pigs were anaesthetized ( intramuscularly ) with 10 mg / kg ketamine hydrochloride and 4 mg / kg xylasine hydrochloride ( vetmarket , israel ). the liver was exposed by a transverse mid - incision . the vena cava caudalis was ligated and a small cut was made . silicon tubing ( teva medical , israel . i . d . 3 mm o . d . 4 mm ) was inserted via the portal vein and connected to a peristaltic pump . the liver was perfused with hanks balanced salts solution ( biological industries , beit ha &# 39 ; emek , israel ) supplemented with 370 mg / liter ethylene diaminetetraacetic acid ( edta , sigma israel ) and 5 u / ml heparin ( sigma israel ) at 150 ml / min for 5 min . at room temperature in order to flush the blood out of the liver . this was followed by a 5 min . perfusion with freezing solution consisting of university of wisconsin ( uw ) solution ( bristol - myers squibb pharmaceutical , ireland ) supplemented with 10 % ethylene glycol ( eg ) ( sigma israel ) at 4 ° c . flow rate was maintained at 150 ml / min . after perfusion , the liver ( about 25 cm × 20 cm × 5 cm ) with its catheter attached , was excised , and transferred to a polyethylene bag ( 37 cm × 26 . 5 cm , with polyethylene thickness of 0 . 1 mm ). a thermocouple was inserted through the tubing of the portal vain for monitoring the liver temperature . the tubing of the excised liver served also in order to ensure continuity between the perfused organ and the surroundings . the bag was transferred to the large static freezing system ( lsfs ) and an additional 250 ml of freezing solution ( 4 ° c .) was added to the bag . the lsfs was tightened to ensure maximal contact between the bag and the lsfs walls , such that the distance between the device cooling plates was about 5 cm . total time of cold ischemia was 15 ± 2 min . during the freezing procedure the lsfs initially cooled the sample to 0 ° c . for 20 min . then seeding was initiated by lowering the lsfs blocks &# 39 ; temperatures to − 10 ° c . for 20 min , this was followed by lowering the lsfs blocks &# 39 ; temperatures to − 20 ° c . at a cooling rate of 1 ° c ./ min , in order to allow quick removal of latent heat . this temperature was also maintained followed by a cooling rate of 0 . 2 ° c ./ min to a final temperature of at least − 40 ° c . in order to ensure directional growth of the ice crystals inside the liver . this cooling protocol was used , based on several earlier experiments that suggested the proper time for the increased heat removal . the total freezing time was 6 hours . the temperature change recorded during freezing is depicted in fig1 , wherein the thin line represents the liver &# 39 ; s temperature as measured during freezing , and the thick line represents the temperature of the freezing device wall . the release of latent heat was observed about 22 minutes into the protocol ( see arrow ). as can be seen , this heat release was extremely short , and temperature reduction commenced almost immediately . it is believed that this is due to the maintenance of the device at the low temperature of − 10 ° c ., a temperature that was maintained until the liver temperature became approximately the same as that of the device . this was apparently facilitated with an additional relatively quick cooling rate of 1 ° c ./ min , down to ca .- 20 ° c . after freezing to − 40 ° c ., the liver was thawed as follows : the frozen bag was immersed in a 10 liter bath of 0 . 9 % nacl in distilled water at 38 ° c . during the thawing process the liver was rubbed and gently shaken to maximize its surface area in order to quicken the thawing process . full thawing was achieved after 15 - 20 min . for viability tests , following full thawing , a cannula was re - connected to the portal vain and the freezing solution was washed out with hanks balanced salts solution for 8 min . at 150 ml / min . hank &# 39 ; s balanced salt solution containing nahco 3 ( 25 mm ), cacl 2 ( 5 mm ), and collagenase ( 0 . 2 wrap was perfused for 8 min . the liver was then shaken gently in a 38 ° c . bath for 10 min , and then filtrated through a 100 μm mesh . the filtrate was centrifuged 3 times for 4 min at 2000 rpm . the pellet of cells was re - suspended in dmem supplemented with 10 % fetal calf serum , penicillin ( 100 u / ml ), streptomycin ( 0 . 