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- Herausgeber: Wiley-VCH
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- Sprache: Englisch
Overview of cutting-edge soft electronics technologies and their clinical and biomedical applications in fields including bioimaging and drug delivery
Soft Electronics for Diagnosis, Therapy, and Integrated Systems summarizes soft bio-integrated electronics in three parts: soft sensors for diagnosis, soft electronics for therapy, and soft systems for interaction, reviewing the latest state-of-the-art research and comprehensively covering topics from device design strategies and materials processing methods to fabrication techniques and electrical measurements.
This book provides information on a wide variety of applications, including flexible sensors for disease diagnosis, flexible electrode for noninvasive brain-computer interface, invasive electrodes, mechanical sensors (transducers) for motion detection of human and organs, smart optoelectronics in health monitoring and human machine interactions, non-invasive detection of bio-analytes, biosensors for blood microbe and virus diagnosis, sensors for bioimaging, self-powered sensors, electrical stimulation, phototherapy, drug delivery, thermotherapy, feedback technology, and soft robots.
Written by a team of highly qualified authors and contributed to by experts in their respective fields, Soft Electronics for Diagnosis, Therapy, and Integrated Systems discusses sample topics such as:
- Island bridge structure-curved lines in flexible sensor mechanics, covering 2D and 3D spiral interconnects as well as 2D fractal structures
- Ocular wearable sensors, covering contact lens sensors, capsule-based tear sensors, wearable eyepatches, and eyeglass sensors
- Materials and structures of soft sensors, covering nanomaterials, liquid conductors, elastomers, hydrogels, and textiles, as well as serpentine, mesh, and coiled structures
- Fundamentals of photodetectors, covering performance parameters, quantum dots, and perovskites and other organic materials
Describing both theory and application, Soft Electronics for Diagnosis, Therapy, and Integrated Systems is an excellent and up-to-date reference on the subject for materials scientists, electronics engineers, biotechnologists, and developers and other professionals in the sensor industry.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright
Preface
Session I: Soft Sensors for Diagnosis
1 Mechanics Design of Flexible Sensors
1.1 Design of Stretchable Flexible Device Structure
1.2 Structural Design of Substrate
1.3 Structural Designs for Spatial Integration of Device Systems
References
2 Epidermal Wearable Biosensors
2.1 Wearable Biosensing Technology
2.2 Epidermal Wearable Biosensors
2.3 Ocular Wearable Sensors
2.4 Wound Sensor
List of Abbreviations
References
3 Soft Sensors for Disease Diagnosis
3.1 Introduction
3.2 Materials and Structures of Flexible Sensors
3.3 Application of Flexible Sensors in Disease Diagnosis
List of Abbreviations
References
Note
4 Wearable Chemical Sensors for Noninvasive Monitoring
4.1 Introduction
4.2 Biofluids of Interest for Wearable Chemical Sensors
4.3 Biofluid Enabled Platforms: Traditional to Wearable
4.4 Sampling and Detection Strategies for Biofluid‐Based Wearable Sensors
4.5 Outlook
References
5 Flexible Electrode for Noninvasive Brain–Computer Interfaces
5.1 Introduction
5.2 Development of Noninvasive BCIs
5.3 Electrode Technologies for Noninvasive BCIs
5.4 Challenges
5.5 Conclusion
References
6 Chronic Neural Interfaces
6.1 Introduction
6.2 Architectures for Mechanical Compliance and Biocompatibility
6.3 Advanced Chronically Stable Materials for Neural Interfaces
6.4 Encapsulation for Stable Operation
6.5 Engineering Strategies for Chronic Active Sensing
6.6 Multimodal Functions of Long‐Term Stable Implants
6.7 Challenges and Future Directions
References
7 Mechanical Sensors (Transducers) for Motion Detection of Humans and Organs
7.1 Introduction
7.2 Classification of Stretchable Mechanical Sensors
7.3 Material Architectures
7.4 Sensing Mechanisms
7.5 Representative Applications
References
8 Smart Optoelectronics in Health Monitoring and Human‐Machine Interactions
8.1 Fundamentals on Photodetectors
8.2 Integrated Optoelectronic Systems
8.3 Flexible Integrated Systems Based on Photodetectors for Advanced Applications
8.4 Future Trend of Photodetectors for Soft Electronics
References
9 Wearable Sensor for Bioimaging
9.1 Introduction
9.2 Wearable Ultrasound Bioimaging Sensor
9.3 Wearable Photoacoustic Imaging Sensor
9.4 Wearable Electrical Impedance Tomography
9.5 Wearable Terahertz Imaging Sensor
9.6 Wearable Bioimaging Device for Biomedical Applications
9.7 Conclusion
References
Session II: Soft Sensors for Therapy
10 Thermotherapy (Resistive Heaters, Photothermal Nanomaterials, Textile Devices, Cryotherapy, etc.)
10.1 Introduction
10.2 Resistive Heaters
10.3 Photothermal Nanomaterials
10.4 Textile Devices
10.5 Cryotherapy
List of Abbreviations
References
11 Soft Electronics for Drug Delivery
11.1 Introduction
11.2 Skin Structure
11.3 Soft Electronics‐Assisted TTDS for Drug Delivery
11.4 Conclusions and Perspectives
List of Abbreviations
References
12 Wearable and Implantable Drug Delivery System
12.1 Introduction
12.2 Categories of Soft Electronics for Drug Delivery
12.3 Challenges and Prospects
References
13 Soft Robotic Sensing and Medicine
13.1 Introduction
13.2 Soft Robotic Tactile Sensing
13.3 Soft Robotic Environmental Sensing
13.4 Miniature Robotic
In Vivo
Medicine
References
Session III: Soft Sensors for Interaction
14 Integration System
14.1 Power Supply Strategy of Soft Electronics
14.2 Encapsulation
14.3 Communication
14.4 Closed‐Loop Control (AI, Deep Learning, Microcontroller Unit, etc.)
List of Abbreviations
References
15 Self‐powered Sensors
15.1 Introduction
15.2 Piezoelectric Sensor
15.3 Triboelectric Sensor
15.4 Piezoionic Sensor
15.5 Electromagnetic Sensor
15.6 Thermoelectric Sensors
15.7 Potentiometric Ion Sensors
15.8 Conclusion
References
Index
End User License Agreement
List of Tables
Chapter 2
Table 2.1 Biomarkers in sweat and ISF.
Chapter 3
Table 3.1 Measured locations, parameters, working principles, and p...
Chapter 4
Table 4.1 Important diagnostic biomarkers and their concentrations ...
Table 4.2 Important diagnostic biomarkers and their concentrations ...
Chapter 6
Table 6.1 Summary of key properties in various device platforms.
Table 6.2 Comparison points for the duration and leakage modes of b...
Chapter 9
Table 9.1 Electrical conductivity of some typical human tissues....
Chapter 14
Table 14.1 Comparison of the major communication methods.
