Patent Abstract:
a method of cryopreserving biological material by administering a cryoprotective agent mixture is provided . the mixture must be produced in a concentration sufficient to permit vitrification . the cryoprotective agent is an alkoxylated organic compound . the method includes administering the cryoprotective agent mixture and cooling the biological material until it is vitrified . preferred cryoprotective mixtures contain 3 methoxy - 1 , 2 propanediol and 2 - methoxy ethanol .

Detailed Description:
generally , the method of the present invention comprises cooling a living biological material to be cryoprotected to a temperature between about 0 - 15 ° c ., perfusing the biological material with a cpa solution , and subsequently perfusing with an inert fluid to replace the cpa solution until the vascular system is cleared of the cpa , then cooling the inert fluid to a desired low temperature and circulating it through the vascular system of the biological material until a desired temperature is achieved for the biological material . the inert fluid functions as an internal heat exchange medium which significantly promotes the cooling / rewarming process by allowing direct heat exchange between the fluid and the wall of the vascular system . compared with the external cooling method of the prior art , the surface area for heat exchange is increased tremendously with the internal cooling method of the present invention and , thus , heat exchange rate is enhanced significantly by increased heat conduction . the cooled inert fluid is circulated continuously through the vascular system which promotes heat exchange by enhanced heat convection . therefore , the overall cooling rate is increased significantly . by replacing vascular water that might otherwise freeze , the inert fluid also provides a cryoprotective effect . the lowest temperature which can be achieved by the internal cooling method of the present invention is determined by the pour point of the inert fluid , which can be as low as - 180 ° c . the inert fluid can be cooled by passage through a heat exchanger that is cooled by refrigeration , by thermal contact with a large cold heat sink , by circulation of a cold fluid , or by cold gas . in the present implementation of the invention , cold nitrogen gas generated from boiling liquid nitrogen is fan - driven past a heat exchange unit to cool the inert fluid . the inert fluid of the present invention can be any chemical or mixtures thereof that remain liquid at low temperatures with low viscosity and low toxicity . preferred inert fluids of the present invention include fluorocarbons , polysiloxanes ( silicones ), fluorosilicones . they can be used in pure form or as mixtures . agents of these classes are typically innocuous to living systems . fluorocarbons , particularly perfluorocarbons , have begun to see wide application in medicine ( j . g . riess , overview of progress in the fluorocarbon approach to in vivo oxygen delivery , biomater - artif - cells - lmmobilization - biotechnol ., 20 ( 2 - 4 ): 183 - 202 ( 1992 )) as blood substitutes ( emulsions ), intra - ocular tamponades , and liquid ventilation media ( as pure agents ). perfluorocarbons are generally insoluble in both lipids and water , and thus ideally suited as inert non - toxic perfusates in biological systems . vascular perfusion with pure fc - 72 ( perfluorohexane ) accompanied with fixation preserves endothelial ultrastructure extremely well ( d . e . sims , m . m . horne , non - aqueous fixative preserves macromolecules on the endothelial cell surface : an in situ study , eur - j - morphol . march ; 32 ( 1 ): 59 - 64 ( 1994 )), consistent with the expectation that pure perfluorocarbon is non - toxic intravascularly . rat hearts have been perfused with pure fluorocarbon and demonstrated subsequent recovery ( f . gollan , l . c . clark , organ perfusion with fluorocarbon fluid . physiologist 9 : 191 , 1966 ). example perfluorocarbons for this invention include fc - 72 ( pour point - 90 ° c ., viscosity 1 . 9 centistokes at - 79 ° c . ), fc - 87 ( pour point - 101 ° c ., viscosity 2 centistokes at - 90 ° c . ), pf - 5050 ( pour point - 115 ° c ., viscosity 5 centistokes at - 100 ° c . ), fc - 77 ( pour point - 95 ° c .) and mixtures thereof . fc - 77 and fc - 87 are isomers and homologues of perfluoroalkanes , and are products of minnesota mining and manufacturing ( 3m ) co . following initial cooling by these or similar agents , it &# 39 ; s then also possible to transition to perfluorobutane isomeric mixtures ( pf - 5040 , pour point - 128 ° c ., boiling point - 2 ° c . ), or perfluoropropane mixture ( pf - 5030 , pour point - 183 ° c ., boiling point - 37 ° c .) to continue cooling into the deep sub - zero range . many perfluorocarbon mixtures freeze ( crystallize ) at temperatures not far below their pour point . such