Designing Power-Efficient, Implantable Medical Devices

Medical devices used for treatment traditionally tend to be big, bulky, and full of wires, making them uncomfortable or inconvenient for the patient to use. For Dr. Rikky Muller and Cortera Neurotechnologies, power-efficient, implantable medical devices provide a viable alternative.

Muller is an assistant professor of electrical engineering and computer sciences at UC Berkeley. She is also a co-founder of Cortera Neurotechnologies, which designs medical devices that are geared toward improving patient care and quality of life and also advancing neuroscientific research.

For Muller, who made MIT Technology Review’s list of 35 Innovators Under 35 in 2015, the opportunity to enhance or even save lives drives her to conduct research into and create innovative implantable medical devices.

“Recent advances in brain/machine interfaces have offered hope to 100 million people worldwide who are living with paralysis,” Muller said during a talk at the Design Automation Conference (DAC) in Austin in June. However, “even state-of-the-art solutions clearly have a long way to go.”

That’s why Muller’s work focuses on small, minimally invasive wireless neural interfaces. “Our work aims to transition these tools into a much smaller form factor, integrating the same functionality on a chip, so it can be fully implanted and remotely powered,” she said.

An eye toward power efficiency
Miniaturization and power efficiency are key criteria for Cortera Neurotechnologies’ work. For the foundation of their efforts, Muller and her team turned to electrocorticography (ECoG), where sensors are placed directly on the surface of the brain. To develop their prototype, the team:

• Replaced wired sensors with small wireless implants made of thin, flexible materials that interface seamlessly with the human body.
• Added an antenna for wireless capabilities. This approach allows the surgical site to be closed, which minimizes the risk of infection.
• Used tiny, ultra-low power circuits to record signals and transmit them wirelessly out of the body.

The team designed high-density ECoG electrodes that could map which region of the brain’s cortex is tuned to hearing which specific frequency. By monolithically integrating the antenna in the same MEMS process, they were able to support a larger diameter and up to 800mW of power without making the resulting device too unwieldy or uncomfortable.

In the wireless IC, neural signal acquisition front-ends perform amplification, filtering, and digitization. To reduce die area and improve power efficiency, the team designed a mixed-signal architecture with a mixed-signal servo loop and a digital loop filter on a 65nm low-power CMOS process. Large time constraints were moved from the analog to the digital domain. This effort resulted in a solution with 64 channels in just 1.6mm² and a 3X improvement in power efficiency.

The team’s prototype, validated via testing on an anesthetized rodent, is a 64-channel, wireless, single-chip ECoG neural sensor. “We believe this technology is an excellent prospect to become a technology of choice for clinically relevant neural recording,” noted Muller.

One great challenge for design automation
During her DAC talk, Muller offered one great challenge to the design automation community: uncover better ways to determine design specifications and validate designs. Currently, the validation process can be long and expensive. Researchers and developers need multi-domain tools that incorporate models of biological systems for safety and efficacy, said Muller. The result, she noted, can save years of development time, millions of dollars, and, potentially, human lives.

“There’s very little doubt that we’re quickly heading toward a future full of connected devices that are going to monitor, learn, diagnose, and even treat our illnesses,” Muller said.

Muller helped form Cortera Neurotechnologies in 2013 to further develop their prototype. The implantable neural interface that is under development will be targeted for treating neuralpsychiatric disorders including major depressive disorder and post-traumatic stress disorder (PTSD). The company also develops high-density microelectrocorticography arrays that can be used for high-resolution subdural or epidural neural recordings.