Original Article
In situ measurements of thin films in bovine serum lubricated contacts using optical interferometry
Proc IMechE Part H: J Engineering in Medicine 2014, Vol. 228(2) 149–158 Ó IMechE 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954411913517498 pih.sagepub.com
Martin Vrbka1, Ivan Krˇupka1, Martin Hartl1, Toma´sˇ Na´vrat1, Jirˇ´ı Gallo2 and Ade´la Galanda´kova´2
Abstract The aim of this study is to consider the relevance of in situ measurements of bovine serum film thickness in the optical test device that could be related to the function of the artificial hip joint. It is mainly focussed on the effect of the hydrophobicity or hydrophilicity of the transparent surface and the effect of its geometry. Film thickness measurements were performed using ball-on-disc and lens-on-disc configurations of optical test device as a function of time. Chromatic interferograms were recorded with a high-speed complementary metal-oxide semiconductor digital camera and evaluated with thin film colorimetric interferometry. It was clarified that a chromium layer covering the glass disc has a hydrophobic behaviour which supports the adsorption of proteins contained in the bovine serum solution, thereby a thicker lubricating film is formed. On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour. In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. Metal and ceramic balls have no substantial effect on lubricant film formation although their contact surfaces have relatively different wettability. It was confirmed that conformity of contacting surfaces and kinematic conditions has fundamental effect on bovine serum film formation. In the ball-on-disc configuration, the lubricant film is formed predominantly due to protein aggregations, which pass through the contact zone and increase the film thickness. In the more conformal ball-on-lens configuration, the lubricant film is formed predominantly due to hydrodynamic effect, thereby the film thickness is kept constant during measurement.
Keywords Total hip replacement, conformity of surfaces, hydrophobicity, hydrophilicity, bovine serum, protein formation, lubrication, film thickness, colorimetric interferometry
Date received: 17 July 2013; accepted: 19 November 2013
Introduction Real hip joint simulators play an important role in the preclinical validation phase. These simulators are especially useful for wear analyses of different types of artificial hip components.1–10 The hip joint wear simulators combine a set of motions, loads and a lubricant that create tribological conditions comparable, but not necessarily identical, to those occurring in vivo.11 Simulators currently in use differ from each other in many parameters: number of stations, loading, degree of freedom, ball-cup relative position and temperaturecontrolled test fluid baths for each hip joint assembly.11 Some simulators are developed in order to measure friction between the joint bodies as well.12–14 However, both types of simulator can be hardly used for the in situ observation of the fundamental lubrication
mechanisms. That is why there is still demand to involve other experimental approaches to enable such a study. It is widely recognized that synovial fluid proteins play an important role in the lubrication processes of artificial hip joints.15–18 Recent results have shown that real-time measurement of protein film formation based
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Faculty of Mechanical Engineering, Brno University of Technology, Brno, Czech Republic 2 Orthopaedic Clinic, University Hospital Olomouc, Olomouc, Czech Republic Corresponding author: M Vrbka, Faculty of Mechanical Engineering, Brno University of Technology, Technicka´ 2896/2, 616 69 Brno, Czech Republic. Email: [email protected]
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on optical interferometry is very suitable method that can help to understand lubrication mechanisms within hip joint replacements.19–23 Fan et al.19 investigated the role of proteins in the lubrication process. Film thickness was measured by optical interferometry in a ball-on-disc device where commercial CoCrMo femoral head was used as a stationary component against a rotating glass disc (mean speed range of 2–60 mm/s). The results for bovine serum (BS) showed a complex time-dependent behaviour, which was not representative of simple fluid. After a few minutes, sliding BS formed a thin adherent film of 10–20 nm, which was attributed to protein absorbance at the surface. This layer was augmented by a hydrodynamic film, which often increased at slow speeds. At the end of the test, deposited surface layers of 20–50 nm were measured. The authors suppose that the film formation is dominated by surface deposition and shear-flow aggregation of protein molecules. These molecules are aggregated in the inlet shear field and come out of the solution to form gel-like deposits in the inlet; this material adheres to the metal surface and periodically passes through the contact, forming a much thicker hydrodynamic film. The authors also note the fact that protein deposits are formed in the inlet of the contact in the low-pressure region. Myant et al.20 performed measurements of central film thickness by optical interferometry as a function of time (constant mean speed 0 and 10 mm/s) and variable mean speed (0–50 mm/s) for series of BS and proteincontaining (albumin, globulin) saline solutions and for CoCrMo femoral component sliding against a glass disc. The effect of load on film thickness was also studied. The results showed that the film thickness increased with time for both the static and sliding tests. In the sliding tests, a wear scar rapidly formed on the implant component for the BS and albumin fluids; negligible wear was observed for globulin solutions. The authors point out that the film thickness decreased rapidly with increasing load for all fluids, and they also supported the idea of the protein-aggregation lubricated mechanism introduced in Fan et al.19 Myant and Cann21 enhanced observations of the film formation mechanisms in a model metal-on-metal hip joint lubricated with BS. They showed that model synovial fluid solutions demonstrate a complex timedependent film thickness behaviour that is not characteristic of a simple Newtonian fluid. The results indicated that two types of films are formed: a boundary layer of adsorbed protein molecules, which are augmented by a high-viscosity fluid film generated by hydrodynamic effects. This high-viscosity film is due to inlet aggregation of protein molecules forming a gel which is entrained into the contact preferentially at low speeds. As the speed increases, this gel appears to shear thin, giving much lower lubricant film thickness. These results suggested that protein-containing fluids do not obey classical Newtonian elastohydrodynamic lubrication (EHL) models.
Fan et al.22 critically examined synovial fluid lubrication mechanisms and the effect of synovial fluid chemistry which includes protein content and pH. They confirmed two distinct film formation mechanisms reported in earlier articles19–21 and concluded that patient synovial fluid chemistry plays an important role in determining implant wear and the likelihood of failure. Moreover, they pointed out that at present, the function of synovial fluid properties in determining implant performance is implicitly ignored in all the lubrication models. In the previous study,23 the authors performed detailed experimental analysis of lubricant film thickness of BS within the contact between the artificial metal and ceramic heads and the glass disc to analyse the effect of proteins on film formation under various rolling/sliding conditions using thin film colorimetric interferometry. Film thickness was studied as a function of time. From the performed experiments, it was confirmed that the formation of protein film thickness is markedly dependent on kinematic conditions acting in the contact, especially on the positive and negative slide-to-roll ratio and the mean speed. From the cited literature,19–23 it is apparent that optical technique has proved to be a valuable experimental tool in the study of lubrication mechanisms in the artificial joints. It is a technique that gives detailed information about the lubricant film distribution within the contact, and especially, only this method is able to provide experimental data about protein film formation. However, one of the contact bodies must be optically transparent (glass or sapphire) and coated with a semi-reflective layer (chromium or silica), thereby the formation of protein film can be significantly affected. Moreover, design of the optical test devices is generally based on classical ball-on-disc EHL simulators where lubrication mechanisms are examined under higher pressures between the ball and the flat transparent surface (non-conformal contact). Therefore, the aim of this study is to verify the use of optical test device for measurement of BS protein film in relation to the function of the artificial hip joint. It is mainly focussed on the effect of the hydrophobicity or hydrophilicity of the transparent surface and the effect of its geometry.
Experimental method and test programme Several optical test rig configurations were involved in this study (Figure 1). Formation of lubricating film of BS solution was observed using an optical test rig in which a circular contact is realized between a glass disc or lens and a metal or ceramic head of total hip joint replacement (Figure 1). BS (Sigma–Aldrich B9433, protein concentration 55.5 mg/mL) and sterile water were used for preparing sample with appropriate w/w concentration. BS concentration of 25% with a total
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Figure 1. Film thickness measurement using optical test rig. BS: bovine serum; CMOS: complementary metal-oxide semiconductor.
