A neuro-chip to manage brain disorders

Mahsa Shoaran of the Integrated Neurotechnologies Laboratory in the School of Engineering collaborated with Stéphanie Lacour in the Laboratory for Soft Bioelectronic Interfaces to develop NeuralTree: a closed-loop neuromodulation system-on-chip that can detect and alleviate disease symptoms. Thanks to a 256-channel high-resolution sensing array and an energy-efficient machine learning processor, the system can extract and classify a broad set of biomarkers from real patient data and animal models of disease in-vivo, leading to a high degree of accuracy in symptom prediction.

"NeuralTree benefits from the accuracy of a neural network and the hardware efficiency of a decision tree algorithm," Shoaran says. "It's the first time we've been able to integrate such a complex, yet energy-efficient neural interface for binary classification tasks, such as seizure or tremor detection, as well as multi-class tasks such as finger movement classification for neuroprosthetic applications."

Their results were presented at the 2022 IEEE International Solid-State Circuits Conference and published in the IEEE Journal of Solid-State Circuits, the flagship journal of the integrated circuits community.

Efficiency, scalability, and versatility

NeuralTree functions by extracting neural biomarkers -- patterns of electrical signals known to be associated with certain neurological disorders -- from brain waves. It then classifies the signals and indicates whether they herald an impending epileptic seizure or Parkinsonian tremor, for example. If a symptom is detected, a neurostimulator -- also located on the chip -- is activated, sending an electrical pulse to block it.

Shoaran explains that NeuralTree's unique design gives the system an unprecedented degree of efficiency and versatility compared to the state-of-the-art. The chip boasts 256 input channels, compared to 32 for previous machine-learning-embedded devices, allowing more high-resolution data to be processed on the implant. The chip's area-efficient design means that it is also extremely small (3.48mm2), giving it great potential for scalability to more channels. The integration of an 'energy-aware' learning algorithm -- which penalizes features that consume a lot of power -- also makes NeuralTree highly energy efficient.

In addition to these advantages, the system can detect a broader range of symptoms than other devices, which until now have focused primarily on epileptic seizure detection. The chip's machine learning algorithm was trained on datasets from both epilepsy and Parkinson's disease patients, and accurately classified pre-recorded neural signals from both categories.

"To the best of our knowledge, this is the first demonstration of Parkinsonian tremor detection with an on-chip classifier," Shoaran says.

Self-updating algorithms

Shoaran is passionate about making neural interfaces more intelligent to enable more effective disease control, and she is already looking ahead to further innovations.

"Eventually, we can use neural interfaces for many different disorders, and we need algorithmic ideas and advances in chip design to make this happen. This work is very interdisciplinary, and so it also requires collaborating with labs like the Laboratory for Soft Bioelectronic Interfaces, which can develop state-of-the-art neural electrodes, or labs with access to high-quality patient data."

As a next step, she is interested in enabling on-chip algorithmic updates to keep up with the evolution of neural signals.

"Neural signals change, and so over time the performance of a neural interface will decline. We are always trying to make algorithms more accurate and reliable, and one way to do that would be to enable on-chip updates, or algorithms that can update themselves."

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Light-based tech could inspire Moon navigation and next-gen farming

Super-thin chips made from lithium niobate are set to overtake silicon chips in light-based technologies, according to world-leading scientists in the field, with potential applications ranging from remote ripening-fruit detection on Earth to navigation on the Moon.

They say the artificial crystal offers the platform of choice for these technologies due to its superior performance and recent advances in manufacturing capabilities.

RMIT University's Distinguished Professor Arnan Mitchell and University of Adelaide's Dr Andy Boes led this team of global experts to review lithium niobate's capabilities and potential applications in the journal Science.

The international team, including scientists from Peking University in China and Harvard University in the United States, is working with industry to make navigation systems that are planned to help rovers drive on the Moon later this decade.

As it is impossible to use global positioning system (GPS) technology on the Moon, navigation systems in lunar rovers will need to use an alternative system, which is where the team's innovation comes in.

By detecting tiny changes in laser light, the lithium-niobate chip can be used to measure movement without needing external signals, according to Mitchell.

"This is not science fiction -- this artificial crystal is being used to develop a range of exciting applications. And competition to harness the potential of this versatile technology is heating up," said Mitchell, Director of the Integrated Photonics and Applications Centre.

He said while the lunar navigation device was in the early stages of development, the lithium niobate chip technology was "mature enough to be used in space applications."

"Our lithium niobate chip technology is also flexible enough to be rapidly adapted to almost any application that uses light," Mitchell said.

"We are focused on navigation now, but the same technology could also be used for linking internet on the Moon to the internet on Earth."

What is lithium niobate and how can it be used?

Lithium niobate is an artificial crystal that was first discovered in 1949 but is "back in vogue," according to Boes.

