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Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II

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A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "A：Physics".

Deadline for manuscript submissions: closed (31 December 2019) | Viewed by 41429

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Special Issue Editor

Dr. Ramses V. Martinez

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Guest Editor

School of Industrial Engineering, Weldon School of Biomedical Engineering, Purdue University, Grissom Hall (GRIS) Rm.284, 315 N. Grant Street, West Lafayette, IN 47907-2023, USA
Interests: wearable devices; self-powered flexible sensors; soft robotics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The development of flexible electronic solutions as wearable and conformable sensors, displays, and powering devices is making a huge impact on the integration of electronic and optical systems in our daily lives. Flexible electronic platforms benefit from their bendability, low profile, and light weight to provide highly adaptable industrial and medical devices and enable the integration of sensitive electronic skins on large objects, robots, or machines with complex 3D shapes. To realize their full potential, however, it is necessary to develop new manufacturing, assembly, and packaging methods to create multifunctional flexible devices at a reduced cost and with an increased resistance to mechanical fatigue. Accordingly, this Special Issue seeks to showcase short communications, research papers, and review articles that focus on novel methodological development for the fabrication and integration of organic and inorganic flexible electronics in healthcare, environmental monitoring, displays and human–machine interactivity, robotics, communication and wireless networks, and energy conversion, management, and storage. Novel, large-area processing methods, such as ink-jet processing, solution deposition, spray pyrolysis, or roll-to-roll processing, and cutting-edge work on flexible designs and form factors are also of interest.

Dr. Ramses V. Martinez
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Micromachines is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Flexible electronics
Stretchable electronics
Printable electronics
Epidermal electronics
Robotic skin
Conformable sensors
Low-cost flexible devices
Flexible displays
Flexible energy storage
Flexible energy conversion
Flexible electronic textiles
Wearable flexible devices

Published Papers (12 papers)

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Research

Jump to: Review

18 pages, 10569 KiB

Open AccessArticle

Aerosol Spray Deposition of Liquid Metal and Elastomer Coatings for Rapid Processing of Stretchable Electronics

by Taylor V. Neumann, Berra Kara, Yasaman Sargolzaeiaval, Sooik Im, Jinwoo Ma, Jiayi Yang, Mehmet C. Ozturk and Michael D. Dickey

Micromachines 2021, 12(2), 146; https://doi.org/10.3390/mi12020146 - 01 Feb 2021

Cited by 29 | Viewed by 5350

Abstract

We report a spray deposition technique for patterning liquid metal alloys to form stretchable conductors, which can then be encapsulated in silicone elastomers via the same spraying procedure. While spraying has been used previously to deposit many materials, including liquid metals, this work focuses on quantifying the spraying process and combining it with silicones. Spraying generates liquid metal microparticles (~5 μm diameter) that pass through openings in a stencil to produce traces with high resolution (~300 µm resolution using stencils from a craft cutter) on a substrate. The spraying produces sufficient kinetic energy (~14 m/s) to distort the particles on impact, which allows them to merge together. This merging process depends on both particle size and velocity. Particles of similar size do not merge when cast as a film. Likewise, smaller particles (<1 µm) moving at the same speed do not rupture on impact either, though calculations suggest that such particles could rupture at higher velocities. The liquid metal features can be encased by spraying uncured silicone elastomer from a volatile solvent to form a conformal coating that does not disrupt the liquid metal features during spraying. Alternating layers of liquid metal and elastomer may be patterned sequentially to build multilayer devices, such as soft and stretchable sensors. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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13 pages, 4647 KiB

Open AccessArticle

Resistance Reduction of Conductive Patterns Printed on Textile by Curing Shrinkage of Passivation Layers

by Tomoya Koshi, Ken-ichi Nomura and Manabu Yoshida

Micromachines 2020, 11(6), 539; https://doi.org/10.3390/mi11060539 - 26 May 2020

Cited by 5 | Viewed by 2331

Abstract

Directly printing conductive ink on textiles is simple and compatible with the conventional electronics manufacturing process. However, the conductive patterns thus formed often show high initial resistance and significant resistance increase due to tensile deformation. Achieving conductive patterns with low initial resistance and reduced deformation-induced resistance increase is a significant challenge in the field of electronic textiles (e-textiles). In this study, the passivation layers printed on conductive patterns, which are necessary for practical use, were examined as a possible solution. Specifically, the reduction of the initial resistance and deformation-induced resistance increase, caused by the curing shrinkage of passivation layers, were theoretically and experimentally investigated. In the theoretical analysis, to clarify the mechanism of the reduction of deformation-induced resistance increase, crack propagation in conductive patterns was analyzed. In the experiments, conductive patterns with and without shrinking passivation layers (polydimethylsiloxane) cured at temperatures of 20–120 °C were prepared, and the initial resistances and resistance increases due to cyclic tensile and washing in each case were compared. As a result, the initial resistance was reduced further by the formation of shrinking passivation layers cured at higher temperatures, and reduced to 0.45 times when the curing temperature was 120 °C. The cyclic tensile and washing tests confirmed a 0.48 and a 0.011 times reduction of resistance change rate after the 100th elongation cycle (10% in elongation rate) and the 10th washing cycle, respectively, by comparing the samples with and without shrinking passivation layers cured at 120 °C. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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12 pages, 2675 KiB

