Embedded 3D Printing of Polymeric and Ceramic Lattices

Abstract

From the insulating properties of cork to the high strength of lightweight bird bones, lattices with exceptional properties are abundant in nature. These natural structures are constructed of materials spanning from purely polymeric (e.g., wood and silk) to highly mineralized ceramics (e.g., shells and bone) that are arranged in astoundingly complex architectures. Recent advances in computational design and 3D printing have enabled the fabrication of synthetic lattices that mimic their natural counterparts. However, light-based methods, such as stereolithography, are typically confined to monolithic photopolymerizable organic and preceramic polymers, while ink- based 3D printing methods, such as direct ink writing, are limited in geometric complexity due to their layerwise build sequences. My Ph.D. dissertation focuses on the freeform fabrication of polymeric and ceramic lattices via embedded three-dimensional (EMB3D) printing. To fully leverage the omnidirectional printing capability of EMB3D printing, we first developed an algorithm, Automated Eulerian Route Optimization (AERO), to generate print paths for arbitrary lattices whereby individual struts can be arbitrarily printed in 3D. This contrasts with the existing layer-based segmentation approaches of conventional slicing typically used in 3D printing. By freeform printing struts we can overcome the fundamental tradeoff between resolution (small steps) and speed (large steps) in layer-based approaches. Our integrated approach, based in graph theory, has enabled the rapid design of the most complex print paths for EMB3D printing to date. Building on this advance, we designed inks for the EMB3D printing of polymeric lattices in periodic and stochastic motifs. We investigated the effects of ink rheology on filamentary printing and effects of the print path and orientation on resultant mechanical properties. By co- printing multiple polymeric materials with different mechanical properties within a silicone support matrix, a broad range of lattice architectures and mechanical responses were produced, opening new avenues for constructing architected 3D lattices. To demonstrate the utility of AERO, a stochastic 3D lattice with more than 1000 struts, based on the Stanford bunny model, is printed. To further demonstrate the versatility of EMB3D printing, we created architected ceramics with spatially controlled composition in freeform shapes. Aqueous colloidal inks are first printed within a silicone support matrix, then rapidly cured via microwave-activated polymerization, and finally dried and sintered into dense architectures composed of one or more oxide materials. As exemplars, both high-temperature resistant multi-ceramic lattices and chain-link architectures are created. Their compressive and tensile properties were tested, and micro-computed tomography was used to assess the presence of defects (e.g., bubbles, delamination, or drying cracks). In summary, we show for the first time that EMB3D printing can be used to create robust multimaterial lattices composed of a broad palette of polymeric and ceramic materials. Specifically, we combined graph-based print path planning, ink rheology optimization, and thermal curing (with and without microwave activation) to generate complex periodic and stochastic lattices in a freeform manner. We envision this method being broadly applied for fabricating lightweight structural components to bone scaffolds to battery electrodes and catalyst supports.