Kezi Cheng, Ph.D. '21: Easing plastic recycling with new polymers

When it comes to recycling plastics, cost rules. One of the biggest hardships plastic recycling faces is that it’s considerably cheaper to produce new plastic products than it is to acquire, treat and re-use plastic waste. As a result, millions of tons of plastic are wasted every year.

“At the end of the day, the hardest thing about recycling is the complexity, contamination and cost,” said Kezi Cheng, Ph.D. ‘21. “For recycling to really work, the material has to improve or at least maintain performance through cycles, recycled materials have to be cheaper than virgin plastics, and you need the recycling to be low-energy, low-cost and modular enough to handle the messy, contaminated waste streams we actually see.”

Cheng, who earned her Ph.D. in materials science and mechanical engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), co-founded FLO Materials in 2021. The Santa Barbara-based startup is producing an alternative plastic polymer that is far easier and cheaper to recycle back into a usable form. Her first target: the plastic used to make frames for eyeglasses, which are cut down from larger blocks of acetate, similar to removing chunks of marble from a block until only the statue remains.

“The eyewear industry is very large, around $180 billion globally,” she said. “The sheet of the material to make one pair of glasses is about probably 100 grams in weight. Your frame, the final product, is about 15-20 grams in weight. So what that means is in cutting that down through CNC machining, you're losing almost 80% of that material. Premium eyewear frames are made of sheet materials that cost $10-20 per kilogram. These are not cheap plastics. These are expensive plastics that you can get back quickly from manufacturers."

Cheng first became interested in materials entrepreneurship after finishing her bachelor’s degree in materials science and engineering at MIT. She spent time working at TIAX and CAMX Power, where the company took early-stage innovations in the areas of functional wearables, non-flourinated coatings and lithium ion batteries, and transformed them into technology-enabled products. 

“I saw that a lot of the project leads were people with PhDs, and they were able to lead and direct these projects and really push forward on the innovation side,” she said. “That definitely gave me a sense of what hard tech, deep tech materials entrepreneurship would look like.”

The direction to work on recyclable plastics came midway through her Ph.D. research on plastics that can heal from damage on their own with David Clarke, Extended Tarr Family Professor of Materials. Her research eventually put her in touch with Alex Chortos, a postdoctoral researcher in Professor Jennifer Lewis’s lab and now principal investigator at Purdue University, who served as a mentor on synthesis and characterization of materials. Around that time, she came across publications on polydiketoenamines, a type of polymer developed at Lawrence Berkeley National Laboratory (LBNL) that can be easily broken down and cleaned of additives for much easier recycling. So intrigued by the research, she traveled to LBNL as a two-week exchange student and met Peter Christensen, the study’s first author and inventor, who co-founded FLO Materials with her.

“I was thinking about the Venn diagram of what I was trained in, what I was passionate about, and what society needs,” she said. “As a material scientist, what struck me was the tradeoff I kept seeing: every time we pushed materials to higher performance or functionality, we made them more complex and harder to deal with at the end of their life. Plastics recycling today is inherently hard because they were never designed to be recycled; they were designed 100 years ago to be cheap and durable. And I knew I had a deep commitment to tackling plastic waste accumulation.”

Cheng co-founded and became Chief Executive Officer of FLO soon after graduating Harvard in 2021, and for the next two years funded it through an Activate Fellowship. She was named to the “Forbes 30 Under 30 – Manufacturing and Industry” list in 2023.

As CEO, Cheng’s responsibilities generally fall into three categories: getting funding; building a good team; and defining strategic vision for how the company should grow.

“The biggest challenge is that adoption is very tough,” she said. “Getting to an agreement with a strategic partner takes a lot. You need to put in a lot of resources before someone is willing to take a chance on you. It requires a lot of patient capital, product development and research and development. You have to do both in parallel, and you have to focus.”

During her time here, Cheng served as both a Graduate Energy and Environment Consortium Fellow and part of the organizing team for the Harvard Circular Economy Symposium. The symposium brought in speakers from multiple Harvard schools, exposing her to both the academic and financial aspects of an economy focused on minimizing waste and reusing already-produced materials.

The chemistry used at FLO Materials is a pivot from Cheng’s Ph.D. research, but her time at SEAS continues to benefit her. Her technical background makes it easier to communicate with the scientists she’s brought on. And more than that, SEAS taught her an approach to problem-solving that’s translated directly to starting a company.

“My PhD was really trying to figure out the right question,” she said. “You spend 80% of the time trying to figure out the question you want to solve, and then 20% of the time solving it. Going through a startup in this open space, and trying to figure out how we take this chemistry and commercialize it at scale, and who wants it, is really like finding the right question, and being able to sit with uncertainty long enough that the answer starts to appear.”