Flexible Electronics: Theory and Method of Structural Design

Description

Flexible electronics are electronics that can be stretched, bent, twisted, and deformed into arbitrary shapes. They break through the bottleneck and monopoly of traditional, rigid IC technologies and represent the next-generation electronics. This book provides an overview of the underlying theory and method of structural design for flexible electronics. Compared to intrinsically flexible and stretchable materials, structural engineering has proven its unique advantages, e.g. stretchable inorganic electronics. Based on the mechanical mechanisms, this book discusses the main structural deformation behaviors of flexible electronics, including mechanics of film-on-substrate and fiber-on-substrate, self-similar design with/without substrate, conformal design on rigid/soft substrate, purely in-plane design of serpentine interconnect with/without substrate, buckling-driven self-assembly and kirigami assembly strategies, neutral layer design, and the new materials-based structure design like liquid metals, etc. Moreover, the related advanced fabrication technology, the devices designs and applications of flexible electronics are also presented. The comprehensive and in-depth content makes this book can be used as a reference book for experienced researchers, as well as a teaching material for graduate students.  Foreword Preface Contents 1 Structural Engineering of Flexible Electronics 1.1 Introduction 1.2 Applications of Flexible Electronics 1.2.1 Wearable Human Healthcare 1.2.2 Robotics and Haptic Interface 1.2.3 Smart Skin in Aircraft 1.3 Structural Strategies 1.3.1 Wavy Strategy 1.3.2 Island-Bridge Strategy 1.3.3 Kirigami and Origami Strategy 1.3.4 Buckling-Driven Assembly Strategy 1.3.5 Structural Designs of Substrate 1.4 Structural Opportunities by Materials 1.5 Summary References 2 Buckling of Film-on-Substrate System in Flexible Electronics 2.1 Introduction 2.2 Formation of Film-on-Substrate Structure 2.3 Island-Bridge Structure of Stretchable Electronics 2.3.1 Mechanical Model for the Bridge Structure 2.3.2 Mechanical Model for the Island Structure 2.4 Temperature-Dependent Global Buckling Analysis and Structural Design 2.4.1 Geometrical Model and Governing Equations 2.4.2 Structure Design Based on Temperature-Dependent Properties 2.4.3 Temperature-Dependent Local Buckling Analysis and Critical Condition 2.5 Summary References 3 Buckling of Fiber-on-Substrate System in Flexible Electronics 3.1 Introduction 3.2 Fabrication of Buckled Fibers 3.2.1 Mechano-Electrospinning (MES) Technique for In-Surface Buckled Devices 3.2.2 Direct-Writing of Fibers onto a Pre-strained PDMS 3.2.3 Materials and Experimental Set-Up 3.3 Buckling Behaviors of 1D Micro/Nanowires 3.3.1 Out-of-Plane and In-Plane Buckling 3.3.2 Mechanics of Out-of-/In-Surface Buckling 3.3.3 Competition of Buckling Modes 3.4 In-Plane Buckled, Highly Stretchable Devices 3.5 Performance of the Fabricated Stretchable Piezoelectric Device 3.6 Summary References 4 Freestanding Fractal-Inspired Design for Stretchable Electronics 4.1 Introduction 4.2 Elasticity of Fractal Inspired Interconnects 4.2.1 Elastic Analysis of Fractal Interconnects 4.2.2 Experiments of the Fractal Structures 4.3 Fractal Designs in Stretchable Electronics 4.3.1 Mechanics and Electronics with Peano-Based Geometries 4.3.2 Fractal-Based Epidermal Electronics 4.3.3 Radio-Frequency Devices with Fractal Layouts 4.4 Summary References 5 Fractal-Inspired Design on Substrate for Stretchable Electronics 5.1 Introduction 5.2 Mechanical Modeling of SSIs on Soft Substrate 5.2.1 Maximum Strain of Order-2 SSI 5.2.2 The Scale Law Formula 5.2.3 FEM Simulation Results 