one preferred embodiment of the invention relates to a gel casting technique for resorbable synthetic alpha - polyesters , which may be used for producing novel bone repair materials or bone substitutes , resorbable medical implants and timed release carriers for medication . formation of a distinct gel phase in the techniques of this invention enables production of relatively thick - section , solid moldings . the gel may be used as a medium to incorporate non - gelling or weakly gelling polymers ( such as low molecular weight species , non - lactide polymers , or lactide copolymers ), in the material composition for control of properties such as resorption rate and density , mechanical properties , shrinkage , and thermal characteristics . gel casting can be used to vary morphology ( e . g . solid versus microporous materials ) and to produce blends of homopolymers , copolymers and particulate fillers , such as tricalcium phosphate . control of material biodegradation rate by variation of polymer type , molecular weight range , crystallinity and morphology may therefore be facilitated . other embodiments of the invention include processes for preparing gels from the gelling polymer and from blends of such polymer with resorbable and nonresorbable homopolymers , copolymers , and filler materials ; processes for obtaining thick - section solid and microporous materials / foams from the gels , methods for controlling the shrinkage of microporous materials containing a substantial amorphous component by controlled drying and extraction of plasticizing molecules ; and resorbable implants in the form of honeycomb structures for repair of long bones , being manufactured from microporous gel cast materials . in a preferred embodiment , the gelling medium is a pla - pga copolymer having an average molecular weight of around 45 , 000 daltons which is sold commercially by medisorb , inc . of cincinnati , ohio . the solvent may be acetone or ethyl acetate . acetone is preferred since polymer dissolution is generally easier in this solvent . dissolution of a potential gel - forming polylactide polymer in acetone appears to be highly dependent on the physical form of the starting material . a decrease in crystallinity of the starting polylactide will also facilitate its dissolution in acetone and increase its potential for use as a gel - forming medium . useful polymers and copolymers for blending with the gelling medium include lactide homopolymers , non - lactide polymers such as poly epsilon carpolactone , lactide copolymers , copolymers produced from mixtures of lactide and non - lactide comonomers such as lactones ( e . g . epsilon caprolactone ) or other hydroxy acids ( e . g . glycolic acid ), lactides , non - lactide polymers ( e . g . polyethylene oxide ), or copolymers containing &# 34 ; soft blocks &# 34 ; of polyether , polyester or other similar polymers . such blending may enable variation of molecular weight distribution , density , shrinkage , and mechanical and thermal characteristics . useful fillers include particulates of bioceramics such as tricalcium phosphate and hydroxyapatite ; non - resorbable discontinuous fibers of alumina , carbon or polyethylene terephthalate ; or resorbable discontinuous fibers such as polyglycolic acid or calcium metaphosphate . in a preferred embodiment of this invention , a method is provided for preparing solid resorbable materials , comprising the steps of : solid materials may alternatively be produced by extracting the solvent with a nonsolvent ( e . g . methanol ) over around 24 hours before drying , then drying the material . shrinkage may result in material consolidation to form a solid core encased in a layer of microporous material . this latter material may be removed by machining if desired . for highly crystalline polymers , e . g . polylactide , the gel casting method described above may be altered to produce microporous materials ( or foams ) of good structural integrity and foam consistency by following the steps ( 1 )-( 4 ) above , then converting the gel to a microporous material by precipitation in a nonsolvent such as methanol , followed by drying . the density of microporous blends containing certain amorphous polymers can be controlled by predrying and water immersion subsequent to gel production . extraction of solvent may remove the plasticizing effect of organic molecules , thereby restricting chain recoiling of the amorphous phase , which may result in material shrinkage on drying . if substantially amorphous polymers or semi - crystalline polymers are included with the basic gelling polymer , the product gel may be converted to a microporous material by precipitation in an organic nonsolvent , and / or immersion in water to remove the plasticizing effect of organic molecules , followed by drying . also , the extent of shrinkage and , therefore , density of microporous materials containing a substantial proportion ( e . g . about 25 %) of amorphous polymer can be controlled by partially drying the gel ( or the methanol - treated gel ) to a desired level and extracting / replacing the solvent or nonsolvent with water prior to drying . this may remove the plasticizing effect of the organic solvent or nonsolvent molecules , which may facilitate chain recoiling in the amorphous phase , resulting in excessive shrinkage of the material on drying . the resorption rate of biodegradable polymers may be influenced by the material form . porosity generally facilitates fluid ingress throughout the material , exposing a large surface area of the material to chain scission by hydrolysis . increased degradation rates can be expected . porous implant surfaces present a favorable surface for cell attachment and growth , enhancing the implant &# 39 ; s function as a biodegradable scaffold for tissue repair or implant fixation . the release rate of medication from resorbable , polymeric delivery systems may also be influenced by the porous character and density of the delivery vehicle . the control of density and , therefore , pore size and structure in resorbable polymers may be achieved by the above - described drying / water treatment stage in the process of manufacture of microporous materials from blends containing an amorphous polymeric component . the resulting solid or microporous materials can be used to form implantable devices of various shapes , preferably discs for nonunion fractures . advantages of a microporous material include the increase in surface area for hydrolysis or breakdown of the implant and the presence of a potentially better surface for attachment of osteoprogenitor cells . as illustrated in fig1 and 2 , the material can be machined to produce a bone graft substitute 20 of macroporous character , for example by drilling 100 - 500 μm bores 22 running the length of the implant and intersecting with the implant end surfaces 24 . in one embodiment , the macropores are about 500 μm in diameter . the center hole is about 2 mm in diameter , and the macropores are spaced about 1 mm from each other , center to center . in repair of long bones , the macropores allow migration throughout the implant to establish a supply of osteoprogenitor cells and growth factor and allow primary bone growth and vascularization along the pore channels . gradual resorption of the implant bridge allows secondary bone formations to be established and bone remodeling to take place by load transfer to the ingrown tissue . a polymer blend of slow - resorbing polymer ( l - pla ) and fast - resorbing polymer [ e . g . poly ( dl - lactide co - glycolide )] can be used to produce an &# 34 ; enduring scaffold &# 34 ; system which provides a support element for immature bone formations during and after resorption of the fast - degrading phase to promote and encourage satisfactory tissue repair , stability and remodeling . in another embodiment of this invention the material used is a 50 : 50 copolymer ( plg50 ) of pla and pga having an intrinsic viscosity of 0 . 65 dl / g and a molecular weight of 45 , 000 daltons , of medisorb inc ., cincinnati , ohio . a circular implant having a thickness of about 2 . 5 mm and a diameter of about 13 . 5 mm is formed , as shown in fig1 and 2 , is formed by gel casting . the polymer is solubilized in acetone , placed in a mold and allowed to stand until a gel is formed . the acetone is extracted by immersing in mixtures of methanol and acetone having successively higher concentrations of methanol . the implant precipitates as a microporous solid with pores approximately 5 to 20 μm in size . in another embodiment , a therapeutically active additive is added to the acetone / polymer solution prior to gel formation . the additive is released in an initial burst which augments initial wound healing , followed by a second , essentially continuous and steady release which fosters differentiation and maturation of the cells to form bone tissue bridging the nonunion . when such an additive is used , the mold is preferably rotated at a rate of about one - fourth to one - half revolution per minute to keep the additive evenly distributed throughout the material and prevent settling while the gel is forming . the following examples are designed to illustrate certain aspects of the present invention . the examples are not intended to be comprehensive of all features and all embodiments of the present invention , and should not be construed as limiting the claims presented herein . material shrinkage was measured with respect to the diameter of the gel on demold . material density was estimated from measurements of the dimensions of a disc of material and its corresponding weight in air . drying of materials was carried out in air under ambient conditions unless otherwise specified . polymer dissolution was aided by stirring with a magnetic stirrer bar and , unless otherwise specified , the mold was a 10 cc plastic syringe body . a single component gel was produced by heating finely divided l - pla ( polysciences , inc ., mw 100 , 000 , batch no . 61490 ) in acetone at a concentration of 7 - 9 % ( w / v ), with agitation at a temperature of 46 °- 52 ° c ., until dissolved ( approximately 15 minutes ). 7 % ( w / v ) gave the best results . the clear solution was poured into a mold and allowed to cool to room temperature under quiescent conditions . the cloud point , or the point at which opacity developed in solution due to crystal formation , was accompanied by a marked increase in solution viscosity , and occurred at approximately 28 °- 32 ° c . for a 10 % ( w / v ) solution . a weak gel was formed at 25 ° c ., which was easily disrupted by stirring . the product gel which formed in situ in the mold had a distinct white color and hardened over time . it was firm enough to withstand demold after 30 minutes at room temperature ( 22 °- 24 ° c .). at that point , the demolded gel was allowed to dry to remove the solvent and produce solid material . a shrinkage in diameter of approximately 50 % relative to the demolded gel occurred after 24 hours air drying . a demolded gel prepared as in example 1 was immersed in methanol to extract solvent and produce a microporous material on subsequent drying . an acetone - based gel produced from a 7 % ( w / v ) solution was immersed in methanol for three days ( 50 cc methanol in 100 cc beaker with a change of immersion medium at 24 hours ). the methanol was allowed to evaporate and the material allowed to dry in the beaker over four days at room temperature . a shrinkage of approximately 40 % occurred relative to the gel on demold and a density of 0 . 35 gm / cm 3 was obtained for the product foam / microporous material . treatment of the demolded gel in methanol for three days , followed by water immersion for three days , with a change in immersion medium after 24 hours , slightly reduced the shrinkage to 37 % and yielded a microporous material density of 0 . 33 gm / cm 3 after drying for four days at room temperature . an acetone - based gel of the preferred l - pla polymer produced from a 7 % ( w / v ) solution was redissolved by heating in an excess of solvent ( e . g . 1 . 1 gm gel in 10 cc acetone ) at 50 ° c . in less than ten minutes . a 2 . 5 % ( w / v ) solution of l - pla ( mw 100 , 000 ) in acetone was produced by dissolving 0 . 