We are Nitinol. Nitinol Medical Devices and ImplantsStoeckelMin Invas Ther & Allied Technol 9(2)pp. 81‐882000www.nitinol.com47533 Westinghouse DriveFremont, California 94539t 510.683.2000f 510.683.2001

Min Invas Ther & Allied TechnoI 2000: 9(2) 81-88Nitinol medical devices andimplantsD. StoeckelCordis Corporation - Nitino/ Devices and Components, Fremont, CA, USASummarySuperelastic Nitinol is now a common and well-known engineering material in the medical industry.While the greater flexibility of the alloy drives many of its applications, there are also a large number oflesser- known advantages of Nitinol in medical devices. This paper reviews 7 of these less-obvious butvery important reasons for Nitinol's success, bot h past and future. Several new medical applicationswill be used to exem plify these points, includ ing the quickly-g rowing and technolog ically-demanding stentapplications. Stents are particularly interesting because they take advantage of the thermoelastic hysteresis ofNitinoLKeywordsNitinol, shape memory, superelasticity, dynamic interference, biased stiffnessIntroductionAlthough both effects are clearly spectacular, theyNickel-titanium (Nitinol) alloys are rapidly becoming are by no means the only important properties of thethe materials of choice for self-expanding stents, graft : material. In this paper some important devicesupport systems, filters, baskets and various other characteristics, not commonly known to the devicedevices for minimally-invasive interventional and designers, will be discussed, all of which can beendoscopic procedures . Most medical device attributed to the specific properties of Nitinol. Theycompanies now offer products the performance of allow interesting solutions for superior medicalwhich is based on the highly unusual properties of devices [2]:these materials. Elastic deploymentNitinol alloys are most qommonly known for theirsuperelasticity and thermal shape memory [1]. The Thermal deployment Kink resistancelerm 'shape-memory' is used to describe thephenomenon of restoring to a predetermined shape Constant stress Dynamic interferenceon heating, after having been 'plastically' deformed; Stress hysteresis (biased stiffness)'superelasticity' refers to the enormous elasticity of Temperature dependence of stressthese alloys. It can be 10 times more than theelasticity of the best stainless steels (SS) used in Other, equal ly important properties, such as MRImed icine today and follows a non-l inear path,compatibility, biocompatibility and corros ion arecharacterised by a pronounced hysteresis (Figure 1).discussed elsewhere in this publication.CorrespCJf'IderJaJ: D. Stoeckel, Cordis CotporatiOn - NftinoI Devices and Com,oonents 47533 West.1lghouse Drive, Fremont CA 94539,USA.C 2000 Isis Medical Media Ltd81

D. Stoeckelcannula, which is inserted into the breast andadjusted unti l its tip is verified to be at the site of thetumour, The hook is then advanced, reforming a tighthook configuration, If necessary, the device can bewithdrawn into the need le, repositioned and redep loyed unti l the position is verified to be markedcorrectly for the surgeon .The concept of elastically dep loying a curveddevice through a straight need le or cannu la isprobably t he most common use of Nitino l in medicalinstrumentat ion. Among the newer devices are theSmartGuide deflectable puncture needle (DaumMedical, Figure 3) , and the radiofrequency interstitialtissue ablation device (R ITA Medical Systems, Inc.,Figure 4). Both devices deliver curved tubular needles.Other devices, such as suture-passers, retractors,deflectable graspers and scissors, have been in usesince the early 1990s in endoscopic surgery [4,5].Sign ificantly more comp lex devices can also besuperelastically deployed. Interesting examples areocclusion devices for repairing defects in the septalwall, or the patent ductus of the heart. The AmplatzerSeptal occlusion device is a Nitinol wire mesh shapedinto a 'doub le mushroom' configuration (Figure 5). Itcan be del ivered through a 6- 9 F catheter [6] . OtherNitinol occlusion devices are the ASDOS (Osypka,Figure 6) , t he AngelWings (MicroVena) and theCardioSeal (Nitinol Med ical Technolog ies) . Thesedevices use an umbrella type design, while the PFMDuct-Occlude device (Figure 7) uses a Nitinol doub lehelix configuration./"'"./Steel.V LLV-NilinollA , , " . . .' --'"'-'MllATStrainFigure 1. Tensile behaviour of stainless steel and Nitinol(schematic).Elastic deploymentThe enormous elasticity of Nitinol allows devices to bebrought into t he body through catheters or otherdelivery systems w ith a small profi le. Once inside thebody, the devices can be released from constrainingmeans and unfo ld or expand to a much larger size.Probably the first such product to be marketed wasthe Homer Mammalok, wh ich radiologists use to'mark' the location of a breast tumour. It consists of aNitinol wire hook and a S8 cannu lated need le (Figure2) [3] . The wire hook is withdrawn into t he needleThermal deploymentMost self-expanding implants, such as stents andfilters , make use of the thermal shape-memory ofNitinol. A device w ith a transition temperature (Af) of30 C can be compressed at room or lowertemperature. It w ill stay compressed until thetemperature is raised to 30 C. It will then expand toFig ure 2. The Homer Mammalok needle/wire localiser [2J .Figure 3. SmartGuide deflectable puncture needle (Daum Medical).82

