Degree of monomer conversion upon light-curing condition, biaxial flexural strength, and surface apatite formation of orthodontic adhesive containing calcium phosphate and nisin

PurposeTo examine the effects of different curing protocols on the degree of monomer conversion (DC), evaluate the 4-week mineral-forming capability, and assess the mechanical strength of experimental orthodontic adhesives containing calcium phosphate and nisin.Materials and methodsFive experimental materials were formulated using varying concentrations of monocalcium phosphate monohydrate (MCPM) and nisin: F1 (10 wt% MCPM, 10 wt% nisin), F2 (10 wt% MCPM, 5 wt% nisin), F3 (5 wt% MCPM, 10 wt% nisin), F4 (5 wt% MCPM, 5 wt% nisin), and F5 (no additives). DC was measured using ATR-FTIR (n = 5) under different curing conditions: conventional LED (SmartLite, 1200 mW/cm2, 20s) or high-intensity LED (Valo Grand, 3200 mW/cm2, 3s), with bracket coverage. The surface apatite formation (n = 1) and biaxial flexural strength (n = 7) after 4-week immersion in simulated body fluid were also investigated.ResultsCuring protocol significantly influenced DC. SmartLite (20s) generally exhibited higher DC than Valo (3s). After 4 weeks, F1 and F2 demonstrated calcium phosphate precipitation but showed reduced mechanical properties. BFS ranged from 48 MPa (F1) to 193 MPa (F5), while BFM ranged from 1.6 GPa (F1) to 7.0 GPa (F5). Increasing concentrations of additives reduced mechanical properties, with nisin showing a greater negative effect than MCPM.ConclusionHigher additive concentrations promoted mineral formation but reduced mechanical properties after 4 weeks. Using an extended-curing time with conventional LED provided higher monomer conversion than high-power LED for experimental materials. Optimizing additive concentrations and curing protocols is crucial for the performance of experimental orthodontic adhesives.