fig1 depicts a soft tissue implant 10 according to the present invention having been implanted in a female breast 12 . the implant or prosthesis 10 is comprised of an envelope or pouch 14 and filler material 16 . the filler material is comprised primarily of collagen and elastin fibers whereas the pouch is made of an elastin / collagen membrane derived from natural animal tissue . preferred pouch membrane starting materials include bovine or porcine peritoneum or omentum . these tissues are cleaned of non - collagenous and non - elastinous proteins by a process which does not destroy the natural collagen / elastin matrix . one suitable cleaning method is described in u . s . pat . no . 5 , 028 , 695 , the disclosure of which is incorporated herein by reference . basically , the cleaning process is carried out by first washing the membranes and subjecting them to a strong alkali treatment , preferably by soaking them in a sodium hydroxide ( naoh ) solution . other suitable alkali treatments might include potassium hydroxide ( koh ) or calcium hydroxide ( caoh ). after washing again with water , the membranes are treated with a strong acid , preferably hydrochloric acid ( hcl ). other suitable acids might include sulfuric acid ( h 2 so 4 ) and acetic acid ( c 2 h 4 o 2 ). washing then removes most of the acid , and that which remains is neutralized by use of a base , preferably bicarbonate of soda ( nahco 3 ). other suitable neutralizing agents might include sodium acetate ( nac 2 h 3 o 2 ) and disodium hydrogen phosphate . the resulting salts are then removed by washing in water , following which the water is removed , for example , by acetone extractions , and the membranes are air dried to remove the residual acetone . while it is believed unnecessary and , indeed , preferable not to cross - link the collagen in the pouch membrane , cross - linking may be used for some applications , for example , it is possible that some cross linking may assist in decreasing the degradation rate . if there is to be cross - linking , the cross - linked collagen should be non - cytotoxic and should not provoke cellular or immune responses . hexamethylene diisocyanate may be one such cross - linking agent , as may some polyepoxy compounds , for example , polyethylene glycol diglycidyl ether and water soluble carbodiimides , for example , 1 - ethyl - 3 3 - dimethyl amino propyl ! carbodiimide . hcl in the presence of n - hydroxysuccinimide . since the cross - linking agents are believed to form very stable bonds with the collagen , they produce cross - linked collagen which is non - cytotoxic and is not likely to provoke any immune or cellular response . preferred filler starting materials include ligaments and tendons . in particular , the starting material believed to be most suitable is bovine ligamentum nuchae because it is made up largely of elastin , with only a relatively small amount of collagen . indeed , more than 70 % of the dry weight of this ligament is elastin . the relatively high elastin content and relatively low collagen content make it an ideal starting material for the filler material of the present invention . as noted above , the term &# 34 ; elastin &# 34 ; is used herein to include elastin together with microfibrils . elastin in natural tissue is normally associated with microfibrils and , for the purpose of the present invention , no effort is made to remove the microfibrils . while , in its broadest scope , the present invention covers the use of elastin which has had the microfibrils removed , it is believed preferable not to remove them . to make a preferred filler material according to the present invention , the ligamentum nuchae may be cleaned using a process similar to that taught in u . s . pat . no . 5 , 028 , 695 . the ligament is first cleaned of blood and adherent tissue and then is chemically treated to remove the non - elastinous and non - collagenous components . the chemical treatment preferably includes subjecting the ligament to a strong alkali treatment then to an acid treatment and then to a neutralizing bath . a preferred alkali treatment involves soaking in a sodium hydroxide ( naoh ) bath . other suitable alkali agents include potassium hydroxide and calcium hydroxide . a preferred acid for the acid treatment is hydrochloric acid ( hcl ) although other acids , such as sulfuric acid and acetic acid , might also be suitable . as for the neutralizing step , many suitable bases are available , the preferred one being bicarbonate of soda ( nahco 3 ), while other . neutralizing agents might include sodium acetate and disodium hydrogen phosphate . the chemical treatment is followed by mechanical manipulation which separates the elastin and collagen fibers . the separated fibers are then dried , preferably without cross - linking . however , it might be possible to decrease the degradation time if the collagen were cross - linked so as to produce a non - cytotoxic material that does not provoke a cellular or immune response . suitable cross - linking agents might include a hexamethylene diisocyanate , a polyepoxy compound , or a water soluble carbodiimide as discussed above . although the preferred filler starting material is the ligamentum nuchae , other starting materials may also be used . for example , the peritoneum and omentum membranes have about 15 % elastin and could also be used to make a filler material in accordance with the present invention . in addition , elastin and collagen from different sources could be mixed together to produce a mix having whatever proportions are deemed advantageous for a particular application . it is believed , however , that the composition should have minimum of perhaps as little as about 10 % elastin ( dry weight ) and might have as much as 90 % or perhaps even higher of elastin . while , as noted , it is believed preferable for the filler to contain both collagen and elastin , elastin alone without collagen might also be suitable . similarly , collagen without elastin might be used , particularly if that collagen is cross linked with an agent that does not leach out and produces a non - cytotoxic composition that does not provoke a significant immune or cellular response . such suitable cross - linking agents might include the hexamethylene diisocyanate , polyepoxy compounds and water soluble carbodiimides referred to above . it has been found that the cleaned membranes prepared in accordance with this invention have a mesothelial side 18 and a loose connective tissue side 20 . the pouch 14 is made with the loose connective tissue side facing out because when implanted with the loose connective tissue side out , it is believed that the pouch will rapidly become anchored in place as a result of fibrovascular ingrowth from the surrounding tissue . unlike the prior art prosthesis , the implant of the present invention employs a porous membrane to form the envelope . a porous membrane is made possible because the filler material 16 is made up of cleaned natural fibers , namely elastin and collagen , which have very low antigenicity and hence cause only minor and transitory inflammatory response . in addition to being porous , the envelope membrane has very low antigenicity and produces only a very minor inflammatory response . moreover , because the loose connective tissue side which faces out promotes vascularization , the membrane quickly becomes integrated into the surrounding tissue without the formation of the fibrous encapsulation and contracture which is found in the prior art . it is believed that once the prosthesis of the present invention is implanted , the collagen in the membrane causes some mild transitory tissue reaction in the surrounding tissue and , therefore , some increased vascular activity . vascularization of the implant membrane begins while the collagen in that membrane is being resorbed and proceeds through the membrane and into the collagen / elastin filler material . this vascular ingrowth through the membrane into the filler material leads to resorption of the collagen , both in the membrane matrix and in the filler material . as vascularization and resorption of the collagen in the filler material is going forward , fibroblasts are activated . those fibroblasts , which appear to have an affinity for elastin , attach themselves to the elastin fibers and lay down an organized matrix of new native connective tissue . the natural vascularization which occurs and the integration of the envelope membrane into the surrounding tissue may make suturing of the envelope in place unnecessary . if however , such suturing is deemed advisable , it can easily be done , preferably with non - absorbable suture material like prolene . in this connection , it should be noted that suturing can be done both easily and without creating any additional risk . the envelope membrane , being composed of a natural elastin / collagen matrix , will readily close any hole made by a needle and suture . moreover , even if the natural sealing of the holes is not complete , slight leaks of filler material are of no concern because , as discussed above , the filler material is made up of cleaned collagen and elastin , both of which are either non - antigenic or , at worst , only minimally antigenic . while it is believed most desirable for the membrane to consist of a natural collagen or collagen / elastin matrix , the filler material herein disclosed can be used in any pouch . for example , if it is determined that the primary problem with present day pouches relates to the filler material rather than the pouch membrane material , the filler of the present invention could be substituted for that prior art filler without changing the pouch membrane . thus , a pouch made of a silicone membrane filled with the filler material of the present invention should be understood as falling within the scope of the present invention . a batch of about 200 porcine peritoneal membranes , each measuring about 20 × 30 cm are obtained from a slaughterhouse and , if necessary , mechanically freed of fatty residues . these membranes , which weigh about 10 kg , are washed for about 4 hours with running tap water at about 45 ° c . in a rotating drum and then washed three more times with 40 l . of room temperature tap water . after the washing steps have been completed , the membranes are soaked in 40 l . of a 0 . 5 % ( w / w ) solution of sodium hydroxide ( naoh ) in tap water for up to 13 days at room temperature until the amido nitrogen content is reduced by about one half to original value or to about 0 . 35 m mol / g . or less . during this period , the naoh solution may be refreshed several times . thereafter , the membranes are again washed 3 times with 40 l . of room temperature tap water . next , the membranes are soaked for 4 hours in a room temperature solution of hydrochloric acid ( hcl ). the hcl solution is prepared by mixing 4 l . of concentrated ( 37 %) hcl with 36 l . of tap water . after completion of the acid treatment , the membranes are washed with tap water until the ph of the wash water is between 2 . 