Nitinol medical devices and implantsTemperaturemonitori ng-Insulated Trocar-1cm markings- - --"X1IFigure 4. Radiofrequency interstitial ti ssue ablation device(RITA Medical Systems. Inc.).Figure 6. Atrial septal defect occlusion system ASDOS(Osypka Medizintechnik).Corporation).Figure 5. Amplatzer septal occlusion device (AGA MedicalFigure 7. Duct-occlude occlusion device (PFM Produkte fU rd ie Medizin AG).its preset shape. If this device could be kept coldduring introduction into the body, it wou ld not expanduntil at the desired location where body heat wouldwarm it up. This is, of course, rather difficu lt toaccomplish. All self-expanding devices, therefore, areconstrained in the delivery systems to preventpremature deployment. Figure 8 shows the deployment of the TrapEase (Cordis) vena cava filter from a 6F delivery system atDevices could, theoretically, be built with transition temperatures of 40 Cor higher. These devices would have to be heatedafter delivery to the site to make them expand .The Simon vena cava filter (Nitinol Med icalTechnologies) was the first shape-memory vascularimplant developed for thermal deployment. The devicehas a transition temperature around or below roomtemperature, it is preloaded in a catheter in its lowtemperature state . Flushing the catheter with ch illedsaline solution keeps the device in this state whi leposit ion ing it at the deployment site. When releasedfrom the catheter, the device is warmed by body heatand recovers its 'pre-programmed' shape [7J .An interesting twist of the thermal deploymentfeature is 'thermal retrieval ' of temporary devices,such as prostat ic stents. Coil stents made byEngineers & Doctors , or by EndoCare, for example,can be retrieved from the prostate by chi lling thedevice with cool solution causing the Nitinol to lose3rc.83

D. Stoeckelits stiffness. The stent becomes soft and pliable andcan be retrieved with a grasping forceps (Figure 9) [8].A multitude of vascular implants have beenapproved by the FDA in recent years, or are bein9investigated or marketed in Europe. Among these arestents and filters by Bard!Angiomed (Memothermstent, Simon vena cava filter), Boston Scientific(Radius, Symphony stents), Medtronie (AneuRx andTalent AAA devices) and Cordis (SMART stent,TrapEase vena cava filter, AngioGuard distalprotection device)Kink resistanceTo some extent this design property stems from theincreased elasticity of superelastic Nitinol, but it isalso a result of the shape of the stress-strain curve.When strains are locally increased beyond theplateau level, stresses increase marked ly. Thiscauses strain to partition to the areas of lower strain,instead of increasing the peak strain itself. Thuskinking, or strain localisation, is prevented by creatinga more uniform strain than could be realised with aconventional material. The first applications to takeadvantage of this feature were guide-wires, whichmust be passed through tortuous paths withoutkinking (9]. Steerability and torquability (the ability totranslate a twist at one end of the guide-wire into aturn of nearly identical degree at the other end) of aguide-wire are directly affected by the straightness ofthe wire. Even very small permanent deformationswill cause the wire to whip and destroy the ability tosteer it through side branches or around sharpbends in the vasculature. Kink-resistant Nitinol wiresplay an important role in interventionai card iology andrad iology. Another early appl ication was in retrievalbaskets with Nitinol kink-resistant shafts, as well as asuperelastic baskets used to retrieve stones fromFigure 8. Deployment ofthe TrapEase vena cava filter (Cordis).Figure 9. Deployment and retrieval of the horizon stool (Endocare) [71.84