"Lithium niobate has new uses in the field of photonics -- the science and technology of light -- because unlike other materials it can generate and manipulate electro-magnetic waves across the full spectrum of light, from microwave to UV frequencies," he said.

"Silicon was the material of choice for electronic circuits, but its limitations have become increasingly apparent in photonics.

"Lithium niobate has come back into vogue because of its superior capabilities, and advances in manufacturing mean that it is now readily available as thin films on semiconductor wafers."

A layer of lithium niobate about 1,000 times thinner than a human hair is placed on a semiconductor wafer, Boes said.

"Photonic circuits are printed into the lithium niobate layer, which are tailored according to the chip's intended use. A fingernail-sized chip may contain hundreds of different circuits," he said.

How does the lunar navigation tech work?

The team is working with the Australian company Advanced Navigation to create optical gyroscopes, where laser light is launched in both clockwise and anticlockwise directions in a coil of fibre, Mitchell said.

"As the coil is moved the fibre is slightly shorter in one direction than the other, according to Albert Einstein's theory of relativity," he said.

"Our photonic chips are sensitive enough to measure this tiny difference and use it to determine how the coil is moving. If you can keep track of your movements, then you know where you are relative to where you started. This is called inertial navigation."

Potential applications closer to home

This technology can also be used to remotely detect the ripeness of fruit.

"Gas emitted by ripe fruit is absorbed by light in the mid-infrared part of the spectrum," Mitchell said.

"A drone hovering in an orchard would transmit light to another which would sense the degree to which the light is absorbed and when fruit is ready for harvesting.

"Our microchip technology is much smaller, cheaper and more accurate than current technology and can be used with very small drones that won't damage fruit trees."

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New chip-based beam steering device lays groundwork for smaller, cheaper lidar

Researchers have developed a new chip-based beam steering technology that provides a promising route to small, cost-effective and high-performance lidar (or light detection and ranging) systems. Lidar, which uses laser pulses to acquire 3D information about a scene or object, is used in a wide range of applications such as autonomous driving, free-space optical communications, 3D holography, biomedical sensing and virtual reality.

"Optical beam steering is a key technology for lidar systems, but conventional mechanical-based beam steering systems are bulky, expensive, sensitive to vibration and limited in speed," said research team leader Hao Hu from the Technical University of Denmark. "Although devices known as chip-based optical phased arrays (OPAs) can quickly and precisely steer light in a non-mechanical way, so far, these devices have had poor beam quality and a field of view typically below 100 degrees."

In Optica, Optica Publishing Group's journal for high-impact research, Hu and co-author Yong Liu describe their new chip-based OPA that solves many of the problems that have plagued OPAs. They show that the device can eliminate a key optical artifact known as aliasing, achieving beam steering over a large field of view while maintaining high beam quality, a combination that could greatly improve lidar systems.

"We believe our results are groundbreaking in the field of optical beam steering," said Hu. "This development lays the groundwork for OPA-based lidar that is low cost and compact, which would allow lidar to be widely used for a variety of applications such as high-level advanced driver-assistance systems that can assist in driving and parking and increase safety."

A new OPA design

OPAs perform beam steering by electronically controlling light's phase profile to form specific light patterns. Most OPAs use an array of waveguides to emit many beams of light and then interference is applied in far field (away from the emitter) to form the pattern. However, the fact that these waveguide emitters are typically spaced far apart from each other and generate multiple beams in the far field creates an optical artifact known as aliasing. To avoid the aliasing error and achieve a 180° field of view, the emitters need to be close together, but this causes strong crosstalk between adjacent emitters and degrades the beam quality. Thus, until now, there has been a trade-off between OPA field of view and beam quality.

To overcome this trade-off, the researchers designed a new type of OPA that replaces the multiple emitters of traditional OPAs with a slab grating to create a single emitter. This setup eliminates the aliasing error because the adjacent channels in the slab grating can be very close to each other. The coupling between the adjacent channels is not detrimental in the slab grating because it enables the interference and beam formation in the near field (close to the single emitter). The light can then be emitted to the far field with the desired angle. The researchers also applied additional optical techniques to lower the background noise and reduce other optical artifacts such as side lobes.

High quality and wide field of view

To test their new device, the researchers built a special imaging system to measure the average far-field optical power along the horizontal direction over a 180° field of view. They demonstrated aliasing-free beam steering in this direction, including steering beyond ±70°, although some beam degradation was seen.

They then characterized beam steering in the vertical direction by tuning the wavelength from 1480 nm to 1580 nm, achieving a 13.5° tuning range. Finally, they showed the versatility of the OPA by using it to form 2D images of the letters "D," "T" and "U" centered at the angles of -60°, 0° and 60° by tuning both the wavelength and the phase shifters. The experiments were performed with a beam width of 2.1°, which the researchers are now working to decrease to achieve beam steering with a higher resolution and a longer range.

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