the present invention provides both devices and methods of employing said devices for regulating ph . this is accomplished by preparing a mixture of an alkaline , acidic or buffering substance with a biodegradable polymer , and preparing an implantable device with the mixture . the mixture may also include a pharmacologically active agent . the invention also provides methods for preparing such devices , as well as methods of using these devices to enhance the biocompatibility of an implantable device over clinically useful periods of time . these and other aspects of the invention will be illustrated in the following examples . however , these examples are intended to illustrate specific embodiments of the present invention only . those skilled in this field will recognize that modifications could be made to the disclosed methods and that other applications would remain within the scope of the present invention . the present example describes the incorporation of materials with a basic nature ( ph & gt ; 7 . 0 ) in implants fabricated from polymers or copolymers belonging to the family of polylactic and polyglycolic acids . this idea can be implemented in several ways , one of which is described below : a solution of a 50 : 50 pla - pga copolymer was prepared in acetone . alternatively , methylene chloride or chloroform may be used to prepare the polymer . the polymer was then precipitated in ethanol . alternatively , the polymer may be precipitated in methanol or other alcohol . the precipitate was extracted and the basic salt ( 30 percent by volume with respect to polymer ) is then rolled / kneaded into the polymer . the polymer was then packed into a mold and dried under heat and vacuum to yield the implant . upon degradation in vivo or in vitro , the pla and pga polymers in the implant will release lactic and glycolic acids . simultaneously , the basic salt will dissolve in the surrounding media and neutralize the acids in the vicinity of the implants . in these implants , the ph - controlling material is evenly distributed throughout the device . methods for preparing these implants constitute still another aspect of the invention . the present example provides an in vitro study offset of changes ( particularly decreases ) in ph surrounding a biodegradable polymer provided by incorporation of salts with a basic nature within the implants . biodegradable implants were fabricated using a 50 : 50 pla - pga copolymer with inherent viscosity of 0 . 71 dl / gm and weight average molecular weight of 53 kd . these implants were divided into 4 groups : a control group , and 3 test groups corresponding to 3 basic salts : calcium carbonate ( cc ), sodium bicarbonate ( sb ), and calcium hydroxyapatite ( ch ). each of these groups were further subdivided into four sets corresponding to test periods of 0 , 3 , 6 , and 9 weeks . for fabrication the polymer was dissolved in acetone and precipitated in ethanol . next , the reagent quality salts ( 30 % v / v ) were added to the gummy polymer and the polymer - salt composite was packed into molds , and placed under 25 mtorr vacuum at room temperature for curing . the control specimens did not contain any salts . prior to testing , the mass of each specimen was recorded . next , each specimen of the 3 , 6 , and 9 week sets was immersed in 10 ml of distilled water and maintained at 37 ° c . the ph of the water was monitored every 2 days . at the end of each test period , the specimens were removed , dried in a vacuum for 72 hours and then analyzed for changes in mass , molecular weight , mechanical properties , and surface morphology . the molecular weight of the polymer was estimated using gel permeation chromatography . the mechanical properties of the specimens were measured under creep conditions using an automated indentation apparatus . the ph of the water of the control specimens remained relatively constant up to 3 . 5 weeks followed by a rapid decrease until approximately 7 weeks ( fig2 ). the sb specimens exhibited only a small decrease in ph up to 5 . 5 weeks . however , between 5 . 