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.