Visible-Light-Fueled Polymerizations for 3D Printing

ConspectusLight-driven polymerizations and their application in 3D printing have revolutionized manufacturing across diverse sectors, from healthcare to fine arts. Despite the popularized notion that with 3D printing "imagination is the only limit", we and others in the scientific community have identified fundamental hurdles that restrict our capabilities in this space. Herein, we describe the ZAP group's efforts in developing photochemical systems that respond to nontraditional colors of light to elicit the rapid, spatiotemporally controlled formation of plastics. Our research addresses key limitations in current photopolymerization methods, such as the reliance on high-energy UV light, oxygen sensitivity, and narrow materials scope. We present a comprehensive overview of our advancements in both light-fueled radical and nonradical chemistry and its implementation in vat photopolymerization 3D printing using panchromatic resins. In radical chemistry, we have developed a class of boron dipyrromethene (BODIPY) dye molecules that act as photoradical generators (PRGs). Upon exposure to visible or near-infrared (NIR) light, these molecules induced efficient polymerization of acrylics. Structural modifications, including the installment of halogens, twisted aromatic groups, nitrogen bridgeheads, and thiophenes, have imbued activity across this wide spectral range. Systematic photophysical characterization of these dyes revealed the presence of long-lived excited (high in energy) states, from which we accredited the enhancements in polymerization efficiency. In turn, curing (converting a liquid to solid) with low intensity visible-to-NIR light was possible in mere seconds; a requirement for many light-based 3D printing technologies. Our efforts in nonradical chemistry have been motivated by the need for new materials with properties and functionality currently inaccessible using radical-based 3D printing approaches (e.g., tough and recyclable), while also providing an avenue toward multimaterial fabrication. We have developed photobase generators (PBGs) - dyes that release basic cargo upon light exposure-to catalyze polymerizations beyond acrylic-only resins. These include coumarinylmethyl- and BODIPY-tetramethylguanidine (TMG) derivatives, as well as onium photocages, which enabled photocuring of thiol-ene and thiol-isocyanate resins. Lastly, we have pioneered rapid, high-resolution visible-to-NIR light-based 3D printing. Our work includes the development of reactive photoredox catalyst systems for speed, additives for oxygen-tolerance, NIR-light reactivity for nanoparticle composites, models for streamlined optimization, and triplet fusion for high resolution. These advancements led to build speeds up to 45 mm/h with features <100 μm, rivaling contemporary UV-based technologies. The impact of our research extends beyond academic interest, offering practical solutions for additive manufacturing of (multi)functional materials. By enabling the use of lower-energy light sources, our work paves the way for environmentally friendly, cost-effective, and versatile 3D printing. It opens new possibilities for printing with previously incompatible materials, including UV-sensitive compounds and high-refractive-index nanocomposites. Nascent developments in multimaterial 3D printing via color- and dose-controlled light exposure are enabling the production of objects with precise placement of materials having disparate composition and properties. As we continue to develop photopolymerizations and light-based 3D printing, we anticipate transformative applications in fields ranging from tissue engineering to advanced electronics manufacturing. This will bring the community one step closer to fulfill the dream of creators only being "limited by imagination".