Table 1. Performed experiments for various material combinations of balls and disc coatings. Experiment
Ball, diameter (mm)
Glass body, coating
BS supply
1 2 3 4 5 6 7
CoCrMo, 36 CoCrMo, 36 Al2O3, 32 Al2O3, 32 CoCrMo, 36 CoCrMo, 36 CoCrMo, 28
Disc, Cr Disc, Cr + SiO2 Disc, Cr Disc, Cr + SiO2 Disc, Cr + SiO2 + hydrophobic modification Disc, Cr Lens with radius of 15.6 mm, Cr
Syringe pump Syringe pump Syringe pump Syringe pump Syringe pump Fully bathed Syringe pump
BS: bovine serum.
protein content of 13.9 mg/mL was prepared in volumes of 12 mL and immediately stored in a freezer at 220 °C. The lower surface of the glass disc or lens was coated with a thin semi-reflective chromium layer with a different thickness according to the use of either the metal or the ceramic head, and the upper side had an antireflective coating. For some experiments, the chromium layer of glass disc was coated with a silica layer (SiO2) with additional thickness of 150 nm. Artificial femoral heads, in the following text marked as balls, Zimmer (CoCrMo – ProtasulÒ-20/ISO 583212/36 mm in diameter), AESCULAP NK561 (Al2O3 – BioloxÒ forte/ISO 6474/32 mm in diameter) and AESCULAP NK430K (CoCr29Mo/ISO 5832-12/28 mm in diameter) were delivered in the original package from the manufacturer. Prior to film thickness experiments, surface topography of the metal and ceramic balls was analysed in detail using the optical measurement method based on phase shifting interferometry that provides topography data with the accuracy below
1 nm (apparatus Bruker ContourGT-X8). Evaluated average surface roughness Ra was between 0.0011 and 0.0038 mm for the metal and between 0.0017 and 0.0021 mm for the ceramic balls. It can be noted that the disc and lens surfaces were optically smooth. The ball (diameter of 36 mm for CoCrMo and diameter of 32 mm for Al2O3) was fixed stationary, and the glass disc was rotated against the ball by a servomotor to provide required pure sliding conditions under mean speed of 10 mm/s (Figure 1, configuration B). The ball was held in a positioning holder, and for each test, it could be rotated in a different position. In the case of the use of the concave lens with radius of 15.6 mm, the CoCrMo ball, having diameter of 28 mm, was rotated against the stationary lens under mean speed of 10 and 40 mm/s (Figure 1, configuration C). Film thickness was studied as a function of time for various material combinations of contact surfaces (Table 1), where the total time of each measurement was 5 min. Experiments were realized at room temperature of 24 °C under steady-state
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load corresponding to mean Hertzian pressure of 153– 177 MPa for ball-on-disc configuration and 37 MPa for ball-on-lens configuration. The room temperature of 24 °C was set to avoid temperature fluctuations due to low volume of BS used in experiments (it was confirmed that BS film behaviour under room temperature of 24 °C and under body temperature of 37 °C has the same tendency). All components, which are in contact with BS (e.g. glass disc, lens, ball, holder), were cleaned in 1% w/w sodium dodecyl sulphate, rinsed in distilled water and then washed in isopropyl alcohol before assembly. This cleaning process was always repeated between the individual measurements. Test fluids were taken out from the freezer 2 h before measurements and then were supplied to the contact zone in the amount of 3.5 mL/min through a syringe coupled with a needle (a syringe pump) for a period of 3 min. Experiment 6 (see Table 1) was realized with a larger amount of the test fluid where the disc and the ball were fully bathed. A new sample of test fluid was used for each measurement. The contact formed between the glass disc or lens and the ball was illuminated by xenon lamp. The obtained chromatic interferograms were recorded with a high-speed complementary metal-oxide semiconductor (CMOS) digital camera and evaluated using a thin film colorimetric interferometry (Figure 1). A detailed description of this technique is given in Hartl et al.24,25 The contact zone was recorded with a frequency of 24 Hz for 5 min. During this time, 7200 interferograms were recorded for each measurement. From each performed measurement, about 50 interferograms were selected and then the film thickness was evaluated.
Results and discussion In the previous study,23 the effect of proteins on film formation under various rolling/sliding conditions was studied using ball-on-disc optical test rig in configuration A (Figure 1). Film thickness measurements as a function of time for three values of slide-to-roll ratio (S = 0, 1.5 and 21.5) are displayed in Figure 2 for CoCrMo ball having diameter of 28 mm and for mean speed of 5.7 mm/s. The individual points in the graph in Figure 2(a) correspond to average film thickness from the central area of the contact zone, and the marked grey region (0–180 s) represents the time of BS supply to the contact zone. Formation and evolution of film thickness in the whole contact zone dependent on time are illustrated in Figure 2(b)–(d) by individual interferograms. Under pure rolling conditions (S = 0), the film thickness increases gradually with time, and at the end of measurement (after 5 min), the central film thickness achieved the value of about 25 nm (Part C of Figure 2(b)). Under rolling/sliding conditions, for the case where the disc is faster than the ball (S = 1.5), at first, the film thickness increases rapidly, and when the maximum film thickness about 140 nm is reached (Part B of Figure 2(c)), then this effect is lost and the film
Figure 2. Film thickness behaviour as a function of time for three values of slide-to-roll ratio (S) and mean speed of 5.7 mm/s.23 (a) Central film thickness development. Chromatic interferograms: (b) S = 0, (c) S = 1.5 and (d) S = 21.5. The inlet region is on the left. BS: bovine serum.
thickness starts to decrease, and finally, at the end of the measurement, the film thickness drops to a few nanometres (Part C of Figure 2(c)). Under rolling/sliding conditions, for the case where the ball is faster than the disc (S = 21.5), an absolutely different formation of BS film thickness is observed. Under these conditions, the average central film thickness about 5 nm remains during the measurement approximately constant even if the fresh BS is supplied to the contact (Figure 2(d)). It should be noted that for ceramic ball, the development of lubricant film thickness was qualitatively very similar as with the metal ball. From the performed experiments under rolling/sliding conditions, it is obvious that the formation of lubricant film thickness is markedly dependent on kinematic conditions acting in the contact, especially on the positive and negative slide-to-roll ratio. However, the behaviour of BS film in the contact can be significantly
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Figure 3. Film thickness behaviour as a function of time for two types of disc coatings and CoCrMo head, under pure sliding conditions (S = 2) and mean speed of 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) glass disc with chromium layer and (c) glass disc with silica layer. The inlet region is on the left. BS: bovine serum.
affected by optically transparent disc, which is coated with a chromium semi-reflective layer. Moreover, in reality, a relative motion between the artificial hip components is mostly pure sliding (S = 62). Therefore, in this study, other experiments were performed on the optical test rig in configuration B, where the metal or ceramic ball is fixed and the disc rotates against the ball, with the aim to explain the effect of disc coating on protein film formation.
Effect of surface wettability At first, film thickness was studied using a combination of CoCrMo ball and glass disc with chromium layer. From the graph in Figure 3(a) (blue points), it is possible to observe that the central film thickness shows an initial intensive growth to the maximal value of 880 nm in time of 50 s. Then, the film thickness starts to decrease, and in the last 100 s, the film thickness reaches the average value of around 50 nm. This measurement was carried out in a standard way when the BS was supplied to the contact zone by the syringe pump with a constant flow rate of 3.5 mL/min for the period of 0–180 s (in Figure 3(a), it is marked as a grey region). Interferograms captured at different times (Figure 3(b)) show a typical composition of lubricating
film during the experiment. After beginning of the measurement, there is a visible massive accumulation of protein aggregations in the inlet region of contact zone, which causes the growth of film thickness (Part A of Figure 3(b)). After that, the protein aggregations gradually pass through the contact zone, and a total amount of accumulated protein in the inlet region is reduced, and subsequently, the lubricant film thickness is decreased (Part B of Figure 3(b)). As the protein aggregations were passing through the contact zone, they were adsorbed especially on the chromium layer of the glass disc. During the experiment, these aggregations then formed a lubricating layer, which affected the film thickness at the end of the measurement (Part C of Figure 3(b)). Next experiment was also carried out with CoCrMo ball, but the glass disc with chromium layer was overlaid by silica (SiO2) with thickness of 150 nm. Development of average central film thickness is plotted in Figure 3(a) (black points), and selected interferograms are displayed in Figure 3(c). Although the experiments were performed under the same kinematic conditions, absolutely different results can be observed. Within the whole experiment, the lubricant film thickness in the contact zone reached the values of only a few nanometres; occasionally, some local spots passing through the contact zone were visible (Figure 3(c)). These spots could be small protein aggregations caused by disruption of a natural protein structure by pressure and shear stress in the contact zone. However, noticeable aggregations and accumulations of proteins in the inlet region of contact zone were not observed. It can be concluded that the change in the material of disc layer causes considerable changes in film thickness behaviour. These results were supported by measurements performed with the ceramic ball (Figure 4). It was suggested from the above-described results that properties of contacting surfaces play an important role and can influence the results obtained from optical test rig. That is why wettability of individual contact surfaces was examined using a contact angle measurement method (Figure 5). A drop of distilled water with a volume of 15 mL was applied on the contact surfaces of disc and ball. Individual drops were subsequently recorded by charge-coupled device (CCD) camera, and the evaluated results were summarized in Table 2. From this table, it is evident that the surface of the disc with a chromium layer is hydrophobic (the contact angle is greater than 90°), whereas the disc with a silica layer is strongly hydrophilic (the contact angle is close to 30°). From this, it can be deduced that the hydrophobicity has a considerable influence on the formation of protein aggregations and possibly on the adsorption of the proteins on a disc layer. For verification of this hypothesis, a new experiment was realized with the glass disc where a micro-emulsion based on silicone with 0.4% w/w sulphuric acid was applied on the silica layer. This micro-emulsion makes a hydrophobic layer with contact angle about 100° (see Table 2).