Open AccessFeature PaperArticle

Strain-Insensitive Elastic Surface Electromyographic (sEMG) Electrode for Efficient Recognition of Exercise Intensities

by Daxiu Tang, Zhe Yu, Yong He, Waqas Asghar, Ya-Nan Zheng, Fali Li, Changcheng Shi, Roozbeh Zarei, Yiwei Liu, Jie Shang, Xiang Liu and Run-Wei Li

Micromachines 2020, 11(3), 239; https://doi.org/10.3390/mi11030239 - 25 Feb 2020

Cited by 9 | Viewed by 2733

Abstract

Surface electromyography (sEMG) sensors are widely used in the fields of ergonomics, sports science, and medical research. However, current sEMG sensors cannot recognize the various exercise intensities efficiently because of the strain interference, low conductivity, and poor skin-conformability of their electrodes. Here, we present a highly conductive, strain-insensitive, and low electrode–skin impedance elastic sEMG electrode, which consists of a three-layered structure (polydimethylsiloxane/galinstan + polydimethylsiloxane/silver-coated nickel + polydimethylsiloxane). The bottom layer of the electrode consists of vertically conductive magnetic particle paths, which are insensitive to stretching strain, collect sEMG charge from human skin, and finally transfer it to processing circuits via an intermediate layer. Our skin-friendly electrode exhibits high conductivity (0.237 and 1.635 mΩ·cm resistivities in transverse and longitudinal directions, respectively), low electrode–skin impedance (47.23 kΩ at 150 Hz), excellent strain-insensitivity (10% change of electrode–skin impedance within the 0–25% strain range), high fatigue resistance (>1500 cycles), and good conformability with skin. During various exercise intensities, the signal-to-noise ratio (SNR) of our electrode increased by 22.53 dB, which is 206% and 330% more than that of traditional Ag/AgCl and copper electrode, respectively. The ability of our electrode to efficiently recognize various exercise intensities confirms its great application potential for the field of sports health. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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7 pages, 2481 KiB

Open AccessArticle

Electrical Re-Writable Non-Volatile Memory Device Based on PEDOT:PSS Thin Film

by Iulia Salaoru and Christos Christodoulos Pantelidis

Micromachines 2020, 11(2), 182; https://doi.org/10.3390/mi11020182 - 10 Feb 2020

Cited by 1 | Viewed by 2017

Abstract

In this research, we investigate the memory behavior of poly(3,4 ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) cross bar structure memory cells. We demonstrate that Al/PEDOT:PSS/Al cells fabricated elements exhibit a bipolar switching and reproducible behavior via current–voltage, endurance, and retention time tests. We ascribe the [...] Read more.

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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12 pages, 6019 KiB

Open AccessArticle

Development of Real-Time Measurement Platform for Stretchable and Rollable Functions of Flexible Electronics under Multiple Dynamic Loads

by Chang-Chun Lee, Jui-Chang Chuang, Ruei-Ci Shih and Chi-Wei Wang

Micromachines 2020, 11(1), 106; https://doi.org/10.3390/mi11010106 - 19 Jan 2020

Cited by 5 | Viewed by 2536

Abstract

Mainstream next generation electronic devices with miniaturized structures and high levels of performance are needed to meet the characteristic requirements of electronics with flexible and stretchable capabilities. Accordingly, several applied fields of innovative electronic component techniques, such as wearable devices, foldable curtain-like displays, and flexible hybrid electronic (FHE) biosensors, are considered. This study presents a novel inspection system with multifunctions of stressing tensile and bending mechanical loads to acquire the stretchable and rollable characteristics of soft specimens. The performance of the proposed measurement platform using samples of three different geometric types is evaluated in terms of its stretchability. The results show a remarkable enhancement of mechanical reliability when the sine wave geometric structure is used. A symmetrical sine wave-shaped sample is designed to measure performance under cyclic rolling. The proposed measurement platform of flexible electronics meets the testing requirements of mechanical reliability for the development of future flexible electronic components and FHE products. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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13 pages, 5345 KiB

Open AccessArticle

A Low-Noise-Level Heart Sound System Based on Novel Thorax-Integration Head Design and Wavelet Denoising Algorithm

by Shuo Zhang, Ruiqing Zhang, Shijie Chang, Chengyu Liu and Xianzheng Sha

Micromachines 2019, 10(12), 885; https://doi.org/10.3390/mi10120885 - 17 Dec 2019

