Power/Performance Bits: May 25

5G energy harvesting
Researchers at Georgia Institute of Technology propose a way to harvest power for IoT devices using 5G networks. The team’s device uses a flexible Rotman lens-based rectifying antenna (rectenna) system capable of millimeter-wave harvesting in the 28-GHz band.

“With this innovation, we can have a large antenna, which works at higher frequencies and can receive power from any direction. It’s direction-agnostic, which makes it a lot more practical,” noted Jimmy Hester, senior lab advisor and the CTO and co-founder of Atheraxon, a Georgia Tech spinoff developing 5G RFID technology.

In the team’s work, all the electromagnetic energy collected by the antenna arrays from one direction is combined and fed into a single rectifier, which maximizes its efficiency.

“People have attempted to do energy harvesting at high frequencies like 24 or 35 Gigahertz before,” said Aline Eid, a senior researcher in the ATHENA lab at Georgia Tech, noting that such antennas only worked if they had line of sight to the 5G base station and that there had been no way to increase their angle of coverage until now.

The team’s Rotman lens provides six fields of view simultaneously. Tuning the shape of the lens allows the structure to map a set of selected radiation directions to an associated set of beam-ports. The lens is then used as an intermediate component between the receiving antennas and the rectifiers for 5G energy harvesting.

A Georgia Tech ATHENA group member holds an inkjet-printed prototype of a mm-wave harvester. The researchers envision a future where IoT devices will be powered wirelessly over 5G networks. (Credit: Christopher Moore, Georgia Tech)

The researchers said their system could allow for passive, long-range, mm-wave 5G-powered RFID for wearable and ubiquitous IoT applications.

“The fact is 5G is going to be everywhere, especially in urban areas. You can replace millions, or tens of millions, of batteries of wireless sensors, especially for smart city and smart agricultural applications,” said Emmanouil (Manos) Tentzeris, Professor in Flexible Electronics in the School of Electrical and Computer Engineering at Georgia Tech, who also noted that power-over-the-air could be a new revenue stream for telecoms.

Protection for flexible electronics
Researchers at Osaka University developed a polymer coating based on cellulose nanofibers to protect flexible electronics from water.

Any protective coating for flexible electronics must also be able to adapt top bending without hampering the protective qualities. The researcher’s circuit protection mechanism retains electrode function underwater and can undergo hundreds of bending cycles.

“In our initial work, an unprotected copper electrode failed after 5 minutes of dripping water onto it,” said Takaaki Kasuga of Osaka University. “Remarkably, a cellulose nanofiber coating prevented failure over at least a day of the same water challenge.”

Instead of repelling water, the cellulose polymer coating migrates in the electrode in such a way to prevent formation of conductive metal filaments that cause short-circuits. The nanofibers are also made from renewable resources.

Water inevitably penetrates water-proof coatings if they are damaged, and water can easily cause malfunctions due to dendrite growth. Cellulose nanofibers (CNFs) migrate toward the anode and gel, thus inhibiting short circuits even if the CNF coating film is damaged. (Credit: Osaka University)

“Our results aren’t attributable to simple ion-exchange or nanofiber length,” added Masaya Nogi of Osaka University. “The nanofibers aggregate in water into a protective layer made cohesive by locally acidic conditions and polymer cross-linking.”

The team tested the polymer coating’s performance after 300 cycles of bending underwater over the course of an hour. A conventional polymer coating usually failed, but the cellulose nanofibers continued to power LEDs. In other tests, a coating 1.5 micrometers thick performed similarly.

“You’ll be able to stretch, bend, and fold electronics with our coating, and they’ll still retain their water resistance,” said Kasuga. “This is critical for use in applications under extreme conditions where device failure is unacceptable–for example, medical devices used in emergency disaster response.”

Wireless brain-computer interface
Researchers from Brown University, Providence VA Medical Center, Stanford University, Massachusetts General Hospital, and Montefiore Hospital and Medical Center are creating brain-computer interfaces (BCIs) that can be used wirelessly.

BCIs are an assistive technology used to enable people with paralysis to do things such as type, interact with computers, and move robotic prothesis based on a sensing array implanted in the brain. However, these BCIs require cables connecting ports linked to the sensors with computers capable of decoding the signals. While wireless systems have been proposed, they have had lower bandwidth and fidelity compared to wired connections.

In the BrainGate project, the researchers developed an intracortical wireless BCI with an external wireless transmitter. It can transmit brain signals at single-neuron resolution and in full broadband fidelity without physically tethering the user to a decoding system. The traditional cables are replaced by a small transmitter about 2 inches in its largest dimension and weighing a little over 1.5 ounces. The unit sits on top of a user’s head and connects to an electrode array within the brain’s motor cortex using the same port used by wired systems, according to the researchers.

In tests, two participants with tetraplegia were able to point, click, and type on a standard tablet computer, while achieving similar accuracy and speed as with a wired connection. The system can record neural signals at 48 megabits per second from 200 electrodes with a battery life of over 36 hours.

“We’ve demonstrated that this wireless system is functionally equivalent to the wired systems that have been the gold standard in BCI performance for years,” said John Simeral, an assistant professor of engineering (research) at Brown University and a member of the BrainGate research consortium. “The signals are recorded and transmitted with appropriately similar fidelity, which means we can use the same decoding algorithms we used with wired equipment. The only difference is that people no longer need to be physically tethered to our equipment, which opens up new possibilities in terms of how the system can be used.”

The researchers noted that the wireless system allowed them, with the assistance of caregivers, to continue the studies during COVD-19. It also allowed for observation of brain activity for prolonged periods of time, including while the participant was asleep. Collecting a broad range of brain signals over time is important for refining the system, said Leigh Hochberg, an engineering professor at Brown, a researcher at Brown’s Carney Institute for Brain Science and leader of the BrainGate clinical trial. “This will help us to design decoding algorithms that provide for the seamless, intuitive, reliable restoration of communication and mobility for people with paralysis.”

Ultimately, a major objective in BCI research is a fully implantable system that can help restore independence for people who have lost the ability to move.