referring to the drawing , there is shown the distal end portion of a stent delivery catheter indicated generally by numeral 10 . the stent 11 itself ( fig2 ) may typically comprise a braided or slotted , non - self - expanding metal or plastic tube whose inside diameter closely conforms to the exterior of the expander member 12 when the expander member is non - inflated ( not shown ). the delivery catheter 13 may be introduced into the vascular system 15 in a conventional way and then advanced through the vascular system until the expander member 12 on the catheter body stock 14 carrying the yet non - expanded stent is juxtaposed relative to a treatment site in the vascular system . once so positioned , the expander member 12 is inflated and , in doing so , the stent is also expanded to a predetermined diameter that is a function of the outside diameter of the expander member 12 at a desired pressure . once the stent has been expanded in the manner described , the expander member 12 is again deflated by aspirating the inflation fluid therefrom and , once deflated , is extracted from the vascular system . it is , of course , desirable that upon aspiration , the expander member 12 deflates so as to closely conform to the outside diameter of the catheter body stock 14 on which it is mounted . so - called winging or pancaking of the expander member is undesirable . furthermore , when the end - use of the balloon catheter is for deploying non - self - expanding stents , it is important that the expander member 12 possess high abrasion resistance so as to prevent rupture as the surface of the expander member 12 frictionally engages the stent during expansion thereof . as shown in fig1 the catheter body stock 14 comprises an elongated , flexible tube having an inflation lumen 16 extending the length thereof . affixed to the distal end portion of the catheter body stock 14 is a generally cylindrical , tubular expander member having conically - shaped end portions as at 18 and 20 which are bonded to the exterior wall of the catheter body stock 14 in zones 22 and 24 to define a hollow chamber 26 when the expander member 12 is inflated . the distal end of the inflation lumen 16 extends beyond the seal zone 22 permitting inflation fluid under pressure to flow into the chamber 26 and expand the expander member . in accordance with the present invention , the expander member 12 is formed by blow - molding and stretching a parison previously formed in a coextrusion process such as is described in the hamlin u . s . pat . no . 5 , 270 , 086 . thus , the resulting expander member 12 has double walls 28 and 30 , respectively . the coextruded parison is designed to have an inner wall comprising pet and an outer wall of polyamide , with nylon - 12 being preferred . several nylons suitable for the expander member are grilamid l25 , ems , vestamide 2101 f , huls and vestamide 1801 f , huls . the pet component may be ici 5822 c or shell traytuf 1006 . when subjected to the stretch / blow - molding operation , both the pet layer and the nylon layer are biaxially oriented within a heated mold until a desired composite wall thickness and outer diameter are attained . typical wall thickness in an unexpanded state may range from about 0 . 0004 - 0 . 0009 , preferably about 0 . 00045 - 0 . 0006 inches . when the end - use of the balloon catheter is stent delivery , it is important that the expander member 12 exhibit a relatively low compliance factor so that the stent will only be expanded to a desired outside diameter . those skilled in the art will recognize that if the expander member is highly compliant , it becomes more difficult to control the extent of expansion of the stent being delivered thereby . it has been determined that if the compliance factor is kept below about 15 percent , and preferably 13 percent or less for pressures in the range of from 18 atmospheres to 18 atmospheres , good control over expanded stent diameter can be realized . it has been determined that when the outer wall 30 of the expander member 12 comprises polyamide , such as nylon - 12 , and when the inner wall 28 is pet , of the percent by weight of the polyamide in the composite is in the range of from 20 to 80 percent , the balloon &# 39 ; s compliance can be maintained in the desired range indicated . the pet layer 28 provides an expander member with a high burst strength while the outer nylon - 12 layer 30 offers excellent abrasion resistance . to improve the conformance parameter of the composite , double - walled expander member , such that upon inflation and subsequent deflation , it conforms closely to the profile of the catheter body stock 14 , temperature annealing of the expander member in accordance with the teachings of the aforereferenced roychowdhury patent application may be used . preferably , a blown expander member is taco - wrapped in a sheath and subjected to a heating cycle at a temperature between 75 ° and 95 ° c . for a period of time in the range of from 1 to 4 hours . the annealed double - walled balloon has been found to closely conform to the outer diameter of the catheter body stock 14 when first expanded to a pressure of about 8 atmospheres and subsequently deflated by aspirating the inflation fluid from the chamber 26 . no appreciable winging results . it has also been found that the distension curve for the composite , double - walled balloon can be tailored based upon the nylon - 12 content and the annealing conditions imposed . an expander member having a diameter of 3 . 