5 and 7 weeks there was a significant decrease in ph followed by a relatively constant value thereafter . the ch specimens displayed a linear decrease in ph up to 9 weeks . the cc specimens exhibited an approximately linear but small decrease in ph over the entire test period . as shown in table i , the sb specimens exhibited the maximum mass loss at 3 weeks . however , at 9 weeks the difference between the control and sb specimens was not significant ( fig4 ). all the sets exhibited virtually a 100 % decrease in molecular weight at 9 weeks even though they underwent different degrees of loss at 3 weeks . the control and ch specimens exhibited an increase in stiffness at 3 weeks . however , the stiffness of sb specimens decreased over this same period of time . based on gross morphology observations , sb specimens exhibited significant swelling compared to control specimens . significant swelling was also observed in cc specimens . table i______________________________________percent loss in mass and molecular weightweek cc ch sb control______________________________________3 2 . 24 ± 0 . 6 / 8 . 28 ± 1 . 6 / 27 . 7 ± 4 . 2 / 4 . 84 ± 0 . 13 / 7 . 0 . 2 ± 4 . 9 34 . 9 ± 2 . 6 58 . 3 ± 2 . 4 60 . 64 ± 6 . 286 34 . 7 ± 9 . 6 / 78 . 7 ± 9 . 2 / 71 . 7 ± 6 . 4 / 55 . 05 ± 6 . 39 / 92 . 1 ± 1 . 5 99 ± 0 . 1 95 . 8 ± 1 . 1 99 . 09 88 . 9 ± 10 / 82 . 5 ± 2 . 8 / 97 . 4 ± 1 . 9 / 99 . 5 ± 0 . 01 / 100 99 ± 0 . 1 100 99 . 0 ± 0 . 02______________________________________ % mass loss /% molecular weight loss ; mean ± s . d . the results demonstrated that all 3 salts investigated in this study were successful in controlling the decrease in ph due to the acidic degradation products of the copolymer . at 9 weeks the cc group exhibited an average ph of 6 . 3 compared to 3 . 0 for the control group . implants containing cc maintained the ph value between 7 . 4 - 6 . 3 throughout the degradation process of the pla - pga copolymer until complete degradation was achieved . implants with ch and sb controlled the ph values between 6 . 9 - 4 . 3 and 8 . 2 - 4 . 5 respectively . thus , the results of this study show that a decrease in ph in the vicinity of pla - pga implants can be effectively controlled by incorporating basic salts . thus , the deleterious effects of such implants reported by other researchers related to a decrease in ph may be offset using the presently disclosed techniques . it is well known that the fabrication process for preparing porous polymeric devices ( e . g ., pla - pga ) often results in the formation of a relatively impermeable and non - porous outer covering or &# 34 ; skin &# 34 ;. while this &# 34 ; skin &# 34 ; does not destroy the internally porous structure , it creates a problem because it impedes to varying degrees the passage of fluids and other substances into and out of the porous internal structure of the polymer . thus , it is desirable to devise a fabrication process that achieves a uniformly porous and permeable implant . according to the presently described technique , enhanced surface porosity may be achieved by removing all or a portion of the non - porous &# 34 ; skin &# 34 ;. a new method to increase the surface porosity of porous devices treated or created with polymeric materials has been designed . this is particularly important in the context of the present invention , as the biodegradable polymeric devices will more readily adapt to and control against ph shifts in the environment where surface porosity is maximized . in the present example , a 50 : 50 pla - pga porous implant was prepared as defined in example 1 . the polymer included an alkaline ph controlling substance , sodium bicarbonate . in the formation of such devices , a &# 34 ; skin &# 34 ; forms at the polymer surface . using a mechanical device devised by the inventors , a quick and consistent removal of the skin was achieved . in one embodiment , the mechanical device is a circular punch with a sharp cutting edge . all of the skin of a rod - like polymeric cylinder was effectively removed by slicing the skin away . the surface porosity of the polymeric implant prepared according to the present invention is dramatically improved by removing the outer layer , or skin , from the polymer implant . the porous outer surface will result in enhanced ingress of fluids into the implant , thus resulting in better release of the buffering agents . it is also expected that body fluids ( e . g ., vascular supplies , marrow , synovial fluid ) will be able to enter the implant much more expediently and thus provide the repair sites with migrating cells ( e . g ., mesenchymal stem cells , chondrocytes , osteoblasts ) or nutrients necessary for tissue ingrowth into the porous implant . in some biodegradable devices , depending on the polymer used for fabrication , the release of acidic by - products occurs in significant quantities only after an initial incubation period . it would be preferable to provide a mechanism for the release of an alkaline agent ( s ) in an amount that would prevent