List of Illustrations
Chapter 1
Figure 1.1 (a) Scanning electron microscopy (SEM) images of wavy S...
Figure 1.2 Pre‐strain preparation method of corrugated structure....
Figure 1.3 (a–d) Schematic diagram of corrugated structure prepare...
Figure 1.4 (a–d) Schematic diagram of film layer structure with mo...
Figure 1.5 The steps of preparing corrugated structure by ultravio...
Figure 1.6 Geometric diagram of buckling delamination of corrugate...
Figure 1.7 Finite element simulation structure of buckling delamin...
Figure 1.8 (a–c) Schematic diagram of corrugated‐substrate bucklin...
Figure 1.9 (a–g) Geometric diagram of buckling of the corrugated s...
Figure 1.10 (a,b) Simplified mechanical model of buckling wrinkle...
Figure 1.11 (a–d) Schematic diagram of double substrate soft laye...
Figure 1.12 (a–c) Structure diagram of nonlinear corrugated‐subst...
Figure 1.13 Effect of membrane spacing on buckling amplitude.
Figure 1.14 Noncoplanar arc line structure.
Figure 1.15 Loading mechanical model of the island on the substra...
Figure 1.16 The three possible deformation modes of a filamentary...
Figure 1.17 (a–d) Experimental diagrams of four buckling modes....
Figure 1.18 Four kinds of buckling mode diagram. (a) Flat. (b) Wr...
Figure 1.19 A schematic diagram of a conformal curved object wrap...
Figure 1.20 (A–C) Shape parameter diagram of snake‐like geometric...
Figure 1.21 (a–f) Optical microscopic images and finite element a...
Figure 1.22 (A,B) Structure diagram of ITO serpentine tape and re...
Figure 1.23 (A,B) Analytical model diagram and tensile results of...
Figure 1.24 (a–d) Schematic diagram of island bridge structure on...
Figure 1.25 3D‐MIMs are used for spatiotemporal measurement and s...
Figure 1.26 Experimental and numerical analysis of the buckling m...
Figure 1.27 (A,B) Schematic diagram and tensile fracture of fully...
Figure 1.28 (a–d) Deformation mode of thick rod serpentine conduc...
Figure 1.29 (a–d) Shear interconnection demonstration of thick ro...
Figure 1.30 (a–c) Step diagram of manufacturing serpentine interc...
Figure 1.31 (A,B) The results of the tensile properties of serpen...
Figure 1.32 Stress distribution of serpentine, self‐similar, and ...
Figure 1.33 (a–f) The deformation and mechanical responses of the...
Figure 1.34 Mechanical properties and applications of 2D helical‐...
Figure 1.35 The influence of interconnecting serpentine structure...
Figure 1.36 Various 3D spiral interconnect structures and mechani...
Figure 1.37 (a–c) 3D interconnects' soft packaging strategy.
Figure 1.38 (a–c) Interconnect structural performance of 2D fract...
Figure 1.39 2D fractal structure expansion process and calculatio...
Figure 1.40 Conformal attachment of Peano curve electrodes to a f...
Figure 1.41 Schematic diagram and optical image of thin stretchab...
Figure 1.42 FEA results and optical images of ion sensors integra...
Figure 1.43 (a) Schematic diagram of the cellular substrate; (b) ...
Figure 1.44 (a) A uniaxially stretched composite designed to mimi...
Figure 1.45 Soft network material design. Nature Publishing Group...
Figure 1.46 Schematic diagram of the components and integration s...
Figure 1.47 Optical image of a multifunctional inflatable balloon...
Figure 1.48 (a) Geometry of the origami tube unit (variation of t...
Figure 1.49 (a) Schematic diagram of GHEGs with excellent deforma...
Figure 1.50 (a) Schematic diagram of 3D‐GHEG assembled into a pyr...
Figure 1.51 Optical pictures of the fabricated origami‐enabled Si...
Figure 1.52 Structure and photographs of the TEGs. (a) Scanning e...
Figure 1.53 A folded Miura‐ori sheet consists of tessellations of...
Figure 1.54 (a) Schematic of Miura folding procedures for 5 × 5 p...
Figure 1.55 (a–d) Assembly and mechanical analysis of 3D photodet...
Figure 1.56 (a) Illustrates an electrically small antenna with la...
Figure 1.57 (a–h) Design, fabrication, experimental measurements,...
Figure 1.58 Morphable 3D mesostructures and integrated circuits b...
Figure 1.59 (a–d) Materials and procedures for assembly of 3D ele...
Figure 1.60 Mechanically guided, deterministic assembly of 3D hie...
Figure 1.61 (a–c) Design and characterization of a four‐layer str...
Chapter 2
Figure 2.1 Schematic representation of enzymatic glucose oxidation...
Figure 2.2 Schematic illustration of label‐free immunosensors.
Figure 2.3 Schematic of ion‐selective electrodes based on a bracel...
Figure 2.4
Flexible and stretchable epidermal sensors
. (a) Schemat...
Figure 2.5
Self‐powered sweat sensors
. (a) Battery‐free swea...
Figure 2.6
Various wearable ocular sensors
. (a) Schematic of (i) a...
Figure 2.7 (a) Screen printing the smart bandage.(b) Wearable ...
Figure 2.8 Scheme of the sensing principle for noninvasive wound m...
Figure 2.9 (a) Embroidered electrochemical sensors on gauze and wo...
Chapter 3
Figure 3.1 The structure composition of flexible sensor.
Figure 3.2 Illustration and practical application of materials use...
Figure 3.3 Structure diagram of 2D structures applied in flexible ...
Figure 3.4 Structure diagram of 3D structures applied in flexible ...
Figure 3.5 (a) Common materials used in flexible sensors for neuro...
Figure 3.6 (a) Images and schematic illustrations of multiplexed p...
Chapter 4
Figure 4.1 Schematic showing the biofluid‐enabled wearable sensors...
Figure 4.2 Schematic depicting the feasible partitioning routes of...
Figure 4.3 (a) A skin‐integrated microfluidic platform with capill...
Figure 4.4 (a) Illustration of ISF extraction methods including ab...
Figure 4.5 (a) Schematic of a graphene‐based biotransferrable oral...
Figure 4.6 (a) Schematic depicting the fabrication process of the ...
Chapter 5
Figure 5.1 The schematics of brain‐computer interface.
Figure 5.2 The development timeline of noninvasive BCIs.
Figure 5.3 Some examples of rigid dry electrodes. (a) A 3D‐printed...
Figure 5.4 Coating technology: conductive materials on a flexible ...
Figure 5.5 3D printing enables flexible structural electrodes. (a)...
Figure 5.6 Textile electrode. (A) Standard electrode (a) and two t...
Figure 5.7 Semi‐dry electrode. (a) A quasi‐dry electrode concept, ...
Chapter 6
Figure 6.1 Conventional structure of bioelectronic systems. (A) SE...
Figure 6.2 (a–c) Flexible filamentary probes. (a) Schematic of fil...
