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Flexible and Stretchable Electronics

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A special issue of Micromachines (ISSN 2072-666X).

Deadline for manuscript submissions: closed (31 December 2016) | Viewed by 127818

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Special Issue Editors

Prof. Dr. Seung Hwan Ko

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Guest Editor

Mechanical Engineering, Seoul National University, Seoul, Republic of Korea
Interests: sustainable energy devices; nanomaterial synthesis and characterization; microscale heat transfer; flexible and stretchable electronics
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Daeho Lee

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Guest Editor

Department of Mechanical Engineering, Gachon University, Seongnam, Gyeonggi, Korea
Interests: Ni-based electronics; NiO nanoparticles; ZnO nanowires; ZnO nanoparticles; CuO nanoparticles; nanoparticle ink; transparent conductors; flexible sensors; laser processing
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Prof. Dr. Zhigang Wu

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Guest Editor

School of Mechanical Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Rd, Wuhan 430074, China
Interests: soft robotics; bio-inspired soft microsystems; stretchable electronics; microfluidics; biomedical devices; non-conventional manufacturing technologies
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Flexible and stretchable electronics are getting tremendous attention as future electronics due to flexibility and light weight especially application in wearable electronics. Flexible electronics are usually fabricated on heat sensitive flexible substrates such as plastic, fabric or even papers while stretchable electronics are ussally fabricated elastomeric substrate to survive large deformation in the practical application scenarios. Therefore, the successful fabrication of flexible electronics needs a low temperature processible novel materials and novel processing development because traditional materials and processes are not compatible with flexible/stretchable electronics. Huge technical challenges and opportunities are behind this dramatically changes from the perspective of new material design and processing, new fabrication technique, large deformation mechanics, new application development and so on. Here, we would like to invite the talented researchers to join us on this new vital field that have a potential to reshape our future life, by contributing the wisdoms from your perspective.

Prof. Dr. Seung Hwan Ko
Prof. Dr. Daeho Lee
Prof. Dr. Zhigang Wu
Guest Editors

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Keywords

flexible electronics
stretchable electronics
wearable electronics
printed electronics
inkjet printing
R2R printing
low temperature process
polymer substrate
nanowires
nanoparticles
Skin Intelligence
Soft Smartness
Bio-inspired Microsystems
Large Deformation Mechanics
Soft Material and Processing
Fabrication Technique for Soft Device

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Published Papers (13 papers)

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Editorial

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1533 KiB

Open AccessEditorial

Opportunities and Challenges in Flexible and Stretchable Electronics: A Panel Discussion at ISFSE2016

by Zhigang Wu, Yongan Huang and Rong Chen

Micromachines 2017, 8(4), 129; https://doi.org/10.3390/mi8040129 - 18 Apr 2017

Cited by 9 | Viewed by 4669

Abstract

The 2016 International Symposium of Flexible and Stretchable Electronics (ISFSE2016), co-sponsored by the Flexible Electronics Research Center, Huazhong University of Science and Technology (HUST) & State Key Laboratory of Digital Manufacturing and Equipment Technology, National Natural Science and Engineering (NSFC), was successfully held [...] Read more.

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

Research

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2076 KiB

Open AccessArticle

Rapid and Effective Electrical Conductivity Improvement of the Ag NW-Based Conductor by Using the Laser-Induced Nano-Welding Process

by Phillip Lee, Jinhyeong Kwon, Jinhwan Lee, Habeom Lee, Young D. Suh, Sukjoon Hong and Junyeob Yeo

