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Evaluation of sliding properties and durability of DLC coating for medical devices
Diamond & Related Materials 96 (2019) 97–103
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Evaluation of sliding properties and durability of DLC coating for medical devices
T
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Kengo Sakuraia, , Masanori Hiratsukab, Hideki Nakamorib, Kazushige Namikic, Kenji Hirakuria a
Department of Electrical and Electronic Engineering, Tokyo Denki University, Japan Nanotec Co, Ltd., Japan c Namiki-Mi Co, Ltd., Japan b
A B S T R A C T
Metallic materials that are used in medical devices such as SUS and brass generate friction against biological tissue. Therefore, when a device is inserted into the body, the patient may feel invaded and uncomfortable because of the friction. In addition, medical devices must be sterilized after use, and the devices may deteriorate with sterilization. Diamond-like carbon (DLC), which possesses advantageous characteristics such as a low friction coefficient, biocompatibility, and chemical stability, has attracted attention as a surface-modification material for medical devices. In this study, the biological characteristics and durability of DLC/ SUS samples formed on SUS substrates by the ionized vapor deposition method were evaluated. From the results of friction-coefficient measurements performed with the ball-on-disc test, the friction coefficient was reduced by factors of approximately 1/4 and 1/5 through DLC coating under atmospheric dry conditions (Dry) and in a physiological saline solution (Wet), respectively. The durability of DLC was evaluated by immersion in an acidic solution. Furthermore, the durability against sterilization treatment was tested using an autoclave. The usefulness of DLC coating was confirmed from the results of observing the surface smoothness of the sample after immersion in an acidic solution and sterilization treatment. The above results suggest that DLC film coating is a useful technique for improving the surfaces of medical devices.
1. Introduction With the development of medical devices and therapeutics, the average life expectancy has increased significantly in developed countries. Japan is no exception, and the average age of Japan's population is currently increasing [1]. Under such circumstances, there has been an increase in elderly patients. For such patients, incisions at the time of surgery are physically harsh and burdensome. Therefore, the use of surgery methods that only use an endoscope without cutting open the abdomen has been increasing [2]. For early-stage stomach cancer, highly effective treatment can be achieved by a simple treatment method that only removes the malignant tumor on the surface of the stomach with an endoscope [3]. In this manner, medical technology is advancing every year, the development of medical devices is progressing, and the scale of the global medical field is increasing. On the other hand, there are several problems with the materials used in medical devices. Currently, metallic materials are often used in indwelling and intubation medical devices that are used to perform therapeutic treatments and examinations [4]. For example, speculums and cervical dilators that are widely used for examinations and treatments in obstetrics and gynecology are made with SUS and brass as raw materials [5]. It is known that metallic materials such as SUS and brass have high friction coefficients against biological tissue of the human
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Corresponding author. E-mail address: [email protected] (K. Sakurai).
https://doi.org/10.1016/j.diamond.2019.03.021 Received 5 March 2019; Accepted 30 March 2019 Available online 12 April 2019 0925-9635/ © 2019 Published by Elsevier B.V.
body [6]. Therefore, when inserting a speculum or cervical dilator into the vaginal cavity, the patient experiences “discomfort” or “pain” due to the friction. Furthermore, this will cause damage to affected parts and a reduction in therapeutic effect in cases involving lacerations or ulcers due to vaginal inflammation caused by a Candida bacterial infection or malignant tumor. In addition, depending on the part of the organ where the device is inserted, the metallic material may elute owing to the acid in the body, which may lead to problems such as deterioration of the material or inflammation of the skin in patients with a metal allergy [7]. Moreover, it is very important for medical devices to be treated after use as an infection-control measure, and in the actual medical field, it is necessary to sterilize medical devices after each use. However, medical devices can deteriorate with sterilization [8]. From this perspective, there is a need for improving various characteristics of medical devices such as the sliding property against body tissue, acid and alkali resistance, and sterilization resistance. Diamond-like carbon (DLC) has a thin-film structure in which a diamond structure, based on the sp3 bonding of carbon atoms, is irregularly mixed with a graphite structure, based on sp2 bonding. Furthermore, some of the films contain 0–40 atm% hydrogen, the properties of which depend on the ratio between sp2 and sp3 bonds as well as the hydrogen content [9]. Therefore, the DLC film is fabricated
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under conditions suitable for the intended use and properties of the material surface. With a low hydrogen content, a film with hardness close to that of diamond is formed. On the other hand, with a high hydrogen content, a film possessing a high gas barrier property and chemical resistance is formed. Since various film-forming methods yield different characteristics such as high hardness, high gas barrier property, and low friction, their practical development, mainly for industrial applications such as cutting tools and surface coatings of optical parts, continues even now. In addition, since DLC possesses properties such as chemical stability and biocompatibility, DLC coatings on biomaterials are drawing attention as surface-modification materials for medical devices [10,11]. The present study aimed to utilize DLC surface coating technology to impart functionalities such as sliding properties, chemical resistance, and sterilization resistance to various medical devices. Therefore, in this experiment, in order to evaluate the sliding properties of a DLC film on a medical device, ball-on-disc tests and friction-coefficient measurements were performed in a simulated biological environment. In addition, to test durability, studies have also been conducted on the chemical resistance of DLC films. Kobayashi et al. investigated the usefulness of a DLC film for application to orthodontic archwires by immersing it in physiological saline [12]. Furthermore, researchers such as Huang et al. [13] and Yamauchi et al. [14] have conducted corrosion resistance tests using hydrochloric acid or sodium hydroxide. However, to the best of our knowledge, no studies have been conducted in which a medical setting was simulated, sterilization treatment was performed, and immersion with chemicals carried out. Therefore, in this study, we examined the sterilization resistance and strong acid resistance of a DLC film for practical medical application. After evaluating the biological characteristics, their relationship with the basic characteristics was investigated by evaluating the basic physical properties of the DLC film.
Table 1 Coating condition of DLC/SUS sample. Sample Source gas Pressure [Pa] Filament current [A] Substrate voltage [kV] Deposition time [min] Film thickness [μm]
SUS304 C6H6 0.2 30 2 220 2
Fig. 2. Schematic of the RF plasma system for surface treatment.
Table 1. The substrates were ultrasonically cleaned in Acetone solution for 20 min. Before DLC film formation, the dry substrates were bombarded with Ar ions in the ionized vapor deposition chamber for 60 min. Friction strongly depends on the surface conditions of the material and the environment in which it is used [16–20]. Therefore, for the measurement of friction coefficient under simulated biological environments, the DLC/SUS sample was subjected to oxygen plasma surface treatment using a high-frequency plasma method. A schematic of the plasma reactor is shown in Fig. 2, and the surface treatment conditions are listed in Table 2. Note that the base material SUS304 is denoted as SUS, the sample in which DLC is formed on SUS304 is denoted as DLC/SUS, and the DLC/SUS sample subjected to oxygen plasma treatment is denoted as O-DLC/SUS. DLC films are coated on all surfaces.
