The molecular builders of 3D printing: developing the next generation of safe and efficient visible light photoinitiators

Photopolymerization-based additive manufacturing, particularly vat photopolymerization (VP), is undergoing a transformation driven by the synergistic evolution of light-emitting diode (LED) technology and innovative photoinitiating systems (PISs). The development of novel Type I (cleavage), Type II (hydrogen abstraction/electron–proton transfer), and cationic photoinitiators has successfully extended the spectral sensitivity from the UV into the visible light regime. This red-shift is not merely an incremental improvement; it is a critical enabler for fabricating objects with greater depth, complexity, and speed while leveraging the energy efficiency, safety, and wavelength control of LEDs. These advances are accelerating innovation in fields from high-fidelity prototyping to biomedical engineering and dental materials. However, the full potential of these technologies is fundamentally constrained by a critical bottleneck: the (cyto)toxicity of many high-performance photoinitiators, which precludes their use in biocompatible and sustainable applications. This review provides a critical assessment of the state-of-the-art in several VP technologies and the chemistry of visible-light PISs that empower them. We critically analyze their initiation mechanisms and performance in 3D printing and, most importantly, provide a comprehensive evaluation of their cytotoxicity. By mapping the intricate relationship between chemical structure, photo-efficiency, and biological response, this review aims to establish a clear roadmap for the rational design of next-generation PISs that are not only highly efficient but also fundamentally safer and more sustainable.