Effective Use of Nitinol for Medical Devices

Transcription

1 Effective Use of Nitinol for Medical Devices N o v e m b e r 2012

2 TABLE OF CONTENTS Abstract... 3 Abbreviations... 4 Market Trends/Challenges... 5 Solution... 5 Best Practices... 7 Common Issues... 8 Conclusion References Author Info... 11

3 Abstract Nitinol is a shape memory alloy widely used in medical devices in the form of guide wires and stents. In the early 1960s, a nickel titanium alloy was developed by W. F. Buehler, a metallurgist investigating nonmagnetic, salt resisting, waterproof alloys for the space program at the Naval Ordnance Laboratory in Silver Springs, Maryland USA. The thermodynamic properties of this intermetallic alloy were found to be capable of producing a shape memory effect when specific controlled heat treatment was undertaken. The alloy was named Nitinol, an acronym for the elements from which the material was composed, NI for nickel, TI for titanium and NOL from the Naval Ordnance Laboratory. Nitinol is the name given to a family of intermetallic alloys of nickel and titanium which have been found to have unique properties of shape memory and super-elasticity. Nitinol offers a unique range of characteristics when designing medical device delivery shafts, including superelastic and shape memory behavior, good crush resistance, and flexibility. Yet, many designers are reluctant to specify Nitinol in design due to its high cost. Instead, more economical material such as stainless steel, which exhibits good overall trackability, is more often specified. Very effective cost savings can be achieved if it is possible to replace the whole or a portion of nitinol wire with some other material such as stainless steel. This paper presents an overview of: o Superiority of nitinol in medical devices o Constraints in replacing nitinol in medical devices o Various joining methods of nitinol and stainless steel The first section describes a few properties of nitinol which are utilized in medical devices. The primary one is its capability to exhibit different shapes, known as shape memory behavior, at different levels of stress and temperature, which is very important for guides and stents. Due to this property, nitinol is categorized as a shape memory alloy. Limitations in replacing nitinol totally are also discussed. The second section describes different methods to join nitinol and other materials, and the challenges involved in the joining process. The metallurgical and mechanical basics of SMA are well developed and understood. So far, however, their applicability has been limited due the lack of available fabrication techniques. Included are joining processes for assembling SMA to themselves, and not at least, to other materials. The development of such cost efficient and quality processes will probably be the key to further increase the engineering applications of SMA. There should be sufficient joint strength without compromise in essential properties. 3

4 Abbreviations Sl. No. Acronyms Full form 1 Nitinol Nickel Titanium Naval Ordinance Laboratory 2 MRI Magnetic Resonance Imaging 3 TiO2 Titanium Dioxide 4 HAZ Heat Affected Zone 5 FEA Finite Element Analysis 6 SMA Shape Memory Alloy 4

5 Market Trends/Challenges Nitinol exhibits shape memory and super-elasticity with good shape retention, biocompatibility, biomechanical compatibility, MRI compatibility, flexibility, kink resistance and fatigue resistance. Shape memory and super elasticity can be triggered thermally or mechanically. Nitinol wire is very costly and its machinability is very poor. By using any alternative material in a device, whether partially or whole, considerable cost savings can be attained. The challenge here is that super-elasticity cannot be compromised in the wire at the distal tip. By using nitinol in the distal section and using an economical metal like stainless steel for proximal section, designers can optimize both performance and cost of the overall design. The challenge here is to join Nitinol and stainless steel without compromising the beneficial properties. Solution Super-elasticity is caused by stress-induced martensite transformation upon loading and by subsequent reverse transformation, i.e. austenite, upon unloading. A diagrammatic representation is shown in Fig. 1. The super-elasticity property of nitinol is utilized to attain certain shapes at the distal end; upon removal of stress, it behaves like a super spring. Fig. 1: Diagrammatic representation of super-elasticity (International Endodontic Journal, 33, , 2000) Due to self passivation, a very stable protective TiO 2 layer reduces the release of nickel a known allergen and possible carcinogen and protects the base material from corrosion. With proper treating (electro polishing), excess nickel from the surface is removed and forms a TiO 2 layer enriched in titanium. In order to optimize the cost, it would be beneficial to join Nitinol with stainless steel. 5