2. Experimental procedures 2.1. Fabrication of DLC film and surface treatment method DLC was formed using an ionized vapor deposition method with C6H6, which has a purity of 99.5%, as a raw-material gas. A schematic of the film-deposition apparatus is shown in Fig. 1 [15]. This experiment used an SUS304 substrate (10 mm × 10 mm × 1.5 mm), which is excellent in terms of cost and ease of production and is handled in actual medical institutions. The film deposition conditions are listed in
2.2. Measurement of friction coefficient by ball-on-disc test The friction coefficients of SUS and DLC/SUS were measured using the ball-on-disc test. A schematic of the ball-on-disc test is shown in Fig. 3, and the measurement conditions are listed in Table 3. SUS balls with a diameter of 6 [mm] were used as targets, and the load was 3 [N]. In this measurement, ISO/DIS 18535 standard was used as the reference [21]. Assuming intubation into biological organs, measurements were made in atmospheric dry conditions (Dry) and in physiological saline Table 2 Treatment condition of O2 plasma. Sample Source gas Pressure [Pa] Supply power [W] Treatment time [min]
Fig. 1. Schematic diagram of ionized deposition system for DLC/SUS film coating. 98
DLC/SUS O2 10 200 2
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(3) A weight was set on the top of the device, and the load applied to the sample was kept constant. (4) The sample sandwiched by urethane rubber was pulled out using a string fitted with a digital force gauge (AD-4932A-50 N). (5) Measurements were performed with varying loads of 0, 100, 200, and 300 g. Immediately after the sample begins to move, the static friction coefficient was calculated using Eq. (1):
F = μN
where F [N] is the numerical value displayed on the digital force gauge (ignoring air resistance), μ is the static friction coefficient, and N [N] is the normal force. In addition, when extracting the sample with the digital force gauge, the device was fixed. Before starting the measurement, the samples and device were immersed in phosphate buffer saline. In this test, since the sample was sandwiched between two plates, the static friction coefficient was calculated using Eq. (2):
Fig. 3. Schematic of the ball-on disc test.
F = F1 + F2 = μ (2N1 + N2 )
Table 3 Ball-on disc test conditions. Object Load [N] Rotational speed [cm/s] Mileage [m]
(1)
(2)
where N1 [N] is the combined weight of the aluminum plates and urethane rubber and N2 [N] is the weight of each sample.
SUS ball φ = 6 [mm] 3 5.00 50
2.4. Immersion in hydrochloric acid and sterilization test Since it is necessary to investigate the acid resistance and durability of the DLC film under environments simulating medical settings, immersion in hydrochloric acid and sterilization tests were conducted. In this test, SUS and DLC/SUS samples were used. A schematic of the high-pressure steam sterilizer is shown in Fig. 5, and the sterilization conditions are listed in Table 4. The detailed protocol is as follows.
conditions (Wet). In atmospheric dry conditions, the humidity was 15% and the temperature was 25 °C. For measurements in physiological saline, the substrate was set in the testing machine, and 2 μL phosphate buffer saline (Wako, 0.01 mol/L, pH of 7.2–7.4 at 25 °C) was dropped onto the substrate before the test was conducted.
(1) Each sample was treated by high-pressure steam sterilization (TOMY, LSX-300). The sterilizing reagent used was general tap water. The sterilization conditions were the same as those currently used in medical institutions. (2) After sterilization, moisture adhering to the sample was wiped off, following which the sample was thoroughly dried. (3) The sample was immersed in hydrochloric acid for the immersion test. Assuming that the sample is used as a medical device, the test utilized hydrochloric acid (Wako, concentration = 35–37%, density = 1.18 g/mL, JIS K8180 special grade) having almost the same concentration as the acid in the body (stomach acid). Prior to the immersion test, the acid concentration was adjusted to approximately the degree of body acidity with a compact pH meter (HORIBA, LAQUATWIN-22B). The temperature was adjusted to a constant value with a digital hot plate (AS ONE, DP-2 M). A schematic of the immersion test with hydrochloric acid is shown in Fig. 6, and the immersion conditions are listed in Table 5. (4) After the immersion test, oxidation of the sample surface was confirmed by visual observation, and the sample was subsequently
2.3. Measurement of friction coefficient under simulated biological environment and contact-angle measurement In order to clarify the friction effects of DLC/SUS and SUS base materials on biomaterials, the friction coefficient was measured in a simulated biological body with urethane. In this test, comparison studies were conducted with four samples: SUS, DLC/SUS, O-DLC/SUS, and urethane rubber. Since urethane rubber has the same material as the opposing material, it was used as the reference evaluation standard. First, before the measurement, the contact angle was measured for SUS, DLC/SUS, and O-DLC/SUS, and the surface wettability was evaluated. The detailed protocol for measuring the friction coefficient under the simulated biological environment is as follows. The test schematic is shown in Fig. 4. (1) Polyester-type urethane rubber (Ra = 6.5 μm, hardness = 50°) was used as the material simulating the biological body, and phosphate buffer saline (Wako, 0.01 mol/L, pH of 7.2–7.4 at 25 °C) was used as the lubricant. (2) Two pieces of urethane rubber were fixed so that they are sandwiched between aluminum plates, and each sample was inserted in between the two pieces.
Fig. 4. Schematic of the measurement of friction coefficient under a constant load.
Fig. 5. Schematic of high-pressure steam sterilization. 99
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0.176. Next, by comparing SUS and DLC/SUS in physiological saline, it was found that DLC coating significantly reduced the friction coefficient by a factor of approximately 1/5, from 0.556 to 0.117. It was found that even a sample subjected to the oxygen plasma treatment (i.e., O-DLC/ SUS) had a sufficiently low friction coefficient. However, the friction coefficient of O-DLC/SUS is higher than that of DLC/SUS because oxygen terminates on the surface. In addition, comparison of DLC/SUS in atmospheric dry conditions and DLC/SUS in physiological saline shows that the friction coefficient of DLC/SUS in physiological saline is lower by approximately 0.06. This is thought to be due to the presence of a liquid layer between the SUS ball and the sample, causing the ball to become smoother on the sample surface than in the atmospheric dry condition. This result confirms that the DLC film has a sufficiently low friction coefficient, even when SUS is the opposing material. This suggests that the mechanical sliding property of the DLC film is suitable for application to medical devices.
Table 4 Sterilization condition. Time [min] Temperature [°C]
15 126
Fig. 6. Schematic of hydrochloric-acid immersion test.
