Evaluating the polymerization effectiveness and biocompatibility of bio-sourced, visible light-based photoinitiator systems.

The use of photopolymerization is expanding across a multitude of biomedical applications, from drug delivery to bioprinting. Many of these current and emerging photopolymerization systems employ visible light, as motivated by safety and energy efficiency considerations. However, the "library" of visible light initiators is limited compared with the wealth of options available for UV polymerization. Furthermore, the synthesis of traditional photoinitiators relies on diminishing raw materials, and several traditional photoinitiators are considered emerging environmental contaminants. As such, there has been recent focus on identifying and characterizing biologically sourced, visible light-based photoinitiator systems that can be effectively used in photopolymerization applications. In this regard, several bio-sourced molecules have been shown to act as photoinitiators, primarily through Type II photoinitiation mechanisms. However, whether bio-sourced molecules can also act as effective synergists in these reactions remains unknown. In this study, we evaluated the effectiveness of bio-sourced synergist candidates, with a focus on amino acids, due to their amine functional groups, in combination with two bio-sourced photoinitiator molecules: riboflavin and curcumin. We tested the effectiveness of these photoinitiator systems under both violet (405 nm) and blue (460-475 nm) light using photo-rheology. We found that several synergist candidates, namely lysine, arginine, and histidine, increased the polymerization effectiveness of riboflavin when used with both violet and blue light. With curcumin, we found that almost all tested synergist candidates slightly decreased the polymerization effectiveness compared with curcumin alone under both light sources. These results show that bio-sourced molecules have the potential to be used as synergists with bio-sourced photoinitiators in visible light photopolymerization. However, more work must be done to fully characterize these reactions and to investigate more synergist candidates. Ultimately, this information is expected to expand the range of available visible light-based photoinitiator systems and increase their sustainability.