Figure 6.3 Schematic of approximate ranges of WVTRs (at 25 °C, 100...
Figure 6.4 Materials (capping layer) and integration strategies fo...
Figure 6.5 (a) Theoretical modeling of reactive diffusion for the ...
Figure 6.6 Ion effect on SiO
2
dissolution and lifetimes for therma...
Figure 6.7 Diagram illustrating (a) direct addressing and (b) acti...
Figure 6.8 (a) Optical image of the capacitively coupled sensing u...
Figure 6.9 (a) Schematic diagram in an angled, exploded view forma...
Chapter 7
Figure 7.1 Low‐strain detection of stretchable strain sensors with...
Figure 7.2 Low‐strain detection of stretchable strain sensors with...
Figure 7.3 Large‐strain detection of stretchable strain sensors fo...
Figure 7.4 Large‐strain detection of stretchable strain sensors fo...
Chapter 8
Figure 8.1 The overall structure, application, and performance eva...
Figure 8.2 Represented photodetectors based on novel materials lik...
Figure 8.3 Photodetectors for self‐powered sensing. (a) All‐printa...
Figure 8.4 Printable fabrication of wearable photodetectors and in...
Figure 8.5 Flexible devices based on LED‐Photodetectors opt pairs ...
Figure 8.6 Flexible devices with photodetectors for human‐machine ...
Chapter 9
Figure 9.1 Transmission and reflection of ultrasound wave in tissu...
Figure 9.2 Schematic illustration of PUT and MUT: (a) PUT, (b) cap...
Figure 9.3 Various types of 1D transducer arrays for wearable ultr...
Figure 9.4 Various types of 2D transducer arrays for wearable ultr...
Figure 9.5 The orthogonal transducer array for wearable ultrasound...
Figure 9.6 A typical PA imaging system.
Figure 9.7 Optical photographs of the soft PA imaging patch under ...
Figure 9.8 Schematics illustration of the stethoscope laminated on...
Figure 9.9 Structure and working principle of the photoacoustic pa...
Figure 9.10 Measurement process of an EIT system with 16 electrod...
Figure 9.11 Various types of wearable EIT devices for pulmonary i...
Figure 9.12 Various types of wearable EIT devices for cancer dete...
Figure 9.13 Various types of wearable EIT devices for gesture rec...
Figure 9.14 Terahertz imaging systems: (a) active terahertz imagi...
Figure 9.15 Wearable terahertz imaging sensor. (a) Terahertz resp...
Figure 9.16 (a) Illustration of the BAUS prob. (b) The structure ...
Figure 9.17 Schematics of the stretchable ultrasonic array lamina...
Figure 9.18 (a–d) Optical, B‐mode images, and strain mapping of v...
Figure 9.19 Exploded view schematics of the wearable imager with ...
Figure 9.20 Diagrams and B‐mode images depicting cardiac anatomie...
Figure 9.21 (a) Stress echocardiography stages: Rest (4 minutes s...
Figure 9.22 Overview of the fully integrated USoP: (a) Photo of e...
Figure 9.23 Autonomous and continuous blood pressure recording in...
Figure 9.24 Principle of photoacoustic imaging.
Figure 9.25
In‐vivo
imaging of blood vessels and venous occ...
Chapter 10
Figure 10.1 (a) The present study showcases a conceptual illustra...
Figure 10.2 (a) The present study showcases three innovative appl...
Figure 10.3 (a) The present study focuses on the utilization of a...
Figure 10.4 (a) The present study presents a schematic illustrati...
Figure 10.5 (a) The current study outlines a schematic illustrati...
Chapter 11
Figure 11.1 Schematic representation of the major structures of t...
Figure 11.2 Diagrammatic representation of penetration pathways, ...
Figure 11.3
MNs for passive drug delivery
. (a) Schematic illustra...
Figure 11.4
Soft electronics‐MN systems for active drug delivery
...
Figure 11.5
Soft electronics‐MN systems for closed‐loop drug deli
...
Figure 11.6
Soft electronics systems for closed‐loop drug deliver
...
Chapter 12
Figure 12.1
The application and composition
of soft electronic dru
...
Figure 12.2
Wearable soft electronic systems for drug delivery.
(...
Figure 12.3
Implantable soft electronic systems for drug delivery
...
Chapter 13
Figure 13.1
Soft electronic system for robotic tactile sensing.
(...
Figure 13.2
Soft robotic environmental sensing.
(a) Exploration o...
Figure 13.3
Miniature devices and soft systems for in vivo roboti
...
Chapter 14
Figure 14.1 Illustration of a typical wireless powering system.
Figure 14.2 Typical energy collection circuit.
Figure 14.3 Design of an LDO.
Figure 14.4 Equivalent circuit of LDO.
Figure 14.5 BUCK topology.
Figure 14.6 BOOST topology.
Figure 14.7
Fabrication and encapsulation method for soft electro
...
Figure 14.8 (a) Tactile IoT device and three common Bluetooth net...
Figure 14.9
Wireless notifying healthcare monitoring systems
. (a)...
Figure 14.10
Closed‐loop controlled healthcare monitoring s
...
Chapter 15
Figure 15.1
Conversion of energy from the human body into electri
...
Figure 15.2
Piezoelectric sensors
. (a,b) Mechanism of a piezoelec...
Figure 15.3
Triboelectric sensors
. (a) The electron‐cloud‐potenti...
Figure 15.4
Piezoionic sensors
. (a) Schematic representation comp...
Figure 15.5
Electromagnetic sensors
. (a) Illustration of the tact...
Figure 15.6
Thermoelectric sensors
. (a) Schematic representation ...
Figure 15.7
Potentiometric ion sensors
. (a) Schematic illustratio...
Figure 15.8 Self‐powered integrated sensing system and future dev...
Guide
Cover
Table of Contents
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Soft Electronics for Diagnosis, Therapy, and Integrated Systems
Edited by Xinge Yu, Jiyu Li, Ya Huang, and Enming Song
Editors
Xinge YuCity University of Hong Kong83 Tat Chee Ave. Kowloon TongYeung Kin Man Academic Bldg.Hong Kong
Jiyu LiSun Yat-sen UniversityNo. 66, Gongchang Road, Guangming DistrictSchool of Biomedical EngineeringShenzhen, China
Ya HuangFudan UniversityState Key Laboratory of Molecular Engineering of PolymersInstitute of Fiber Materials and DeviceChina
Enming SongFudan University2005 Songhu Rd, Yangpu DistrictInterdisciplinary Research BuildingShanghai, China
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Preface
The rapid advancement of soft electronics has revolutionized modern biomedical technology, forging a critical link between flexible materials science and innovative healthcare solutions. Soft Electronics for Diagnosis, Therapy, and Integrated Systems stands at the vanguard of this interdisciplinary field, assembling cutting‐edge research that underscores the transformative potential of soft electronic devices in disease diagnosis, therapeutic intervention, and integrated biomedical systems. Published by Wiley, this authoritative volume synthesizes contributions from global thought leaders, demonstrating how soft electronics address healthcare challenges through non‐invasive, patient‐friendly, and efficient solutions.
