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Volumetric Additive Manufacturing (VAM) is a novel light-based 3D printing technique which radially exposes a volume of material from 0 to 360º with different grayscale images to fabricate arbitrary 3D geometries within the material. Here, we leverage the ability to print grayscale structures into a solid volume of material with VAM to 3D print gradient index (GRIN) lens refractive index profiles into a flexible photopolymer. To realize 3D printed flexible GRIN lenses, the patterning optics and the gradient index material were co-designed and experimentally characterized. We also discuss further work needed to 3D print high quality, flexible GRIN lenses.
VAM offers new approaches to additive manufacturing (AM) that are not possible with traditional AM techniques. One advantage over other AM techniques is the lack of layering effects due to the simultaneous printing of an entire 3D part as opposed to layer-by-layer deposition printing. Like other light-based AM techniques, control over local material properties is realized by modulating local polymer conversion. In a region of material, the product of exposure intensity and time determines the degree of conversion of that region, giving a method to spatially control conversion and thus the refractive index locally within a print. Additionally, due to the ability to print into a volume of material with VAM, we can print into solid volumes of materials.
A solid, flexible photopolymer consisting of a low refractive index polyurethane host matrix, a high refractive index acrylate writing monomer, and a low absorption photoinitiator was developed for solid VAM printing. Upon exposure, polymerization and subsequent monomer transport occurs, changing the composition and index of refraction of that region dependent on exposure conditions. The grayscale images required to fabricate a flexible GRIN lens are computed by filtered back projection and optimized through an iterative algorithm for a more accurate GRIN profile.
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