fig1 illustrates a guidewire 10 which includes a core member 11 having an outer sheath 12 formed of a co — ni — cr alloy and an inner member 13 formed of ni — ti , a helical coil 14 on the distal end of the core member with a shaping ribbon 15 extending between the distal end of the core member and a rounded plug 16 which connects the distal end of the shaping ribbon with the distal end of the helical coil . the distal section 17 of the core member 11 , which is disposed primarily within the coil 14 , is tapered to sequentially smaller diameters to provide gradually increasing flexibility along the length of the distal portion of the guidewire 10 . the taper is formed by removing the sheath 12 formed of high strength co — ni — cr alloy which exposes the inner niti alloy member 13 having moderate strength and substantial flexibility which may then be ground in a conventional manner to one or more smaller diameter sections . fig2 depicts a guidewire 30 with a construction wherein the tapered distal section 31 of the core member 32 extends to the plug 33 which connects the distal end of the core member to the distal end of the helical coil 34 disposed about the distal section of the core member . the proximal section 35 of the core member 32 is of composite construction as in the prior embodiment with a sheath 36 of high strength co — ni — cr alloy and an inner member 37 of a pseudoelastic niti alloy . the high strength sheath 36 is removed from the core member to form the tapered distal section 31 to increase the flexibility of the distal section of the guidewire 30 . the elongated proximal portions of the guidewires are generally about 130 to about 140 cm in length with an outer diameter of about 0 . 006 to about 0 . 018 inch for coronary use . larger diameter guidewires may be employed in , peripheral arteries and other body lumens . the lengths of the smaller diameter and tapered sections can range from about 2 to about 20 cm , depending upon the stiffness or flexibility desired in the final product . the helical coil is about 20 to about 45 cm in length , has an outer diameter about the same size as the diameter of the elongated proximal portion , and is made from wire about 0 . 002 to about 0 . 003 inch in diameter . the shaping ribbon and the flattened distal section of distal portion have rectangular transverse cross - sections which usually have dimensions of about 0 . 001 by 0 . 003 inch . the overall length of a guidewire is typically about 175 cm . a presently preferred cobalt - nickel containing alloy is commercially available as mp35n from carpenter technology corporation which has a nominal composition of about 35 % cobalt , about 35 % nickel , about 20 % chromium and about 10 % molybdenum . other commercially available alloys include elgiloy from elgiloy limited partnership and haynes 188 from haynes international . the following example is given to illustrate the method of forming the core member of a guidewire in accordance with the invention . a niti alloy rod having a composition of about 55 . 9 % ni and 44 . 1 % ti was drawn to a diameter of about 0 . 06 inch . the as - drawn wire , which was in a cold worked condition ( e . g . 50 % cold work ), was ground or etched to remove tenaceous surface oxides and then annealed at 700 ° c . for about one hour . a tubular sheath having a nominal composition of 35 % co , 35 % ni , 20 % cr and 10 % mo was formed with an outer diameter of about 0 . 114 inch and an inner diameter of about 0 . 068 inch in an annealed condition . the niti wire was disposed within the inner lumen of the high strength sheath and the assembly was drawn in a series of five stages with a 50 % reduction in area followed by heat treating at 750 ° c . for 15 minutes in each stage . the fifth stage was followed by a sixth stage which included drawing with a cold work of about 16 % followed by heat treating at 750 ° c . and a seventh stage which included drawing with a cold work of about 50 % but with no heat treating . the final cold worked product was aged at temperatures of about 650 ° for about one minute to develop maximum bending , yield and modulus with minimum spring back . the composite core member of the invention provides a number of favorable properties and characteristics . the outer sheath of high strength cobalt - nickel alloy provides the necessary stiffness and push and the inner niti alloy member provides the desirable distal flexibility . another advantage of the composite product of the present invention , when utilized as a core member of a guidewire , is that the proximal end of the flexible coil can be soldered or brazed to the co — ni — cr alloy sheath so as to avoid the problems with soldering the coil to a niti alloy which is very difficult to bond to by conventional soldering techniques because of the tenaceous oxide which usually forms on the surfaces of titanium containing alloys . fig3 and 4 illustrate another embodiment of the invention wherein the high strength co — ni — cr alloy