5 gm of the polymer in 20 cc of solvent at 50 ° c . the solution was poured into molds . on standing at room temperature , opacity developed in solution after 45 minutes , denoting crystal formation . on demold after 19 hours , a strong , white gel was evident , which exhibited a 65 % shrinkage in diameter after five hours air drying at room temperature . l - pla of molecular weight 200 , 000 ( polysciences , inc . ), when heated in acetone at 50 ° c . at a concentration of 2 gm polymer to 20 cc of solvent , did not dissolve completely . instead , a milky suspension of powder in solvent was obtained . a weak gel was , however , formed from this suspension after one hour at room temperature , presumably due to dissolution of a certain fraction of the starting polymer . this same polymer dissolved readily in p - dioxane to give a 10 % ( w / v ) solution , but did not gel under quiescent conditions at room temperature . l - pla , with a weight average molecular weight of 50 , 000 ( polysciences , inc .) when heated in acetone at a 9 % ( w / v ) concentration also resulted in incomplete dissolution ( milkiness persisted ). after standing at room temperature for 17 hours , a very weak , easily damaged gel was evident on demold which collapsed under its own weight . doubling the concentration of 50 , 000 l - pla in acetone also produced a weak gel on standing which cracked or crumbled on drying . l - pla , mw 100 , 000 , exhibited a crystalline melting point at approximately 160 ° c . rapid cooling in the molten state from 230 ° c . resulted in increasing amorphous content , evidenced by the absence of a recrystallizing transition on cooling . on reheating , no recrystallization of the amorphous component occurred . in contrast , a weakly gelling lactide polymer ( mw 50 , 000 ), which was substantially unsuitable for use , exhibited a recrystallizing transition at around 125 ° c ., followed by a crystalline melting transition at 170 ° c . on reheating the amorphous form produced by rapid cooling . this reflected a greater chain mobility for this polymer . these tests suggest that operable semi - crystalline polymers suitable for producing the gelling medium in the disclosed gel casting technique may be characterized by a limited chain mobility due to molecular weight distribution , which impedes recrystallization of the amorphous form of the polymer on heating . formation of a distinct gel phase immediately following polymer dissolution tends to enable production of thick solid moldings . an 11 mm diameter cylinder of the 100 , 000 mw l - pla was produced by air drying for 17 hours a gel produced from a 9 % ( w / v ) solution in acetone , followed by annealing for 56 hours at 72 ° c . final shrinkage ( based on diameter ) was 62 %. in contrast , the absence of gelation in a 7 % ( w / v ) solution of the preferred l - pla polymer in chloroform at room temperature resulted in the formation of a film or coating on the mold walls on solvent evaporation over four days . shrinkage forces developed during drying of the gel at room temperature , yielding a homogenous , thick - section solid product with a state of consolidation visually similar to that of a thermoplastic processed in the melt at high temperatures and pressures by injection molding or extrusion . material consolidation in gel casting was achieved in a &# 34 ; cold &# 34 ; system through a combination of factors . the gel &# 39 ; s liquid medium provided crystal mobility and chain flexibility in the amorphous phase by a plasticizing effect . this , coupled with the forces generated by the tendency of macromolecules in the solvent - swollen , amorphous phase to recoil on evaporation of the swelling medium , resulted in the observed good material consistency . an 11 % ( w / v ) solution of low molecular weight l - pla ( mw 2000 , polysciences , inc .) was produced by dissolving 2 . 2 gm polymer in 20 cc acetone at 52 ° c . and allowed to stand at room temperature for 21 hours . an extremely weak gel was evident on demold , which did not retain the molded form but collapsed under its own weight , eventually drying to a weak , brittle solid . 12 % ( w / v ) solutions in acetone of poly ( dl - lactic acid ) ( mw 20 , 000 , polysciences , inc .) or 70 : 30 poly ( dl - lactide co - glycolide ) copolymer ( mw 30 , 000 - 60 , 000 , polysciences , inc .) or 90 : 10 poly ( dl - lactide co - glycolide ) copolymer ( mw 30 , 000 - 60 , 000 , polysciences , inc .) or 85 : 15 poly ( dl - lactide co - glycolide ) copolymer ( mw 40 , 000 - 100 , 000 , dupont ) did not gel on standing at room temperature . gradual evaporation of solvent occurred from the bulk solution resulting in formation of a viscous fluid . these polymers were combined with the preferred gelling l - pla polymer , for example , in the ratio 25 ( l - pla ): 75 ( other ), by simultaneous dissolution in acetone to produce a blended polymer gel . the gel was subsequently converted to a solid material by drying / solvent extraction . for predominantly crystalline polymer additions , e . g . lower molecular weight l - pla , the gel was converted to a microporous material by precipitation in a nonsolvent , followed by drying . for substantially amorphous polymer additions to the basic gelling polymer , the gel was converted to a microporous material by precipitation in a nonsolvent and / or immersion in water ( to remove the plasticizing effect of organic molecules ), followed by drying . non - gelling or weakly gelling polymers were incorporated into a particular material composition by using l - pla as the gelling medium . a 30 % solid , particulate - filled , resorbable material consisting of tricalcium phosphate in l - pla was produced by dissolving 1 . 4 gm l - pla ( mw 100 , 000 ) in 20 cc acetone at 52 ° c . on dissolution of the polymer ( in approximately 15 minutes ), 0 . 6 gm of tricalcium phosphate ( tcp ) ( j . t . baker ) was added , with stirring until a uniform dispersion was obtained . the suspension was poured into a mold and allowed to stand at room temperature . a strong , firm gel was obtained on demold after 11 / 2 hours , which exhibited a shrinkage value of 55 % over 29 hours air drying to yield a solid , particulate - filled material having a density of 1 . 