Triplet Fusion Upconversion for Photocuring 3D Printed Particle-Reinforced Composite Networks.

High energy photons (λ<400 nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge to additive manufacturing of particle-reinforced composite networks since deep light penetration of short-wavelength photons limits the homogeneous modification of physicochemical and mechanical properties. Herein, we employ the unconventional, yet versatile multiexciton process of triplet-triplet annihilation upconversion (TTA-UC) for curing opaque hydrogel composites created by direct-ink-write (DIW) 3D printing. TTA-UC converts low-energy red light (λ max = 660 nm) for deep penetration into higher-energy blue light to initiate free radical polymerizations within opaque objects. As proof-of-principle, hydrogels containing up to 15 wt% TiO 2 filler particles and doped with TTA-UC chromophores were readily cured with red light, while composites without the chromophores and TiO 2 loadings as little as 1-2 wt% remained uncured. Importantly, this method has wide potential to modify the chemical and mechanical properties of complex DIW 3D printed composite polymer networks. This article is protected by copyright. All rights reserved.