Sometimes the introduction of important new applications involving materials requires simultaneous advancements in several disciplines. The recent application of metal stents, which not only resist corrosion by body fluids but also support the opening of arteries and permit the release of pharmaceuticals to treat atherosclerosis (deposition of plaques on artery walls), provides an interesting example. The discovery of a unique class of alloys, corresponding advances in the electroplating of alloy compositions, and participation by the medical/pharmaceutical community have provided a real multidisciplinary solution to this important medical problem.
W. J. Buehler of the US Naval Ordinance discovered the shape memory effect for the binary intermetallic compound NiTi in 1963. In the meantime, alloying additions (Hf, Zr, Au, Pd, or Pt) have been shown to affect the temperature and nature of the desired phase transformation for the so-called nitinol alloy. As Gunther Eggeler has explained to me, shape memory alloys have two fascinating properties: the one-way effect and pseudoelasticity. Both rely on a diffusionless (lattice shearing) transformation from a high-temperature ‘austenite’ (face-centered cubic) phase to a low-temperature ‘martensite’ (body-centered tetragonal) phase. When the load is removed from a deformed nitinol alloy, it does not change shape. However, when heated, all atoms return to their original lattice positions. This produces a fascinating one-way ‘shape memory’ effect.
But it is the second anomaly, pseudoelasticity, that has led to today's breakthrough in shape memory technology. Here, the austenite phase is deformed at a temperature above that where the martensite forms. The alloy responds by forming the martensite phase in different variants, which permits the system to accommodate the strain and do work. The stress-induced martensitic transformation produces a stress-strain plateau with recoverable strains (upon isothermal unloading) of up to 10% (100 times greater than normal elasticity would provide).
Prior to the use of nitinol stents, a balloon and stainless steel stent were placed across the plaque. The balloon was expanded leaving the stent to prop open the artery. However, after a short time, restenosis would set in, whereby scar tissue builds up around the stent, causing flow restriction. Today, a nitinol stent can be inserted in a relatively thin constraining guide tube, which is positioned by the doctor. When it is deployed, the nitinol stent, with a composition keyed to the body temperature of 38°C, expands to its original shape, transforming to the austenite phase.
But, before these stents could become economically successful in the past 5 years, two metallurgical advances had to be achieved: (i) small precise tubing needed to be produced following an ingot metallurgy route, and (ii) laser cutting procedures had to be established to cut these thin tubes to produce slotted stents. Also, because bare nitinol alloy is corroded by body fluids to release carcinogenic Ni, it has to have a passivating surface layer to be a biocompatible metal approved by the US Food and Drugs Administration.
Despite tremendous advances in the coating of stents with polymers that slowly release antistenosis pharmaceuticals, there remain many practical limitations. These include the type of pharmaceutical that can be delivered and its rate of delivery, as well as the lingering question regarding the stability of particular protein-based pharmaceuticals. For medical devices under continuous stress and exposed to a corrosive environment, adhesion of polymers is also a problem. Mordechay Schlesinger has shown that electrochemical deposition of thin metallic films on nitinol stents that contain and release a wide range of pharmaceuticals over long periods in vitro is a viable option. While previously unexploited in medicine, electrochemical methods can accomplish many desirable biomimetic effects: alter surface morphology, release drugs, enhance radio opacity, and prevent corrosion, all with the same coating. Although the insertion of nitinol stents is expensive (not so much because of materials costs, but because of the required processing), the material costs represent only a small fraction of the overall expense (hospitalization, operation, etc.). Once again, a multidisciplinary approach has proved to provide a unique solution to a real problem.