02 gm / cm 3 . an acetone - based gel of tcp - filled l - pla , produced as described above , was immersed in methanol on demold for five days , then air dried for four days . the microporous material obtained exhibited a shrinkage of 26 % and a density of 0 . 27 gm / cm 3 . a 29 % tcp particulate - filled microporous blend of l - pla with 70 : 30 poly ( dl - lactide co - glycolide ) copolymer ( mw 30 , 000 - 60 , 000 , polysciences , inc .) was produced by dissolving 0 . 8 gm and 2 . 4 gm of each polymer , respectively , in 20 cc acetone at 52 ° c . particulate filler ( 1 . 28 gm ) was dispersed in the solution by stirring . the suspension was transferred to molds and allowed to stand at room temperature for 22 hours before demold , whereupon a weak , sticky gel was obtained . ( sedimentation of tcp filler was limited prior to gel formation by the viscosity of the blended polymer solution .) the gel was subsequently immersed in 50 cc methanol in a 100 cc beaker for two days with a solvent change at 24 hours . the nonsolvent was then allowed to evaporate and the material allowed to dry in the beaker over three days . the tcp particulate - filled l - pla : plg microporous blend obtained exhibited a shrinkage of 28 % and a density of 0 . 6 gm / cm 3 . solid l - pla material ( mw 100 , 000 , polysciences , inc .) was produced by dissolving 1 . 8 gm of this gelling polymer in 20 cc acetone at 52 ° c . the solution was transferred to cylindrical molds and allowed to stand at room temperature . the gel obtained on demold after 39 minutes was air dried under ambient conditions to consolidate the polymer as a solid rod . thermal transitions were determined by differential scanning calorimetry ( dsc ). on heating at 20 ° c ./ min ., from 10 ° c . to 230 ° c . a single melting peak was observed at 159 ° c . no recrystallization peak was evident on immediate cooling of the sample at 50 ° c ./ min . to 10 ° c . reheating the sample at 50 ° c ./ min to 230 ° c . revealed only a glass transition ( tg ) at 70 ° c ., indicating that the polymer exists mainly in the amorphous phase after rapid cooling from the melt . a 50 : 50 blend of l - pla ( mw 100 , 000 ) and poly ( dl - lactide ) ( mw 20 , 000 , polysciences , inc .) was produced by codissolving 1 gm of each polymer in 20 cc acetone at 52 ° c . the solution was poured into cylindrical plastic molds and allowed to stand at room temperature . the product blended gel was demolded after 55 minutes and air dried to consolidate the material as a solid rod . thermal transitions were revealed by dsc , using the test procedure described in example 10 . a broad , spiky melting peak was observed on heating , extending from 120 °- 160 ° c . and centered around 140 ° c . no recrystallization peak was evident on cooling the sample . a glass transition was observed on remelting at 53 ° c . the ( dl - lactide ) polymer showed a glass transition at 35 ° c . on heating , which shifted to 45 ° c . after cooling from 230 ° c . and reheating . a 25 : 75 blended solution of high molecular weight l - pla ( mw 100 , 000 ) and low molecular weight l - pla ( mw 2 , 000 , polysciences , inc .) was produced by dissolving 0 . 6 gm and 1 . 8 gm of each polymer , respectively , in 20 cc acetone at 50 ° c . the solution was transferred to molds and allowed to stand at room temperature . the resultant firm gel was demolded in 30 minutes , following a rapid cloud point of five minutes . air drying the gel over five days resulted in a shrinkage of 44 % and production of a hard , waxy solid . a 25 : 75 blended gel of high and low molecular weight l - pla , demolded after 19 hours , was immersed in 50 cc methanol in a 100 cc beaker for two days with a change of medium after 24 hours . the methanol was allowed to evaporate and the material allowed to dry in the beaker over four days at room temperature . the blended , microporous material obtained exhibited a shrinkage of 19 % and a density of 0 . 29 to 0 . 43 gm / cm 3 . the thermal testing procedure described in example 10 revealed melting peaks at 136 ° c . and 155 ° c . for 25 : 75 microporous material , roughly corresponding to the individual homopolymer components , and no recrystallizing transition on cooling . on reheating , a glass transition was observed at 63 ° c ., a broad recrystallizing transition at 130 ° c . and a melting peak at 155 ° c . solid , low molecular weight l - pla obtained by air drying the weak gel produced from an 11 % ( w / v ) solution in acetone revealed low broad melting peaks centered around 104 ° c . and 118 ° c . and the main melting peak at 140 ° c . no recrystallization transition was observed on cooling . on reheating , a glass transition was evident at 50 ° c . and a small melting peak at 143 ° c . the lower molecular weight polymer species introduced into the blend allowed sufficient chain mobility for recrystallization to occur from the amorphous form on heating . recrystallization did not occur for the amorphous form of the single higher molecular weight polymer . it may be possible to vary the crystallinity of a blend by isothermal conditioning . a 25 : 75 blended solution of l - pla and a 70 : 30 poly ( dl - lactide co - glycolide ) copolymer ( mw 30 , 000 - 60 , 000 , polysciences , inc .) was produced by co - dissolution of 0 . 6 and 1 . 8 gm of each polymer , respectively , in 20 cc of acetone at 52 ° c . the solution was transferred to molds and allowed to stand at room temperature . an acetone seal was applied to the gel after one hour to prevent surface drying at long demold times . the blended gel produced on demold after 21 hours was subsequently immersed in methanol for four days , 18 hours before drying at room temperature for one week . shrinkage of the methanol - based material occurred on drying to the extent of 48 %, to yield a practically solid polymer core . an acetone - based gel was immersed in methanol for five days , 18 hours , then immersed in water for 21 hours prior to drying . in this case , the microporous blend obtained exhibited a shrinkage of only 13 % and a density of 0 . 