Nature achieves sophisticated complexity in function and composition. This is typically from spatiotemporal control over incorporation of key elements from a vast array of fundamental precursors. In this way, multimaterial composition is constructed “bottom-up,” from molecules to systems. This was inspiring to us as we considered how we might achieve single 3D printed parts bearing controlled, varied physicochemical properties as a function of their geometry. Across the additive manufacturing (AM) landscape, several powerful techniques are found for selective deposition of materials. These include material extrusion with multiple extruders, multi-jetting techniques, and dual-powder fusion systems (Figure 1).
Figure 1. Generalized depictions of common multimaterial AM technologies.
Vat photopolymerization methods, such as Digital Light Processing AM (DLP-AM), were particularly exciting to us as chemists because these systems convert liquid monomer photoresins into polymer network structures during the printing process. In this way, the printing process might be viewed as spatially controlled synthesis, rather than material deposition. It was that perspective that triggered our thinking about using an all-in-one multicomponent vat and differentiating material synthesis based upon patterns of different wavelengths of light (Figure 2). Using two different wavelengths to initiate two orthogonal polymerization mechanisms would provide access to spatial control of chemical composition simply by changing the wavelength patterns and combinations. Notably, DLP-based AM systems can take advantage of patterned images across an entire 2D build layer in parallel and project whatever colors of light are available from their color wheels. At the outset of our project, our vision was that we could take any multicolor image that we could create in PowerPoint and reflect that diversity of color with diversity of material composition. Changing the patterns and colors of sequential images would ultimately give complete compositional control along all three axes of the printed object. Of course, that was overly optimistic although it did help motivate us through this initial demonstration of multimaterial actinic spatial control (MASC) 3D printing.
Figure 2. An idealized depiction of MASC AM.
Instead of spanning a broad range of chemistries with MASC, we focused instead on demonstrating the concept with resin compositions that were already generally known in the vat photopolymerization communities. Specifically, curing of acrylates via radical mechanisms and epoxides via cationic mechanisms. To keep the chemistry simple, we met the demand part way between chemical and engineering optimization. That is, we adjusted the formulations’ chemistry and also built a custom dual-projector 3D printer that could provide UV and visible light inputs (Figure 3, top). This enabled us to use simple photoinitiator systems. Under visible light irradiation, only radical-initiated acrylate monomers polymerize. Under UV irradiation, both the radical and cationic photoinitiators absorb, resulting in polymerization of both the acrylate and epoxide monomers. Therefore, patterned combinations of light input were expected to result in a patterned 2D multimaterial layers. We envisioned the printing process to involve synthesis of interpenetrating networks with compositional variations that correlate to the patterned combinations of UV and visible light. In our system, the epoxide monomers produce stiff hydrophobic materials and the acrylates produce soft (flexible) materials that swell in various solvents. The contrast in mechanical and physical properties was chosen largely as an easily observable outcome of the spatial control – the source of the differences in properties, of course, originating at the molecular level.
Figure 3. (top) A schematic of our dual-projector MASC DLP-AM system. (bottom) Representative examples of multimaterial parts printed with MASC DLP-AM.
We were successful in demonstrating that MASC DLP-AM enables spatial control of composition along all three axes of printing, making complex objects not immediately accessible by other techniques (Figure 3, bottom). We demonstrated this within our paper by creating a complex lattice framework with composition-controlled differences in response to mechanical loading based upon the axis of compression. Along one axis, the object is ca. 26 times stiffer than the other. Swelling-induced actuation was also demonstrated from sea stars printed with MASC DLP-AM. In these specimens, combinations of epoxy- versus acrylate-dominated regions guided the differential swelling to cause the sea stars to curl up in water or organic solvents.
Figure 4. A multimaterial lattice that displays anisotropic mechanical properties despite high geometric symmetry.
We think there is considerable potential for MASC AM. This initial demonstration signifies viability of the concept and encourages us to pursue many avenues of development. AM has a wonderful characteristic of being a centerpiece for interdisciplinary work. As we move forward, we see unmet needs and exciting opportunities to improve the chemical efficiency of curing, broaden the scope of materials combinations, increase the efficiency and accessibility of the custom equipment, develop models for the non-equilibrium processes involved in graded network formation, and integrate software that enables cohesion from the designers’ imaginations to the product in the hands of the makers.
For more details, see our paper “Multimaterial Actinic Spatial Control 3D and 4D Printing” here.