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Proc IMechE Part H: J Engineering in Medicine 228(2) Table 2. Measurement of wettability of ball and disc contact surfaces. Contact surface
Contact angle (°)
Ball, CoCrMo Ball, Al2O3 Glass disc, Cr layer Glass disc, SiO2 layer Glass disc, SiO2 layer + hydrophobic modification
71.6 6 2.5 60.4 6 1.7 94.5 6 2.5 28.7 6 2.0 100.5 6 2.4
Figure 4. Film thickness behaviour as a function of time for two types of disc coatings and Al2O3 head, under pure sliding conditions (S = 2) and mean speed of 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) glass disc with chromium layer and (c) glass disc with silica layer. The inlet region is on the left. BS: bovine serum.
Figure 6. Film thickness behaviour as a function of time for SiO2 and modified SiO2 disc coatings and CoCrMo head, under pure sliding conditions (S = 2) and mean speed of 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) glass disc with modified silica layer and (c) glass disc with silica layer. The inlet region is on the left. BS: bovine serum.
Figure 5. Wettability examination using a contact angle measurement method.
The results of the experiment with the modified disc are summarized in Figure 6(a) (blue points) and for clearness are compared with the disc without modification (see Figure 6(a), black points). Development of lubricant film thickness is very similar to the experiment, where the disc contains only chromium layer (see Figure 3(a), blue points). After beginning of the measurement, a rapid increase can be seen in the film thickness, up to 510 nm within the period of 50 s, and then the film thickness starts to decrease, and at the end of the measurement, it drops to the value of about 20 nm.
The above-mentioned behaviour of the lubricant film can be observed in the selected interferograms (Figure 6(b)), where especially the accumulation of protein aggregations in the inlet region is clearly visible (Part A of Figure 6(b)). These aggregations gradually pass through the contact zone, partially adsorbed on the modified silica layer of disc and form a lubricating film with thickness of about tens of nanometres (Parts B and C of Figure 6(b)). When the disc is without mentioned hydrophobic modification, these effects are not observed (see Figure 6(c)). From the presented results, a significant effect of materials of the contact pairs on the lubricant film
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formation was observed. Sethuraman et al.26 referred that with the decreasing wettability, a degree of protein adsorption has an increasing tendency. Consequently, the contact of BS solution with the hydrophobic disc surface (Cr layer) leads to the adsorption of proteins, and the thickness of this adsorbed layer can take the values from a few to tens of nanometres. Nakanishi et al.27 studied the protein adsorption on rigid surfaces; they concluded that the thickness of the human serum albumin (HSA) layer, adsorbed on a relatively hydrophobic surface of titanic oxide, is 18 nm. They also indicated that the adsorbed amount of proteins at the room temperature is dependent on the type of protein, surface and adsorption conditions. The adsorption force also plays an important role in the protein adsorption process. Nakashima et al.28,29 studied the friction of lubricants with a different amount of albumin and g-globulin proteins. They concluded that a different rate of adsorption is dependent on the weight proportion of individual proteins, whereas the individual proteins show a different adsorption force. While albumin binds to the surface with a relatively low force, g-globulin has a stronger adsorption. The authors also stated that albumin can generate a low shear layer, while g-globulin can form a tight layer which protects against wear. Heuberger et al.30 studied the adsorption of HSA in natural and denatured forms on articulating surfaces. They found out that the presence of denatured albumin in the solution leads to a significant reduction in the adsorbed amount. They supposed that the denatured albumin is adsorbed more easily onto the hydrophobic surface than the albumin in its natural form. A passivation layer formed by the adsorbed denatured albumin then protects from further adsorption. On the other hand, albumin in its natural form creates a thicker adsorbed film. Fang et al.31 carried out friction experiments using a pin-on-disc tribometer with various materials of the artificial replacements in the presence of the albumin lubricant. They showed that the presence of the denatured albumin leads to the increase of a friction coefficient where a layer of denatured albumin on friction surfaces was observed at the end of experiments. Fang et al. also confirmed that the denatured albumin forms a compact layer which attempts to maintain more on a hydrophobic surface compared to adsorbed natural albumin layer which decreases the coefficient of friction. The influence of the surface hydrophobicity on the protein adsorption and binding of denatured proteins on frictional surfaces is also supported by other studies.32 It is obvious that hydrophobicity and hydrophilicity of the disc surface play an important role in the film formation process. At the beginning of the measurement, the natural proteins, contained in the BS, were gradually adsorbed on the hydrophobic surface, that is, Cr layer of the disc (experiments 1, 3 and 5). However, the adsorption forces of the proteins in natural form are quite weak; therefore, due to the rotation of disc
against the stationary ball, the adsorbed proteins are wiped off. These are then accumulated in the inlet region of the contact zone and lead to a strong growth of lubricant film thickness and creation of a new protein phase. The subsequent decrease in film thickness is probably caused by formation of a passivation layer, which protects the contact surfaces from further adsorption of proteins in the lubricant. The layer can be composed of adsorbed denatured proteins or another type of protein, for example, g-globulin having a higher adsorption force. Generally, the problems of different albumin and g-globulin adsorption are considerably complicated, especially for BS fluid, as is interpreted in Nakashima et al.28 and Yarimitsu et al.33,34 On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour (experiments 2 and 4). In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. Therefore, for obtaining relevant data about BS film formation, the wettability of glass disc coating must be very close to the real artificial hip joint components.
Effect of surface geometry The effect of wettability of surfaces is not the only parameter that can influence the obtained results within optical test rig. The other one is the geometry of transparent disc. It should be mentioned that it could influence lubricant supply to the contact. Once the disc is slowly rotating, BS on its surface can change properties – it can be dried out before entering the contact again. So that observed protein film formation can be connected with the disc geometry and its movement. Therefore, for confirmation of this hypothesis, another experiment was realized where both the disc with chromium layer and the CoCrMo ball were fully bathed in BS lubricant. The results of this experiment are plotted in Figure 7(a) (black points) and are also compared with experiment where BS was supplied to the contact through the syringe pump (see Figure 7(a), blue points). From the comparison of the results, it can be obvious that the use of full bath leads to a rapid increase of film thickness up to 640 nm already at 5 s of measurement. After that, lubricant film thickness is gradually decreased to a few nanometres, and at the end of the measurement, the film thickness periodically oscillates around 20 nm. Characteristic interferograms, which describe the composition of lubricant film in the contact zone under fully bathed conditions, are shown in Figure 7(c). A smaller accumulation of proteins can be seen in the inlet region (see Part A of Figure 7(c)) compared to the experiment, where BS was supplied through syringe pump (see Part A of Figure 7(b)); nevertheless, the global behaviour of lubricant film within the contact zone is almost identical. It can be concluded that a fully bathed disc surface in BS lubricant has a relatively negligible effect on the formation of thicker
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Figure 7. Film thickness behaviour as a function of time for different types of BS supply to the contact, under pure sliding conditions (S = 2) and mean speed of 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) BS was supplied to the contact zone by the syringe pump with a constant flow rate of 3.5 mL/min within a period of 0–180 s and (c) the disc and ball were fully bathed. The inlet region is on the left. BS: bovine serum.
lubricating film. Reduced accumulation of protein aggregations in the inlet region and lower maximum values of film thickness can be caused by washing of protein aggregations away from the disc surface after their passage through the contact zone. Moreover, disc geometry influences the pressure between contacting bodies. Previous experiments were realized using optical test rig in the ball-on-disc configuration A or B (Figure 1), so that the mean Hertzian pressures between the metal or ceramic heads and the glass disc were relatively high (153–177 MPa) than pressures that are common for more conformal surfaces of artificial hip components. Therefore, the optical test rig was modified on configuration C (Figure 1), and following experiments were realized under more conformal geometry – that is, between lens with radius of 15.6 mm (coated with chromium layer) and CoCrMo ball with diameter of 28 mm. This geometric configuration then causes requested low mean Hertzian pressure about 37 MPa. Moreover, in the ball-on-lens configuration, the lens is fixed stationary, and the ball rotates against the lens (S = 22); therefore, the kinematic conditions are different from configuration B, while quite close to a function of total hip replacement.
