EVIDENCES OF THERMAL STRESS WAVE INDUCED IN DEEPLY FROZEN BIOMATERIALS BY A STRONG AND INSTANTANEOUS HEATING

Abstract

Mechanical stress that develops in biological tissues during freezing and subsequent heating has been identified as an important cause of tissue damage. One such interesting phenomenon is the recently observed concentric ring induced around a probe in biomaterials through alternatively switching between strong freezing and heating. Interpretation of the mechanisms thus involved is critical for the successful operation of combined cryosurgery and hyperthermia on target tumor. To detect the possible stress wave induced in the tissues, an electrode was positioned at a distance far enough from the probe applicator to measure the dynamic low-frequency electrical impedance of the in vitro biological materials (fresh pork and pig liver) subject to strong freezing and heating. It was found that a sudden impedance jump appears in the liquid-phase tissue immediately after the probe freezing was switched to heating; meanwhile, the temperature still remains almost unchanged. Because the magnitude for this impedance jump is so large, and cannot be accounted for by the temperature-dependent effects, it is revealed that a certain shock wave has been produced in the frozen ice ball and is then transmitted into the unfrozen tissues. To theoretically explain the experimental observation, a thermal stress model was applied to characterize the wave feature of the shock stress induced in the strong freezing and heating process. Results obtained in this paper are expected to be significant for better understanding the practices of cryosurgery or cryopreservation.