3.2. Measurement of friction coefficient under simulated biological environment and contact-angle measurement
Table 5 Immersion conditions with hydrochloric acid. Time [min] Acid concentration Temperature [°C]
30 pH = 1.023 at 22.1 °C 70 ± 5 °C
The friction coefficient under a simulated biological environment and the contact angle were measured for SUS, DLC/SUS, and O-DLC/ SUS. Table 7 lists the contact-angle measurement results. First, the contact-angle measurement result of each sample will be described. The contact angle of the samples against purified water was measured using the θ/2 method. The amount of purified water (TRUSCO, W-20) was 2 μL. SUS was slightly hydrophilic with a contact angle of 36.7° (n = 3). On the other hand, the contact angle of DLC/SUS was 81.6° (n = 3), indicating that it is water-repellent, and O-DLC/SUS was hydrophilic with a contact angle of 26.0° (n = 3). Although SUS is generally known to be water-repellent, the surface of SUS304 used had a bright annealed (BA) finish and had been subjected to bright annealing treatment. In bright annealing treatment, the process of annealing in a reducing or inert gas can eliminate the formation of an oxide film on the surface. Therefore, it is thought that annealing increases the surface energy of SUS and makes it hydrophilic. As for ODLC/SUS, it had been shown that the outermost surface becomes hydrophilic on modifying the surface of the DLC film with oxygen plasma treatment [22,23]. It can be said that similar results were obtained for this sample. Furthermore, for the DLC/SUS sample, since it is known that the DLC film is generally water repellent, the same results were also obtained. Next, the result of friction-coefficient measurement under a simulated biological environment will be described. The measurement results are listed in Table 8. Since the opposing material is urethane rubber, the measurement result with a urethane rubber sample is used as the evaluation standard. First, we consider the friction coefficient of SUS. When comparing urethane rubber and DLC/SUS, it can be confirmed that SUS has a reduced friction coefficient with respect to the weight. As described above, the surface of the SUS304 used had a BA finish, and the oxide film on the surface had been removed. Therefore, it is thought that the surface energy of SUS is increased and that it is hydrophilic [24]. Next, we consider the friction coefficient of DLC/SUS. Compared to the urethane rubber sample, which is the evaluation standard, it can be confirmed that the friction coefficient with respect to the weight was also reduced in DLC/SUS. However, when compared with the SUS sample, it was found that the friction coefficient is higher
cleaned with a cotton swab containing ethanol, if necessary. (5) Steps 1–4 above constitute one cycle, and a total of 100 cycles were carried out. High-pressure steam sterilization was selected because it is the most widely used technique at present. The immersion test with hydrochloric acid was carried out at a high rate by setting the temperature to be higher than the general temperature. Considering that basic analysis is performed every 0, 10, 30, 50, 70, and 100 cycles, samples were prepared for each of these cycle numbers and used for analysis. To analyze the physical properties of the sample, the basic structure of the film was evaluated using a Raman spectroscope (NRS-4100, JASCO). The surface conditions of the sample were observed with a Schottky field-emission scanning electron microscope (FE-SEM: JEOL, JSM-7100 F) and visual photographs. The surface roughness was measured with an atomic force microscope (AFM: JEOL, JSPM-4200). The results were compared between SUS and DLC/SUS samples, and the usefulness of DLC films was investigated.
3. Results and discussion 3.1. Measurement of friction coefficient by ball-on-disc test The friction coefficients of SUS and DLC/SUS were measured through the ball-on-disc test. The measurement results are listed in Table 6. According to the results in Table 6, comparing the atmospheric dry conditions of SUS and DLC/SUS, the DLC coating reduced the friction coefficient by a factor of approximately 1/4, from 0.529 to Table 6 Friction coefficient of each sample. Sample
SUS DLC/SUS O-DLC/SUS
Friction coefficient (Average)
Table 7 Contact angle of each sample (n = 3).
Dry: 0.529 Wet: 0.556 Dry: 0.176 Wet: 0.117 Dry: 0.265 Wet: 0.233
Temp: 25 °C, RH: 15%, saline: 250 μL. 100
Sample
Contact angle [deg]
SUS DLC/SUS O-DLC/SUS
36.7 81.6 26.0
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3.3. Immersion in hydrochloric acid and sterilization test
Table 8 Friction coefficient of each sample under simulated biological environment. Weight [g]
In order to evaluate the durability of the substrate of each sample after the tests of sterilization and hydrochloric-acid immersion, the surface condition was visually observed by SEM. SEM images of SUS at each test cycle are shown in Fig. 7, SEM images of DLC/SUS are shown in Fig. 8, and visual photographs are shown in Fig. 9. In Fig. 7, from cycle 0 to 50 in SUS, a process can be observed in which unevenness appeared on the surface owing to deterioration from the sterilization immersion test. Furthermore, from cycle 50 to 100, the convex portion of the surface eluted and became flat again. The reason for this is thought to be that elution was promoted because the convex portion that is in contact with the hydrochloric acid has a large surface area. In contrast, the deterioration of the surface of DLC/SUS was not observed from cycle 0 to 100. In addition, the results in Fig. 9 show that, although the deterioration of the substrate can be confirmed for each SUS cycle, this was not the case for DLC/SUS. In order to measure the change in surface roughness of the substrate in each sample, the surface roughness of the substrate after the test was evaluated using AFM. Fig. 10 lists the mean roughness of SUS and DLC/ SUS at each cycle. In Fig. 10, from cycle 0 to 50 in SUS, it can be seen that the mean roughness (Ra) increases because of the unevenness caused by deterioration from the immersion in hydrochloric acid and sterilization tests. Furthermore, as the number of repetitions is further increased, it can be confirmed that the mean roughness Ra gradually decreases from cycle 50 to 100. The cause of Ra decreasing from cycle 50 is thought to be that the convex portion is eluted owing to the large surface area in contact with hydrochloric acid, becoming flatter. Furthermore, it can be seen that the mean roughness Ra at cycle 50 of SUS is the highest Ra value among all cycle numbers of the sample at 106 nm. This can be said to be a reasonable result even when combined with the SEM result (SUS substrate at cycle 50) shown above. In contrast, it can be seen that there is no significant change in the value of the arithmetic mean roughness Ra from cycle 0 to 100 for DLC/SUS. The film structure of the sample after immersion in hydrochloric acid and sterilization tests was evaluated by Raman spectroscopy. The analysis result is shown in Fig. 11. DLC was evaluated by generally focusing on confirming the presence or absence of the G-band and Dband. From Fig. 10, the G-band attributable to the graphite structure unique to the DLC film near 1530 cm−1 and the D-band attributable to defects of the graphite structure around 1310 cm−1 were detected. This result confirmed that the structure of the DLC film was maintained even after the sterilization immersion tests. In addition, almost no change in the crystal structure was observed at each cycle number. In order to measure the coefficient of friction of the substrate in
Friction coefficient Urethane rubber
SUS
DLC/SUS
1.14 0.73 0.87 0.87
2.29 1.33 1.14 0.97
O-DLC/SUS
(n = 5) 0 100 200 300
2.79 1.54 1.24 1.27
1.01 0.67 0.59 0.60
※Object: Urethane rubber.
in DLC/SUS. This result appears to suggest that the wettability of the surface is largely attributed to the friction coefficient being measured an under simulated biological environment. In this test, measurements are made before immersion in a phosphate buffer saline solution. It is thought that, if the sample has good surface wettability, a slight liquid interlayer is formed between the opposing material and the sample, and the contact region becomes slippery. On the other hand, even if a sample with high water repellency on the surface is immersed in phosphate buffer physiological saline, it will immediately repel the liquid. Therefore, it is thought that the surface energy of the opposing material and the specimen is increased, resulting in an increase in the friction coefficient. From the above, it is conceivable that the wettability of the sample surface is important in the measurement of the friction coefficient under the simulated biological environment. From this, it is thought that DLC/SUS has a relatively high friction coefficient. Next, we consider the friction coefficient of O-DLC/SUS. It can be confirmed that the friction coefficient of O-DLC/SUS is greatly reduced when compared to those of urethane rubber, SUS, and DLC/SUS. When the weight of the sample is 200 g, it can be seen that the friction coefficient of urethane rubber is 1.24, whereas that for O-DLC/SUS is 0.59, which is reduced to approximately half or less. Even compared to SUS, which was found to have a low friction coefficient, the friction coefficient of O-DLC/SUS is approximately 0.3 lower. As described above, this result is thought to be due to the good wettability of the surface of O-DLC/SUS and the formation of an interlayer at the interface with physiological saline. This can be said to be a logical result even when combined with the results of contact-angle measurement. The above results suggest that the friction coefficient with respect to the biological body is reduced if the surface of the DLC film is subjected to hydrophilic treatment.