Figure 8. Film thickness behaviour as a function of time for ball-on-lens configuration, under pure sliding conditions (S = 22) and mean speeds of 40 and 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) for mean speed of 40 mm/s and (c) for mean speed of 10 mm/s. The inlet region is on the left.
From the graph (Figure 8(a)), it is evident that film thickness behaviour is absolutely different from results performed on the ball-on-disc test device (see, for example, Figure 3(a), blue points). At the beginning of measurement, the relatively thick lubricant film is formed due to proteins, which are adsorbed on contact surfaces (Part A of Figure 8(b) and (c)). After very short time, the lubricant film is stabilized, and average central film thickness is kept about constant value of 40 nm for mean speed of 40 mm/s and 30 nm for mean speed of 10 mm/s (Part B of Figure 8(b) and (c)). In this phase of experiments, the lubricant film is formed predominantly due to a hydrodynamic effect (without massive accumulation of protein aggregations), which is more pronounced under higher mean speed. Mentioned behaviour of lubricating film can be caused not only due to more conformal geometry of contacting bodies and lower pressure but also due to kinematic conditions (the ball rotates against the stationary chromium coated lens) and due to sufficient amount of BS
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lubricant in the surrounding of contacting bodies (dimension of lens is much smaller than the disc). Relatively thick protein film that is formed at the beginning of experiments (i.e. under start up conditions) can be very beneficial because it can protect the rubbing surfaces of artificial hip joint components against a wear, for example, in the case of change of gait cycle from a stance phase to a swing phase. Then, during the swing phase, where the hip components have adequate mean speed, the lubricant film formation is based only on the hydrodynamic effect. It is obvious that geometry of the transparent contact surface has significant effect on the BS lubricant film formation. Therefore, for obtaining relevant data about BS film formation, the conformity of transparent body and the artificial head must be very close. Hence, use of non-conformal ball-on-disc configuration is not suitable for another experimental research programme.
Conclusion The aim of this study was to consider the relevance of in situ measurements of BS film thickness in the optical test device that could be related to the function of the artificial hip joint. Film thickness measurements were performed using different configurations of optical test device as a function of time. The individual experiments were focussed especially on influence of material and geometry of contacting surfaces and influence of kinematic conditions. From the obtained results, the following conclusions can be drawn.
thickness. In the more conformal ball-on-lens configuration, the lubricant film is formed predominantly due to hydrodynamic effect; thereby, the film thickness is kept constant during measurement. Only within very short time period after beginning of the measurement, the film is formed by thicker layer of adsorbed proteins, which is quite quickly taken away. From performed study, it can be concluded that optical test rig and thin film colorimetric interferometry can be successfully used for in situ observations of lubricant film formation inside the artificial hip joints. However, for obtaining relevant results, the wettability and conformity of transparent surface and also kinematic conditions must be very close to the real artificial hip joints. Such experimental approach then enables further study of other factors such as surface topography, transient speed and loading, composition, temperature and oxidation of the BS solution. Some of them are currently under investigation. Acknowledgements The authors thank D Bosa´k and J Lasˇ tu˚vka for the film thickness measurement. Declaration of conflicting interests The authors declare that there is no conflict of interest.
Funding 1.
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It was clarified that a chromium layer covering the glass disc shows a hydrophobic behaviour, which supports the adsorption of proteins contained in the BS solution; thereby, a thicker lubricating film is formed. On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour. In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. A relatively different wettability of the metal and ceramic balls has no substantial effect on global behaviour of lubricating film for both types of chromium and silica disc coatings. It was confirmed that either the supply of BS lubricant to the contact zone using a syringe pump or a fully bathed disc and ball contact surfaces in BS lubricant show a qualitatively similar film thickness behaviour; however, the fully bathed disc surface causes washing away of protein aggregations; thereby, the film thickness reaches lower values. It was confirmed that conformity of contacting surfaces and kinematic conditions has fundamental effect on BS film formation. In the ball-on-disc configuration, the lubricant film is formed predominantly due to protein aggregations, which pass through the contact zone and increase the film
This research was supported by the project ‘The influence of joint fluid composition on formation of lubricating film in THA’ (NT/14267-3/2013) financed by the Internal Grant Agency of the Ministry of Health of the Czech Republic and by the project ‘NETME – New Technologies for Mechanical Engineering’ (CZ.1.05/ 2.1.00/01.0002) financed by the European Regional Development Fund.
References 1. Goldsmith AAJ, Dowson D, Isaac GH, et al. A comparative joint simulator study of the wear of metal-onmetal and alternative material combinations in hip replacements. Proc IMechE, Part H: J Engineering in Medicine 2000; 214: 39–47. 2. Kaddick C and Wimmer MA. Hip simulator wear testing according to the newly introduced standard ISO 14242. Proc IMechE, Part H: J Engineering in Medicine 2001; 215: 429–442. 3. Dowson D, Hardaker C, Flett M, et al. A hip joint simulator study of the performance of metal-on-metal joints. Part I: the role of materials. J Arthroplasty 2004; 19: 118–123. 4. Dowson D, Hardaker C, Flett M, et al. A hip joint simulator study of the performance of metal-on-metal joints. Part II: design. J Arthroplasty 2004; 19: 124–130.
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5. Catelas I, Medley JB, Campbell PA, et al. Comparison of in vitro with in vivo characteristics of wear particles from metal-metal hip implants. J Biomed Mater Res B Appl Biomater 2004; 70: 167–178. 6. Essner A, Sutton K and Wang A. Hip simulator wear comparison of metal-on-metal, ceramic-on-ceramic and crosslinked UHMWPE bearings. Wear 2005; 259: 992– 995. 7. Li CX, Hussain A and Kamali A. A hip simulator study of metal-on-metal hip joint device using acetabular cups with different fixation surface conditions. Proc IMechE, Part H: J Engineering in Medicine 2011; 225: 877–887. 8. Fisher J. A stratified approach to pre-clinical tribological evaluation of joint replacements representing a wider range of clinical conditions advancing beyond the current standard. Faraday Discuss 2012; 156: 59–68. 9. Saikko V, Ahlroos T, Revitzer H, et al. The effect of acetabular cup position on wear of a large-diameter metal-on-metal prosthesis studied with a hip joint simulator. Tribol Int 2013; 60: 70–76. 10. Loving L, Lee RK, Herrera L, et al. Wear performance evaluation of a contemporary dual mobility hip bearing using multiple hip simulator testing conditions. J Arthroplasty 2013; 28: 1041–1046. 11. Affatato S, Spinelli M, Zavalloni M, et al. Tribology and total hip joint replacement: current concepts in mechanical simulation. Med Eng Phys 2008; 30: 1305–1317. 12. Hall RM and Unsworth A. Friction in hip prostheses. Biomaterials 1997; 18: 1017–1026. 13. Scholes SC, Unsworth A, Hall RM, et al. The effects of material combination and lubricant on the friction of total hip prostheses. Wear 2000; 241: 209–213. 14. Brockett C, Williams S, Jin Z, et al. Friction of total hip replacements with different bearings and loading conditions. J Biomed Mater Res B Appl Biomater 2007; 81: 508–515. 15. Mazzucco D and Spector M. The John Charnley Award Paper: the role of joint fluid in the tribology of total joint arthroplasty. Clin Orthop Relat Res 2004; 429: 17–32. 16. Karuppiah KSK, Sundararajan S, Xu Z-H, et al. The effect of protein adsorption on the friction behavior of ultra-high molecular weight polyethylene. Tribol Lett 2006; 22: 181–188. 17. Scholes SC and Unsworth A. The effects of proteins on the friction and lubrication of artificial joints. Proc IMechE, Part H: J Engineering in Medicine 2006; 220: 687–693. 18. Roba M, Naka M, Gautier E, et al. The adsorption and lubrication behavior of synovial fluid proteins and glycoproteins on the bearing-surface materials of hip replacements. Biomaterials 2009; 30: 2072–2078. 19. Fan J, Myant CW, Underwood R, et al. Inlet protein aggregation: a new mechanism for lubricating film formation with model synovial fluids. Proc IMechE, Part H: J Engineering in Medicine 2011; 225: 696–709.
