Tag Archives: flexible electronics

Silver nanowires have a surprising ability to self-heal

It seems there could be a new member of the flexible electronics materials community, silver nanowires, according to a Jan. 23, 2015 news item on ScienceDaily,

Wth its high electrical conductivity and optical transparency, indium tin oxide is one of the most widely used materials for touchscreens, plasma displays, and flexible electronics. But its rapidly escalating price has forced the electronics industry to search for other alternatives.

One potential and more cost-effective alternative is a film made with silver nanowires–wires so extremely thin that they are one-dimensional–embedded in flexible polymers. Like indium tin oxide, this material is transparent and conductive. But development has stalled because scientists lack a fundamental understanding of its mechanical properties.

A Jan. 23, 2015 Northwestern University news release (also on EurekAlert), which originated the news item, explains what makes silver nanowires a candidate as an alternative to indium tin oxide for use in flexible electronics,

… Horacio Espinosa, the James N. and Nancy J. Farley Professor in Manufacturing and Entrepreneurship at Northwestern University’s McCormick School of Engineering, has led research that expands the understanding of silver nanowires’ behavior in electronics.

Espinosa and his team investigated the material’s cyclic loading, which is an important part of fatigue analysis because it shows how the material reacts to fluctuating loads of stress.

“Cyclic loading is an important material behavior that must be investigated for realizing the potential applications of using silver nanowires in electronics,” Espinosa said. “Knowledge of such behavior allows designers to understand how these conductive films fail and how to improve their durability.”

By varying the tension on silver nanowires thinner than 120 nanometers and monitoring their deformation with electron microscopy, the research team characterized the cyclic mechanical behavior. They found that permanent deformation was partially recoverable in the studied nanowires, meaning that some of the material’s defects actually self-healed and disappeared upon cyclic loading. These results indicate that silver nanowires could potentially withstand strong cyclic loads for long periods of time, which is a key attribute needed for flexible electronics.

“These silver nanowires show mechanical properties that are quite unexpected,” Espinosa said. “We had to develop new experimental techniques to be able to measure this novel material property.”

The findings were recently featured on the cover of the journal Nano Letters. Other Northwestern coauthors on the paper are Rodrigo Bernal, a recently graduated PhD student in Espinosa’s lab, and Jiaxing Huang, associate professor of materials science and engineering in McCormick.

“The next step is to understand how this recovery influences the behavior of these materials when they are flexed millions of times,” said Bernal, first author of the paper.

Here’s a link to and citation for the paper,

Intrinsic Bauschinger Effect and Recoverable Plasticity in Pentatwinned Silver Nanowires Tested in Tension by Rodrigo A. Bernal, Amin Aghaei, Sangjun Lee, Seunghwa Ryu, Kwonnam Sohn, Jiaxing Huang, Wei Cai, and Horacio Espinosa. Nano Lett., 2015, 15 (1), pp 139–146 DOI: 10.1021/nl503237t Publication Date (Web): October 3, 2014
Copyright © 2014 American Chemical Society

This particular version of the paper is behind a paywall. However, access to the paper is possible although I make no claims as to which version it is or whether it will continue to be freely accessible.

Flexible electronics and Inorganic-based Laser Lift-off (ILLO) in Korea

Korean scientists are trying to make the process of creating flexible electronics easier according to a Nov. 25, 2014 news item on ScienceDaily,

Flexible electronics have been touted as the next generation in electronics in various areas, ranging from consumer electronics to bio-integrated medical devices. In spite of their merits, insufficient performance of organic materials arising from inherent material properties and processing limitations in scalability have posed big challenges to developing all-in-one flexible electronics systems in which display, processor, memory, and energy devices are integrated. The high temperature processes, essential for high performance electronic devices, have severely restricted the development of flexible electronics because of the fundamental thermal instabilities of polymer materials.

A research team headed by Professor Keon Jae Lee of the Department of Materials Science and Engineering at KAIST provides an easier methodology to realize high performance flexible electronics by using the Inorganic-based Laser Lift-off (ILLO).