mixtures are not preferred for this invention . mixtures containing fc - 77 are a notable exception . fc - 77 is found to vitrify rather than freeze during deep cooling . fc - 77 behaves as a fluorocarbon cryoprotectant , depressing the pour point , and enhancing glass formation in other perfluorocarbons to which it is added . for example a mixture of 20 % fc - 77 and 80 % fc - 87 remains liquid from + 30 ° c . down to - 140 ° c ., which is an ideal temperature span for this invention . this mixture vitrifies at temperatures below - 140 ° c ., avoiding any damage that would be caused by fluorocarbon ice crystals . clearly other perfluorocarbon mixtures affording similar mutual cryoprotective properties are possible in the spirit of this invention . other fluorocarbons , such as fluoroethers , hydrofluoroethers , or hydrofluorocarbons may also be used for the invention , although they are less preferred due to decreased inertness . siloxanes , particularly low molecular weight dimethyl siloxane polymers ( silicones ), are non - reactive , non - toxic fluids that retain low viscosity well into the sub - zero range . octamethyltrisiloxane , for example , remains fluid down to - 80 ° c . the associated monomer , tetramethylsilane ( although less inert ) remains a liquid to - 99 ° c . proprietary siloxane mixtures with even lower pour points are available from dow chemical company . siloxanes are suitable for inert fluid cooling when the target temperature is relatively high , and a fluid of low physical density is desired . for lower temperatures , fluorocarbons are preferred . fluorosilicones ( such as polymethyl - 3 , 3 , 3 - trifluoropropylsiloxane ) are a class of compounds intermediate in density between silicones and fluorocarbons , and may be used at relatively high temperatures when a fluid of density near that of water is preferred . the present invention also discloses the use of new cpa solutions containing glycol ethers in a cryopreservation process . for this invention , &# 34 ; glycol ethers &# 34 ; are understood to comprise chemical compounds containing alkoxy , and particularly methoxy functional groups . exemplary classes include alkoxylated alkanes , alkoxylated alcohols and polyols , as well as other alkoxylated organics . the particular compounds shown above are examples only , and clearly do not exhaust the possibilities for each class . it will be understood by those skilled in the art that various modifications to the these example classes are possible , such as methylation to increase glass forming properties . thus , there are a number of additional compounds which can be effectively used within the spirit and scope of this invention . any compounds which are produced as a result of alkoxylation to improve cryoprotective properties are contemplated . cpas protect biological systems during cooling by interacting with water ( hydrogen bonding ) in a manner that prevents the ordering of water molecules ( freezing ) at low temperatures . interaction with water is usually achieved by including hydroxyl ( oh ), amine ( nh2 ), or other polar groups as part of the cpa molecule . the disadvantage of polar groups with highly localized positive and negative charge is that such groups not only bond with water , but also hydrogen bond with similar groups on adjacent cpa molecules . this strong interaction between cpa molecules is undesirable because it increases solution viscosity ( making perfusion and tissue permeation difficult ), and decreases ice - inhibition and glass forming ability . as an alternative to the hydroxylation of the prior art , methoxylation ( inclusion of o - ch3 groups ) offers the advantage of decreased interaction between cpa molecules while still preserving strong water interaction . the decreased interaction between cpa molecules is achieved by more widely distributing the positive charge that would otherwise be highly localized on a single hydrogen in a hydroxyl group . without a localized positive charge on the cpa molecule , oxygen atoms in adjacent cpa molecules no longer have a bonding target . however the localized negative charge on the oxygen atoms remain , providing a bonding target for the hydrogen in water molecules . this reduction in cpa - cpa interaction maximizes the availability of bonding sites for water , which increases ice inhibition and glass forming properties . viscosity is also greatly reduced , and permeation kinetics are improved . as expected , glycol ethers are a highly penetrating class of compounds that typically pass through cell membranes faster than other molecules of similar size , including amides and alkylamides ( the most penetrating class of cpas previously known ). this rapid penetration allows rapid equilibration between intracellular and extracellular cpa concentration , minimizing tissue dehydration and osmotic injury . it also minimizes