Researchers at UC Berkeley have developed a method—Computed Axial Lithography (CAL)—that can synthesize arbitrary geometries volumetrically through photopolymerization. In brief, it uses light to transform gooey liquids into complex solid objects in only a matter of minutes.

The CAL approach, which is described in an open-access paper in Science, has several advantages over conventional layer-based printing methods, the authors said. The method may be used to circumvent support structures as it can print into high viscosity fluids or even solids. Printing 3D structures around preexisting solid components is also possible with CAL. CAL is scalable to larger print volumes, and is several orders of magnitude faster, under a wider range of conditions, than layer-by-layer methods.

We demonstrated concurrent printing of all points within a three-dimensional object by illuminating a rotating volume of photosensitive material with a dynamically evolving light pattern. We print features as small as 0.3 mm in engineering acrylate polymers, as well as printing soft structures with exceptionally smooth surfaces into a gelatin methacrylate hydrogel. Our process enables us to construct components that encase other pre-existing solid objects, allowing for multi-material fabrication. We developed models to describe speed and spatial resolution capabilities. We also demonstrated printing times of 30–120 s for diverse centimeter-scale objects.

—Kelly et al.

CAL volumetric fabrication. (A) Underlying concept: patterned illumination from many directions delivers a computed 3D exposure dose to a photoresponsive material. (B) Schematic of CAL system used in this work. (C) Sequential view of the build volume during a CAL print. A 3D geometry is formed in the material in less than a minute. (D) The 3D part shown in (C) after rinsing away uncured material. (E) The part from (D) painted for clarity. (F) A larger (40 mm-tall) version of the same geometry. (G) Opaque version of the geometry in (F), using crystal violet dye in the resin. Scale bars: 10 mm. Kelly et al.

Nicknamed the “replicator” by the inventors—after the Star Trek device that can materialize any object on demand—a CAL 3D printer can create objects that are smoother, more flexible and more complex than what is possible with traditional 3D printers. It can also encase an already existing object with new materials—for example, adding a handle to a metal screwdriver shaft—which current printers struggle to do.

The technology has the potential to transform how products from prosthetics to eyeglass lenses are designed and manufactured, the researchers say.

I think this is a route to being able to mass-customize objects even more, whether they are prosthetics or running shoes. The fact that you could take a metallic component or something from another manufacturing process and add on customizable geometry, I think that may change the way products are designed.

—Hayden Taylor, assistant professor of mechanical engineering at UC Berkeley and senior author

Most 3D printers, including other light-based techniques, build up 3D objects layer by layer. This leads to a “stair-step” effect along the edges. They also have difficulties creating flexible objects because bendable materials could deform during the printing process, and supports are required to print objects of certain shapes, such as arches.

The new printer relies on a viscous liquid that reacts to form a solid when exposed to a certain threshold of light. Projecting carefully crafted patterns of light onto a rotating cylinder of liquid solidifies the desired shape all at once.

Taylor and the team used the printer to create a series of objects, from a tiny model of Rodin’s “The Thinker” statue to a customized jawbone model. Currently, they can make objects up to four inches in diameter.

This is the first case where we don’t need to build up custom 3D parts layer by layer. It makes 3D printing truly three-dimensional.

—Brett Kelly, co-first author

The new printer was inspired by the computed tomography (CT) scans that can help doctors locate tumors and fractures within the body. CT scans project X-rays or other types of electromagnetic radiation into the body from all different angles. Analyzing the patterns of transmitted energy reveals the geometry of the object.

Essentially we reversed that principle. We are trying to create an object rather than measure an object, but actually a lot of the underlying theory that enables us to do this can be translated from the theory that underlies computed tomography.

—Hayden Taylor

Besides patterning the light, which requires complex calculations to get the exact shapes and intensities right, the other major challenge faced by the researchers was how to formulate a material that stays liquid when exposed to a little bit of light, but reacts to form a solid when exposed to a lot of light.

The 3D printing resin is composed of liquid polymers mixed with photosensitive molecules and dissolved oxygen. Light activates the photosensitive compound which depletes the oxygen. Only in those 3D regions where all the oxygen has been used up do the polymers form the “cross-links” that transform the resin from a liquid to a solid. Unused resin can be recycled by heating it up in an oxygen atmosphere, Taylor said.

Our technique generates almost no material waste and the uncured material is 100 percent reusable. This is another advantage that comes with support-free 3D printing.

—Hossein Heidari, co-first author

The objects also don’t have to be transparent. The researchers printed objects that appear to be opaque using a dye that transmits light at the curing wavelength but absorbs most other wavelengths.

This work was supported by UC Berkeley faculty startup funds and by Laboratory-Directed Research and Development funds from Lawrence Livermore National Laboratory. The team has filed a patent application on the technique.

Resources

• Brett E. Kelly, Indrasen Bhattacharya, Hossein Heidari, Maxim Shusteff, Christopher M. Spadaccini, Hayden K. Taylor (2019) “Volumetric additive manufacturing via tomographic reconstruction” Science doi: 10.1126/science.aau7114