A perfect battery for bendable electronic devices?

The battery can be twisted, stretched and bent without breaking off the power supply. (Photographs: Niederberger Group, ETH Zurich)

Markus Niederberger’s team of researchers at ETH has used stretchable materials to develop a battery that can be bent, stretched and twisted. For applications in bendable electronic devices, this is precisely the kind of battery they need.

Today’s electronics industry is increasingly focusing on computers or smartphones with screens that can be folded or rolled. Smart clothing items make use of wearable micro-devices or sensors to monitor bodily functions, for example. However, all these devices need an energy source, which is usually a lithium-ion battery. Unfortunately, commercial batteries are typically heavy and rigid, making it fundamentally unsuitable for applications in flexible electronics or textiles.

A remedy for this problem is now being created by Markus Niederberger, Professor for Multifunctional Materials at ETH Zurich, and his team. The researchers have developed a prototype for a flexible thin-film battery that can be bent, stretched and even twisted without interrupting the supply of power.

What makes this new battery special is its electrolyte – that part of the battery through which lithium-ions move when the battery is charged or discharged. This electrolyte was discovered by ETH doctoral student Xi Chen, lead author of the study that recently appeared in the scientific journal Advanced Materials.

Systematically employing bendable components

Following the design of commercial batteries, this new type of battery is built in layers like a sandwich. However, it marks the first time that researchers have used flexible components to keep the whole battery bendable and stretchable. “To date, no one has employed exclusively flexible components as systematically as we have in creating a lithium-ion battery,” Niederberger says.

The two current collectors for the anode and the cathode consist of bendable polymer composite that contains electrically conductive carbon and that also serves as the outer shell. On the interior surface of the composite, the researchers applied a thin layer of micronsized silver flakes. Due to the way the flakes overlap like roof tiles, they don’t lose contact with one another when the elastomer is stretched. This guarantees the conductivity of the current collector even if it is subjected to extensive stretching. And in the event that the silver flakes do in fact lose contact with each other, the electrical current can still flow through the carbon-containing composite, albeit more weakly.

With the help of a mask, the researchers then sprayed anode and cathode powder onto a precisely defined area of the silver layer. The cathode is composed of lithium manganese oxide and the anode is a vanadium oxide.

Water-based gel electrolyte

In the final step, the scientists stacked the two current collectors with the applied electrodes on top of each other, separated by a barrier layer similar to a picture frame, while the gap in the frame was filled with the electrolyte gel.

Niederberger emphasises that this gel is environmentally more friendly than the commercial electrolytes: “Liquid electrolyte in today’s batteries are flammable and toxic.” In contrast, the gel electrolyte that his doctoral student Chen developed contains water with a high concentration of a lithium salt, which not only facilitates the flow of lithium ions between cathode and anode while the battery is charging or discharging, but also keeps the water from electrochemical decomposition.

The scientists joined the various parts of their prototype together with adhesive. “If we want to market the battery commercially, we’ll have to find another process that will keep it sealed tight for a longer period of time,” Niederberger says.

Numerous potential applications

More and more applications for a battery like this are emerging every day. Well-known manufacturers of mobile phones are vying with each other to produce devices with foldable screens. Other possibilities include rollable displays for computers, smartwatches and tablets, or functional textiles that contain bendable electronics – and all of these require a flexible power supply. “For instance, you could sew our battery right into the clothing,” Niederberger says. What’s important is, in the event of battery leakage, to ensure that the liquids that come out cause no damage. This is where the team’s electrolyte offers a considerable advantage.

However, Niederberger stresses that more research is necessary to optimise the flexible battery before they consider commercialising it. Above all, the team has to increase the amount of electrode material it can hold. A new doctoral student has recently begun refining the stretchable power supply. The inventor of the initial prototype, Xi Chen, returned to his homeland of China after completing his doctoral thesis to take up a new job – as a consultant for the battery industry.

Learn more: A battery with a twist

 

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Flexible, stretchable electronics that can change size and shape dynamically are poised to open doors to innovation

Medical implants of the future may feature reconfigurable electronic platforms that can morph in shape and size dynamically as bodies change or transform to relocate from one area to monitor another within our bodies. In Applied Physics Letters, a group of researchers reports a silicon honeycomb-serpentine reconfigurable electronic platform that can dynamically morph into three different shapes: quatrefoils (four lobes), stars and irregular ones. This image shows: (a) the serpentine-honeycomb reconfigurable platform. (b) The design with the eight reconfiguring nodes highlighted. (c) The irregular configuration. (d) The quatrefoil configuration. (e) The star configuration. CREDIT Muhammad Hussain

Reconfigurable electronics show promise for wearable, implantable devices

Medical implants of the future may feature reconfigurable electronic platforms that can morph in shape and size dynamically as bodies change or transform to relocate from one area to monitor another within our bodies.