5.3 Self-Similar Design of Surface Electrodes 5.3.1 Electromechanical Design of Self-Similar Surface Electrodes 5.3.2 Electromechanical Optimization Model 5.3.3 Feasible Range of the Geometric Parameters 5.3.4 Characterization of Mechanical Performance 5.4 Self-Similar Design for Stretchable Wireless LC Strain Sensors 5.4.1 Self-Similar Design for Wireless LC Strain Sensor 5.4.2 Structural Stretchability of the Self-Similar Strain Sensor 5.4.3 Strain-Induced Tunable Inductance 5.4.4 Experimental Platform and Sensitivity Analysis 5.5 Summary References 6 Conformal Design on Rigid Curved Substrate 6.1 Introduction 6.2 Theoretical Analysis Based on Energy Method 6.2.1 Energy Components of Thin Film 6.2.2 Energy Components of Substrate 6.2.3 Total Energy of Flexible Electronics 6.3 1D Conformability of Membranes on Rigid Wavy Substrates 6.3.1 Analytical Interface Model by Work of Adhesion 6.3.2 Analytical Interface Model by Traction-Separation Relations 6.4 2D Conformability of Island-Bridge Structures on Non-Developable Surfaces 6.4.1 Analytical Model 6.4.2 Adhesion Experiment 6.4.3 Adhesion Experiment for Island on Rigid Surface 6.5 Summary References 7 Conformal Design on Soft Curved Substrate 7.1 Introduction 7.2 1D Conformability on Soft Substrates 7.2.1 1D Conformability of Epidermal Electronics on Soft Skin 7.2.2 1D Conformability of Epidermal Electronics on Soft Skin Under External Strain 7.3 2D Conformability on Wavy Soft Substrates 7.3.1 2D Conformability of Epidermal Electronics on Soft Skin 7.3.2 The Effects of the Roughness and Elastic Modulus of the Skin on Conformability 7.3.3 The Effects of the Substrate Thickness on Conformability 7.3.4 The Effects of the Areal Coverage of Electrode on Conformability 7.3.5 The Effects of the External Load on Conformability 7.4 2D Conformability on Complex Soft Substrates 7.4.1 2D Conformability of Island on a Bicurvature Soft Substrate 7.4.2 The Effects of Geometry Parameters on Stable Conformal Contact 7.4.3 The Effects of Materials Parameters on Conformal Contact 7.4.4 Contact Pressure in Conformal Contact 7.5 Local Failure Analysis of Island During Conformal Process 7.5.1 Conformal Strain in Island During Conformal Process 7.5.2 Wrinkling and Buckling Delamination During Conformal Process 7.5.3 Adhesion Experiment for Island on Soft Surface 7.6 Summary References 8 In-Plane Design of Serpentine Interconnect on Substrate 8.1 Introduction 8.2 Thick Interconnects for Ultra-Large Stretchability 8.3 Transition Between Wrinkling, Buckling and Scissoring 8.3.1 Transition from Wrinkling to Buckling 8.3.2 Transition from Buckling to Scissoring 8.4 Criteria for Three Modes 8.4.1 Stretchability in the Wrinkling Mode 8.4.2 Stretchability in the Buckling Mode 8.4.3 Stretchability in the Scissoring Mode 8.5 Some Applications of Thick Interconnects 8.5.1 Interconnects for Stretchable Arrays of LEDs 8.5.2 Interconnects for Stretchable Arrays of Solar Cells 8.5.3 Traces for Stretchable RF Antennas 8.6 Summary References 9 In-Plane Design for Serpentine Interconnect Without Substrate 9.1 Introduction 9.2 Buckling of Stretchable Serpentine Interconnects 9.3 FPD Buckling Theory of Beams 9.3.1 Geometric Relations for the Finite Deformation of 3D Beams 9.3.2 Governing Equations for the FPD Buckling Analysis 9.4 Application of Three Specific Cases 9.4.1 Lateral Buckling of a Three-Point-Bending Beam 9.4.2 Lateral Buckling of a Pure Bending Beam 9.4.3 Euler Buckling 9.5 Sample Fabrication and Experimental Verification 9.6 Summary References 10 Self-Assembly of Self-Similar Fibers for Stretchable Electronics 10.1 Introduction 10.2 HE-Printing Technique for Fabrication of Self-Similar Nano/Microfibers 10.2.1 HE-Printing