1 mg / ml ), and insulin ( 100 nm ). hepatocytes viability was assessed by trypan - blue exclusion and cfda fluorostaining using a hemocytometer . the results of viability tests for post thaw isolated hepatocytes taken from three different experiments are summarized in table 1 below : lewis rats , 220 - 300 gr ., were anaesthetized with ketamine - xylasin ( intra peritoneal ). the liver was exposed by a transverse mid incision . the vena cava caudalis was ligated and a small cut was made . 200 tubing was inserted via the portal vein ; then the perfusion was started . for collection of bile fluid a polyethylene tube ( length : 5 cm , i . d . 0 . 28 mm , o . d . 0 . 61 mm , becton dickinson , sparks , md ., usa ) was placed in the common bile duct ( ductus choledochus ). the excised liver was about 5 cm × 5 cm and between 1 . 6 cm and 2 cm thick . the liver was first perfused with 1 ml of phosphate buffered saline containing 200 units of heparin , followed by 5 minutes of perfusion using a perfusing solution consisting of hank &# 39 ; s balanced salt solution containing edta ( 0 . 5 mm ). this was followed by a 3 min . perfusion with uw solution supplemented with 10 % eg at 4 ° c . flow rate was maintained at 23 ml / min . after in situ perfusion the liver was excised and transferred to a 25 mm diameter glass tube that contained the same freezing solution used for perfusion . the tubing of the excised liver ensured continuity between the perfused organ and the surroundings . a multi - thermal and multi - velocity freezing protocol was employed using the mtg freezing apparatus ( imt ltd . israel ). first , seeding was initiated by plunging the tip of the tube into ln for 10 sec . then the glass tube was inserted into the mtg freezing apparatus . this device is constructed essentially in accordance with u . s . pat . no . 5 , 873 , 254 , being adapted to perform the following protocol on 25 mm diameter glass tubes , and having only two thermally conductive blocks . the tube was cooled down to − 14 ° c . at 0 . 2 ° c ./ min ., then to − 30 ° c . at 10 ° c ./ min . and then to − 40 ° c . at 0 . 2 ° c ./ min , all the time moving at 0 . 02 mm / sec ). the temperature change recorded during freezing is depicted in fig2 . when the freezing procedure was completed the frozen glass tube was transferred either to a − 80 ° c . freezer ( thermoforma , u . s . a . ), or to a standard − 196 ° c . ln tank , for storage . the livers were stored for 1 - 21 days . for the thawing process , the frozen tube was left at room temperature for 5 . 5 min and then dipped in a 38 ° c . bath for ca . 30 seconds . then the contents of the tube ( including the frozen liver ) are transferred to a turbulent thawing bath which contained phosphate buffered saline at 38 ° c . for 2 . 5 min . for viability tests , following full thawing , a cannula was re - connected to the portal vain and the freezing solution was washed out for 5 min . bile production , as indicator of liver function , was calculated per min . per weight of the liver tissue ( volume of bile produced per min . divided by the weight of the liver ). results varied from 45 - 98 % compared to fresh liver bile production . following washing , biopsies were taken from the rat liver for histology ( fig4 a , 4 b ) and hank &# 39 ; s balanced salt solution containing nahco 3 ( 25 mm ), cacl 2 ( 5 mm ), and collagenase ( 0 . 2 u / ml ) was perfused for 8 min . the liver was then shaken gently in a 37 ° c . bath for 10 min . then the liver was filtrated through a 100 μm mesh . the filtrate was centrifuged 3 times for 4 min at 2000 rpm . the pellet of cells was re - suspended in dmem supplemented with 10 % fetal calf serum , penicillin ( 100 u / ml ), streptomycin ( 0 . 1 mg / ml ) and insulin ( 100 nm ). hepatocytes viability was assessed by trypan - blue exclusion and cfda fluorostaining using a hemocytometer . as can be seen in table 2 , the thawed cells were viable after thawing , displaying up to 92 % viability . bile production after thawing was also substantive , and up to 98 % of fresh liver . in fact , the liver remained viable even after 21 days of storage ( in frozen state ), suggesting that it may remain so practically indefinitely under appropriate storing conditions ( e . g . ln storage ). in addition , biopsies of thawed rat liver ( after 3 - 7 days of storage ), were fixed in 4 % fresh 4 ° c . paraformaldehyde in pbs . 