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DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203158 Drescher Philipp*, Oldorf Paul, Peters Rigo and Seitz Hermann Microstructured ceramic and metallic implant surfaces and their impact on the viscosity of a synovia fluid substitute Abstract: This study evaluates the effect of ring-shaped 10]. In order to improve joint lubrication, the surfaces of the microstructures on the rheological properties of a synovial articulating components can be modified by pulsed fluid substitute. Two different materials that are frequently femtosecond-laser microstructuring. Femtosecond-lasers are used in endoprostheses have been chosen in order to study the an ideal tool for such microstructures, due to their accuracy. lubricating effect of femtosecond-laser microstructured The microstructuring of the implant surface can lead to an implant surfaces by measuring the apparent dynamic viscosity increase of the viscosity of the synovial fluid between the of the lubrication fluid. The two different materials are the articulating joint surfaces and therefore have a positive effect ceramic Al2O3 and the metal alloy CoCrMo. The results show on lubrication . that an increase in the viscosity of the synovia fluid substitute The goal of this approach is to understand and improve can be achieved by specific microstructuring. An increase of hydrodynamic lubrication of joint prostheses by introducing viscosity of up to 40% compared to an unstructured reference microstructures and therefore lower particle abrasion through was observed with ring-shaped microstructures with a diameter of 100 µm, a texture area density of 2.5 % and an wear. Joints exhibit complex and transient load as well as aspect ratio of 0.66. The measurements have also shown that kinematic multi-axis motions that operate in a mixed, the ceramic material resulted in slightly higher viscosity elastohydrodynamic lubrication regime . Despite extensive values, compared to the metal alloy which can explained by research, these mechanisms are still not fully understood. dimensional deviations of the microstructures caused by the Especially the wear of the surfaces in endoprostheses remains laser microstructuring. a major problem . In order to better understand such lubrication mechanisms, an investigation on the impact of Keywords: Surface modification, rheology, joint implants microstructures on the viscosity of the non-Newtonian lubricating fluid, called synovia fluid, is carried out. The https://doi.org/10.1515/cdbme-2020-3158 synovia fluid between the articulating joints is produced by the surrounding tissue of the joint  and plays a vital role in the longevity of a joint replacement . 1 Introduction In a previous study, it was shown that the use of various microstructures has an improving effect on the apparent The demand for better endoprostheses has encouraged viscosity of the synovial substitute . The aim of this study many studies that try to understand the underlying tribological is to investigate the effect of different medical grade materials and lubrication mechanisms of articulating joints as well as and microstructures on the rheological properties of a synovia improving the longevity of endoprostheses [2, 8, 9, 12]. Many fluid substitute. The pulsed laser structuring process of approaches such as better materials or material modifications different materials can lead to dimensional deviations of the like microstructuring were investigated [1, 13]. Research has microstructures, which may result in different rheological already been carried out on the effect of different properties. Besides the most used soft-hard combination of microstructures in endoprostheses [2, 3]. Results have shown joint replacements, hard-hard combinations are still relevant to that micro-pockets lead to a significant improvement in the this day . In order to improve such hard-hard lubricant film thickness compared to untreated surfaces [7, combinations, the microstructuring of medical grade metal and ceramic components were carried out. ______ *Corresponding author: Drescher Philipp: University of 2 Materials and methods Rostock, Justus-von-Liebig Weg 6, Rostock, Germany, e-mail: firstname.lastname@example.org Hermann Seitz: University of Rostock, Rostock, Germany The disks for the laser structuring and the subsequent viscosity Paul Oldorf: SLV GmbH, Rostock, Germany measurements have a diameter of 25 mm and a height of 6 mm. Rigo Peters: SLV GmbH, Rostock, Germany Open Access. © 2019 Drescher Philipp