Nitinol medical devices and implantsFigure 10. Superelastic Nitinol baskets (Epflex GmbH).Figure 11. Inlraartic balloon pump with superelastic centrallumen.Figure 12. Biopsy forceps (BaCher).kidneys, bladders, bile ducts, etc. (Figure 10).Since superelastic tubing became available in theearly to mid 1990s, a variety of catheter products andother endovascular devices which use Nitinol as the 'inner lumen have appeared on the market. Aninteresting example is the intra-aortic balloon pump(IABP, Figure 11), used in cardiac assist procedures.The use of Nitinol allowed the reduction of the size ofthe device, compared with polymer-tu be baseddesigns, and increased the flexibility and kin kFigure 13. Crush -recoverable Nitinal stent.resistance, compared with S8 tube designs.Kinking of thin-wall steel tubing limits the use ofmany interventional devices. Biopsy forceps madefrom 88, for example, are very delicate instruments stressed in bending should be at least 10% of thethat can be destroyed by even very slight outer diameter, to withstand buckling [10).mishandl ing . Nitinol instruments, on the other hand,Kink resistance or, more appropriately, crushcan handle serious bending without buckling, kinkingrecoverability, is an important feature of Nitinol foror permanent deformation. Figure 12 shows 1.5 mm stents in superficial vessels that could be deform edbiopsy forceps that consist of a thin wall Nitinol tubingthrough outside fo rces. The carotid artery is a primewith a Niti nol actuator wire inside. Together t hey are example. There is a perceived risk for balloonable to be bent around radii of 3 em without kinkingexpandable stents in carotid arteries to beand still allow opening and closing of the distalpermanently deformed through outside pressure,grasper jaws without increased resistance. This resulting in a partially or completely blocked vesselinstrument continues to operate smoothly even while once the buckling strength of the stent is exceeded.bent around tortuous paths. It should be pOinted outAlthough Nitinol stents typically don't have thehowever, that the wall thickness of a Nitinol tube buckling strength of 85 stents, they cannot be85

D. Stoeckelpermanently deformed through outside forces . Nitinolstents can be completely compressed (crushed) flatand wil l return to their original diameter when thedeforming force is removed (Figure 13).The resistance of Nitinol to kinking anddeformation is used in the Vidamed TUNA (transurethral need le ablation) catheter to deploy a straightneedle through a curved gu iding channel with a smallradius of curvature. This allows the advancement ofthe ablation need le out of t he catheter perpendicularto the catheter axis.Constant stressAn im portant feature of superelast ic Nitinol al loys ist hat their loading and un load ing curves aresubstantially flat over large strains. This allows thedesign of devices that apply a constant stress over awide range of shapes. The orthodontic archwire wast he first product to make use of this property - morespecifical ly the constant unloading stresses. 88 andot her convent ional wires are tightened by theorthodontist regu larly. As treatment continues, theteeth move and the force applied by 85 w ires quicklyrelaxes, accord ing to Hook's law. This causestreatment to slow, retard ing tooth movement. Nitinolwires, on the other hand, are able to 'move w ith theteeth', applying a constant force over a very broadtreatment time and tooth position.Constant stress upon load ing is used as 'overloadprotection' in hingeless graspers (or forceps) madefrom Nit inol. Hingeless instruments use t he elasticityof spring materials, instead of pivoting joints, to openand close the jaws of grasping forceps or the bladesof scissors. Their simple design, without moving partsand hidden crevices, makes them easier to clean andsterilise. A new generation of hingeless instrumentsuse superelastic Nitinol for the actuating component,resulting in an increased opening span and/orred uced displacement of the constraining tube forergonom ic handli ng [11 ]. In many cases the functionaltip can be a monolithic superelastic component,rather than the multiple intricate, precision-mach . Th is allows t he design of instrumentswith very smal l profiles. The substantial ly constantload ing stress of Nitinol provides constant forcegripping of large and small objects and built-inoverload protection . This reduces t M risk of tissuedamage (Figure 14).Dynamic interferenceDynam ic interference refers to the long-range natureof Nitinol stresses and can be clearly illust rated usingself-expanding stents as an example. Un like balloonexpandable 55 stents , self-expand ing Nitinol stentswill always expand to their pre-set diameters withoutrecoil. Balloon -expandable stents, on the other hand,have to be over-expanded to achieve a certaindiameter as a resu lt of elast ic spring -back afterdeflation. This spring -back, or loosening, is calledacute recoil and is a high ly undesi rable feature. Theover-expansion may damage the vesse l and causerestenosis. Moreover, if the vessel diameter relaxeswith time, or undergoes a temporary spasm, a 58stent w ill not fol low the vessel wall . The interferencestresses w ill be reduced and the stent couldembolise.The Nitinol stent will continue to gently pushoutward against the vessel wall after deployment andfo ll ows vessel movements. Typically, the pre-setdiameter of a Nitinol stent is "'1 - 2 mm greater thanthe target vessel diameter. It will therefore try to reachthis diameter. Should the vessel increase in diameter,the Nit inol stent wil l also expand unti l it reaches itsfinal diameter. A more complete description of th isfeature can be found elsewhere in this publication .Biased stiffness (stress hysteresis)The most unusual feat ure of Nitinol alloys is stresshysteresis (see Figure 1). Wh ile in most engineeringmaterials stress increases with strain upon loading ina linear way and decreases along the same path uponunloading, Nitinol exhib its a disti nctly differentbehaviour. Upon loading, stress first increases linearlywith strain, up to ",1 % strain. After a first 'yield point',several percentage points of strain can beaccumulated with on ly a little stress increase. The endDeflectionFigure 14. Constant gripping force of a hingeless Nitinolgrasper (schematic).86

