Cryopreservation method and apparatus
The present invention relates to methods and apparatus for the cryopreservation of biological samples involving a density assisted vitrification wherein a sample of biological material in a cryopreservation agent is cooled from its top surface, causing an ice layer to form thereon. As cooling continues the ice layer grows downwards through the sample to provide a cryoprotectant and biological material rich layer below the ice layer that undergoes vitrification as cooling continues to below the glass transition temperature.
1. A method for cryopreservation of an initially liquid sample comprising a biological material in a cryopreservation medium, the method comprising the step of cooling the top surface of the sample to selectively form an ice layer at the top surface of the sample, and further cooling the sample to provide formation of ice from said ice layer at the top surface progressively downwardly toward a base of the sample whereat the biological material is substantially located or concentrated; wherein the cooling applied the top surface of the sample is continued until a layer of the sample below the ice layer solidifies as a glass to form a vitrified, or amorphous solid, layer as a result of cooling below the glass transition temperature, wherein the cryopreservation medium is an aqueous solution comprising at least one cryoprotectant.
2. The method according to claim 1 , further comprising the step of cooling a layer of the sample below the ice layer to a temperature just above that of the ice layer of less than 5° C. above that of the ice layer.
3. The method according to claim 1 , wherein the at least one cryoprotectant is selected from the group consisting of dimethyl sulfoxide, formamide, acetamide, C1-C3 alcohols, 1,2-isopropyldiol, 1,2-propanediol, ethylene glycol, propylene glycol, glycerol, glucose, mono-saccharides, disaccharides, sucrose, trehalose, lactose, polysaccharides, raffinose, dextran, ficoll, polyethylene glycol and polyvinylpyrollidine.
4. The method according to claim 1 , wherein the cryopreservation medium further comprises DMSO, or a sugar, or a combination of DMSO and a sugar.
5. The method according to claim 1 , further comprising the step of adding additional cryoprotectant to the layer of the sample below the ice layer after the ice front on the top surface of the sample has formed.
6. The method according to claim 5 , wherein the additional cryoprotectant is added in a progressive fashion as a function of the decrease in temperature of the ice layer or the sample liquid below the ice layer.
7. The method according to claim 1 , wherein a mechanical agitation is applied to the sample during cooling.
8. The method according to claim 1 , wherein cooling is provided by thermal conduction through a cooling element in the form of a heat conducting member located within the sample.
9. A cryopreservation apparatus for performing a cryopreservation method according to claim 1 .
10. The apparatus according to claim 9 , comprising a housing, the housing comprising a cavity and a primary cooling element that is incorporated in the roof of the housing above the cavity.
11. The apparatus according to claim 9 , wherein the housing is insulated by a vacuum or partial vacuum.
12. The apparatus according to claim 9 , further wherein the degree of cooling provided by the primary cooling element can be controlled manually or automatically by a control unit.
13. The apparatus according to claim 9 , further comprising at least one sensor adapted to provide a signal characteristic of the physical condition of a sample located in the cavity.
14. The apparatus according to claim 13 , wherein the at least one sensor adapted to provide a signal characteristic of the physical condition of a sample located in the cavity is electrically connected to a control unit, wherein the signal indicates the presence of an ice layer in, or the temperature of, a sample located in the cavity.
15. The apparatus according to claim 10 , wherein the cavity comprises one or more liquid inlet and/or outlet ports.
16. The apparatus according to claim 15 , comprising a pump configured to supply fluid to an inlet port or draw fluid from an outlet port.
17. The apparatus according to claim 16 , wherein fluid flow to an inlet port or from an outlet port is controlled by a control unit in response to a signal received from a sensor located in the cavity.
18. The apparatus according to claim 9 , comprising means for mechanically agitating any sample located within the cavity.
19. The apparatus according to claim 9 , comprising a secondary cooling element adapted to cool a section of the wall(s) or floor of the cavity.
20. A cryopreserved sample obtained by a method according to claim 1 .
21. The cryopreserved sample according to claim 20 , wherein the sample is a tissue sample, a biopsy, an organ or a tissue construct.
22. A sample container for use in cryopreservation in an apparatus according to claim 9 .
23. The apparatus according to claim 9 , further comprising at least one cooling member located to be in contact with sample liquids in the cavity arranged to homogenise thermal gradients within the sample.
24. The apparatus according to claim 23 , wherein the member includes at least one rod of heat conductive material surrounded by one or more discrete bands of material which is less thermally conductive than the rod.
25. The apparatus according to claim 24 , wherein multiple rods are used.
26. The method according to claim 1 , wherein a solute or combination of solutes with density >2 g per cm 3 , solubility of >0.5 g/cml water at 4° C. are used with concentration >5% w/v.