is in the form of an intraluminal stent 40 , which as shown in fig3 is expanded by the balloon 41 of catheter 42 within a stenosis 43 . after permanent expansion of the stent 40 within the body lumen 44 , such as a coronary artery , the balloon 41 is deflated and the catheter 42 withdrawn . the high strength developed by the co — ni — cr alloy allows the stent to be formed of thinner material , yet provide the radial rigidity to hold the body lumen upon deflation of the balloon . the balloon utilized to expand the stent is similar in many respects to a dilatation balloon used in angioplasty procedures in that it is a generally ineleastic balloon formed of a suitable polymeric material such as a high density polyethylene , polyethylene terephthalate and polyolefin , e . g . surlyn ®. a particularly suitable stent design is disclosed in copending application ser . no . 08 / 164 , 986 , filed on dec . 9 , 1993 which is assigned to the present assignee and which is incorporated herein by reference . fig4 is an enlarged perspective view of a stent 50 as shown in fig3 and 5 - 7 with one end of the stent shown in an exploded view to illustrate in greater detail the placement of interconnecting elements 53 between adjacent radially expandable cylindrical elements 52 . each pair of the interconnecting elements 53 on one side of a cylindrical element 52 are preferably placed to achieve maximum flexibility for a stent . in the embodiment shown in fig8 the stent 50 has three interconnecting elements 53 between adjacent radially expandable cylindrical elements 52 which are 120 degrees apart . each pair of interconnecting elements 53 on one end of a cylindrical element 52 are offset radially 60 degrees from the pair on the other side of the cylindrical element . the alternation of the interconnecting elements results in a stent which is longitudinally flexible in essentially all directions . various configurations for the placement of interconnecting elements are possible , and several examples are illustrated schematically in fig9 - 12 . however , as previously mentioned , all of the interconnecting elements of an individual stent should be secured to either the peaks or valleys of the undulating structural elements in order to prevent shortening of the stent during the expansion thereof . fig5 illustrates a stent 50 incorporating features of the invention which is mounted onto a delivery catheter 51 . the stent generally comprises a plurality of radially expandable cylindrical elements 52 disposed generally coaxially and interconnected by elements 53 disposed between adjacent cylindrical elements . the delivery catheter 51 has an expandable portion or balloon 54 for expanding of the stent 50 within an artery 55 . the artery 55 , as shown in fig5 has a dissected lining 56 which has occluded a portion of the arterial passageway . the delivery catheter 51 onto which the stent 50 is mounted , is essentially the same as a conventional balloon dilatation catheter used for angioplasty procedures . the balloon 54 may be formed of suitable material such as polyethylene , polyethylene terephthalate , polyvinyl chloride , nylon and ionomers such as surlyn ® manufactured by the polymer products division of the du pont company . other polymers may also be used . in order for the stent 50 to remain in place on the balloon 54 during delivery to the site of the damage within the artery 55 , the stent 50 is compressed onto the balloon . a retractable protective delivery sleeve 20 as described in co - pending application ser . no . 07 / 647 , 464 , filed on apr . 25 , 1990 and entitled stent delivery system may be provided to further ensure that the stent stays in place on the expandable portion of the delivery catheter 51 and prevent abrasion of the body lumen by the open surface of the stent 50 during delivery to the desired arterial location . other means for securing the stent 50 onto the balloon 54 may also be used , such as providing collars or ridges on the ends of the working portion , i . e ., the cylindrical portion , of the balloon . each radially expandable cylindrical element 52 of the stent 50 may be independently expanded . therefore , the balloon 54 may be provided with an inflated shape other than cylindrical , e . g . tapered , to facilitate implantation of the stent 50 in a variety of body lumen shapes . in a preferred embodiment , the delivery of the stent 50 is accomplished in the following manner . the stent 50 is first mounted onto the inflatable balloon 54 on the distal extremity of the delivery catheter 51 . the balloon 54 is slightly inflated to secure the stent 50 onto the exterior of the balloon . the catheter - stent assembly is introduced within the patient &# 39 ; s vasculature in a conventional seldinger technique through a guiding catheter ( not shown ). a guidewire 58 is disposed across the damaged arterial section with the detached or dissected lining 56 and then the catheter - stent assembly is advanced over a guidewire 58 within