The effects of heat-treatment and braiding parameters on the flexural fatigue property of Ni-Ti braided cables for an implantable lead

Abstract

In this study, the flexural fatigue property of a novel implantable lead with a Ni–Ti braided cable was investigated. The effects of heat-treatment and braiding parameters on the flexural fatigue life of the braided cable were evaluated through modifying quantity of filaments, braiding angle and core diameter. The results of flexural fatigue testing demonstrated that (a) heat-treatment improved the flexural fatigue life of the Ni–Ti braided cable from a few thousand to more than 10 million cycles and (b) flexural fatigue property of the super-elastic Ni–Ti braided cable can be improved with the increase in quantity of filaments and braiding angle.

Keywords

Implantable lead Ni-Ti braided cable flexural fatigue property heat-treatment braiding parameters

1. Introduction

Deep brain stimulation (DBS) is one of four types of therapeutic neuromodulation methods approved by the US FDA [1]. DBS has been widely used in Parkinson’s disease, essential tremor, and dystonia, with further applications currently being studied [1–3]. However, challenges still exist when patients are in a complex electromagnetic environment especially when undergoing MRI scans with implanted DBS devices. The DBS lead in the human body absorbs the energy of the radiofrequency electromagnetic field of the MRI because of the antenna effect. This changes the distribution of the induced electric field, which concentrates around the lead, especially in the vicinity of the tip contact of the lead, and leads to thermal damage in patients [4–7]. In order to avoid such risks, a novel implantable lead has been developed to shield the radiofrequency field by covering a Ni–Ti braided cable around the traditional lead. The antenna effect of the lead is greatly reduced and it protects the patients from harm caused by electromagnetic interference. However, it is necessary to keep the braided cable intact so that it can reduce the radiofrequency heat at an optimal level [8]. Due to the complex alternating loads in the human body, the implanted lead used for DBS is required to be carefully tested for its fatigue property before clinical application [9,10]. Therefore, a flexural fatigue test is required with novel implanted leads to study fatigue property. In this study, the effects of heat-treatment and braiding parameters on the flexural fatigue property of braided cables were explored and the optimized design was proposed.

Fig. 1.

Test materials.

2. Materials and methods

2.1. Materials

The 24 and 48-filament cables were braided on both a silicon rubber tube (outer diameter: 2.6 mm, with 4 cavities of inner diameter: 0.5 mm) and a polyurethane tube (outer diameter: 1.3 mm, inner diameter: 0.9 mm) with different braiding angles (𝛼 = 40°, 43°, 46°; and 𝛼 = 20°, 23°, 26.4°, respectively). As shown in Fig. 1 (a)(b), cables were braided by 0.05 mm diameter Ni-Ti filaments (Ni-55.8%, Ti-44.2%) of cold-work (Fort Wayne Metals Corp.). The quantity of filaments n is the sum of filaments in the two directions (n ₁ and n ₂). The characteristic austenite finish temperature of the Ni-Ti filament was 15 °C. Several braided cables were heat treated at 350 °C (10 min) to study the effects of heat-treatment following the process shown in Fig. 1(c).

2.2. Methods

Following the method described in ISO 14708: 2-2005 standard (Fig. 2(a)), a customized lead flexural fatigue test system was used (Fig. 2(b)(c)) [10]. The test specimen was fixed on the holding fixture and driven to bend repeatedly at 90° bilaterally by a motor, at a rate of 2 Hz, around a specific bending radius R. A weight of 70 g was hung to the bottom of the specimen to induce a bend of test samples to 90°. The sample was checked at set intervals and the test was finished once one filament of the braided cable was found broken under inspection with a microscope, with the cycle number at that time noted as the fatigue life N. In order to simulate the temperature in a human body, a hot air gun was used to keep the ambient temperature around the test segment at 37 ± 1.5 °C and a temperature sensor was used for temperature calibration as shown in Fig 2(d).

Fig. 2.

Methods.