sudden changes in ph that would occur as a consequence , such as through the release of matching amounts of said alkaline agent ( s ) at the same or similar rates . such would provide an implantable device having ph control over a clinically useful period of time . in order to achieve an appropriate release rate of an alkaline agent , the alkaline agents may be incorporated in the implant in layers . for example , each layer of alkaline agent would be overlaid by a layer of the biodegradable polymer , alternating alkaline agent layer , polymer layer , etc . as each layer degrades , the alkaline or other ph - controlling substance is released , thus controlling against an overly acidic ph surrounding the implant over an extended period of time . any variety of configurations of the layered polymeric implant may be created using this technique and tailored for the particular application desired . for example , a base amount of a ph controlling substance may be included in polymeric layers closest the core of an implant , with succeeding layers containing either progressively lower or higher amounts of the ph controlling substance . although the present invention has been described in some detail by way of illustration and example for purposes of clarity and understanding , it will be obvious that certain changes and modifications may be practiced within the scope of the claims . the references listed below are incorporated herein by reference to the extent they supplement , explain , provide a background for or teach methodology , techniques and / or compositions employed herein . 2 . suganuma , j . and alexander , h . ( 1993 ), j . appl . biomaterials , 4 : 13 - 27 . 3 . daniels et al . ( 1992 ), proc . orthop . res . soc ., pg . 88 . 4 . taylor et al . ( 1994 ), &# 34 ; six bioadsorbaliler polymers / in vitro acute toxicity of accumulated degradated materials ,&# 34 ; j . appl . &# 39 ; d bio . materials , 5 : 151 - 157 . 5 . tencer et al . ( 1986 ), &# 34 ; bone ingrowth into polymer coated porous synthetic coralline hydroxyapatite ,&# 34 ; ieee / engineering in med . and biol . soc ., annual conference , pp . 1668 - 1671 . 6 . fong , j . w ., u . s . pat . no . 4 , 479 , 911 , oct . 30 , 1984 , &# 34 ; process for preparation of microspheres and modification of release rate of core material ; mixing alkalinity agent with polymer - core - solvent system .&# 34 ; 7 . kampner s . l ., u . s . pat . no . 4 , 990 , 161 , ( 1991 ) &# 34 ; bone implant with resorbable stem has biodegradable anchor with exterior surface to engage interior of bone canal .&# 34 ; 8 . allemann et al . ( 1993 ), &# 34 ; in vitro extended - release properties of drug - loaded poly ( dl - lactic acid ) nanoparticles produced by a salting - out procedure ,&# 34 ; pharm . res ., 10 : 12 . 9 . younes et al . ( 1988 ), &# 34 ; biodegradable pela block copolymers : in vitro degradation and tissue reaction ,&# 34 ; bimater . artif . cells artif . organs , 16 ( 4 ): 705 - 719 . 10 . mariette et al . ( 1993 ), &# 34 ; release of the grf29nh2 analog of human grf44nh2 from a pla / ga matrix ,&# 34 ; j . control release , 24 ( 1 - 3 ): 237 - 246 . 11 . vasenius et al . ( 1992 ), &# 34 ; do intramedullary rods of self - reinforced poly - l - lactide or poly - dl / l - lactide cause lactic acid acidosis in rabbits ?&# 34 ; clin . mater . 10 ( 4 ): 213 - 218 . 12 . gilding and reed ( l979 ), &# 34 ; biodegradable polymers for use in surgery - polyglycolic / poly ( lactic acid ) homo - and copolymers ,&# 34 ; polymer , 20 : 1459 - 1464 .
the invention discloses ph - controlling devices that comprise a biodegradable polymer and a ph - controlling substance , particularly an alkaline , acidic or buffering agent . by way of example , such alkaline agents include calcium carbonate and sodium bicarbonate . methods of preparing such devices are also described . methods for enhancing biocompatibility of an implantable device are also provided , as neutralizing alkaline materials are released at a rate that offsets changes in ph typically observed as polymers degrade to various acidic or alkaline by - products . by way of example , biodegradable polymers include pla , pga , polycaprolactone , copolymers thereof , or mixtures thereof . a new technique is also disclosed to increase the surface porosity of porous implants which have a tendency to form relatively impermeable coverings . this technique entails the use of mechanical means to remove at least part of said covering , thus increasing the implant &# 39 ; s surface porosity and permeability .