrast to top-down technologies that transfer functional ink onto substrates directly, the bottom-up method achieves a great improvement in the adhesion between substrates and metal electrodes. In this paper, the challenges of top-down technologies, including cost, synthesis, and choice of ink for printing technologies, the limited choice of metal for bottom-up technologies and the mass production of these methods, are also discussed

Soft actuator and agile soft robot

Robots play an important part in many aspects of our society by doing repetitive, dangerous, or precision tasks. Most existing robots are made of rigid components, which lack passive compliance and pose a challenge in adapting to the environment and safe human-robot interaction. Rigid robots may be equipped with sensors and programmed with proprioceptive feedback control to achieve active compliance, but this may fail in the event of unforeseen situations or sensor failure. In contrast, animals have evolved flexible or soft body parts to help them adapt to changing environments. Soft robotics is an emerging field in robotics, drawing inspiration from nature by integrating soft material into the actuator and mechanical design. With the inclusion of soft material, soft actuators and robots can deform actively/passively, making it possible to sense, absorb impact, and adapt to its environment with deformation. However, while soft actuators/robots have superior properties to rigid ones, they are often challenging to manufacture and control precisely. In addition, they may suffer from slow speed and material degradation. Thus, in this thesis, we aim to address the issues in developing high-performance soft actuators and soft robots. The thesis is divided into two parts. In the first part, we focus on improving the manufacturability and performance of a self-contained soft actuator originated in the Creative Machines Lab. The soft actuator is composed of a cured silicone-ethanol mixture embedded with heating coils. When the coils are electrically actuated, ethanol trapped inside undergoes liquid-vapor transitions, and thus the actuator undergoes extreme volume change. While this actuator exhibits high strain and high stress, it is very slow to actuate, has limited life cycles, and requires molds to manufacture. The first part of the thesis will address these issues. Specifically, in chapter 2, we discuss using multi-material 3D printing to automate the manufacturing of silicone-ethanol composite. In chapter 3, we discuss using laser-cut flexible Kirigami patterns to improve the manufacturability of its heating element. Chapter 4 characterizes its actuation profile and addresses improvements to the thermal conductivity by infusing thermally conductive fillers. Soft actuation is an actively researched area; however, many high-performance soft actuators are challenging to manufacture and thus are less accessible to the general robotics community. Conventional actuators such as electric motors are widely available but lack flexibility. Therefore, the second part of the thesis aims at combining rigid motors with soft materials to design and control high-performance hybrid soft robots. Simulation is a good way to evaluate and optimize robot design and control. However, existing simulators that support motor-driven soft robots have limited features. Chapter 5 discusses this issue and presents a physically based real-time soft robot simulator capable of simulating motor-driven soft robots. In addition, chapter 5 presents the design and control of a 3D printed hybrid soft quadruped robot. Chapter 6 presents the design and control of a 3D printed hybrid soft humanoid robot. The two parts of the thesis aim to improve aspects in soft actuators and soft robots. In conclusion, we summarize the lessons learned in developing soft actuators/robots and new possibilities and challenges for advancing soft robotics research