Surface Conditions of Nitinol Wires, Tubing, and As-Cast Alloys. The Effect of Chemical Etching, Aging in Boiling Water, and Heat Treatment S. A. Shabalovskaya, 1 J. Anderegg, 1 F. Laab, 1 P. A. Thiel, 1 G. Rondelli 2 1 Institute for Physical Research and Technology and Ames Laboratory, Iowa State University, Ames, Iowa 50011 2 CNR - Institute of Energy and Interface, Milano, Italy Received 25 March 2002; revised 3 September 2002; accepted 2 October 2002 Abstract: The surface conditions of Nitinol wires and tubing were evaluated with the use of X-ray photoelectron spectroscopy, high-resolution Auger spectroscopy, electron backscatter- ing, and scanning-electron microscopy. Samples were studied in the as-received state as well as after chemical etching, aging in boiling water, and heat treatment, and compared to a mechanically polished 600-grit-finish Nitinol surface treated similarly. General regularities in surface behavior induced by the examined surface treatments are similar for wires, tubing, and studied as-cast alloy, though certain differences in surface Ni concentration were ob- served. Nitinol wires and tubing from various suppliers demonstrated great variability in Ni surface concentration (0.5–15 at.%) and Ti/Ni ratio (0.4 –35). The wires in the as-received state, with the exception of those with a black oxide originating in the processing procedure, revealed nickel and titanium on the surface in both elemental and oxidized states, indicating a nonpassive surface. Shape-setting heat treatment at 500 °C for 15 min resulted in tremen- dous increase in the surface Ni concentration and complete Ni oxidation. Preliminary chemical etching and boiling in water successfully prevented surface enrichment in Ni, initially resulting from heat treatment. A stoichiometric uniformly amorphous TiO 2 oxide generated during chemical etching and aging in boiling water was reconstructed at 700 °C, revealing rutile structure. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 65B: 193–203, 2003 Keywords: 600-grit polished Nitinol and wire/tubing surface conditions; surface treatment; chemical etching; aging in boiling water and heat treatment; amorphous oxide INTRODUCTION Extensive use of Nitinol wire in the production of stents, especially delicate ones like vascular stents, imposes severe requirements on surface homogeneity and stability. Small surface defects tolerable in thick fixation plates can be of real danger in the case of tiny wires whose surfaces are subjected to complex manufacturing processes. Surface chemistry is another aspect related to surface stability and biocompatibil- ity, in particular. Elimination of surface nickel and prepara- tion of completely passive surface layers that are structurally and chemically uniform is a desirable target to pursue in the development of stable Nitinol surfaces for long-term implan- tation. Nitinol wires currently offered commercially exhibit variable corrosion resistance. 1,2 The same observation applies to the corrosion studies of an as-cast NiTi alloy with mechan- ically polished finish surface. 3 In the absence of a standard procedure for the treatment of Nitinol surface a 600-grit finish surface is a final state in state– of-the-art studies on Nitinol implantation. 4 The surface conditions of Nitinol are also modified during heat treatment at  500 °C required for shape setting of an implant or device or through sterilization procedures. Previous studies on the surface of Nitinol as-cast alloys have shown that surface chemistry critically depends on oxidation media (air, water/steam, hydrogen peroxide), though general tendency is toward the formation of Ti-based oxide surface films. 5,6 The effect of certain sterilization pro- cedures was a subject of a discussion in a review article. 7 In the present study the surface conditions of Nitinol wires and tubing are evaluated with the help of X-ray photoelectron spectroscopy, Auger electron spectroscopy, electron back- scattering, and scanning electron microscopy in as-received, heat-treated, and chemically treated states and compared with Correspondence to: S. A. Shabalovskaya, Institute for Physical Research and Technology and Ames Laboratory, Iowa State University, Ames, IA 50011 (e-mail: shabalov@ameslab.gov) Contract grant sponsor: National Institutes of Health (NHLBI); contract grant number: 1 R01 HL67632-01 Contract grant sponsor: U.S. Department of Energy; contract grant number: W-4705-ENG-82 © 2003 Wiley Periodicals, Inc. 193