The number of endoprosthetic implants for both large and small joints is increasing at a steady rate, thereby creating a growing demand for durable products that closely replicate the functionality of human joints. Notwithstanding the aforementioned advancements, challenges pertaining to implant fixation and tribological surfaces persist. The advent of progressive technologies, such as three-dimensional printing, offers a promising avenue for addressing these challenges in implant design and surface engineering. Ti6Al4V, renowned for its biocompatibility and osseointegration properties, represents a promising printable material, although it is susceptible to wear on articulating surfaces. In order to mitigate the effects of abrasion, it is essential to implement surface treatments to facilitate the formation of a robust lubricating film. This research investigates the potential of texturing and electrochemical polishing to enhance protein aggregation in the contact area. The study employs a reciprocating simulator and colourimetric interferometry to observe the contact area and measure the coefficient of friction (CoF) of modified surfaces. The findings demonstrate that textured surfaces and the combination of electrochemical polishing result in an increase in the thickness of the protein lubrication film, which may potentially reduce wear. These outcomes suggest the potential for the utilization of Ti6Al4V alloy implants with fewer elements manufactured by additive technology.
Research Article
Effect of surface texturing on friction and lubrication of Ti6Al4V biomaterials for joint implants
https://doi.org/10.21203/rs.3.rs-4868535/v1
This work is licensed under a CC BY 4.0 License
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Ti6Al4V
micro texture
implant
optical interferometry
friction
film thickness
Joint arthroplasty is a common surgery with an increasing incidence. The materials used in arthroplasty have a high standard, with a good biocompatibility, and osseointegration ability. The topic of joint wear remains a significant area of research, with the development of new materials leading to a reduction in the amount of loose material. [1]. Additive Manufacturing (AM) have advanced this field by customizing the implants for better comfort and a longer service life [2]. Nevertheless, the advent of additive technologies has also introduced a number of new challenges. One such challenge is the identification of the optimal manufacturing parameters for the production of structures suitable for osseointegration or the improvement of the tribological properties of articulating surfaces [3].
The longevity of implants is affected by the tribological behavior of the articulating surfaces, and interaction with the counterbody surface. CoCrMo and Ti6Al4V alloys are typically used for endoprostheses manufactured by the selective laser melting (SLM) additive technology [4]. The CoCrMo alloy is known for its high wear resistance, relatively strong mechanical properties, and a good corrosion resistance [5]. Furthermore, the formation of a passive oxide film on the metal surface in the human body also contributes to a better wear resistance of the CoCrMo implants [6]. This oxide layer is mainly composed of cobalt, chromium, and molybdenum oxides. Among these, Cr2O3 inhibits both the anodic and cathodic reactions. It acts as a physical barrier, limiting the transport of cations and anions to the metal surface, and serves as an electronic barrier for electrons [7, 8]. Ti6Al4V is frequently used in oncological implants due to its mechanical properties, which are similar to those of a human bone. This minimizes the stress shielding and promotes the osseointegration [9]. However, the alloy is susceptible to a high abrasion on the articulating surfaces. This issue can be potentially resolved by various surface modifications, such as a coating or micro texturing. Both coating and micro texturing can enhance the mechanical properties and the durability of the implants. However, the coating techniques may suffer from an instability of the coating layer. The micro texturing can modify the behavior of the lubricating layer for a more sustainable performance, while also allowing the use of various materials.
The surface texturing has a positive effect on the tribological properties by increasing the hydrodynamic pressure and reducing the surface wear. This leads to the separation of the contact surfaces by a thicker layer of a lubricant containing proteins, which alters the lubrication regime and reduces wear. Simultaneously, the textures serve as lubricant reservoirs, facilitating the desired separation of the articulating surfaces [10, 11]. Additionally, the textures can capture and remove wear particles from the contact area. Aseptic wear particles are produced as a result of the abrasive wear of the materials, primarily in the boundary and mixed lubrication regimes. Carefully chosen textures can efficiently remove the particles from the contact area or retain them within the texture, thereby enabling the articulating surfaces to maintain a smooth surface topography for longer periods in vivo [12, 13].