20. Myant C, Underwood R, Fan J, et al. Lubrication of metal-on-metal hip joints: the effect of protein content and load on film formation and wear. J Mech Behav Biomed Mater 2012; 6: 30–40. 21. Myant C and Cann P. In contact observation of model synovial fluid lubricating mechanisms. Tribol Int 2013; 63: 97–104. 22. Fan J, Myant C, Underwood R, et al. Synovial fluid lubrication of artificial joints: protein film formation and composition. Faraday Discuss 2012; 156: 69–85. 23. Vrbka M, Na´vrat T, Krˇ upka I, et al. Study of film formation in bovine serum lubricated contacts under rolling/ sliding conditions. Proc IMechE, Part J: J Engineering Tribology 2013; 227: 459–475. 24. Hartl M, Krˇ upka I, Polisˇ cˇuk R, et al. An automatic system for real-time evaluation of EHD film thickness and shape based on the colorimetric interferometry. Tribol T 1999; 42: 303–309. 25. Hartl M, Krˇ upka I, Polisˇ cˇuk R, et al. Thin film colorimetric interferometry. Tribol T 2001; 44: 270–276. 26. Sethuraman A, Han M, Kane RS, et al. Effect of surface wettability on the adhesion of proteins. Langmuir 2004; 20: 7779–7788. 27. Nakanishi K, Sakiyama T and Imamura K. On the adsorption of proteins on solid surfaces, a common but very complicated phenomenon. J Biosci Bioeng 2001; 91: 233–244. 28. Nakashima K, Sawae Y and Murakami T. Study on wear reduction mechanisms of artificial cartilage by synergistic protein boundary film formation. JSME Int J C: Mech Sy 2005; 48: 555–561. 29. Nakashima K, Sawae Y and Murakami T. Effect of conformational changes and differences of proteins on frictional properties of poly(vinyl alcohol) hydrogel. Tribol Int 2007; 40: 1423–1427. 30. Heuberger MP, Widmer MR, Zobeley E, et al. Proteinmediated boundary lubrication in arthroplasty. Biomaterials 2005; 26: 1165–1173. 31. Fang H-W, Hsieh M-C, Huang H-T, et al. Conformational and adsorptive characteristics of albumin affect interfacial protein boundary lubrication: from experimental to molecular dynamics simulation approaches. Colloids Surf B Biointerfaces 2009; 68: 171–177. 32. Mishina H and Kojima M. Changes in human serum albumin on arthroplasty frictional surfaces. Wear 2008; 265: 655–663. 33. Yarimitsu S, Nakashima K, Sawae Y, et al. Effects of lubricant composition on adsorption behavior of proteins on rubbing surface and stability of protein boundary film. Tribol Online 2008; 3: 238–242. 34. Yarimitsu S, Nakashima K, Sawae Y, et al. Influences of lubricant composition on forming boundary film composed of synovia constituents. Tribol Int 2009; 42: 1615– 1623.
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In situ measurements of thin films in bovine serum lubricated contacts using optical interferometry
Proc IMechE Part H: J Engineering in Medicine 2014, Vol. 228(2) 149–158 Ó IMechE 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954411913517498 pih.sagepub.com
Martin Vrbka1, Ivan Krˇupka1, Martin Hartl1, Toma´sˇ Na´vrat1, Jirˇ´ı Gallo2 and Ade´la Galanda´kova´2
Abstract The aim of this study is to consider the relevance of in situ measurements of bovine serum film thickness in the optical test device that could be related to the function of the artificial hip joint. It is mainly focussed on the effect of the hydrophobicity or hydrophilicity of the transparent surface and the effect of its geometry. Film thickness measurements were performed using ball-on-disc and lens-on-disc configurations of optical test device as a function of time. Chromatic interferograms were recorded with a high-speed complementary metal-oxide semiconductor digital camera and evaluated with thin film colorimetric interferometry. It was clarified that a chromium layer covering the glass disc has a hydrophobic behaviour which supports the adsorption of proteins contained in the bovine serum solution, thereby a thicker lubricating film is formed. On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour. In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. Metal and ceramic balls have no substantial effect on lubricant film formation although their contact surfaces have relatively different wettability. It was confirmed that conformity of contacting surfaces and kinematic conditions has fundamental effect on bovine serum film formation. In the ball-on-disc configuration, the lubricant film is formed predominantly due to protein aggregations, which pass through the contact zone and increase the film thickness. In the more conformal ball-on-lens configuration, the lubricant film is formed predominantly due to hydrodynamic effect, thereby the film thickness is kept constant during measurement.
Keywords Total hip replacement, conformity of surfaces, hydrophobicity, hydrophilicity, bovine serum, protein formation, lubrication, film thickness, colorimetric interferometry
Date received: 17 July 2013; accepted: 19 November 2013
Introduction Real hip joint simulators play an important role in the preclinical validation phase. These simulators are especially useful for wear analyses of different types of artificial hip components.1–10 The hip joint wear simulators combine a set of motions, loads and a lubricant that create tribological conditions comparable, but not necessarily identical, to those occurring in vivo.11 Simulators currently in use differ from each other in many parameters: number of stations, loading, degree of freedom, ball-cup relative position and temperaturecontrolled test fluid baths for each hip joint assembly.11 Some simulators are developed in order to measure friction between the joint bodies as well.12–14 However, both types of simulator can be hardly used for the in situ observation of the fundamental lubrication
mechanisms. That is why there is still demand to involve other experimental approaches to enable such a study. It is widely recognized that synovial fluid proteins play an important role in the lubrication processes of artificial hip joints.15–18 Recent results have shown that real-time measurement of protein film formation based
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Faculty of Mechanical Engineering, Brno University of Technology, Brno, Czech Republic 2 Orthopaedic Clinic, University Hospital Olomouc, Olomouc, Czech Republic Corresponding author: M Vrbka, Faculty of Mechanical Engineering, Brno University of Technology, Technicka´ 2896/2, 616 69 Brno, Czech Republic. Email: [email protected]
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on optical interferometry is very suitable method that can help to understand lubrication mechanisms within hip joint replacements.19–23 Fan et al.19 investigated the role of proteins in the lubrication process. Film thickness was measured by optical interferometry in a ball-on-disc device where commercial CoCrMo femoral head was used as a stationary component against a rotating glass disc (mean speed range of 2–60 mm/s). The results for bovine serum (BS) showed a complex time-dependent behaviour, which was not representative of simple fluid. After a few minutes, sliding BS formed a thin adherent film of 10–20 nm, which was attributed to protein absorbance at the surface. This layer was augmented by a hydrodynamic film, which often increased at slow speeds. At the end of the test, deposited surface layers of 20–50 nm were measured. The authors suppose that the film formation is dominated by surface deposition and shear-flow aggregation of protein molecules. These molecules are aggregated in the inlet shear field and come out of the solution to form gel-like deposits in the inlet; this material adheres to the metal surface and periodically passes through the contact, forming a much thicker hydrodynamic film. The authors also note the fact that protein deposits are formed in the inlet of the contact in the low-pressure region. Myant et al.20 performed measurements of central film thickness by optical interferometry as a function of time (constant mean speed 0 and 10 mm/s) and variable mean speed (0–50 mm/s) for series of BS and proteincontaining (albumin, globulin) saline solutions and for CoCrMo femoral component sliding against a glass disc. The effect of load on film thickness was also studied. The results showed that the film thickness increased with time for both the static and sliding tests. In the sliding tests, a wear scar rapidly formed on the implant component for the BS and albumin fluids; negligible wear was observed for globulin solutions. The authors point out that the film thickness decreased rapidly with increasing load for all fluids, and they also supported the idea of the protein-aggregation lubricated mechanism introduced in Fan et al.19 Myant and Cann21 enhanced observations of the film formation mechanisms in a model metal-on-metal hip joint lubricated with BS. They showed that model synovial fluid solutions demonstrate a complex timedependent film thickness behaviour that is not characteristic of a simple Newtonian fluid. The results indicated that two types of films are formed: a boundary layer of adsorbed protein molecules, which are augmented by a high-viscosity fluid film generated by hydrodynamic effects. This high-viscosity film is due to inlet aggregation of protein molecules forming a gel which is entrained into the contact preferentially at low speeds. As the speed increases, this gel appears to shear thin, giving much lower lubricant film thickness. These results suggested that protein-containing fluids do not obey classical Newtonian elastohydrodynamic lubrication (EHL) models.