25 gm / cm 3 . the density of microporous materials could be varied by water treatment subsequent to acetone extraction in methanol . extraction of organic liquid from the material and its substitution by water removed the plasticizing effect of the organic molecules , raised the tg and thereby restricted chain recoiling of the amorphous phase , which resulted in material shrinkage on drying . 25 : 75 acetone - based gels of l - pla and 70 : 30 poly ( dl - lactide co - glycolide ) were demolded after 24 hours and immersed in methanol ( 50 cc methanol in 100 cc beaker ) for three days , with a change of immersion medium at 24 hours . samples were then air dried for time periods of 0 , 60 and 80 minutes before immersion in water for three days , with a change in immersion medium at 24 hours . samples were finally air dried under ambient conditions for three days to give uniformly microporous materials with the final shrinkage and density values shown in table 1 . table 1______________________________________air drying time ( minutes ) 0 60 80prior to water immersionfinal foam shrinkage (%) 5 15 19foam density ( gm / cm . sup . 3 ) 0 . 19 0 . 22 0 . 29______________________________________ a 25 : 75 blended solution of l - pla and 85 : 15 poly ( dl - lactic co - glycolide ) ( mw 40 , 000 - 100 , 000 , dupont ) was produced by dissolution of 0 . 8 gm and 2 . 4 gm of each polymer , respectively , in 20 cc acetone at 52 ° c . the solution was transferred to molds and allowed to stand at room temperature . a cloud point was observed after approximately 15 minutes . on demold after 24 hours , gel samples were air dried for time periods of 0 , 15 , 45 , and 75 minutes before immersion in water for three days , with a change of immersion medium after 48 hours . drying of the microporous materials resulted in the final shrinkage and density values shown in table 2 , with foam properties ranging from tough and pliable to hard - yet - tough as density increased with predry time . table 2______________________________________air drying time ( minutes ) 0 15 45 75prior to water immersionfinal foam shrinkage (%) 8 10 17 20foam density ( gm / cm . sup . 3 ) 0 . 22 0 . 25 0 . 32 0 . 38______________________________________ poly ( dl - lactide ) ( mw 40 , 000 - 100 , 000 , dupont ) was dissolved in acetone to produce a 12 % ( w / v ) solution , transferred to a 10 cc syringe body and allowed to stand at room temperature . gelation did not result and solvent evaporation occurred over seven days , to leave a coating on the mold walls . a 25 : 75 blended solution of l - pla and poly ( dl - lactide ) was produced by co - dissolution of 0 . 8 gm and 2 . 4 gm of each polymer , respectively , in 20 cc acetone at 52 ° c . the solution was transferred to molds and allowed to stand at room temperature . a cloud point was observed after approximately 10 minutes . an acetone seal was applied to the gel after 30 minutes to prevent surface drying at long demold times . on demold after 21 hours , a soft gel was obtained , which was immersed in methanol for three days with a change of immersion medium after 22 hours . immersion of the methanol - based material for three days in water , with a change in medium after 24 hours , prior to air drying resulted in a white , microporous material which exhibited a shrinkage of 1 % and a density of 0 . 18 gm / cm 3 . on demold after 21 hours , an acetone - based gel sample was air dried for 65 minutes before immersion in water for three days , with a change of immersion medium after 24 hours . drying of the microporous material so obtained resulted in a final shrinkage of 26 % and a density of 0 . 40 gm / cm 3 . l - pla can be used as a gelling medium to incorporate non - gelling polymers in a particular material composition for adjustment of resorption rates , for example . a 50 : 50 blended solution of l - pla and low molecular weight polycaprolactone ( mw 15 , 000 , polysciences , inc .) was prepared by co - dissolution of 0 . 7 gm of each polymer in 10 cc acetone at 52 ° c . the solution was transferred to a mold and allowed to stand at room temperature . a cloud point was observed after 30 minutes . a firm , white damage - tolerant gel was obtained on demold after 25 hours , which was immersed in methanol for two days , 21 hours . drying of this methanol - based material at room temperature for two days , 19 hours resulted in a shrinkage of 30 % and a density of 0 . 45 gm / cm 3 for the resulting firm , white , microporous material . treatment of the methanol - based material in a 50 % methanol / water mixture for two days , 19 hours , then water for six days prior to air drying for four days resulted in a microporous material which exhibited a shrinkage of 15 % relative to the demolded gel and a density of 0 . 25 - 0 . 29 l gm / cm 3 . dsc revealed melting transitions at 64 ° c . and 160 ° c ., corresponding to the individual blend components . a 50 : 50 blended solution of l - pla ( mw 100 , 000 polysciences , inc .) and dl - lactide ( mw 144 . 12 , polysciences , inc .) was prepared by co - dissolution of 0 . 7 gm of each material in acetone at 52 ° c . the solution was transferred to a cylindrical mold and allowed to stand at room temperature . an acetone seal was applied after 30 minutes . a firm , white , damage - tolerant gel was obtained on demold after 24 hours , which was immersed in methanol for two days , 21 hours . drying of this methanol - based material for two days , 19 hours resulted in a hard , white microporous material exhibiting a shrinkage of 37 % relative to the demolded gel and a density of 0 . 38 gm / cm 3 . treatment of the methanol - based material for two days , 19 hours in a 50 % methanol / water mixture , followed by immersion in water for six days , prior to air drying ( four days ), resulted in a firm , white microporous material which exhibited a shrinkage of 19 % and a density of 0 . 