Machining the Surface of Orthopedic Stent Wire Using a Non-Toxic Abrasive Compound in a Magnetic Abrasive Finishing Process

Abstract

The orthopedic stent wire is one of the critical medical components, which is mainly used for the replacement of physically damaged parts in the human body. Therefore, a smooth surface and lack of toxic substances on the surface of this component are highly demanded. In this study, a magnetic abrasive finishing (MAF) process was carried out using a non-toxic abrasive compound (a mixture of iron powder, diamond particles, cold cream, and eco-friendly oils) to achieve high-quality surface finishing of orthopedic stent wire. The surface roughness (Ra) of the stent wire was investigated according to various processing parameters: different rotational speeds (500, 1000, and 2000 rpm), diamond particle sizes (1.0 µm), and three eco-friendly oils (olive oil: C₉₈H₁₈₄O₁₀; grapeseed oil: C₁₈H₃₂O₂; and castor oil: C₅₇H₁₀₄O₉) within 300 s of the finishing time. The results showed that the surface roughness of the wire was reduced to 0.04 µm with a rotation speed of 1000 rpm and a diamond particle size of 1 µm when using grapeseed oil. SEM microimages and EDS analysis showed that the MAF process using a non-toxic abrasive compound could improve the surface quality of orthopedic Ni-Ti stent wire with a lack of toxic substances on the surface finish.

1. Introduction

Ni-Ti is highly biocompatible, and its properties are suitable for application in orthopedic implants such as stents, catheters, and super-elastic needles. Because this material is directly exposed to the human body, it must have an ultra-smooth and clean surface to prevent surface contamination and corrosion and to reduce its frictional features [1]. Several processing methods can be used to achieve the requisite ultra-smooth surface of this material, namely grinding [2], turning [3], and milling [4]. However, Ni-Ti alloy is classified as difficult to machine due to its high toughness, high strength, severe work hardening, sensitivity to temperature, phase transformation, and high strain rate [5,6]. In particular, when the workpiece to be processed is a wire with a 0.5-mm diameter, these general machining processes are not feasible for achieving the needed finish properties. Because they entail the application of high pressure, they might actually damage the surface to be finished, or microcracks can form on the final finished surface. The magnetic abrasive finishing (MAF) process is an advanced surface machining technique wherein a magnetic field controls magnetic abrasive particles to remove microcracks or unevenness from the product’s surface [7,8]. Unlike the other conventional machining processes, the MAF process uses very little cutting force with the flexible magnetic abrasive compound, which can produce a high-quality, damage-free surface. The MAF process has been used to achieve high surface fineness and dimensional accuracy in products with different shapes such as circular pipes, cylindrical bars, planes, and micro-scale diameter wire [9,10,11]. Owing to its major advantages, the MAF process has been applied, for example, in medical applications, automotive, aerospace, and nuclear industries, and in the hydrogen supply system [9,12]. The MAF process generally uses a mixture of abrasive particles (e.g., diamond abrasive, carbon nanotube (CNT) abrasive, and Al₂O₃ abrasive), iron powder, and industrial oil [13,14]. This magnetic abrasive compound has the advantage of improving the surface and the machining dimensional precision regardless of the workpiece shape [15]. However, this combination cannot be used for machining the surface of biomaterials, such as Ni-Ti wire, because the abrasive particles and industrial oils are toxic to humans. These components remain on the workpiece surface or have direct or indirect harmful effects on the human body. Therefore, when processing biomaterials, a very low surface roughness and a lack of toxic substances on the surface finish of the material are required [16]. Accordingly, the MAF process uses a non-toxic abrasive mixture of iron powder, diamond particles, cold cream, and eco-friendly oil to achieve a high surface finish on the Ni-Ti wire.

In this study, we explored various finishing parameters and investigated the characteristics of high-precision MAF that were obtained with grapeseed oil, olive oil, and castor oil.

2. Experiment and Equipment for MAF

The 2D view and the test apparatus, which was used to confirm the feasibility of MAF using a non-toxic abrasive compound, are shown in Figure 1 and Figure 2, respectively. A small but strong permanent magnet (Nd-Fe-B) was used to generate magnetic force to control the magnetic abrasive compound. To perform the high-precision MAF process, the moving Ni-Ti stent wire was inserted inside a particulate brush loaded with the mixture of iron powder, diamond particles, cold cream, and eco-friendly oil. This loaded brush was then vibrated at an 8-Hz frequency by the electric slider while it was rotated up to 2000 rpm by the stepper motor. Using this procedure, an ultra-smooth and clean surface for orthopedic Ni-Ti stent wire can be achieved. The 2D view of the experimental apparatus for surface finishing of the Ni-Ti (nitinol) wire using the MAF process is shown in Figure 1. This apparatus was 800 mm in width and 850 mm long. The experimental set-up of the MAF process for surface finishing of orthopedic stent wire is shown in Figure 2, in which (a) shows a photograph of the magnetic field rotating system and (b) shows a photograph of a front view of the finishing part.