The process is described in a Nov. 26, 2014 KAIST news release on ResearchSEA, which originated the news item (despite the confusion of the date, probably due to timezone differentials), provides more detail about the technique for ILLO,

The ILLO process involves depositing a laser-reactive exfoliation layer on rigid substrates, and then fabricating ultrathin inorganic electronic devices, e.g., high density crossbar memristive memory on top of the exfoliation layer. By laser irradiation through the back of the substrate, only the ultrathin inorganic device layers are exfoliated from the substrate as a result of the reaction between laser and exfoliation layer, and then subsequently transferred onto any kind of receiver substrate such as plastic, paper, and even fabric.

This ILLO process can enable not only nanoscale processes for high density flexible devices but also the high temperature process that was previously difficult to achieve on plastic substrates. The transferred device successfully demonstrates fully-functional random access memory operation on flexible substrates even under severe bending.

Professor Lee said, “By selecting an optimized set of inorganic exfoliation layer and substrate, a nanoscale process at a high temperature of over 1000 °C can be utilized for high performance flexible electronics. The ILLO process can be applied to diverse flexible electronics, such as driving circuits for displays and inorganic-based energy devices such as battery, solar cell, and self-powered devices that require high temperature processes.”

Here’s a link to and a citation for the research paper,

Flexible Crossbar-Structured Resistive Memory Arrays on Plastic Substrates via Inorganic-Based Laser Lift-Off by Seungjun Kim, Jung Hwan Son, Seung Hyun Lee, Byoung Kuk You, Kwi-Il Park, Hwan Keon Lee, Myunghwan Byun and Keon Jae Lee. Advanced Materials Volume 26, Issue 44, pages 7480–7487, November 26, 2014 Article first published online: 8 SEP 2014 DOI: 10.1002/adma.201402472

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Here’s an image the researchers have made available,

This photo shows the flexible RRAM device on a plastic substrate. Courtesy: KAIST

This photo shows the flexible RRAM device on a plastic substrate. Courtesy: KAIST

Finally, the research paper is behind a paywall.

Bend it, twist it, any way you want to—a foldable lithium-ion battery

Feb. 26, 2013 news item on ScienceDaily features an extraordinary lithium-ion battery,

Northwestern University’s Yonggang Huang and the University of Illinois’ John A. Rogers are the first to demonstrate a stretchable lithium-ion battery — a flexible device capable of powering their innovative stretchable electronics.

No longer needing to be connected by a cord to an electrical outlet, the stretchable electronic devices now could be used anywhere, including inside the human body. The implantable electronics could monitor anything from brain waves to heart activity, succeeding where flat, rigid batteries would fail.

Huang and Rogers have demonstrated a battery that continues to work — powering a commercial light-emitting diode (LED) — even when stretched, folded, twisted and mounted on a human elbow. The battery can work for eight to nine hours before it needs recharging, which can be done wirelessly.

The researchers at Northwestern have produced a video where they demonstrate the battery’s ‘stretchability’,

The Northwestern University Feb. 26, 2013 news release by Megan Fellman, which originated the news item, offers this detail,

“We start with a lot of battery components side by side in a very small space, and we connect them with tightly packed, long wavy lines,” said Huang, a corresponding author of the paper. “These wires provide the flexibility. When we stretch the battery, the wavy interconnecting lines unfurl, much like yarn unspooling. And we can stretch the device a great deal and still have a working battery.”

The power and voltage of the stretchable battery are similar to a conventional lithium-ion battery of the same size, but the flexible battery can stretch up to 300 percent of its original size and still function.

Huang and Rogers have been working together for the last six years on stretchable electronics, and designing a cordless power supply has been a major challenge. Now they have solved the problem with their clever “space filling technique,” which delivers a small, high-powered battery.

For their stretchable electronic circuits, the two developed “pop-up” technology that allows circuits to bend, stretch and twist. They created an array of tiny circuit elements connected by metal wire “pop-up bridges.” When the array is stretched, the wires — not the rigid circuits — pop up.

This approach works for circuits but not for a stretchable battery. A lot of space is needed in between components for the “pop-up” interconnect to work. Circuits can be spaced out enough in an array, but battery components must be packed tightly to produce a powerful but small battery. There is not enough space between battery components for the “pop-up” technology to work.

Huang’s design solution is to use metal wire interconnects that are long, wavy lines, filling the small space between battery components. (The power travels through the interconnects.)

