Biokältetechnik: Verfahren der Gefrierkonservierung in der Medizin

Phenomena at the advancing ice-liquid interface: solutes, particles and biological cells

Ice formation in aqueous solutions and suspensions involves a number of significant changes and processes in the residual liquid. The resulting effects were described concerning the redistribution of dissolved salts, the behaviour of gaseous solutes and bubble formation, the rejection and entrapment of second-phase particles. This set of conditions is also experienced by biological cells subjected to freezing. The influences of ice formation in that respect and their relevance for cryopreservation were considered as well. A model of transient heat conduction and solute diffusion with a planar ice front, propagating through a system of finite length was found to be in good agreement with measured salt concentration profiles. The spacing of the subsequently developing columnar solidification pattern was of the same order of magnitude as the pertubation wavelengths predicted from the stability criterion. Non-planar solidification of binary salt solutions was described by a pure heat transfer model under the assumption of local thermodynamic equilibrium. The rejection of gaseous solutes and the resulting gas concentration profile ahead of a planar ice front has been estimated by means of a test bubble method, yielding a distribution coefficient of 0.05 for oxygen. The nucleation of gas bubbles has been observed to occur at slightly less than 20-fold supersaturation. The subsequent radial growth of the bubbles obeys a square-root time dependence as expected from a diffusion controlled model until the still expanding bubbles become engulfed by the advancing ice-liquid interface. The maximum bubble radii decrease for increasing ice front velocities. The transition between repulsion and entrapment of spherical latex particles by an advancing planar ice-front has been characterized by a critical value of the velocity of the solidification interface. The critical velocity is inversely proportional to the particle radius as suggested by models assuming an undisturbed ice front. The increase of the critical velocity for increasing thermal gradients shows good agreement with a theoretically predicted square-root type of dependence. Critical velocities have also been measured for yeast and red blood cells. The effect of freezing on biological cells has been analyzed for human lymphocytes and erythrocytes. The reduction of cell volume observed during non-planar freezing agrees reasonably well with shrinkage curves calculated from a water transport model. The probability of intracellular ice formation has been characterized by threshold cooling rates above which the amount of water remaining within the cell is sufficient for crystallization.(ABSTRACT TRUNCATED AT 400 WORDS)

Basic investigations on the freezing of human lymphocytes

Human lymphocytes were frozen at constant cooling rates in the range 2.4 to 1000 degrees K/min without cryoadditive on the cold stage of a thermally defined cryomicroscope. The volume loss due to water efflux was quantified optically for the cooling rates 2.4, 12, 48, and 120 degrees K/min. The likelihood of the formation of intracellular ice was determined as function of the cooling rate. Intracellular crystallization temperatures were obtained for ice formation during both cooling and rewarming. A theoretical analysis of the cell volume loss during freezing was compared to the experimental data and used for an indirect determination of the water permeability of the cells. A relative optimum of the cooling rate is predicted theoretically under the assumption of a critical level of intracellular salt concentration near the eutectic temperature. The dependence of survival and cooling rate was determined cryomicroscopically by simultaneously applying the FDA/EB fluorescence viability test. The optimal cooling rate of about 35 degrees K/min was also found for 2-ml samples frozen within the range of cooling rates of interest. The results show that for freezing in physiological saline solution (1) the optimum of the cooling rate is theoretically predictable, (2) cryomicroscopical data are significant for freezing of samples of larger volume, and (3) the lethal type of intracellular crystallization is cooling rate dependent and distinguishable from innocuous types.

Thermally defined cryomicroscopy and some applications on human leucocytes

A freezing-stage has been developed for use on a standard light-microscope, which can provide reproducible, precisely linear cooling and warming rates in the range from 0.1 to 10,000 K/min. Biological cells in aqueous solutions can be observed during the freeze-thaw cycle; the volume loss due to osmotic efflux of water and the intracellular crystallization of water are detected by video-monitoring. The temperature field generated in the observed samples is comparable to extended cylindrical probes and allows the transfer of cryomicroscopic data to technically used vial geometries. Lymphocytes and granulocytes were observed during freezing using the system described. They were separated and washed, and then frozen on the cold stage of the cryomicroscope at cooling rates ranging from 2 to 500 K/min. Shrinkage of the cells was observed up to 100 K/min and intracellular ice formation could be detected starting at 10 K/min. The results show that human leucocytes show excessive shrinkage up to 36% of their initial volume; the probability of intracellular ice formation exhibits a sharp increase from 10 to 100 K/min where nearly all cells contain ice.