Special Issue "Flexible Electronics: Fabrication and Ubiquitous Integration"

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

Deadline for manuscript submissions: closed (30 April 2018)

Special Issue Editor

Guest Editor
Dr. Ramses V. Martinez

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
Website | E-Mail
Phone: +1-765-496-0399
Interests: wearable devices; self-powered flexible sensors; soft robotics

Special Issue Information

Dear Colleagues,

Flexible Electronics platforms are increasingly used in the fields of sensors, displays, and energy conversion with the ultimate goal of facilitating their ubiquitous integration in our daily lives. Some of the key advantages associated with flexible electronic platforms are: bendability, lightweight, elastic, conformally shaped, nonbreakable, roll-to-roll manufacturable, and large-area. To realize their full potential, however, it is necessary to develop new methods for the fabrication of multifunctional flexible electronics 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 flexible electronics in healthcare, environmental monitoring, displays and human-machine interactivity, robotics, communication and wireless networks, and energy conversion, management, and storage.

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 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 1200 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 (11 papers)

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Editorial

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Open AccessEditorial Editorial for Special Issue on Flexible Electronics: Fabrication and Ubiquitous Integration
Micromachines 2018, 9(11), 605; https://doi.org/10.3390/mi9110605 (registering DOI)
Received: 26 October 2018 / Accepted: 8 November 2018 / Published: 19 November 2018
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Abstract
Based on the premise “anything thin is flexible”, the field of flexible electronics has been fueled from the ever-evolving advances in thin-film materials and devices. [...] Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)