The Evolution and Significance of Soft Electronics
Characterized by flexibility, stretchability, and biocompatibility, soft electronics have disrupted the paradigm of rigid biomedical devices. Unlike conventional technologies, these systems conform to the body's contours, enabling prolonged wearability and minimally invasive interaction with biological tissues. The book opens by dissecting the fundamental materials and designs underpinning soft electronics—from nanocomposites and hydrogels to innovative architectures like serpentine patterns and three‐dimensional porous networks. These components form the basis for devices that seamlessly integrate with the human body, unlocking new frontiers in real‐time health monitoring and targeted therapeutic strategies.
Diagnosis: Non‐Invasive Sensing and Biomarker Detection
Part I delves into the application of soft electronics in non‐invasive diagnostics. Chapters detail wearable chemical sensors for real‐time monitoring of biofluids (sweat, interstitial fluid, and saliva), enabling the detection of disease biomarkers for conditions such as diabetes, cardiovascular disorders, and inflammatory diseases. For example, flexible epidermal sensors facilitate continuous glucose monitoring via interstitial fluid, while microneedle arrays enable minimally invasive sampling of metabolic markers. The integration of microfluidics and electrochemical sensing technologies enhances device sensitivity and specificity, enabling point‐of‐care diagnostics without reliance on laboratory infrastructure.
Therapy: Targeted Interventions and Therapeutic Systems
Part II focuses on therapeutic applications, where soft electronics enable precise, patient‐specific interventions. Chapters explore soft robotic systems for minimally invasive surgery, implantable drug delivery devices with tunable release kinetics, and thermal therapy platforms for localized treatment. For example, stretchable electrodes combined with hydrogel matrices enable non‐invasive neuromodulation, while biodegradable implants provide controlled drug release for chronic disease management. The book also highlights the role of soft electronics in rehabilitation, such as wearable exoskeletons and haptic feedback systems for motor function recovery.
Integrated Systems: From Sensing to Intelligent Healthcare Networks
Part III emphasizes the integration of soft electronics into comprehensive healthcare ecosystems. Chapters discuss self‐powered sensor networks, wireless data transmission modules, and artificial intelligence‐driven analytics that transform raw biological data into actionable insights. For instance, fully integrated wearable systems combine physiological sensors with AI algorithms to predict disease exacerbations or optimize treatment plans in real time. The book also addresses the challenges of power management, data security, and clinical translation, ensuring that these technologies can transition from the lab to commercial applications.
Outlook and Challenges
The concluding chapters envision the future of soft electronics in healthcare, identifying opportunities in personalized medicine, telehealth, and global health equity. While these technologies offer unprecedented advantages in comfort and functionality, challenges persist in scalable manufacturing, long‐term biocompatibility, and multi‐modal sensing integration. The book calls for cross‐disciplinary collaboration—across materials science, electrical engineering, and medicine—to surmount these hurdles and realize the full potential of soft electronics in revolutionizing patient care.
The editors extend gratitude to contributing authors for their expertise and dedication, as well as to the Wiley team for their support in realizing this comprehensive work. We also acknowledge the foundational research of countless scientists who have paved the way for soft electronics to become a cornerstone of modern biomedical innovation.
This book serves as an essential reference for researchers, engineers, and clinicians at the intersection of flexible electronics and healthcare. It offers a comprehensive exploration of the principles, applications, and future directions of soft electronics in diagnostics, therapy, and integrated systems—guiding the development of next‐generation, patient‐centric intelligent healthcare technologies.
Xinge Yu, Jiyu Li, Ya Huang, Enming Song
Session ISoft Sensors for Diagnosis
1Mechanics Design of Flexible Sensors
Li Yuhang1, Zhao Zhao2, and Wu Wenbin1
1Beihang University (BUAA), Institute of Solid Mechanics, School of Aeronautic Science and Engineering, XueYuan Road, HaiDian District, Beijing 100191, P.R. China
2China Special Equipment Inspection and Research Institute, HePing Street, ChaoYang District, Beijing 100029, P.R. China
1.1 Design of Stretchable Flexible Device Structure
The physical synthesis of fragile inorganic semiconductor materials has been realized with the development of nanofabrication technology, which leads to explosive growth in the research of high‐performance stretchable flexible devices [1–3]. Due to their excellent flexibility, ductility, and better mechanical properties, inorganic semiconductor materials can form three‐dimensional (3D) structures through self‐assembly or indirect guidance, including tubular, wrinkled, buckling, and other delicate structures [4, 5]. Inorganic semiconductor materials in lines, bands, films, sheets, and strips can be obtained [6]. These materials can be used for high‐performance transistors and circuit components of flexible, stretchable electronic devices. Integrating these materials into flexible substrates to prepare high‐performance flexible electronic devices has become an important research issue. Therefore, relevant scholars have proposed different mechanical structure guidance strategies based on the principles of mechanics, including the ripple method, island bridge connection method, etc., which meet the high‐performance requirements of electronic devices for complex surfaces [7].
1.1.1 Ripple Method
Buckling means the instability and failure of the structure. However, the design method of controllable buckling can effectively improve the ability of flexible electronics to resist tensile and compressive failure. Based on the buckling principle, the ripple method can make the flexible substrate and the film attached to it produce large wavy deformation simultaneously through the pre‐stretching and strain release of the flexible substrate to adapt to and withstand more significant deformation [8].
There are three main preparation methods for corrugated structure: substrate pre‐strain releasing, closed colloidal solution expansion, and ultraviolet radiation method [9–11]. The substrate pre‐strain release method mainly uses single‐crystal etching to pattern the silicon film. The silicon film array is placed on the pre‐stretched polydimethylsiloxane (PDMS) substrate based on the transfer printing. Then, the pre‐stretched strain of the substrate is released to generate compressive stress, and a corrugated silicon film‐PDMS double‐layer structure is obtained, as shown in Figure 1.1.
Figure 1.1 (a) Scanning electron microscopy (SEM) images of wavy Si ribbons on PDMS substrates. (b) Optical microscopy images and atomic force microscopy images of wavy Si nanofilms.
Source: Song et al. [2]/with permission of AIP Publishing.
Yu et al. [10] from Arizona State University fabricated a novel stretchable temperature sensor using the substrate pre‐strain release method. Figure 1.2 shows that the ultra‐thin single‐crystal silicon film was prepared by traditional lithography technology. The Cr‐Au thin layer thermistor was prepared by the sputtering deposition method and patterned by the boosting method. Then, the single‐crystal silicon band was prepared by reactive ion etching as the bonding layer between the thermistor and the rubber substrate. Based on the van der Waals force, the thermistor‐monocrystalline silicon adhered to PDMS is peeled off from the silicon‐on‐insulator (SOI) film, and another PDMS substrate with pre‐strain after radiation treatment is prepared. Finally, the pre‐strain is released to obtain a corrugated flexible temperature sensor device.
