Continuous Volumetric 3D Printing: Xolography in Flow

Various printing techniques have emerged since Charles Hull's introduction of commercial 3D printing in 1980, with vat polymerization and technologies like stereolithography (SLA) and digital light processing (DLP) standing out with high resolution and fine surface finish. Despite impressive resolutions of up to 600 nm for SLA and about 75 µm for DLP, a fundamental trade-off between resolution and printing speed persists, limiting the applicability of 3D printing in high-throughput scenarios like scaffolds in bottom-up tissue engineering. For growing specific tissues on centimeter scale, millions of biocompatible building blocks with controllable micron-sized porosity are required to enable cell interpenetration and nutrient supply. Traditional AM methods that rely on the layer-by-layer approach, are not yet able to close the gap between high production rates and high resolution.<br/>The next step in rapid additive manufacturing is represented by volumetric 3D printing, such as xolography. Unlike traditional layer-by-layer approaches, volumetric printing directly polymerizes a defined resin volume, allowing for arbitrary geometries while maintaining high resolution at rapid printing speeds. Xolography is a linear volumetric 3D printing method that utilizes dual-color polymerization for precise volumetric curing. A photoswitchable photoinitiator switches from a dormant to a latent state by illumination with the first wavelength in the UV-light spectrum. Upon illumination with the second wavelength, polymerization is initiated. Based on this principle, the polymerized volume can be set by projecting an image of the second wavelength onto a UV-light sheet. By moving the resin through the UV-light sheet and synchronizing a sequence of images with this movement, complex 3D geometries can be produced in seconds to minutes.<br/>Xolography emerges as a possible choice for medical applications like tissue engineering, owing to its ability to achieve high printing speeds while maintaining high resolution, even with marginal photoinitiator concentrations (0.01wt%) - which can be a safety concern in biomedical contexts - and the use of high-viscosity resins, making it compatible with biodegradable high-molecular photopolymers.<br/>In this study, Xolography is transformed from a batch process into a continuous fabrication method by vertically flowing resin through the UV-light sheet. The sequence of the projected images is synchronized to the resin velocity at the area of both intersecting wavelengths. The flow profile is visualized by computational fluid dynamic simulations. By designing a symmetrical flow cell with four inlets, the flow profile can be adjusted by the ratio of the volume flow through each inlet and is subsequently flattened in the intersecting area of both wavelengths for an enhanced printing resolution.<br/>A second prerequisite for continuously printing objects via xolography is the circumvention of unwanted polymerization by the UV-light only. At increased radiant fluxes, polymerization occurs without the second wavelength. This is particularly the case at the glass-resin boundary where the resin velocity is reduced, and the radiant flux received by the resin is maximized. By integrating oxygen-permeable side windows to the flow cell, oxygen can diffuse into the resin, quench the photoinitiator in the boundary layers and thus prevent unwanted curing.<br/>With these modifications, the continuous xolography process achieves recognizable feature sizes of up to 10 µm in x/y and 25 µm in z-direction. Objects can be printed in parallel at a minimum object distance of 80 µm to utilize the entire printing area with constant resolution and print speed, showcasing the flexibility and potential for upscaling continuous volumetric 3D printing via xolography. This innovative approach closes the gap between high volume generation rate at high resolutions and holds great promise for biomedical applications.