Special Issue "Printed Flexible and Stretchable Electronics"

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

Deadline for manuscript submissions: closed (20 September 2018)

Special Issue Editors

Guest Editor
Prof. Matti Mäntysalo

Laboratory of Electronics and Communications Engineering, Tampere University of Technology, Korkeakoulunkatu 10, 33720, Finland
Website | E-Mail
Phone: +358-40-757-6800
Interests: electronics, printed and hybrid electronics, stretchable and wearable electornics, materials and applications
Guest Editor
Dr. Enrico Sowade

Department Digital Printing, Textile Auxiliaries Division, Zschimmer&Schwarz Mohsdorf GmbH & Co. KG, Chemnitztalstr. 1, 09217 Burgstädt, Germany
Website | E-Mail
Phone: +49 (0)3724 67-254
Interests: Digital textile printing, inkjet printing, printed electronics, functional printing

Special Issue Information

Dear Colleagues,

Today, electronic devices are evolving from their rigid, heavy, and stationary origins to smart, flexible, and wearable applications. Thus, the area of flexible, printable, and potentially also stretchable electronics is attracting increasing interest. Various applications are predicted, such as smart skin electronics, smart textile electronics, and imperceptible and conformal electronics. All of these application scenarios will allow a more seamless integration of electronics into everyday life. Research on the topic of non-rigid electronics started more than 20 years ago, and became of high importance in research again in recent years. However, there are many challenges concerning materials and processes required for the manufacturing of flexible, printable, and potentially stretchable electronic devices.

Until now, traditional evaporation technologies known from semiconductor industry were mainly used for the manufacturing of flexible and stretchable high-performance electronics. These evaporation technologies are not qualified for large-area processing, and lead to high costs. To make efficient use of the properties of flexibility and stretchability, the area should be comparably large. Methods based on liquid deposition such as printing and coating technologies (among others, screen, inkjet, gravure and flexo printing; slot-die and knife coating) enable large-area deposition of functional materials, and are thus advantageous over evaporation technologies. This Special Issue focuses on functional materials deposited by printing and/or coating technologies for flexible and potentially stretchable applications, on specialized deposition technologies for these applications, and novel material functional materials developments including flexible and potentially stretchable ink formulations, substrates, encapsulation, etc.

Prof. Matti Mäntysalo
Dr. Enrico Sowade
Guest Editors

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 papers will be 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 1400 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

  • Liquid deposition: Printing and coating
  • Functional materials and post-processing
  • Materials for flexible and stretchable electronics
  • Flexible and potentially stretchable applications

Published Papers (9 papers)