In Applied Physics Letters, from AIP Publishing, a group of researchers from King Abdullah University of Science and Technology and University of California, Berkeley reports a silicon honeycomb-serpentine reconfigurable electronic platform that can dynamically morph into three different shapes: quatrefoils (four lobes), stars and irregular ones.

“Quatrefoils can be used for rectangular object-based operation, while stars are for more intricate architectures, and irregular-shaped ones are specifically for implanted bioelectronics,” said Muhammad Hussain, co-author and a visiting professor at the University of California, Berkeley.

With their work, the researchers are introducing a new branch of flexible, stretchable electronics — opening the door to new engineering challenges and providing opportunities for innovation in biomedical technologies that can be used for drug delivery, health monitoring, diagnosis, therapeutic healing, implants and soft robotics.

Inspiration for the group’s honeycomb-shaped platform comes from nature. “Think of how flowers bloom. Based on the same principle, we gathered many videos of flowers blossoming, analyzed their geometric pattern and used them for our first set of designs,” Hussain said. “In particular, we analyzed their stress distribution in an iterative manner, taking design architecture, materials and their properties into consideration. It’s a tedious process to reach the optimal balance, but this is where engineering helps.”

Reconfigurable electronic platforms are designed to undergo physical deformation, such as stretching, bending, folding or twisting to morph into another shape. “Imagine that a lab-on-chip platform is implanted within your body to monitor the growth of a tumor in the shoulder area,” said Hussain. “While it is implanted, if we observe some abnormality in lung function, a platform that is equipped enough can change its shape and size, and relocate or expand to go monitor lung function.”

Another idea the researchers are actively pursuing is a wearable heart sleeve to monitor heart activity with the ability to mechanically pump the heart by repeated expansion and contraction when needed.

“We still have a long way to go to integrate soft robotics with embedded high-performance flexible complementary metal-oxide semiconductor (CMOS) electronics on a variety of reconfigurable electronic platforms, which will be of immense importance,” Hussain said. “It offers wonderful engineering challenges, requires true multidisciplinary efforts and has the ability to bind a variety of disciplines into applications that are simply not possible with the existing electronics infrastructure.”

Learn more: Reconfigurable electronics show promise for wearable, implantable devices

 

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Monitoring the healing of cerebral aneurysms with a stretchable wireless monitor in the brain

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A wireless sensor small enough to be implanted in the blood vessels of the human brain could help clinicians evaluate the healing of aneurysms — bulges that can cause death or serious injury if they burst. The stretchable sensor, which operates without batteries, would be wrapped around stents or diverters implanted to control blood flow in vessels affected by the aneurysms.

To reduce costs and accelerate manufacturing, fabrication of the stretchable sensors uses aerosol jet 3D printing to create conductive silver traces on elastomeric substrates. The 3D additive manufacturing technique allows production of very small electronic features in a single step, without using traditional multi-step lithography processes in a cleanroom. The device is believed to be the first demonstration of aerosol jet 3D printing to produce an implantable, stretchable sensing system for wireless monitoring.

“The beauty of our sensor is that it can be seamlessly integrated onto existing medical stents or flow diverters that clinicians are already using to treat aneurysms,” said Woon-Hong Yeo, an assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering and the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University. “We could use it to measure an incoming blood flow to the aneurysm sac to determine how well the aneurysm is healing, and to alert doctors if blood flow changes.”

Inserted using a catheter system, the sensor would use inductive coupling of signals to allow wireless detection of biomimetic cerebral aneurysm hemodynamics. The research was reported August 7 in the journal Advanced Science.

Monitoring the progress of cerebral aneurysms now requires repeated angiogram imaging using contrast materials that can have harmful side effects. Because of the cost and potential negative effects, use of the imaging technique must be limited. However, a sensor placed in a blood vessel could allow more frequent evaluations without the use of imaging dyes.

“For patients who have had a procedure done, we would be able to tell if the aneurysm is occluding as it should without using any imaging tools,” Yeo said. “We will be able to accurately measure blood flow to detect changes as small as 0.05 meters per second.”