Research

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Open AccessArticle Tunnel Encapsulation Technology for Durability Improvement in Stretchable Electronics Fabrication
Micromachines 2018, 9(10), 519; https://doi.org/10.3390/mi9100519
Received: 18 September 2018 / Revised: 10 October 2018 / Accepted: 12 October 2018 / Published: 14 October 2018
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Abstract
Great diversity of process technologies and materials have been developed around stretchable electronics. A subset of them, which are made up of zigzag metal foil and soft silicon polymers, show advantages of being easy to manufacture and low cost. However, most of the
[...] Read more.
Great diversity of process technologies and materials have been developed around stretchable electronics. A subset of them, which are made up of zigzag metal foil and soft silicon polymers, show advantages of being easy to manufacture and low cost. However, most of the circuits lack durability due to stress concentration of interconnects entirely embedded in elastic polymer silicone such as polydimethylsiloxane (PDMS). In our demonstration, tunnel encapsulation technology was introduced to relieve stress of these conductors when they were stretched to deform in and out of plane. It was realized by dissolving the medium of Polyvinyl Alcohol (PVA), previous cured together with circuits in polymer, to form the micro-tunnel which not only guarantee the stretchability of interconnect, but also help to improve the durability. With the protection of tunnel, the serpentine could stably maintain the designed shape and electrical performance after 50% strain cycling over 20,000 times. Finally, different materials for encapsulation were employed to provide promising options for applications in portable biomedical devices which demand duplicate distortion. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Open AccessFeature PaperArticle Rapid Fabrication of Epidermal Paper-Based Electronic Devices Using Razor Printing
Micromachines 2018, 9(9), 420; https://doi.org/10.3390/mi9090420
Received: 12 May 2018 / Revised: 6 June 2018 / Accepted: 7 June 2018 / Published: 22 August 2018
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Abstract
This work describes the use of a benchtop razor printer to fabricate epidermal paper-based electronic devices (EPEDs). This fabrication technique is simple, low-cost, and compatible with scalable manufacturing processes. EPEDs are fabricated using paper substrates rendered omniphobic by their cost-effective silanization with fluoroalkyl
[...] Read more.
This work describes the use of a benchtop razor printer to fabricate epidermal paper-based electronic devices (EPEDs). This fabrication technique is simple, low-cost, and compatible with scalable manufacturing processes. EPEDs are fabricated using paper substrates rendered omniphobic by their cost-effective silanization with fluoroalkyl trichlorosilanes, making them inexpensive, water-resistant, and mechanically compliant with human skin. The highly conductive inks or thin films attached to one of the sides of the omniphobic paper makes EPEDs compatible with wearable applications involving wireless power transfer. The omniphobic cellulose fibers of the EPED provide a moisture-independent mechanical reinforcement to the conductive layer. EPEDs accurately monitor physiological signals such as ECG (electrocardiogram), EMG (electromyogram), and EOG (electro-oculogram) even in high moisture environments. Additionally, EPEDs can be used for the fast mapping of temperature over the skin and to apply localized thermotherapy. Our results demonstrate the merits of EPEDs as a low-cost platform for personalized medicine applications. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Open AccessArticle The Conformal Design of an Island-Bridge Structure on a Non-Developable Surface for Stretchable Electronics
Micromachines 2018, 9(8), 392; https://doi.org/10.3390/mi9080392
Received: 25 June 2018 / Revised: 27 July 2018 / Accepted: 2 August 2018 / Published: 7 August 2018
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Abstract
Conformal design of the island-bridge structure is the key to construct high-performance inorganic stretchable electronics that can be conformally transferred to non-developable surfaces. Former studies in conformal problems of epidermal electronics are mainly focused on soft surfaces that can adapt to the deformation
[...] Read more.
Conformal design of the island-bridge structure is the key to construct high-performance inorganic stretchable electronics that can be conformally transferred to non-developable surfaces. Former studies in conformal problems of epidermal electronics are mainly focused on soft surfaces that can adapt to the deformation of the electronics, which are not suitable for applications in hard, non-developable surfaces because of their loose surface constraints. In this paper, the conformal design problem for the island-bridge structure on a hard, non-developable surface was studied, including the critical size for island and stiffness and the demand for stretchability for the bridge. Firstly, the conformal model for an island on a part of torus surface was established to determine the relationship between the maximum size of the island and the curvatures of the surface. By combining the principle of energy minimization and the limit of material failure, a critical non-dimensional width for conformability was given for the island as a function of its thickness and interfacial adhesion energy, and the ratio of two principal curvatures of the surface. Then, the dependency of the tensile stiffness of the bridge on its geometric parameters was studied by finite element analysis (FEA) to guide the deterministic assembly of the islands on the surface. Finally, the location-dependent demands for the stretchability of the bridges were given by geometric mapping. This work will provide a design rule for stretchable electronics that fully conforms to the non-developable surface. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Open AccessFeature PaperArticle Stretchability—The Metric for Stretchable Electrical Interconnects
Micromachines 2018, 9(8), 382; https://doi.org/10.3390/mi9080382
Received: 2 July 2018 / Revised: 24 July 2018 / Accepted: 27 July 2018 / Published: 1 August 2018
Cited by 1 | PDF Full-text (3501 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Stretchable circuit technology, as the name implies, allows an electronic circuit to adapt to its surroundings by elongating when an external force is applied. Based on this, early authors proposed a straightforward metric: stretchability—the percentage length increase the circuit can survive while remaining
[...] Read more.
Stretchable circuit technology, as the name implies, allows an electronic circuit to adapt to its surroundings by elongating when an external force is applied. Based on this, early authors proposed a straightforward metric: stretchability—the percentage length increase the circuit can survive while remaining functional. However, when comparing technologies, this metric is often unreliable as it is heavily design dependent. This paper aims to demonstrate this shortcoming and proposes a series of alternate methods to evaluate the performance of a stretchable interconnect. These methods consider circuit volume, material usage, and the reliability of the technology. This analysis is then expanded to the direct current (DC) resistance measurement performed on these stretchable interconnects. A simple dead reckoning approach is demonstrated to estimate the magnitude of these measurement errors on the final measurement. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Open AccessArticle Flexible Thermo-Optic Variable Attenuator based on Long-Range Surface Plasmon-Polariton Waveguides
Micromachines 2018, 9(8), 369; https://doi.org/10.3390/mi9080369
Received: 28 April 2018 / Revised: 10 June 2018 / Accepted: 24 July 2018 / Published: 26 July 2018
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Abstract
A flexible thermo-optic variable attenuator based on long-range surface plasmon-polariton (LRSPP) waveguide for microwave photonic application was investigated. Low-loss polymer materials and high-quality silver strip were served as cladding layers and core layer of the LRSPP waveguide, respectively. By using finite element method
[...] Read more.
A flexible thermo-optic variable attenuator based on long-range surface plasmon-polariton (LRSPP) waveguide for microwave photonic application was investigated. Low-loss polymer materials and high-quality silver strip were served as cladding layers and core layer of the LRSPP waveguide, respectively. By using finite element method (FEM), the thermal distribution and the optical field distribution have been carefully optimized. The fabricated device was characterized by end-fire excitation with a 1550 nm laser. The transmission performance of high-speed data and microwave modulated optical signal was measured while using a broadband microwave photonics link. The results indicated that the propagation loss of the LRSPP waveguide was about 1.92 dB/cm. The maximum attenuation of optical signal was about 28 dB at a driving voltage of 4.17 V, and the variable attenuation of microwave signals was obviously observed by applying different driving voltage to the heater. This flexible plasmonic variable attenuator is promising for chip-scale interconnection in high-density photonic integrated circuits and data transmission and amplitude control in microwave photonic systems. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Open AccessArticle An Organic Flexible Artificial Bio-Synapses with Long-Term Plasticity for Neuromorphic Computing
Micromachines 2018, 9(5), 239; https://doi.org/10.3390/mi9050239
Received: 19 April 2018 / Revised: 10 May 2018 / Accepted: 11 May 2018 / Published: 15 May 2018
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Abstract
Artificial synapses, with synaptic plasticity, are the key components of constructing the neuromorphic computing system and mimicking the bio-synaptic function. Traditional synaptic devices are based on silicon and inorganic materials, while organic electronics can open up new opportunities for flexible devices. Here, a
[...] Read more.
Artificial synapses, with synaptic plasticity, are the key components of constructing the neuromorphic computing system and mimicking the bio-synaptic function. Traditional synaptic devices are based on silicon and inorganic materials, while organic electronics can open up new opportunities for flexible devices. Here, a flexible artificial synaptic device with an organic functional layer was proposed. The organic device showed good switching behaviors such as ON/OFF ratio over 100 at low operation voltages. The set and reset voltages were lower than 0.5 V and −0.25 V, respectively. The long-term plasticity, spike-timing-dependent plasticity learning rules (STDP), and forgetting function were emulated using the device. The retention times of the excitatory and inhibitory post-synaptic currents were both longer than 60 s. The long-term plasticity was repeatable without noticeable degradation after the application of five voltage pulse cycles to the top electrode. These results indicate that our organic flexible device has the potential to be applied in bio-inspired neuromorphic systems. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Open AccessFeature PaperArticle Stretchable Tattoo-Like Heater with On-Site Temperature Feedback Control
Micromachines 2018, 9(4), 170; https://doi.org/10.3390/mi9040170
Received: 20 February 2018 / Revised: 20 March 2018 / Accepted: 27 March 2018 / Published: 8 April 2018
Cited by 3 | PDF Full-text (25715 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Wearable tissue heaters can play many important roles in the medical field. They may be used for heat therapy, perioperative warming and controlled transdermal drug delivery, among other applications. State-of-the-art heaters are too bulky, rigid, or difficult to control to be able to
[...] Read more.
Wearable tissue heaters can play many important roles in the medical field. They may be used for heat therapy, perioperative warming and controlled transdermal drug delivery, among other applications. State-of-the-art heaters are too bulky, rigid, or difficult to control to be able to maintain long-term wearability and safety. Recently, there has been progress in the development of stretchable heaters that may be attached directly to the skin surface, but they often use expensive materials or processes and take significant time to fabricate. Moreover, they lack continuously active, on-site, unobstructive temperature feedback control, which is critical for accommodating the dynamic temperatures required for most medical applications. We have developed, fabricated and tested a cost-effective, large area, ultra-thin and ultra-soft tattoo-like heater that has autonomous proportional-integral-derivative (PID) temperature control. The device comprises a stretchable aluminum heater and a stretchable gold resistance temperature detector (RTD) on a soft medical tape as fabricated using the cost and time effective “cut-and-paste” method. It can be noninvasively laminated onto human skin and can follow skin deformation during flexure without imposing any constraint. We demonstrate the device’s ability to maintain a target temperature typical of medical uses over extended durations of time and to accurately adjust to a new set point in process. The cost of the device is low enough to justify disposable use. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Review

