The chemist building the world's most sensitive protein test
Feng Xuan runs Spear Bio out of the Boston area, where his company is trying to answer a deceptively simple question: how do you measure a protein when there is almost none of it to measure? For diseases that begin quietly - years before symptoms - the signal in a person's blood can be vanishingly small. Spear Bio's job is to catch it anyway.
As co-founder and CEO, Xuan sits at the center of that effort. His company's platform, called SPEAR, is designed to detect minute quantities of protein biomarkers from samples as small as a single microliter, and to do it on the quantitative PCR machines that most research labs already own. In 2024 the approach drew an oversubscribed $45 million Series A. In the period that followed, the U.S. Food and Drug Administration granted the company a Breakthrough Device Designation for a blood-based pTau 217 test aimed at the early detection of Alzheimer's disease.
"Our goal is to build the world's most sensitive immunoassay platform and deliver assays we are proud to put in our customers' hands to address critical scientific and clinical challenges."
— Feng Xuan, Co-Founder & CEO, Spear Bio
An invention that came before the company
Most founders start with a market and go looking for a product. Xuan started with the science. Before Spear Bio existed, he was a Technology Development Fellow at Harvard's Wyss Institute for Biologically Inspired Engineering, working in the lab of core faculty member Peng Yin. It was there that he invented the technology the company is named after: SPEAR, short for Successive Proximity Extension Amplification Reaction. He developed it alongside co-inventors Cherry (Tsz Wing) Fan and Yu Wang.
The idea borrows from DNA nanotechnology. Two probes are designed to latch onto nearby spots on a target protein. When both bind, they trigger an engineered reaction that stitches together a unique DNA sequence - a molecular receipt confirming the protein was there. That sequence is then amplified and counted on a standard qPCR machine. The result is a test with extreme sensitivity, a wash-free workflow, and a broad dynamic range, all in volumes down to a single microliter.
What SPEAR is built to do
Illustrative comparison of SPEAR's design goals. Not to scale.
"The extreme sensitivity in very small sample volumes provided by SPEAR, and the fact that it can be read out using common quantitative PCR equipment, offer unique potential for creating microsampling-based in vitro diagnostics that can transform academic and clinical research in multiple disease areas."
— Feng Xuan
From the bench to the corner office
Xuan's path to the platform ran through a long stretch of pure research. He earned a Ph.D. in chemical engineering from the Hong Kong University of Science and Technology, completed postdoctoral work at Harvard Medical School, and then spent years at the Wyss Institute refining techniques in molecular and immuno-diagnostics. By his own account he spent roughly a decade researching and inventing in the field before he ever incorporated a company.
Spear Bio was founded in 2021 by Harvard researchers from the Wyss Institute, and in 2022 the technology was licensed exclusively to the young startup through Harvard's Office of Technology Development. Xuan made the move that many academic inventors never do: he stepped away from the lab bench to run the business built on his work, first as chief technology officer and then as chief executive.
Betting on a blood test for Alzheimer's
The clearest expression of Spear Bio's ambition is in neuroscience. Diagnosing neurodegenerative disease early has traditionally meant invasive spinal taps or expensive PET scans. Xuan's company is chasing a less invasive route: measuring biomarkers like Neurofilament light, pTau 217, and pTau 231 directly from small blood samples, at concentrations reported in the sub-femtogram-per-milliliter range.
That work is what earned the company its FDA Breakthrough Device Designation for a pTau 217 blood test, a signal that regulators see promise in a scalable tool for early Alzheimer's detection. In mid-2026 the company said it would unveil new Alzheimer's immunoassays and a fully automated SPEAR instrument platform at the Alzheimer's Association International Conference - a step toward taking the technology from specialized labs to routine use.
The investors and the endorsement
When Spear Bio closed its $45 million round in July 2024, it was led by Foresite Capital and Bio-Techne Corporation, with participation from Maverick Ventures, Yonjin Capital, and CDH Investments. The capital was earmarked to expand the company's menu of assays and its commercial offerings. Xuan framed the raise less as a financial milestone than a validation of the underlying science.
"Their investment is a strong endorsement of SPEAR technology and the company's vision."
— Feng Xuan on the 2024 Series A
There is a consistency to how Xuan talks about the work. He tends to return to two things: sensitivity, and adoption. The first is the hard science - seeing what others cannot. The second is a design choice - making the test simple enough, and compatible enough with existing equipment, that scientists and clinicians will actually use it. A technology that requires exotic new hardware stays in a handful of labs. One that runs on machines already sitting on the bench can spread.
That combination - a chemical engineer's instinct for the reaction and an operator's instinct for the workflow - is what Xuan has spent the better part of a decade assembling. Whether SPEAR becomes a fixture of early disease diagnostics is still an open question, and the answer will play out over years of validation and commercialization. But the wager is clear, and it is his: that the difference between missing a disease and catching it early can come down to a single drop of blood, read the right way.