Several studies have investigated the effect of the micro texturing on the coefficient of friction [14–16]. Some of these studies suggest an increase in friction, which is mainly caused by the behavior of the protein components of the synovial fluid as it passes through the contact. This behavior is due to the shear stress of the aggregated proteins adhering to the surface (γ-globulin) or to the further layering of the albumin with already a lower shear. Nevertheless, it is important to supplement the given conclusions with kinematic conditions, which have a significant influence on the behavior of the lubricating layer and the design of the texture [17–19].
The potential applications of micro texturing extend to a range of implants, with the possibility of it becoming a standard treatment for all articulating surfaces in the future. However, the current focus is mainly on small joint replacements, where the number of surgeries is steadily increasing year on year [20]. Developments in this field often provide solutions that do not reflect current trends in endoprosthetics and replace them with proven procedures that are often at the expense of patient comfort. A clear example is the metatarsophalangeal joint, which is often affected by hallux valgus and hallux rigidus. In such cases, two treatment options are simultaneously offered to the patient. One is arthrodesis, which results in the loss of joint functionality. The other option is a mobile replacement of the affected joint. Despite a number of disadvantages, arthrodesis is the most common solution due to its simplicity and reliability. This is because it does not contain interlocking moving parts [21]. These statistics also indicate the necessity for further development in the field of functional MTP replacements in order to make them the surgeons' preferred solution due to their reliability.
Micro texturing is largely dependent on the load and the rate of movement of the interacting surfaces. As for loading, the contact conformity, contact pressure and material are important. Micro texturing on hard materials such as the CoCrMo or Ti6Al4V is especially promising for the small joint implants, where the articulating surfaces are less exposed to contact pressure and ranges of motion. A study by Shereff et al. [22] described the kinematics of the metatarsophalangeal joint for patients with a joint disability. The average total range of motion in the sagittal plane was 111 degrees, with approximately 76 degrees of dorsal flexion and 34 degrees of plantar flexion. The implant attempts to maintain a full range of motion, which has a significant impact on a person's stability and proper foot function. Zhang et al. [23] conducted a numerical study on the pressure distribution. They found that increase in the contact pressure for patients affected by the hallux valgus disease, which also led to a higher risk of the joint damage. The pressure for the normal joint is 1.53 MPa and affected joint 2.21 MPa. In the case of the Morgan et al. research [24] (číslo citace), the contact pressures were higher. In the case of the cadaver tests, the values were in excess of 30 MPa, with numerical simulations showing values as low as 10 MPa in the 200–230 MPa load range. The values were significantly higher for joint implants, depending on the material used. The pressure on the implant was recalculated in relation to the geometry of the pair, based on the predictions from the healthy joint [16, 25].
The CoCrMo alloy shows a good abrasion resistance. However, since its mechanical properties differ significantly from those of a human bone, it is susceptible to tribocorrosion and aseptic loosening. Wang et al. [26] compared the CoCrMo alloys with the Ti alloys, specifically in terms of the tribocorrosion. The study concluded that the Ti alloy was better alternative to the CoCrMo alloy, due to its lower wear and potential for the health hazardous ions, such as Co(III) and Cr(VI). The Ti6Al4V exhibits a high wear rate [27, 28], which can be resolved by surface modifications or modifications that create a lubricating film to separate the joint surfaces.
