USC Researchers Develop First Pulse-Mode Electron Paramagnetic Resonance Chip

From different roasts of coffee to multiple grades of coal, Ray Sun has tested his microchip on a variety of free radicals. His sensor chip may be tiny – measuring only 1.5 x 2 millimeters – but it holds the capability of instruments many times its size. The chip is the first-ever chip-based sensor capable of pulse-mode electron paramagnetic resonance (EPR).

What is EPR and why is this chip unique?

EPR is a powerful scientific method for analyzing materials with unpaired electrons. By manipulating and detecting the interactions between magnetic fields and a property of electrons known as spin, EPR allows a spectrum to be measured, like a fingerprint for a specific material.

EPR is an invaluable tool for a variety of industries due to its capability for studying material properties. In biomedical research, EPR can be used to analyze DNA and protein structures. Other examples of the wide-ranging applications of EPR include analyzing crude oil at drilling sites and investigating the purity of silicon in solar cells. Controlling the spin of electrons is also vital for quantum computing applications.

EPR is typically performed in one of two modes, continuous-wave (CW) or pulse mode. Typically, the specialized spectrometer instruments used for EPR are large, bulky and extremely expensive, requiring permanent installation in dedicated labs. Changing between the two sensing modes also requires labor-intensive reconfiguration of the equipment.

Chip-based EPR sensors have been developed for portable, miniaturized spectrometers with the potential to expand the applicability of EPR. However, Sun’s chip is the first to incorporate pulse-mode functionality in addition to the CW features of prior EPR chips, allowing miniaturized spectrometers to leverage the capability of pulse-mode to measure properties not obtainable with CW.

Sun, entering his fifth-year as a Ming Hsieh Department of Electrical and Computer Engineering PhD student, says his microchip would allow researchers to leverage both modes of EPR simultaneously, without the hardware reconfiguration needs of conventional spectrometers. Moreover, it paves the way for broader applications of this technology. Traditional spectrometers are limited to analyzing a single sample at a time, but the dual-mode chip surpasses this limitation.

One of the chip’s key innovations is its two independent sensing cells on a single device, unlike traditional commercial systems that typically feature only one. This enables the chip to perform two simultaneous EPR experiments and has the potential to scale up to tens or even hundreds of experiments with different samples in a future version of the chip extended to feature an array of sensors.

“For example, when testing multiple variants of a drug to determine their efficacy, using a commercial system requires measuring each sample one at a time—removing one, inserting the next—which is a highly labor-intensive process. With our chip, we can include many sensor cells, and because of its small size, the overall system remains compact, even with significantly increased capacity,” said Constantine Sideris, an Assistant Professor of Electrical and Computer Engineering and holder of the Andrew and Erna Viterbi Early Career Chair.

Additionally, the chip’s compact design, combined with the ability to mass-produce silicon chips, makes the dual-mode spectrometer chip a significantly more cost-effective, reducing the price tag by more than three orders of magnitude compared to traditional spectrometer machines.

The chip’s early development and its selection for the 2024 IEEE International Solid-State Circuits Conference (ISSCC)

Sun, who is an international student from Taiwan, previously completed his undergraduate degree at the California Institute of Technology (Caltech). His research currently focuses on integrated circuit chips for biomedical applications, such as biosensors that are used to detect and analyze chemical substances.

His project began in 2020, when he entered USC as a graduate student. Sun works as a graduate researcher at the USC’s Analog/RF Integrated Circuits, Microsystems, and Electromagnetics (ACME) Lab, under the leadership of Sideris. Since then, he has worked closely with Sideris to bring the chip to life.

In February, Sun presented a demo of his chip at the IEEE International Solid-State Circuits Conference (ISSCC) in San Francisco, which is widely considered to be the largest and most prestigious conference in the field of integrated circuits with an acceptance rate of just 27% in 2024.

His paper, titled “A Portable 14GHz Dual-Mode Pulse and Continuous-Wave Electron Paramagnetic Resonance Spectrometer Using a Subharmonic Direct Conversion Receiver,” received significant recognition and was invited to a special issue of the IEEE Transactions on Biomedical Circuits and Systems (TBioCAS). An extended version of the conference paper was recently published in this special issue in December 2024.

Sun’s dual-mode chip also won an outstanding poster award in the PhD student category at the 14th Annual Electrical and Computer Engineering (ECE) Research Festival in November. 

The silicon chip for the dual-mode device is manufactured by Taiwan Semiconductor Manufacturing Company (TSMC), a global leader in chip production. TSMC manufactures chips for many of the world’s top technology companies, including Apple, NVIDIA, and AMD. Sun said the design of the chip alone took about a year and a half.

Overcoming Challenges in Developing the Groundbreaking EPR Chip

Despite the groundbreaking nature of the chip, the development of this technology also came with its own challenges. Sun, who was born with a severe vision impairment, said his poor vision has made it difficult for him to work with the chip at times. However, his passion for electrical engineering has continued to push him through. He said that even with his vision problem, he faced few difficulties with soldering and assembling the proof-of-concept EPR system.

“I knew even before starting college that I wanted to work in electrical engineering. Electronics are often at the heart of making things move, and I sought to be at the center of that,” Sun said.

In 2019, Sun sought to pursue a graduate degree following his graduation from Caltech. It was during that time that Sun was introduced to Sideris, a fellow Caltech alumnus, through a lecturer who highlighted Sideris’ research at USC.

“I wrote an email to Professor Sideris to introduce myself and express my interest in joining his lab. I had no experience in working with chips at the time,” Sun said.

“When the time came for me to decide where to go for graduate school, I realized that I could see Professor Sideris as a potential advisor and that I liked his focus on making integrated circuits for impactful biomedical applications, so I came to USC,” he added.

The technology behind the EPR chip is currently protected under a provisional patent and the full patent application is expected to be finalized in February 2025. This will be Sun’s first patent.

“Today, EPR is predominantly employed as an analytical method in established research environments. I am hopeful that the techniques pioneered by our chip will lead to miniaturized, portable EPR spectrometers for a wide range of applications, especially wearable devices and point-of-care diagnostics,” Sun said.

Sun plans to pursue an academic career in integrated circuits after graduating from USC. He anticipates completing his PhD in the spring of 2027.

“Perhaps the essence of my PhD experiences is the potential of integrated circuits to dramatically reduce the size and cost of systems such as biomedical devices. I look forward to designing more chips to accomplish this as I wrap up my PhD and start my professional career,” he added.

Published on January 10th, 2025

Last updated on January 10th, 2025