Figure 1.2 Pre‐strain preparation method of corrugated structure.
Source: Yu et al. [10]/with permission of AIP Publishing.
Yang et al. [9] used a closed colloidal solution expansion method to prepare a PDMS flexible double‐layer structure with wavy wrinkled films and proposed a strategy to control wavy patterns and characteristic wavelengths to achieve various applications of corrugated structures, including adjustable adhesion, wetting, microfluidics, and microlens arrays. Figure 1.3 shows that the solvent or monomer solution is used to expand and seal the elastic‐plastic polymer or hydrogel film. Because the bottom of the film is connected to the rigid substrate and cannot deform, an anisotropic osmotic pressure is generated along the thickness direction of the film. When the net pressure exceeds the critical pressure, the outer surface of the outer film will buckle and form a corrugated structure pattern (Figure 1.4).
Figure 1.3 (a–d) Schematic diagram of corrugated structure prepared by closed colloidal solution expansion method.
Source: Yang et al. [9]/with permission of John Wiley & Sons.
Figure 1.4 (a–d) Schematic diagram of film layer structure with modulus gradient prepared by photocrosslinking method.
Source: Yang et al. [9]/with permission of John Wiley & Sons.
Yu and Jiang [11] from Arizona State University processed the pre‐stretched PDMS substrate by ultraviolet radiation to prepare an SIO2 film with a wavy structure. The effects of pre‐strain, radiation duration, and modulus on the wavy profile were obtained, and the accuracy of the mechanical analysis was confirmed by experimental comparison. The researchers first prepared a pre‐stretched PDMS substrate and placed it under the radiation of an ultraviolet lamp in the atmospheric environment. The chemical composition of PDMS can be changed by reacting with oxygen. After reaching a sufficient exposure time, the pre‐strained PDMS surface will produce a wavy structure (Figure 1.5).
Figure 1.5 The steps of preparing corrugated structure by ultraviolet radiation.
Source: Yu and Jiang [11]/with permission of Elsevier.
To further study the buckling properties of corrugated structures in stretchable electronic devices, scholars have investigated through experiments, theories, and finite element methods and made a reasonable explanation for the formation and evolution mechanism of periodic buckling corrugated structures on macro‐ and microscales [8–10, 12–21]. Zhang and Yin [12] from Temple University carried out related research on the periodic delamination mechanism of spontaneous buckling of thin films on flexible substrates under significant compression. Firstly, the geometric evolution mechanical model of the periodic corrugated structure is established based on the energy method and verified by experiments and finite element simulation. According to the size of the compressive strain, the geometric deformation process of the corrugated structure can be divided into three stages: the generation of buckling delamination under minor compression conditions, the expansion of buckling delamination under medium compression conditions, and the post‐buckling phenomenon of delamination stopping under extensive compression conditions. In the experimental study, a thin metal film Au (thickness of 40 nm) was deposited on a uniaxially pre‐stretched PDMS (thickness of 2 mm) substrate. The delamination process of the buckling surface morphology was characterized by scanning electron microscopy to evaluate the evolution process of the microbuckling delamination release strain (Figure 1.6).
Figure 1.6 Geometric diagram of buckling delamination of corrugated structure.
Source: Zhang and Yin [12]/with permission of Elsevier.
Considering the large deformation of the silicone rubber substrate, it is assumed that the film is an elastic thin plate and the substrate is a semi‐infinite solid that satisfies the Neo‐Hookean constitutive law. The total energy Utol in the film‐substrate system is composed of the tensile energy in the film Ustr, the bending energy Ubend, the elastic energy in the substrate Usub, and the adhesion energy between the film and the substrate Uadh, as shown in the following formula:
The tensile strain and bending energy of the film can be written as:
The approximate relationship between the shape of the corrugated structure and the applied strain is ε ≈ π2h2/4δ2. So, the tensile strain energy in the film can be approximately ignored under the assumption of the nonexpandable film. The strain energy of nonlinear elastic substrate can be given as:
Since the strain energy in the substrate and the strain energy of the film delamination can be neglected, the effect of nonlinear buckling of the material in the elastic substrate on the buckling delamination morphology can be neglected (Figure 1.7).
Figure 1.7 Finite element simulation structure of buckling delamination under different tensile levels.
Source: Zhang and Yin [12]/with permission of Elsevier.
Given the limitations of the original semi‐infinite substrate model, Ma et al. [13] studied the buckling critical pre‐strain of a finite‐thickness substrate. The pre‐strain is released and bonded to the polyimide film on the flexible substrate into a sinusoidal corrugated structure. The tensile stiffness of the substrate is much smaller than that of the film; the substrate cannot shrink back to the initial length after pre‐strain release, so the film‐substrate system may bend (Figure 1.8).
Figure 1.8 (a–c) Schematic diagram of corrugated‐substrate buckling structure with finite thickness.
Source: Ma et al. [13]/with permission of Royal Society of Chemistry.
Based on the finite thickness substrate assumption, the critical pre‐strain can be obtained as follows. The critical buckling strain of the polyimide film on the PDMS substrate increases with the ratio h/H of the film height to the substrate height, consistent with the finite element analysis (FEA) results.
Yan et al. [14] studied the mechanical buckling of thin films bonded to pre‐strained finite‐thickness substrates. The maximum strain analysis method and the critical buckling load analysis model were established, respectively, and the maximum tensile and compressibility evaluation criteria of the buckling structure were established (Figures 1.9 and 1.10).
Figure 1.9 (a–g) Geometric diagram of buckling of the corrugated structure under different tensile levels.
Source: Yan et al. [14]/with permission of Elsevier.
Figure 1.10 (a,b) Simplified mechanical model of buckling wrinkles.
Source: Yan et al. [14]/with permission of Elsevier.
The stretchability of the curved corrugated structure is:
where L6 is the maximum tensile length, and the length between the two ends of the top film is L3, as shown in Figure 1.9, which can be approximated as the length of the two ends of the neutral surface of the curved film/substrate structure. According to the buckling analysis, the critical buckling load is positive only when the thickness ratio of the substrate/film exceeds the critical value, and the larger thickness ratio makes the critical load larger. The maximum strain of the film mainly depends on the substrate and strain. Based on the critical buckling load and maximum strain criterion, the maximum compressibility based on buckling analysis and maximum stress analysis is 7% and 20%, respectively.
Jiang et al. [15] from Arizona State University studied the post‐buckling mechanical behavior of corrugated structures based on the energy method and obtained the buckling wavelength and peak value of the ripple. When the applied strain reaches the critical strain, the buckling wavelength increases, the amplitude disappears, and the film strain is equal to the critical strain. The additional applied strain relaxes the film and eventually stretches to a breaking point.
