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Effective Use of Nitinol for Medical Devices

Effective Use of Nitinol for Medical Devices

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:
 
  • Superiority of nitinol in medical devices
  • Constraints in replacing nitinol in medical devices
  • 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.

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