Sodium Hypochlorite Treatment and Nitinol Performance for Medical Devices

J D Weaver; E J Gutierrez; S Nagaraja; P R Stafford; S Sivan; M Di Prima

doi:10.1007/s11665-017-2880-7

Sodium Hypochlorite Treatment and Nitinol Performance for Medical Devices

J Mater Eng Perform. 2017 Sep 1;26(9):4245-4254. doi: 10.1007/s11665-017-2880-7.

Authors

J D Weaver¹, E J Gutierrez², S Nagaraja¹, P R Stafford³, S Sivan², M Di Prima¹

Affiliations

¹ Division of Applied Mechanics, Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, U.S. Food and Drug Administration, Silver Spring, MD.
² Oak Ridge Institute for Science and Education, Oak Ridge, TN.
³ Winchester Engineering and Analytical Center, Office of Regulatory Affairs, U.S. Food and Drug Administration, Winchester, MA.

Abstract

Processing of nitinol medical devices has evolved over the years as manufacturers have identified methods of reducing surface defects such as inclusions. One recent method proposes to soak nitinol medical devices in a 6% sodium hypochlorite (NaClO) solution as a means of identifying surface inclusions. Devices with surface inclusions could in theory then be removed from production because inclusions would interact with NaClO to form a visible black material on the nitinol surface. To understand the effects of an NaClO soak on performance, we compared as-received and NaClO-soaked nitinol wires with two different surface finishes (black oxide and electropolished). Pitting corrosion susceptibility was equivalent between the as-received and NaClO-soaked groups for both surface finishes. Nickel ion release increased in the NaClO-soaked group for black oxide nitinol, but was equivalent for electropolished nitinol. Fatigue testing revealed a lower fatigue life for NaClO-soaked black oxide nitinol at all alternating strains. With the exception of 0.83% alternating strain, NaClO-soaked and as-received electropolished nitinol had similar average fatigue life, but the NaClO-soaked group showed higher variability. NaClO-soaked electropolished nitinol had specimens with the lowest number of cycles to fracture for all alternating strains tested with the exception of the highest alternating strain 1.2%. The NaClO treatment identified only one specimen with surface inclusions and caused readily identifiable surface damage to the black oxide nitinol. Damage from the NaClO soak to electropolished nitinol surface also appears to have occurred and is likely the cause of the increased variability of the fatigue results. Overall, the NaClO soak appears to not lead to an improvement in nitinol performance and seems to be damaging to the nitinol surface in ways that may not be detectable with a simple visual inspection for black material on the nitinol surface.

Keywords: corrosion; fatigue; nickel ion release; nitinol; sodium hypochlorite; surface inclusions.

Grants and funding

FD999999/Intramural FDA HHS/United States