The above studies focus on brittle materials on soft elastic substrates and endow the substrate‐film system with stretchable mechanical properties through controllable buckling. However, the typical elastomer is permeable to the fluid, so it cannot provide a waterproof function. Moreover, since the primary system mechanical strength of the soft substrate usually cannot meet the practical application, a double‐layer substrate is introduced. That is, there is a soft layer on the relatively hard layer in the substrate, which can ensure the mechanical properties of the equipment without losing its stretchability (Figure 1.11).
Figure 1.11 (a–d) Schematic diagram of double substrate soft layer film‐substrate system.
Source: Cheng et al. [16]/with permission of Elsevier.
Cheng et al. [16] established a simple analytical model of a rigid film on a double‐layer substrate. The amplitude and wavelength of the buckling corrugated structure are obtained by the energy method. The Young's modulus of the top substrate layer is more critical than the Young's modulus of the bottom substrate layer for the wavelength, amplitude, and maximum strain of the buckling berth structure.
For the amplitude and wavelength of the buckled corrugated structure, Cheng and Song [17] from the University of Miami also considered the nonlinear constitutive model of the pre‐strained substrate under different conditions: (i) the geometric change of the film/substrate system in the nonstrain state; (ii) the finite strain of the substrate has a nonlinear strain‐displacement relationship; (iii) the nonlinear constitutive model of the substrate. The results show that the post‐buckling amplitude of the film decreases with the increase in the applied strain. In contrast, the wavelength increases with the increase in the applied strain, and the finite geometric change plays a dominant role in the post‐buckling (Figure 1.12).
Figure 1.12 (a–c) Structure diagram of nonlinear corrugated‐substrate system.
Source: Cheng et al. [16]/with permission of Elsevier.
Song et al. [18] from the University of Illinois used the perturbation method to analyze the mechanical properties of the buckling film on the flexible substrate. To further study the influence parameters of the buckling mechanical behavior of thin films on flexible substrates, Jiang et al. [20] from Arizona State University conducted systematic experiments and analysis on the buckling behavior of finite‐width rigid films on flexible substrates. The results show that the amplitude and wavelength of the buckling film increase with the increase in the film width (Figure 1.13).
Figure 1.13 Effect of membrane spacing on buckling amplitude.
Source: Jiang et al. [20]/with permission of Elsevier.
The above studies assume that the buckling structure of the film is a sinusoidal buckling geometry. Chen et al. [21] abandoned the original sinusoidal geometric buckling assumption and proposed a new theoretical model to describe the deformation of the buckling film. The mechanical model shows that the previous mechanical model overestimates the deflection and curvature of the film. The results can provide design guidance for many applications, such as stretchable electrons to micronanoscale surface patterns and precision metrology.
1.1.2 Island Bridge Structure‐Curved Line
To further improve the stretchability of flexible electronic devices, Song et al. [22] proposed a noncoplanar grid design, which uses a semiconductor island and an arc line connected to an elastic substrate. The maximum strain on the arc line and the island is predicted, and the tensile row and compressibility of the entire system are studied. The maximum strain in the arc line decreases with the increase in the pre‐strain. Since the island devices are mostly rigid electronic components, the elastic modulus is much larger than the arc line structure. So, the peak strain in the island is much smaller than the strain in the arc line (Figure 1.14).
Figure 1.14 Noncoplanar arc line structure.
Source: Song et al. [22]/with permission of AIP Publishing.
Li et al. [23] established an analytical mechanical model of the island bridge structure. Based on the unified scaling law, the relationship between the normalized maximum strain of the island bridge structure and the strain of the substrate is revealed, which provides a theoretical basis for the fracture safety of stretchable electronic components (Figure 1.15).
Figure 1.15 Loading mechanical model of the island on the substrate.
Source: Li et al. [23]/with permission of Royal Society of Chemistry.
Among them is the scaling law of the maximum strain on the island:
According to the adhesion between the banded material and the substrate, there may be three de‐deformation modes: global, local, and nonbuckling [24]. Wang et al. [25] established the critical length and thickness ratio of the arc‐shaped banded structure for the banded material structure that is selectively bonded to the elastic substrate, and global buckling can occur without material failure (Figure 1.16).
Figure 1.16 The three possible deformation modes of a filamentary ribbon.
Source: Ko et al. [24]/with permission of John Wiley & Sons.
Wang and Wang [26] studied the four buckling modes of curved films. When the soft tissue shrinks, the electronic components are compressed and bent into various modes. Due to the different stiffness of the tissue and the degree of adhesion of the interface, buckling occurs. The phenomenon of delamination and the conditions for generating different buckling modes are verified theoretically and experimentally, as shown in Figure 1.17.
Figure 1.17 (a–d) Experimental diagrams of four buckling modes.
Source: Wang and Wang [26]/with permission of Elsevier.
Various buckling modes can be divided into four modes, as shown in Figure 1.18:
(1) In the case of no compression to slight compression, the film does not buckle and remains flat;
(2) With the increase of compression, the film wrinkles at the top of the elastomer into multiple wavelets but does not delaminate from the interface, which we call the wrinkling mode;
(3) Under further compression, multiple waves are merged into one, resulting in partial delamination of the film from the interface, which is a partial delamination mode;
(4) The more considerable compression eventually leads to the complete delamination of the film from the interface, which is defined as the complete delamination mode in this study.
Figure 1.18 Four kinds of buckling mode diagram. (a) Flat. (b) Wrinkling. (c) Partial delamination. (d) Total delamination.
Source: Wang and Wang [26]/with permission of Elsevier.
The island bridge structure with curved lines can be used for conformal packaging planes, such as placing silicon‐based circuits on complex surfaces. Wang et al. [27] established a reliable method to predict the buckling mode of interconnected island‐bridge structures on arbitrary axisymmetric surfaces. First, the stress distribution on the curved surface is obtained analytically. Then, the buckling modes of the interconnection bridge along different directions and different positions of the curve are determined by using the stress distribution. Finally, the maximum strain of the interconnection island bridge structure is analytically obtained using the stress distribution and buckling mode (Figure 1.19).
Figure 1.19 A schematic diagram of a conformal curved object wrapped by a compressible circuit grid structure and an elastic transfer element.
Source: Wang et al. [27]/with permission of Royal Society of Chemistry.
1.1.3 Island Bridge Structure‐Serpentine Line
Compared with the wave structure and the island bridge structure connected by arc lines, the island bridge structure connected by serpentine lines can effectively solve the problem of insufficient island spacing and increase the effective coverage area of circuit components [28–35].
Widlund et al. [29] studied the influence of serpentine wire geometry on its stretchability and flexibility through theoretical, numerical, and experimental methods. The results show that the narrower the band, the larger the arc radius and arc angle, and the longer the arm length, the smaller the intrinsic strain and effective stiffness. When the arm length is close to infinity, the stretchability can be improved by several orders of magnitude (Figure 1.20).
Figure 1.20 (A–C) Shape parameter diagram of snake‐like geometric structure.