time necessary for equilibration , so that tissues need not be exposed to high cpa concentrations at high temperatures for great lengths of time , thereby minimizing toxic effects . glycol ethers possess very low viscosity compared to conventional cpas , and exhibit good ( non - colligative ) freezing point depression . this makes them ideally suited for introduction and removal from tissue at sub - zero temperatures , where toxicity can be minimized . glycol ethers generally , and 2 - methoxyethanol in particular , strongly inhibit ice formation during both cooling and rewarming . this is consistent with the present commercial use of 2 - methoxyethanol as a jet fuel de - icing additive . glycol ethers are good to excellent glass formers . in tables i - a through i - d , experimental data for conventional cryoprotectants and the cpas according to the present invention are provided . glycol ethers achieve high penetration while in many cases still retaining a modest lipid / water partition coefficient ( a measure of hydrophobicity , which correlates with toxicity ). table i - e shows toxicity data for several cpa mixtures containing glycol ethers . table i - a______________________________________conventional cryoprotectancts stab - perm - agent structure cnv ility visc . eance______________________________________ethylene hoch . sub . 2 ch . sub . 2 oh 54 % stable 25 3 . 4glycolpropylene ch . sub . 3 ( choh ) ch . sub . 2 oh 47 % unstable 60 1 . 8glycolglycerol hoch . sub . 2 ( choh ) ch . sub . 2 oh 59 % unstable 1400 0 . 6______________________________________ table i - b______________________________________methoxylated analogs of cryoprotectants sta - bil - perm - agent structure cnv ity visc . eance______________________________________2 - hoch . sub . 2 ch . sub . 2 och . sub . 3 49 % stable 1 . 7 12methoxyethanol1 - ch . sub . 3 ( choh ) ch . sub . 2 och . sub . 3 45 % semi - 2 . 0 7methoxy - stable2 - propanol3 - hoch . sub . 2 ( choh ) ch . sub . 2 och . sub . 3 54 % semi - 80 1 . 0methoxy - stable - propane - diol______________________________________ table i - c______________________________________other examples of glycol ethers of the present invention stab - perm - agent structure cnv ility visc . eance______________________________________dimeth - ch3och . sub . 2 ch . sub . 2 och . sub . 3 42 % stable 0 . 5 20oxyethanediglyme ch . sub . 3 o ( ch . sub . 2 ch . sub . 2 o ). sub . 2 ch . sub . 3 50 % stable 1 10tetra - ch . sub . 3 o ( ch . sub . 2 ch . sub . 2 o ). sub . 4 ch . sub . 3 51 % stable 5 6 . 8glyme______________________________________ table i - d______________________________________exemplary mixture stab - perm - agent cnv ility visc . eance______________________________________2 : 1 ( v / v ) 56 % semi - 60 4 . 4 ( mean ) glycerol : stablemethoxyethanol______________________________________ cnv : minimum concentration ( w / w ) needed to vitrify in pure water during cooling at a rate of 10 ° c . per minute . stability : stability ( resistance to ice formation ) of vitreous solution a cnv during rewarming at a rate of 10 ° c . per minute from - 60 ° c . visc : viscosity ( centipoise ) of pure agent at 20 ° c . permeance : relative rate of penetration across human red blood cell membrane . table i - e______________________________________toxicity of glycol ether mixturesmixture viability______________________________________control 100 % vs4 / 2 - methoxyethanol 98 % vs4 / 2 - ethoxyethanol 95 % vs4 / triglyme 93 % vs4 / 1 , 3 - dimethoxy - 2 - propanol 93 % vs4 / 1 - methoxy - 2 - propanol 89 % vs4 / 1 , 2 - dimethoxyethane 85 % ______________________________________ vs4 / glycol ether refers to a vs4 mixture ( as per u . s . pat . no . 5 , 217 , 860 ) in which propylene glycol is substituted by the glycol ether . viability i measured in a kidney slice model as in the above patent after exposure to a peak cpa concentration of 40 % for 10 minutes at 0 ° c . large animals can be perfused with high concentrations of glycol ethers near 0 ° c . with rapid equilibration , no dehydration , no edema or other visible evidence of toxic effects . histologic preservation is excellent at microscopic and ultrastructural levels . the inclusion of glycol ethers in perfusion solutions is a new and promising approach to the reduction of cryoinjury in organs , tissues and humans cooled to sub - zero temperatures . in preferred embodiments of the present invention , 2 - methoxyethanol and 1 - methoxy - 2 - propanol have been used in perfusion solutions . two glycol ethers ( 2 - methoxyethanol and 1 , 3 - dimethoxy - 2 - propanol ) have occasionally been used for cryopreservation of embryos and cell suspensions ( refs . 