Production and impact of the micro textures
Micro textures are frequently used in the arthroplasty of large joints as they offer a better surface wear resistance and lubrication compared to the smooth surfaces [27, 29–31]. For small replacements, the texturing has an even greater potential due to lower contact pressures. The most used texture shape is a circular or rounded dimple oriented in the slip direction. This is due to its locally increasing hydrodynamic pressures and manufacturing efficiency. The selected parameter is the aspect ratio between the texture depth (hp) and the diameter (dp), commonly referred to as ε. This ratio, in combination with the texture's surface coverage density (Sp) and the shape, determine the efficiency in different lubrication regimes during the implant cycle [32, 33]. It is expected that the friction reduction will be more pronounced for shallow dimples (3–10 µm) having smaller diameters (100–200 µm) [34]. The effect of surface textures on friction will be further driven mainly by the parameters of the textures. Depending on the aspect ratio ε, the textures will either provide an enhanced hydrodynamic effect
(ε < 0.1) or serve as a lubricant reservoir (ε > 0.1). Several studies suggest that an optimal ratio value is 0.1 or lower, depending on the material used [35]. However, the conclusions regarding the coverage density are not clear. Some studies proved that tribological properties can improve with an increasing texture density, while others showed the opposite trend, with improvement occurring with a decreasing density [36]. Qui et al. [37] conducted experiments on the conformal contact system under the boundary lubrication conditions. They investigated three texture densities of 26%, 41%, and 58% at an ε ratio of 0.1. The results showed that the lowest friction coefficient was achieved at 58%. Li et al. [38] investigated the effect of three different densities of hemispherical pits (5%, 13%, and 35%) at the ε ratio of 0.01. They found that the lowest friction coefficient was achieved with a density of 13%. Similarly, Raeymaekers et al. [39] found that densities of approximately 15% and ε ratios in the range of 0.1 to 0.3 produced the best results. Zhang et al. [40] proposed a patelloid texture produced by a pulsed laser ablation. Pin-on-plate tests demonstrated a long-term decrease in the coefficient of friction, resulting in a reduction of the wear rate in the hydrodynamic lubrication regime.
The lubrication properties are influenced not only by the density of the micro texture coverage but also by the arrangement of the texture. The textures are arranged according to the direction of movement, with longitudinal, transverse, or oblique orientation. The textures are arranged mainly in a square, triangular (hexagonal), or a random pattern. Choudhury et al. [41] investigated the distribution of the micro textures in square, circular, and triangular arrays on the hip replacements. They concluded that the square arrangement provided the best tribological properties. According to Braun et al. [42], the use of the circular textures of a suitable size in a triangular arrangement can lead to a reduction in the friction of up to 80%. Schneider et al. [35] concluded that a pitting aspect ratio of 10% results in a reduction in friction of 0.1. A triangular pattern is superior to a square arrangement, provided that the texture design is of high quality.
The texturing technology has a major influence on the overall function of the texture. Among micro machininig tilling method [43], laser machining is the most common technique. However, it has a drawback: it forms sharp corners that act as stress concentrators. This leads to two-body abrasive wear, where the sharp rims cause micro-scratches on the counter-surface, increasing the coefficient of friction (citace). To mitigate this issue, efforts are made to round the rims and reduce their negative impact. This text discusses different methods of the surface modification or rough surface preparation in titanium implants. The methods are based on the mechanical, thermal, chemical, electrochemical, and laser techniques. It should be noted that these methods do not only remove the rims caused by the laser machining but also modify the overall surface. [44, 45]. Over the past decade, the patented DLyte technology has garnered a significant attention. Unlike the conventional technologies, the DLyte only smooths the peaks of the roughness, not the valleys, through a selective surface smoothing. The DLyte operates on the principle of an ion transport by free solids, which is a combination of an electrical flow and a particle movement through an electrolytic medium [46–48].
Aim of the study
Additive methods together with the micro texturing and the DLyte technology have the potential to introduce a customized, on-order manufacturing of the implants. However, it is crucial to investigate closer the behavior of the specific alloys that are suitable for additive production, such as the CoCrMo and Ti6Al4V alloys, in joint implant simulations.
This study investigates the behavior of synovial fluid in the contact region of textured specimens made of Ti6Al4V alloy using colorimetric interferometry and friction coefficient. Colorimetric interferometry was proven to bring an essential insight into assessing lubrication mechanisms in hip replacements before [49]. Subsequently, the results are compared with those of a conventional CoCrMo alloy. In addition, attention is paid to the DLyte method, which has the potential to modify the final surface in terms of local irregularities.