6 gm / cm 3 . dsc revealed a small melting peak at 60 ° c . and the main melting transition at 160 ° c . a bone repair device potentially suitable for general bone augmentation and reconstruction or for repairing large segmental defects and nonunion fractures was fabricated from a microporous 25 : 75 blend of l - pla and 85 : 15 poly ( dl - lactide co - glycolide ) produced by the disclosed gel casting technique . 0 . 6 gm of l - pla ( mw 100 , 000 ) and 1 . 8 gm of 85 : 15 poly ( dl - lactide co - glycolide ) copolymer ( medisorb , mw 40 , 000 - 100 , 000 , dupont ) were dissolved with stirring in 20 cc acetone at 52 ° c . in approximately 15 minutes . the solution was transferred to molds and allowed to stand at room temperature for 24 hours before demold . the gel obtained was dried in air for 45 minutes to give a shrinkage of 19 % then immersed in water for three days , with a change in immersion medium after 24 hours . air drying of the microporous material obtained over four days resulted in a final shrinkage of 17 % and a density of 0 . 32 gm / cm 3 . this stock material was machined further to produce a particular honeycomb design for repair of long bones . fabrication of implants : a 50 %- 50 % copolymer ( plg50 ) of pla and pga ( intrinsic viscosity 0 . 65 dl / g , mw 45 , 000 daltons , medisorb inc ., cincinnati , ohio ) was used to fabricate the specimens . a 15 % ( w / v ) solution of plg50 was prepared in hplc grade acetone under continuous stirring and low heat ( 40 ° c ). this solution was poured into a specially designed teflon ® mold having piano wires spaced corresponding to the holes shown in fig2 and a central channel corresponding to the central hole shown in fig2 . the mold was then capped and allowed to stand at room temperature until the solution formed a gel . a small amount of acetone was added to the top of the gel to prevent it from drying . after 24 hours the gel was extracted from the mold and processed through several stages of solvent exchange as follows : immersion for 24 hours in methanol : acetone 1 : 2 ( v / v ), followed by soaking in 1 : 1 and 2 : 1 ( v / v ) methanol : acetone mixtures for 24 hours each . during this process the implant precipitated as a microporous solid with pores approximately 5 to 20 μm in size . the resulting disc - shaped implant as shown in fig1 and 2 was 13 . 5 mm in diameter , 2 . 5 mm in thickness and weighed approximately 150 mg . protein ( 15 mg / implant ) was incorporated into the implant prior to gel formation . bmp was purified from canine bone using a modification of the method of urist et al . (&# 34 ; purification of bovine bone morphogenetic protein by hydroxyapatite chromatography &# 34 ;, natl . acad . sci . ( 1984 ) 81 : 371 - 375 ) as described previously ( heckman , j . d . et al ., &# 34 ; the use of bone morphogenetic protein in the treatment of non - union in a canine model ,&# 34 ; j . bone and joint surg . ( 1991 ) 73 - a : 750 - 764 ). when bmp is isolated in this manner it is hydrophobic and dissolves in organic solvents like acetone or chloroform . due to the low availability of recombinant bmp and the high cost of purified growth factors , we also examined release kinetics of a model protein , soybean trypsin inhibitor ( ti ). ti was selected because of its hydrophobic character , the fact that it has a molecular weight comparable to that of bmp , and its availability ( sigma chemical company , st . louis , mo .). ti was used for the studies described below ; a small number of implants were also fabricated with purified canine bmp . the amount of protein actually retained in the implant following the fabrication process was determined . the utensils , glassware and mold used for fabrication were washed with ultrapure distilled water . the washes were retained for measurement of protein content using the micro bca method . the results indicated that approximately 80 percent of the protein added to the implant was lost during fabrication . activity of bmp in the implant was verified as the ability of the implant to induce bone formation when placed in the thigh muscle of nude mice . representative implants made with each batch of bmp were implanted as described by heckman et al . (&# 34 ; the use of bone morphogenetic protein in the treatment of non - union in a canine model ,&# 34 ; j . bone and joint surg . ( 1991 ) 73 - a : 750 - 764 ). after 28 days , the tissue was excised and examined by routine histology . all implants containing bmp supported bone formation , whereas those without bmp did not . ti implants were not tested in this manner . protein release kinetics : a total of 48 test specimens were fabricated with ti , weighed and then randomly divided into eight groups of six specimens each . these groups were subjected to hydrolytic degradation in phosphate buffered saline ( pbs ), ph 7 . 4 , at 37 ° c . for test periods of 0 , 2 , 22 , 32 , 42 , 52 , 62 , and 72 days respectively . another 12 implants were fabricated without any ti . these were divided into two control groups of six specimens each and subjected to hydrolytic degradation in pbs for 0 and 72 days respectively . a separate group of six implants was fabricated with 15 mg of bmp each and was analyzed for a total of 20 days . each specimen was immersed in 15 ml of pbs . for the first 48 hours 1 . 5 ml of pbs were removed several times a day and stored at - 20 ° c . for later analysis . after this initial period , 1 . 