Fig. 7. SEM images of SUS substrate after dipping and sterilization test at (a)0 cycle, (b)10 cycles, (c)30 cycles, (d)50 cycles, (e)70 cycles, and (f)100 cycles. 101
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Fig. 8. SEM images of DLC/SUS substrate after dipping and sterilization test at (a)0 cycle, (b)10 cycles, (c)30 cycles, (d)50 cycles, (e)70 cycles, and (f)100 cycles.
Fig. 9. Photographs of SUS and DLC/SUS substrates after dipping and sterilization test at (a)0 cycle, (b)10 cycles, (c)30 cycles, (d)50 cycles, (e)70 cycles, and (f) 100 cycles.
Mean roughness
200
SUS DLC/SUS
150 100 50 0
0
20
40
60
80
100
Cycle numbers Fig. 10. Mean roughness Ra of each sample at different cycle numbers. Fig. 11. Raman spectra of each sample after immersion and sterilization tests.
each sample, a ball-on-disc test was performed on the tested substrates. The results of the friction coefficient values at 0 cycle, 10 cycles, 50 cycles, and 100 cycles are shown in Fig. 12. It can be seen that the friction coefficient value of SUS increases up to 10 cycles. After 10 cycles of the test, the friction coefficient is saturated. On the other hand, the friction coefficient of the DLC/SUS sample is slightly elevated at 10 cycles. The saturation value of the friction coefficient is almost 0.3. This result demonstrate that the DLC films were stable even after the
sterilization and immersion test. . From the results, the DLC film produced in this study did not deteriorate because of sterilization, and corrosion due to strong hydrochloric acid was not observed. This suggests that the durability was good against sterilization treatment and acidic solutions.
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Friction coeffcient
1
Industrial Research Division (2004) 93–95. [5] Mitsuo Niinomi, Fatigue characteristics of metallic biomaterials, Int. J. Fatigue 29 (2007) 992–1000. [6] Chengxiong Lin, Qingyuan Yu, Juan Wang, Wei Ji, Wei Li, Zhongrong Zhou, Friction behavior between endoscopy and esophageal internal surface, Wear 376377 (2017) 272–280. [7] Mei-Eng Tu, Yu-Hung Wu, Multiple allergies to metal alloys, Dermatol. Sin. 29 (2011) 41–43. [8] Craig A. Wallace, New developments in disinfection and sterilization, Am. J. Infect. Control 44 (2016) e23–e27. [9] Geoffrey Dearnaley, James H. Arps, Biomedical applications of diamond-like carbon (DLC) coatings: a review, Surf. Coat. Technol. 200 (2005) 2518–2524. [10] Roland Hauert, A review of modified DLC coatings for biological applications, Diam. Relat. Mater. 12 (2003) 583–589. [11] Ritwik Kumar Roy, Kwang-Ryeol Lee, Biomedical applications of diamond-like carbon coatings: a review, J. Biomed. Mater. Res. B Appl. Biomater. 83 (2007) 72–84. [12] Shinya Kobayashi, Yasuharu Ohgoe, Kazuhide Ozeki, Keisuke Sato, Touru Sumiya, Kenji K. Hirakuri, Hideki Aoki, Diamond-like carbon coatings on orthodontic archwires, Diam. Relat. Mater. 14 (2003) 1094–1097. [13] Guifang Huang, Zhou Lingping, Huang Weiqing, Zhao Lihua, Li Shaolu, Li Deyi, The mechanical performance and anti-corrosion behavior of diamond-like carbon film, Diam. Relat. Mater. 12 (2003) 1406–1410. [14] Naohiko Yamauchi, Kei Demizu, Nobuhiro Ueda, Nguyen K. Cuong, Takumi Sone, Yukio Hirose, Friction and wear of DLC films on magnesium alloy, Surf. Coat. Technol. 193 (2005) 277–282. [15] Hideki Nakamori, Akihiko Homma, Kenji Hirakuri, Yasuharu Ohgoe, Effect of Oxygen Plasma Treatment for Different Types of DLC Film, the 79th JSAP Autumn Meeting, (2016). [16] Mohammad L. Rahaman, Liangchi Zhang, Mei Liu, Weidong Liu, Surface roughness effect on the friction and wear of bulk metallic glasses, Wear 332-333 (2015) 1231–1237. [17] Pradeep L. Menezes, Satish V. Kailas, Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate, Wear 267 (2009) 1534–1549. [18] Shoufu Tian, Longtao Jiang, Qiang Guo, Gaohui Wu, Effect of surface roughness on tribological properties of TiB2/Al composites, Mater. Des. 53 (2014) 129–136. [19] Jeeho Lee, Muyang He, Chang-Dong Yeo, Golden Kumar, Zhonglue Hu, Edward L. Quitevis, Vidura D. Thalangamaarachchige, Friction and wear of Pd-rich amorphous alloy (Pd43Cu27Ni10P20) under dry and ionic liquid (IL) lubricated conditions, Wear 408-409 (2018) 190–199. [20] A. Morina, A. Neville, Understanding the composition and low friction tribofilm formation removal in boundary lubrication, Tribol. Int. 40 (2007) 1696–1704. [21] ISO18535, Diamond-like Carbon Films – Determination of Friction and Wear Characteristics of Diamond-like Carbon Films by Ball-on Disc Method, (2015). [22] Martin Grischke, André Hieke, Frank Morgenweck, Heinz Dimigen, Variation of the wettability of DLC-coatings by network modification using silicon and oxygen, Diam. Relat. Mater. 7 (2–5) (1998) 454–458. [23] Kazuya Kamasugi, Yasuharu Ohgoe, Kenji Hirakuri, Akio Funakubo, Yasuhiro Fukui, Control of cell growth on DLC films, Surface Volume 45 (12) (2007) 9–16. [24] Chinmay Ghoroi, Lakxmi Gurumurthy, Darrell J. McDaniel, Laila J. Jallo, Rajesh N. Dave, Multi-faceted characterization of pharmaceutical powders to discern the influence of surface modification, Powder Technol. 236 (2013) 63–74.
SUS DLC/SUS
0.8 0.6 0.4 0.2 0
0
50
Cycle numbers
100
Fig. 12. Friction coefficient of each sample at different cycle numbers.
