Additive manufacturing – also called 3D printing – has revolutionized many fields and has spurred researchers to investigate manufacturing techniques at the micro- and nanoscale.
One such technique is multi-photon lithography, where a volume of resin is exposed to a series of focused high-intensity laser pulses, which causes the resin at the focal point to polymerize almost instantly. By moving the laser focal point and/or the resin, 3D shapes can be built. While useful for research and proof-of-concept applications, this method of scanning a single laser point has limited potential for scaling up printing throughput. Therefore, a significant challenge is presented in how to make this unique printing process more feasible for large scale manufacturing.
In a new paper published in Light: Science & Applications, a team of scientists at Purdue University have combined multi-photon lithography with spatiotemporal two-dimensional projection imaging of femtosecond laser pulses to demonstrate rapid, continuous, and repeatable 3D printing of complex 3D structures.
"Multi-photon lithography has been studied for more than 20 years," said Paul Somers, PhD student and lead author of the paper. "It has great potential, but it's still slow. Our goal is to speed it up and scale it up, so that it can be used to manufacture useful structures with high fidelity."
To achieve this fidelity, they implement spatiotemporal focusing in a continuous manner for printing. A digital micro-mirror device (an array of tiny mirrors controlled by a computer) separates the laser into multiple wavelengths of light and then recombines them, accurately forming an image in the area of the resin they want to print. With this technique, they can print an entire 2D layer instantly, while continuously moving the platform up to print many layers.
"You see this with typical 3D printers, where it deposits a layer of material, and then moves up to deposit another layer, leaving you with jagged edges," said Somers. "But the breakthrough in our technique is that the platform moves continuously; rather than distinct layers, it's one smooth shape all the way up. This allows us to print much faster, with no layering artifacts, and make complex shapes that traditional 3D printing otherwise can't do." They demonstrated this technique by printing complex, shapes such as a microscale trefoil knot, and a miniature replica of the Cloud Gate sculpture in Chicago – each printed in less than a second.
To print larger structures, they used repetition and demonstrated construction of a metamaterial-like structure: printing a single shape, and then moving the platform to repeat the process multiple times. They printed 74,088 tiny shapes or unit cells into a 42 × 42 × 42 unit cube, nearly 1 millimeter in width. This demonstrated that multi-photon lithography has the potential to build highly complex structures, with customized designs, in a practical time frame.
"This work is a result of collaboration with two other research groups," the lead-Principal Investigator of the project, Prof. Xianfan Xu, the James J. and Carol L. Shuttleworth Professor of Mechanical Engineering at Purdue University added, "Prof. Liang Pan's group in Mechanical Engineering of Purdue University and Prof. Bryan Boudouris's group in Chemical Engineering of Purdue University. The teamwork among a number of researchers made this work possible".
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