Heat Transfer In Biological Tissues During Freezing

Cryopreservation is the most effective method for the storage of biomaterials. Biomaterials can be preserved as long as several decades after prepared with cryoprotective agents and storage in liquid nitrogen. Cryopreservation generally is achieved by slow-freezing and vitrification. Vitrification is an ice-free method and has no expansion, it can also intactly preserve the morphological structure of tissues and organs, and is the most potential cryopreservation method. The cryopreservation of biomaterials is based on the different kinds and concentrations of cryoprotective agents, as well as cooling and rewarming rate. Determining available kinds and concentrations of cryoprotective agents can reduce the toxic injury and ice formation rate, along the optimal cooling rate can also improve the probability of biomaterials living through dangerous temperature regions. However, most of researches are focused on the selection of kinds and concentrations of cryoprotective agents, but few for the cooling process of biomaterials. The researches on cooling processes are only concerned on theoretical exploration by numerical simulation, and the thermophysical properties are assumed as bulk calculation value. The cooling process model should be constructed in order to predict and optimize the cooling process. Moreover, the thermophysical properties of tissues, organs and cryoprotective agents are temperature dependent and over remain infrequent, make the results of numerical simulation besides actual situation possibly be different.Our work obtained the thermophysical properties of tissues and cryoprotective agents with experiments and calculations on the cooling process of biomaterials plunged into liquid nitrogen combined theoretical modeling prediction and experimental verification. Detailed research contents and results are as follows:Initially, the thermal conductivity of fresh hog liver was measured, and the experiment data was processed with two methods to verify the veracity, and obtained the applicable range of the two methods of data processing. The thermal conductivities of cryoprotective agents for tissues, organs and hog liver (which is prepared with VS55 solution) are measured subsequently along with complementing the blank data.Secondly, our studies perceived on the convective heat transfer coefficient of common cryopreservation container. The model of cooling process for cryovials and straw plunged into liquid nitrogen was established with biological heat transfer theory. When object (close to room temperature) immersed in liquid nitrogen, four regimes come across i.e. film boiling, transition boiling, nucleate boiling and convective, which will follow in sequential, the convective heat transfer coefficients of different regimes are fairly different. Although, the most of studies on cooling process are usually use only one convective heat transfer coefficient for simulation, therefore, the difference appears. The cooling process with the film boiling regime followed by nucleate boiling was simulated, and compared with one regime only and the convective heat transfer coefficients were obtained by inverse problem technique, also evaluated with the residual error. Results ascertained that the calculation of theoretical cooling process with film boiling followed by nucleate boiling is better than only one regime.At last, based on the critical parameters of the above two sections, further research on the cooling process of biomaterials plunghed into liquid nitrogen was did. Taking the example of 1.8 mL cryovial and VS55 solution, firstly, the theoretical cooling process and experiment results of cryovial filled with VS55 solutions plunged into liquid nitrogen were compared, the theoretical results can well predict the experiment results. When studies on the distribution of the temperature field on the vertical and radial direction and different moments, it was observed that the temperature field distributions are quite uniform except for the hemisphere region at the bottom, therefore, the cryovial was divided into two sections, the straight wall section and hemisphere section. The biomaterials are suggested to place at the straight wall sections to get more uniform cooling. Afterward, the hog liver is added into the cryovial and modelling, the theoretical cooling curve can well predict the experiment results. Thus the thermophysical properties obtained with experiments and bio-heat transfer method can well predict the cooling process for biomaterials plunged into liquid nitrogen, it provides the basic foundations for optimization of cooling rate.