4. Conclusion The present study aimed to impart functionalities such as sliding properties, chemical resistance, and sterilization resistance to various medical devices by applying DLC surface coating technology on medical devices. As a result, it was found that the mechanical sliding property of the material was enhanced and friction against the body was greatly reduced. Furthermore, it was verified that the DLC film possesses good chemical resistance against strong acidity as well as sterilization resistance. Acknowledgement This work was supported by JKA and its promotion funds from KEIRIN RACE. References [1] Chadwick C. Curtis, Steven Lugauerm, Nelson C. Mark, Demographics and aggregate household saving in Japan, China, and India, J. Macroecon. 51 (2017) 175–191. [2] Nabeel Merali, Sukhpal Singh, Abdominal access techniques (including laparoscopic access), Abdom. Surg. 36:5. [3] Philip Wai, Yan Chiu, Novel endoscopic therapeutics for early gastric cancer, Clin. Gastroenterol. Hepatol. 12 (1) (2014) 120–125. [4] Katsuya Hio, Survey report on metal biomaterials – new stainless steels as biomaterials, Reports of the Mie Prefectural Science and Technology Promotion Center
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Evaluation of sliding properties and durability of DLC coating for medical devices
T
⁎
Kengo Sakuraia, , Masanori Hiratsukab, Hideki Nakamorib, Kazushige Namikic, Kenji Hirakuria a
Department of Electrical and Electronic Engineering, Tokyo Denki University, Japan Nanotec Co, Ltd., Japan c Namiki-Mi Co, Ltd., Japan b
A B S T R A C T
Metallic materials that are used in medical devices such as SUS and brass generate friction against biological tissue. Therefore, when a device is inserted into the body, the patient may feel invaded and uncomfortable because of the friction. In addition, medical devices must be sterilized after use, and the devices may deteriorate with sterilization. Diamond-like carbon (DLC), which possesses advantageous characteristics such as a low friction coefficient, biocompatibility, and chemical stability, has attracted attention as a surface-modification material for medical devices. In this study, the biological characteristics and durability of DLC/ SUS samples formed on SUS substrates by the ionized vapor deposition method were evaluated. From the results of friction-coefficient measurements performed with the ball-on-disc test, the friction coefficient was reduced by factors of approximately 1/4 and 1/5 through DLC coating under atmospheric dry conditions (Dry) and in a physiological saline solution (Wet), respectively. The durability of DLC was evaluated by immersion in an acidic solution. Furthermore, the durability against sterilization treatment was tested using an autoclave. The usefulness of DLC coating was confirmed from the results of observing the surface smoothness of the sample after immersion in an acidic solution and sterilization treatment. The above results suggest that DLC film coating is a useful technique for improving the surfaces of medical devices.
1. Introduction With the development of medical devices and therapeutics, the average life expectancy has increased significantly in developed countries. Japan is no exception, and the average age of Japan's population is currently increasing [1]. Under such circumstances, there has been an increase in elderly patients. For such patients, incisions at the time of surgery are physically harsh and burdensome. Therefore, the use of surgery methods that only use an endoscope without cutting open the abdomen has been increasing [2]. For early-stage stomach cancer, highly effective treatment can be achieved by a simple treatment method that only removes the malignant tumor on the surface of the stomach with an endoscope [3]. In this manner, medical technology is advancing every year, the development of medical devices is progressing, and the scale of the global medical field is increasing. On the other hand, there are several problems with the materials used in medical devices. Currently, metallic materials are often used in indwelling and intubation medical devices that are used to perform therapeutic treatments and examinations [4]. For example, speculums and cervical dilators that are widely used for examinations and treatments in obstetrics and gynecology are made with SUS and brass as raw materials [5]. It is known that metallic materials such as SUS and brass have high friction coefficients against biological tissue of the human
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Corresponding author. E-mail address: [email protected] (K. Sakurai).
https://doi.org/10.1016/j.diamond.2019.03.021 Received 5 March 2019; Accepted 30 March 2019 Available online 12 April 2019 0925-9635/ © 2019 Published by Elsevier B.V.
body [6]. Therefore, when inserting a speculum or cervical dilator into the vaginal cavity, the patient experiences “discomfort” or “pain” due to the friction. Furthermore, this will cause damage to affected parts and a reduction in therapeutic effect in cases involving lacerations or ulcers due to vaginal inflammation caused by a Candida bacterial infection or malignant tumor. In addition, depending on the part of the organ where the device is inserted, the metallic material may elute owing to the acid in the body, which may lead to problems such as deterioration of the material or inflammation of the skin in patients with a metal allergy [7]. Moreover, it is very important for medical devices to be treated after use as an infection-control measure, and in the actual medical field, it is necessary to sterilize medical devices after each use. However, medical devices can deteriorate with sterilization [8]. From this perspective, there is a need for improving various characteristics of medical devices such as the sliding property against body tissue, acid and alkali resistance, and sterilization resistance. Diamond-like carbon (DLC) has a thin-film structure in which a diamond structure, based on the sp3 bonding of carbon atoms, is irregularly mixed with a graphite structure, based on sp2 bonding. Furthermore, some of the films contain 0–40 atm% hydrogen, the properties of which depend on the ratio between sp2 and sp3 bonds as well as the hydrogen content [9]. Therefore, the DLC film is fabricated
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under conditions suitable for the intended use and properties of the material surface. With a low hydrogen content, a film with hardness close to that of diamond is formed. On the other hand, with a high hydrogen content, a film possessing a high gas barrier property and chemical resistance is formed. Since various film-forming methods yield different characteristics such as high hardness, high gas barrier property, and low friction, their practical development, mainly for industrial applications such as cutting tools and surface coatings of optical parts, continues even now. In addition, since DLC possesses properties such as chemical stability and biocompatibility, DLC coatings on biomaterials are drawing attention as surface-modification materials for medical devices [10,11]. The present study aimed to utilize DLC surface coating technology to impart functionalities such as sliding properties, chemical resistance, and sterilization resistance to various medical devices. Therefore, in this experiment, in order to evaluate the sliding properties of a DLC film on a medical device, ball-on-disc tests and friction-coefficient measurements were performed in a simulated biological environment. In addition, to test durability, studies have also been conducted on the chemical resistance of DLC films. Kobayashi et al. investigated the usefulness of a DLC film for application to orthodontic archwires by immersing it in physiological saline [12]. Furthermore, researchers such as Huang et al. [13] and Yamauchi et al. [14] have conducted corrosion resistance tests using hydrochloric acid or sodium hydroxide. However, to the best of our knowledge, no studies have been conducted in which a medical setting was simulated, sterilization treatment was performed, and immersion with chemicals carried out. Therefore, in this study, we examined the sterilization resistance and strong acid resistance of a DLC film for practical medical application. After evaluating the biological characteristics, their relationship with the basic characteristics was investigated by evaluating the basic physical properties of the DLC film.
Table 1 Coating condition of DLC/SUS sample. Sample Source gas Pressure [Pa] Filament current [A] Substrate voltage [kV] Deposition time [min] Film thickness [μm]
SUS304 C6H6 0.2 30 2 220 2
Fig. 2. Schematic of the RF plasma system for surface treatment.
Table 1. The substrates were ultrasonically cleaned in Acetone solution for 20 min. Before DLC film formation, the dry substrates were bombarded with Ar ions in the ionized vapor deposition chamber for 60 min. Friction strongly depends on the surface conditions of the material and the environment in which it is used [16–20]. Therefore, for the measurement of friction coefficient under simulated biological environments, the DLC/SUS sample was subjected to oxygen plasma surface treatment using a high-frequency plasma method. A schematic of the plasma reactor is shown in Fig. 2, and the surface treatment conditions are listed in Table 2. Note that the base material SUS304 is denoted as SUS, the sample in which DLC is formed on SUS304 is denoted as DLC/SUS, and the DLC/SUS sample subjected to oxygen plasma treatment is denoted as O-DLC/SUS. DLC films are coated on all surfaces.
