Thermal breakdown properties of indocyanine green

Because indocyanine green (ICG) dye is being used in tissue soldering, investigation of its degradation properties has become important. Studies have demonstrated the kinetics of indocyanine green degradation on light exposure. Temperature dependent changes have not been characterized. Since tissue soldering is a thermal process, we examined how absorption characteristics of aqueous ICG solutions change after heating. A 3.23 mM ICG stock solution in 2.5% human serum albumin (HSA) was prepared daily. From the stock solution, 1.5 ml samples were light shielded and heated to 100 degree(s)C for intervals of up to 5.5 hours. Control samples were maintained at room temperature. After heating, samples were diluted to 16.6 (mu) M ICG with 2.5% HSA for scanning. Absorbance from 200 to 900 nm was measured using an absorbance spectrophotometer. Peak absorption intensity decreased as heating time of samples increased from 1 to 5 hours, with the first consistently detectable change occurring after 1 hour. Absorption peak was stable at 805 - 808 nm with no novel peaks and absorbance magnitude was reduced only 2% over 1 hour. Lower temperatures more commonly seen in laser welding (60 - 80 degree(s)C) produced even smaller reductions in absorption intensity. Measurable thermal degradation of aqueous ICG solutions occurs at 100 degree(s)C after a period of hours. Therefore, thermal degradation of ICG in the context of tissue soldering is probably insignificant.