Cited by 7 | Viewed by 3304

Abstract

Along with the great performance in diagnosing cardiovascular diseases, current stethoscopes perform unsatisfactorily in controlling undesired noise caused by the surrounding environment and detector operation. In this case, a low-noise-level heart sound system was designed to inhibit noise by a novel thorax-integration head with a flexible electric film. A hardware filter bank and wavelet-based algorithm were employed to enhance the recorded heart sounds from the system. In the experiments, we used the new system and the 3M™ Littmann® Model 3200 Electronic Stethoscope separately to record heart sounds in different noisy environments. The results illustrated that the average estimated noise ratio represented 21.26% and the lowest represented only 12.47% compared to the 3M stethoscope, demonstrating the better performance in denoising ability of this system than state-of-the-art equipment. Furthermore, based on the heart sounds recorded with this system, some diagnosis results were achieved from an expert and compared to echocardiography reports. The diagnoses were correct except for two uncertain items, which greatly confirmed the fact that this system could reserve complete pathological information in the end. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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13 pages, 4198 KiB

Open AccessFeature PaperArticle

Ultra-Sensitive Flexible Tactile Sensor Based on Graphene Film

by Xiaozhou Lü, Liang Qi, Hanlun Hu, Xiaoping Li, Guanghui Bai, Jun Chen and Weimin Bao

Micromachines 2019, 10(11), 730; https://doi.org/10.3390/mi10110730 - 28 Oct 2019

Cited by 10 | Viewed by 2742

Abstract

Flexible tactile sensor can be integrated into artificial skin and applied in industrial robot and biomedical engineering. However, the presented tactile sensors still have challenge in increasing sensitivity to expand the sensor’s application. Aiming at this problem, this paper presents an ultra-sensitive flexible tactile sensor. The sensor is based on piezoresistive effect of graphene film and is composed of upper substrate (PDMS bump with a size of 5 mm × 7 mm and a thickness of 1 mm), medial Graphene/PET film (Graphene/PET film with a size of 5 mm × 7 mm, PET with a hardness of 2H) and lower substrate (PI with fabricated electrodes). We presented the structure and reduced the principle of the sensor. We also fabricated several sample devices of the sensor and carried out experiment to test the performance. The results show that the sensor performed an ultra high sensitivity of 10.80/kPa at the range of 0–4 kPa and have a large measurement range up to 600 kPa. The sensor has 4 orders of magnitude between minimum resolution and maximum measurement range which have great advantage compared with state of the art. The sensor is expected to have great application prospect in robot and biomedical. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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12 pages, 5348 KiB

Open AccessArticle

Requirements for Durability Improvement of Conductive Patterns Permeated in Textiles under Cyclic Tensile Deformation

by Tomoya Koshi, Ken-ichi Nomura and Manabu Yoshida

Micromachines 2019, 10(11), 721; https://doi.org/10.3390/mi10110721 - 25 Oct 2019

Cited by 6 | Viewed by 2781

Abstract

Conductive patterns on textiles are one of the key components for electronic textiles (E-textiles). The patterns with deeper permeation of inks into the textiles show better durability against cyclic tensile deformation. However, other requirements for improving the durability and the behavior of resistance under deformation are still unclear. In this study, the resistance during cyclic tensile deformation was measured with changing conditions, and the resistance variation was analyzed while considering the stress variation. Silver inks were printed on a plain weave, and the pattern width and tensile direction against weft yarns were changed. Measurements confirmed that the resistance increased less with wider pattern widths and when the tensile direction was horizontal to the axis of the weft yarns. Through scanning electron microscopy (SEM) observation, we also confirmed that the growth rate of cracks, at the crossing point of yarns, was changed by the tensile direction. These results indicate that the durability is improved when the electricity path redundancy within the pattern is robust, and the crack growth rate at the yarn crossing points is low. The analysis also confirmed both increasing and decreasing behavior of resistance during stretching in the cyclic tensile deformation, indicating the behavior results from the stress variation of a plain weave. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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26 pages, 3153 KiB

Open AccessArticle

Evolutionary Computation for Parameter Extraction of Organic Thin-Film Transistors Using Newly Synthesized Liquid Crystalline Nickel Phthalocyanine

by Juan A. Jiménez-Tejada, Adrián Romero, Jesús González, Nandu B. Chaure, Andrew N. Cammidge, Isabelle Chambrier, Asim K. Ray and M. Jamal Deen

Micromachines 2019, 10(10), 683; https://doi.org/10.3390/mi10100683 - 10 Oct 2019

