the stent of the present invention is employed to support or otherwise treat a targeted site within the vasculature . such stent is introduced into the vasculature , advanced therethrough to the deployment site and expanded using conventional techniques and delivery systems . once in position and subject to the continuous flow of blood therethrough , it gradually degrades , substantially without the risk inherent in previously known biodegradable stents or stents with biodegradable coatings of breaking up into or releasing sizeable particles that may be swept downstream and cause emboli . the material employed in the manufacture of the stent of the present invention is a polymer that is simultaneously hydrophobic and has water - labile linkages interconnecting its monomers that are further fortified by ester or imide bonds . the hydrophobic nature of the polymer precludes the incursion of water into its interior while the water - labile bonds that are exposed on its surface nonetheless cause the polymer to degrade . degradation thereby exclusively progresses from the material &# 39 ; s surface inwardly to yield a much more uniform degradation rate and to preclude bulk erosion . the incorporation of the imide or ester bonds serves to impart sufficient strength to the material to enable it to provide the support that is required of a stent . alternatively , if the material is used as stent coating , the incorporation of the imide or ester bonds impart sufficient strength to the material to prevent it from flaking off or otherwise becoming detached as the underlying stent undergoes the distortion attendant its being expanded by for example the inflation of a balloon . many of the stent &# 39 ; s ultimate performance characteristics are controllable by the appropriate selection of the various dimensional parameters of the stent . increasing the dimensions of various structural elements of the stent will generally serve to increase strength and decrease flexibility . such effect would result from both an increase in the width or in the wall thickness of the stent &# 39 ; s structural elements . the time period in which the stent would become totally degraded or absorbed is a function of the wall thickness of the various elements while the degradation rate is a function of the total area exposed to contact with the blood . by for example selecting a stent configuration which employs a large number of relatively narrow spine and strut elements to achieve a particular level of strength , the time in which the stent degrades when subjected to the blood flow can be substantially accelerated . conversely , a stent configuration in which a relatively few , wide structural elements are employed causes the degradation rate to be somewhat retarded . the stent &# 39 ; s ultimate performance characteristics are of course also controllable by the appropriate selection of chemical variables . for example , the number of imide or ester bonds that are incorporated in the polymer material not only affects the ultimate strength and flexibility characteristics of the stent , but also has an effect on the rate at which the material degrades when subjected to blood flow . an increased bond content enhances strength , decreases flexibility and increases degradation time . the specific requirements of a particular application will ultimately determine the optimal combination of the stent configuration , wall thickness and ester or imide bond content . polymers that satisfy the above - described requirements include polyanhydrides and polyorthoesters . representative examples of polyanhydride polymers suitable for use in the construction of a stent or formulation of a stent coating in accordance with the present invention include anhydride - co - imide ter polymers containing trimellitylimido - l - tyrosine , sebacic acid ( sa ) and 1 , 3 bis ( carboxyphenoxy ) propane . other examples of suitable polyanhydrides include poly ( fatty acid — sebacic acid ) synthesized from erucic acid and sebacic anhydride p ( ead : sa ) and poly ( l - lactic acid - co - l - aspartic acid ). representative examples of polyorthoester polymers suitable for use in the construction of a stent or formulation of a stent coating in accordance with the present invention include poly ( 4 - hydroxy - l - proline ester ), poly ( 1 , 10 decanediol - 1 , 10 decanediol dilactide ) and poly ( 1 , 2 , 6 hexanetriol - trimethylorthoacetate ). an ester or imide content of 20 %- 40 % has been found to be effective to provide sufficient strength for a stent application . the process for forming a polymer stent is well known in the art . a stent of the present invention is formed by first causing the appropriate reagents to react to form the desired polyanhydride or polyorthoester composition . during copolymer synthesis , the imide content of such composition is increased by incorporating higher imide containing monomers like trimellitylimido - l - tyrosine . increasing imide content results in higher strength material . flexibility of polyanhydrides like p ( ead : sa ) can be increased by increasing the percentage of erucic acid dimer ( ead ) during polymer synthesis . the ester content of such composition is increased by incorporating higher ester containing monomers such as l - proline ester or trimethyl orthoacetate . selected pharmacological agents can be added to the reagents so as to incorporate such materials throughout the polymer to thereby provide for the gradual dispensation of the drug over the service life of the stent . the blending may be accomplished either in solution or in a melt state . drugs such as for example heparin or other proteins can readily be added to the reactants before or during the polymerization process . alternatively , some drugs may be infused throughout the polymer after polymerization is completed . if desired , the drug may be applied to the surface of the cured polymer to cause the entire dosage to be released shortly after implantation . the stent may be formed by any of a number of well known methods including the extrusion of the polymer into the shape of a tube . preselected patterns of voids are then formed into the tube in order to define a plurality of spines and struts that impart a degree of flexibility and expandability to the tube . alternatively , the drug loaded polymer may be applied to the selected surfaces of a stent formed of , for example , stainless steel or nitinol . in order to coat all of the surfaces of the stent , the stent is immersed in the molten polymer . alternatively , the polymer may be extruded in the form of a tube which is then co - drawn with a tube of stainless steel or nitinol . by co - drawning two tubes of the polymer with a metal tube , one positioned about the exterior of the metal tube and another positioned within such metal tube , a tube having multi - layered walls is formed . subsequent perforation of the tube walls to define a preselected pattern of spines and struts imparts the desired flexibility and expandability to the tube to create a stent . while a particular form of the invention has been illustrated and described , it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention . accordingly , it is not intended that the invention be limited except by the appended claims .
a stent is fabricated utilizing a polymer that is selected for its tendency to degrade from the surface inwardly rather than undergo bulk erosion so as to substantially reduce the risk of large particles becoming detached and being swept downstream . such polymer is hydrophobic yet has water - labile linkages interconnecting the monomers . ester or imide bonds are incorporated in the polymer to render the surface degrading materials suitable for use in stent applications . the stent may be coated with such polymer or may be wholly formed therefrom .