5 ml of pbs were sampled every 48 hours after vortexing . after sampling the pbs in the tubes was replaced with fresh pbs in order to prevent a decrease in ph and thus simulate physiologic buffering . a large fraction ( 66 %) of ti in the specimens was eluted in the first 48 hours ( fig5 ). the elution rate was the lowest between 12 and 24 days , and then increased marginally . the bmp release in the first 48 hours followed a pattern similar to that of ti although the amount released was less ( fig6 ). bmp specimens released an average of 322 μg protein in the initial two days . significant portions of this amount were released in the first 24 hours ( fig6 ). a comparison of bmp and ti elution over a period of 20 days again exhibited similar release patterns ( fig7 ), even though the amount of ti released in the first four days was higher than bmp . the cumulative protein loss from the ti specimens during the test period is shown in fig8 . these specimens released approximately 680 μg of ti over a test period of 72 days post - immersion . the elution of bmp and ti from the biodegradable implants exhibited similar trends even though the amounts released in the first four days were significantly different in magnitude . after day 6 the differences in elution rate of the two proteins were only marginal . thus , ti appears to be a reasonable model to predict the elution characteristics of bmp from the present biodegradable implant . ti and bmp release from the implants included an initial burst during the first 48 hours . since the specimens did not experience any significant loss in mass or molecular weight during this time period , it can be deduced that the protein release during this period is primarily diffusion controlled and not due to degradation of the implant . it is possible that the majority of the protein in this initial &# 34 ; burst release &# 34 ; comes from the surface of the specimen which subsequently becomes depleted of protein . the later phase of increased protein release beginning at approximately 24 days was accompanied by significant decreases in both the molecular weight and mass of the specimens indicating rapid breakdown of the polymer due to hydrolysis . the release of ti from the test specimens in the present study appears to be two phased in nature : the first phase is diffusion controlled and is dominant at the beginning of the degradation process . the second phase is more dependent on degradation of the polymer and the protein release occurs in concert with polymer breakdown . the present implant provides an initial burst of protein which serves to augment the in vivo wound healing response . as mesenchymal cells migrate to the wound site and undergo differentiation , the second stage release provides continued exposure to differentiation factors . many growth factors act on cells at different points in their differentiation and maturation with varying effects . for example , a factor may be mitogenic for undifferentiated mesenchymal cells , promoting their proliferation at the wound site . following the proliferative phase , the growth factor may promote differentiation of a chondrogenic or osteogenic phenotype . this is particularly important in prevention and treatment of nonunions , where it is often necessary for undifferentiated cells to migrate long distances and to proliferate and differentiate in hypovascularized regions . studies examining healing of critical size craniotomy defects , suggest that lack of an adequate supply of needed factors may contribute to fibrocartilage development typical of nonunions ( schmitz , j . d . et al . ( 1990 ) &# 34 ; characterization of rat calvarial nonunion defects ,&# 34 ; acta anatomica , pp 185 - 192 ). in the reported model , this occurred approximately 17 - 20 days after creation of the critical size defect , about the same time that the second phase of protein release was seen in the in vitro assays reported here . polymer degradation : at the end of each test period described above the specimens were removed from pbs , vacuum dried for 72 hours and massed on an electronic balance . the polymer was analyzed for molecular weight using gel permeation chromatography with chloroform as the mobile phase and polystyrene standards ( aldrich , milwaukee , wis .). the amount of ti or bmp released was measured as a function of protein content using a micro bca assay ( pierce , rockford , ill . ; lot no . 931112150 ). a student &# 39 ; s t - test was performed to detect significant differences . all the specimens subjected to hydrolytic degradation in pbs exhibited a decrease in mass and molecular weight as a function of time . the test specimens with ti lost approximately 33 % of their mass in the first 32 days ( fig3 ). mass loss increased in a steady fashion thereafter and reached 74 % in 72 days . however , the control group with no protein content lost approximately 90 % of its mass during the same period . compared to the mass , the molecular weight ( weight average ) of the protein specimens decreased 65 % in the first 32 days and 94 % in 72 days ( fig4 ). at the end of 72 days the control specimens had suffered a 97 % loss in their molecular weight . the implants underwent degradation under the experimental conditions used , whether or not protein was incorporated into the polymer . the starting weight average molecular weight of the polymer was approximately 45 , 000 daltons . in 72 days it decreased to approximately 2 , 700 daltons for the test implants with ti and to 910 daltons in the absence of ti . at this time only small portions of these implants remained , which were gel - like and sticky in nature , indicative of low molecular weight . the loss in the molecular weight of the implants exhibited a more rapid decrease than the corresponding mass loss . this behavior is typical of the degradation characteristics of pla - pga polymers . hydrolytic scission of the molecular chains of these polymers starts upon contact with water . however , the initial degradation products are too large to freely diffuse from the bulk implant and as a result the mass loss is minimal . after a period of time , as degradation continues , the molecular chains are sufficiently reduced in size to exit the bulk and cause a significant decrease in mass . for example schankenraad et al . (&# 34 ; enzymatic activity toward poly ( l - lactic acid ) implants ,&# 34 ; j . biomed . mat . res . ( 1990 ) 24 : 529 - 545 ) have reported that for l - pla there is an exponential decrease in mass once the molecular weight decreases below 5 , 000 daltons . the presence of protein affected both loss of molecular weight and loss of mass . after a degradation period of 72 days the specimens with the protein lost approximately 95 percent of their molecular weight compared to a 98 percent loss for the control specimens with no protein . this statistically significant difference ( p & lt ; 0 . 