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Nitinol ‐ a Material with Unusual Properties Stoeckel Endovascular We are Nitinol.™ Nitinol ‐ A Material with unusual Properties Stoeckel Endovascular Update Issue 1 1998 www.nitinol.com 47533 Westinghouse Drive Fremont, California 94539 t 510.683.2000 f 510.683.2001 25 March 2009 Letterhead (scale 80%) Option #1 NDC Business System R2 Nitinol - A material with unusual properties Dieter StOckel Cordis - Nicinol Devices & Components , Inc., Fremont, CA, USA Nitinol (nickel~titanium) alloys exhibit a combination of properties which make them particularly suitable for the manufacture of self-expanding stents. Some of these properties are not possessed by other materials currently used to manufacture stents. This paper describes the fundamental nitinol properties of shape memory and superelasticity. Material properties and device characteristics such as elastic deployment. thermal deployment, kink resistance, constancy of stress, dynamic interference, biased stiffness, magnetic resonance imaging (MRl) compatibility, radiopacity and biocompatibility are discussed. Introduction an d other interventional devices, exhibit a distinctly Nitinol alloys are rapidly becoming the materials of different elastic deformation behaviour from that of the choice for use in self-expanding stents, graft support structural materials of the human body. The elastic systems, filters, baskets and va ri ous other devices for deformation of these metals and alloys is li mited to ", 1% interventional procedures. Companies such as Bard­ strain and elonga tion typically increases and decreases Angiomed (Memotherm), Boston Scientific (Symphony li nearly (proportionally) with the applied force. In a.o.), Medtronic-AI\euRx , Nitinol Medical Technologies, contrast, natural materials such as hair, tendon and bone Worl d Medical Technologies and Cordis offer nitinol can be elasticall y deformed, in some cases, up to 10% produc ts, the performance of which is based on the highly strain in a non-linear way (2). When the deforming mess unusual properties of nitinol alloys. is released, the strain is recovered at lower stresses. As The bc.st-known properties of nitinol all oys are their shown in Fig. I, the loading/unloading cycle is superelasticity and [hennal shape memory. While the term characteri zed by a pronounced hysteresis. 'shape memory' describes the phenomenon of restoring a A similar behaviour is found with nitinol alloys, wh ich predetennined shape by means of hearing, having are equiaromic or near-equiatomic intennetanic compounds 'plasticall y' deformed that shape, the tenn superelasticity of ci[3nium and nickel. Figu re 2 shows a characteristic refers to the enormous elasticity of these alloys, which can load/deflection (stress/strain) curve for a nitinol alloy wire be IO times greater than the best stainless steels used in at body temperaru re (T in Fig. 2; as will be shown later, the medicine today. Although both effects are clearly properties of nitinol alloys are strongly temperature­ spectacular, they are not the onl y important properties of dependent). As wi th natura l marerials, the loading and the ma teri al. In this paper, features such as biomechanical unloading curves show plateaus, along which large compatibility, constancy of stress, dynamic interference and deflections (strains) can be accumulated on loading, or 'biased stiffness' will be described. In combination with recovered on unloading, wi thout a significant increase or strength, fatigue resistance, biocompatibility and MRI decrease respectively in load (stress ). Becau se a deformation compatibility, these nitinol-specific properties allow of more than 10% strain can be elastically recovered, this interesting solu tions for the design of superior medical behaviour is called 'superelasti city' or sometimes more devices (I J. scientifically 'pse udoelasticity'. It is the basis for most applications of nitinol in medical devices. Superelasticity and shape memory of nitinol If the temperature is raised by, for example, to·C, the Conventional metalliC materials such as stainless steel, complete hysteresis loop, i. e. the loading and unloading titanium and Eigi lloy a.o., which are used in stents, filters curves, shifts to a higher level (denoted T +.1.T in Fig. 2). ENOOVASCUl AR U PDATE Deflection Deflection Figure 1 . Deformation characteristics of natural Figure 2. Influence of temperature on the materials and nitinol {2]. deformation characteristics of nitinot However, the qualitative appearance of the curves is properties and used advantageously in the manufacture of maintained. Lowering the temperature by W'C, however, self-expanding stents and other medical devices. will shift the hysteresis loop to a lower level (T- LlT). Lowering the temperature even further will cause the load El astic depl oyment to reach zero before the deflection is recovered, i.e. the The enormous elasticity of nitino! allows such alloy sample will stay deformed at this temperature (T- xilT), If devices to be introduced into the bod)' through catheters