5 μm sections , from livers that were frozen and thawed as described above , were dehydrated in graded ethanol solutions , cleared in chloroform and embedded in paraplast ( sigma , israel ). the sections were stained either with eosin / hematoxylin ( fig4 a ) or taken for immunohistochemistry using α - von willebrand factor ( vwf , factor viii ) antibody ( fig4 b ). factor viii related antigen was detected using polyclonal anti - human willebrand factor antibody ( zymed laboratories , israel ) diluted in 1 % normal goat serum in phosphate buffered saline at 1 : 700 dilution and lsab2 detection kit ( dako corp . santa barbara , calif ., usa ) according to the manufacturer &# 39 ; s instructions . in each of fig4 a and 4 b , an arrow points to the central vain . as can be seen , the thawed liver shows normal architecture of the parenchyma and the endothelial cells present a normal shape . hepatocytes , were isolated from rat livers ( frozen and thawed as detailed above ) after 1 - 21 days of storage , fluorostained with fda ( fluorescein diacetate , sigma , israel ). glowing viable hepatocytes are seen in fig5 , representing an example from a liver that was stored for 7 days . similar results were obtained for samples stored up to 21 days ( not shown ). male sprague dauley rats ( 280 - 320 g ) were obtained from harlan laboratories ( jerusalem , israel ). the rats were anticoagulated with heparin sodium ( 500 u / rat , intra peritoneal and 30 minutes later anesthetized with pentobarbital sodium ( 30 mg / rat , intra peritoneal ). hearts were immediately removed and placed in heparinized ice - cold saline solution . the hearts dimensions were about 2 - 2 . 75 cm in length , with a diameter of about 1 cm . the aorta was cannulated to a langendorff perfusion apparatus . the pulmonary artery was cut open to provide drainage . haemodynamic parameters were assayed for each heart during perfusion ( before freezing ) and reperfusion ( after thawing ), when the hearts were connected to the langendorff apparatus , as follows : a latex balloon - tipped catheter was inserted through an incision in the left atrium and advanced through the mitral valve into the left ventricle and connected to a pressure transducer placed at equivalent height to the heart and a recording system ( powerlab , adinstruments , australia ). the balloon was inflated and equilibrated to give an end - diastolic pressure of 0 mmhg . left ventricular systolic and diastolic pressures and time derivatives of pressure were measured during contraction (+ dp / dt ) and relaxation (− dp / dt ). left ventricular developed pressure ( lvdp ) was calculated as the difference between the systolic and diastolic pressures . hearts were perfused with oxygenated ( 95 % o 2 / 5 % co 2 ) krebs - henseleit ( kh ) solution at a constant pressure of 90 cm h 2 o with the following composition ( mm ): kc 1 - 4 . 9 , cacl 2 2 . 5 , nacl 118 , mgso 4 1 . 2 , kh 2 po 4 1 . 2 , nahco 3 25 , glucose 11 . 1 . hearts were subjected to 20 min of perfusion at a constant temperature of 37 ° c . the hearts were then perfused for 1 min with 10 ml of 4 ° c . uw solution followed by 3 min perfusion of 10 % ethylene glycol solution dissolved in uw ( uweg ) at 4 ° c . the hearts were removed from the langendorff apparatus and placed in a glass chamber containing uweg solution at 4 ° c . and transferred to a small static freezing system ( ssfs ). for freezing , the following protocol was used : first the ssfs was cooled to 0 ° c . and remained at that temperature for 15 min , followed by cooling to − 5 ° c . at 0 . 5 ° c ./ min . this was followed by a reduction in the temperature of the ssfs to − 10 ° c . at 1 . 