20 - 23 ). however , to our knowledge , the special properties and utility ( particularly glass forming ability ) of these compounds , or methoxylated compounds generally , for cryoprotection of vascular tissue have not been documented until now . according to the present invention , the step to introduce a cpa solution into an organ can be carried out in different ways , but usually through vascular perfusion . the perfusion of an organ or any other biological materials usually starts with a dilute cpa solution . concentration of the cpa solution is gradually increased to a predetermined level while the perfusion proceeds . concentrations of the cpa both at the inlet and the outlet of the vascular system are monitored by concentration sensors , which help determine the completion of the cpa perfusion ( achieved when inflow and outflow concentrations are nominally identical ). any cpas or mixtures thereof known to the art can be used in the present invention . but it is preferred to include the glycol ethers mentioned earlier , more preferred to use or include 2 - methoxyethanol or 1 - methoxy - 2 - propanol . cpa perfusion is usually conducted at a temperature between about - 30 to 15 ° c . depending on several factors such as temperature dependence of toxicity and viscosities of particular cpas , the size of organs , and the desired perfusion rate . for example , lower temperature usually reduces the toxic effects of cpa , but also reduces flow rate of the cryoprotectant which may not be desirable . the rate of cpa perfusion is limited by several factors including permeability of the agent into the cell and the size of the organ to be perfused . the time required to complete a cpa perfusion varies from few minute to few hours . for example , in one preferred embodiment of the present invention , the perfusion of a 20 kg dog takes 45 minutes to complete with 2 - methoxyethanol , 2 - 3 hours with 3 - methoxy - 1 , 2 - propanediol . it is not necessary to conduct the perfusion isothermally , i . e . during perfusion the temperature can be reduced gradually which gives the benefit of high perfusion rate at low cpa contractions and low temperature at high cpa concentration . according to the present invention , in a typical cooling application , an organ is perfused with isothermic inert fluid at the completion of cpa perfusion . perfusion with the inert fluid is continued until the vascular system is substantially cleared of the cpa solution . as there is no need to let the inert fluid permeate into cells , perfusion with the inert fluid can be conducted as fast as possible . the perfusion rate of the inert fluid is usually in the range of the basal blood flow rate , depending on the biological material to be cryoprotected and the viscosity of the inert fluid used . this perfusion is conducted at temperatures similar to those used for cpa perfusion . it is very important to wash out the cpa perfusate completely from the vascular system , otherwise the remaining cpa perfusate will block the flow path of the vascular system when the inert fluid is cooled to very low temperatures . effective replacement of aqueous cpa perfusate with inert fluid is difficult due to the differing physical properties of the two liquids . the inert fluid is usually immiscible with cpa solution and water , typically 50 % 100 % more dense than water , and viscosities of those fluids suitable for deep cooling are typically lower than water . two serious problems are caused by these differences in physical properties : ( 1 ) density differences and immiscibility lead to the formation of water &# 34 ; bubbles &# 34 ; in some parts of the vasculature . these aqueous &# 34 ; bubbles &# 34 ; will freeze ( or vitrify ) during deep cooling and plug pathways in the vasculature causing failure of inert fluid perfusion . ( 2 ) the low viscosity inert fluid tends to shunt flow , preferentially following the first clear arterio - venous pathways , leaving downstream vasculature filled with higher viscosity aqueous perfusate . these downstream areas of failed inert fluid perfusion will not experience proper heat transfer during inert fluid cooling / rewarming . several techniques are developed according to the present invention to minimize these problems . the shunt flow problem is most easily addressed by employing a series of inert fluids with different viscosities , i . e . initially perfusing with a high viscosity inert fluid comparable to that of the aqueous cpa perfusate to wash out the aqueous perfusate , followed by progressive dilution of the high viscosity fluid with a miscible low viscosity fluid suitable for deep cooling . although the high viscosity inert fluid used for the initial perfusion is preferably miscible with the inert fluid for later deep cooling , an immiscible initial fluid can be used as long as it does not freeze at deep cooling temperatures . furthermore , as different compounds have different temperature dependence of viscosity , a proper perfusion temperature sometimes is available to help mach the viscosities . thus , use of a series of inert fluids with different viscosities can be used to aid introduction and removal of inert fluids from the vascular system . the problem of immiscibility ( aqueous bubble formation ) can be solved by the inclusion of water / inert fluid surface active agents ( surfactants ) to emulsify the water / inert fluid phases . typical water / inert fluid surfactants used in the present invention include polyoxypropylene / polyoxyethylene copolymers , although other surfactants ( particularly ionic fluorocarbon surfactants ) can also be used . concentrations of surfactants are usually in a range of several percent . the inert fluid can also be introduced and removed as an aqueous emulsion of progressively increasing and decreasing density respectively . in such aqueous emulsions , the inert fluid content preferably varies from 0 - 100 % w / w , more preferably 30 - 95 %, still more preferably 50 - 90 %. the cpa solution content preferably varies from 100 - 0 % w / w , more preferably from 70 - 5 %, still more preferably from 50 - 10 %. the surfactant content is preferably in a range of 0 - 10 % w / w , more preferably from 1 - 5 %. in one preferred embodiment of the present invention , the inert fluid is introduced initially as an emulsion mixture of fc - 87 , 46 % w / w aqueous solution of 1 - methoxy - 2 - propanol and pluronic f - 68 surfactant in which fc - 87 counts 60 % w / w , the 46 % w / w aqueous solution of 1 - methoxy - 2 - propanol counts 39 % w / w , and the rest is the surfactant . ( the transition to pure inert fluid being made following perfusion with maximum - density emulsion .) alternatively , the densities of the aqueous and inert fluid phases can be altered by mixing with compatible solutes or colloids at the time of fluid introduction and removal . following complete replacement of aqueous perfusate by inert fluid of composition suitable for deep cooling , the input temperature of the fluid is then reduced until a desired temperature difference with respect to the organ is achieved . in one preferred embodiment of the present invention , the initial temperature difference between the inert fluid and the organ is preferably set to be within the range from approximately 20 ° c . to approximately 200 ° c ., more preferably this difference is within the range from approximately 50 ° c . to approximately 150 ° c . in a particularly preferred embodiment , the temperature difference is 100 ° c . perfusion of the inert fluid continues until the organ reaches the desired target temperature . the inert fluid can be a pure compound or a mixture . during deep cooling , the composition of the inert fluid can be changed easily . there are many ways to manipulate the combinations of different inert fluids . for example , the deep cooling can be started with an inert fluid with a relatively high pour point such as fc - 72 and replaced later with an inert fluid with a relatively low pour point such as pf - 5030 . it is impossible to exhaust all possible combinations of different inert fluid and cpas according to the present invention , but it is obvious according to the present invention that different combinations can be used for different purposes , which is within the spirit and scope of the present invention . the rate at which an organ can be cooled depends upon the temperature difference between the organ and fluid ( at input ), the mass of the organ , the fluid flow rate , and the heat capacity of the inert fluid . basal blood flow rates in various organs and the whole body of a 55 kg adult are given by guyton ( textbook of medical physiology , a . c . guyton , w . b . saunders company , 1986 .) as in the following table ii : table ii______________________________________organ liters / min . liters / min ./ kg______________________________________kidney 0 . 55 3 . 6liver 1 . 35 0 . 95brain 0 . 7 0 . 5body 5 . 0 0 . 09______________________________________ clearly a high flow rate of the inert fluid is desirable . because viscosities of some of the inert fluids of the present invention are less than that of blood , a flow rate of the inert fluid as high as several times basal blood flow rate can be easily achieved . if necessary , the perfusion pressure can be increased beyond normal physiologic values to further enhance the flow rate of the inert fluid . in addition , once the organ is cooled to a sufficiently low temperature that it is nearly solid , a pump can be used at the outlet of the vascular system to generate a low pressure on the outlet side so as to further promote the inert fluid circulation . the heat capacity of perfluorocarbons is typically half that of water per unit volume . the passage of perfluorocarbon volume equal to an organ &# 39 ; s own volume would thus be expected to lower the organ temperature approximately halfway toward the fluid input temperature . assume the initial temperature of tissue is 0 ° c . perfusion with fluid is begun with an input temperature of - 100 ° c ., and flow rate equal to twice the basal blood flow rate . expected initial and final cooling rates are shown in table iii . ( a temperature of - 90 ° c . is taken to be the final target because below this temperature cpa toxicity and risk of freezing during vitrification are greatly reduced , and cooling can proceed more leisurely .) table iii______________________________________ initial cooling rate final cooling rateorgan ( at 0 ° c .) ( at - 90 ° c .) ______________________________________kidney 360 ° c ./ min . 