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Open AccessFeature PaperReview Tunable Adhesion for Bio-Integrated Devices
Micromachines 2018, 9(10), 529; https://doi.org/10.3390/mi9100529
Received: 28 September 2018 / Revised: 14 October 2018 / Accepted: 16 October 2018 / Published: 18 October 2018
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Abstract
With the rapid development of bio-integrated devices and tissue adhesives, tunable adhesion to soft biological tissues started gaining momentum. Strong adhesion is desirable when used to efficiently transfer vital signals or as wound dressing and tissue repair, whereas weak adhesion is needed for
[...] Read more.
With the rapid development of bio-integrated devices and tissue adhesives, tunable adhesion to soft biological tissues started gaining momentum. Strong adhesion is desirable when used to efficiently transfer vital signals or as wound dressing and tissue repair, whereas weak adhesion is needed for easy removal, and it is also the essential step for enabling repeatable use. Both the physical and chemical properties (e.g., moisture level, surface roughness, compliance, and surface chemistry) vary drastically from the skin to internal organ surfaces. Therefore, it is important to strategically design the adhesive for specific applications. Inspired largely by the remarkable adhesion properties found in several animal species, effective strategies such as structural design and novel material synthesis were explored to yield adhesives to match or even outperform their natural counterparts. In this mini-review, we provide a brief overview of the recent development of tunable adhesives, with a focus on their applications toward bio-integrated devices and tissue adhesives. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Open AccessFeature PaperReview Novel Nano-Materials and Nano-Fabrication Techniques for Flexible Electronic Systems
Micromachines 2018, 9(6), 263; https://doi.org/10.3390/mi9060263
Received: 30 April 2018 / Revised: 19 May 2018 / Accepted: 24 May 2018 / Published: 28 May 2018
Cited by 1 | PDF Full-text (10167 KB) | HTML Full-text | XML Full-text
Abstract
Recent progress in fabricating flexible electronics has been significantly developed because of the increased interest in flexible electronics, which can be applied to enormous fields, not only conventional in electronic devices, but also in bio/eco-electronic devices. Flexible electronics can be applied to a
[...] Read more.
Recent progress in fabricating flexible electronics has been significantly developed because of the increased interest in flexible electronics, which can be applied to enormous fields, not only conventional in electronic devices, but also in bio/eco-electronic devices. Flexible electronics can be applied to a wide range of fields, such as flexible displays, flexible power storages, flexible solar cells, wearable electronics, and healthcare monitoring devices. Recently, flexible electronics have been attached to the skin and have even been implanted into the human body for monitoring biosignals and for treatment purposes. To improve the electrical and mechanical properties of flexible electronics, nanoscale fabrications using novel nanomaterials are required. Advancements in nanoscale fabrication methods allow the construction of active materials that can be combined with ultrathin soft substrates to form flexible electronics with high performances and reliability. In this review, a wide range of flexible electronic applications via nanoscale fabrication methods, classified as either top-down or bottom-up approaches, including conventional photolithography, soft lithography, nanoimprint lithography, growth, assembly, and chemical vapor deposition (CVD), are introduced, with specific fabrication processes and results. Here, our aim is to introduce recent progress on the various fabrication methods for flexible electronics, based on novel nanomaterials, using application examples of fundamental device components for electronics and applications in healthcare systems. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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Other

