The Furukawa NT Alloys Supporting Advanced Medical Devices
Nickel-titanium (Ni-Ti) alloys (hereinafter referred to as NT alloys ) are known as alloys that possess both shape memory and super-elastic properties. NT alloys are currently being used in the development of various types of products. In particular, in the medical field NT alloys have recently been used as a material for medical devices such as stents and guide wires in the catheter treatment of vascular diseases such as myocardial infarction and cerebral infarction. They significantly contribute to minimally invasive treatments that reduce the burden of medical procedures on the patient's body by utilizing properties characterized by flexibility (super-elasticity) and the ability to return to their original shape even after being excessively deformed.
Shape Memory Alloys and Super-Elastic Alloys
Metals differ in their ability to return to their original shape after removing an external force (stress) that had been applied to it. The property of being able to return to an original shape after being deformed is referred to “elasticity”. Likewise, the range of deformation is referred to as the “region of elasticity” and the maximum value of the region of elasticity as the “limit of elasticity”. If the deformation exceeds the limit of elasticity, the metal will not be able to return to its original shape and will thus be permanently deformed. NT alloys deformed at a low temperature return to their original shape after being heated to a specified temperature or higher. This type of alloy is referred to as a shape memory alloy. By combining a general-purpose metal spring with a shape memory alloy spring, it is possible to achieve mechanical switching at a specific temperature. Shape memory alloys are especially good at certain tasks such as being used to release heat and steam by closing plugs at low temperatures and opening them at high temperatures.
Furthermore, NT alloys exhibit super-elastic properties at room temperature. Super-elastic alloys can be restored to their original shape even after applying a large deformation to such an extent that would permanently deform general metals such as stainless steel and aluminum. The shape memory and super-elastic properties can be changed by adjusting the composition ratio of Ni and Ti.
Shape Memory Alloy and Super-Elastic Alloy StressStrain Curve
Conventional Metal Materials
Does not return to its original shape after applying a force beyond its region of elasticity.
Shape Memory Alloy
Returns to its original shape via heating, despite being deformed by a force.
Returns to its original shape similar to rubber after removing the deforming force.
Deformation Behavioral Differences between Super-Elastic Alloys and Stainless Steel
Super-elastic alloy and stainless steel deformed similarly
Unlike the stainless steel, NT super-elastic alloys return to their original shape after removing the force.
Application of Super-Elastic Alloys to Medical Devices
Stents that Self-Expand in Blood Vessels
Blood vessels are pervasive in the body and serve as a conduit for circulating blood throughout the entire body. However, when the passage of blood becomes constricted or blocked, oxygen and nutrients are prevented from reaching certain tissues, and this can lead to disease. Sometimes blood vessels rupture, and this can result in very serious symptoms. In such a case, catheter treatment may be used to remove the blockage, widen the blood vessels, and maintain blood flow.
Devices such as guide wires and balloons are used to widen blood vessels, and structures called stents, which are shaped like mesh tubes, are inserted into blood vessels to maintain the lumen in them. Stents made from super-elastic NT alloys are deployed to the affected area through a catheter using a thin tube. When released from the catheter at the affected area, stents will self-expand to a given diameter to support the blood vessels from the inside. The use of a super-elastic alloy can greatly facilitate the maintenance of lumen and significantly reduce stress on the blood vessels since it flexibly adapts to the movements of the body.
Furukawa Techno Material (a member of the Furukawa Electric Group) has been supplying NT alloy tubes for use in the manufacture of mesh-type stents. One of the challenges of NT alloys is that mere deviation of 0.1% in the composition ratio will change the transformation temperature by about 10°C, thereby changing the mechanical properties of the material. NT alloys are high-tech products that require a high level of cleanliness, circularity, biostability, and durability since they are placed in the body after being processed in a stent.
Guide Wires for Delivering the Stent or Catheter to the Affected Area
A guide wire is used to carry a stent or catheter to its destination in the blood vessel. The guide wire makes use of a flexible super-elastic NT alloy coated with a smooth resin that helps it move smoothly in the blood vessel.
In order to pass deep inside the complex and winding blood vessels of the body, the guide wire must bend flexibly along the path of the blood vessel, while still maintaining just enough stiffness to allow it to move through the blood vessel without rupturing it when a force is applied at the insertion point of the blood vessel. Furthermore, the tip of the guide wire needs to rotate in sync with the rotation of the wire at the insertion point of the blood vessel. There are many different types of guide wires depending on the treatment method and affected area. Therefore, in order to meet these requirements, Furukawa Techno Material provides NT alloy wires that possess various characteristics.
In this regard, Hiroshi Horikawa, Director of the Special Metals Business Division at Furukawa Techno Material Co., Ltd., made the following statement:
“Our Company's biggest advantage is that we have an integrated manufacturing system that includes in-house melting and casting of Ni-Ti ingots, hot processing and cold wire drawing, molding, memory processing, and secondary types of processing such as parts processing. As a result, we are able to provide meticulous quality assurance and speedy development and manufacturing of products for our customers.
‘F-ELI (Furukawa Extra Low Inclusion Wire)’ is a product that we are currently commercializing that not only achieves high durability through the suppression of inclusions such as oxides inside the alloy, but is also characterized by its perfect traceability via our integrated manufacturing system. A large number of catheter-type minimally invasive treatments that use stents and guide wires are actively being developed since they reduce the burden of patients and suppress medical costs. Furukawa Techno Material is committed to developing high performance products in response to customer needs and in expectation that our products will contribute to the treatment of an increasing number of patients. “