The unique mechanism is a “spring within a spring”: The line connecting the components is a large “S” shape and within that “S” are many smaller “S’s.” When the battery is stretched, the large “S” first stretches out and disappears, leaving a line of small squiggles. The stretching continues, with the small squiggles disappearing as the interconnect between electrodes becomes taut.

“We call this ordered unraveling,” Huang said. “And this is how we can produce a battery that stretches up to 300 percent of its original size.”

The stretching process is reversible, and the battery can be recharged wirelessly. The battery’s design allows for the integration of stretchable, inductive coils to enable charging through an external source but without the need for a physical connection.

Huang, Rogers and their teams found the battery capable of 20 cycles of recharging with little loss in capacity. The system they report in the paper consists of a square array of 100 electrode disks, electrically connected in parallel.

I’d like to see this battery actually powering a device even though the stretching is quite alluring in its way. For those who are interested here’s a citation and a link to the research paper,

Stretchable batteries with self-similar serpentine interconnects and integrated wireless recharging systems by Sheng Xu, Yihui Zhang, Jiung Cho, Juhwan Lee, Xian Huang, Lin Jia, Jonathan A. Fan, Yewang Su, Jessica Su, Huigang Zhang, Huanyu Cheng, Bingwei Lu,           Cunjiang Yu, Chi Chuang, Tae-il Kim, Taeseup Song, Kazuyo Shigeta, Sen Kang, Canan Dagdeviren, Ivan Petrov  et al.   Nature Communications 4, Article number: 1543 doi: 10.1038/ncomms2553  Published 26 February 2013

The article is behind a paywall.

Transparent image sensor has no electronics or internal components

This shows the world's first flexible and completely transparent image sensor. The plastic film is coated with fluorescent particles. Credit: Optics Express.

This shows the world’s first flexible and completely transparent image sensor. The plastic film is coated with fluorescent particles. Credit: Optics Express.

Stunning isn’t it? The work is from researchers at the Johannes Kepler University Linz in Austria and is featured in an article being published in Optics Express. From the Feb. 20, 2013 news release about the Optics Express article on EurekAlert,

Digital cameras, medical scanners, and other imaging technologies have advanced considerably during the past decade. Continuing this pace of innovation, an Austrian research team has developed an entirely new way of capturing images based on a flat, flexible, transparent, and potentially disposable polymer sheet. The team describes their new device and its possible applications in a paper published today in the Optical Society’s (OSA) open-access journal Optics Express.

The new imager, which resembles a flexible plastic film, uses fluorescent particles to capture incoming light and channel a portion of it to an array of sensors framing the sheet. With no electronics or internal components, the imager’s elegant design makes it ideal for a new breed of imaging technologies, including user interface devices that can respond not to a touch, but merely to a simple gesture.

The news release goes on to describe the technology,

The sensor is based on a polymer film known as a luminescent concentrator (LC), which is suffused with tiny fluorescent particles that absorb a very specific wavelength (blue light for example) and then reemit it at a longer wavelength (green light for example). Some of the reemitted fluorescent light is scattered out of the imager, but a portion of it travels throughout the interior of the film to the outer edges, where arrays of optical sensors (similar to 1-D pinhole cameras) capture the light. A computer then combines the signals to create a gray-scale image. “With fluorescence, a portion of the light that is reemitted actually stays inside the film,” says Bimber. [Oliver Bimber of the Johannes Kepler University Linz in Austria, co-author of the Optics Express paper] “This is the basic principle of our sensor.”

For the luminescent concentrator to work as an imager, Bimber and his colleagues had to determine precisely where light was falling across the entire surface of the film. This was the major technical challenge because the polymer sheet cannot be divided into individual pixels like the CCD camera inside a smartphone. Instead, fluorescent light from all points across its surface travels to all the edge sensors. Calculating where each bit of light entered the imager would be like determining where along a subway line a passenger got on after the train reached its final destination and all the passengers exited at once.

The solution came from the phenomenon of light attenuation, or dimming, as it travels through the polymer. The longer it travels, the dimmer it becomes. So by measuring the relative brightness of light reaching the sensor array, it was possible to calculate where the light entered the film. This same principle has already been employed in an input device that tracks the location of a single laser point on a screen.