Nitinol medical devices and implantsRRF9 . ?:.:ir. .:. -. .- :'.COF I Niti nol50.8 at % NiA14.5 MPalK'I 0 (7 ), r-.MITATTemperature differenceLumen interlerence'---/Figure 16. Influence of temperature on plateau stress(schematiC).Figure 15. 'Biased stiffness ' of a stent as a result of thestress hysteresis.simplicity isothermal cond it ions are assumed). Uponrelease from the delivery system inside the vessel, itexpands fo llowing the un load ing path of thestress- strain curve. At point B it reaches the diameterof the vessel lumen, appositioning itself against thevessel wall with a low outward force (chron ic outwardforce, COF) . As can be seen from the figure, this forceremains nearly constant, even if the vessel wou ldincrease in diameter (dynamic interference). If thevessel contracts, through spasms for example, or iscompressed from the outside, the stent resistsdeformation with a higher force (RRF, radial resistiveforce). The stress hysteresis of Nit inol allows thedesign of self-expand ing stents with biased stiffness,mean ing that the stents exert on ly low outward force,but resist deformation with a much higher force.of this plateau ('loading plateau') is reached at about8% strain . After that, there is another linear increase ofstress with strain . Un loading from the end of theplateau region causes the stress to decrease rapidly,until a lower plateau ('un loading plateau') is reached.St rain is recovered in this region with only a smalldecrease in stress. The last portion of the deform ingstrain is finally recovered in a li near fash ion again . Theunloading stress can be as low as 25% of the loadingstress.The pronounced stress hysteresis can be ut ilisedadvantageously for a variety of medical devices. Franket at. [121 describe a detachab le bowel clamp wh ichuses a Nitinol spring . When the clamp is applied, it w illbe fi rst opened to a si2.e larger than the boweldiameter, following the loading curve. When releasedto occlude the bowel, the force decreases, fol lowingthe unloading curve. Whenever bowel contents arepropelled by peristalsis and increase the distractionforce on the clamp, the force, again, follows theloading part of the stress- strain curve. Thus, the forcerequ ired to open the clamp can be considerably largerthan the force appl ied by the clamp under staticcond itions. Simi larly, the holding force of hingelessforceps decreases when tissue is displaced, thusallowing atraumatic grasping.In se lf-expanding Nitinol stents, the concept of'biased stiffness' is most obvious. As illustrated inFigure 15, a stent is compressed into the deliverysystem fo llowing the loading curve to point A (forTemperature-dependent stiffnessThe plateau stresses are strongly temperaturedependent above the transition temperature of thealloy. As a result, superelastic devices become stifferwhen temperature increases . The stiffness of asuperelastic device of a given design at a specifictemperature, body temperature for example , can bemod ified to some extent by adjusting the t ransitiontemperature of the Nitinol alloy used through a heattreatment [1 ]. Lowering the transition temperaturemakes the device stiffer at body temperature . Plottingthe loading plateau stress (at a defined strain) versusIH (body temperature minus transition temperature)87

D. StoeckelPelton AR, Hodg son D, Russell SM, Duerig T, editors.Proceedings 2nd International Conference on ShapeMemory and Supere/astie Technologies (SMST) ;PacificGrove: MIAS, 1997: 509- 14.6 AGA Medical Corporation website: www.agamedical.com7 Nitinol Medical Technolog ies, Inc. website: www.nitinolmed.com8 Endocare Inc. website: www.ecare.org9 Zadno R, Simpson JW, The effect of material selectionon torquability of guidewires. In: Pelton AR, Hodgson D,Russell SM, Duerig T, editors. Proceedings 2ndInternational Conference on Shape Memory andSuperelastie Technologies (SMST):Pacific Grove: MIAS,1997: 437--42.10 Pelton AR, Rebelo N, Duerig TW, Wick A. Experimentaland FEM analysis of the bending behavior ofsuperelastic tubing. In: Pelton AR, Hodgson D, DuerignN, editors. The 1st International Conference on ShapeMemory and Supere/astie Technologies. Pacific Grove:MIAS, 1994: 353-8.11 Stoeckel D, Melzer A. The use of Ni-Ti alloys for surgicalinstruments. In: Vincenzini P, editor. Materials in clinicalapplications, Techn