2. Experimental procedures 2.1. Fabrication of DLC film and surface treatment method DLC was formed using an ionized vapor deposition method with C6H6, which has a purity of 99.5%, as a raw-material gas. A schematic of the film-deposition apparatus is shown in Fig. 1 [15]. This experiment used an SUS304 substrate (10 mm × 10 mm × 1.5 mm), which is excellent in terms of cost and ease of production and is handled in actual medical institutions. The film deposition conditions are listed in
2.2. Measurement of friction coefficient by ball-on-disc test The friction coefficients of SUS and DLC/SUS were measured using the ball-on-disc test. A schematic of the ball-on-disc test is shown in Fig. 3, and the measurement conditions are listed in Table 3. SUS balls with a diameter of 6 [mm] were used as targets, and the load was 3 [N]. In this measurement, ISO/DIS 18535 standard was used as the reference [21]. Assuming intubation into biological organs, measurements were made in atmospheric dry conditions (Dry) and in physiological saline Table 2 Treatment condition of O2 plasma. Sample Source gas Pressure [Pa] Supply power [W] Treatment time [min]
Fig. 1. Schematic diagram of ionized deposition system for DLC/SUS film coating. 98
DLC/SUS O2 10 200 2
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(3) A weight was set on the top of the device, and the load applied to the sample was kept constant. (4) The sample sandwiched by urethane rubber was pulled out using a string fitted with a digital force gauge (AD-4932A-50 N). (5) Measurements were performed with varying loads of 0, 100, 200, and 300 g. Immediately after the sample begins to move, the static friction coefficient was calculated using Eq. (1):
F = μN
where F [N] is the numerical value displayed on the digital force gauge (ignoring air resistance), μ is the static friction coefficient, and N [N] is the normal force. In addition, when extracting the sample with the digital force gauge, the device was fixed. Before starting the measurement, the samples and device were immersed in phosphate buffer saline. In this test, since the sample was sandwiched between two plates, the static friction coefficient was calculated using Eq. (2):
Fig. 3. Schematic of the ball-on disc test.
F = F1 + F2 = μ (2N1 + N2 )
Table 3 Ball-on disc test conditions. Object Load [N] Rotational speed [cm/s] Mileage [m]
(1)
(2)
where N1 [N] is the combined weight of the aluminum plates and urethane rubber and N2 [N] is the weight of each sample.
SUS ball φ = 6 [mm] 3 5.00 50
2.4. Immersion in hydrochloric acid and sterilization test Since it is necessary to investigate the acid resistance and durability of the DLC film under environments simulating medical settings, immersion in hydrochloric acid and sterilization tests were conducted. In this test, SUS and DLC/SUS samples were used. A schematic of the high-pressure steam sterilizer is shown in Fig. 5, and the sterilization conditions are listed in Table 4. The detailed protocol is as follows.
conditions (Wet). In atmospheric dry conditions, the humidity was 15% and the temperature was 25 °C. For measurements in physiological saline, the substrate was set in the testing machine, and 2 μL phosphate buffer saline (Wako, 0.01 mol/L, pH of 7.2–7.4 at 25 °C) was dropped onto the substrate before the test was conducted.
(1) Each sample was treated by high-pressure steam sterilization (TOMY, LSX-300). The sterilizing reagent used was general tap water. The sterilization conditions were the same as those currently used in medical institutions. (2) After sterilization, moisture adhering to the sample was wiped off, following which the sample was thoroughly dried. (3) The sample was immersed in hydrochloric acid for the immersion test. Assuming that the sample is used as a medical device, the test utilized hydrochloric acid (Wako, concentration = 35–37%, density = 1.18 g/mL, JIS K8180 special grade) having almost the same concentration as the acid in the body (stomach acid). Prior to the immersion test, the acid concentration was adjusted to approximately the degree of body acidity with a compact pH meter (HORIBA, LAQUATWIN-22B). The temperature was adjusted to a constant value with a digital hot plate (AS ONE, DP-2 M). A schematic of the immersion test with hydrochloric acid is shown in Fig. 6, and the immersion conditions are listed in Table 5. (4) After the immersion test, oxidation of the sample surface was confirmed by visual observation, and the sample was subsequently
2.3. Measurement of friction coefficient under simulated biological environment and contact-angle measurement In order to clarify the friction effects of DLC/SUS and SUS base materials on biomaterials, the friction coefficient was measured in a simulated biological body with urethane. In this test, comparison studies were conducted with four samples: SUS, DLC/SUS, O-DLC/SUS, and urethane rubber. Since urethane rubber has the same material as the opposing material, it was used as the reference evaluation standard. First, before the measurement, the contact angle was measured for SUS, DLC/SUS, and O-DLC/SUS, and the surface wettability was evaluated. The detailed protocol for measuring the friction coefficient under the simulated biological environment is as follows. The test schematic is shown in Fig. 4. (1) Polyester-type urethane rubber (Ra = 6.5 μm, hardness = 50°) was used as the material simulating the biological body, and phosphate buffer saline (Wako, 0.01 mol/L, pH of 7.2–7.4 at 25 °C) was used as the lubricant. (2) Two pieces of urethane rubber were fixed so that they are sandwiched between aluminum plates, and each sample was inserted in between the two pieces.
Fig. 4. Schematic of the measurement of friction coefficient under a constant load.
Fig. 5. Schematic of high-pressure steam sterilization. 99
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0.176. Next, by comparing SUS and DLC/SUS in physiological saline, it was found that DLC coating significantly reduced the friction coefficient by a factor of approximately 1/5, from 0.556 to 0.117. It was found that even a sample subjected to the oxygen plasma treatment (i.e., O-DLC/ SUS) had a sufficiently low friction coefficient. However, the friction coefficient of O-DLC/SUS is higher than that of DLC/SUS because oxygen terminates on the surface. In addition, comparison of DLC/SUS in atmospheric dry conditions and DLC/SUS in physiological saline shows that the friction coefficient of DLC/SUS in physiological saline is lower by approximately 0.06. This is thought to be due to the presence of a liquid layer between the SUS ball and the sample, causing the ball to become smoother on the sample surface than in the atmospheric dry condition. This result confirms that the DLC film has a sufficiently low friction coefficient, even when SUS is the opposing material. This suggests that the mechanical sliding property of the DLC film is suitable for application to medical devices.
Table 4 Sterilization condition. Time [min] Temperature [°C]
15 126
Fig. 6. Schematic of hydrochloric-acid immersion test.