05 ) was even more pronounced for the case of mass loss ( fig5 ). these results suggest that the presence of ti in the specimen decreases the degradation rate of the polymer . the degree and nature of micropores may have contributed to this effect of protein on polymer degradation . implants with proteins exhibited a more evenly distributed porosity with small pores with an approximate size of 20 μm . on the other hand , the control specimens exhibited larger ( 60 μm ) but fewer number of pores . it is possible that the pores in the protein specimens were caused by dissolution of protein aggregates during the fabrication process . a greater number of pores would usually imply a larger surface area exposed to water and hence a faster rate of degradation . however , the results of this study indicate a higher rate of degradation for implants with the lower porosity implying that factors other than porosity may be dominant . the crystallinity of the implant polymer can play a significant role in determining its rate of degradation . according to gilding and reed , copolymers of dl - pla and pga are predominantly amorphous when the pga content is more than 30 percent . however , as reported in coombes , a . g . a . and heckman , j . d . ( 1992 ) &# 34 ; gel casting of resorbable polymers 1 . processing and applications .&# 34 ; biomaterials 13 : 217 - 224 ; and coombes , a . g . a and heckman , j . d . ( 1992 ) &# 34 ; gel casting of resorbable polymers 2 . in vitro degradation of bone graft substitutes ,&# 34 ; biomaterials 13 : 297 - 307 , both gel cast and as - received plg50 exhibit several small melting peaks on a thermogram . such melting points are indicative of some degree of crystallization and are signs of crystallization of chain segments of the copolymer ( murdoch , j . r . and loomis , g . l ., &# 34 ; polylactide compositions ,&# 34 ; u . s . pat . no . 4 , 719 , 246 1988 ). coombes , a . g . a . and heckman , j . d . ( 1992 ) &# 34 ; gel casting of resorbable polymers 1 . processing and applications .,&# 34 ; biomaterials 13 : 217 - 224 suggest that acetone based gels of plg 50 similar to those used in the present study are formed due to &# 34 ; effective entanglements &# 34 ; and small quantities of lamellar crystals which serve as anchor points for the 3 - dimensional network of polymeric chains . in the present study it was observed that specimens containing a protein gelled in approximately 3 to 4 hours , while the control specimens ( without any proteins ) took approximately 5 to 6 hours to form a stable gel . it is possible that the protein molecules aid in the formation of the 3 - dimensional network that is necessary for gel formation . the proteins may either act as tie molecules and provide &# 34 ; effective entanglements &# 34 ; between anchor points of the gel , or they may serve as &# 34 ; seeds &# 34 ; for polymer crystallization and possibly increase the number of crystalline sites . such an increase would then be reflected in a slower rate of degradation . alternatively it is possible that ti binds to the polymer by some complex mechanism and hinders its hydrolysis . implant morphology : representative control specimens as well as specimens with ti and bmp were subjected to cryogenic fracture at liquid nitrogen temperatures to obtain surfaces free of deformation for microstructural studies . these specimens had not been subjected to degradation in pbs . the exposed cross - sectional fracture surfaces were sputter coated with gold - palladium and examined in a scanning electron microscope . the scanning electron microscopy showed that the specimens with bmp and ti were microporous in nature with an even distribution of similar sized pores . no differences as a function of protein used were observed . the control specimens on the other hand did not reveal the same degree of even porosity . a 100 , 000 molecular weight poly ( l - lactide ) obtained from polysciences , inc . ( batch no . 61490 ) was supplied in a fine , string or fibrous precipitate form and dissolved readily in acetone at 52 ° c . poly ( l - lactide ) polymer obtained from dupont [ batch no . 59010 l051 , weight average molecular weight ( mw )- 100 , 000 ] was supplied in the form of solid platelets or pellets ( approximately 4 × 15 × 1 mm ), which were presumably produced by melt extrusion . dissolution of the pellets was not achieved in acetone ( 0 . 4 gm in 10 cc solvent ) at 52 ° c . in one hour and they retained their starting form without coalescing . in contrast , the dupont material in precipitate form dissolved readily in acetone at 52 ° c . to produce a 4 % ( w / v ) solution . gelation occurred on standing at room temperature in less than 15 minutes . the poly ( l - lactide ) precipitate can be prepared , for example , by dissolution of 2 gm of polymer in 20 cc methylene chloride at room temperature , followed by precipitation in 40 cc methanol which is agitated by a stirrer bar . the precipitate is dried at room temperature before use . dissolution of the above - mentioned dupont poly ( l - lactide ) in acetone was also facilitated by using the film form of the polymer . as - received pellets were converted to a film by casting a 10 % ( w / v ) solution of the polymer in methylene chloride on a glass surface , followed by drying . the instant invention has been disclosed in connection with specific embodiments . however , it will be apparent to those skilled in the art that variations from the illustrated embodiments may be undertaken without departing the spirit and scope of the invention .
a biodegradable implant for placement in nonunion bone fractures as a substitute for bone graft material is provided which is a flat plate or disk having a thickness of between about 1 mm and about 15 % of the length of the bone , interconnected micropores , and canals substantially equivalent in size and spacing to naturally - occurring haversian canals . the implant may additionally comprise additives such as growth factors , including bone morphogenic protein . in a preferred embodiment the implant is formed from a biodegradable polymer such as a polylactic acid - polyglycolic acid copolymer by a gel casting technique followed by solvent extraction to precipitate the implant as a microporous solid .