Fan et al.22 critically examined synovial fluid lubrication mechanisms and the effect of synovial fluid chemistry which includes protein content and pH. They confirmed two distinct film formation mechanisms reported in earlier articles19–21 and concluded that patient synovial fluid chemistry plays an important role in determining implant wear and the likelihood of failure. Moreover, they pointed out that at present, the function of synovial fluid properties in determining implant performance is implicitly ignored in all the lubrication models. In the previous study,23 the authors performed detailed experimental analysis of lubricant film thickness of BS within the contact between the artificial metal and ceramic heads and the glass disc to analyse the effect of proteins on film formation under various rolling/sliding conditions using thin film colorimetric interferometry. Film thickness was studied as a function of time. From the performed experiments, it was confirmed that the formation of protein film thickness is markedly dependent on kinematic conditions acting in the contact, especially on the positive and negative slide-to-roll ratio and the mean speed. From the cited literature,19–23 it is apparent that optical technique has proved to be a valuable experimental tool in the study of lubrication mechanisms in the artificial joints. It is a technique that gives detailed information about the lubricant film distribution within the contact, and especially, only this method is able to provide experimental data about protein film formation. However, one of the contact bodies must be optically transparent (glass or sapphire) and coated with a semi-reflective layer (chromium or silica), thereby the formation of protein film can be significantly affected. Moreover, design of the optical test devices is generally based on classical ball-on-disc EHL simulators where lubrication mechanisms are examined under higher pressures between the ball and the flat transparent surface (non-conformal contact). Therefore, the aim of this study is to verify the use of optical test device for measurement of BS protein film in relation to the function of the artificial hip joint. It is mainly focussed on the effect of the hydrophobicity or hydrophilicity of the transparent surface and the effect of its geometry.
Experimental method and test programme Several optical test rig configurations were involved in this study (Figure 1). Formation of lubricating film of BS solution was observed using an optical test rig in which a circular contact is realized between a glass disc or lens and a metal or ceramic head of total hip joint replacement (Figure 1). BS (Sigma–Aldrich B9433, protein concentration 55.5 mg/mL) and sterile water were used for preparing sample with appropriate w/w concentration. BS concentration of 25% with a total
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Figure 1. Film thickness measurement using optical test rig. BS: bovine serum; CMOS: complementary metal-oxide semiconductor.
Table 1. Performed experiments for various material combinations of balls and disc coatings. Experiment
Ball, diameter (mm)
Glass body, coating
BS supply
1 2 3 4 5 6 7
CoCrMo, 36 CoCrMo, 36 Al2O3, 32 Al2O3, 32 CoCrMo, 36 CoCrMo, 36 CoCrMo, 28
Disc, Cr Disc, Cr + SiO2 Disc, Cr Disc, Cr + SiO2 Disc, Cr + SiO2 + hydrophobic modification Disc, Cr Lens with radius of 15.6 mm, Cr
Syringe pump Syringe pump Syringe pump Syringe pump Syringe pump Fully bathed Syringe pump
BS: bovine serum.
protein content of 13.9 mg/mL was prepared in volumes of 12 mL and immediately stored in a freezer at 220 °C. The lower surface of the glass disc or lens was coated with a thin semi-reflective chromium layer with a different thickness according to the use of either the metal or the ceramic head, and the upper side had an antireflective coating. For some experiments, the chromium layer of glass disc was coated with a silica layer (SiO2) with additional thickness of 150 nm. Artificial femoral heads, in the following text marked as balls, Zimmer (CoCrMo – ProtasulÒ-20/ISO 583212/36 mm in diameter), AESCULAP NK561 (Al2O3 – BioloxÒ forte/ISO 6474/32 mm in diameter) and AESCULAP NK430K (CoCr29Mo/ISO 5832-12/28 mm in diameter) were delivered in the original package from the manufacturer. Prior to film thickness experiments, surface topography of the metal and ceramic balls was analysed in detail using the optical measurement method based on phase shifting interferometry that provides topography data with the accuracy below
1 nm (apparatus Bruker ContourGT-X8). Evaluated average surface roughness Ra was between 0.0011 and 0.0038 mm for the metal and between 0.0017 and 0.0021 mm for the ceramic balls. It can be noted that the disc and lens surfaces were optically smooth. The ball (diameter of 36 mm for CoCrMo and diameter of 32 mm for Al2O3) was fixed stationary, and the glass disc was rotated against the ball by a servomotor to provide required pure sliding conditions under mean speed of 10 mm/s (Figure 1, configuration B). The ball was held in a positioning holder, and for each test, it could be rotated in a different position. In the case of the use of the concave lens with radius of 15.6 mm, the CoCrMo ball, having diameter of 28 mm, was rotated against the stationary lens under mean speed of 10 and 40 mm/s (Figure 1, configuration C). Film thickness was studied as a function of time for various material combinations of contact surfaces (Table 1), where the total time of each measurement was 5 min. Experiments were realized at room temperature of 24 °C under steady-state
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load corresponding to mean Hertzian pressure of 153– 177 MPa for ball-on-disc configuration and 37 MPa for ball-on-lens configuration. The room temperature of 24 °C was set to avoid temperature fluctuations due to low volume of BS used in experiments (it was confirmed that BS film behaviour under room temperature of 24 °C and under body temperature of 37 °C has the same tendency). All components, which are in contact with BS (e.g. glass disc, lens, ball, holder), were cleaned in 1% w/w sodium dodecyl sulphate, rinsed in distilled water and then washed in isopropyl alcohol before assembly. This cleaning process was always repeated between the individual measurements. Test fluids were taken out from the freezer 2 h before measurements and then were supplied to the contact zone in the amount of 3.5 mL/min through a syringe coupled with a needle (a syringe pump) for a period of 3 min. Experiment 6 (see Table 1) was realized with a larger amount of the test fluid where the disc and the ball were fully bathed. A new sample of test fluid was used for each measurement. The contact formed between the glass disc or lens and the ball was illuminated by xenon lamp. The obtained chromatic interferograms were recorded with a high-speed complementary metal-oxide semiconductor (CMOS) digital camera and evaluated using a thin film colorimetric interferometry (Figure 1). A detailed description of this technique is given in Hartl et al.24,25 The contact zone was recorded with a frequency of 24 Hz for 5 min. During this time, 7200 interferograms were recorded for each measurement. From each performed measurement, about 50 interferograms were selected and then the film thickness was evaluated.
Results and discussion In the previous study,23 the effect of proteins on film formation under various rolling/sliding conditions was studied using ball-on-disc optical test rig in configuration A (Figure 1). Film thickness measurements as a function of time for three values of slide-to-roll ratio (S = 0, 1.5 and 21.5) are displayed in Figure 2 for CoCrMo ball having diameter of 28 mm and for mean speed of 5.7 mm/s. The individual points in the graph in Figure 2(a) correspond to average film thickness from the central area of the contact zone, and the marked grey region (0–180 s) represents the time of BS supply to the contact zone. Formation and evolution of film thickness in the whole contact zone dependent on time are illustrated in Figure 2(b)–(d) by individual interferograms. Under pure rolling conditions (S = 0), the film thickness increases gradually with time, and at the end of measurement (after 5 min), the central film thickness achieved the value of about 25 nm (Part C of Figure 2(b)). Under rolling/sliding conditions, for the case where the disc is faster than the ball (S = 1.5), at first, the film thickness increases rapidly, and when the maximum film thickness about 140 nm is reached (Part B of Figure 2(c)), then this effect is lost and the film
Figure 2. Film thickness behaviour as a function of time for three values of slide-to-roll ratio (S) and mean speed of 5.7 mm/s.23 (a) Central film thickness development. Chromatic interferograms: (b) S = 0, (c) S = 1.5 and (d) S = 21.5. The inlet region is on the left. BS: bovine serum.
thickness starts to decrease, and finally, at the end of the measurement, the film thickness drops to a few nanometres (Part C of Figure 2(c)). Under rolling/sliding conditions, for the case where the ball is faster than the disc (S = 21.5), an absolutely different formation of BS film thickness is observed. Under these conditions, the average central film thickness about 5 nm remains during the measurement approximately constant even if the fresh BS is supplied to the contact (Figure 2(d)). It should be noted that for ceramic ball, the development of lubricant film thickness was qualitatively very similar as with the metal ball. From the performed experiments under rolling/sliding conditions, it is obvious that the formation of lubricant film thickness is markedly dependent on kinematic conditions acting in the contact, especially on the positive and negative slide-to-roll ratio. However, the behaviour of BS film in the contact can be significantly
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Figure 3. Film thickness behaviour as a function of time for two types of disc coatings and CoCrMo head, under pure sliding conditions (S = 2) and mean speed of 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) glass disc with chromium layer and (c) glass disc with silica layer. The inlet region is on the left. BS: bovine serum.
affected by optically transparent disc, which is coated with a chromium semi-reflective layer. Moreover, in reality, a relative motion between the artificial hip components is mostly pure sliding (S = 62). Therefore, in this study, other experiments were performed on the optical test rig in configuration B, where the metal or ceramic ball is fixed and the disc rotates against the ball, with the aim to explain the effect of disc coating on protein film formation.