et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. Drescher Philipp et al., Microstructured ceramic and metallic implant surfaces and their impact on the viscosity of a synovia fluid substitute — 2 The metal disks are made of a medical CoCrMo alloy type The surface of the disks was investigated in regard to Ergiloy 9.9135HL from the company Zapp Precision Metals roughness by a laser scanning microscope with a λc of 80 µm GmbH (Ratingen, Germany) and the ceramic discs are made (OLS4000, Olympus, Tokyo, Japan). The surface structure in of a zirconiumoxide-enriched aluminumoxide ELEC PLUS® regard to depth, lateral distance and gap width was measured from the company HiPer Ceramics GmbH (Oberkrämer, at least three times at a magnification of 10x and 50x, using a Germany). Dimensions of the structuring area on the samples laser scanning microscope (VK-X200, Keyence Corporation, are given in Error! Reference source not found., with d as Osaka, Japan). The ring-shaped microstructures consist of a the microstructure’s diameter, t as the microstructure depth, w width of 15 µm with the following variations of the for the microstructure distance and a for the microstructure’s microstructure dimensions on the disks are given in Table 1. width. Microstructuring was carried out with a femtosecond fiber Table 1: Specifications of the ring-shaped microstructures laser of the type TruMicro 5050 Femto Edition (Trumpf Sample Diameter Distance Depth Aspect Density GmbH & Co. KG, Ditzingen, Germany) and a pulse duration in µm in µm in µm Ratio in % of 800 fs. A ring area of the disks with an outer radius of 12.5 S1 100 200 20 1.33 5.9 mm and an inner radius of 5 mm was microstructured in order S2 100 150 20 1.33 10.5 to reduce the shear rate reducing effect. Figure 1 shows how S3 150 300 20 1.33 2.5 the structuring of the disks are to be carried out. S4 100 200 10 0.66 5.9 S5 100 150 10 0.66 10.5 S6 150 300 10 0.66 2.5 3 Results and discussion The roughness of the unstructured CoCrMo and Al2O3 disks was measured at R = 0.34 µm and R = 0.36 µm, respectively. a a Microstructuring of CoCrMo disks was carried out successfully using a femtosecond laser, as seen in Figure 2. The microstructures show a typical laser-generated groove Fig. 1: Schematic depiction of microstructures on the disk profile (see Figure 3) with high qualities and a minimized burr, samples. resulting in an average width of a = 14.9 ± 1.3 µm for the CoCrMo specimens and 15.2 ± 0.9 µm for the Al2O3 The rheometer MCR 702 (Anton Paar Germany GmbH, specimens, respectively Ostfildern, Germany) with its software RheoCompass (Version 1.20.493) was used for the parallel plate rheological measurements. The measurement of the apparent dynamic viscosity η’ were carried out with a reduced gap of 0.1 mm using a modified rheometer setup which is described in a previous study . Each disk sample was rheologically tested at least 3 times for statistical reasons. As a synovial fluid substitute, a bovine serum-based fluid exhibiting comparable non-Newtonian behaviour was used. The synovial fluid substitute consists of a foetal bovine serum from the company Capricorn Scientific GmbH Fig. 2: Microstructured disks (S3) by a femtosecond laser for (Ebsdorfergrund, Germany) with a total protein content of 3.7 rheological parallel plate testing (left: CoCrMo, right: Al2O3). g/dL and hyaluronic acid (HA) with a molecular weight of 2x10 Da with a ratio of 3 g/L. The substitute fluid was applied The measurements of the apparent dynamic viscosity η’ were with the pipette VITLAB®micropipette (VITLAB GmbH, carried out at a temperature of 20 °C and at a constant shear Grossostheim, Germany). rate of 100 s-1, even though a shear rate dependency of the viscosity of joint fluid is well known. Nonetheless, it was Drescher Philipp et al., Microstructured ceramic and metallic implant surfaces and their impact on the viscosity of a synovia fluid substitute — 3 necessary to measure the properties in the chosen range in ceramic and metallic samples. Additionally, the ceramic order to effectively compare the samples to each other. specimens exhibit a slightly rougher surface of approx. 