Cited by 4 | Viewed by 2224

Abstract

In this work, the topic of the detrimental contact effects in organic thin-film transistors (OTFTs) is revisited. In this case, contact effects are considered as a tool to enhance the characterization procedures of OTFTs, achieving more accurate values for the fundamental parameters of the transistor threshold voltage, carrier mobility and on-off current ratio. The contact region is also seen as a fundamental part of the device which is sensitive to physical, chemical and fabrication variables. A compact model for OTFTs, which includes the effects of the contacts, and a recent proposal of an associated evolutionary parameter extraction procedure are reviewed. Both the model and the procedure are used to assess the effect of the annealing temperature on a nickel-1,4,8,11,15,18,22,25-octakis(hexyl)phthalocyanine (NiPc₆)-based OTFT. A review of the importance of phthalocyanines in organic electronics is also provided. The characterization of the contact region in NiPc₆ OTFTs complements the results extracted from other physical–chemical techniques such as differential scanning calorimetry or atomic force microscopy, in which the transition from crystal to columnar mesophase imposes a limit for the optimum performance of the annealed OTFTs. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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9 pages, 3030 KiB

Open AccessArticle

High Precision Thermoforming 3D-Conformable Electronics with a Phase-Changing Adhesion Interlayer

by Kang Wu, Qifeng Zhou, Huaping Zou, Kangmin Leng, Yifan Zeng and Zhigang Wu

Micromachines 2019, 10(3), 160; https://doi.org/10.3390/mi10030160 - 26 Feb 2019

Cited by 8 | Viewed by 3409

Abstract

Modern design-conscious products have raised the development of advanced electronic fabricating technologies. These widely used industrial technologies show high compatibility for inorganic materials and capacity for mass production. However, the morphology accuracy is hard to ensure and cracks happen easily, which could cause the degradation of device performance and life span. In order to make high precision 3D conformable electronics, a thermal phase-changing adhesion interlayer and modified fabricating processes are used in self-developed equipment. The working principles and influencing factors such as heating time and geometry parameters are studied quantitatively. The accuracy of fabricated patterns is enhanced by this new technology and serpentine designed structures. The delamination or detachment are significantly alleviated. Due to the operation convenience and compatibility with existing materials, the presented fabrication method has great potential for mass production of 3D curved conformable electronics. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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Review

Jump to: Research

19 pages, 4572 KiB

Open AccessReview

Recent Advances on Thermal Management of Flexible Inorganic Electronics

by Yuhang Li, Jiayun Chen, Shuang Zhao and Jizhou Song

Micromachines 2020, 11(4), 390; https://doi.org/10.3390/mi11040390 - 09 Apr 2020

Cited by 4 | Viewed by 3319

Abstract

Flexible inorganic electronic devices (FIEDs) consisting of functional inorganic components on a soft polymer substrate have enabled many novel applications such as epidermal electronics and wearable electronics, which cannot be realized through conventional rigid electronics. The low thermal dissipation capacity of the soft polymer substrate of FIEDs demands proper thermal management to reduce the undesired thermal influences. The biointegrated applications of FIEDs pose even more stringent requirements on thermal management due to the sensitive nature of biological tissues to temperature. In this review, we take microscale inorganic light-emitting diodes (μ-ILEDs) as an example of functional components to summarize the recent advances on thermal management of FIEDs including thermal analysis, thermo-mechanical analysis and thermal designs of FIEDs with and without biological tissues. These results are very helpful to understand the underlying heat transfer mechanism and provide design guidelines to optimize FIEDs in practical applications. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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33 pages, 16934 KiB

Open AccessReview

Recent Developments of Flexible and Stretchable Electrochemical Biosensors

by Xudong Yang and Huanyu Cheng

Micromachines 2020, 11(3), 243; https://doi.org/10.3390/mi11030243 - 26 Feb 2020

Cited by 56 | Viewed by 8123

Abstract

The skyrocketing popularity of health monitoring has spurred increasing interest in wearable electrochemical biosensors. Compared with the traditionally rigid and bulky electrochemical biosensors, flexible and stretchable devices render a unique capability to conform to the complex, hierarchically textured surfaces of the human body. With a recognition element (e.g., enzymes, antibodies, nucleic acids, ions) to selectively react with the target analyte, wearable electrochemical biosensors can convert the types and concentrations of chemical changes in the body into electrical signals for easy readout. Initial exploration of wearable electrochemical biosensors integrates electrodes on textile and flexible thin-film substrate materials. A stretchable property is needed for the thin-film device to form an intimate contact with the textured skin surface and to deform with various natural skin motions. Thus, stretchable materials and structures have been exploited to ensure the effective function of a wearable electrochemical biosensor. In this mini-review, we summarize the recent development of flexible and stretchable electrochemical biosensors, including their principles, representative application scenarios (e.g., saliva, tear, sweat, and interstitial fluid), and materials and structures. While great strides have been made in the wearable electrochemical biosensors, challenges still exist, which represents a small fraction of opportunities for the future development of this burgeoning field. Full article

(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration, Volume II)

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