Figure 25 (a) Schematic illustration of the photooxidation of KCD/TA and photoreduction of KCD/NPG upon visible light irradiation, respectively. (b) Schematic illustration of the bottom-up DLP 3D printing. (c) 3D-printed objects photomediated by KCD/TA and KCD/NPG, respectively.[93]
At present, UV and blue lights are the main irradiation sources in photopolymerization 3D printing.[94] Despite the convenience offered by UV and blue lights, intensity of short wavelength lights is a trade-off for better mechano-properties by uniformed polymerization and health concern of UV exposure.[63] Among the photopolymerization methodologies, NIR has a more salient role in rapid deep-curing for its remarkable penetration in various media by employing up-conversion strategies.[60, 95-96] Liu et al.[63] reported a NIR photopolymerization 3D printing strategy, and the fusion of NIR photocurable material and DIW 3D printing technology could achieve in-situ curing of thick filament with high penetration. The structure of NIR-induced DIW setup, chemical structures and photoreaction mechanism used in NIR DIW printing, and monitoring of NIR photopolymerization are exhibited in Figure 26. This enables the photocuring of deposited filaments up to 4 mm in diameter, which far exceeding any existing UV-assisted DIW, in addition, the strategy also possesses the capability of parallel fabrication to multi-color filaments and freestanding objects simultaneously.