Abstract:
The invention relates to a high-pressure freezing system in which the pressure build-up ensues from a prestressed pneumatic cylinder. This results in a particularly rapid pressure rise in the sample holder.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of the Swiss Patent Application CH1260/99. 
     FIELD OF THE INVENTION 
     The invention relates to a high-pressure freezing system in accordance with the characterizing clause of claim  1 . In the sense of the invention, a high-pressure freezing system is understood to be a freezing device for the rapid freezing (vitrification) of aqueous samples under high pressure. 
     BACKGROUND OF THE INVENTION 
     Information on the physics of such a freezing process can be found in the German Patent DE-PS-1806741 and the European Patent Application EP-A-0 853238. 
     In professional circles, for example, such a known system of the applicant is employed, which is successfully marketed under the name “Leica EM HPF”™ and described in the publication “LEICA EM HPF, High Pressure Freezer,  1 .K.-LEICA EM HPF-E-6/94, Jun. 1994”. 
     The “Leica EM HPF”™ allows the vitrification of conventional samples under a pressure of ca. 2000 bar at a cooling rate of 10 3 -10 5  K/s. For the said appliance, the crucial cooling phase from ambient temperature to −100° C. takes ca. 10 ms (cooling rate 10 4  K/s) at the surface of the sample. Consequently, all samples with a thickness of ca. 200μm are cooled to -100° C. in ca. 50 ms. 
     The known cooling technology and the physical processes show clearly: Applying an infinitely high cooling rate to the surface of a biological sample only partially determines the rate of cooling in its centre. A sample of thickness 200 μm, for example, exhibits a cooling rate in its centre of ca. 6000 K/s, regardless of whether an infinite cooling rate is employed at the surface or a cooling rate of ca. 10,000 K/s. The cooling rate is not determined by the pressure. By increasing the pressure, a lower cooling rate is sufficient for vitrification. Therefore, the cooling rate for the vitrification of biological samples is under standard pressure ca. 10 5 - 10 6  K/s, tinder 2000 bar, however, the cooling rate is only 10 3- 10 4 K/s, i.e., it is still possible to vitrify biological samples under high pressure using cooling rates a hundredfold lower. 
     The inventor referred in the article “A NEW CONCEPT AND MACHINE FOR HIGH PRESSURE FREEZING” from 15.3.1999, published in the internet under the address “www-mem.unibe.ch/˜ danis/abstract HPF”, to these and further aspects of high-pressure freezing. He proposed increasing the pressure from 1 bar to 2000 bar within 10 ms by employing a compressed-air cylinder in place of the well-known very bulky and heavy equipment. 
     Pneumatic cylinders are already employed in a wide variety of applications. Using them to quickly build up high pressures, however, necessitates the use of large and heavy pumps. 
     SUMMARY OF THE INVENTION 
     Consequently, the first problem to be solved in the invention is the construction of a high-pressure freezing system, which is lighter than the hitherto known systems but which nevertheless allows a rapid build-up of pressure. 
     The above problem is solved by the features of independent claim  1  and claim  11  which is a reduction in the constructional size and yet an accelerated pressure build-up. 
     The dependent claims  2  to  6  and  12  to  18  refer to improved solutions with extensive integration and extensive advantages over the state of the art. 
     However, the invention is based on a further problem: The aim is a simple pressure adjustment in the range of ca. 1500-2000 bar. The reason for this being that it is known from technical literature that biological samples exposed to pressures exceeding 1600 bar can exhibit irreversible damages. In accordance with the invention, this is possible by merely varying the pneumatic pressure at the prestressed pneumatic cylinder; i.e., altering the input pneumatic pressure at the pneumatic cylinder during the prestressing phase can result in a different driving power at the output end leading to a different or adjustable pressure build-up in the high-pressure cylinder. 
     In addition, as, in accordance with the invention, pressure build-up and cooling are separated, it is possible to cool the sample using liquid nitrogen to 196° C. In boundary ranges of 200μm, in particular, the cell structure remains undamaged using the procedure of the present invention, which can be allowed for with no difficulty in the selection of the sample dimensions. 
     During vitrification, very low cooling temperatures, as acquired using liquid and undercooled nitrogen, for example, are of course advantageous, provided that the nitrogen itself —as indicated sporadically in the state of the art —is not exposed to these high pressures. If the undercooled nitrogen were to be used as pressure carrier, this would constitute a potential source of danger at the high pressures and it would not be possible to attain a temperature of -196° C. as the nitrogen is already solid at -196° C. under pressures exceeding 1500 bar. 
     The dependent claims  7  to  10  and  19  to  24  relate to improvements which can be applied independently to advantage other known high-pressure freezing systems or those still to be designed. This concerns in particular a novel and improved holding device for a sample container in accordance with the European Patent Application EP-A-0 853 238 of the inventor. A new holding device should facilitate the handling of the known sample container and accelerate the changing of sample containers or samples. In particular, this should accelerate the throughput of samples through a vitrification plant and thus achieve an improvement in economic efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter of the invention is described with reference to the embodiments shown in the drawings: 
     FIG. 1 shows a symbolic drawing of a complete high-pressure freezing system in accordance with the invention; 
     FIG. 2 shows a graph of the course of a high-pressure freezing process for a system in accordance with the invention; 
     FIG. 3 discloses an oblique view of a new type of arm log holder for a sample container; 
     FIG. 4 discloses a sectional view of the arm log holder of FIG.  3  and 
     FIG. 5 shows a holder for the arm log holder of FIG. 3 as view, top view and left-hand and right-hand side views. 
