Grundlagen der elektrisch induzierten Eiskeimbildung und ihre Anwendung in der Kryobiologie
- Basic investigations of electrically induced ice nucleus formation ('Electrofreezing') and its application in cryobiology
Petersen, Ansgar; Rau, Günter (Thesis advisor)
Aachen : Publikationsserver der RWTH Aachen University (2008)
Dissertation / PhD Thesis
Aachen, Techn. Hochsch., Diss., 2007
Electrically induced ice nucleus formation describes the effect of an on-demand ice nucleus generation in a supercooled aqueous solution. The nucleus forms at the surface of an electrode when a high voltage pulse is applied. Even though this effect has been known for more than 50 years, the possible underlying mechanism is still discussed controversially. In cryobiology the supercooling of a sample is an important parameter since it determines the temperature changes inside the sample and thus the ice crystal structure that develops during the freezing process. In this thesis the basic principles of the electrically induced ice nucleus formation and the potential for an application in the field of cryopreservation and freeze drying are investigated. At first, systematic experimental investigations are performed to find the parameters influencing the electrically induced nucleation. One important parameter which hinders the nucleus formation is the presence of additives like salts and cryoprotective agents, as shown in this work. Especially in respect to an application in cryobiology, where these additives are essential for cell survival, this is an critical issue. The critical field strength which is necessary for ice nucleus generation in pure water was determined experimentally to be 3*10^9 V/m at a temperature of -1°C. Comparing this result to the value derived by a specially developed thermodynamic model there is good agreement. In contrast to many other models found in literature, the approach used in this work is based on the energetic favorability of ice nucleus formation in an electric field. The energy is reduced during the phase change of the system. Based on this thermodynamical description of the system, the influence of the investigated experimental parameters is discussed. The new development of a special electrode cap makes it possible to start crystallization reliably in aqueous solutions, even those containing salts and additives. Thus, for the first time electrically induced nucleation can be applied to the field of cryobiology. The post-cryopreservation cell survival for human fibroblasts and human progenitor cells (CD34+) can be increased significantly by the optimization of nucleation temperature through systematic parameter variation. The comparison of experimental results with a simulation of the water transport through the cell membrane helps to explain the mechanisms of cell damage at suboptimal nucleation temperatures. Freeze drying is a further potential field of application for electrically induced nucleation. It is shown in this thesis that the drying time of a freeze drying process can be drastically reduced by a minimized supercooling during the freezing process. For the chosen freeze drying parameters, the freeze drying time could be reduced to approximately half the value compared to uncontrolled nucleation. The analysis of the sample structure shows that a different sample morphology develops during the freezing process. This structure is advantageous for drying and leads to a reduced drying time.