Hydrogen embrittlement of nickel-titanium alloy in biological environment
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INTRODUCTION
TITANIUM and its alloys have been developed and used as biomaterials for dental and medical devices, instruments, and tools since the 1960s.[1] In this research and development, nickel-titanium alloys have been designed, examined, and tested for dental and medical applications, because the combination of good biocompatibility, good strength, and ductility with specific functional properties, such as the shape memory effect and superelasticity, enables the generation of a unique material.[2] For example, in dental clinics, Ni-Ti alloys have been used as orthodontic archwires, specific endosseous implants, dental root implants, adjustable dental abutments, and springs to create space or close gaps between elements in the mouth. For orthopedic devices, wires, nails, staples, plates, rods, screws, rings, and connectors have been designed and used in surgical clinics. Many kinds of instruments and tools such as needles, stylets, guidewires, catheters, stents, filters, tissue anchors and connections, flow control devices, and rhinosurgical instruments have also been developed and used.[3,4] During the development of Ni-Ti alloys, their mechanical, chemical, and electrochemical properties have been extensively studied. Biocompatibility, toxicity, and hypersensitivity to the alloy systems have been concurrently focused on in the research.[5–8] However, degradation other than by surface corrosion of the alloys as biomaterials is not considered, because the alloys are thought to be absolutely reliable. It is known, though, that the titanium alloy is susceptible to environmental embrittlement in a corrosive atmosphere. For example, some Ni-Ti arch wires may break in the oral cavity a few months after setting.[9–12] Some dentists recognized K. ASAOKA, Professor, and K. YOKOYAMA, Assistant Professor, are with the School of Dentistry, The University of Tokushima, Tokushima, Japan 770-8504. M. NAGUMO, Professor, is with the Department of Materials Science and Engineering, Waseda University, Tokyo, Japan 169-8555. Manuscript submitted April 23, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
that the alloy is pliable during setting but the broken wire has lost its ductility. In spite of the ductility of the alloy, reduction in the size of the cross-area and in dimple feature was not observed from the fracture surface of the broken wire.[13,14] Fracture was also reported for dental and orthopedic implant devices.[15–19] In these cases, we suspected that hydrogen embrittlement is the probable cause of the fracture. In regard to the hydrogen absorption of the alloy in biological circumstances, the following are conceivable: (1) hydrogen evolves on the positive electrochemical pole (the pole where the electrons enter) when restorated alloys from Volta’s cell; (2) hydrogen evolves during repassivation of the passive oxide film in cases in which films had been damaged by friction (fretting) or by excessive deformation of the alloy; (3) hydrogen ions in saliva or in bioliquid are absorbed into the alloy; and (4) hydrogen e
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