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Research Papers

Biocompatibility and Corrosion Response of Laser Joined NiTi to Stainless Steel Wires

[+] Author and Article Information
Grant Brandal

Mechanical Engineering Department,
Columbia University,
500 W 120th Street, Mudd Rm 220,
New York, NY 10023
e-mail: gbb2114@columbia.edu

Y. Lawrence Yao

Fellow ASME
Columbia University,
New York, NY 10023
e-mail: yly1@columbia.edu

Syed Naveed

Endoscopy Division,
Boston Scientific Corporation,
Marlborough, MA 01752
e-mail: Syed.naveed@bsci.com

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received May 6, 2014; final manuscript received January 20, 2015; published online March 5, 2015. Assoc. Editor: Wei Li.

J. Manuf. Sci. Eng 137(3), 031015 (Jun 01, 2015) (9 pages) Paper No: MANU-14-1272; doi: 10.1115/1.4029766 History: Received May 06, 2014; Revised January 20, 2015; Online March 05, 2015

The biocompatibility of nickel titanium (NiTi) wires joined to stainless steel (SS) wires via laser autogenous brazing has been evaluated. The laser joining process is designed to limit the amount of mixing of the materials, thus preventing the formation of brittle intermetallic phases. This process has the potential for manufacturing implantable medical devices; therefore, the biocompatibility must be determined. Laser joined samples underwent nickel release rate, polarization, hemolysis, and cytotoxicity testing. Competing effects regarding grain refinement and galvanic effects were found to influence the corrosion response. After 15 days of exposure to a simulated body fluid, the total nickel released is less than 2 ug/cm2. Numerical modeling of the corrosion currents along the wires, by making use of polarization data, helped to explain these results. Microbiological testing found a maximum hemolytic index of 1.8, while cytotoxicity tests found a zero toxicity grade. All of these results indicate that the autogenous laser brazing process results in joints with good biocompatibility.

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Figures

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Fig. 1

Schematic diagram illustrating the autogenous laser brazing setup. The wires are rotated, while the laser simultaneously scans toward the interface, but is turned off before crossing over to the SS side. The angle θ corresponds to the apex angle of the cup/cone configuration.

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Fig. 2

Visual description of the numerical model used for prediction of the galvanic current. Laplace's equation is solved in the electrolyte PBS. Material properties are taken into account as the relevant boundary conditions along the bottom border. The function fi(φ) is the polarization response for the respective material.

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Fig. 3

Polarization response of metal samples in PBS solution. The solid lines indicate measurements, while the dashed lines are approximations. The more electronegative equilibrium potential of the NiTi will cause it to experience anodic effects, while the SS is cathodic. When the transition from the base materials goes through the two intermediary phases, the corrosion current gets significantly reduced. This data was used for fi(φ) in Fig. 2.

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Fig. 4

Numerical simulation results for the distribution of the electric potential in the PBS solution. The gradient of this field gives the direction of electron flow and is denoted via the arrows. At either extreme along the bottom boundary, the potential approaches the equilibrium potential of the respective material nearby.

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Fig. 5

Distribution of electron flow along the lower boundary of Fig. 4, corresponding to the corrosion current. The dotted lines are for a electrolytic conductivity that is only 10% that of the solid lines, causing a more nonuniform distribution.

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Fig. 6

Total amount of nickel released from joined samples as a function of time. The two different processing parameters of low and high energy inputs have similar profiles, but are noticeably higher than the base NiTi. This NiTi has undergone a previous heat treatment; nontreated NiTi has a release profile higher than the treated samples in here in our case.

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Fig. 7

Optical images comparing the interface regions for two different laser power levels. Both underwent a rotational speed of 3000 deg/s, (a) had a laser power of 15 W while (b) was irradiated at 13 W. The HAZ of the NiTi progresses through several regions before the material starts to bow out at the interface.

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Fig. 8

SEM image of the outside interface for a joined sample. Roughness and nonuniformity is evident in this region, making it susceptible to effects such as pitting and crevice corrosion.

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Fig. 9

Surface corrosion at the mixed interface region after 6 days (a) and 15 days (b) of exposure to the simulated body fluid. Both samples were joined at 17 W and 1500 deg/s. Rather uniform texture is seen in (a), but this surface becomes much rougher as corrosion proceeds, as evidenced by the sharper regions found in (b).

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Fig. 10

Optical micrograph at the interface of a longitudinally sectioned sample. Distinct grains are clearly visible in the NiTi on the left hand side, with size increasing nearer to the interface. The inset shows an enlarged image near the outer edge, where the combination of stress and elevated temperatures resulted in dynamic recrystallization. This is a region that recrystallized without having reached the melting temperature.

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Fig. 11

EBSD unique grain map in a region of NiTi far from the interface. The rather small grain size may be an effect of previous cold-working. When compared with the sizes of Fig. 10, it is clear that the thermal accumulation of the interface results in significant grain growth.

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Fig. 12

EBSD phase map in a HAZ of the SS. At elevated temperatures, SS may become sensitized by precipitation of chromium along grain boundaries. This is a representative image of the whole region, and chromium precipitation was never detected. Thus, the biocompatibility of the SS is not harmed by the laser joining process.

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Fig. 13

Images of the border of the HAZ and base material in the NiTi, as indicated by the inset region of Fig. 1. More corrosion is evident in the lower power sample of (a) than for the higher power of (b). This is evidence that more intermetallic formation at the interface may act as an electrical resistance, effectively reducing the flow of the galvanic current.

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Fig. 14

Hemolysis testing for two different laser processing parameters. Slightly more hemolytic properties are found in the sample irradiated with higher energy, but both are well within the biocompatible, safe zone. Error bars indicate standard error.

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