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3D Printing: How Materials Are Printed Layer-By-Layer Using Stereolithography

Engineered materials consisting of specific patterns of nano- or microparticles embedded in a matrix can exhibit unique mechanical, electrical, thermal, acoustic, and/or electromagnetic properties, which are attributed to the specific type, geometry, and spatial pattern of the embedded particles.

Such materials have been designed for application in cloaking, sub-wavelength imaging, and multifunctional composite materials with tailored electrical and mechanical properties, among others. Manufacturing engineered materials consisting of patterns of nano- or microparticles embedded in a matrix material has been achieved via three categories of techniques, including subtractive and additive techniques, and self-assembly. These techniques are primarily constrained by material choice, the patterns of particles or features that can be fabricated, long fabrication times, dimensional scalability, and limited control of the macroscale geometry of the material specimen.

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In contrast with existing manufacturing techniques, this work uses ultrasound directed self-assembly (DSA), which relies on the acoustic radiation force associated with an ultrasound wave field to assemble patterns of particles with user-specified alignment and spatial arrangement, independent of their material properties and shape. Furthermore, combining ultrasound DSA with photo-curing enables organizing patterns of particles within a thin layer of liquid photopolymer resin, and subsequently photo-curing to polymerize the resin and fixate the pattern of particles in place. This manufacturing process enables, for the first time, 3D printing macroscale engineered materials layer-by-layer using stereolithography (SLA). In each printed layer a user-specified pattern of particles is organized using ultrasound directed self-assembly.

We have demonstrated the manufacturing process using an octagonal reservoir, lined with eight ultrasound transducers around its perimeter, which contains carbon particles dispersed in liquid photopolymer resin. Solving the inverse ultrasound DSA problem using existing theory enables computing the ultrasound transducer settings (amplitude and phase) to assemble almost any pattern of particles in an arbitrary-shaped fluid reservoir lined with any number of transducers.

A Digital Light Processing projector exposes the liquid photopolymer resin to visible/ultraviolet (UV) light through the transparent reservoir floor, which causes the liquid photopolymer resin to cross-link and fixate the pattern of particles in place. We repeat the process to 3D print the engineered material layer-by-layer, and each layer contains a user-specified pattern of particles to enable tailoring the microstructure of the material. Furthermore, we use the SLA process to control the macroscale geometry of the material.

We have illustrated the capability of the manufacturing process by 3D printing multi-layer engineered materials containing a user-specified Bouligand microstructure and engineered materials with electrically-conductive lines of nickel-coated carbon fibers. These examples demonstrate that the ultrasound DSA/SLA manufacturing process enables fabricating engineered materials with both macroscale complex 3D geometries and user-specified microstructure.

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In contrast with previous demonstrations that are limited to laboratory-scale and/or 2D implementations, this process bridges the gap between tailoring the material microstructure to obtain designer physical properties and fabricating macroscale specimens with complex 3D geometry required for implementation in engineering applications. This platform technology has significant implications in a wide range of applications including, but not limited to, manufacturing of composite materials, engineered materials for acoustic and electromagnetic cloaking and sub-wavelength imaging, and 3D printing structures with embedded electrical wiring.

This study, 3D Printing Macroscale Engineered Materials Using Ultrasound Directed Self-Assembly and Stereolithography was recently published by Bart Raeymaekers in the journal Advanced Materials Technologies. This work was funded by the Army Research Office.

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