Effect of surface wettability At first, film thickness was studied using a combination of CoCrMo ball and glass disc with chromium layer. From the graph in Figure 3(a) (blue points), it is possible to observe that the central film thickness shows an initial intensive growth to the maximal value of 880 nm in time of 50 s. Then, the film thickness starts to decrease, and in the last 100 s, the film thickness reaches the average value of around 50 nm. This measurement was carried out in a standard way when the BS was supplied to the contact zone by the syringe pump with a constant flow rate of 3.5 mL/min for the period of 0–180 s (in Figure 3(a), it is marked as a grey region). Interferograms captured at different times (Figure 3(b)) show a typical composition of lubricating
film during the experiment. After beginning of the measurement, there is a visible massive accumulation of protein aggregations in the inlet region of contact zone, which causes the growth of film thickness (Part A of Figure 3(b)). After that, the protein aggregations gradually pass through the contact zone, and a total amount of accumulated protein in the inlet region is reduced, and subsequently, the lubricant film thickness is decreased (Part B of Figure 3(b)). As the protein aggregations were passing through the contact zone, they were adsorbed especially on the chromium layer of the glass disc. During the experiment, these aggregations then formed a lubricating layer, which affected the film thickness at the end of the measurement (Part C of Figure 3(b)). Next experiment was also carried out with CoCrMo ball, but the glass disc with chromium layer was overlaid by silica (SiO2) with thickness of 150 nm. Development of average central film thickness is plotted in Figure 3(a) (black points), and selected interferograms are displayed in Figure 3(c). Although the experiments were performed under the same kinematic conditions, absolutely different results can be observed. Within the whole experiment, the lubricant film thickness in the contact zone reached the values of only a few nanometres; occasionally, some local spots passing through the contact zone were visible (Figure 3(c)). These spots could be small protein aggregations caused by disruption of a natural protein structure by pressure and shear stress in the contact zone. However, noticeable aggregations and accumulations of proteins in the inlet region of contact zone were not observed. It can be concluded that the change in the material of disc layer causes considerable changes in film thickness behaviour. These results were supported by measurements performed with the ceramic ball (Figure 4). It was suggested from the above-described results that properties of contacting surfaces play an important role and can influence the results obtained from optical test rig. That is why wettability of individual contact surfaces was examined using a contact angle measurement method (Figure 5). A drop of distilled water with a volume of 15 mL was applied on the contact surfaces of disc and ball. Individual drops were subsequently recorded by charge-coupled device (CCD) camera, and the evaluated results were summarized in Table 2. From this table, it is evident that the surface of the disc with a chromium layer is hydrophobic (the contact angle is greater than 90°), whereas the disc with a silica layer is strongly hydrophilic (the contact angle is close to 30°). From this, it can be deduced that the hydrophobicity has a considerable influence on the formation of protein aggregations and possibly on the adsorption of the proteins on a disc layer. For verification of this hypothesis, a new experiment was realized with the glass disc where a micro-emulsion based on silicone with 0.4% w/w sulphuric acid was applied on the silica layer. This micro-emulsion makes a hydrophobic layer with contact angle about 100° (see Table 2).
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Proc IMechE Part H: J Engineering in Medicine 228(2) Table 2. Measurement of wettability of ball and disc contact surfaces. Contact surface
Contact angle (°)
Ball, CoCrMo Ball, Al2O3 Glass disc, Cr layer Glass disc, SiO2 layer Glass disc, SiO2 layer + hydrophobic modification
71.6 6 2.5 60.4 6 1.7 94.5 6 2.5 28.7 6 2.0 100.5 6 2.4
Figure 4. Film thickness behaviour as a function of time for two types of disc coatings and Al2O3 head, under pure sliding conditions (S = 2) and mean speed of 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) glass disc with chromium layer and (c) glass disc with silica layer. The inlet region is on the left. BS: bovine serum.
Figure 6. Film thickness behaviour as a function of time for SiO2 and modified SiO2 disc coatings and CoCrMo head, under pure sliding conditions (S = 2) and mean speed of 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) glass disc with modified silica layer and (c) glass disc with silica layer. The inlet region is on the left. BS: bovine serum.
Figure 5. Wettability examination using a contact angle measurement method.
The results of the experiment with the modified disc are summarized in Figure 6(a) (blue points) and for clearness are compared with the disc without modification (see Figure 6(a), black points). Development of lubricant film thickness is very similar to the experiment, where the disc contains only chromium layer (see Figure 3(a), blue points). After beginning of the measurement, a rapid increase can be seen in the film thickness, up to 510 nm within the period of 50 s, and then the film thickness starts to decrease, and at the end of the measurement, it drops to the value of about 20 nm.
The above-mentioned behaviour of the lubricant film can be observed in the selected interferograms (Figure 6(b)), where especially the accumulation of protein aggregations in the inlet region is clearly visible (Part A of Figure 6(b)). These aggregations gradually pass through the contact zone, partially adsorbed on the modified silica layer of disc and form a lubricating film with thickness of about tens of nanometres (Parts B and C of Figure 6(b)). When the disc is without mentioned hydrophobic modification, these effects are not observed (see Figure 6(c)). From the presented results, a significant effect of materials of the contact pairs on the lubricant film
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formation was observed. Sethuraman et al.26 referred that with the decreasing wettability, a degree of protein adsorption has an increasing tendency. Consequently, the contact of BS solution with the hydrophobic disc surface (Cr layer) leads to the adsorption of proteins, and the thickness of this adsorbed layer can take the values from a few to tens of nanometres. Nakanishi et al.27 studied the protein adsorption on rigid surfaces; they concluded that the thickness of the human serum albumin (HSA) layer, adsorbed on a relatively hydrophobic surface of titanic oxide, is 18 nm. They also indicated that the adsorbed amount of proteins at the room temperature is dependent on the type of protein, surface and adsorption conditions. The adsorption force also plays an important role in the protein adsorption process. Nakashima et al.28,29 studied the friction of lubricants with a different amount of albumin and g-globulin proteins. They concluded that a different rate of adsorption is dependent on the weight proportion of individual proteins, whereas the individual proteins show a different adsorption force. While albumin binds to the surface with a relatively low force, g-globulin has a stronger adsorption. The authors also stated that albumin can generate a low shear layer, while g-globulin can form a tight layer which protects against wear. Heuberger et al.30 studied the adsorption of HSA in natural and denatured forms on articulating surfaces. They found out that the presence of denatured albumin in the solution leads to a significant reduction in the adsorbed amount. They supposed that the denatured albumin is adsorbed more easily onto the hydrophobic surface than the albumin in its natural form. A passivation layer formed by the adsorbed denatured albumin then protects from further adsorption. On the other hand, albumin in its natural form creates a thicker adsorbed film. Fang et al.31 carried out friction experiments using a pin-on-disc tribometer with various materials of the artificial replacements in the presence of the albumin lubricant. They showed that the presence of the denatured albumin leads to the increase of a friction coefficient where a layer of denatured albumin on friction surfaces was observed at the end of experiments. Fang et al. also confirmed that the denatured albumin forms a compact layer which attempts to maintain more on a hydrophobic surface compared to adsorbed natural albumin layer which decreases the coefficient of friction. The influence of the surface hydrophobicity on the protein adsorption and binding of denatured proteins on frictional surfaces is also supported by other studies.32 It is obvious that hydrophobicity and hydrophilicity of the disc surface play an important role in the film formation process. At the beginning of the measurement, the natural proteins, contained in the BS, were gradually adsorbed on the hydrophobic surface, that is, Cr layer of the disc (experiments 1, 3 and 5). However, the adsorption forces of the proteins in natural form are quite weak; therefore, due to the rotation of disc
against the stationary ball, the adsorbed proteins are wiped off. These are then accumulated in the inlet region of the contact zone and lead to a strong growth of lubricant film thickness and creation of a new protein phase. The subsequent decrease in film thickness is probably caused by formation of a passivation layer, which protects the contact surfaces from further adsorption of proteins in the lubricant. The layer can be composed of adsorbed denatured proteins or another type of protein, for example, g-globulin having a higher adsorption force. Generally, the problems of different albumin and g-globulin adsorption are considerably complicated, especially for BS fluid, as is interpreted in Nakashima et al.28 and Yarimitsu et al.33,34 On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour (experiments 2 and 4). In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. Therefore, for obtaining relevant data about BS film formation, the wettability of glass disc coating must be very close to the real artificial hip joint components.
