A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 1 7 2 3D PRINTSHOP Tiny Eiffel Tower produced by 3D printing method that can change printing direction on the fly. The printing path is shown on the right. Courtesy of Northwestern University. ‘ON THE FLY’ 3D PRINTING WITH A TWIST A team of engineers from Northwestern University have developed a new method that uses light to improve 3D printing speed and precision while also, in combination with a high-precision robot arm, providing the freedom to move, rotate, or dilate each layer as the structure is being built. The method introduces the ability to manipulate the original design layer by layer and pivot the printing direction without recreating the model. This “onthe-fly” feature enables the printing of more complicated structures and significantly improves manufacturing flexibility. The Northwestern process uses a robotic arm and a liquid photopolymer that is activated by light. Sophisticated 3D structures are pulled out from a bath of liquid resin by a high-precision robot with enhanced geometric complexity, efficiency, and quality compared to the traditional printing process. The arm is used to change the printing direction dynamically. “We are using light to do the manufacturing,” says Cheng Sun, associate professor of mechanical engineering at Northwestern’s McCormick School of Engineering. “Shining light on the liquid polymer causes it to crosslink, or polymerize, converting the liquid to a solid. This contributes to the speed and precision of our 3D printing process.” In a paper published in the journal Advanced Materials, researchers demon- strate several applications, including 3D printing a customized vascular stent and printing a soft pneumatic gripper made of two different materials, one hard and one soft. A double helix and a tiny Eiffel Tower are two other printed examples in the study. northwestern.edu. 3D PRINTING TO FINETUNE PHOTONIC CRYSTAL FIBERS Fiber optics are conventionally produced by drawing thin filaments out of molten silica glass down to microscale dimensions. By infusing these fibers with long narrow hollow channels, a new class of optical devices termed “photonic crystal fibers” were introduced. “Photonic crystal fibers allow you to confine light in very tight spaces, increasing the optical interaction,” explains Andrea Bertoncini, a postdoc at King Abdullah University of Science and Technology. “This enables the fibers to massively reduce the propagation distance needed to realize particular optical functions, like polarization control or wavelength splitting.” One way researchers tune the optical properties of photonic crystal fibers is by varying their cross-sectional geometry or arranging them into fractal designs. Typically, these patterns are made by performing the drawing process on scaled-up versions of the final fiber. Not all the geometries are possible with this method, however, due to gravity and surface tension. To overcome such limitations, the group turned to 3D printing. Using a laser to transform photosensitive polymers into transparent solids, the team built up photonic crystal fibers layer by layer. Characterizations revealed that this technique could successfully replicate the geometrical pattern of several types of microstructured optical fibers at faster speeds than conventional fabrications. Bertoncini explains that the new process also makes it easy to combine multiple photonic units together. They 3D printed a series of photonic crystal fiber segments that split the polarization components of light beams into separated fiber cores. A custom-fabricated tapered connection between the beam splitter and a conventional fiber optic ensured efficient device integration. “Photonic crystal fibers offer scientists a type of ‘tuning knob’ to control light-guiding properties through geometric design,” says Bertoncini. “However, people were not fully exploiting these properties because of the difficulties of producing arbitrary hole patterns with conventional methods. The surprising thing is that now, with our approach, you can fabricate them. You design the 3D model, you print it, and that’s it.” www.kaust.edu.sa/en. Photonic crystal fibers are built layer by layer at much faster speeds than conventional methods. Courtesy of Anastasia Serin/KAUST.
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