By Michael Floyd & Susan Gladwin (we also want to acknowledge the contribution to this article by the Fall 2015 University of California Berkeley Greener Solutions class, whose report is available here.)
The process of stereolithographic (SLA) 3D printing entails using UV (or near-UV) light to convert liquid monomers to solid polymers, accreting layer upon layer to build parts. The fundamentals of this process have changed little in the 24 years since the publication of Paul Jacobs’s seminal book, Rapid Prototyping & Manufacturing: Fundamentals of SteroLithography, which defined the field of SLA 3D printing.
Though SLA’s underlying process has changed little over the years, 3D printing has begun to revolutionize modern manufacturing. We have witnessed the rise of large-scale applications of 3D printing, properly known as “additive manufacturing” (AM), which now enables the rapid creation of customized, functional, and locally produced products.
Despite this remarkable progress, AM has yet to reach its full potential. Products manufactured using 3D printing should be safe for humans and the environment throughout their lifespan: during their production, use, and disposal. Unfortunately, the chemical hazards associated with many of today’s SLA 3D printing materials present considerable drawbacks in all these areas. We can print an incredible range of products with amazing geometries, but process and product safety issues persist. The development of ‘biofriendly’ materials—that are safe and sustainable—for 3D printing will allow for new use cases and open up whole new markets. Solving this piece of the puzzle is therefore critical to unlocking AM’s larger potential.
Using PR48 as a Starting Point for Innovation
Autodesk Standard Clear Prototyping Resin, PR48, consists of reactive oligomers, reactive monomers, a photoinitiator, and a UV-blocker. Like most SLA resins, PR48 uses an acrylate-based monomer, which polymerizes in the presence of UV light, changing from liquid to solid. The acrylates and the photoinitiator both present hazards to human health and the environment.
Using PR48 as a best-in-class point of departure, the Greener Solutions research team at University of California Berkeley’s Center for Green Chemistry (BCGC), and researchers at Autodesk collaborated to explore potential for more sustainable, biofriendly alternatives. Below, we share insights from the BCGC Greener Solutions class (Fall 2015), which is described more fully in their class report), along with those of our own research team. It is our hope that this work will spur innovation in materials for SLA in promising biofriendly directions.
Three Approaches To Improving SLA Materials
Under the guidance of several Autodesk team members, including Chris Venter of the Bio/Nano team and Brian Adzima of the Digital Manufacturing Group, the Greener Solutions researchers looked at three approaches to improving the hazard profile of an SLA formulation like PR48:
- Replace the photoinitiator
- Modify the resin acrylates
- Create an entirely new resin formulation
The team looked to nature’s own design strategies for inspiration, studying materials ranging from castor oil, to spices, to derivatives of oyster shells. By incorporating certain properties of natural materials, or in some cases the materials themselves, they confirmed that there are many ways to concoct alternative resin formulations that are safer for life.
Approach 1: Replace the Photoinitiator
This and the second approach can be considered incremental changes to the PR48 formulation, addressing either the photoinitiator or aspects of the acrylates. These alternative bio-based components have the potential to be more sustainably sourced than their petroleum-based counterparts, which are currently used in commercial SLA resins. When used in combination, these incremental changes could increase safety and decrease costs, thus lowering hazards significantly and enabling new applications.
Approach 2: Modify the Resin Acrylates
Since photo-curable monomers such as acrylates make up nearly all of a formulation like PR48 (>99% by weight), only replacing the photoinitiator with a curcumin-riboflavin system will not greatly alter potential exposure hazards during resin use and disposal. However, combining that approach with acrylate modifications has the potential to significantly improve the overall hazard profile of PR48 and resin formulations based upon it.
Approach 3: Create an Entirely New Resin Formulation
In order to develop much safer and truly sustainable 3D printing resins, we need to look beyond acrylate-based resins and consider how to create entirely new formulations. The reproductive and aquatic toxicity of acrylates at the materials handling and disposal stage is reason enough to pursue such a new and different approach to photopolymerization.
To that end, the research team investigated ways to use localized pH change in an SLA (light-initiated) process to solidify non-acrylate liquid resin. This is an exciting, uncharted area of exploration that could change additive manufacturing greatly. It represents an opportunity to additively manufacture in a process much closer to biological systems. Non-acrylate-based formulas could have a much more benign effect on human and environmental health. It may even be possible to source their chemical inputs from waste streams and recycled material, in a cradle-to-cradle fashion.
This ground-breaking research inquiry is part of an early stage in the evolution of additive manufacturing. Materials developers, manufacturers, product designers and others in the additive manufacturing ecosystem will all have a role to play in creating and implementing the most appropriate and biofriendly solutions. The collaboration between industry and academia demonstrated here advances a deeper understanding of what’s possible. We hope that by sharing insights from this collaboration, we can foster the development of innovative SLA resins that are safer and healthier for people and for our planet.
 Compatible with living tissue, not eliciting negative tissue or system responses
 A connectivity transition from a set of independent molecules to a percolated (connected) network that is effectively a single molecule.