3. Materials and Methods

Magnetic Abrasive Conpound and Workpiece Materials

In this study, the magnetic abrasive compound was fabricated by mixing 14 wt% of diamond particles (1.0 µm), 72 wt% of electrolytic iron powder (200 µm), 14 wt% of cold cream, and 0.2 mL of eco-friendly oil (

Table 1. Composition of the magnetic abrasive compound according to the type of processing oil.

Composition of the Magnetic Abrasive Compound
Type of lubricant (viscosity)	Diamond particles	Electrolytic iron particles	Cold cream (Bioderma-ABCDerm product)
Olive oil: 0.2 mL (0.0562 Pa.s)	Grain size: 1 µm Weight percentage: 14 wt%	Grain size: 200 µm Weight percentage: 72 wt%	Weight percentage: 14 wt%
Grapeseed oil: 0.2 mL (0.0466 Pa.s)
Castor oil: 0.2 mL (0.58 Pa.s)

). The addition of eco-friendly processing oils (olive oil: C₉₈H₁₈₄O₁₀; grapeseed oil: C₁₈H₃₂O₂; and castor oil: C₅₇H₁₀₄O₉) and cold cream (Bioderma-ABCDerm product) to the magnetic abrasive compound mixture helped reduce both the friction force and the high temperature that occurred during the MAF process, resulting in the production of a smooth surface for the orthopedic stent wire without toxic substances on the finished surface. An SEM image and EDS analysis of the diamond abrasive particles (1.0 µm) are shown in Figure 3a,b, respectively. The workpiece was the orthopedic Ni-Ti (nitinol) wire, a biomaterial that is commonly used in a variety of medical applications. Nowadays, biodegradable Ni-Ti and its alloys are mainly used in stent and orthopedic applications and are preferable for orthopedic implants and biomedical implements such as dental brace wire, SMA orthodontic wire, staples, palatal arches, orthodontic distracters, and biliary stents [17,18]. Figure 4 shows some actual and possible applications of biodegradable Ni-Ti implants. However, because they are implanted inside the human body, their surfaces directly contact human flesh; hence, any contaminants or friction can cause physical harm.

4. Experimental Methods

Table 2. Experimental conditions.

Workpiece Materials	Ni-Ti Wire (L = 200 mm, D = 0.5 mm)
Electrolytic iron powder	200 µm (72 wt%)
Diamond powder	1 µm (14 wt%)
Cold cream	14 wt%
Lubricant	0.2 mL (olive oil, grapeseed oil, castor oil)
Nd-Fe-B permanent magnet	Size: 20 mm × 10 mm × 10 mm
Magnetic pole vibration	8 Hz, amplitude: 2 mm
Rotational speed	500, 1000, 2000 rpm
Finishing time	0, 60, 120, 180, 240, 300 s

shows the experimental conditions. As mentioned above, biocompatible Ni-Ti wire was prepared as the workpiece (L = 200 mm, D = 0.5 mm). A set of two Nd-Fe-B permanent magnets (20 mm × 10 mm × 10 mm) was used to generate the magnetic force. The vibration frequency of the Ni-Ti wire was 8 Hz with an amplitude of 2 mm. The three described processing oils were applied separately for comparison. The process was repeated 5 times for 60 s each for a total of 300 s. After each 60-s pass, the changes in surface roughness were confirmed by a surface roughness tester (Mitutoyo SJ-400 by Mitutoyo Sakado, Kawasaki, Japan).

In order to confirm the effects of the non-toxic magnetic abrasive compound on the finished surface of orthopedic Ni-Ti wire, SEM microimages and EDS analysis were used for the evaluation.

5. Results and Discussion

5.1. Characteristics of MAF According to the Type of Processing Oil

Figure 5 shows the effects of the MAF process on the surface finish of the Ni-Ti stent wire with the different processing oils. The initial experiment was carried out at 500 rpm and a workpiece vibration frequency of 8 Hz, and the finishing time of each wire workpiece was a total of 350 s. The surface roughness was reduced with all three oils, which demonstrates the effect of the processing oil in the MAF process. With the castor oil, the Ra was reduced from 0.11 µm to 0.07 µm. With the olive oil, it was reduced from 0.12 µm to 0.06 µm, and with the grapeseed oil, it was reduced from 0.13 µm to 0.05 µm. The best result was obtained with the grapeseed oil, followed by the olive oil and the castor oil. This can be explained by grapeseed oil having the lowest viscosity. Here, the grapeseed oil’s superior physical properties (low viscosity) made it feasible for ultra-finishing of orthopedic Ni-Ti stent wire. Table 1 shows that the grapeseed oil had the lowest viscosity when compared with the olive oil and castor oil. The viscosities of the grapeseed oil, olive oil, and castor oil were 0.0466 Pa.s, 0.0562 Pa.s, and 0.58 Pa.s, respectively.