The six-layer sensor is fabricated from biocompatible polyimide, two separate layers of a mesh pattern produced from silver nanoparticles, a dielectric and soft polymer-encapsulating material. The sensor would be wrapped around the stent or flow diverter, which must be less than two or three millimeters in diameter to fit into the blood vessels.

The sensor includes a coil to pick up electromagnetic energy transmitted from another coil located outside the body. Blood flowing through the implanted sensor changes its capacitance, which alters the signals passing through the sensor on their way to a third coil located outside the body. In the laboratory, Yeo and his collaborators have measured capacitance changes six centimeters away from a sensor implanted in meat to simulate brain tissue.

“The flow rate is correlated really well with the capacitance change that we can measure,” Yeo said. “We have made the sensor very thin and deformable so it can respond to small changes in blood flow.”

Use of the aerosol jet 3D printing technique was essential to producing the stretchable and flexible electronics necessary for the sensor. The technique uses a spray of aerosol particles to create patterns, allowing narrower feature sizes than conventional inkjet printing.

“We can control the printing speed, the printing width, and the amount of material being jetted,” Yeo said. “The parameters can be optimized for each material, and we can use materials that have a broad range of viscosities.”

Because the sensor can be fabricated in a single step without costly cleanroom facilities, it could be manufactured in higher volume at lower cost.

The next phase of the aneurysm sensor will be able to measure blood pressure in the vessel along with the flow rates. “We will be able to measure how pressure contributes to flow change,” Yeo explained. “That would allow the device to be used for other applications, such as intracranial pressure measurements.”

Yeo’s research team has also developed a flexible and wearable health monitor able to provide ECG and other information. He says the success of the monitoring technique demonstrates the potential for smart and connected wireless soft electronics based on nanomaterials, stretchable mechanics, and machine learning algorithms.

“We are excited that people are now recognizing the potential of this technology,” Yeo added. “There are a lot of opportunities to integrate this sensing mechanism into ultrathin membranes that are implantable within the body.”

Learn more: Stretchable Wireless Sensor Could Monitor Healing of Cerebral Aneurysms

 

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3D stretchable electronics with a multitude of functions while staying thin and small

By stacking and connecting layers of stretchable circuits on top of one another, engineers have developed an approach to build soft, pliable “3D stretchable electronics” that can pack a lot of functions while staying thin and small in size.

The work is published in the Aug. 13 issue of Nature Electronics.

As a proof of concept, a team led by the University of California San Diego has built a stretchable electronic patch that can be worn on the skin like a bandage and used to wirelessly monitor a variety of physical and electrical signals, from respiration, to body motion, to temperature, to eye movement, to heart and brain activity. The device, which is as small and thick as a U.S. dollar coin, can also be used to wirelessly control a robotic arm.

“Our vision is to make 3D stretchable electronics that are as multifunctional and high-performing as today’s rigid electronics,” said senior author Sheng Xu, a professor in the Department of NanoEngineering and the Center for Wearable Sensors, both at the UC San Diego Jacobs School of Engineering.

Xu was named among MIT Technology Review’s 35 Innovators Under 35 list in 2018 for his work in this area.

To take stretchable electronics to the next level, Xu and his colleagues are building upwards rather than outwards. “Rigid electronics can offer a lot of functionality on a small footprint—they can easily be manufactured with as many as 50 layers of circuits that are all intricately connected, with a lot of chips and components packed densely inside. Our goal is to achieve that with stretchable electronics,” said Xu.

The new device developed in this study consists of four layers of interconnected stretchable, flexible circuit boards. Each layer is built on a silicone elastomer substrate patterned with what’s called an “island-bridge” design. Each “island” is a small, rigid electronic part (sensor, antenna, Bluetooth chip, amplifier, accelerometer, resistor, capacitor, inductor, etc.) that’s attached to the elastomer. The islands are connected by stretchy “bridges” made of thin, spring-shaped copper wires, allowing the circuits to stretch, bend and twist without compromising electronic function.

Making connections

This work overcomes a technological roadblock to building stretchable electronics in 3D. “The problem isn’t stacking the layers. It’s creating electrical connections between them so they can communicate with each other,” said Xu. These electrical connections, known as vertical interconnect accesses or VIAs, are essentially small conductive holes that go through different layers on a circuit. VIAs are traditionally made using lithography and etching. While these methods work fine on rigid electronic substrates, they don’t work on stretchable elastomers.

