The Use of Superelasticity in Modern Medicine
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The Use of
Superelasticity in Modern Medicine T.W. Duerig
Abstract The value of Nitinol (Ni-Ti) in the field of medicine has proven to be far greater than just as the simple “springy metal” it was once considered to be. In particular, its use in vascular implants highlights many valuable yet subtle behaviors, including a “biased stiffness,” enhanced fatigue resistance, low magnetic susceptibility, and good biocompatibility. Nitinol today is nearly as well known to medical-device designers and physicians as are stainless steel and titanium, and it is the enabling ingredient in a growing number of successful and profitable life-saving devices. Keywords: alloys, biomedical materials, intermetallic, shape memory, superelasticity.
Introduction It has long been understood that “mechanical memory” and “thermal memory” are simply different manifestations of the same thermoelastic behavior, but the commercial potential of the former (later called “superelasticity,” or “pseudoelasticity”) was not considered in its own right until the mid-1970s. Until then, the use of Nitinol (Ni-Ti) as simply a more efficient spring was not considered interesting, due to the very narrow temperature range over which the shape-memory effect could be observed and because one could just add more mass to a conventional spring, achieving the same result at a lower cost. In 1976, it was recognized that the constant temperature of the human body was ideally suited to the superelastic temperature range of Nitinol.1 Not only did this resolve the temperature limitations of its use, but the drive toward minimally invasive instruments, devices, and surgical techniques also provided a perfect response to the second concern: the enormous value placed on compact, simple designs made the use of a larger spring highly undesirable. The medical field, however, does not quickly adopt new materials, and the 50% nickel content of Nitinol (nickel being a known carcinogen2) raised concerns about biocompatibility. Two major breakthroughs occurred in this same time period: Castleman et al.2 wrote the first overview of Nitinol’s biocompatibility, concluding
MRS BULLETIN/FEBRUARY 2002
that while more work should be done, the alloy appeared to be biologically compatible; and Nonnenmann3 completed his thesis under Andreasson, in which the use of Nitinol as an orthodontic archwire was explored (this was soon to become the first commercial “medical” application of Nitinol). Many concepts using Nitinol were conceived during the next decade, but it was several years before guidewires4 and a surgical anchor5 known as Mammalok® joined the ranks of commercially successful products that used superelasticity as an enabling technology. Meanwhile, other nonmedical superelastic products began to emerge in high volumes, such as eyeglass frames and brassieres, then later, fishing lures and cellular telephone antennae. The success of these products created the much-needed production volume and acceptance of the material, and by the early 1990s, Nitinol began to appear in a host of
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