Photoinitiators are compounds that, when added to a formulation, convert light energy into chemical energy. These compounds produce reactive species that can initiate or catalyze chemical reactions by absorbing light, altering the physical properties of different materials. To be used in biomedical applications, photoinitiators must be biocompatible, soluble in water, and with low cytotoxicity [1,2].
Photopolymerization is a simple, clean, and convenient method for obtaining covalently crosslinked hydrogels using photoinitiators with an absorption peak at a specific wavelength to initiate radical polymerization. This method is used to control the formation and structure of hydrogels spatially and temporally. Photopolymerization is widely implemented using ultraviolet (UV) light; however, this type of light can cause damage to cells during exposure . Alternatively, near-UV light can be used (~ 405 nm), which is less harmful to cells.
A replacement for UV light is visible light. With it, the hydrogel system can achieve higher cell compatibility and broader application possibilities. In addition, its high penetration depth makes hydrogels result in a more uniform structure. The visible light initiators can be classified into free radicals and cationic according to their active polymerized species. Free radical photoinitiators can be divided into type I (one component pyrolysis) and type II. Type I photoinitiators absorb the incident photons, splitting them into two primary radicals when exposed to light. Type II, known as photosensitizing photoinitiators or co-initiators, extract hydrogen from the co-initiator to generate secondary radicals. In the current scenario, the ruthenium bipyridine complex, and camphorquinone, are photoinitiators that are attracting significant attention and are widely applied in tissue engineering .
Figure 1: The photoinitiation process using: A. type I initiator, B. type II initiator .
Free radical polymerization is the most used in biomedical applications since cationic photopolymerization generates strong protonic acids during initiation, negatively affecting cell cultures. The selection of a photoinitiator and the consideration of its absorption spectrum, solubility in water, ability to generate free radicals, and stability is crucial for the process of formulating excellent bioinks for 3D bioprinting approaches. This selection is vital since the type of photoinitiator and the duration of exposure to visible light can affect cell viability and the efficiency of the bioink .