0 ° c ./ min . following adjustment to − 20 ° c . dependent upon the heart &# 39 ; s recorded temp on the screen . if the heart &# 39 ; s temperature on screen showed a rise ( indicating a release of latent heat ) then the ssfs temperature set point was lowered to − 20 ° c . until the heart &# 39 ; s recorded temp dropped again to the desired temperature . in one heart , in order to confirm the homogeneity of freezing inside the heart , two thermocouples were used for monitoring the heart temperature during freezing : one was inserted through an incision in the left atrium and the other in the intramyocardium ( fig3 , dotted line and thin line , respectively ). as seen in fig3 , the temperature profile during freezing was almost identical at both points of measurement . in all other experiments ( including the control freezing example ), only a single thermocouple was used , measuring temperature only in the left atrium . after freezing was completed the chamber was removed for thawing from the ssfs to a bath containing 0 . 9 % saline solution at 37 ° c . for 30 sec . hearts were removed from the chamber and were re - connected to the langendorff apparatus for 60 min of reperfusion . as a control , a heart was prepared as described above , with the exception that the ssfs was set to a constant cooling rate of 0 . 26 ° c ./ min , from 0 ° c . to − 20 ° c . the temperature of this heart at the left atrium was measured during freezing , and depicted in fig3 as a dashed line . surprisingly , during re - perfusion the hearts that were frozen according to the present invention began beating spontaneously . the recovery rate of the heart was derived by multiplication of lvdp by heart rate ( hr ). the thawed hearts had a recovery rate ( lvdp × hr , measured during reperfusion ) of 43 . 3 ± 120 % ( assayed for 13 hearts ) of the recovery rate measured for the same hearts when they were fresh ( during perfusion ). in order to further assay the viability of the hearts frozen and thawed as described above , additional assays were performed , as detailed below , and the results are summarized in table 3 . for this set of tests , hypothermically preserved hearts were used as a control . these hearts were harvested and perfused as described above , and then maintained at 4 ° c . for 50 minutes . these conditions were chosen in order to approximate donated hearts as they are currently stored and transported before implantation . reperfusion was the same as for the thawed hearts . after conclusion of the reperfusion procedures , while the hearts are still beating , left ventricular slices ( 1 cubic mm ) were taken from frozen / thawed hearts for viability and analyzed by 3 [ 4 , 5 - dimethylhiazol - 2 - yl ]- 2 , 5 - diphenyltetrazolium bromide ( mtt , sigma chemical co .). tissue viability was expressed as the ratio between the optical density of treated hearts to that of its aerobic controls ( 8 control hearts perfused with kh for 20 min = 100 % viability ), normalized to dry weight of myocardial tissue . at the end of reperfusion hearts were rapidly frozen with stainless steel blocks precooled in liquid nitrogen . perchloric acid extracts were prepared from tissue samples . atp and phosphocreatine levels were assessed by enzymatic assay ( nutt et al ., 1991 ), and expressed as micromoles per gram of dry weight of myocardial tissue ( dried at 90 ° c . for 24 h ). the average results are depicted in table 3 as a percentage from fresh hearts that were assayed immediately after harvest . while it is noted that the results of the frozen - thawed hearts are lower than those of fresh hearts , they are comparable to results of hypothermia hearts . those skilled in the art will readily appreciate that various modifications and changes can be applied to the embodiments of the invention as hereinbefore exemplified without departing from its scope defined in and by the appended claims .
disclosed is a method of freezing bulky biological material , comprising transferring heat out of said material to cool the material and increasing the rate of heat transfer during a time period when the inner portion of said biological material freezes and releases latent heat . this method allows prolonged ex - vivo preservation of solid mammalian organs , including a liver or significant portion thereof and a heart . the invention also provides a bank of deposited body organs or tissues as well as methods for operating such a bank .