In order to study the effects of heat-treatment and braiding parameters (quantity of filaments n and braiding angle 𝛼) on the fatigue life (N) of Ni-Ti braided cables, two groups of experiments were conducted separately with the samples shown in Tables 1 and 2. All samples were tested 3 times. In experiment 1, the bending radius R was 6 mm, while in experiment 2, the bending radius R was at 3, 4.5 and 6 mm.

3. Discussions

3.1. Effects of heat-treatment

From experiment 1 (Fig. 3), it was found that flexural fatigue life of the braided cables (S1.2 and S1.4) without heat-treatment was less than 10,000 times (Fig. 3(a)), which is lower than the standard required in the ISO 14708: 2-2005 [10]. However, specimens with heat-treatment (S1.1 and S1.3) had a better fatigue property of more than 10 million cycles, which meets the ISO 14708: 2-2005 standard. The photos of the specimens taken under the microscope in Table 1 after certain times of flexural fatigue test are shown in Fig. 3(b). The samples with heat-treatment remained intact, but samples without heat- treatment were broken as indicated in the red circle, with the fracture section of the specimens shown in Fig. 3 (c). Due to repeated bending, defect occurred at the surface of the Ni-Ti filament at point a. With continued experimentation, the defect expanded gradually in area b. As the crack developed to a certain area, the filament fractured rapidly and the scaly textile could be found in area c.

Table 1
Samples of experiment 1: Explore the effects of Heat-Treatment

No. D (mm) n 𝛼(°) Heat-treatment

S1.1 2.60 48 46 With

S1.2 Without

S1.3 1.30 48 26.4 With

S1.4 Without

No.	D (mm)	n	𝛼(°)	Heat-treatment
S1.1	2.60	48	46	With
S1.2				Without
S1.3	1.30	48	26.4	With
S1.4				Without

Table 2

Samples of experiment 2: Explore the effects of Braiding Parameters

No.	D (mm)	n	𝛼(°)	Heat-treatment
S2.1	2.60	48	40	With
S2.2			43	With
S2.3			46	With
S2.4		24	40	With
S2.5			43	With
S2.6			46	With
S2.7	1.30	48	20	With
S2.8			23	With
S2.9			26.4	With
S2.10		24	20	With
S2.11			23	With
S2.12			26.4	With

Fig. 3.

The effects of Heat-Treatment.

Fig. 4.

The effects of Braiding Parameters.

To further analyse the effects of heat-treatment, a tensile test was conducted on single filaments both with and without heat-treatment. The stress-strain figure is given in Fig. 3(d). It was found that the tensile strength remained at around 1900 MPa after heat-treatment but the fracture elongation increased from 7% to 17%, while the elastic modulus decreased from 51.2 Gpa to 27.9 GPa. Furthermore, a stress platform formed in the stress-strain curve of the filament with heat-treatment, indicating that the Ni-Ti filament with heat-treatment became super-elastic. Therefore, in a certain strain range, the Ni-Ti filament had smaller stress and was able to revert to the original shape after bending, so that the fatigue property was improved greatly. Furthermore, as demonstrated in Fig. 3(e), the braided cable without heat-treatment could not maintain its structure when it was cut at one end, while the cable with heat-treatment could maintain its structure. The result showed that the heat-treatment could eliminate the stress in the braiding process which also improves the fatigue life.