7 %. The CoCrMo specimens show an average width discrepancy of 2 % and an average depth discrepancy of 5 %, compared to the Al2O3 specimens. However, sample S3 shows a slightly higher discrepancy with the depth of a 9 % and width of a 5 % deviation between both materials. This could explain why this specimen is exhibiting a larger deviation than the other specimens. Fig. 3: Exemplary optical analysis of the microstructures of In regard to the shape of the microstructures, the symmetrical specimen S5: CoCrMo (left: 10x magnification, right: 50x) geometry of the ring-shaped structure can be regarded as a positive attribute and could therefore lead to a steadier As seen in Figure 4 of the rheological measurements, the formation of a lubricant film. The reason for higher apparent microstructures have resulted in an overall increase in the dynamic viscosities at microstructured surfaces in comparison apparent dynamic viscosity of the synovial substitute with to a smooth surface is the local change in gap size which ring-like structures, in comparison to the reference with an results in a change in the shear rate of the fluid. The unstructured surface. Differences in the microstructures such microstructures function as wells on surfaces that are a gap as texture density and aspect ratio can be observed to have an enlargement for the flow . The fluid can flow into the impact on the apparent dynamic viscosity, with the CoCrMo wells, lowering shear rates and increase viscosity. sample S6M (η’ = 49.5 ± 0.3 mPas) and the Al2O3 sample S4C Furthermore, these textures can act as wear traps for particles (η’ = 61.1 ± 0.8 mPas) showing the largest increase of 30% and subsequently improve the life cycle of artificial joints. and 42%, respectively, compared to the reference sample (η = 35.4 ± 0.7 mPas). Furthermore, the specimens with less distance between the ring-shaped microstructures and 4 Conclusion therefore a higher texture density result in a marginally higher apparent dynamic viscosity with the exception of specimen S6. Microstructures were created using a pulsed femtosecond laser with well-defined and sharp contours. The measurement of the apparent dynamic viscosity of a synovia substitute fluid was carried out successfully. The results of the rheological measurements have shown that ring-like microstructures have an increasing effect on the apparent dynamic viscosity of the non-Newtonian test fluid which is hypothesized to improve lubrication and lower the wear rate of the implant resulting in a longer life cycle. The results also show that the influence of different materials is not marginal, and the differences are probably due to the dimensional deviations of the microstructures caused by the laser microstructuring. Further inspection of more microstructures can help to better Fig. 4: Results of the parallel plate rheological measurements understand the influence of such parameters and even lead to an optimum value for improved joint lubrication. A possible This could be due to some combinatory effects between aspect correlation between the friction coefficient, film thickness, ratio, texture density and microstructure shape, which needs to hydrodynamic pressure and viscosity likewise needs to be be investigated further. Looking at the aspect ratio, the investigated in future studies to further understand the impact samples with 0.66 show a generally higher measurement of the of microstructures on the lubrication of joint prostheses. apparent dynamic viscosity, compared to the samples with an aspect ratio of 1.33. However, this trend is not as prevalent in Author Statement the Al2O3 specimens. It can also be observed that the Al2O3 Research funding: This research was funded by the Federal specimens reach overall higher dynamic viscosities of approx. Ministry of Education and Research (funding code: 20% on average. This could be explained by slight variations 03WKCU4B; Innovative Regional Growth Cores “MikroLas” in the geometric accuracy of the microstructures between which is part of the initiative “Entrepreneurial Regions”- “The Drescher Philipp et al., Microstructured ceramic and metallic implant surfaces and their impact on the viscosity of a synovia fluid substitute — 4 BMBF Innovation Initiative for the New German Länder”). Properties after Joint Arthroplasty. Lubricants 3, 2, This work was supported by the German Research Foundation 394–412. (DFG, grant number INST 264/133-1 FUGB. Conflict of  Langhorn, J., Borjali, A., Hippensteel, E., Nelson, W., interest: Authors state no conflict of interest. Informed and Raeymaekers, B. 2018. Microtextured CoCrMo consent: Informed consent has been obtained from all alloy for use in metal-on-polyethylene prosthetic joint individuals included in this study. 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Current Directions in Biomedical Engineering – de Gruyter
Published: Sep 1, 2020
Keywords: Surface modification; rheology; joint implants
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