Alternative Co-Initiators for Photocurable Biomaterials: Polymerisation, Quantum Yield of Conversion and Cytotoxicity

Cyclic acetals such as 1,3 benzodioxole (BZD), piperonyl alcohol (PA), and N-phenyl glycine (NPG) are naturally occurring compounds capable of acting as co-initiators during free-radical polymerisation, and potentially serve to offer non-allergic and biologically less toxic alternatives to conventional (tertiary) amines. The current study aimed to evaluate the use of cyclic acetals in concentrations of 0.5-6.0 mol % for photopolymerisation of an unfilled model dimethacrylate resin system, bisphenol-A-glycidyldimethacrylate (BisGMA), and triethylene glycol dimethacrylate (tegDMA), 1:1 wt.%) using the photosensitiser camphorquinone (CQ; λmax = 469 nm) in concentrations of 0.5-1.5 mol %. Conventional tertiary amines, namely, ethyl-4-dimethyamino benzoate (EDMAB) (aromatic) and dimethylaminoethyl methacrylate (DMAEMA) (aliphatic) were utilised for comparison purposes. Real-time degree of conversion (DC) was evaluated using Fourier transform near-infrared spectroscopy. To measure the effectiveness of these alternative compounds in facilitating CQ photon absorption, quantum yield of conversion of CQ per absorbed photon was calculated using UV-Vis spectroscopy. Cytotoxicity of NPG and cyclic acetals were assessed using MTT and Calcien AM to determine metabolic activity and cell viability, respectively. The result indicated that cyclic acetals were capable of facilitating free radical polymerisation as co-initiators at all three CQ concentrations (0.5, 1.0 and 1.5 mol %). Further, the use of NPG as a co-initiator resulted in post-irradiation DC values that were comparable to both EDMAB and DMAEMA across all three CQ concentrations. Based on the quantum yield of conversion of CQ, alternative compounds facilitated the hydrogen abstraction process, which provided high conversion of CQ molecules. UV-Vis absorption and quantum yield data revealed that NPG exhibited competitive effects in absorbing blue light photons, which might be attributed to the photolytic degradation of NPG and the formation of N-methylaniline. Indirect treatment of human dental pulp cells (HDPCs) with eluted extracts of NPG, BZD and PA exhibited relatively lower cytotoxicity when compared with HDPCs groups treated with no eluted extracts. The relative cytotoxicity of DMAEMA was comparable to that of PA. However EDMAB, CQ and the neat model resin media extracts resulted in significant cytotoxicity, with highest relative reduction in the number of viable cells observed in groups treated with neat model resin extracts, suggesting that bisGMA and tegDMA monomers played significant role in the overall cytotoxicity of photocurable systems involving HDPCs.