Effect of surface geometry The effect of wettability of surfaces is not the only parameter that can influence the obtained results within optical test rig. The other one is the geometry of transparent disc. It should be mentioned that it could influence lubricant supply to the contact. Once the disc is slowly rotating, BS on its surface can change properties – it can be dried out before entering the contact again. So that observed protein film formation can be connected with the disc geometry and its movement. Therefore, for confirmation of this hypothesis, another experiment was realized where both the disc with chromium layer and the CoCrMo ball were fully bathed in BS lubricant. The results of this experiment are plotted in Figure 7(a) (black points) and are also compared with experiment where BS was supplied to the contact through the syringe pump (see Figure 7(a), blue points). From the comparison of the results, it can be obvious that the use of full bath leads to a rapid increase of film thickness up to 640 nm already at 5 s of measurement. After that, lubricant film thickness is gradually decreased to a few nanometres, and at the end of the measurement, the film thickness periodically oscillates around 20 nm. Characteristic interferograms, which describe the composition of lubricant film in the contact zone under fully bathed conditions, are shown in Figure 7(c). A smaller accumulation of proteins can be seen in the inlet region (see Part A of Figure 7(c)) compared to the experiment, where BS was supplied through syringe pump (see Part A of Figure 7(b)); nevertheless, the global behaviour of lubricant film within the contact zone is almost identical. It can be concluded that a fully bathed disc surface in BS lubricant has a relatively negligible effect on the formation of thicker
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Figure 7. Film thickness behaviour as a function of time for different types of BS supply to the contact, under pure sliding conditions (S = 2) and mean speed of 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) BS was supplied to the contact zone by the syringe pump with a constant flow rate of 3.5 mL/min within a period of 0–180 s and (c) the disc and ball were fully bathed. The inlet region is on the left. BS: bovine serum.
lubricating film. Reduced accumulation of protein aggregations in the inlet region and lower maximum values of film thickness can be caused by washing of protein aggregations away from the disc surface after their passage through the contact zone. Moreover, disc geometry influences the pressure between contacting bodies. Previous experiments were realized using optical test rig in the ball-on-disc configuration A or B (Figure 1), so that the mean Hertzian pressures between the metal or ceramic heads and the glass disc were relatively high (153–177 MPa) than pressures that are common for more conformal surfaces of artificial hip components. Therefore, the optical test rig was modified on configuration C (Figure 1), and following experiments were realized under more conformal geometry – that is, between lens with radius of 15.6 mm (coated with chromium layer) and CoCrMo ball with diameter of 28 mm. This geometric configuration then causes requested low mean Hertzian pressure about 37 MPa. Moreover, in the ball-on-lens configuration, the lens is fixed stationary, and the ball rotates against the lens (S = 22); therefore, the kinematic conditions are different from configuration B, while quite close to a function of total hip replacement.
Figure 8. Film thickness behaviour as a function of time for ball-on-lens configuration, under pure sliding conditions (S = 22) and mean speeds of 40 and 10 mm/s. (a) Central film thickness development. Chromatic interferograms: (b) for mean speed of 40 mm/s and (c) for mean speed of 10 mm/s. The inlet region is on the left.
From the graph (Figure 8(a)), it is evident that film thickness behaviour is absolutely different from results performed on the ball-on-disc test device (see, for example, Figure 3(a), blue points). At the beginning of measurement, the relatively thick lubricant film is formed due to proteins, which are adsorbed on contact surfaces (Part A of Figure 8(b) and (c)). After very short time, the lubricant film is stabilized, and average central film thickness is kept about constant value of 40 nm for mean speed of 40 mm/s and 30 nm for mean speed of 10 mm/s (Part B of Figure 8(b) and (c)). In this phase of experiments, the lubricant film is formed predominantly due to a hydrodynamic effect (without massive accumulation of protein aggregations), which is more pronounced under higher mean speed. Mentioned behaviour of lubricating film can be caused not only due to more conformal geometry of contacting bodies and lower pressure but also due to kinematic conditions (the ball rotates against the stationary chromium coated lens) and due to sufficient amount of BS
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lubricant in the surrounding of contacting bodies (dimension of lens is much smaller than the disc). Relatively thick protein film that is formed at the beginning of experiments (i.e. under start up conditions) can be very beneficial because it can protect the rubbing surfaces of artificial hip joint components against a wear, for example, in the case of change of gait cycle from a stance phase to a swing phase. Then, during the swing phase, where the hip components have adequate mean speed, the lubricant film formation is based only on the hydrodynamic effect. It is obvious that geometry of the transparent contact surface has significant effect on the BS lubricant film formation. Therefore, for obtaining relevant data about BS film formation, the conformity of transparent body and the artificial head must be very close. Hence, use of non-conformal ball-on-disc configuration is not suitable for another experimental research programme.
Conclusion The aim of this study was to consider the relevance of in situ measurements of BS film thickness in the optical test device that could be related to the function of the artificial hip joint. Film thickness measurements were performed using different configurations of optical test device as a function of time. The individual experiments were focussed especially on influence of material and geometry of contacting surfaces and influence of kinematic conditions. From the obtained results, the following conclusions can be drawn.
thickness. In the more conformal ball-on-lens configuration, the lubricant film is formed predominantly due to hydrodynamic effect; thereby, the film thickness is kept constant during measurement. Only within very short time period after beginning of the measurement, the film is formed by thicker layer of adsorbed proteins, which is quite quickly taken away. From performed study, it can be concluded that optical test rig and thin film colorimetric interferometry can be successfully used for in situ observations of lubricant film formation inside the artificial hip joints. However, for obtaining relevant results, the wettability and conformity of transparent surface and also kinematic conditions must be very close to the real artificial hip joints. Such experimental approach then enables further study of other factors such as surface topography, transient speed and loading, composition, temperature and oxidation of the BS solution. Some of them are currently under investigation. Acknowledgements The authors thank D Bosa´k and J Lasˇ tu˚vka for the film thickness measurement. Declaration of conflicting interests The authors declare that there is no conflict of interest.
Funding 1.
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It was clarified that a chromium layer covering the glass disc shows a hydrophobic behaviour, which supports the adsorption of proteins contained in the BS solution; thereby, a thicker lubricating film is formed. On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour. In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. A relatively different wettability of the metal and ceramic balls has no substantial effect on global behaviour of lubricating film for both types of chromium and silica disc coatings. It was confirmed that either the supply of BS lubricant to the contact zone using a syringe pump or a fully bathed disc and ball contact surfaces in BS lubricant show a qualitatively similar film thickness behaviour; however, the fully bathed disc surface causes washing away of protein aggregations; thereby, the film thickness reaches lower values. It was confirmed that conformity of contacting surfaces and kinematic conditions has fundamental effect on BS film formation. In the ball-on-disc configuration, the lubricant film is formed predominantly due to protein aggregations, which pass through the contact zone and increase the film
This research was supported by the project ‘The influence of joint fluid composition on formation of lubricating film in THA’ (NT/14267-3/2013) financed by the Internal Grant Agency of the Ministry of Health of the Czech Republic and by the project ‘NETME – New Technologies for Mechanical Engineering’ (CZ.1.05/ 2.1.00/01.0002) financed by the European Regional Development Fund.
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