Source: Widlund et al. [29]/with permission of Elsevier.
Electronics for wearable applications require soft, flexible, and stretchable materials and designs to overcome the mechanical mismatch between the human body and the device. The critical requirement of this wearable electronic device is reliable operation with high performance and robustness in various deformations caused by motion. Son et al. [30] proposed materials and device design strategies for core components of wearable electronic products, such as transistors, charge trap floating gate memory cells, and various logic gates with stretchable shape factors (Figure 1.21).
Figure 1.21 (a–f) Optical microscopic images and finite element analysis results of the serpentine interconnection inverter.
Source: Son et al. [30]/with permission of American Chemical Society.
Yang et al. [31] developed a low‐cost, dehydrated manufacturing process to successfully integrate the brittle indium tin oxide (ITO) serpentine tape on a stretchable substrate. In situ electromechanical experiments measured the tensile properties of ITO ribbons. It was found that the tensile properties were not only related to the shape of the ribbons but also to the adhesion between the ribbons and the substrate. When the adhesion is weak, up to 200% stretchability can be achieved. When the adhesion is strong, a new failure mechanism is observed, which can be used for design criteria under different adhesion conditions (Figure 1.22).
Figure 1.22 (A,B) Structure diagram of ITO serpentine tape and resistance‐strain test results.
Source: Yang et al. [31]/with permission of Elsevier.
Based on the finite deformation of the plane bending beam, Fan et al. [32] established an analytical model of serpentine connection. The FEA of serpentine interconnects with various geometric parameters is carried out to verify the established model. By comparing the predicted stretchability with the estimation of the linear model, the influence of finite deformation can be quantitatively studied. Both theoretical and numerical results show that the linear model can cause considerable overestimation for many serpentine interconnects with representative shapes. In addition, a simplified analytical solution of stretchability is obtained using the approximate model of the nonlinear effect (Figure 1.23).
Figure 1.23 (A,B) Analytical model diagram and tensile results of the serpentine structure.
Source: Fan et al. [32]/with permission of Elsevier.
Xiao et al. [33] studied the conformal design of island bridges on undevelopable surfaces, including the critical size and stiffness and the tensile requirements of bridges. First, the conformal model of the island on the surface of the torus is established to determine the relationship between the maximum size of the island and the surface curvature. The critical dimensionless width of the island is given, which is a function of the thickness of the island, the interface adhesion energy, and the ratio of the two principal curvatures of the surface. Then, the FEA method is used to study the relationship between the tensile stiffness of the bridge and the geometric parameters to guide the deterministic assembly of the surface islands. Finally, the position‐dependent bridge stretchability requirements are given by geometric mapping. This work will provide design rules for stretchable electronic devices fully compliant with nonexpandable surfaces (Figure 1.24).
Figure 1.24 (a–d) Schematic diagram of island bridge structure on nondeployable surface.
Source: Xiao et al. [33]/CC BY 4.0/MDPI.
Xu et al. [34] developed a 3D elastic membrane that accurately matches the epicardium of the heart by using 3D printing as a platform for multifunctional sensors with deformable arrays of electronic and optoelectronic components. This skin device completely suitably encapsulates the heart and has inherent elasticity to provide a mechanically stable biotic/biotic interface during a normal cardiac cycle (Figure 1.25).
Figure 1.25 3D‐MIMs are used for spatiotemporal measurement and stimulation of the epicardial surface. (a) Graphical description of critical steps in device design and manufacturing. (b) Langendorff perfusion of a representative 3D multifunctional capsule (3D‐mim) image on the rabbit heart. (c) Amplification of functional elements in conformal contact with the epicardium.
Source: Xu et al. [34]/with permission of Springer Nature.
Lithography‐defined electrical interconnections with thin, filamentary serpentine layouts have been widely explored for stretchable electronics supported by elastic substrates. Zhang et al. [35] studied the buckling physical properties of this stretchable suspended serpentine wire structure by analyzing the model, finite element calculation, and quantitative experiments and designed the serpentine layout of the super‐stretchable electrode. The buckling initiation and post‐buckling behavior are studied, and the scaling laws of critical buckling strain and elastic behavior limit are determined. Two buckling modes, symmetric and antisymmetric modes, are identified and analyzed. The experimental images and numerical results show that the relevant post‐buckling processes are significantly consistent (Figure 1.26).
Figure 1.26 Experimental and numerical analysis of the buckling mechanism of serpentine conductors with a strain of 0–80%.
Source: Zhang et al. [35]/with permission of Royal Society of Chemistry.
Zhang et al. [36] bonded the serpentine interconnect to the substrate and performed fracture tests on structures formed with and without pre‐strain, quantitatively demonstrating the possible tensile enhancement effect. FEA illustrates the effects of various materials and geometric parameters. As the thickness of the metal increases, the elastic tensile properties decrease sharply, which is due to the change in the buckling mode. The experiment shows no wrinkling from the local wrinkling at the small thickness to the large thickness. An analytical model was created to quantitatively predict the wavelength of this wrinkling and explain the thickness dependence of the buckling behavior (Figure 1.27).
Figure 1.27 (A,B) Schematic diagram and tensile fracture of fully bonded serpentine wire.
Source: Zhang et al. [36]/with permission of John Wiley & Sons.
Su et al. [37] developed a different stretchable structural path in which the coarse rod geometry replaces the ribbon layout, resulting in scissor‐like deformation rather than in‐plane or out‐of‐plane buckling mode. The experimental and analytical models show that the metal and silicon structures with this rod‐like structure can be stretched to 350% and 90% strain without fracture, respectively. Compared with previous studies, this is a significant improvement, achieving 54% stretchability in pure bent, narrow metal interconnects. The metal features of these layouts have additional advantages; they provide low resistance as interconnects to enhance operations in the examples, including high‐power LED lights and solar cell arrays (Figures 1.28 and 1.29).
Figure 1.28 (a–d) Deformation mode of thick rod serpentine conductor under buckling state.
Source: Su et al. [37]/with permission of John Wiley & Sons.
Figure 1.29 (a–d) Shear interconnection demonstration of thick rod serpentine wire in stretchable LED lamp array.
Source: Su et al. [37]/with permission of John Wiley & Sons.
Pan et al. [38] studied the influence of the modulus and thickness of the elastic substrate on the tensile properties of the serpentine interconnect material. FEA shows that the elastic tensile properties significantly increase, and the substrate thickness is reduced. The low‐cycle fatigue test confirms this trend by examining the tension required to form fatigue cracks associated with plastic deformation. To clarify the mechanical principle, the buckling behavior of deformed serpentine interconnections on substrates with different thicknesses was studied. Scanning electron microscopy and 3D optical profiler studies confirmed the transformation of buckling behavior. The overall buckling found in thin substrates can accommodate large stretches before the serpentine plastic deformation, which dramatically improves the stretchability of these systems (Figures 1.30 and 1.31).
Figure 1.30 (a–c) Step diagram of manufacturing serpentine interconnect wire on soft substrate.