3.2. Effects of braiding parameters

From experiment 2, we found that most breakpoints appeared in the filament-intersections of the area in contact with the holding fixture. According to this phenomenon, a mechanical model was built for the bending segment of the lead. The mechanical analysis of the braided cable in a bent state is shown in Fig. 4(a). Due to the load under the lead, the test segment conformed to the bending radius. As the gravity of the segment and the friction can be ignored compared to the pressure, the segment was pressed evenly by the holding fixture. According to the mechanical equilibrium, the pressure p in unit length was calculated from Eqs (1) and (2). $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle G-\displaystyle \int _{0}^{\frac{{\pi}}{2}}dF_{x}=G-\int _{0}^{\frac{{\pi}}{2}}pR\cos {\theta}d{\theta}=0\end{eqnarray}$ (1) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle p=\displaystyle \frac{G}{R}.\end{eqnarray}$ (2) According to the structure of the braided cable, the number of filament-intersections N ₀ in unit length was obtained from the following equation (3): $\begin{eqnarray}\displaystyle N_{0}\propto \displaystyle \frac{n\tan {\alpha}}{{\pi}D}. & & \displaystyle\end{eqnarray}$ (3) Therefore, the pressure P in a single filament-intersection followed the relation: $\begin{eqnarray}\displaystyle P\propto \displaystyle \frac{p}{N_{0}}=\frac{{\pi}DG}{nR\tan {\alpha}}. & & \displaystyle\end{eqnarray}$ (4)

An index S ^′ could be extracted from Eq. (4) as Eq. (5): $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle S^{\prime }=\frac{D}{nR\tan {\alpha}}\end{eqnarray}$ (5) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle \lg N=-5.781\lg S^{\prime }-4.501.\end{eqnarray}$ (6) S ^′ is the fatigue stress of the test segment in the bent state, reflecting the effects of the braiding parameters and the test conditions. Therefore, according to the results of experiment 2, the relationship between the fatigue stress (S ^′) and the fatigue life (N) is shown in Fig. 4(b) and the fit curve could be derived as Eq. (6). According to the Fig. 4 (b) and Eqs (5)–(6), it was concluded that the fatigue life of the braided cable can be improved by increasing the quantity of filaments n and braiding angle 𝛼.

4. Conclusions

The fatigue property of Ni-Ti braided cable for a novel implantable lead was improved significantly after heat-treatment, due to the decrease of the elastic modulus, the increase of the fracture elongation and the stress release of the braiding process. Furthermore, the flexural fatigue life of the super-elastic Ni–Ti braided cable can be improved with an increase in the quantity of filaments and braiding angle.

Footnotes

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51407103, 51777115), Major Achievements Transformation Project of Beijing’s College, The National Key Research and Development Program of China (No. 2016YFC0105502), Tsinghua University Initiative Scientific Research Program.

References

DiLorenzo

D.J.

and Bronzino

J.D.

, Neuroengineering (2008), 12.

Breit

, Deep brain stimulation, Cell and tissue research 318(1) (2004), 275–288.

Deeb

, Proceedings of the fourth annual deep brain stimulation think tank: A review of emerging issues and technologies, Frontiers in Integrative Neuroscience (2016), 1–21.

Mattei

, Complexity of MRI induced heating on metallic leads: Experimental measurements of 374 configurations, BioMedical Engineering Online (2008), 1–16.

Rezai

A.R.

, Neurostimulation systems for deep brain stimulation: In vitro evaluation of magnetic resonance imaging-related heating at 1.5 tesla, J Magn Reson Imaging 15(3) (2002), 241–250.

Baker

K.B.

, Variability in RF-induced heating of a deep brain stimulation implant across MR systems, Journal of Magnetic Resonance Imaging 24(6) (2006), 1236–1242.

Nyenhuis

J.A.

Kildishev

A.V.

Bourland

J.D.

Foster

K.S.

and Grabber

, Heating near implanted medical devices by the MRI RF-magnetic field, IEEE Trans. Magn 35(5) (1999), 4133–4135.

, Research on the magnetic resonance imaging radiofrequency-induced heating on deep brain stimulation lead. PhD. Dissertation, Tsinghua University, 2017.

Jiang

Dong

Meng

Hao

Zhang

Feng

and Li

, An experimental study of deep brain stimulation lead fracture: Possible fatigue mechanisms and prevention approach, Neuromodulation 18 (2015), 243–248.

10.

International Organization for Standardization. Implants for surgery: Active implantable medical devices. Part 2: cardiac pacemakers (2005) ISO 14708-2:2005.