Power/Performance Bits: Aug. 23

Connecting implanted devices

University of Washington researchers developed a new method for communication between devices such as brain implants, contact lenses, credit cards and smaller wearable electronics with other devices such as smartphones and watches.

Using only reflections, an interscatter system requires no specialized equipment, relying solely on mobile devices to generate Wi-Fi signals using 10,000 times less energy than conventional methods.

“Instead of generating Wi-Fi signals on your own, our technology creates Wi-Fi by using Bluetooth transmissions from nearby mobile devices such as smartwatches,” said co-author Vamsi Talla, a research associate in the UW Department of Computer Science & Engineering.

The team’s process relies on a communication technique called backscatter, which allows devices to exchange information simply by reflecting existing signals. Because the new technique enables inter-technology communication by using Bluetooth signals to create Wi-Fi transmissions, the team calls it “interscattering.”

In one example the team demonstrated, a smartwatch transmits a Bluetooth signal to a smart contact lens outfitted with an antenna. To create a blank slate on which new information can be written, the UW team developed a way to transform the Bluetooth transmission into a “single tone” signal that can be further manipulated and transformed. By backscattering that single tone signal, the contact lens can encode data – such as health information it may be collecting – into a standard Wi-Fi packet that can then be read by a smartphone, tablet, or laptop.

In interscatter communication, a backscattering device such as a smart contact lens converts Bluetooth transmissions from a device such as a smartwatch to generate Wi-Fi signals that can be read by a phone or tablet. (Source: University of Washington)

“Bluetooth devices randomize data transmissions using a process called scrambling,” said lead faculty Shyam Gollakota, assistant professor of computer science and engineering. “We figured out a way to reverse engineer this scrambling process to send out a single tone signal from Bluetooth-enabled devices such as smartphones and watches using a software app.”

The challenge is that the backscattering process creates an unwanted mirror image copy of the signal, which consumes more bandwidth as well as interferes with networks on the mirror copy Wi-Fi channel. So, the team developed a technique called “single sideband backscatter” to eliminate the unintended byproduct.

“That means that we can use just as much bandwidth as a Wi-Fi network and you can still have other Wi-Fi networks operate without interference,” said electrical engineering doctoral student Bryce Kellogg.

The researchers built three proof-of-concept demonstrations, including a smart contact lens, an implantable neural recording device that can communicate directly with smartphones and watches, and credit card prototypes.

Purifying carbon nanotubes

Researchers at McMaster University developed a new way to purify carbon nanotubes. A major problem standing in the way of replacing the silicon in chips with carbon nanotubes has been untangling metallic and semiconducting carbon nanotubes, since both are created simultaneously in the process of producing the microscopic structures, which typically involves heating carbon-based gases to a point where mixed clusters of nanotubes form spontaneously as black soot.

Only pure semiconducting or metallic carbon nanotubes are effective in device applications, but efficiently isolating them has proven to be a challenging problem to overcome. Even when the nanotube soot is ground down, semiconducting and metallic nanotubes are knotted together within each grain of powder. Both components are valuable, but only when separated.

While previous researchers had created polymers that could allow semiconducting carbon nanotubes to be dissolved and washed away, leaving metallic nanotubes behind, there was no such process for doing the opposite: dispersing the metallic nanotubes and leaving behind the semiconducting structures.

Artistic rendition of a metallic carbon nanotube being pulled into solution, in analogy to the work described by the group. (Source: Alex Adronov/McMaster University)

The group managed to reverse the electronic characteristics of a polymer known to disperse semiconducting nanotubes while leaving the rest of the polymer’s structure intact. By so doing, they reversed the process, leaving the semiconducting nanotubes behind while making it possible to disperse the metallic nanotubes.

The next step for the team is to find a way to develop even more efficient polymers and scale up the process for commercial production.

Solid-state batteries

Researchers at ETH Zurich developed a type of battery that, unlike conventional ones, consists entirely of solid chemical compounds and is non-flammable.

The potential fire dangers of conventional lithium-ion batteries have been widely reported. In conventional lithium-ion batteries as well in most other batteries, the positive and negative poles – the two electrodes – are made of solid conductive compounds; charges move between these electrodes in a liquid or gel electrolyte. If you charge such a battery improperly (overcharging) or leave it sitting out in the sun, the liquid can ignite or the gel can swell up.

This is not the case with solid-state batteries. “Solid electrolytes do not catch fire even when heated to high temperatures or exposed to the air,” explains Jennifer Rupp, who, as Professor of Electrochemical Materials at ETH Zurich, is leading the development of this new type of battery.

One of the challenges in developing solid-state batteries is to connect the electrodes and electrolyte in such a way that the charges can circulate between them with as little resistance as possible. The ETH researchers say they’ve developed an improved electrode-electrolyte interface.

In the laboratory, the team constructed a sandwich-like battery featuring a layer of lithium-containing compound (lithium garnet), which acts as a solid electrolyte between the two electrodes. Lithium garnet is one of the materials with the highest known conductivity for lithium ions.

A slice of (white) lithium garnet electrolyte coated with a (black) lithium compound acting as the battery’s minus pole in the laboratory of the ETH researchers. (Source: ETH Zurich/Fabio Bergamin)

“During production, we made sure that the solid electrolyte layer obtained a porous surface,” said Jan van den Broek, a master’s student and one of the authors of the study. The researchers then applied the material of the negative pole in a viscous form, allowing it to seep into the pores. Finally, the scientists tempered the battery at 100 degrees Celsius. “With a liquid or gel electrolyte, it would never be possible to heat a battery to such high temperatures,” says van den Broek. Thanks to the trick with the pores, the researchers were able to significantly enlarge the contact area between the negative pole and the solid electrolyte, which ultimately means that the battery can be charged faster.

Batteries produced like this could theoretically operate at a normal ambient temperature, but they work best at 95 degrees Celsius and above at the current development state. This characteristic could be put to use in battery storage power plants. “Today, the waste heat that results from many industrial processes vanishes unused,” said Semih Afyon, a professor at the Izmir Institute of Technology in Turkey. “By coupling battery power plants with industrial facilities, you could use the waste heat to operate the storage power plant at optimal temperatures.”

Further research involves optimizing the battery, with a focus on increasing the conductivity of the electrode-electrolyte interface, and the development of thin-film batteries based on the technology.