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Research

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Open AccessFeature PaperArticle
Slot-Die Coating of Double Polymer Layers for the Fabrication of Organic Light Emitting Diodes
Micromachines 2019, 10(1), 53; https://doi.org/10.3390/mi10010053
Received: 13 December 2018 / Accepted: 9 January 2019 / Published: 14 January 2019
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Abstract
This study presents the slot-die coating process of two layers of organic materials for the fabrication of organic light emitting diodes (OLEDs). Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), which is commonly used in OLEDs and in organic photovoltaic devices as the hole injection layer (HIL), [...] Read more.
This study presents the slot-die coating process of two layers of organic materials for the fabrication of organic light emitting diodes (OLEDs). Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), which is commonly used in OLEDs and in organic photovoltaic devices as the hole injection layer (HIL), has been deposited via slot-die coating. Uniform films of PEDOT:PSS were obtained after optimizing the slot-die processing parameters: substrate temperature, coating speed, and ink flow rate. The film quality was examined using optical microscopy, profilometry, and atomic force microscopy. Further, poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), a well know polymer blend F8:F8BT, which is used as an emissive layer in OLEDs, has been slot-die coated. The optoelectronic properties of the slot-die coated F8:F8BT films were examined by means of photoluminescence (PL) and electroluminescence (EL) studies. The fabricated OLEDs, consisting of slot-die coated PEDOT:PSS and F8:F8BT films, were characterized to record the brightness and current efficiency. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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Open AccessArticle
The Effect of Encapsulation Geometry on the Performance of Stretchable Interconnects
Micromachines 2018, 9(12), 645; https://doi.org/10.3390/mi9120645
Received: 29 September 2018 / Revised: 18 November 2018 / Accepted: 29 November 2018 / Published: 5 December 2018
Cited by 1 | PDF Full-text (7744 KB) | HTML Full-text | XML Full-text
Abstract
The stretchability of electronic devices is typically obtained by tailoring the stretchable interconnects that link the functional units together. The durability of the interconnects against environmental conditions, such as deformation and chemicals, is therefore important to take into account. Different approaches, including encapsulation, [...] Read more.
The stretchability of electronic devices is typically obtained by tailoring the stretchable interconnects that link the functional units together. The durability of the interconnects against environmental conditions, such as deformation and chemicals, is therefore important to take into account. Different approaches, including encapsulation, are commonly used to improve the endurance of stretchable interconnects. In this paper, the geometry of encapsulation layer is initially investigated using finite element analysis. Then, the stretchable interconnects with a narrow-to-wide layout are screen-printed using silver flake ink as a conductor on a thermoplastic polyurethane (TPU) substrate. Printed ultraviolet (UV)-curable screen-printed dielectric ink and heat-laminated TPU film are used for the encapsulation of the samples. The electromechanical tests reveal a noticeable improvement in performance of encapsulated samples compared to non-protected counterparts in the case of TPU encapsulation. The improvement is even greater with partial coverage of the encapsulation layer. A device with a modified encapsulation layer can survive for 10,000 repetitive cycles at 20% strain, while maintaining the electrical and mechanical performance. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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Open AccessFeature PaperArticle
Manufacturing of All Inkjet-Printed Organic Photovoltaic Cell Arrays and Evaluating Their Suitability for Flexible Electronics
Micromachines 2018, 9(12), 642; https://doi.org/10.3390/mi9120642
Received: 22 October 2018 / Revised: 27 November 2018 / Accepted: 29 November 2018 / Published: 4 December 2018
Cited by 1 | PDF Full-text (4653 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The generation of electrical energy depending on renewable sources is rapidly growing and gaining serious attention due to its green sustainability. With fewer adverse impacts on the environment, the sun is considered as a nearly infinite source of renewable energy in the production [...] Read more.
The generation of electrical energy depending on renewable sources is rapidly growing and gaining serious attention due to its green sustainability. With fewer adverse impacts on the environment, the sun is considered as a nearly infinite source of renewable energy in the production of electrical energy using photovoltaic devices. On the other end, organic photovoltaic (OPV) is the class of solar cells that offers several advantages such as mechanical flexibility, solution processability, environmental friendliness, and being lightweight. In this research, we demonstrate the manufacturing route for printed OPV device arrays based on conventional architecture and using inkjet printing technology over an industrial platform. Inkjet technology is presently considered to be one of the most matured digital manufacturing technologies because it offers inherent additive nature and last stage customization flexibility (if the main goal is to obtain custom design devices). In this research paper, commercially available electronically functional inks were carefully selected and then implemented to show the importance of compatibility between OPV material stacks and the device architecture. One of the main outcomes of this work is that the manufacturing of the OPV devices was accomplished using inkjet technology in massive numbers ranging up to 1500 containing different device sizes, all of which were deposited on a flexible polymeric film and under normal atmospheric conditions. In this investigation, it was found that with a set of correct functional materials and architecture, a manufacturing yield of more than 85% could be accomplished, which would reflect high manufacturing repeatability, deposition accuracy, and processability of the inkjet technology. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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Open AccessArticle
Development of Flexible Robot Skin for Safe and Natural Human–Robot Collaboration
Micromachines 2018, 9(11), 576; https://doi.org/10.3390/mi9110576
Received: 9 October 2018 / Revised: 31 October 2018 / Accepted: 3 November 2018 / Published: 5 November 2018
Cited by 2 | PDF Full-text (7540 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
For industrial manufacturing, industrial robots are required to work together with human counterparts on certain special occasions, where human workers share their skills with robots. Intuitive human–robot interaction brings increasing safety challenges, which can be properly addressed by using sensor-based active control technology. [...] Read more.
For industrial manufacturing, industrial robots are required to work together with human counterparts on certain special occasions, where human workers share their skills with robots. Intuitive human–robot interaction brings increasing safety challenges, which can be properly addressed by using sensor-based active control technology. In this article, we designed and fabricated a three-dimensional flexible robot skin made by the piezoresistive nanocomposite based on the need for enhancement of the security performance of the collaborative robot. The robot skin endowed the YuMi robot with a tactile perception like human skin. The developed sensing unit in the robot skin showed the one-to-one correspondence between force input and resistance output (percentage change in impedance) in the range of 0–6.5 N. Furthermore, the calibration result indicated that the developed sensing unit is capable of offering a maximum force sensitivity (percentage change in impedance per Newton force) of 18.83% N−1 when loaded with an external force of 6.5 N. The fabricated sensing unit showed good reproducibility after loading with cyclic force (0–5.5 N) under a frequency of 0.65 Hz for 3500 cycles. In addition, to suppress the bypass crosstalk in robot skin, we designed a readout circuit for sampling tactile data. Moreover, experiments were conducted to estimate the contact/collision force between the object and the robot in a real-time manner. The experiment results showed that the implemented robot skin can provide an efficient approach for natural and secure human–robot interaction. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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Open AccessArticle