3.2. Measurement of friction coefficient under simulated biological environment and contact-angle measurement
Table 5 Immersion conditions with hydrochloric acid. Time [min] Acid concentration Temperature [°C]
30 pH = 1.023 at 22.1 °C 70 ± 5 °C
The friction coefficient under a simulated biological environment and the contact angle were measured for SUS, DLC/SUS, and O-DLC/ SUS. Table 7 lists the contact-angle measurement results. First, the contact-angle measurement result of each sample will be described. The contact angle of the samples against purified water was measured using the θ/2 method. The amount of purified water (TRUSCO, W-20) was 2 μL. SUS was slightly hydrophilic with a contact angle of 36.7° (n = 3). On the other hand, the contact angle of DLC/SUS was 81.6° (n = 3), indicating that it is water-repellent, and O-DLC/SUS was hydrophilic with a contact angle of 26.0° (n = 3). Although SUS is generally known to be water-repellent, the surface of SUS304 used had a bright annealed (BA) finish and had been subjected to bright annealing treatment. In bright annealing treatment, the process of annealing in a reducing or inert gas can eliminate the formation of an oxide film on the surface. Therefore, it is thought that annealing increases the surface energy of SUS and makes it hydrophilic. As for ODLC/SUS, it had been shown that the outermost surface becomes hydrophilic on modifying the surface of the DLC film with oxygen plasma treatment [22,23]. It can be said that similar results were obtained for this sample. Furthermore, for the DLC/SUS sample, since it is known that the DLC film is generally water repellent, the same results were also obtained. Next, the result of friction-coefficient measurement under a simulated biological environment will be described. The measurement results are listed in Table 8. Since the opposing material is urethane rubber, the measurement result with a urethane rubber sample is used as the evaluation standard. First, we consider the friction coefficient of SUS. When comparing urethane rubber and DLC/SUS, it can be confirmed that SUS has a reduced friction coefficient with respect to the weight. As described above, the surface of the SUS304 used had a BA finish, and the oxide film on the surface had been removed. Therefore, it is thought that the surface energy of SUS is increased and that it is hydrophilic [24]. Next, we consider the friction coefficient of DLC/SUS. Compared to the urethane rubber sample, which is the evaluation standard, it can be confirmed that the friction coefficient with respect to the weight was also reduced in DLC/SUS. However, when compared with the SUS sample, it was found that the friction coefficient is higher
cleaned with a cotton swab containing ethanol, if necessary. (5) Steps 1–4 above constitute one cycle, and a total of 100 cycles were carried out. High-pressure steam sterilization was selected because it is the most widely used technique at present. The immersion test with hydrochloric acid was carried out at a high rate by setting the temperature to be higher than the general temperature. Considering that basic analysis is performed every 0, 10, 30, 50, 70, and 100 cycles, samples were prepared for each of these cycle numbers and used for analysis. To analyze the physical properties of the sample, the basic structure of the film was evaluated using a Raman spectroscope (NRS-4100, JASCO). The surface conditions of the sample were observed with a Schottky field-emission scanning electron microscope (FE-SEM: JEOL, JSM-7100 F) and visual photographs. The surface roughness was measured with an atomic force microscope (AFM: JEOL, JSPM-4200). The results were compared between SUS and DLC/SUS samples, and the usefulness of DLC films was investigated.
3. Results and discussion 3.1. Measurement of friction coefficient by ball-on-disc test The friction coefficients of SUS and DLC/SUS were measured through the ball-on-disc test. The measurement results are listed in Table 6. According to the results in Table 6, comparing the atmospheric dry conditions of SUS and DLC/SUS, the DLC coating reduced the friction coefficient by a factor of approximately 1/4, from 0.529 to Table 6 Friction coefficient of each sample. Sample
SUS DLC/SUS O-DLC/SUS
Friction coefficient (Average)
Table 7 Contact angle of each sample (n = 3).
Dry: 0.529 Wet: 0.556 Dry: 0.176 Wet: 0.117 Dry: 0.265 Wet: 0.233
Temp: 25 °C, RH: 15%, saline: 250 μL. 100
Sample
Contact angle [deg]
SUS DLC/SUS O-DLC/SUS
36.7 81.6 26.0
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3.3. Immersion in hydrochloric acid and sterilization test
Table 8 Friction coefficient of each sample under simulated biological environment. Weight [g]
In order to evaluate the durability of the substrate of each sample after the tests of sterilization and hydrochloric-acid immersion, the surface condition was visually observed by SEM. SEM images of SUS at each test cycle are shown in Fig. 7, SEM images of DLC/SUS are shown in Fig. 8, and visual photographs are shown in Fig. 9. In Fig. 7, from cycle 0 to 50 in SUS, a process can be observed in which unevenness appeared on the surface owing to deterioration from the sterilization immersion test. Furthermore, from cycle 50 to 100, the convex portion of the surface eluted and became flat again. The reason for this is thought to be that elution was promoted because the convex portion that is in contact with the hydrochloric acid has a large surface area. In contrast, the deterioration of the surface of DLC/SUS was not observed from cycle 0 to 100. In addition, the results in Fig. 9 show that, although the deterioration of the substrate can be confirmed for each SUS cycle, this was not the case for DLC/SUS. In order to measure the change in surface roughness of the substrate in each sample, the surface roughness of the substrate after the test was evaluated using AFM. Fig. 10 lists the mean roughness of SUS and DLC/ SUS at each cycle. In Fig. 10, from cycle 0 to 50 in SUS, it can be seen that the mean roughness (Ra) increases because of the unevenness caused by deterioration from the immersion in hydrochloric acid and sterilization tests. Furthermore, as the number of repetitions is further increased, it can be confirmed that the mean roughness Ra gradually decreases from cycle 50 to 100. The cause of Ra decreasing from cycle 50 is thought to be that the convex portion is eluted owing to the large surface area in contact with hydrochloric acid, becoming flatter. Furthermore, it can be seen that the mean roughness Ra at cycle 50 of SUS is the highest Ra value among all cycle numbers of the sample at 106 nm. This can be said to be a reasonable result even when combined with the SEM result (SUS substrate at cycle 50) shown above. In contrast, it can be seen that there is no significant change in the value of the arithmetic mean roughness Ra from cycle 0 to 100 for DLC/SUS. The film structure of the sample after immersion in hydrochloric acid and sterilization tests was evaluated by Raman spectroscopy. The analysis result is shown in Fig. 11. DLC was evaluated by generally focusing on confirming the presence or absence of the G-band and Dband. From Fig. 10, the G-band attributable to the graphite structure unique to the DLC film near 1530 cm−1 and the D-band attributable to defects of the graphite structure around 1310 cm−1 were detected. This result confirmed that the structure of the DLC film was maintained even after the sterilization immersion tests. In addition, almost no change in the crystal structure was observed at each cycle number. In order to measure the coefficient of friction of the substrate in
Friction coefficient Urethane rubber
SUS
DLC/SUS
1.14 0.73 0.87 0.87
2.29 1.33 1.14 0.97
O-DLC/SUS
(n = 5) 0 100 200 300
2.79 1.54 1.24 1.27
1.01 0.67 0.59 0.60
※Object: Urethane rubber.
in DLC/SUS. This result appears to suggest that the wettability of the surface is largely attributed to the friction coefficient being measured an under simulated biological environment. In this test, measurements are made before immersion in a phosphate buffer saline solution. It is thought that, if the sample has good surface wettability, a slight liquid interlayer is formed between the opposing material and the sample, and the contact region becomes slippery. On the other hand, even if a sample with high water repellency on the surface is immersed in phosphate buffer physiological saline, it will immediately repel the liquid. Therefore, it is thought that the surface energy of the opposing material and the specimen is increased, resulting in an increase in the friction coefficient. From the above, it is conceivable that the wettability of the sample surface is important in the measurement of the friction coefficient under the simulated biological environment. From this, it is thought that DLC/SUS has a relatively high friction coefficient. Next, we consider the friction coefficient of O-DLC/SUS. It can be confirmed that the friction coefficient of O-DLC/SUS is greatly reduced when compared to those of urethane rubber, SUS, and DLC/SUS. When the weight of the sample is 200 g, it can be seen that the friction coefficient of urethane rubber is 1.24, whereas that for O-DLC/SUS is 0.59, which is reduced to approximately half or less. Even compared to SUS, which was found to have a low friction coefficient, the friction coefficient of O-DLC/SUS is approximately 0.3 lower. As described above, this result is thought to be due to the good wettability of the surface of O-DLC/SUS and the formation of an interlayer at the interface with physiological saline. This can be said to be a logical result even when combined with the results of contact-angle measurement. The above results suggest that the friction coefficient with respect to the biological body is reduced if the surface of the DLC film is subjected to hydrophilic treatment.