0 mm and comprising coextruded pet , and nylon - 12 in ratios from about 55 - 45 to 65 - 35 were formed and during the stretch / blow - molding thereof were stretched a distance of from 5 : 1 to 8 : 1 in the radial direction . the expander members exhibited a wall thickness in the range of from 0 . 0005 to 0 . 001 inches and in an unexpanded state and were found to exhibit the following composite hoop stresses and burst pressures for the indicated wall thicknesses . hoop stress was calculated as a σ = pd / 2t , wherein t is the balloon wall thickness measured in an unexpanded state , p is the burst pressure measured at 37 ° c ., and d is the diameter at 10 atmospheres and room temperature . ______________________________________ hoop stresswall ( in .) nylon (%) burst ( psi ) ( psi ) ______________________________________ . 00058 38 . 2 441 . 8 44 , 941 . 0007 38 . 5 436 . 4 36 , 782 . 0009 41 . 7 483 . 0 35 , 621______________________________________ fig3 a and 3b are graphs reflecting the variation in the compliance characteristics of average value for five groups of five samples each of catheter expander members when each group of five was subjected to annealing temperatures varying from 75 ° for the first group to 95 ° c . for the fifth group in each case for one hour . the expander members were each 3 . 0 mm in diameter and 20 mm in length . they comprised 40 % nylon , 60 % pet coextrusions . these curves show that by proper control of annealing temperatures , it is possible to tailor the expander members to exhibit desired compliance characteristics in a range of from 10 % to 18 % between 8 and 18 atmospheres . the relative percentages of nylon and pet in the co - extrusion also has an effect on the compliance characteristics . measurements were taken at room temperature and using water as the inflation medium . a summary of the results is shown in table 1 below . table i__________________________________________________________________________1 hour @ 75 ° c . 1 hour @ 80 ° c . 1 hour @ 85 ° c . 1 hour @ 90 ° c . 1 hour @ 95 ° c . psi average psi average psi average psi average psi average__________________________________________________________________________14 . 7 2 . 921 14 . 7 2 . 879 14 . 7 2 . 805 14 . 7 2 . 754 14 . 7 2 . 64929 . 4 2 . 935 29 . 4 2 . 893 29 . 4 2 . 827 29 . 4 2 . 783 29 . 4 2 . 67244 . 1 2 . 946 44 . 1 2 . 905 44 . 1 2 . 847 44 . 1 2 . 797 44 . 1 2 . 68758 . 8 2 . 953 58 . 8 2 . 915 58 . 8 2 . 861 58 . 8 2 . 809 58 . 8 2 . 69873 . 5 2 . 960 73 . 5 2 . 925 73 . 5 2 . 864 73 . 5 2 . 821 73 . 5 2 . 71088 . 2 2 . 970 88 . 2 2 . 939 88 . 2 2 . 886 88 . 2 2 . 830 88 . 2 2 . 719102 . 5 2 . 982 102 . 5 2 . 956 102 . 5 2 . 902 102 . 5 2 . 850 102 . 5 2 . 732117 . 6 2 . 996 117 . 6 2 . 976 117 . 6 2 . 916 117 . 6 2 . 864 117 . 6 2 . 748132 . 3 3 . 013 132 . 3 2 . 997 132 . 3 2 . 937 132 . 3 2 . 892 132 . 3 2 . 769147 . 0 3 . 029 147 . 0 3 . 018 147 . 0 2 . 967 147 . 0 2 . 923 147 . 0 2 . 806161 . 7 3 . 042 161 . 7 3 . 034 161 . 7 2 . 996 161 . 7 2968 161 . 7 2 . 857176 . 4 3 . 056 176 . 4 3 . 047 176 . 4 3 . 022 176 . 4 3 . 002 176 . 4 2 . 919191 . 1 3 . 069 191 . 1 3 . 061 191 . 1 3 . 046 191 . 1 3 . 036 191 . 1 2 . 978205 . 8 3 . 082 205 . 8 3 . 079 205 . 8 3 . 065 205 . 8 3 . 061 205 . 8 3 . 019220 . 5 3 . 101 220 . 5 3 . 089 220 . 5 3 . 065 220 . 5 3 . 079 220 . 5 3 . 054235 . 8 3 . 116 235 . 8 3 . 104 235 . 8 3 . 101 235 . 8 3 . 095 235 . 8 3 . 079249 . 9 3 . 132 249 . 9 3 . 118 249 . 9 3 . 118 249 . 9 3 . 115 249 . 9 3 . 104264 . 6 3 . 153 264 . 6 3 . 135 264 . 6 3 . 138 264 . 6 3 . 136 264 . 6 3 . 123279 . 3 3 . 173 279 . 3 3 . 151 279 . 3 3 . 154 279 . 3 3 . 155 279 . 3 3 . 141294 . 0 3 . 192 294 . 0 3 . 171 294 . 0 3 . 170 294 . 0 3 . 179 294 . 0 3 . 160308 . 7 3 . 214 308 . 7 3 . 192 308 . 7 3 . 189 308 . 7 3 . 197 308 . 7 3 . 184323 . 4 3 . 234 323 . 4 3 . 215 323 . 4 3 . 208 323 . 4 3 . 222 323 . 4 3 . 209338 . 1 3 . 250 338 . 1 3 . 233 338 . 1 3 . 227 338 . 1 3 . 237 338 . 1 3 . 228__________________________________________________________________________ this invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required . however , it is to be understood that the invention can be carried out by specifically different equipment and devices , and that various modifications , both as to the equipment details and operating procedures , can be accomplished without departing from the scope of the invention itself .
an expander member for a balloon catheter to be used in stent delivery applications comprises a double - walled tubular member that is adapted for attachment to a distal end portion of an elongated , flexible , tubular catheter body . the outer wall is preferably a polyamide , such as nylon - 12 , and the inner wall is pet . by properly controlling the percentage of the polyamide relative to the pet content , the distension characteristics of the resulting expander member can be tailored to fall in a range less than 15 percent . the outer nylon - 12 layer provides the expander member with high abrasion resistance . by appropriately temperature annealing the expander member , any tendency toward winging upon deflation thereof is reduced .