the device of the present invention may be fabricated into various forms depending on intended use . some contemplated forms include solid products such as orthopedic pins , clamps , screws , or plates , clips , staples , vascular implants or supports and nerve channels or supports . other medical devices could include fibrillar products , knitted , woven or felted such as velours , burn dressings , hernia patches , absorbant papers or swabs , medicated dressings , facial substitutes , gauze , fabric , sheet , felt or sponge for hemostasis , dental packs and breast prostheses . a good description of the formation of bioresorbable materials as matted surgical dressings may be found in u . s . pat . no . 3 , 937 , 223 to roth . the present improvement could also be useful for bioresorbable polymers in the form of a flake or powder for burns or abrasions , foam , film spray for prosthetic devices , and slowly digestible ion - exchange resins and slow release devices in the form of pills or pellets . particularly useful are tubes of varying shapes , lengths and diameters , to be implanted temporarily or permanently . of these tubular protheses may be mentioned vascular and nerve guidance channels and the like . in the case of the nerve guidance channel , the particular configuration of such tubes may vary according to the size and shape of the nerve to be repaired , and whether the intended repair is to take place in human surgery or in surgery involving other animal species . with respect to nerve guidance channels , u . s . pat . no . 3 , 833 , 002 to palma discloses various sizes and shapes that may be employed . lengths of the tubes , internal diameters , and tubular wall thicknesses may vary according to intended use . the length of the tube would ordinarily be commensurate with the size of the gap to be repaired , also allowing extra tubing in which to insert nerve stumps . the present inventors have found that particularly useful internal diameters commonly range from 0 , 013 mm to 5 . 00 mm . it is also desirable to obtain tubes whose wall thicknesses fall within a specific range , such as 0 . 08 mm to 3 . 0 mm . a preferred range is 0 . 5 mm to 1 . 5 mm in thickness . the bioresorbable polymers of the invention include homopolymers and copolymers of one or more of the alpha hydroxy carboxylic acids , such as glycolic acid , l - lactic acid , d - lactic acid , or dl - lactic acid . preferred polymers include homopolymers of the lactic acid monomers , most preferred being dl - lactide . as used herein , the term &# 34 ; polylactide &# 34 ; is equivalent to &# 34 ; poly ( lactic acid )&# 34 ; as meaning a polymer of lactic acid . in particular , dl - lactide is a lactide derived from a roughly racemic mixture of lactic acid , and this nomenclature is interchangeable with ( dl ) lactic acid . similarly , the terms polyglycolide and poly ( glycolic acid ) are equivalent . to achieve beneficial biological effects , the desired weight average molecular weight range of the polymer is greater than about 150 , 000 . a preferred range is about 150 , 000 - 500 , 000 . more preferred is a range of about 175 , 000 - 350 , 000 . most preferred is a molecular weight range of about 200 , 000 - 250 , 000 . the polymerization process should be carried out in such manner as to achieve the desired polymer molecular weight range . for example , in the case of polylactide , polymerization of the monomeric lactide units can be effected by any acceptable process , such as by using ring opening polymerization , and the like . it is preferred to use a melt polymerization procedure with stannous octoate as the polymerization catalyst , as it is postulated that the polymer molecular weight increases and molecular weight distribution decreases with its use . when using stannous octoate as the catalyst , the requisite parts per million ( ppm ) range from about 5 to about 800 . a preferred amount is about 75 ppm - 200 ppm . reaction time ranges from about 4 hours to about 168 hours , with 6 hours being preferred reaction temperatures range from about 75 ° to 240 ° c ., with about 180 ° c . being preferred . to obtain polymers of different molecular weight , fractional precipitation of the polymer can be achieved using a &# 34 ; good - solvent &# 34 ; such as chloroform or dioxane and a &# 34 ; non - solvent &# 34 ; such as water , methanol , or the like . polymers of narrow molecular weight distribution are also obtainable in this manner . polymers are generally polydisperse or heterogeneous in molecular weight . to improve the physical properties of a polymer product , it is thus desirable to control the molecular weight distribution by the use of fractionation . the molecular weight distribution is commonly calculated as a dispersity number , which is the weight average molecular weight divided by the number average molecular weight ( number of polymer units of a particular molecular weight ). dispersity of a polymer for use in implantation devices is preferred to be less than about 10 . 0 ; more preferred is a dispersity number of less than about 3 . 0 ; most preferred is 1 . 0 - 1 . 9 . further , polymers of different weight average molecular weights and distribution could be judiciously combined to obtain a material of desired weight average molecular weight and distribution . a biocompatible plasticizer or plasticizers may be added to impart greater flexibility to the finished device . such plasticizers include , e . g ., but are not limited to , acetyl tributyl citrate , acetyl triethyl citrate , tri - n - butyl citrate , triethyl citrate , and triacetin . in particular , bioresorbable triethyl citrate has been found to be useful . the polymeric materials of the present invention may also be used in conjunction with biodurable materials . such a construction may serve as a means of extending the in vivo longetivity of a particular implant . it is contemplated that a composite may be formed by the use of a mixture , or a coating film , or layer , or the like of the bioresorbable polymers of the specific molecular weight range and of a suitable molecular weight distribution with one or more biodurable materials such as silicon , silicon rubber , polyethylene , polyethylene terephthalate , polyfluoroethylene , polyphosphazene , polyurethane , segmented polyurethane , or the like . for some applications , such as the nerve channel , it is preferred that the bioresorbable material form a continuous medium of the device . the desired devices may be formed from the polymer by any suitable means including solution formation or heat formation . in general , standard polymer processing techniques can be used to fabricate