Abstracts

Photopolymer 3D printing methods face a constraint: the need for sequential layer deposition, preventing multi-material fabrication. Our study introduces a novel approach enabling multi-material stereolithography [1], transcending this layer-by-layer limitation. This advancement hinges on selectively crosslinking voxels within the stereolithography (SLA) resin vat at any depth and position. The technique uses the invisibility windows and upconversion for targeted volumetric crosslinking. A developed photopolymer resin incorporates a visible light photoinitiator (fluorinated diphenyl titanocene) with broad absorption, fine-matched with an optical absorber (naphthalimide class dye, peak absorption at 445 nm) and upconversion emissions from lanthanide-doped. To harness not only UV but also the more pronounced blue light (425-500 nm) emitted by Tm3+-doped phosphor, we incorporated a visible light initiator that boasts broad absorption up to 500 nm, while providing high initiating activity, storage stability, and low toxicity. Consequently, this allowed for the use of a lower concentration (5 mg mL-1) of UC phosphor compared to previous studies, leveraging the 425-500 nm emissions effectively for photopolymer cross-linking.Employing NIR light for upconversion phosphor excitation facilitates remarkable penetration depths of up to 5.8 cm. This has enabled for the first time the application of UC as a cost-efficient pathway for multi-colour and multi-material in-volume stereolithography. To enhance resolution and mitigate over-curing effects, we introduced a naphthalimide class dye, tailored for dissolution in acrylic resins. A minor adaptation of a desktop FDM printer [2] and a 980 nm laser diode enabled in-volume 3D printing of photopolymer containing Tm3+-doped UC phosphor at a power density of 14 W cm−2, significantly lower than the used by femtosecond lasers for TPP. Moreover, a modified resin enabling copper plating was utilized to print a track within a cavity of a different material, resulting in a conductive path with low sheet resistance[3]. Demonstrations include volumetric SLA printing of intertwined multi-colour samples, versatile rigid/flexible (acrylate/elastomer) samples, and dielectric/metallic plated samples.This breakthrough unlocks diverse avenues in stereolithography, enabling the creation of samples with varied materials, 3D printing within cavities using different materials, crafting 3D circuitry, repairing broken objects, and more.[1] Adilet Zhakeyev, Mansour Abdulrhman, et al, “Upconversion 3D printing enables single-immersion multi-material stereolithography”, Applied Materials Today, 32, 2023, 101854.[2] Adilet Zhakeyev, Rohith Devanathan, Jose Marques-Hueso, “Modification of a desktop FFF printer via NIR laser addition for upconversion 3D printing”, HardwareX, 18, 2024, e00520[3] Adilet Zhakeyev, Fenella Walker, et al, “Upconversion 3D printing enhancement via silver sensitization to enable selective metallization”, Optical Materials, 142, 2023, 114044