The researchers were able to scale up this basic principle by measuring how much light arrives from every direction at each position on the image sensor at the film’s edge. They could then reconstruct the image by using a technique similar to X-ray computed tomography, more commonly known as a CT scan.

“In CT technology, it’s impossible to reconstruct an image from a single measurement of X-ray attenuation along one scanning direction alone,” says Bimber. “With a multiple of these measurements taken at different positions and directions, however, this becomes possible. Our system works in the same way, but where CT uses X-rays, our technique uses visible light.”

Currently, the resolution from this image sensor is low (32×32 pixels with the first prototypes). The main reason for this is the limited signal-to-noise ratio of the low-cost photodiodes being used. The researchers are planning better prototypes that cool the photodiodes to achieve a higher signal-to-noise ratio.

By applying advanced sampling techniques, the researchers can already enhance the resolution by reconstructing multiple images at different positions on the film. These positions differ by less than a single pixel (as determined by the final image, not the polymer itself). By having multiple of these slightly different images reconstructed, it’s possible to create a higher resolution image. “This does not require better photodiodes,” notes Bimber, “and does not make the sensor significantly slower. The more images we combine, the higher the final resolution is, up to a certain limit.”

The researchers discuss applications,

The main application the researchers envision for this new technology is in touch-free, transparent user interfaces that could seamlessly overlay a television or other display technology. This would give computer operators or video-game players full gesture control without the need for cameras or other external motion-tracking devices. The polymer sheet could also be wrapped around objects to provide them with sensor capabilities. Since the material is transparent, it’s also possible to use multiple layers that each fluoresce at different wavelengths to capture color images.

The researchers also are considering attaching their new sensor in front of a regular, high-resolution CCD sensor. This would allow recording of two images at the same time at two different exposures. “Combining both would give us a high-resolution image with less overexposed or underexposed regions if scenes with a high dynamic range or contrast are captured,” Bimber speculates. He also notes that the polymer sheet portion of the device is relatively inexpensive and therefore disposable. “I think there are many applications for this sensor that we are not yet aware of,” he concludes.

Here’s a citation and a link,

“Towards a transparent, flexible, scalable and disposable image sensor using thin-film luminescent concentrators,” A. Koppelhuber and O. Bimber, Optics Express, Vol. 21, Issue 4, pp. 4796-4810 (2013) (link: http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-21-4-4796).

Canada’s Queen’s University strikes again with its ‘paper’ devices

Roel Vertegaal at Queen’s University (Ontario, Canada) has released a ‘paper’ tablet. Like the bendable, flexible ‘paper’ phone he presented at the CHI 2011 meeting in Vancouver, Canada (my May 12, 2011 posting), this tablet offers some intriguing possibilities but is tethered. The Jan. 9, 2013 news item on phys.org provides more information about the new ‘paper’ device (Note: Links have been removed),

Watch out tablet lovers – a flexible paper computer developed at Queen’s University in collaboration with Plastic Logic and Intel Labs will revolutionize the way people work with tablets and computers.

The PaperTab tablet looks and feels just like a sheet of paper. However, it is fully interactive with a flexible, high-resolution 10.7-inch plastic display developed by Plastic Logic and a flexible touchscreen. It is powered by the second generation I5 Core processor developed by Intel.

Vertegaal and his team have produced a video demonstrating their ‘paper’ tablet/computer:

The Jan. 8, 2013 Queen’s University news release, which originated the news item, provides descriptions (for those who don’t have time to watch the video),

“Using several PaperTabs makes it much easier to work with multiple documents,” says Roel Vertegaal, Director of Queen’s University’s Human Media Lab. “Within five to ten years, most computers, from ultra-notebooks to tablets, will look and feel just like these sheets of printed color paper.”

“We are actively exploring disruptive user experiences. The ‘PaperTab’ project, developed by the Human Media Lab at Queen’s University and Plastic Logic, demonstrates novel interactions powered by Intel processors that could potentially delight tablet users in the future,” says Intel’s Experience Design Lead Research Scientist, Ryan Brotman.

PaperTab’s intuitive interface allows users to create a larger drawing or display surface by placing two or more PaperTabs side by side. PaperTab emulates the natural handling of multiple sheets of paper. It can file and display thousands of paper documents, replacing the need for a computer monitor and stacks of papers or printouts.

