Effects of Ultrasonic Irradiation on Ice Formation in Biological Tissue

A method to actively controlling crystallization is one of promising technique for cryopreservation. The object of this paper is to study the effect of ultrasonic irradiation on ice formation during freezing of biological tissue with supercooling. In the experiments, agar gel was frozen under irradiation of ultrasound at frequency of 28kHz. The measurements of temperature and the microscopic observation of ice crystals using fluorescent indicator were carried out. Firstly, it was found that the supercooled state was released by applying ultrasound at high intensity level, since ultrasonic cavitation effects were augmented. On the other hand, a weakly delay in the nucleation temperature was induced by applying ultrasound at low intensity level. Secondly, ice structure size decreased with increasing ultrasonic power under given supercooling degree at nucleation stage. The effect of ultrasound was enhanced by using large supercooling condition. Thirdly, paying attention to the thermal effect of ultrasound as changing temperature profile, sound and heat conduction in agar gel immersed in low temperature solution were numerically calculated. It was suggested that ultrasound at high frequency is effective to enlarge supercooled region.