Engineers from the prestigious Massachusetts Institute of Technology (MIT) recently unveiled a revolutionary method in the field of three-dimensional printing that promises to transform the way complex objects are manufactured, while significantly reducing waste. This innovation is based on the development of a special resin that changes its properties depending on the type of light it is exposed to, opening up new possibilities for faster and more environmentally friendly production.

A New Era of Photopolymerization in 3D Printing

Vat photopolymerization (VP) is one of the fundamental technologies of additive manufacturing, better known as 3D printing. This process uses light, most commonly ultraviolet (UV), to selectively cure liquid photopolymer resin, layer by layer, thereby building the desired three-dimensional object. The technology is valued for its ability to produce high-resolution objects with fine details and smooth surfaces, making it ideal for products such as personalized hearing aids, precise dental implants, mouthguards, and various other components that require individual customization and complex geometries.

The classic VP process, however, faces an inherent challenge: the need for support structures. These structures are printed along with the main product, using the same material, to ensure the stability of overhanging parts and prevent deformation during fabrication. After printing is complete, these supports must be manually removed, which is often a painstaking and time-consuming process. Furthermore, the removed supports usually end up as waste, as their reuse is rarely possible, contributing to the environmental footprint of production.

Revolutionary Resin with Dual Properties

A team of experts from MIT, located in Cambridge, Massachusetts, has offered an elegant solution to this problem. They have developed an innovative resin that can form two different types of solid materials depending on the wavelength of light acting on it. When exposed to UV light, the resin cures into an extremely strong and durable material, ideal for the final product. On the other hand, when the same resin is exposed to visible light, it also transitions to a solid state, but forms a material that is easily soluble in certain solvents.

This duality allows for the simultaneous printing of a solid object and its supports in a single integrated process. By using precisely directed patterns of UV light to create the product itself and patterns of visible light to form the support structures, engineers have eliminated the need for mechanical removal of supports. Once printing is complete, the entire assembly is simply immersed in the appropriate solvent. The supports dissolve quickly, leaving behind only the desired, solid part made with UV light.

Interestingly, various safe-to-use liquids can be used as solvents, including even baby oil. More significantly, the supports can also be dissolved in the main liquid component of the original resin, much like an ice cube melts in water. This discovery opens the door to continuous recycling of the material used for supports. After the support material dissolves, this mixture can be directly returned to fresh resin and reused for printing subsequent parts and their soluble supports, drastically reducing waste and material costs.

Demonstration of Advanced Capabilities

To demonstrate the effectiveness of their method, the researchers successfully applied the new technique to create a series of complex structures. These included functional gear assemblies, intricate lattice structures, and other objects that would traditionally require careful and lengthy removal of supports. One example was a model of a dinosaur inside an egg-shaped support structure, which elegantly dissolved to reveal a perfectly preserved figure.

"Now you can, in a single printing process, create multi-part, functional assemblies with moving or interconnected parts, and simply rinse away the supports," explains Nicholas Diaco, a graduate student and one of the lead researchers on the project. "Instead of throwing that material away, you can recycle it on-site and generate significantly less waste. That's our ultimate hope."

Details of this innovative method were published in the scientific journal Advanced Materials Technologies at the end of 2023. In addition to Diaco, co-authors of the study from MIT include Carl Thrasher, Max Hughes, Kevin Zhou, Michael Durso, Saechow Yap, Professor Robert Macfarlane, and Professor A. John Hart, Head of the Department of Mechanical Engineering at MIT.

Overcoming the Challenges of Conventional Photopolymerization

Conventional vat photopolymerization (VP) begins with a 3D computer model of the structure to be printed. In addition to the object itself, the model includes detailed support structures located around, under, and between parts of the object to ensure stability during printing. The computer model is then "sliced" into hundreds or thousands of thin digital layers that are sent to the VP printer.