the artery 55 until the stent 50 is directly under the detached lining 56 . the balloon 54 of the catheter is expanded , expanding the stent 50 against the artery 55 , which is illustrated in fig2 . while not shown in the drawing , the artery 55 is preferably expanded slightly by the expansion of the stent 50 to seat or otherwise fix the stent 50 to prevent movement . in some circumstances during the treatment of stenotic portions of an artery , the artery may have to be expanded considerably in order to facilitate passage of blood or other fluid therethrough . the stent 50 serves to hold open the artery 55 after the catheter 51 is withdrawn , as illustrated by fig7 . due to the formation of the stent 50 from an elongated tubular member , the undulating component of the cylindrical elements of the stent 50 is relatively flat in transverse cross - section , so that when the stent is expanded , the cylindrical elements are pressed into the wall of the artery 55 and as a result do not interfere with the blood flow through the artery 55 . the cylindrical elements 52 of the stent 50 which are pressed into the wall of the artery 55 will eventually be covered with endothelial cell growth which further minimizes blood flow interference . the undulating portion of the cylindrical sections 52 provide good tacking characteristics to prevent stent movement within the artery . furthermore , the closely spaced cylindrical elements 52 at regular intervals provide uniform support for the wall of the artery 55 , and consequently are well adapted to tack up and hold in place small flaps or dissections in the wall of the artery 55 as illustrated in fig6 and 7 . fig1 illustrates a stent of the present invention wherein three interconnecting elements 53 are disposed between radially expandable cylindrical elements 51 . the interconnecting elements 52 are distributed radially around the circumference of the stent at a 120 degree spacing . disposing four or more interconnecting elements 53 between adjacent cylindrical elements 52 will generally give rise to the same considerations discussed above for two and three interconnecting elements . the properties of the stent 50 may also be varied by alteration of the undulating pattern of the cylindrical elements 53 . fig1 illustrates an alternative stent structure in which the cylindrical elements are in serpentine patterns but out of phase with adjacent cylindrical elements . the particular pattern and how many undulations per unit of length around the circumference of the cylindrical element 52 , or the amplitude of the undulations , are chosen to fill particular mechanical requirements for the stent such as radial stiffness . the number of undulations may also be varied to accommodate placement of interconnecting elements 53 , e . g . at the peaks of the undulations or along the sides of the undulations as shown in fig8 and 13 . the stent 50 of the present invention can be made in many ways . however , the preferred method of making the stent is to coat a thin - walled tubular member , such as stainless steel hypotubing , with a material which is resistive to chemical etchants , and then to remove portions of the coating to expose the underlying hypotubing which is to be removed but to leave coated portions of the hypotubing in the desired pattern for the stent so that subsequent etching will remove the exposed portions of the metallic tubing , but will leave relatively untouched the portions of the metallic tubing which are to form the stent . the coated portion of the metallic tube is in the desired shape for the stent . an etching process avoids the necessity of removing burrs or slag inherent in conventional or laser machining processes . it is preferred to remove the etchant - resistive material by means of a machine - controlled laser as illustrated schematically in fig1 . while the present invention has been described herein in terms of certain preferred embodiments , those skilled in the art will recognize that a variety of modifications and improvements can be made to the present invention without departing from the scope thereof .
this invention is directed to an intracorporeal device formed of a high strength co — ni — cr alloy and is particularly suitable for forming a composite product with a pseudoelastic member formed of niti alloy . suitable intracorporeal products include guidewires and stents . the high strength alloy consists essentially of about 28 to about 65 % cobalt , about 2 to about 40 % nickel , about 5 to about 35 % chromium , up to about 12 % molybdenum , up to about 20 % tungsten , up to about 20 % iron and the balance inconsequential amounts of impurities and other alloying constituents , with a preferred alloy composition including about 30 to about 45 % cobalt , about 25 to about 37 % nickel , about 15 to about 25 % chromium and about 5 to about 15 % molybdenum . intravascular devices such as guidewires , stents and the like can be formed of this high strength co — ni — cr alloy .