Why is Nitinol Used in Stents?

Nearly every stent on the market is made from nitinol.  I’ve talked to numerous product designers who would love to save money by using stainless steel rather than nitinol in their stent projects.  However, every single one of them turns around and ends up using nitinol to build their stent—in spite of the enormous cost of doing so.

Of course, this begs the question, why is nitinol ubiquitous in stent production?  The answer is deformation resilience.  Stainless steel (depending on which alloy), tends to fail at 0.5% strain.  Nitinol, on the other hand, can recover 50% strain and, at 5% strain, has a fatigue life of 100,000 cycles.  Further strain reductions can push the fatigue life into the billion cycle range.  This means that nitinol can withstand 100x as much abuse as stainless steel.

This translates into two major benefits:

  1. Larger ratio of compressed to deployed sizes.  Using nitinol, procedures can be less invasive because a smaller incision is required for insertion.
  2. Longevity.  One of the huge problems of implanting devices inside patients is that they move—a lot.  The most extreme case is within the heart, which cycles 60+ times per second (resting).  This means that if a heart product is to last 20 years, it will be subjected to more than 630 million cycles.  Nitinol is the only material able to withstand this cycle requirement.

Now, there is a second benefit that only nitinol can bring and that arises from the nonlinearity of nitinol.  Most materials closely follow Hooke’s law—meaning that as displacement increases, the force increases linearly.  Nitinol, however, levels out substantially after it reaches a certain strain.  This levelling out is referred to as the superelastic plateau.  As long as the displacement stays within the plateau region, the force remains constant (Note: this explanation has been simplified and a more detailed explanation will be published as a separate article). 

This phenomenon is incredibly useful because it prevents overstraining of tissue and it allows parts in valves to maintain positive sealing forces while requiring a minimal cracking pressure.

How will this empower your application?  Find out by going to www.KelloggsResearchLabs.com for more information.

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