Multi-material 3D printing produces expandable microlattices
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hree-dimensional (3D) printing technology has transformed the way we conceive materials as it allows for bottom-up fabrication of customized designs and 3D shapes. However, 3D printing is generally limited to the printing of a single material composition. Being able to spatially vary the materials composition and mechanical properties along with the 3D architecture would be a major step forward to harness unusual behaviors such as enhanced ductility or fracture toughness, zero or negative thermal expansion, or to achieve biomimetic structures. Ductility, for example, can be amplified by building gradients from soft to hard components, which is observed in bone-tendon or bone-ligament connective tissues. But forming 3D printed materials with dissimilar compositions and mechanical properties is challenging. The interfaces need to be tight enough to prevent unbinding, while remaining sharp enough to avoid blending of the two materials and homogenization. In an article published recently in Scientific Reports (doi:10.1038/s41598-018-26980-7), Xiaoyu (Rayne) Zheng and his student Da Chen from Virginia Tech introduced an additive manufacturing method where 3D porous architectures can have spatially varying mechanical properties. The reported additive manufacturing tool is a 3D printer with multiple extrusion nozzles that deposit struts onto a transparent window (Figure a). The struts are blends of acrylate monomers mixed at various ratios to offer low or high stiffness. After each layer is deposited, the structure is strengthened by UV-curing using UV light placed below the transparent window. To change the chemical composition without cross-contamination between the inks, a robotic device cleans the printing tip quickly and efficiently. This novel addition is unique in 3D printing systems and is pending an international patent. Exploiting the full capabilities of traditional 3D
printing, the researchers a printed porous microlattices of 1–2 cm, with a strut diameter of ca. 100 µm and with different geometries. One major interest of Zheng’s research group is in “morphing structures for lightweight flight structure designs, bioinspired morphing b 0 materials, flexible body c –0.1 armor with enhanced energy absorption, and –0.2 fracture resistance, as –0.3 well as active materials.” –0.4 Numerical Therefore, building miAnalytical –0.5 Experimental crolattices enables light–0.6 weighting due to the high 10–3 10–2 10–1 1 10 102 103 porosity, whereas control Er/Ev of the local mechanical stiffness and microar(a) Schematic of the multi-material 3D printing apparatus. chitecture can enable (b) Variation of the Poisson’s ratio in function of the ratio uncommon properties. Er/Ev, which is the ratio between the Young’s modulus of two orthogonal phases in the lattice presented in the inset (c). An example discussed Credit: Scientific Reports. in the article is auxetic structures with negative Poisson’s ratio—when the structure is stretched, the width can She is very curious to see if the concept extend up to seven times! This non
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