An Introduction to the Unique Material Properties of Nitinol

This article will appear in Journal of ASTM International.  I hope you find it useful.

Xiao-Yan Gong, President, medical implant mechanics LLC

www.medicalimplantmech.com

Follow this link to the details of the coming ASME webnar.

http://catalog.asme.org/Education/Webinar/Nitinol_Webinar.cfm

Xiao-Yan Gong, PhD

When a mechanical engineer and a material scientist were asked for the root cause of an in-vivo fracture. Mechanical engineer pointed to the loading and the material scientist pointed to the processing. While they both are correct, they both also missed the real ROOT cause, the design.

It is very common that medical device design engineers are so focused on the device functionality that often the very basic mechanics is overlooked. Lack of knowledge on the in-vivo environment (Design Requirements) is another subject to blame. However, it is common that even technology driven companies have gaps between design department and duarability deparment. Up front design engineers do not necessarily keep up with the fast paces of material advances. On the other hand, downstram subject matter experts, device tesing teams or often the R&D departments are not informed of design changes before the design is fixed. The problem is worse often in industrial leaders than in start-ups, but the sympton is the same, problem found in animal studies and/or clinical trials before they reached industrial subject matter experts.

The major challenge in medical implant industry is the knowledge about human body. Had we know the human body and its functions better, we can make better and reliable implants. Below are two examples that I have learned.

Let's start from stent, a small, lattice-shaped, metal tube that is inserted permanently into an artery. The stent opens the narrowed artery so that an adequate supply of blood can be restored. See this FDA site for further detail.

Stent has revolutionized the treatments for cardiovascular disease and the interventional system. However, stent fractures are commonly observed in-vivo in the past years and has become a concern for patient wellness and therefore a challenge/opportunity for mechanical engineering. Both the engineering and the medical care societies have to work together to solve this issue. It is very surprising that little publications are available to study the key issues such as artery deformation, motion, its mechanical properties and its variations among patient age, race, and other factors. As a result, current stents, even they have been proven to be lifesavers for many patients, they are not necessarily a satisfactory product for a mechanical engineer. We can not wait for the medical care society to give us the information because they often concern and focus on different issues than us. In addition, they can not work alone to come up with the necessary equipments. Therefore, we need proactive to interact and help each other to get what we want. The day we know our interventional system better is the day that we can make better stents because stents can only be as good as our knowledge to the interventional system.