Apparatus
The analysis of the lubrication film formation and the friction was conducted using a reciprocating motion simulator with a pin-on-plate configuration. This allowed for simultaneous observation of the contact and in-situ friction measurements. Figure 1 displays the design of the device, which was modified from the one used by Čípek et al. for cartilage analysis [50, 51]. The device was equipped with a colorimetric interferometry apparatus to observe the thickness of the lubricant film. The simulator bath was heated to a temperature of 37°C to simulate the human body environment. A glass plate was mounted on a movable carriage, which performed a reciprocating motion while the specimen remained stationary. The formation of the lubrication film was observed using an optical imaging system that included a microscope, a halogen light source, a CMOS digital high-speed camera (Phantom v710), and a PC. When the metal pin and the glass plate came into a contact and were illuminated, the color Newton rings were observed. Hartl et al. used a thin film colorimetric interferometry to evaluate the film thickness [52]. In our study, the contact surface of the glass plate was coated with a semi-reflective chromium layer to increase the contrast of the interference fringes. The film thickness evaluation was based on three steps:
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The calibration curves were obtained from an interferogram of a lightly loaded static contact, which was then matched with the measured contact profile. This provided information about the relationship between the color and the film thickness.
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Interferograms of a fully loaded contact during the translation of the carriage were captured using a high-speed camera (Phantom V710, Vision Research, USA).
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The thickness at any arbitrary location of the contact could be determined by matching the captured interferograms with the calibration curves.
Samples
A sample of an optical glass BK270 with dimensions of 155x44x4 mm was used as a plate. One side of the glass was coated with a semi-reflective chromium layer and the other with an anti-reflective layer. The experimental pins were made of two main materials used in the implantology: the medical grade cast CoCrMo alloy (ASTM-F75) and the Ti6Al4V alloy (ISO 5832-3).
The CoCrMo pin was manufactured from a cold-drawn bar, cut and turned under the same cutting conditions as the conventional joint replacement implants. The radius of the head of the pin was R100 and the diameter of the pin was 9.7 mm. The ball surface was further polished to the required minimum roughness Rq of 0.012 ± 0.005 µm. The Ti6Al4V pin was manufactured from a 10 mm diameter cold-drawn bar and then machined and polished to achieve the same geometry and surface finish as the CoCrMo pin.
The surface geometry of the samples was evaluated before the experiments. An optical profilometer based on phase-shifting interferometry (Bruker, Contour GT-X8) was used to analyze the surface roughness (Rq) in the contact area with dimensions of 1 x 1.2 mm covering a theoretically calculated circular contact area with radii of 0.06 mm.
The distribution of the micro textures was chosen to be triangular, with the basic element of this lattice being an equilateral triangle of side length 42,7µm. The shape of the micro texture itself was circular with radius and depth hp, while the bottom of the texture was flat. A schematic of the arrangement and shape of the texture is shown in Fig. 2. A constant surface coverage density Sp of 15% was chosen, with the texture depth as the variable, while keeping the diameter the same. Based on studies describing the beneficial effect of textures on lubrication properties, 5 texture parameters were selected (depth 0.4–6 µm). The textures were selected based on ε ratios in the range of 0.01–0.2. The distance between the centers of dimples was 74 µm, and the radii of the dimples were 15 µm.
The textures were created using a laser micromachining with a picosecond laser (Perla 100 (Hilase)) to minimize the thermal impact on the surface. The laser operated at a wavelength of 1030 nm, a pulse length of 1 ps, a repetition rate of 60 kHz, and a maximum pulse energy of 1 mJ. The laser beam was guided through a harmonic frequency attenuator and an optical combiner. The laser beam was guided through a system of mirrors to an Intelliscan 14 (Scanlab) scanning head, which was placed on a sliding z-axis stage. The textures were fabricated at an average power of 72 mW, moving the laser beam along a spiral trajectory at speeds up to 400 mm⸱s-1. Ten samples were created for each material, with each texture (0.4–6 depths). The DLyte method was used to remove the sharp rims that were produced during the process on the texture. The CoCrMo pins were polished using CoCr DLyte MIX MSA-S H FOR S100 electrolyte, while Ti DLyte MIX MSA PLUS-S electrolyte was used for polishing titanium samples.