Micromachines 2017, 8(5), 164; https://doi.org/10.3390/mi8050164 - 19 May 2017

Cited by 17 | Viewed by 6277

Abstract

To date, the silver nanowire-based conductor has been widely used for flexible/stretchable electronics due to its several advantages. The optical nanowire annealing process has also received interest as an alternative annealing process to the Ag nanowire (NW)-based conductor. In this study, we present an analytical investigation on the phenomena of the Ag NWs’ junction and welding properties under laser exposure. The two different laser-induced welding processes (nanosecond (ns) pulse laser-induced nano-welding (LINW) and continuous wave (cw) scanning LINW) are applied to the Ag NW percolation networks. The Ag NWs are selectively melted and merged at the junction of Ag NWs under very short laser exposure; these results are confirmed by scanning electron microscope (SEM), focused-ion beam (FIB), electrical measurement, and finite difference time domain (FDTD) simulation. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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3679 KiB

Open AccessArticle

Large-Area Compatible Laser Sintering Schemes with a Spatially Extended Focused Beam

by Habeom Lee, Jinhyeong Kwon, Woo Seop Shin, Hyeon Rack Kim, Jaeho Shin, Hyunmin Cho, Seungyong Han, Junyeob Yeo and Sukjoon Hong

Micromachines 2017, 8(5), 153; https://doi.org/10.3390/mi8050153 - 11 May 2017

Cited by 12 | Viewed by 6396

Abstract

Selective laser sintering enables the facile production of metal nanoparticle-based conductive layers on flexible substrates, but its application towards large-area electronics has remained questionable due to the limited throughput of the laser process that originates from the direct writing nature. In this study, modified optical schemes are introduced for the fabrication of (1) a densely patterned conductive layer and (2) a thin-film conductive layer without any patterns. In detail, a focusing lens is substituted by a micro lens array or a cylindrical lens to generate multiple beamlets or an extended focal line. The modified optical settings are found to be advantageous for the creation of repetitive conducting patterns or areal sintering of the silver nanoparticle ink layer. It is further confirmed that these optical schemes are equally compatible with plastic substrates for its application towards large-area flexible electronics. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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5196 KiB

Open AccessArticle

Ultrasonic Spray-Coating of Large-Scale TiO₂ Compact Layer for Efficient Flexible Perovskite Solar Cells

by Peng Zhou, Wangnan Li, Tianhui Li, Tongle Bu, Xueping Liu, Jing Li, Jiang He, Rui Chen, Kunpeng Li, Juan Zhao and Fuzhi Huang

Micromachines 2017, 8(2), 55; https://doi.org/10.3390/mi8020055 - 14 Feb 2017

Cited by 30 | Viewed by 8730

Abstract

Flexible electronics have attracted great interest in applications for the wearable devices. Flexible solar cells can be integrated into the flexible electronics as the power source for the wearable devices. In this work, an ultrasonic spray-coating method was employed to deposit TiO₂ nanoparticles on polymer substrates for the fabrication of flexible perovskite solar cells (PSCs). Pre-synthesized TiO₂ nanoparticles were first dispersed in ethanol to prepare the precursor solutions with different concentrations (0.5 mg/mL, 1.0 mg/mL, 2.0 mg/mL) and then sprayed onto the conductive substrates to produce compact TiO₂ films with different thicknesses (from 30 nm to 150 nm). The effect of the different drying processes on the quality of the compact TiO₂ film was studied. In order to further improve the film quality, titanium diisopropoxide bis(acetylacetonate) (TAA) was added into the TiO₂-ethanol solution at a mole ratio of 1.0 mol % with respect to the TiO₂ content. The final prepared PSC devices showed a power conversion efficiency (PCE) of 14.32% based on the indium doped tin oxide coated glass (ITO-glass) substrate and 10.87% on the indium doped tin oxide coated polyethylene naphthalate (ITO-PEN) flexible substrate. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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4558 KiB

