Upconversion 3D printing enables single-immersion multi-material stereolithography

Photopolymer 3D printing techniques have a common limitation: the layers have to be deposited in a sequential order, which constrains multi-material fabrication to techniques where the vats have to be exchanged many times, in addition to repeating the cleaning step for each vat change. In this work, we present a technique that allows multi-material stereolithography by overcoming the layer-by-layer limitation, enabling one single im-mersion per resin. This is enabled by selective crosslinking of voxels at any depth and position inside the ster-eolithography (SLA) resin vat. The concept uses invisibility windows and upconversion to achieve selective volumetric crosslinking. This enables printing inside and through previously 3D printed parts. A photopolymer resin is developed, in which a visible light photoinitiator (fluorinated diphenyl titanocene) with a broad ab-sorption and an optical absorber (naphthalimide class dye, maximum absorption at 445 nm) are carefully matched with the upconversion emissions of lanthanide-doped phosphors (both micron-sized phosphors and customized core/shell/shell nanoparticles). The lateral printing resolution of the formulation was found to be 134 +/- 13 mu m, which was improved to 103 +/- 11 mu m through the addition of light-absorbing dye. The vertical resolution was defined by the layer thickness achieved by lowering the build platform, which was set to 100 mu m. The NIR light used for the excitation of upconversion phosphors enabled enhanced penetration depths of up to 5.8 cm, which allowed SLA printing of interwoven multi-color samples, multi-material rigid/flexible (acrylate/ elastomer) samples and dielectric/metallic plated samples. This opens up a myriad of new possibilities, such as 3D printing objects inside cavities in a different material, restoration of broken objects and artefacts, 3D circuitry, insitu bioprinting, etc.