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Nitinol Properties Affecting Uses in Interventional Radiology

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BACKGROUND

Nitinol is an alloy comprised of approximately 50% nickel and 50% titanium. Nitinol has properties of superelasticity and shape memory. Superelasticity refers to the enhanced ability of a material to be deformed without irreversible change in shape. Shape memory is the ability of a material to regain its shape after plastic deformation at a lower temperature. These physical properties of nitinol allow complex device configurations and high expansion ratios enabling percutaneous delivery through

• Structural Basics

Superelasticity and shape memory are based on the ability of nitinol to exist in several crystal forms or phases. Phase transitions between liquid and solid phases are commonly known phenomena, such as water freezing to ice or solid metal melting into liquid. What makes nitinol unique is its ability to exist in two distinctly different, reversible crystal phases in its solid state at clinically useful temperatures. The alignment of crystals at the higher temperature is called austenite (A)

BIOCOMPATIBILITY

Nitinol is a highly biocompatible and nontoxic material (8, 9), although nitinol alloys contain approximately 50% nickel, which is considered toxic. However, assessment of nickel metabolism in humans indicates that the body burden on nickel in normal adults averages 0.5 mg (7 micrograms per kg for a 70-kg adult person). The maximum recommended permissible amount of nickel administered to a human as a contaminant in intravenous fluids should not exceed 0.5 micrograms per kg per day (35

GENERAL CONSIDERATIONS

Nitinol devices can generate tremendous force (up to 80 kg/mm2)if they meet resistance during their recovery. Varying the thickness of the nitinol wire, the overall shape of the device, and the spatial arrangement of the nitinol wire within this shape can regulate the force. These design features, as well as the characteristics of the nitinol alloy used, should be taken into account when designing, selecting, and deploying interventional devices for specific clinical applications.

Self-expanding

CONCLUSION

The features of superelasticity and shape memory make nitinol an excellent material for interventional devices. These physical characteristics permit manufacture of complex shapes of devices and introduction through low-profile delivery systems. Familiarity with the characteristics of nitinol aids in selecting the most suitable device for a specific clinical indication.

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