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Open AccessPerspective Liquid-Metal Enabled Droplet Circuits
Micromachines 2018, 9(5), 218; https://doi.org/10.3390/mi9050218
Received: 13 April 2018 / Revised: 29 April 2018 / Accepted: 2 May 2018 / Published: 5 May 2018
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Abstract
Conventional electrical circuits are generally rigid in their components and working styles, which are not flexible and stretchable. As an alternative, liquid-metal-based soft electronics offer important opportunities for innovation in modern bioelectronics and electrical engineering. However, their operation in wet environments such as
[...] Read more.
Conventional electrical circuits are generally rigid in their components and working styles, which are not flexible and stretchable. As an alternative, liquid-metal-based soft electronics offer important opportunities for innovation in modern bioelectronics and electrical engineering. However, their operation in wet environments such as aqueous solution, biological tissue or allied subjects still encounters many technical challenges. Here, we propose a new conceptual electrical circuit, termed as droplet circuit, to fulfill the special needs described above. Such unconventional circuits are immersed in a solution and composed of liquid metal droplets, conductive ions or wires, such as carbon nanotubes. With specifically-designed topological or directional structures/patterns, the liquid-metal droplets composing the circuit can be discrete and disconnected from each other, while achieving the function of electron transport through conductive routes or the quantum tunneling effect. The conductive wires serve as electron transfer stations when the distance between two separate liquid-metal droplets is far beyond that which quantum tunneling effects can support. The unique advantage of the current droplet circuit lies in the fact that it allows parallel electron transport, high flexibility, self-healing, regulation and multi-point connectivity without needing to worry about the circuit break. This would extend the category of classical electrical circuits into newly emerging areas like realizing room temperature quantum computing, making brain-like intelligence or nerve–machine interface electronics, etc. The mechanisms and potential scientific issues of the droplet circuits are interpreted and future prospects in this direction are outlined. Full article
(This article belongs to the Special Issue Flexible Electronics: Fabrication and Ubiquitous Integration)
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