36 ° c ./ min . liver 95 ° c ./ min . 10 ° c ./ min . brain 50 ° c ./ min . 5 ° c ./ min . body 9 ° c ./ min . 1 ° c ./ min . ______________________________________ the cooling rates shown in table iii are more than ten times greater than can be achieved by previous external cooling methods . such rapid cooling will allow significant decreases in the concentration of cryoprotectants needed to vitrify , enhancing the prospects for successful cryopreservation of organs with non - toxic cpa mixtures . these cooling rates also for the first time open the possibility of vitrifying whole humans . finally , the invention can also be used for rapid rewarming of organs previously cooled by inert fluid perfusion . in this application , the organ would first be rewarmed by external means to a temperature at which the previously perfused fluid becomes liquid (- 100 ° c . for fc - 87 ). the organ would then be perfused with warm inert fluid at a temperature within the range from about - 30 to about 15 ° c ., more preferably from about - 5 to about 5 ° c . in one particularly preferred embodiment of the present invention , the rewarming process is conduced with a warm inert fluid at about 0 ° c . a rewarming process would give initial and final rewarming rates comparable to the cooling rates in table iii ( except that the initial and final temperatures would be reversed ). with rewarming rates in excess of 300 ° c ./ minute achievable for well - vascularized organs such as the kidney , the invention is a possible alternative technology to rf heating for rewarming vitrified organs , or for rewarming frozen organs without recrystallization injury . all the techniques used for deep cooling according to the present invention are applicable to the rewarming . for the rewarming application , it is crucial that dissolved gases that typically accumulate in cold inert fluids be removed during rewarming to prevent gas embolism . this can be achieved by transiently lowering the pressure on the warm side of the perfusion circuit ( such as by stirring , venturi flow , or a vertical excursion of the perfusion circuit ) causing dissolved gases to come out of solution and be captured in a bubble trap . transient overheating ( followed by re - cooling ) in part of the circuit is also effective . in this example , two 20 kg leaving adult dogs are tested for cooling and rewarming the whole body . one of the dogs is cooled and rewarmed using the internal cooling / rewarming method of the present invention . the other is cooled and rewarmed using the conventional external cooling / rewarming method as control test . first the dogs are anaesthetized and subjected to surface cooling with ice bags . the next step is to access the animal &# 39 ; s vasculature via the femoral vessels . then the whole body is cooled to about 15 ° c . the blood is washed out using mshp2 base perfusate , and the body is further cooled to 5 ° c ., then perfused with 2 - methoxyethanol at 5 ° c . the introduction of the cryoprotectant 2 - methoxyethanol is conducted with two containers : at the beginning the first container contains base perfusate with no 2 - methoxyethanol , the second container contains a 35 % w / w 2 - methoxyethanol solution in base perfusate . the first container is connected to the inlet of the vascular system of the dog and pumped into the vascular system , while the second container continuously provides the first container with the 35 % 2 - methoxyethanol cpa solution , so that concentration of 2 - methoxyethanol in the first container increases gradually and reaches to the level of 35 % of 2 - methoxyethanol . the perfusion of 2 - methoxyethanol last about 45 minutes . after the completion of 2 - methoxyethanol solution perfusion , the 2 - methoxyethanol solution within the vascular system of one of the two dogs is replaced with fc - 72 by circulating the fc - 72 through the vascular system at about 5 ° c . the flow rate of the fc - 72 is about 7 litter / min . the replacement process lasts about ten minutes , and the 2 - methoxyethanol is completely removed . then the two dogs are put into a alcohol - dry ice bath separately , ready for deep cooling . for the deep cooling , the initial bath temperature is about - 35 ° c . one dog is allowed to cool solely by contact with the alcohol bath ( the external cooling method of the prior art ). the other dog is cooled by circulating the inert fluid fc - 72 at a flow rate of about 6 liter / min , at a starting temperature of - 10 ° c ., and subsequent adjustment of the fluid temperature to remain 2 ° c . below the measured esophageal temperature . this temperature differential resulted in a cooling rate of about 0 . 