Source: Pan et al. [38]/with permission of John Wiley & Sons.
Figure 1.31 (A,B) The results of the tensile properties of serpentine wires varying with the thickness and modulus of the elastic substrate.
Source: Pan et al. [38]/with permission of John Wiley & Sons.
1.1.4 2D Spiral Interconnects
With the more profound research in two‐dimensional planar stretchable structures, a typical 2D spiral structure in nature has attracted much attention due to its excellent tensile properties. Compared with serpentine interconnects, the spiral‐based interconnects can offer higher stretchability [39]. When the 2D spiral interconnect is subjected to transverse force, the spiral structure rotates around the circle's center to release the curly interconnect. The structure shows excellent tensile deformation on the macro level. Once the external force is removed, the strain energy of the structure is released, and the interconnect returns to its original position and shape. The mechanical principles of such structures as tape measures are very similar. They both have low elastic modulus (significant tensile rate) and high yield strength (elastic deformation capacity) properties. Therefore, the 2D spiral structure is highly matched with the mechanical behavior of flexible materials and has been widely used in flexible electronics.
Archimedes spiral is the most common 2D spiral structure in flexible electronics, and its slight curvature and uniform variation are very conducive to sizeable tensile deformation. Lv et al. [39] used Finite element analysis (FEM) to analyze three structures (serpentine, self‐similar, and helical) tensile properties. Among the interconnects of the three structures shown in Figure 1.32, the Archimedean spiral structure has the largest elastic stretchability, reaching 200%. In contrast, the elastic stretchability of the periodic snake structure and the self‐similar structure is only 112% and 90%, respectively. The out‐of‐plane deformation of the curly interconnects compensates for the in‐plane tension. The structure only has some areas that have plastic deformation under large deformation. However, the plastic deformation in the limited area does not affect the overall elasticity. So, this structure can return to underformed shape after external unloading.
Figure 1.32 Stress distribution of serpentine, self‐similar, and helical structures in plane.
Source: Lv et al. [39]/with permission of Elsevier.
To further meet the unique requirements of different applications for spiral structures, Yuan and Wang [40] developed a theoretical model for the mechanical responses of Archimedean‐spiral interconnects under the in‐plane stretching based on the finite deformation plane‐strain beam theory. As shown in Figure 1.33, the key parameters of the mechanical responses including the effective tensile stress, maximum strain, and deformed configurations are analytically derived and validated by FEA for a wide range of geometric variables.
Figure 1.33 (a–f) The deformation and mechanical responses of the Archimedean‐spiral interconnect.
Source: Yuan and Wang [40]/with permission of Elsevier.
The 2D spiral structure central has excellent stability. The 2D helix‐node design proposed by Huang et al. [41] not only ensures the strain isolation of the node region but also limits the out‐of‐plane deformation of the spiral interconnect during the development process. The structure wraps 1.6 μm‐thin silicon spiral bands around circular silicon islands (200 μm). The helix is pulled out under in‐plane tensile load while the silicon island rotates around the circle's center. The force‐displacement relationship of the structure obtained by FEM is shown in Figure 1.34. According to the above research results, the helix rotates around the center of the node in the plane under the in‐plane tensile load, and the structure center is relatively static. Therefore, the interconnection of many islands‐bridges adopts the Archimedean helix structure. Alcheikh et al. [43] studied the relationship between the tensile properties of island bridge structures and the geometric parameters of interconnects through mathematical modeling and experimental verification. As shown in Figure 1.34b, when the three interconnects reach 100% tensile rate, the Archimedean structure's maximum stress is the smallest, σmax = 273 MPa, while the maximum stress of spiral structure and serpentine structure is 454 and 518 MPa, respectively. The results show that the Archimedes interconnect has the lowest stress and, thus, the highest ductility. Sung et al. [42] used the island bridge design to develop a large‐area chip network using stretchable electroplated copper helical wires as interconnects and realized the conformal deformation of rigid functional devices and two‐dimensional surfaces. As shown in Figure 1.34c, functional devices are directly integrated on nodes distributed in a two‐dimensional chip network that are mechanically and electrically connected to surrounding nodes by stretchable copper interconnects. Interconnectors can expand the distance between functional devices by several orders of magnitude to build an extensive area chip network of interconnected devices. Figure 1.34d shows the SEM image of the Archimedes' interconnect with copper plating on a silicon substrate.
Figure 1.34 Mechanical properties and applications of 2D helical‐node interconnect designs.
Source: Sung et al. [42]/with permission of IOP Publishing.
The helix structure achieves a large in‐plane tensile ratio due to the spiral folding around the circle's center, but the helix itself is not designed to be stretchable. The tensile ratio of the structure reaches the limit after the helix is completely pulled out [44]. To further explore the tensile properties of 2D spiral structures, Rehman and Rojas [45] further optimized the design of 2D spiral interconnects and added a serpentine structure to the interconnects to improve the tensile properties, as shown in Figure 1.35a,b. The structure considers the integrated design of a double‐arm helix with serpentine and horseshoe structural variants, and its mechanical response under applied deformation is analyzed by FEA. As shown in Figure 1.35e,f, the proposed composite structure provides excellent tensile capacity compared to the original, unoptimized helical structure. It shows a 55% reduction in stress/strain, as well as a uniform distribution of stress/strain changes. The maximum stress of the optimized composite spiral structure is reduced to 1779 MPa, which is 58.47% lower than that of the original spiral structure.
Figure 1.35 The influence of interconnecting serpentine structure on tensile properties.
Source: Rehman and Rojas [45]/with permission of Elsevier.
1.1.5 3D Spiral Interconnects
Flexible electronic devices have pursued more excellent stretchability and stronger mechanical robustness. To further improve the device's performance, the researchers broke through the 2D plane to conduct in‐depth research on the 3D interconnect structure. As a highly innovative design in stretchable electronics, the 3D helix interconnect provides structures with extremely high elastic stretchability and mechanical stability, it is an ideal platform for flexible wearable devices.
The 3D interconnection structure is the spatial extension of the 2D structure. Huang's group [46, 47] proposed a strategy to transform 2D structure geometry into 3D structure through compression buckling and designed and verified more than 40 representative 3D interconnected structures by combining theoretical analysis, FEA, and experiments. This novel geometric transformation strategy is based on the theory of minimizing the total strain energy of the system after the 2D structure is spatially deformed. The 2D filamentous precursors of different shapes form covalent bonds with the in‐plane pre‐stretched elastomer through predefined bonding sites to achieve the transfer on the elastic substrate. When the pre‐stretched substrate strain is released, the 2D filament precursor is subjected to in‐plane pressure, triggering both in‐plane and out‐of‐plane translational/rotational motion and bending/torsional deformation in the nonbonded region simultaneously. Finally, the 3D structure achieves the balance between the adhesive force of the substrate and the strain energy of the bent and twisted strips, forming a stable 3D helical structure.