Unlike traditional tablets, PaperTabs keep track of their location relative to each other, and to the user, providing a seamless experience across all apps, as if they were physical computer windows.

“Plastic Logic’s flexible plastic displays allow a natural human interaction with electronic paper, being lighter, thinner and more robust compared with today’s standard glass-based displays. This is just one example of the innovative revolutionary design approaches enabled by flexible displays,” explains Indro Mukerjee, CEO of Plastic Logic.

The partners are saying that ‘paper’ tablets may be on the market in foreseeable future  according to Emma Wollacott’s Jan. 8, 2013 article for TG Daily,

The bendy tablet has been coming for quite a while now, but a version to be shown off today at CES [Consumer Electronics Show] could be ready for the market within three years, say its creators.

You can find out more about the Human Media Lab at Queen’s University here, Plastic Logic here, and Intel Core I5 Processors here.

COllaborative Network for Training in Electronic Skin Technology (CONTEST) looking for twelve researchers

The CONTEST (COllaborative Network for Training in Electronic Skin TechnologyCOllaborative Network for Training in Electronic Skin Technology) project was launched today in Italy. According to the Aug. 21, 2012 news item on Nanowerk,

“Flexible electronics” is one of the most significant challenges in the field of future electronics. The possibility of realizing flexible and bendable electronic circuits, that can be rolled up, twisted or inserted in films around objects, would introduce a range of infinite applications in multiple fields, including healthcare, robotics and energy.

In this area, the Fondazione Bruno Kessler of Trento will coordinate the CONTEST project (COllaborative Network for Training in Electronic Skin Technology), an Initial Training Network (ITN) Marie Curie project funded by the European Commission involving European research, academic and business players. These include seven full partners (Fondazione Bruno Kessler, Italy; ST Microelectronics, Italy; Technical University Munich, Germany; Fraunhofer EMFT, Germany; University College London, UK; Imperial College London, UK; and Shadow Robotics Company, UK) and two associate partners (University of Cambridge, UK, and University of Tokyo, Japan).

The CONTEST project page at the Fondazione Bruno Kessler website offers more details,

At the heart of the CONTEST programme lies the multidisciplinary research training of young researchers. The CONTEST network will recruit twelve excellent Early-Stage Researchers (e.g. PhD students) and two Experienced Researchers (e.g. Post-Doc fellows). Information for submitting applications is available at the project’s website: http://www.contest-itn.eu/.
CONTEST activities will be coordinated by Ravinder S. Dahiya, researcher at the Bio-MEMS Unit (BIO-Micro-Electro-Mechanical-Systems) of  the Center for Materials and Microsystems (Fondazione Bruno Kessler) and by Leandro Lorenzelli, head of the Bio-MEMS Unit.
“The disruptive flexible electronics technology – says Ravinder S. Dahiya – will create change and improve the electronic market landscape and usher in a new revolution in multifunctional electronics. It will transform to an unprecedented degree our view of electronics and how we, as a society, interact with intelligent and responsive systems.”
“The investigation, in a very multidisciplinary framework, of technological approaches for thin flexible components – explains Leandro Lorenzelli - will generate new paradigms and concepts for microelectronic devices and systems with new functionalities tailored to the needs of a wide range of applications including robotics, biomedical instrumentations and smart cities.”

Here’s more about the 12 researchers they’re recruiting, excerpted from the Job Openings page on the CONTEST project website (Note: I have removed some links),

We have been awarded a large interdisciplinary project on electronic skin and applications, called CONTEST (COllaborative Network for Training in Electronic Skin Technology). We are therefore looking for 12 excellent Early-Stage Researchers (e.g. PhD students) and 2 Experienced Researchers (e.g. Post-Doc), associated to:

