Available on-demand - *S.SM09.01.03
Material Horizons for Volumetric Additive Manufacturing
Maxim Shusteff1,Caitlyn Cook1,Johanna Schwartz1,Erika Fong1,James Oakdale1,Bryan Moran1,Allison Kaczmarek1,2,Hossein Heidari3,S.M. Luk3,Charles Rackson4,Robert McLeod4,Hayden Taylor3
Lawrence Livermore National Laboratory1,Clemson University2,University of California, Berkeley3,University of Colorado Boulder4
As additive manufacturing (AM, also known as 3D printing) technologies have proliferated, approaches based on solidification of photosensitive liquid resins have showed great promise due to their superior resolution and precision. However, these techniques have been largely limited to prototyping applications due to the constraints on available materials and their mechanical properties, as well as slow builds and poor surface quality resulting from layer-wise fabrication. With the advent of volumetric AM (VAM), complete 3D structures with complex geometries can now be produced in a single step, leaping over the traditional limitations of layer-by-layer approaches to 3D printing.
Volumetric 3D printing generates a 3D distribution of absorbed optical energy within a volume of photosensitive material, concurrently curing all points within a target geometry, on a timescale of 10s to 100s of seconds. No substrate is required as the part forms unsupported in the resin container. The most versatile implementation of VAM is known as computed axial lithography (CAL) which adapts the principles of computed tomography (CT) to generate a sequence of intensity-modulated projections, which are then beamed sequentially into a rotating resin container via a DLP projector to create the required energy dose. Because there is no fluid motion, and no hydrodynamic forces during a build, this approach is particularly compelling for very soft materials such as the hydrogels widely used in tissue engineering and regenerative medicine applications. The absence of support structure also enables complex geometries such as vasculature to be built.
Successful volumetric 3D printing requires spatio-temporal control over the polymerization reaction at all points within the fabrication volume. This demands a more detailed understanding of a host of process parameters, compared with traditional layered approaches. Perhaps the most important such parameters are the absorbed volumetric energy dose EVOL, as well as the rate at which this dose is delivered. The progress of the photochemical reaction may then be characterized in terms of a variety of metrics, such as double-bond conversion, or the evolution of mechanical and optical properties of the material. We investigate the inter-relationship of these properties, obtained from ex-situ measurements such as real-time FTIR spectroscopy, photo-rheology, and mechanical testing, developing a quantitative framework for predicting volumetric structure formation. An additional resin characteristic critical for CAL printing is nonlinear threshold behavior. This allows resin that receives a sub-threshold dose to remain liquid, and derives from the interaction of the generated radicals and inhibitory species present in the resin. For some chemistries the inhibitory effect is provided by dissolved molecular oxygen, but for other formulations, deliberately including another inhibitor is necessary. We discuss the implications of these interacting factors for photo-resin design within the volumetric AM paradigm, as well as more broadly in photopolymer-based AM methods. We also discuss several material classes for VAM, including high-performance engineering materials, as well as cell-culture compatible hydrogels.
Kelly, B. E., Bhattacharya, I., Heidari, H., Shusteff, M., Spadaccini, C. M., Taylor, H. K., “Volumetric additive manufacturing via tomographic reconstruction” Science 2019, 363, 1075-1079.