    
    
     FIG. 1 shows a symbolic drawing of the arrangement in accordance with the invention. A high-pressure freezing system is used to attain an amorphous state during rapid freezing of aqueous samples, in particular biological samples  33 . 
     DETAILED DESCRIPTION OF THE INVENTION 
     The exceptional feature of the depicted pneumatic cylinder  1  is the ability to stop the piston by means of the locking devicc  3 . In the locked state, the locking device  3  interacts with a thrust bearing  11 . This new arrangement allows a prestressing of the pneumatic cylinder  1  by fully pressurizing the latter in a locked state. After the locking device  3  is released as abruptly as possible, the piston  12  of the pneumatic cylinder accelerates explosively to impinge on a high-pressure piston  13  of a high-pressure cylinder  2 . 
     The high-pressure cylinder  2  comprises a special pressure liquid  34  for building up a pressure exceeding 2000 bar. The pressure liquid  34  does not solidify at low temperatures. It is pressed under pressure onto the sample which is held in a sample holder  4 . A practical example of such a liquid is methylcyclohexane as proposed in the European Patent Application EP-A-0853238. 
     The pressure liquid is topped-up from a refill container  5 . A manometer  6  is used to measure the pressure. The sample holder  4  is cooled externally by spraying the cooling medium  15  under pressure from a second pneumatic cylinder  7  via a piston  9  through nozzles  8  against the sample holder  4 . In accordance with the invention, the resulting cooling should synchronize exactly with the pressure build-up in the sample holder  4 . In the present embodiment, this timing is accomplished by means of a control cylinder  10  which controls the timed sequence of the pressure build-up and the spray cooling. 
     In accordance with the invention, firstly, the locking device  3  is released abruptly and after a —preferably mechanically — adjustable time interval (ca. 5 ms) the nozzles  8  are opened by means of the annulus gate valve  14  and the cooling medium  15  is simultaneously pressurized by piston  9 , propelled by the second pneumatic cylinder  7 . The annulus gate valve  14  should be formed such that the area around the nozzles  8  is permanently cooled to prevent as far as possible any heating of the cooling medium  15  prior to its extrusion from the nozzles  8 . Liquid nitrogen usually serves as cooling medium  15  although the design is not restricted to its sole use. Other types of cooling media are also possible. 
     FIG. 2 depicts the preferred pressure rise required for certain samples—and attained for the first time with the arrangement of the present invention. 
     FIG. 3 illustrates a new type of arm log  16  which accommodates the sample holder  4  and holds it during high-pressure freezing. The sample holder  4  is tubular in order to build up the pressure. In the drawing, the tube  17  is not depicted from end to end as it is very thin. A sample  33  is placed inside the tube. One end of the tube  17  (FIG. 3) is connected to the high-pressure connection of the high-pressure cylinder  2  (FIG. 1) by a conical metal/metal-connection. 
     The other end of the tube is pressure-sealed by a taper plug  18  (FIG.  4 ). Consequently, the sample holder  4  is held under tension between the two connections. A spring  19  in the arm log  16  presses the taper plug  18  against the sample holder  4 . Attached to the sample holder  4  is a bossed flange  20  with which it is held in the locked state by a spring grab  21 , as is shown. On releasing the arm log  16 , the spring grab  21  opens allowing the sample holder  4  to drop out. To this end, a pressure head  22  is pushed forwards relative to the housing  23  of the arm log  16  and against the tension of the spring  19 . This also results in a relative displacement of the taper plug  18  which is arranged on a rod  24 . 
     In practice, this release process occurs after the vitrification of the sample  33 . During this process, the arm log  16  is held in a holder  25  in an approximately horizontal position, as illustrated in FIG.  5 . The holder  25  has a bed at its disposal  26  which accommodates the arm log  16 . The wall of the holder  25  comprises two guide notches  27  which accommodate a bearing axle  28  of the arm log  16 . The bearing axle  28  is rigidly connected to the rod  24  and is supported loosely in a longitudinal slot  29  (in the housing  23  of the arm log  16 ). Thus, the rod  24  can be drawn backwards on the bearing axle  28  relative to the housing  23  to free the other opening of the tube of the sample container  4 , which can be effected by the pressure head  22 . 
     Consequently, the present invention allows the sample container to be released from the arm log  16  by one operator with a single movement or by pressing the pressure head. This is simplified further by the arrangement and effect in accordance with the invention, described in the following: 
     After releasing one end of the tube  17  by the high-pressure connection of the high-pressure cylinder  2 , the arm log  16  tilts downwards around the bearing axle  28  as the holder  25  possesses no bottom in the bearing area and thus no support for the arm log  16 . After the downward swivel, the operator simply presses the pressure head  22  to release the sample holder  4 . This is of particular advantage as the invention shows that a container with liquid cooling medium is positioned beneath the sample holder  4  so that after vitrification, it is possible, within seconds, to immerse or lay the sample holder  4  in the cooling medium in to maintain the cooling temperature. 
     In accordance with a specific embodiment, all parts in the area around the sample holder  4  and this itself are in addition electrically conductive. It is thus possible by performing a resistance test between the high-pressure cylinder  2  and the holder  25  to measure whether the tube (sample holder  4 ) is correctly restrained between its two ends. This electrical contact is callipered via a connection  31  on the holder  25 . The holder  25  is attached to the device on helical bores  32 , which is not described in greater detail. 
     The invention has been described in detail with particular reference to certain preferred embodiments hereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.