Bio-inspired chemistries can make for safer, more sustainable Additive Manufacturing solutions

The photopolymer formulations used in stereolithography 3D-printers (SLA and DLP) have been optimized throughout the years to achieve higher throughputs and very fine control of the printed features, down to a few microns. However, the photosensitive chemicals commonly used in these processes (methacrylates) have a downside: they can be toxic to both humans and the environment.

To solve this problem, researchers at the Berkeley Center for Green Chemistry are investigating new materials and processes inspired by nature with the goal of making 3D printing more sustainable at scale.

An opportunity to innovate

The photopolymer resins commonly used in vat photopolymerization consist of photoreactive monomers (methacrylates), oligomers and a photo initiator. When exposed to light (most frequently UV light), the methacrylates crosslink or polymerize, leading the resin to harden.

However, these powerful crosslinking properties that enable speedy photopolymerization come at a price: methacrylates can cause eye and skin irritation and aquatic toxicity, which makes them problematic for workers in the manufacturing process and hazardous to the environment.

To be fair, Additive Manufacturing (AM) is inherently more sustainable than conventional manufacturing for a variety of reasons. It produces less waste, lowers energy consumption, and enables decentralized production, hence decreasing transportation and the resulting greenhouse gas emissions. With just-in-time production capabilities, it also reduces the need to stockpile inventory, which helps to lower a manufacturer’s overall footprint.

But using less toxic materials in the AM process could further improve sustainability and lower the risk to humans and the environment, particularly as the industry gains momentum.

For more than a decade, Tom McKeag has been on a mission to do just that. As a senior advisor and now-retired executive director at the Berkeley Center for Green Chemistry, he’s led research to devise new additive manufacturing techniques to make for safer and more sustainable processes. With a background in city planning and design and landscape architecture, Tom has been passionate about incorporating biologically-inspired design into cityscapes, industrial design and engineering for nearly three decades.

“Now is the perfect opportunity for us to innovate in the 3D printing industry to devise safer solutions as the industry is still scaling up, rather than waiting until chemicals of concern become widespread and we have a big problem,” McKeag says. “As 3D printing becomes more accessible to the public and commonplace in schools and in homes, this will expose even more vulnerable populations who may not even be aware of the risks or have access to proper PPE. So, now is the time to act.”

Improving safety and sustainability

McKeag’s work in sustainable 3D printing began as a consultant with AutoDesk, working jointly with the Berkeley Center for Green Chemistry to devise nature-inspired innovations for their Ember demo printer. When the company made both the printer hardware and resin chemistry open source—an unusual move—it opened the door to broader research and collaboration to uncover issues and find solutions.

“We found that the crosslinkers in the resin were concerning, causing both eye and skin sensitivity, aquatic toxicity and possible endocrine problems,” McKeag recalls. “We pointed AutoDesk toward some alternatives and that work got me excited about the creative potential of working at the molecular level and collaborating with chemists on earth-friendly solutions.”

Now at the Berkeley Center for Green Chemistry, McKeag and his academic team educate, research and engage with industry to transform how companies design, use and dispose of chemicals to improve safety. Working with interdisciplinary teams of students, the program has worked with over two dozen companies, including Steelcase, Patagonia, Nike and more to examine their chemical hazard problems and find alternatives.

A systems-based approach

Part of the challenge with incorporating safer alternatives is that it’s not as simple as just using a different polymer. McKeag says companies have to look at the entire system—the hardware, resins, crosslinkers, UV blockers, etc.—because making one change may dictate others.

“For example, one way to minimize the risk is to use a resin with a higher molecular weight because it’s less likely to be taken up by the body,” he says. “But in 3D printing, viscosity is directly related to performance and speed. The higher the molecular weight of the material, the slower it will move, which is problematic. There’s no such thing as a drop-in alternative. We have to find a balance between technical performance and environmental safety and health.”

Specifically, his team is now looking at an alternative polymerization process where instead of using acrylates and methacrylates as cross-linkers in resin formulations, the new system would use a photobase generator (PBG) and light activation with a mineral/protein mix and pH activation. Inspired by the mechanisms by which oysters and mussels attach themselves to the ocean floor, the team has outlined a process in which a UV-activated PBG in the resin would alter the pH of the resin, inducing polymerization.

That system has since been enhanced to remove dihydroxyphenylalanine (DOPA) as part of the organic protein component and replace it with a composite of collagen-ribose-hydroxyapatite, which are all naturally occurring compounds with low to no harm to human health. In this method, the quantity of PBGs, which can cause skin, eye and respiratory irritation, would also be lower, thereby offering a potentially safer alternative to the current use of acrylate cross-linkers in SLA resins while offering comparable technical performance.

The pathway to progress

While there is still much work to be done to make SLA 3D printing greener and safer, McKeag and his team, along with others, are making progress. For example, the Nelson Research Laboratory at the University of Washington focuses on the use of protein platforms based on bio-bovine serum albumin—a whey protein found in milk—to print hydrogels and thermosets that are biocompatible.

Still, McKeag says three key factors are required in order to scale up greener SLA solutions:

• A greater awareness of the health and safety concerns with the conventional chemistry, and the development of a chemical matchmaking system that would help guide alternative formulations.
• A comprehensive approach to design and formulations where environmental, health and safety and technical performance are equally balanced from the beginning.
• Broader access to intellectual property that would provide more transparency about the materials and enable sharing and further development of innovations for the greater good.

“By collaborating on hardware and chemistry, we can help more organizations develop more sustainable options that meet minimum standards for efficiency, efficacy, sustainability and profitability,” McKeag says. “And with greater consumer demand for transparency, we could see more market pull for greener solutions and that would be good for all of us.”

To learn more about the Berkeley Center and its latest work, visit https://bcgc.berkeley.edu/.