Bending Limit Tests for Ultra-Thin Liquid Crystal Polymer Substrate Based on Flexible Microwave Components
Micromachines 2018, 9(10), 531; https://doi.org/10.3390/mi9100531
Received: 19 September 2018 / Revised: 12 October 2018 / Accepted: 16 October 2018 / Published: 20 October 2018
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Abstract
In this paper, bending limit tests for one ultra-thin liquid crystal polymer (LCP) substrate (Rogers 3850) based on the mechanical properties of flexible microwave microstrip components are presented. First, a set of 50 Ω microstrip lines, a band-pass filter, and a stepped impedance [...] Read more.
In this paper, bending limit tests for one ultra-thin liquid crystal polymer (LCP) substrate (Rogers 3850) based on the mechanical properties of flexible microwave microstrip components are presented. First, a set of 50 Ω microstrip lines, a band-pass filter, and a stepped impedance filter in X-band, are designed by using double clapped LCPs with 50 μm thickness of substrate and 18 μm thickness of copper, which is fabricated by conventional photolithography. Then, the limit tests of the flexibility of the LCP microwave microstrip components are presented, and the range of the bending limit radius, from 1 mm to 0.75 mm, is demonstrated from the testing results. It is found that the cause for component failure is fracture of the copper (18 μm thickness) laminate, according to the bending limit test experiments. Finally, the analysis of the reasons for the collapse of the microwave components, under bending situations, is explored. The results from this work would be useful for further designs of the flexible microwave devices and systems on LCP substrates, with compact sizes and good performance. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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Open AccessArticle
Experimental Study of the Influence of Ink Properties and Process Parameters on Ejection Volume in Electrohydrodynamic Jet Printing
Micromachines 2018, 9(10), 522; https://doi.org/10.3390/mi9100522
Received: 19 August 2018 / Revised: 29 September 2018 / Accepted: 1 October 2018 / Published: 16 October 2018
Cited by 1 | PDF Full-text (3240 KB) | HTML Full-text | XML Full-text
Abstract
Electrohydrodynamic jet (e-jet) printing has very promising applications due to its high printing resolution and material compatibility. It is necessary to know how to choose the printing parameters to get the right ejection volume. The previous scaling law of the ejection volume in [...] Read more.
Electrohydrodynamic jet (e-jet) printing has very promising applications due to its high printing resolution and material compatibility. It is necessary to know how to choose the printing parameters to get the right ejection volume. The previous scaling law of the ejection volume in e-jet printing borrows the scaling law of the ejection volume of an unstable isolated droplet charged to the Rayleigh limit. The influence of viscosity, applied voltage amplitude, and nozzle-to-substrate distance on the ejection volume in e-jet printing was not taken into account in the scaling law. This study investigated the influence of viscosity, conductivity, applied voltage, and nozzle-to-substrate distance on the ejection volume. The ejection volume increases with viscosity and decreases with applied voltage and nozzle-to-substrate distance. The average electric field was kept unchanged while changing the nozzle-to-substrate distance by changing the applied voltage according to the electric field model of a semi-infinite wire perpendicular to an infinite large planar counter electrode. The ejection volume decreases with conductivity as V ~ K 0.6 , which is different from the previous scaling law, which concludes that V ~ K 1 . Finally, a model about the relation between the ejection volume and four parameters was established by regression analysis using a third-order polynomial. Two more experiments were done, and the predicted results of the fitted model accorded well with the experiments. The model can be used to choose the ink properties and process parameters to get the right ejection volume. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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Open AccessFeature PaperArticle
Universal Testing Apparatus Implementing Various Repetitive Mechanical Deformations to Evaluate the Reliability of Flexible Electronic Devices
Micromachines 2018, 9(10), 492; https://doi.org/10.3390/mi9100492
Received: 2 September 2018 / Revised: 19 September 2018 / Accepted: 24 September 2018 / Published: 25 September 2018
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Abstract
A requirement of flexible electronic devices is that they maintain their electrical performance during and after repetitive mechanical deformation. Accordingly, in this study, a universal test apparatus is developed for in-situ electrical conductivity measurements for flexible electrodes that are capable of applying various [...] Read more.
A requirement of flexible electronic devices is that they maintain their electrical performance during and after repetitive mechanical deformation. Accordingly, in this study, a universal test apparatus is developed for in-situ electrical conductivity measurements for flexible electrodes that are capable of applying various mechanical deformations such as bending, twisting, shearing, sliding, stretching, and complex modes consisting of two simultaneous deformations. A novel method of deforming the specimen in an arc to induce uniform bending stress in single and alternating directions is also proposed with a mathematically derived control method. As an example of the arc bending method, the changes in the resistance of the printed radio frequency identification (RFID) tag antennas were measured by applying repetitive inner bending, outer bending, and alternating inner-outer bending. After 5000 cycles, the increases in resistance of the specimens that were subjected to inner or outer bending only were under 30%; however, specimens that were subjected to alternating inner-outer bending showed an increase of 135% in resistance. It is critical that the reliability of flexible electronic devices under various mechanical deformations be determined before they can be commercialized. The proposed testing apparatus can readily provide various deformations that will be useful to inform the design of device shapes and structures to accommodate deformations during use. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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Open AccessArticle
The Use of a Water Soluble Flexible Substrate to Embed Electronics in Additively Manufactured Objects: From Tattoo to Water Transfer Printed Electronics
Micromachines 2018, 9(9), 474; https://doi.org/10.3390/mi9090474
Received: 20 August 2018 / Revised: 6 September 2018 / Accepted: 12 September 2018 / Published: 17 September 2018
Cited by 3 | PDF Full-text (5896 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The integration of electronics into the process flow of the additive manufacturing of 3D objects is demonstrated using water soluble films as a temporary flexible substrate. Three process variants are detailed to evaluate their capabilities to meet the additive manufacturing requirements. One of [...] Read more.
The integration of electronics into the process flow of the additive manufacturing of 3D objects is demonstrated using water soluble films as a temporary flexible substrate. Three process variants are detailed to evaluate their capabilities to meet the additive manufacturing requirements. One of them, called water transfer printing, shows the best ability to fabricate electronics onto 3D additively manufactured objects. Moreover, a curved capacitive touchpad hidden by color films is successfully transferred onto the 3D objects, showing a potential application of this technology to fabricate fully additively manufactured discrete or even hidden electronic devices. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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Review