Open AccessArticle

Direct Silver Micro Circuit Patterning on Transparent Polyethylene Terephthalate Film Using Laser-Induced Photothermochemical Synthesis

by Chen-Jui Lan, Song-Ling Tsai and Ming-Tsang Lee

Micromachines 2017, 8(2), 52; https://doi.org/10.3390/mi8020052 - 13 Feb 2017

Cited by 5 | Viewed by 6597

Abstract

This study presents a new and improved approach to the rapid and green fabrication of highly conductive microscale silver structures on low-cost transparent polyethylene terephthalate (PET) flexible substrate. In this new laser direct synthesis and pattering (LDSP) process, silver microstructures are simultaneously synthesized and laid down in a predetermined pattern using a low power continuous wave (CW) laser. The silver ion processing solution, which is transparent and reactive, contains a red azo dye as the absorbing material. The silver pattern is formed by photothermochemical reduction of the silver ions induced by the focused CW laser beam. In this improved LDSP process, the non-toxic additive in the transparent ionic solution absorbs energy from a low cost CW visible laser without the need for the introduction of any hazardous chemical process. Tests were carried out to determine the durability of the conductive patterns, and numerical analyses of the thermal and fluid transport were performed to investigate the morphology of the deposited patterns. This technology is an advanced method for preparing micro-scale circuitry on an inexpensive, flexible, and transparent polymer substrate that is fast, environmentally benign, and shows potential for Roll-to-Roll manufacture. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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3367 KiB

Open AccessArticle

Suspended Graphene-Based Gas Sensor with 1-mW Energy Consumption

by Jong-Hyun Kim, Qin Zhou and Jiyoung Chang

Micromachines 2017, 8(2), 44; https://doi.org/10.3390/mi8020044 - 1 Feb 2017

Cited by 15 | Viewed by 5961

Abstract

This paper presents NH3 sensing with ultra-low energy consumption for fast recovery and a graphene sheet based on a suspended microheater. Sensitivity and repeatability are important characteristics of functional gas sensors embedded in mobile devices. Moreover, low energy consumption is an essential requirement in flexible and stretchable mobile electronics due to their small dimension and fluctuating resistivity during mechanical behavior. In this paper, we introduce a graphene-based ultra-low power gas detection device with integration of a suspended silicon heater. Dramatic power reduction is enabled by a duty cycle while not sacrificing sensitivity. The new oscillation method of heating improves the sensitivity of 0.049 (ΔR/R0) measured at a flow rate of 18.8 sccm NH3(g) for 70 s. Our experimental tests show that a 60% duty cycle does not sacrifice sensitivity or recovery by dropping the total power consumption from 1.76 mW to 1.05 mW. The aforementioned low energy consuming gas sensor platform not only attracts environmentally-related industries, but also has the potential to be applied to flexible and stretchable mobile electronic devices. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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3684 KiB

Open AccessArticle

Wrinkled Graphene–AgNWs Hybrid Electrodes for Smart Window

by Ki-Woo Jun, Jong-Nam Kim, Jin-Young Jung and Il-Kwon Oh

Micromachines 2017, 8(2), 43; https://doi.org/10.3390/mi8020043 - 1 Feb 2017

Cited by 13 | Viewed by 6422

Abstract

Over the past few years, there has been an increasing demand for stretchable electrodes for flexible and soft electronic devices. An electrode in such devices requires special functionalities to be twisted, bent, stretched, and deformed into variable shapes and also will need to have the capacity to be restored to the original state. In this study, we report uni- or bi-axially wrinkled graphene–silver nanowire hybrid electrodes comprised of chemical vapor deposition (CVD)-grown graphene and silver nanowires. A CVD-grown graphene on a Cu-foil was transferred onto a biaxially pre-strained elastomer substrate and silver nanowires were sprayed on the transferred graphene surface. The pre-strained film was relaxed uni-(or bi-)axially to produce a wrinkled structure. The bi-axially wrinkled graphene and silver nanowires hybrid electrodes were very suitable for high actuating performance of electro-active dielectric elastomers compared with the wrinkle-free case. Present results show that the optical transparency of the highly stretchable electrode can be successfully tuned by modulating input voltages. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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1554 KiB