A standard VP printer consists of a small vat of liquid resin above which a light source is located. Each layer of the model is translated into a corresponding light pattern that is projected onto the resin, causing it to cure according to that pattern. Layer by layer, a solid, light-printed version of the model and its supports is formed on a build platform. Upon completion of printing, the platform lifts the finished part above the resin bath. After rinsing off excess uncured resin, the phase of manual support removal follows, most often using cutting and grinding tools. This support material, as mentioned earlier, almost always ends up as waste.

"In most cases, these supports generate a large amount of waste," Diaco points out, highlighting one of the key problems the new method solves.

The Chemistry Behind the Innovation: Printing and Dissolving

Diaco and his team were looking for a way to simplify and speed up the removal of printed supports, with the ideal goal of recycling them. They devised a general concept of a resin that, depending on the type of light it was exposed to, could assume one of two phases: a durable phase that would form the desired 3D structure and a secondary phase that would serve as support material but would be easily soluble.

After detailed chemical research, the team discovered that they could create such a two-phase resin by mixing two commercially available monomers, the chemical building blocks present in many types of plastics. When ultraviolet light illuminates the mixture, the monomers bond into a tightly crosslinked structure, forming a robust, solvent-resistant material. When the same mixture is exposed to visible light, the same monomers also cure, but at the molecular level, the resulting polymer chains remain separate from each other. The solid material thus formed dissolves quickly when immersed in certain solvents.

During laboratory tests with small test tubes of the new resin, the researchers confirmed that the material indeed transformed into insoluble and soluble forms in response to UV and visible light, respectively. However, when transitioning to a 3D printer with LEDs of lower intensity than those used in laboratory conditions, the material cured by UV light disintegrated in solution. The weaker light only partially linked the monomers, leaving them too weakly intertwined to maintain structural integrity.

The solution was found in adding a small amount of a third, "bridging" monomer. This addition enabled stronger bonding of the two original monomers under UV light, creating a significantly stronger network. This modification allowed the researchers to simultaneously print durable 3D structures and soluble supports using time-coordinated pulses of UV and visible light in a single printing cycle.

Broad Spectrum of Applications and Future Directions

The new method opens the door to the production of extremely complex parts, including the aforementioned gears, lattices, and even a ball inside a square frame, where internal supports are crucial, but their removal presents a major challenge. "With all these structures, you need a network of supports inside and out during printing," says Diaco. "Removing those supports normally requires careful, manual labor. This shows that we can print multi-part assemblies with many moving parts and detailed, personalized products like hearing aids and dental implants in a fast and sustainable way."

Professor of Mechanical Engineering John Hart, one of the research leaders, adds: "We will continue to explore the limits of this process and want to develop additional resins with this wavelength-selective behavior and the mechanical properties needed for durable products. Together with automated parts handling and closed-loop recycling of dissolved resin, this is an exciting path towards resource-efficient and cost-effective large-scale 3D printing of polymers."

The potential applications of this technology are vast. In medicine, in addition to the already mentioned hearing aids and dental implants, possibilities are opening up for the creation of anatomical models for surgical planning with incredible precision, as well as for the production of orthodontic devices. In industry, rapid prototyping of complex parts, functional assemblies for machines, or even components for robotics and the aerospace industry could be revolutionized. The ability to create intricate internal channels and cavities without the risk of damage during support removal is particularly valuable.

This research, which pushes the boundaries of additive manufacturing, is supported, among others, by the Centre for Perceptual and Interactive Intelligence (InnoHK) in Hong Kong, the U.S. National Science Foundation (NSF), the U.S. Office of Naval Research, and the U.S. Army Research Office. Such broad support indicates the recognized importance and potential of this technology, not only for the scientific community but also for industrial application and defense technologies. The innovation from Cambridge thus sets new standards in the sustainability and efficiency of 3D printing.

Further development will likely focus on expanding the range of materials that exhibit this selective behavior to different wavelengths of light, as well as on optimizing the mechanical, thermal, and chemical properties of the materials thus obtained to meet the specific requirements of various industries. Integration with advanced software solutions for automated generation of support structures optimized for dissolution and the development of systems for automated parts handling and resin recycling in a completely closed loop are key steps towards the widespread industrial application of this promising 3D printing technology.

Source: Massachusetts Institute of Technology