So Xu and his colleagues turned to lasers. They first mixed silicone elastomer with a black organic dye so that it could absorb energy from a laser beam. Then they fashioned circuits onto each layer of elastomer, stacked them, and then hit certain spots with a laser beam to create the VIAs. Afterward, the researchers filled in the VIAs with conductive materials to electrically connect the layers to one another. And a benefit of using lasers, notes Xu, is that they are widely used in industry, so the barrier to transfer this technology is low.

Multifunctional ‘smart bandage’

The team built a proof-of-concept 3D stretchable electronic device, which they’ve dubbed a “smart bandage.” A user can stick it on different parts of the body to wirelessly monitor different electrical signals. When worn on the chest or stomach, it records heart signals like an electrocardiogram (ECG). On the forehead, it records brain signals like a mini EEG sensor, and when placed on the side of the head, it records eyeball movements. When worn on the forearm, it records muscle activity and can also be used to remotely control a robotic arm. The smart bandage also monitors respiration, skin temperature and body motion.

“We didn’t have a specific end use for all these functions combined together, but the point is that we can integrate all these different sensing capabilities on the same small bandage,” said co-first author Zhenlong Huang, who conducted this work as a visiting Ph.D. student in Xu’s research group.

And the researchers did not sacrifice quality for quantity. “This device is like a ‘master of all trades.’ We picked high quality, robust subcomponents—the best strain sensor we could find on the market, the most sensitive accelerometer, the most reliable ECG sensor, high quality Bluetooth, etc.—and developed a clever way to integrate all these into one stretchable device,” added co-first author Yang Li, a nanoengineering graduate student at UC San Diego in Xu’s research group.

So far, the smart bandage can last for more than six months without any drop in performance, stretchability or flexibility. It can communicate wirelessly with a smartphone or laptop up to 10 meters away. The device runs on a total of about 35.6 milliwatts, which is equivalent to the power from 7 laser pointers.

The team will be working with industrial partners to optimize and refine this technology. They hope to test it in clinical settings in the future.

Learn more: ‘Building up’ Stretchable Electronics to be as Multipurpose as Your Smartphone

 

 

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Liquid metal printing has taken a key step toward the rapid manufacture of flexible computer screens and other stretchable electronic devices

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Researchers in Oregon State University’s College of Engineering have taken a key step toward the rapid manufacture of flexible computer screens and other stretchable electronic devices, including soft robots.

The advance by a team within the college’s Collaborative Robotics and Intelligent Systems Institute paves the way toward the 3D printing of tall, complicated structures with a highly conductive gallium alloy.

Researchers put nickel nanoparticles into the liquid metal, galinstan, to thicken it into a paste with a consistency suitable for additive manufacturing.

“The runny alloy was impossible to layer into tall structures,” said Yi?it Mengüç, assistant professor of mechanical engineering and co-corresponding author on the study. “With the paste-like texture, it can be layered while maintaining its capacity to flow, and to stretch inside of rubber tubes. We demonstrated the potential of our discovery by 3D printing a very stretchy two-layered circuit whose layers weave in and out of each other without touching.”

Findings were recently published in Advanced Materials Technologies.

Gallium alloys are already being used as the conductive material in flexible electronics; the alloys have low toxicity and good conductivity, plus they’re inexpensive and “self-healing” – able to attach back together at break points.

But prior to the modification developed at OSU, which used sonication – the energy of sound – to mix the nickel particles and the oxidized gallium into the liquid metal, the alloys’ printability was restricted to 2-dimensional.

For this study, researchers printed structures up to 10 millimeters high and 20 millimeters wide.

“Liquid metal printing is integral to the flexible electronics field,” said co-author Do?an Yirmibe?o?lu, a robotics Ph.D. student at OSU. “Additive manufacturing enables fast fabrication of intricate designs and circuitry.”

The field features a range of products including electrically conductive textiles; bendable displays; sensors for torque, pressure and other types of strain; wearable sensor suits, such as those used in the development of video games; antennae; and biomedical sensors.

“The future is very bright,” Yirmibe?o?lu said. “It’s easy to imagine making soft robots that are ready for operation, that will just walk out of the printer.”

The gallium alloy paste demonstrates several features new to the field of flexible electronics, added co-corresponding author Uranbileg Daalkhaijav, Ph.D. candidate in chemical engineering.

“It can be made easily and quickly,” Daalkhaijav said. “The structural change is permanent, the electrical properties of the paste are comparable to pure liquid metal, and the paste retains self-healing characteristics.”

Future work will explore the exact structure of the paste, how the nickel particles are stabilized, and how the structure changes as the paste ages.

Learn more: Modified, 3D-printable alloy shows promise for flexible electronics, soft robots

 

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