Open AccessArticle

One-Dimensional Thermal Analysis of the Flexible Electronic Devices Integrated with Human Skin

by Yun Cui, Yuhang Li, Yufeng Xing, Tianzhi Yang and Jizhou Song

Micromachines 2016, 7(11), 210; https://doi.org/10.3390/mi7110210 - 18 Nov 2016

Cited by 22 | Viewed by 5023

Abstract

A one-dimensional analytic thermal model for the flexible electronic devices integrated with human skin under a constant and pulsed power is developed. The Fourier heat conduction equation is adopted for the flexible electronics devices while the Pennes bio-heat transfer equation is adopted for the skin tissue. Finite element analysis is performed to validate the analytic model through the comparison of temperature distributions in the system. The influences of geometric and loading parameters on the temperature increase under a pulsed power are investigated. It is shown that a small duty cycle can reduce the temperature increase of the system effectively. A thin substrate can reduce the device temperature but increase the skin surface temperature. The results presented may be helpful to optimize the design of flexible electronic devices to reduce the adverse thermal influences in bio-integrated applications. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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Review

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6015 KiB

Open AccessReview

Progress in Research of Flexible MEMS Microelectrodes for Neural Interface

by Long-Jun Tang, Ming-Hao Wang, Hong-Chang Tian, Xiao-Yang Kang, Wen Hong and Jing-Quan Liu

Micromachines 2017, 8(9), 281; https://doi.org/10.3390/mi8090281 - 18 Sep 2017

Cited by 17 | Viewed by 7508

Abstract

With the rapid development of Micro-electro-mechanical Systems (MEMS) fabrication technologies, many microelectrodes with various structures and functions have been designed and fabricated for applications in biomedical research, diagnosis and treatment through electrical stimulation and electrophysiological signal recording. The flexible MEMS microelectrodes exhibit excellent characteristics in many aspects beyond stiff microelectrodes based on silicon or metal, including: lighter weight, smaller volume, better conforming to neural tissue and lower fabrication cost. In this paper, we reviewed the key technologies in flexible MEMS microelectrodes for neural interface in recent years, including: design and fabrication technology, flexible MEMS microelectrodes with fluidic channels and electrode–tissue interface modification technology for performance improvement. Furthermore, the future directions of flexible MEMS microelectrodes for neural interface were described, including transparent and stretchable microelectrodes integrated with multi-functional aspects and next-generation electrode–tissue interface modifications, which facilitated electrode efficacy and safety during implantation. Finally, we predict that the relationships between micro fabrication techniques, and biomedical engineering and nanotechnology represented by flexible MEMS microelectrodes for neural interface, will open a new gate to better understanding the neural system and brain diseases. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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9747 KiB

Open AccessReview

Mechanisms and Materials of Flexible and Stretchable Skin Sensors

by Yicong Zhao and Xian Huang

Micromachines 2017, 8(3), 69; https://doi.org/10.3390/mi8030069 - 28 Feb 2017

Cited by 51 | Viewed by 15580

Abstract

Wearable technology has attracted significant public attention and has generated huge societal and economic impact, leading to changes of both personal lifestyles and formats of healthcare. An important type of devices in wearable technology is flexible and stretchable skin sensors used primarily for biophysiological signal sensing and biomolecule analysis on skin. These sensors offer mechanical compatibility to human skin and maximum compliance to skin morphology and motion, demonstrating great potential as promising alternatives to current wearable electronic devices based on rigid substrates and packages. The mechanisms behind the design and applications of these sensors are numerous, involving profound knowledge about the physical and chemical properties of the sensors and the skin. The corresponding materials are diverse, featuring thin elastic films and unique stretchable structures based on traditional hard or ductile materials. In addition, the fabrication techniques that range from complementary metal-oxide semiconductor (CMOS) fabrication to innovative additive manufacturing have led to various sensor formats. This paper reviews mechanisms, materials, fabrication techniques, and representative applications of flexible and stretchable skin sensors, and provides perspective of future trends of the sensors in improving biomedical sensing, human machine interfacing, and quality of life. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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5807 KiB