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Open AccessReview
Liquid Metal Enabled Electrobiology: A New Frontier to Tackle Disease Challenges
Micromachines 2018, 9(7), 360; https://doi.org/10.3390/mi9070360
Received: 14 June 2018 / Revised: 9 July 2018 / Accepted: 18 July 2018 / Published: 21 July 2018
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Abstract
In this article, a new conceptual biomedical engineering strategy to tackle modern disease challenges, called liquid metal (LM) enabled electrobiology, is proposed. This generalized and simple method is based on the physiological fact that specially administrated electricity induces a series of subsequent desired [...] Read more.
In this article, a new conceptual biomedical engineering strategy to tackle modern disease challenges, called liquid metal (LM) enabled electrobiology, is proposed. This generalized and simple method is based on the physiological fact that specially administrated electricity induces a series of subsequent desired biological effects, either shortly, transitionally, or permanently. Due to high compliance within biological tissues, LM would help mold a pervasive method for treating physiological or psychological diseases. As highly conductive and non-toxic multifunctional flexible materials, such LMs can generate any requested electric treating fields (ETFields), which can adapt to various sites inside the human body. The basic mechanisms of electrobiology in delivering electricity to the target tissues and then inducing expected outputs for disease treatment are interpreted. The methods for realizing soft and conformable electronics based on LM are illustrated. Furthermore, a group of typical disease challenges are observed to illustrate the basic strategies for performing LM electrobiology therapy, which include but are not limited to: tissue electronics, brain disorder, immunotherapy, neural functional recovery, muscle stimulation, skin rejuvenation, cosmetology and dieting, artificial organs, cardiac pacing, cancer therapy, etc. Some practical issues regarding electrobiology for future disease therapy are discussed. Perspectives in this direction for incubating a simple biomedical tool for health care are pointed out. Full article
(This article belongs to the Special Issue Printed Flexible and Stretchable Electronics)
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