25 ° c . per minute , a relatively slow rate which was chosen to avoid intracellular freezing ( the animal not having been perfused with a vitrifiable concentration of 2 - methoxyethanol ). the cooling rates for the inert fluid cooled dog and externally cooled dog are substantially different . it takes about 210 minutes for rectal temperature to drop to - 17 ° c ., for esophageal temperature to drop to - 10 ° c . in the case of the external cooling . while in the case of the internal cooling of the present invention , temperatures decrease sharply at the beginning of the cooling process , - 10 ° c . is almost immediately reached , and at 210 minute rectal temperature already drops to about - 47 ° c . and esophageal temperature to about - 45 ° c . the cooling rate of the method of the present invention is significantly higher than that of the external cooling method of the prior art , especially , at beginning stage . the initial quick drop of the temperature to certain low value helps to minimize the toxic effects of the cpa solution and , therefore , is very desirable . the cooled dogs are also tested with rewarming from - 90 to about - 5 ° c . bath temperature is set at about - 5 ° c . initially in the rewarming process . the initial temperature of the dogs is about - 80 ° c . during the rewarming process , for one dog , fc - 72 at a inlet temperature of about 0 ° c . is circulated through the vascular system , while the other dog is warmed only by placement in the bath . the rewarming rate of the present invention is much faster than that of the external rewarming method of the prior art . after the first 15 minutes , temperatures of right tympanic , esophageal and rectal probes have reached - 14 ° c ., - 26 ° c . and - 42 ° c . respectively in the case of the present invention , while in the case of the external rewarming method , the corresponding temperatures are about - 28 ° c ., - 73 ° c . and - 60 ° c . to reach similar temperatures as the present invention , it takes several hours more for the external rewarming method . the rewarming rates achieved in this example are representative of the cooling rates that can also be achieved if the inert fluid / tissue temperature differential is maximized on cooling ( as would be done if vitrification was the objective ). in addition to extremely rapid cooling , the invention can also be used to cool with extreme uniformity . by providing a large heat exchange surface area and maintaining a small temperature difference between the organ and the fluid input , organs can be cooled or rewarmed in a uniform controlled manner without the temperature gradients that would otherwise accompany external cooling methods . in still other applications , the invention can be used to maintain an organ at low temperature in situ without removal from a living animal . for example , protocols for central nervous system cryopreservation might be studied by selectively perfusing and cooling the brain to low sub - zero temperature while maintaining the rest of the animal at a higher temperature compatible with later recovery . the method of the present invention allows cooling and subsequent rewarming from temperatures lower than - 100 ° c . at rates exceeding 100 ° c . per minute for some organs . these rates are much higher than can be achieved by external heat transfer methods , and will allow significant reduction of the concentration of cryoprotective agents needed to achieve reversible vitrification of organs for long - term banking . heat transfer by inert fluid perfusion is also beneficial for reducing ice crystal damage and cryoprotectant toxicity during ordinary freezing and thawing . the present invention also provides a class of new cryoprotective agent ( glycol ethers ) for reduction and prevention of ice formation during cooling of vascular tissues and organs . glycol ethers generally , and methoxylated compounds in particular , are highly penetrating agents that equilibrate rapidly upon perfusion , and exhibit strong ice inhibition and glass forming properties . the low viscosity and freezing point of these compounds also make them well - suited for sub - zero perfusion to minimize toxic effects . toxicities are compatible with the potential use of glycol ethers in perfusate solutions for reversible cryopreservation of organs and large organisms by freezing or vitrification . 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