Visible light-driven photopolymerization for 3D and 4D printing: mechanisms, materials, and programmable...

Visible light-driven 3D printing has evolved beyond a mere substitution of ultraviolet irradiation and now represents a mechanistically redefined platform for additive manufacturing. As outlined in this review, the transition to visible wavelengths has required a fundamental reconsideration of photophysical and photochemical principles that govern photopolymerization, reshaping the design of photoinitiating systems, and processing strategies. We first discuss how the transition from ultraviolet to visible irradiation has reshaped molecular design strategies in photopolymerization, prompting the development of new photoinitiating systems tailored to the lower-energy visible regime. This shift has expanded the range of accessible activation pathways and enabled diverse polymerization mechanisms, encompassing free-radical, controlled radical, cationic, and other non-radical processes. We examine how these chemically distinct approaches have been translated into practical visible light-based 3D printing platforms. Beyond reaction mechanisms, we analyze formulation and optical considerations that uniquely influence curing performance under visible light, including spectral matching, light penetration, inhibition effects, and the balance between resolution and build speed. Finally, we highlight strategies that leverage wavelength selectivity and multi-component photochemistry to enable multimaterial fabrication and advanced applications. Collectively, this review integrates mechanistic insight with materials and process design, providing a comprehensive perspective on the current landscape and future directions of visible light-based 3D printing.