human or animal tissue can be used as the biological tissue in the method of the invention , for example skin , dura mater , fascia lata , tendons , vessels , cartilage , pericard , bones and plates made of bone , nails , pins , screws . this tissue consists of collagen or of collagen and mineral components . the transplants produced in accordance with the method of the invention are available to the surgeon at any time . the tissue is preferably dehydrated in the first step with the organic solvent which can be mixed with water to a water content in the range from 10 weight percent to 25 weight percent . with soft tissue such as skin , dura mater , fascia lata , dehydration is preferably carried out to a water content in the range from 17 weight percent to 20 weight percent . with hard tissue such as bone , in particular spongiosa bone , dehydration is preferably carried out to a water content in the range from 10 weight percent to 15 weight percent , with , as is to be expected , the structure of the native tissue being maintained . however , it is surprising that the subsequent freeze drying in the second step for the further dehydration up to the desired water content of less than 8 weight percent , which is as low as possible , does not have a negative effect on the tissue structure . methanol , ethanol , propanol , isopropanol , acetone , methyl ethyl ketone or their mixtures can be used , for example , as solvents in a manner known per se . preferably , acetone is used as an organic solvent which can be mixed with water . the solvent used should have a water content which is as low as possible , it should preferably be free of water . the dehydration with the solvent is carried out at temperatures in the range from 0 ° c . to 70 ° c . depending on the solvent used . the dehydration of the first step preferably takes place at room temperature . the tissue is preferably exposed to a vacuum after the dehydration of the first step before it is deep frozen to temperatures from approximately − 25 ° c . to − 40 ° c . the organic solvent is thereby largely removed from the tissue . in the dehydration in particular of spongiosa bone , it can be advantageous to simultaneously carry out a treatment with ultrasound , vibrators or rockers in the first step during the dehydration with the organic solvent . this promotes a better penetration of the solvent into the fine passages of the spongiosa bone and thereby the degreasing and the dehydration . for the same purpose , an overpressure , alternating pressure or underpressure can also be applied . it can furthermore be advantageous to carry out a vacuum treatment before the first extraction step and after every extraction step before dehydration is carried out with fresh solvent in the next step . this also promotes degreasing and a better exchange of the aqueous organic solvent in the passages with fresh organic solvent . all these measures can also be carried out with soft tissues . the freeze drying in the second step takes place in a conventional freeze drying unit . the partly dehydrated tissue is therein gradually brought to temperatures from , for example , − 25 ° c . to − 40 ° c . and the ice produced in the tissue is removed by sublimation by applying a vacuum . as already stated further above , a vacuum is preferably applied before the freeze drying , that is before the cooling of the tissue to low temperatures . in this way , the solvent is removed from the tissue in part . the freeze drying follows on from this . the invention will be explained in more detail with reference to the following examples and to fig1 and 2 . fig1 and 2 are diagrams in which the time curve of the dehydration of the tissue in the examples is shown . the time in hours or days is entered on the abscissa and the water content in weight percent relative to the total weight of the material to be dehydrated on the ordinate . dura mater is removed from the human body and liberated in a manner known to one skilled in the art from antigenic substances and enzymes . for preservation , the tissue parts cleaned in this manner are treated twice for six hours at a time by being placed into anhydrous acetone at room temperature . the solvent quantity amounts in each case to 500 % of the wet weight of the tissue , with a de - swelling of the tissue taking place from 0 . 65 mm to 0 . 57 mm . the water content at the completion of this first dehydration step amounts to 20 weight percent . in the second step , the tissue is cooled for three hours to − 40 ° c . in a freeze drying unit . then , a vacuum of 1 . 2 mbar is applied for removing the ice formed in the tissue by sublimation . the shelf temperature amounts to 35 ° c . the water content after the second step , which takes a total of 15 hours , amounts to 6 weight percent . the thickness of the tissue amounts to 0 . 54 mm and the inner surface is 20 m2 / g . the course of dehydration is shown in fig1 . after packing in moisture - tight pouches and after sterilizing with gamma rays with a minimum dosage of 15 kgry , the preserved dura mater can practically be stored without limitation and is ready for use for transplants . if , instead , the dehydration of the dura mater is carried out only with acetone to the same water content of 6 weight percent , three extraction steps of 12 hours each must be conducted . a spongiosa bone is prepared in a suitable manner known to one skilled in the art . for preservation , the prepared bone is treated five times for 24 hours each time with anhydrous acetone at room temperature . the solvent quantity amounts to 500 % of the wet weight of the bone in each case . after this treatment , the water content amounts to 12 weight percent . the bone is subsequently cooled for 3 hours to − 40 ° c . in the second step and then a vacuum of 1 . 2 mbar applied for removing the ice formed in the bone by sublimation . the shelf temperature amounts to 35 ° c . the water content after the second step , which takes a total of 48 hours , amounts to 2 weight percent . the dehydration course is shown in fig2 . if , instead , the dehydration of the spongiosa bone is carried out only with acetone , then up to 20 extraction steps of 24 hours each are required , with approximately 60 ltrs . of acetone being required per 1 kg of bone .
the invention relates to a two - step method for dehydrating biological tissues for producing preserved transplants . in a first step , the tissue is partially dehydrated with an organic , water - miscible solvent . in a second step , the tissue is dehydrated further by freeze drying .