Technique 10.2.2 Fabrication of Self-Similar Nano/Microfibers 10.3 Buckling-Driven Self-Assembly of Self-Similar Fiber-Based Structures 10.3.1 Buckling of Serpentine Fibers Under Uniaxial Prestrain 10.3.2 Buckling of Serpentine Fibers Under Biaxial Prestrain 10.3.3 Self-Assembly by Tuning In-/Out-of-Surface Buckling 10.4 Hyper-Stretchable Self-Powered Sensors Based on Self-Similar Piezoelectric Nano/Microfibers 10.4.1 Hyper-Stretchable Self-Powered Sensors 10.4.2 Architecture of an HSS and HE-Printing Technique 10.4.3 Characterizations of the HSS 10.4.4 Applications of the HSS 10.5 Summary References 11 Kirigami Strategy for Conformal Electronics 11.1 Introduction 11.2 Self-Healing Kirigami Assembly Strategy 11.2.1 Conformal Criterion for Kirigami Geometry 11.2.2 Geometrical Design Algorithm for 2D-to-3D Conformal Mapping 11.2.3 Preparation and Characterization of the Ag/PCL Self-Healing Materials 11.2.4 Kirigami-Based Conformal Heater 11.2.5 Multifunctional Wind Sensing System 11.3 Soft-Hinge Kirigami Metamaterials for Self-Adaptive Conformal Electronic Armor 11.3.1 Deformation Mechanism of Soft-Hinge Kiri-MMs 11.3.2 Stretchability, Flexibility and Conformability of Soft-Hinge Kiri-MMs 11.3.3 Electrical Enhancements with Conductive Polymer Composite 11.3.4 Functional Soft-Hinge Kiri-MM E-armor Systems 11.4 Summary References 12 Neutral Layer Design for Flexible Electronics 12.1 Introduction 12.2 Mechanics of Neutral Mechanical Plane 12.3 The Effect of Length on Splitting of the Multiple Neutral Mechanical Plane 12.4 The Effect of Boundary Conditions on Splitting of the Neutral Mechanical Plane 12.4.1 Given Slopes Are Imposed at the Ends of the Hard Layers 12.4.2 Given Slopes Are Imposed at the End Sections 12.5 Effects of the Membrane Energy and Bending Energy of the Middle Layer 12.5.1 The Model Incorporating the Shear Energy, Membrane Energy and Bending Energy of the Middle Layer 12.5.2 The Model Neglecting the Shear Energy of the Middle Layer 12.6 Summary References 13 Liquid Metal-Based Structure Design for Stretchable Electronics 13.1 Introduction 13.2 Microfluidic Serpentine Antennas with Designed Mechanical Tunability 13.2.1 Galium-Based Eutectic Alloys 13.2.2 The Design of Serpentine Microfluidic Antenna 13.2.3 Fabrication of the Serpentine Microfluidic Antennas 13.2.4 Characterization of the Serpentine Microfluidic Antennas 13.3 Liquid–Metal Antennas with Stable Working Frequency for RFID Applications 13.3.1 Serpentine Liquid–Metal Antennas for RFID 13.3.2 Design of Stretchable RF Antennas 13.3.3 Relationship Between Stretchability and Resonant Frequency 13.4 Liquid Metal Nanoparticles (LMNPs) for Ultrathin, Flexible Metasurface 13.4.1 Liquid Metal Metasurface 13.4.2 Sintering Process of LMNPs 13.4.3 Electromagnetic Performance 13.5 Summary References 14 Applications of Flexible Electronics 14.1 Introduction 14.2 Application of Flexible Electronics as E-tattoos 14.2.1 Low-Cost, μm-Thick, and Tape-Free E-tattoos 14.2.2 Characterization of Wearability and Motion Artifacts 14.2.3 Applications of the Large-Area Epidermal Electrodes on Human Skin 14.3 Application of Flexible Electronics as Implantable Cardiac Membranes 14.3.1 3D Multifunctional Integumentary Cardiac Membranes 14.3.2 Design of Conformability 14.3.3 Spatiotemporal Cardiac Measurements 14.4 Application of Flexible Electronics as Aircraft Smart Skin 14.4.1 Design of the Multifunctional, Flexible Sensing Skin 14.4.2 External Airflow Multifunctional Perception 14.4.3 Internal Structural Health Monitoring 14.5 Application of Flexible Electronics as Robotic Interface 14.5.1 Design and System Architecture of 3D-Shaped E-skin 14.5.2 Fabrication of the 3D-Shaped E-skin 14.6 Summary References