5 and 3 . the remaining acid is then neutralized by soaking the membranes overnight in 50 l . of tap water to which 350 g . sodium hydrogen carbonate ( nahco 3 ) has been added . there then follows another tap water wash to remove the salts formed during neutralization . samples of the wash water are periodically mixed with a silver nitrate ( agno 3 ) solution . when mixing with the agno 3 solution no longer produces precipitates , removal of salts is deemed complete . water is then removed from the membranes by means of acetone extractions . finally , the acetone is removed by air drying while the membranes are kept in constant motion . a 10 kg batch of bovine ligamentum nuchae which has been mechanically freed of fatty residues is soaked overnight in about 40 l . of tap water at room temperature . this initial soak removes adherent blood and other water soluble components and also assures a more or less natural degree of hydration which is believed to facilitate the subsequent chemical treatments . the ligaments are then washed twice for about 10 minutes each with 50 l . of tap water , before being placed in 50 l . of a 4 % ( w / w ) solution of sodium hydroxide ( naoh ) in tap water . they are permitted to remain in this strongly alkaline soak for 48 hrs . at room temperature . the alkaline soak is followed by three 10 minute washes in 50 l . of tap water . the ligaments are then subjected to a second alkaline soak , this one in 50 l . of a 2 % ( w / w ) solution of naoh in tap water at room temperature for 72 hrs . after removal from the second alkaline soak they are washed three more times for 10 minutes in 30 l . of tap water to remove the solubilized components . next , the ligaments are placed in a solution of hydrochloric acid ( hcl ) for about 4 hr . the hcl solution for this soak is prepared by mixing 4 l . concentrated ( 37 %) hcl with 36 l . of tap water . the acid soaked ligaments are then washed in tap water until the ph of the wash water is between about 2 . 5 and 3 . the ligaments are then placed in an nahco 3 soak to neutralize the remaining acid . the nahco 3 soak is prepared by adding 350 gm . of nahco 3 to 50 l . of tap water . the ligaments are left in this neutralizing bath overnight and then they are again washed in tap water to remove the resulting salts . washing continues until mixing with a silver nitrate ( agno 3 ) solution produces no precipitates . a kolloid mill is then used to separate the natural elastin fibers . fiber length may also be reduced in the kolloid mill . acetone extractions are then used to remove the water and , finally , the fibers are air dried in an oven at about 75 ° c . the filler material which emerges from this process is a fluffy mass of elastin and collagen fibers . they may be used in that state or they may be compressed and molded to any desired shape . two dried membranes , prepared in accordance with example 1 above , are cut into circles of about 12 cm . in diameter . the edges are sewn together using prolene suture material to form an envelope or pouch . while non - adsorbable suture material like prolene is thought to be preferable , absorbable sutures like polylactide or polyglycolide may also be suitable . when the membrane edges are sewn together , one section of the periphery is left unsewn and 30 gm . of filler material , prepared in accordance with example 2 above , is inserted through the unsewn section . the unsewn section is then sewn shut . in the examples above , both the membranes and the filler material are dried at the end of the cleaning processes . this facilitates handling during the sewing and filling of the pouch . a pouch made of dry materials is more easily sterilized than is one made of wet components . also , pouches are more easily stored in the dry state and sterility is more easily maintained over extended periods of time when all the materials are dry . nevertheless , although drying is preferred before preparation of the implant pouch , it is not absolutely necessary . finally , approximately 30 minutes before implant surgery , the sterile , dry pouch is hydrated , for example , in sterile water or phosphate buffered saline . the pouch is then implanted in the wet state . with implants according to the present invention , even after implantation has been completed , the degree of augmentation can still be changed . for example , if the amount of augmentation is found to be insufficient , additional filling material can be injected into the pouch through an hypodermic needle . for such purpose , the filling material would normally be suspended in a biocompatible carrier such as glycerine and water . because of the nature of the pouch membrane , the hole made by the needle is self sealing and hence seepage of filling material into the surrounding tissue would be minimal to non - existent . moreover , since the filling material is non - cytotoxic , whatever seepage that might occur would be of no concern . similarly , if the degree of augmentation is too great , an hypodermic needle could be used to extract some of the filling material . were this to be done , however , it would have to be done before there has been significant ingrowth . while it is believed that breast augmentation and breast reconstruction are the fields in which the present invention will likely find its most immediate utility , this invention is not limited to such uses . for example , implants in accordance with the present invention could be used to build up portions of a person &# 39 ; s face , arms , thighs or buttocks . in fact , an envelope made in accordance with this invention could be implanted into any soft tissue in the body . it will readily be apparent to those skilled in the art that numerous modifications , alterations and changes can be made without departing from the inventive concept described herein . accordingly , all such variants should be viewed as being within the scope of the invention as set forth in the claims below .
soft tissue implants can be constructed by forming an outer membrane into a closed pouch and filling material in that pouch . a composition for use as a filler material in such a soft tissue implant may include elastin and collagen . the filling material has collagen as a component thereof , and this collagen can be cross - linked or not cross - linked . methods of making such implants are described .