  • Fondazione Bruno Kessler, Trento, Italy (2 Early-Stage Researcher positions on silicon based flexible sensors (e.g. touch sensors), electronic circuits and 1 Experienced Researcher position on system integration)  …,
  • ST Microelectronics, Catania, Italy (2 Early-Stage Researcher positions on chemical/physical sensors on flexible substrates, and metal patterned substrates for integrating flexible sensing elements)…,
  • Technical University Munich, Germany (3 Early-Stage Researcher positions on organic semiconductor based electronics devices and circuits, modeling of flexible devices and sensors … , and artificial skin in humanoids…,
  • Fraunhofer EMFT, Munich, Germany (1 Early-Stage Researcher position on assembly on film substrates and foil integration as well as 1 Experienced Researcher position on reliability and ESD issues of components during flex integration) … ,
  • University College London, UK (2 Early-Stage Researcher positions on organic semiconductor based interconnects, solutions processed sensors, alternative on-skin energy schemes, patterning of e-skin and stretchable interconnects using blends of graphene in polymeric materials …
  • Imperial College London, UK (1 Early-Stage Researcher position on human sensori-motor control and robotics) …, and
  • Shadow Robotics Company, UK (1 Early-Stage Researcher position on biorobotics and mechatronics) ….

Mobility rules apply to all these positions. Researchers can be of any nationality. They are required to undertake trans-national mobility (i.e. move from one country to another) when taking up their appointment. One general rule applies to the appointment of researchers: At the time of recruitment by the host organization, researchers must not have resided or carried out their main activity (work, studies, etc.) in the country of their host organization (i.e. recruiting institute) for more than 12 months in the 3 years immediately prior to the reference date. Short stays such as holidays and/or compulsory national service are not taken into account.

Good luck to all who apply! Priority will be given to applications received by Sept. 30, 2012.

Bake and shake your t-shirt to make a flexible electronic device

I don’t think you actually need to shake but you do need to bake your cotton t-shirt, albeit in a special way, to create a wearable battery  or so the University of South Carolina’s Xiaodong Li says. Excerpted from the June 29, 2012 news item on Nanowerk,

Over the years, the telephone has gone mobile, from the house to the car to the pocket. The University of South Carolina’s Xiaodong Li envisions even further integration of the cell phone – and just about every electronic gadget, for that matter – into our lives.

“We wear fabric every day,” said Li, a professor of mechanical engineering at USC. “One day our cotton T-shirts could have more functions; for example, a flexible energy storage device that could charge your cell phone or your iPad.”

Li is helping make the vision a reality. He and post-doctoral associate Lihong Bao have just reported in the journal Advanced Materials (“Towards Textile Storage from Cotton T-Shirts”) how to turn the material in a cotton T-shirt into a source of electrical power.

I’ve been following the ‘wearable battery’ story for a while (the May 9, 2012 posting is the most recent) but Li’s approach is a little different.  Excerpted from the June 29, 2012 University of South Caroline news release by Steven Powell,

Starting with a T-shirt from a local discount store, Li’s team soaked it in a solution of fluoride, dried it and baked it at high temperature. They excluded oxygen in the oven to prevent the material from charring or simply combusting.

The surfaces of the resulting fibers in the fabric were shown by infrared spectroscopy to have been converted from cellulose to activated carbon. Yet the material retained flexibility; it could be folded without breaking.

“We will soon see roll-up cell phones and laptop computers on the market,” Li said. “But a flexible energy storage device is needed to make this possible.”

The once-cotton T-shirt proved to be a repository for electricity. By using small swatches of the fabric as an electrode, the researchers showed that the flexible material, which Li’s team terms activated carbon textile, acts as a capacitor. Capacitors are components of nearly every electronic device on the market, and they have the ability to store electrical charge.

Here’s what makes the approach different; it’s ‘green’ according to Powell’s news release,

Li is particularly pleased to have improved on the means by which activated carbon fibers are usually obtained. “Previous methods used oil or environmentally unfriendly chemicals as starting materials,” he said. “Those processes are complicated and produce harmful side products. Our method is a very inexpensive, green process.”

Somehow I’ve always seen ‘wearable batteries and/or electronics’ as opportunities for electrocution but I seem to be alone with this fear as there’s never any discussion about the safety issues might arise.

ETA July 3, 2012: Dexter Johnson in his June 29, 2012 posting on Nanoclast (a blog on the IEEE [Institute of Electrical and Electronics Engineers] website) notes that the simplicity of Li’s process may be specially exciting,

While Li makes mention of the environmentally friendly chemicals used to impart this capability to a t-shirt, it is perhaps the simplicity of the process that will likely be the most intriguing aspect to manufacturers.