Open AccessReview

Towards Flexible Transparent Electrodes Based on Carbon and Metallic Materials

by Minghui Luo, Yanhua Liu, Wenbin Huang, Wen Qiao, Yun Zhou, Yan Ye and Lin-Sen Chen

Micromachines 2017, 8(1), 12; https://doi.org/10.3390/mi8010012 - 4 Jan 2017

Cited by 42 | Viewed by 9509

Abstract

Flexible transparent electrodes (FTEs) with high stability and scalability are in high demand for the extremely widespread applications in flexible optoelectronic devices. Traditionally, thin films of indium thin oxide (ITO) served the role of FTEs, but film brittleness and scarcity of materials limit its further application. This review provides a summary of recent advances in emerging transparent electrodes and related flexible devices (e.g., touch panels, organic light-emitting diodes, sensors, supercapacitors, and solar cells). Mainly focusing on the FTEs based on carbon nanomaterials (e.g., carbon nanotubes and graphene) and metal materials (e.g., metal grid and metal nanowires), we discuss the fabrication techniques, the performance improvement, and the representative applications of these highly transparent and flexible electrodes. Finally, the challenges and prospects of flexible transparent electrodes will be summarized. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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4143 KiB

Open AccessFeature PaperReview

Materials, Mechanics, and Patterning Techniques for Elastomer-Based Stretchable Conductors

by Xiaowei Yu, Bikram K. Mahajan, Wan Shou and Heng Pan

Micromachines 2017, 8(1), 7; https://doi.org/10.3390/mi8010007 - 27 Dec 2016

Cited by 56 | Viewed by 14651

Abstract

Stretchable electronics represent a new generation of electronics that utilize soft, deformable elastomers as the substrate or matrix instead of the traditional rigid printed circuit boards. As the most essential component of stretchable electronics, the conductors should meet the requirements for both high conductivity and the capability to maintain conductive under large deformations such as bending, twisting, stretching, and compressing. This review summarizes recent progresses in various aspects of this fascinating and challenging area, including materials for supporting elastomers and electrical conductors, unique designs and stretching mechanics, and the subtractive and additive patterning techniques. The applications are discussed along with functional devices based on these conductors. Finally, the review is concluded with the current limitations, challenges, and future directions of stretchable conductors. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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13447 KiB

Open AccessFeature PaperReview

Recent Advancements in Liquid Metal Flexible Printed Electronics: Properties, Technologies, and Applications

by Xuelin Wang and Jing Liu

Micromachines 2016, 7(12), 206; https://doi.org/10.3390/mi7120206 - 30 Nov 2016

Cited by 163 | Viewed by 25118

Abstract

This article presents an overview on typical properties, technologies, and applications of liquid metal based flexible printed electronics. The core manufacturing material—room-temperature liquid metal, currently mainly represented by gallium and its alloys with the properties of excellent resistivity, enormous bendability, low adhesion, and large surface tension, was focused on in particular. In addition, a series of recently developed printing technologies spanning from personal electronic circuit printing (direct painting or writing, mechanical system printing, mask layer based printing, high-resolution nanoimprinting, etc.) to 3D room temperature liquid metal printing is comprehensively reviewed. Applications of these planar or three-dimensional printing technologies and the related liquid metal alloy inks in making flexible electronics, such as electronical components, health care sensors, and other functional devices were discussed. The significantly different adhesions of liquid metal inks on various substrates under different oxidation degrees, weakness of circuits, difficulty of fabricating high-accuracy devices, and low rate of good product—all of which are challenges faced by current liquid metal flexible printed electronics—are discussed. Prospects for liquid metal flexible printed electronics to develop ending user electronics and more extensive applications in the future are given. Full article

(This article belongs to the Special Issue Flexible and Stretchable Electronics)

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