6 Achieving a sound joint is very challenging since it is not limited to the joint strength, but also comprises the shape memory effect and super elasticity. Heat involved in the joining processes causes a reduction of initial properties. While joining nitinol to other metals, brittle intermetallic compounds will be formed at the joints, which make the joints brittle, i.e. less strong. Various joining methods of nitinol and stainless steel 1) Soldering: An aggressive flux like aluminum paste flux should be used to remove the tough oxide layer on nitinol in combination with Sn- 3.5Ag solder alloy (lead-free) for better results. Use a preheated soldering iron set to 315 O C with the solder in place. 2) Welding Laser beam welding is promising in welding nitinol to stainless steel. Diagrammatic representation is shown in Fig. 2. Strong joints have been made using nickel filler between the nitinol and stainless steel. The Nickel filler prevents formation of intermetallic compounds and reduces cracking possibilities to produce strong and more ductile welds. Fig. 2: Diagrammatic representation of welding 3) Adhesive bonding Nitinol can be bonded to other materials using medical grade epoxies and adhesives. 4) Mechanical joining Techniques such as crimping and swaging can also be used. Another technique is to use nitinol s shape memory or super elasticity property for bonding. A nitinol tube connector can be used as a shrink fit to connect the two mating parts. 6

7 Best Practices When designers choose nitinol wire in medical device, it is typically in the distal section, that the unique properties of the metal are employed. But due to lack of proper joining methods, nitinol will be selected for the entire length. It is important for clinicians to be aware of the metallurgy of the NiTi alloy in order that the characteristics of instruments constructed from this alloy can be appreciated and to encourage research to maximize their clinical potential. Welding of SMA to other metals may be challenging due to the extensive formation of intermetallic compounds. These tend to be very brittle. In such cases, it may be required to use additives that prevent such formation. It is noted that some processes are very promising, but also that there is more work needed to be able to maintain the initial base material properties. In fact, it may be wise to include the joining in the design for certain applications, where shape memory effect, or superelasticity, is of primary concern. Such conceptual design will be much more cost effective since it may be possible to integrate alloy manufacturing and joining operations. Even though there are comparative advantages for welding with nickel filler material, joining between nitinol and stainless steel is not practically used in medical devices due to the risks involved. As explained in this paper, a number of joining processes have been applied to SMA. However, to some extent, there is still a lack of systematic studies on the effects of joining parameters. There is still a way to go concerning the process and performance optimization. 7

8 Common Issues Joining strength is the main concern. If insufficient, the nitinol distal end gets detached from the stainless steel and is a deadly situation. The practice of joining nitinol and stainless steel in medical devices is not recommended due to this very high risk. 1) Soldering a) Solder alloy and flux should be biocompatible b) Post-soldering cleaning of flux residue using detergent, water or mechanical scrubbing should be performed 2) Welding a) Formation of brittle intermettalic compound may reduce properties of nitinol b) HAZ. Refer to Figs. 3 and 4 Fig. 3: Laser welding (narrow) Fig. 4: Arc welding (wide) (Dong et al, 2006). (Dong et al, 2006). c) Due to excessive thermal stress and strain, cracking may occur at the joint. Refer to Fig. 5. Fig. 5: Crack at the joint (van der Eijk,2004b) 8

9 3) Adhesive bonding a) Joining strength is less b) Adhesive may degrade based on the service environment 4) Mechanical joining a) Joining strength is comparatively less b) May cause deformation and crack c) Requires manufacturing to close tolerance 9

10 Conclusion Different joining methods are discussed in this paper. The joining methods included are soldering, welding, adhesive bonding, and mechanical joining. All joining techniques that involve heat to the base metals tend generally to cause reduction of their initial properties, but the most severe reduction in shape and superelasticity recovery is achieved in welding. In joining SMA to other metals, formation of brittle intermetallic compounds is difficult to avoid. Each method has its drawbacks in terms of the joint strength and process. Among these, laser welding with Nickel filler is the most promising. It results in narrower HAZ and a strong and ductile weld joint. It should be noticed that, although degradation of properties occurs in most joining processes using heat, this deterioration may take place locally. The joining process involves an in-depth study of the chemical composition and microstructure of nitinol, stainless steel and filler metal. In conventional alloys, successful joints are obtained in spite of differences in the chemical compositions and the microstructure of weld metal and heat affected zone (HAZ) on one hand, and the base metal on the other. In welding SMA, this may not be the case, as the matching of chemical composition and microstructure, as well as transformation temperature, is very important because this controls the mechanical properties and the behavior of the joint. The significance of local strength or ductility changes in a component is not very clear, and there is a need for finite element modeling to simulate the impact on the structural integrity of the component. Moreover, with brittle materials or local brittle constituents, formation of micro cracks has been observed in joining, especially when joining NiTi to other materials. The significance of such micro cracks should be assessed by fracture mechanics. With proper validation of the weld strength by the FEA and physical testing, laser welding of nitinol and stainless steel wire can definitely be introduced into medical devices without the risk of failure, and considerable cost savings can be attained 10

11 References Thompson SA, An overview of nickel titanium alloys used in dentistry. International Endodontic Journal,33, , Odd M. Akselsen SINTEF Materials and Chemistry, Norway Author Info Mr. Bibin Kuruvilla K has four years of experience in the development of relays and APQP guidelines and one year of experience in medical device design improvement. 11

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