devices for implantation into living tissue . hygenic and clean - room conditions for manufacturing of medical articles should be followed . for example , when solution formation is utilized , polymer solutions are commonly filtered before use in a laminar - flow hood to insure that the devices are prepared under clean - room conditions . the devices of the present invention may also include the addition of &# 34 ; tropic factors &# 34 ; desirable for the growth and survival of various classes of cells in tissue culture , these factors most often being macromolecular proteins . of particular interest are neuronotrophic factors . of these growth factors may be mentioned such substances as collagen , fibrinogen , fibronectin , and laminin . the devices of the present invention may also be sterilized by means of the techniques usually employed by surgery as long as extensive decomposition of the material does not result . for example , sterilization with ethylene oxide at room temperature may be employed . the following examples illustrate certain preferred embodiments of the invention and are not definitive of scope . a catalyst solution containing 2 . 49 mg / ml of stannous octoate dissolved in thf was prepared . two mls of the stannous octoate solution was added to 25 g of dl - lactide to achieve 200 parts per million . this mixture was then heated under an inert atmosphere for 6 hours at 180 ° c . the molecular weight average of the resulting polymer was determined to be approximately 178 , 000 in the absence of solvent weight . molecular weight was determined by gel permeation chromatography , calibrated against polystyrene standards in thf . seventy four grams of recrystallized dl lactide was charged to a teflon ® reactor along with 74 μl of 10 % stannous octoate in toluene . the reactor was fitted with a nitrogen inlet , thermocouple and an anchor stirrer . the vessel contents were heated by means of an oil bath . a servodyne gauge and chart recorder were used to monitor the viscosity of the polymer melt . after stirring for 70 minutes under a nitrogen blanket , the viscosity rose rapidly . the oil bath temperature was held at 190 °- 200 ° c . for five hours while the internal thermocouple registered 155 ° c . a 30 g aliquot of the polymer was dissolved in acetone and then precipitated with water in a waring blender . the recovered solids were washed thoroughly with methanol and further granulated in a waring blender . finally , the solids were dried in a vacuum oven for 2 days at room temperature , and 24 g of polymer were recovered . reduced viscosity of the polymer was ηsp / c 2 . 10 , 0 . 1 % in dioxane . the molecular weight average was determined by gel permeation chromatography to be approximately 207 , 000 in the absence of solvent weight . polymers of various molecular weights and distribution were obtained by fractional precipitation . nerve channels composed of polymers prepared substantially as above , ranging in molecular weight from about 177 , 000 to about 320 , 000 , were readily prepared by the usual multiple solution dipping method using inert metal mandrels or glass mandrels for dipping . polymer with a weight average molecular weight normally of about 90 , 000 was obtained commerically ( polyscience ). nerve channels were then prepared from a fractionation , with a weight average molecular weight of 113 , 000 , to serve as a comparison . a thf solution of the polymer was normally used with a plasticizer . polymer solutions were filtered in a laminar - flow hood before use . alternatively , the standard method of melt extrusion of polymers was applied to these polymers to obtain tubings of the desired size . in either case , clean - room conditions were maintained during the preparation of the nerve channels . fig1 illustrates a perspective view of a nerve guidance channel prepared according to the present invention , into which severed nerve ends may be inserted . shown therein is a cylindrical nerve guidance tube 1 having open opposite ends 3 and 5 into which severed distal nerve end 7 and severed proximal nerve end 9 may be inserted . after the nerve ends are inserted into the tube , they may be surgically sutured in place with sutures commonly available . fig2 further illustrates the experimental design of the mouse sciatic nerve regeneration study . therein is shown an adult anesthetized c57bl / 6j mouse with a sciatic nerve 2 transected and with nerve guide 1 in place . in each mouse , both the proximal stump 9 and distal stump 7 ( detailed in fig1 ) were secured by a single 10 - 0 nylon suture 11 and were inserted into a 5 - 6 mm length of nerve guide tube 1 to give a final gap length of 3 - 4 mm . the tubes were composed of poly ( dl - lactide ) of molecular weights of about 177 , 000 , 234 , 000 , or 320 , 000 . poly ( dl - lactide ) tubes with a molecular weight of approximately 113 , 000 were inserted into other mice for comparison purposes . at 2 , 4 or 6 weeks postoperatively , the sciatic nerve of the animal , appropriately perfused for tissue studies , was again exposed and retransected 3 mm distal to the nerve guide tube . nerve guides with enclosed regenerated nerves were then dissected out , post - fixed in 2 % osmium tetroxide and processed for plastic embedding ( der , ted pella inc .). just before embedding , the tissue was divided into several segments for sampling at multiple cross - section levels . for most implants , five levels were sampled by one micron sections . these levels were : proximal sciatic stump at 1 to 2 mm proximal to the implant ; three levels - ( proximal , central , distal ) within the tube 1 through the original gap , and the distal stump 1 to 2 mm distal to the implant . data obtained in the central section was used for comparison . the number of myelinated axons in these sections was determined with a computer - control system . selected blocks were then resectioned for electron microscopy . the following table summarizes the myelinated axon count of regenerated sciatic nerve ( through the 3 - 4 mm gap ) of the study : ______________________________________polymer 2 4 6 ( mw ) week week week______________________________________113k 0 304 ± 102 ( n = 5 ) 627 ± 185 ( n = 5 ) 177k 0 827 ± 188 ( n = 5 ) 759 ± 512 ( n = 4 ) 234k 0 1457 ± 124 ( n = 3 ) 1844 ± 429 ( n = 5 ) 320k 0 821 ± 416 ( n = 3 ) 1637 ± 418 ( n = 5 ) ______________________________________ k = mw × 1000 n = no . of animals
nerve channels made from high molecular weight lactic acid polymers , preferably polymers having a molecular weight between about 234 , 000 and 320 , 000 , provide an unexpectedly beneficial effect on cell growth and regeneration of function .