An overview on potential of novel photoinitiators for vat photopolymerization-based 3D/4D printing formulations

In recent years, three-dimensional (3D) printing has emerged as an outstanding technique for creating intricate 3D structures with complex geometries. Additionally, four-dimensional (4D) printing technology has attracted attention due to its ability to reshape of 3D printed structures under external stimuli over time. The application of this technique in various fields such as polymer recycling, coating, energy storage systems, medical tools, and hydrogels is due to recent advances in functional materials. Vat photopolymerization-based 3D (VP-based 3D) printing relies on photopolymerization reactions, which are commonly used in techniques such as digital light processing (DLP) and stereolithography (SLA). These techniques are associated with critical parameters such as curing kinetics, printing speed, and resolution. Photoinitiators (PIs) as one of the components of the formulation of photo-resins and photo-inks can be very effective in optimizing these parameters. There are challenges in using common photoinitiators, such as yellowing effect, migration to the surface, toxicity, and bio-incompatibility. To address these challenges, recent researches have been directed towards the synthesis of new photoinitiators based on bio-inspired compounds, inorganic particles, aliphatic structures, transition metals, fluorescent compounds, photo-sensitizers, and macro-structure-based photoinitiators. This review article aims to explore recent studies on the synthesis of new photoinitiators in the application of 3D and 4D printing. Also, it examines various chemistries in the design of photoinitiators and describes the relationship between the curing kinetic, type of monomers, and printing speed. Moreover, it highlights the potential of synthesized photoinitiators in addressing challenges with mechanical properties, resolution, and transparency of the printed part. Furthermore, this article effectively categorizes various types of photoinitiators based on organic and inorganic compounds. Finally, the article aims to generate interest and stimulate more research in the use of new photoinitiators in industrial applications and formulation towards sustainable 3D and 4D printed structures.