Figure 6 shows the results when the revolution speed of the magnetic field was changed to from 500 to 1000 rpm, the finishing time was reduced to 180 s, and the workpiece vibration frequency was still 8 Hz. The surface roughness was reduced with all three oils (from 0.13 µm to 0.04 µm with the grapeseed oil and to 0.05 µm and 0.07 µm with the olive oil and castor oil, respectively). When the revolution speed of the magnetic field was increased to 1000 rpm, the magnetic abrasive compound had more time to remove the unevenness from the Ni-Ti stent wire’s surface, resulting in reducing the surface roughness. Figure 7 shows the results when the revolution speed of the magnetic field was increased to 2000 rpm while still at the 8-Hz frequency for 180 s. Similar to the 500 and 1000-rpm conditions, the surface roughness was reduced by all three processing oils (from 0.12 µm to 0.06 µm with the olive oil, from 0.13 µm to 0.05 µm with the grapeseed oil, and from 0.12 µm to 0.06 µm with the castor oil). When the revolution speed was increased to 2000 rpm, the centrifugal force acting on the abrasive particles was increased, which caused the displacement of the abrasive particles from the finished areas, resulting in a low surface roughness. Nevertheless, it was difficult to improve the surface roughness beyond 0.05 µm, even at this higher speed. Therefore, it can be concluded that the optimal surface roughness was obtained when the revolution speed was 1000 rpm under the other given experimental conditions.

5.2. Workpiece Surface Condition Analysis

In order to confirm the effects of the non-toxic magnetic abrasive compound on the surface finishing of orthopedic Ni-Ti wire, SEM microimages were used. Figure 8 shows the SEM images of the Ni-Ti wire workpieces before and after finishing under the optimal conditions. Before the finishing process, many grooves, scratches, and uneven patches were visible, and the surface roughness (Ra) value was 0.13 µm (Figure 8a). After the MAF process with grapeseed oil at 1000 rpm for 180 s was completed, all of the initial defects were completely removed, and the surface’s condition was smooth and clean (Figure 8b). The final surface roughness (Ra) value was 0.04 µm. This confirmed that grapeseed oil can be used as the processing oil for high-precision finishing of Ti-Ni wire material.

5.3. EDS Confirmation of Ingredient Changes on the Workpiece Surface

In order to confirm whether the wire components changed after the MAF process, energy dispersive X-ray spectroscopy (EDS) testing was performed. Figure 9 shows the EDS results of the Ni-Ti stent workpiece surface conditions before and after the MAF process when grapeseed oil was used. Before the MAF process, 44.28% Ti and 55.72% Ni were detected on the Ni-Ti surface (Figure 9a), and afterward, these values were 49.49% Ti and 55.51% Ni. According to the results of the EDS test, the components of the processing oil and the lack of toxic substances on the finished surface of the Ni-Ti stent wire were not found. Thus, grapeseed oil and cold cream may safely be used in an abrasive compound for the MAF process.

6. Conclusions

The experiments were conducted to evaluate the MAF properties of a non-toxic abrasive compound (diamond powder + cold cream + processing oil (olive oil: C₉₈H₁₈₄O₁₀, grapeseed oil: C₁₈H₃₂O₂, or castor oil: C₅₇H₁₀₄O₉). The following results were obtained:

• The best surface roughness (Ra) value was 0.04 µm, obtained when using iron powder + diamond particles + cold cream + grapeseed oil under the optimal conditions (rotational speed: 1000 rpm, finishing time: 180 s, diamond grain size: 1 µm, and vibration: 8 Hz).
• The best result in terms of surface roughness was found with grapeseed oil, followed by olive oil and castor oil. This can be explained by the low viscosity of grapeseed oil that makes it feasible for ultra-finishing of orthopedic Ni-Ti stent wire.
• SEM microimages showed that the MAF process using grapeseed oil and cold cream could improve the surface finish quality of orthopedic Ni-Ti stent wire.
• EDS testing was conducted to determine whether the components of the workpiece changed after MAF due to the various components in the abrasive compound. There were no changes in the workpiece composition when using the grapeseed oil + iron powder + diamond particles + cold cream compound, which confirmed that grapeseed oil could be used as a processing oil for high-precision finishing of orthopedic Ti-Ni wire material.