Lateral Contrast Enhancement in Tomographic Volumetric 3D-Printing via Binary Photoinhibition
Tomographic volumetric 3D-printing (TVP) utilizes a nonlinear photoresponse of polymer precursor to cure all points in a three-dimensional (3D) object in parallel. A key challenge in TVP is to build up dose contrast between in-part and out-of-part points in a lateral plane, which relies on coordinated illumination from various projecting angles. This challenge has mainly been tackled by projection optimization. Here we show that designing material responses to photo-excitation can be a more effective way of addressing this challenge. By introducing a secondary photo-inhibitory species that reacts to external ultraviolet (UV) stimulus, we create a binary photoinhibition (BPI) system that greatly enhances the achievable dose contrast in a lateral plane. We first show that, in theory, combining dose subtraction with sufficient projection angles can guarantee an exact mathematical reconstruction of any greyscale design. We then propose a theoretical framework for BPI, in which a single stationary state with swappable stability can be used to realize dose subtraction. We use oxygen-lophyl radical pair as an approximation to show improvements in print quality enabled by enhanced dose contrast. In situ shadowgraphy shows that BPI improves the lateral patterning with various geometric features, creating differentiable changes in refractive index within 54 um or less. We show qualitative improvements in surface features and internal hollowness in physical prints. The direct impacts of UV light on the formation of positive and negative features on vertical and lateral planes of 5 workpieces are analyzed quantitatively. We conclude that introducing BPI with UV irradiation grants us direct control over the formation of negative features on the lateral plane.