the temperature is increased to 0?:2S'C aftcr unloading, the or other delivery systems with a small profite. Once inside deformation will be recovered therma\ly. This effect is the body, the devices can be released from their constraints called thermal shape memory, or simply shape memory. and unfolded or expanded to a much larger size. Figure 3A The temperature at which the material can no longer shows the elastic deployment of a stent of 20 mm diameter recover the elastic strain depends upon the alloy from a 3 mm i.d. cartridge. In order to fully expand at body composition and processing and can be adjusted to temperature (37"C) the transition temperature of the alloy between = - 20·C and =+ IOI,YC. This transition shou ld be :530OC. If full deployment is required at room temperature is an important characteristic of nitinol temperature (w·e) the transition temperature of the alloy components used in medical applications. Nitinol alloys should be :515°C. Typical expansion ratios for self­ are superelastic over a temperature range of .. SO·C above expanding nitinol stents range between 1:2- 1:5. the transition temperature. As with stents, filters and occlusion devices (atrial septal At higher temperatures, nitinol alloys gradua ll y lose their defect occlusion, Botalli duct occlusion) can be deployed ability to recover the deforming strain until, at a cerrain superelastically through small-sized catheters. Nitinol is maximum temperature (typically> lOOOC), they behave like also used in retricval baskets and snares. a 'nonnal' material. An alloy with a transition temperature of 25"C will recover all but ",(l.5% of the defonning strain Thermal deployment after being defamed by 8% in the temperature range 25- A stent with a transition temperature of 30"C can be 7Ye. The same alloy can be defamed 'plastically' up to 8% compressed at :520·C. It will stay compressed until the (under ideal circumstances) below 25"C and its shape temperature is increased to >30"C. It wi ll then expand to its restored by heating to above 25 ·C. (Note: this description is pre-set shape. If this stent could be kept cold during simplistic. The transition temperature is not a distinct introduction into the body it would not expand. Whcn temperature but a temperature range.) positioned at the desired location it would warm up by The mechanism responsible (or both superelasticity and means of body heat and expand. However, this is difficult to shape memory is a solid-state phase transformation, known accomplish. All self-expanding stents are therefore as the 'thermoelastic martensitic transformation'. Detailed constrained in the delivery system to prevent premature explanations can be found in Ref. 3. In the fo llowing deployment. Stents could theoretically be built with a sections some important device characteristics wil l be transition temperature of 40·C. These stents would have to discussed, all of which can be attributed to specific nitinol be heated after delivery to the site to make them expand. 2 EN DOVASCULAR UPDATE A B T=30'C T<lS'C T=30'C ~ T=30 ' C T=30'C Figure 3. (A) Elastic deployment of a 'slotted-tube' type nitinol stent. (B) Cold deployment and thermal recovery of the stent (demonstration device). Figu re JB shows the stent in Fig. JA being released (rom a stents have to be over-expanded to achieve a certain cooled delivery cartridge. TI,e stem stays compressed until its diameter (d ue to elastic spring-back after deflation). The· temperature exceeds the transition tempera ture of 30"C. niti nol stent will continue to gentl y push outwards against The Simon Vena Cava Filcer (Nitinol Medical the vessel wall after deployment. Typicall y, the pre-set Te<:hnologies) was the first shape memory vascular implanc diameter of a nitinol stent is ,., 1-2 mm greater than the to use the property of [hennal deployment. The device is target vessel diameter. It witl therefore try to reach this preloaded into a catheter in its low-temperature state. diameter. Should the vessel increase in diameter the nirinol Flushing chilled saline solution through the catheter kccps stent will also expand until it reaches its final diameter. the device in this state while positioning to the deployment si te. Upon release from the catheter the device is warmed by Biased stiffness (force hysteresis) body heat and recovers its 'pre-programmed' shape. The most unusual feature of nitinol alloys is force or load hysteresis. While in most engineering materials load (or Constant force (stress) stress, if nonnalizLod) increases li nearly with deflection As shown in Fig. 2, an important feature of superelasric (strain) upon loading and decreases along the same path nitinol alloys is that their unloading curves are flat over a upon unloading, nitinol exhibits distinctly differem wide defl ection (strain) range. This all ows the design of behaviour. After an initial linear increase in load with devices that apply a constant force or load (stress) over a deflection, large deflections can be obtained with only a wide range of shapes.
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