What does the future hold for Nitinol? In this chapter, Tom speculates about the potential of shape memory applications, R-phase, and such emerging technologies as thin-film constructs, and porous Nitinol. Read on for more…
1.4 The Future
This post is an excerpt from Nitinol: The Book, a working draft of an upcoming publication by Tom Duerig, Alan Pelton, and others. Visit the Table of Contents or Introduction for more information.
Fifteen years ago, it is doubtful that 1 in 10,000 physicians had ever heard of Nitinol. Nowadays, if one goes to any major conference on peripheral vascular disease, the term is used in nearly every presentation, and it is bandied about more freely than “stainless steel.” In retrospect, the explosion of interest in medical devices should have been quite predictable—certainly the fit between the alloy’s inherent properties and the needs of the industry are well-grounded and real. There is every reason to believe that the marriage is here to stay for many years to come. Undoubtedly there will be new directions taken:
- Thin film devices will certainly have an impact in many areas. Though not biodegradable, Nitinol offers a rare combination of being robust in thin film form, and is quickly endothelialized,
- Porous Nitinol offers potential as an extraordinarily compliant material that promotes boney in-growth. Many unanswered questions still exist regarding mechanical integrity, but there is undoubtedly potential here, and
- New alloys, both more radiopaque and stiffer can be developed. Alloys of greater purity may also be of value as devices get smaller and smaller in size.
Probably more importantly, the explosion in usage we are now seeing radiates from a very narrow field of application: the treatment of peripheral vascular disease. There is every reason to believe that the same advantages will eventually be realized in other areas, such as orthopedics, cardiology and endoscopy.
But what about the lost children of shape memory, the actuator and coupling applications that we now hear so little about? Is the lack of activity because there really is no potential, or simply because our attention has temporarily been drawn elsewhere? Applications as fluid fittings have diminished over the years, even though alloys with “heat to shrink” capability have been perfected. Costs were the primary reason for the decline, but costs are coming down and will likely continue to do so, so perhaps these types of applications should be revisited. Fasteners, though still only a modest market, continue to grow in importance, and perhaps represent the larger opportunity.
Actuators still represent the largest potential market for shape memory, but the fundamental problems that will be outlined in Chapter 18 have still not been adequately addressed: accidental actuation, protection from thermal excursions, cycle time, efficiency, hysteresis, etc. The panacea for successful electrical actuation would seem to be a true high temperature Nitinol alloy, with high enough transformation temperatures to avoid self-actuation, yet that retains the stability, strength and fatigue resistance of binary alloys. Despite a great deal of work and reasonable progress with Ni-Ti-Hf, we are simply not there yet. High temperatures can be achieved, but only by making significant sacrifices in mechanical performance and efficiency.
For thermal control, it would seem that the R-phase might have some value though it would be helpful to move the effect to higher temperatures and/or increase the size of the effect. It would seem, however, that the rather tidal shift towards superelastic medical devices has left an inappropriately low level of effort on actuator devices.