A new bendable battery can stretch and twist without interrupting the supply of power.

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.

All these devices need an energy source, however, 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.

What makes this new battery prototype special is its electrolyte—that part of the battery through which lithium-ions move when the battery is charged or discharged. Lead author Xi Chen, a doctoral student at ETH Zurich, discovered the electrolyte.

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,” says Markus Niederberger, professor for multifunctional materials, whose team developed the battery prototype.

The two current collectors for the anode and the cathode consist of a 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 micron-sized 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.

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 they filled the gap in the frame with the electrolyte gel.

Niederberger emphasizes that this gel is more environmentally friendly than the commercial electrolytes: “Liquid electrolyte in today’s batteries are flammable and toxic.” In contrast, the gel electrolyte that 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.

More and more applications for a battery like this are emerging every day, the researchers say. 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. It’s important to ensure that, in the event of battery leakage, the liquids that come out cause no damage. This is where the team’s electrolyte offers a considerable advantage.

Niederberger stresses that more research is necessary to optimize the flexible battery before they consider commercializing it. Above all, the team has to increase the amount of electrode material it can hold.

The study appears Advanced Materials.

Source: ETH Zurich

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A new way for wearable devices to connect incorporates conductive textiles into clothing to dynamically connect several devices at once.

Over the past decade, a major trend in electronics has been the development of sensors, displays, and smart devices which are seamlessly integrated onto the human body. Most of these wearable devices are singularly connected to a user’s smart phone and transmit all data via Bluetooth or Wi-Fi signals.

As consumers wear increasing numbers of wearable devices, and as the data they transmit increases in sophistication, however, researchers are seeking more innovative connection methods.

The new “wireless body sensor network” allows devices to transmit data with a 1,000 times stronger signal than conventional technologies, meaning the system dramatically improves battery life of all devices. Wireless networks of these wearable devices on a body have future applications in health monitoring, medical interventions, and human-machine interfaces.

Stronger signals for wearable devices

Currently, almost all body sensors like smart watches connect to smartphones and other wearable electronics via radio-waves like Bluetooth and Wi-Fi. These waves radiate outwards in all directions, meaning that most of the energy is lost to the surrounding area. This method of connectivity drastically reduces the efficiency of the wearable technology as most of its battery life is consumed in attempting the connection.

John Ho, an assistant professor from the Institute for Health Innovation & Technology at the National University of Singapore (NUS) and NUS Engineering, and his team wanted to confine the signals between the sensors closer to the body to improve efficiency.

The signal between devices is so strong that it is possible to wirelessly transmit power from a smartphone to the device itself—opening the door for battery-free wearable devices.

Their solution was to enhance regular clothing with conductive textiles known as metamaterials. Rather than sending waves into surrounding space, these metamaterials are able to create “surface waves” which can glide wirelessly around the body on the clothes. This means that the energy of the signal between devices is held close to the body rather than spread in all directions. The wearable electronics use much less power than normal, and the devices can detect much weaker signals.

“This innovation allows for the perfect transmission of data between devices at power levels that are 1,000 times reduced. Or, alternatively, these metamaterial textiles could boost the received signal by 1,000 times which could give you dramatically higher data rates for the same power,” Ho says.

In fact, the signal between devices is so strong that it is possible to wirelessly transmit power from a smartphone to the device itself—opening the door for battery-free wearable devices.

Crucially, this signal boost does not require any changes to either the smartphone or the Bluetooth device—the metamaterial works with any existing wireless device in the designed frequency band.

This inventive way of networking devices also provides more privacy than conventional methods. Currently, radio-waves transmit signals several meters outwards from the person wearing the device, meaning that personal and sensitive information could be vulnerable to potential eavesdroppers. By confining the wireless communication signal to within 10 centimeters (just under 4 inches) of the body, Ho and his team have created a network which is more secure.

Bendy and foldable

The team has a first-year provisional patent on the metamaterial textile design, which consists of a comb-shaped strip of metamaterial on top of the clothing with an unpatterned conductor layer underneath. The researchers can arrange strips on clothing in any pattern necessary to connect all areas of the body. The metamaterial itself is cost-effective, in the range of a few dollars per meter, and can be bought readily in rolls.

“We started with a specific metamaterial that was both flat and could support surface waves. We had to redesign the structure so that it could work at the frequencies used for Bluetooth and Wi-Fi, perform well even when close to the human body, and could be mass produced by cutting sheets of conductive textile,” Ho explains.

The team created their particular design with the aid of a computer model to ensure successful communication in the radio frequency range and to optimize overall efficacy. They then fabricate the smart clothing by laser-cutting the conductive metamaterial and attaching the strips with fabric adhesive.

Once made, the “smart” clothes are highly robust. They can fold and bend with minimal loss to the signal strength, and the conductive strips can even be cut or torn without inhibiting the wireless capabilities. The garments can also be washed, dried, and ironed just like normal clothing.

The team is talking to potential partners to commercialize this technology, and in the near future Ho is hoping to test the “smart” textiles as specialized athletic clothing and for hospital patients to monitor performances and health. Potential applications could range dramatically—from measuring a patient’s vital signs without inhibiting their freedom of motion, to adjusting the volume in an athlete’s wireless headphones with a single hand motion.

“We envision that endowing athletic wear, medical clothing, and other apparel with such advanced electromagnetic capabilities can enhance our ability to perceive and interact with the world around us,” Ho says.

A paper on the research appears in Nature Electronics.

Source: National University of Singapore

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