Green-to-Red Photopolymerization via Dual-Role Dyes for Transparency-Tunable 3D-Printed Hydrogels

Light-based 3D printing typically achieves high speed and resolution through multicomponent photopolymer resins that combine a separate photoinitiator and photoabsorber. Here, we show that a single visible-light dye can perform both roles at onceacting as a photoinitiator and as an optical attenuatorso that print speed, cure depth, resolution, and the final optical clarity of the construct are all governed by one component. The photobleaching and in situ photorheology of five green-to-red dyes (methylene blue, Azure A, thionine, Eosin Y, and Erythrosin B) are compared to establish which dyes combine efficient gelation with near-complete loss of their colored band. Using methylene blue as a model, dye concentration alone is shown to tune critical exposure energy (E _c = 199-909 mJ cm^-2), cure depth, transparency, and printed morphology across distinct, reproducible printing regimes, without any auxiliary photoabsorber. Postcured green- and red-light DLP prints become nearly colorless, and their color stability is shown to depend on pH. This single-dye strategy provides a simple, generalizable route to traceless, optically tunable hydrogels for biomedical applications ranging from transparent corneal implants to opaque dermal scaffolds.