Fig. 7. SEM images of SUS substrate after dipping and sterilization test at (a)0 cycle, (b)10 cycles, (c)30 cycles, (d)50 cycles, (e)70 cycles, and (f)100 cycles. 101
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Fig. 8. SEM images of DLC/SUS substrate after dipping and sterilization test at (a)0 cycle, (b)10 cycles, (c)30 cycles, (d)50 cycles, (e)70 cycles, and (f)100 cycles.
Fig. 9. Photographs of SUS and DLC/SUS substrates after dipping and sterilization test at (a)0 cycle, (b)10 cycles, (c)30 cycles, (d)50 cycles, (e)70 cycles, and (f) 100 cycles.
Mean roughness
200
SUS DLC/SUS
150 100 50 0
0
20
40
60
80
100
Cycle numbers Fig. 10. Mean roughness Ra of each sample at different cycle numbers. Fig. 11. Raman spectra of each sample after immersion and sterilization tests.
each sample, a ball-on-disc test was performed on the tested substrates. The results of the friction coefficient values at 0 cycle, 10 cycles, 50 cycles, and 100 cycles are shown in Fig. 12. It can be seen that the friction coefficient value of SUS increases up to 10 cycles. After 10 cycles of the test, the friction coefficient is saturated. On the other hand, the friction coefficient of the DLC/SUS sample is slightly elevated at 10 cycles. The saturation value of the friction coefficient is almost 0.3. This result demonstrate that the DLC films were stable even after the
sterilization and immersion test. . From the results, the DLC film produced in this study did not deteriorate because of sterilization, and corrosion due to strong hydrochloric acid was not observed. This suggests that the durability was good against sterilization treatment and acidic solutions.
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Friction coeffcient
1
Industrial Research Division (2004) 93–95. [5] Mitsuo Niinomi, Fatigue characteristics of metallic biomaterials, Int. J. Fatigue 29 (2007) 992–1000. [6] Chengxiong Lin, Qingyuan Yu, Juan Wang, Wei Ji, Wei Li, Zhongrong Zhou, Friction behavior between endoscopy and esophageal internal surface, Wear 376377 (2017) 272–280. [7] Mei-Eng Tu, Yu-Hung Wu, Multiple allergies to metal alloys, Dermatol. Sin. 29 (2011) 41–43. [8] Craig A. Wallace, New developments in disinfection and sterilization, Am. J. Infect. Control 44 (2016) e23–e27. [9] Geoffrey Dearnaley, James H. Arps, Biomedical applications of diamond-like carbon (DLC) coatings: a review, Surf. Coat. Technol. 200 (2005) 2518–2524. [10] Roland Hauert, A review of modified DLC coatings for biological applications, Diam. Relat. Mater. 12 (2003) 583–589. [11] Ritwik Kumar Roy, Kwang-Ryeol Lee, Biomedical applications of diamond-like carbon coatings: a review, J. Biomed. Mater. Res. B Appl. Biomater. 83 (2007) 72–84. [12] Shinya Kobayashi, Yasuharu Ohgoe, Kazuhide Ozeki, Keisuke Sato, Touru Sumiya, Kenji K. Hirakuri, Hideki Aoki, Diamond-like carbon coatings on orthodontic archwires, Diam. Relat. Mater. 14 (2003) 1094–1097. [13] Guifang Huang, Zhou Lingping, Huang Weiqing, Zhao Lihua, Li Shaolu, Li Deyi, The mechanical performance and anti-corrosion behavior of diamond-like carbon film, Diam. Relat. Mater. 12 (2003) 1406–1410. [14] Naohiko Yamauchi, Kei Demizu, Nobuhiro Ueda, Nguyen K. Cuong, Takumi Sone, Yukio Hirose, Friction and wear of DLC films on magnesium alloy, Surf. Coat. Technol. 193 (2005) 277–282. [15] Hideki Nakamori, Akihiko Homma, Kenji Hirakuri, Yasuharu Ohgoe, Effect of Oxygen Plasma Treatment for Different Types of DLC Film, the 79th JSAP Autumn Meeting, (2016). [16] Mohammad L. Rahaman, Liangchi Zhang, Mei Liu, Weidong Liu, Surface roughness effect on the friction and wear of bulk metallic glasses, Wear 332-333 (2015) 1231–1237. [17] Pradeep L. Menezes, Satish V. Kailas, Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate, Wear 267 (2009) 1534–1549. [18] Shoufu Tian, Longtao Jiang, Qiang Guo, Gaohui Wu, Effect of surface roughness on tribological properties of TiB2/Al composites, Mater. Des. 53 (2014) 129–136. [19] Jeeho Lee, Muyang He, Chang-Dong Yeo, Golden Kumar, Zhonglue Hu, Edward L. Quitevis, Vidura D. Thalangamaarachchige, Friction and wear of Pd-rich amorphous alloy (Pd43Cu27Ni10P20) under dry and ionic liquid (IL) lubricated conditions, Wear 408-409 (2018) 190–199. [20] A. Morina, A. Neville, Understanding the composition and low friction tribofilm formation removal in boundary lubrication, Tribol. Int. 40 (2007) 1696–1704. [21] ISO18535, Diamond-like Carbon Films – Determination of Friction and Wear Characteristics of Diamond-like Carbon Films by Ball-on Disc Method, (2015). [22] Martin Grischke, André Hieke, Frank Morgenweck, Heinz Dimigen, Variation of the wettability of DLC-coatings by network modification using silicon and oxygen, Diam. Relat. Mater. 7 (2–5) (1998) 454–458. [23] Kazuya Kamasugi, Yasuharu Ohgoe, Kenji Hirakuri, Akio Funakubo, Yasuhiro Fukui, Control of cell growth on DLC films, Surface Volume 45 (12) (2007) 9–16. [24] Chinmay Ghoroi, Lakxmi Gurumurthy, Darrell J. McDaniel, Laila J. Jallo, Rajesh N. Dave, Multi-faceted characterization of pharmaceutical powders to discern the influence of surface modification, Powder Technol. 236 (2013) 63–74.
SUS DLC/SUS
0.8 0.6 0.4 0.2 0
0
50
Cycle numbers
100
Fig. 12. Friction coefficient of each sample at different cycle numbers.
4. Conclusion The present study aimed to impart functionalities such as sliding properties, chemical resistance, and sterilization resistance to various medical devices by applying DLC surface coating technology on medical devices. As a result, it was found that the mechanical sliding property of the material was enhanced and friction against the body was greatly reduced. Furthermore, it was verified that the DLC film possesses good chemical resistance against strong acidity as well as sterilization resistance. Acknowledgement This work was supported by JKA and its promotion funds from KEIRIN RACE. References [1] Chadwick C. Curtis, Steven Lugauerm, Nelson C. Mark, Demographics and aggregate household saving in Japan, China, and India, J. Macroecon. 51 (2017) 175–191. [2] Nabeel Merali, Sukhpal Singh, Abdominal access techniques (including laparoscopic access), Abdom. Surg. 36:5. [3] Philip Wai, Yan Chiu, Novel endoscopic therapeutics for early gastric cancer, Clin. Gastroenterol. Hepatol. 12 (1) (2014) 120–125. [4] Katsuya Hio, Survey report on metal biomaterials – new stainless steels as biomaterials, Reports of the Mie Prefectural Science and Technology Promotion Center
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