Lubricant
The model solution comprised bovine serum albumin (BSA, Sigma Aldrich A7030, Darmstadt, Germany), bovine serum y-globulin (BSG, Sigma Aldrich G5009, Darmstadt, Germany), hyaluronic acid with a molecular weight of 1000 kDa (HA), and phospholipids. The constituents were added to a phosphate-buffered saline (PBS). Concentrations corresponded to the composition of the synovial fluid of patients after a total joint arthroplasty) albumin 26.3 mg/ml, y-globulin 8.2 mg/ml, Phospholipids 0.35 mg/ml, Hyaluronic acid 0.82 mg/ml) [53]. The lubricants were thawed prior to the testing and stored in a refrigerator to ensure a complete protein dissolution in the PBS. Each experiment used a total volume of 14 ml of lubricant and was conducted under fully flooded conditions to avoid contact starvation.
Experimental Design
The kinematic conditions of the experiment were derived from the kinematics of the first metatarsophalangeal (MTP) joint. According to Durrant et al.'s study [54], the MTP joint undergoes a 65° rotation. The angular values of the initial and the final declination differ according to the physiology, gait type, and joint damage of each person. However, an angle of 65° represents the mean value for an average person [39, 40]. For the MTP joint implants, a head and a socket with a radius of R100 were used. The arc path length was calculated as the product of the radius and angle in radians, resulting in an approximate length of 11.3 mm. However, due to the tribometer conditions, the length was increased to 20 mm. Each cycle involved a back-and-forth motion, resulting in a total cycle length of 40 mm. The frequency of the movement was 0.5 Hz, resulting in a speed of 20 mm/s for a 40 mm path. The kinematic conditions of the experiment are summarized in Table 1. The load of 0.5 N was applied to both micro textures. For the CoCrMo compared to glass, the contact area diameter was 180 µm and the contact pressure was 29.6 MPa. For the Ti6Al4V compared to glass, the contact area diameter was 191 µm and the contact pressure was 26.3 MPa. The contact area diameter and pressure were calculated using the contact Hertz theory. For each texture, including the reference without texture (six samples), a single measurement was conducted, comprising three consecutive cycles. A schematic of the experiment is presented in Fig. 3.
Table 1 Experimental conditions A) sample characteristics B) kinematic conditions
Data Processing
The tribological behavior of the lubricant and the implant materials was evaluated based on two main parameters: the coefficient of friction (CoF) and the film thickness. The coefficient of friction was measured in three consecutive cycles, which were then combined into a single graph. The discontinuity of the measurements into individual cycles was necessary due to the memory capacity of the high-speed camera. The sampling frequency was 50 Hz, resulting in the friction coefficient values obtained for time periods of 0.02 seconds. Filtering and data processing were conducted using MATLAB software. The data were filtered based on a 5% deviation from the mean velocity to remove the outliers and the data from the motion change sections where the null velocity was reached. The film thickness was quantified using the colorimetric interferometry. The fundamental principle of this method is the optical recording of the contact area, captured at a rate of 100 frames per second. The frames were extracted from the scanned data at regular intervals. The glass part of the simulator was moved at a constant speed along the pin. This resulted in 20 frames for evaluation at 20 cycles. The interferograms were evaluated using a custom software [52]. In each of the 20 frames, three reference locations in the contact area were selected, and the average thickness of the lubricating film was determined. The final value for one cycle was calculated as an arithmetic mean of these three values. For each measurement, a total of 20 average thicknesses were obtained, resulting in 60 values in three cycles for each material pair tested
The results presented concern the micro textured surface and its influence on the coefficient of friction, with the objective of describing the lubrication regime using the observed film thickness in the contact area. The main contribution is the use of Ti6Al4V alloy, which has the potential for future use in the fabrication of individualized implants using additive manufacturing. The research is mainly concerned with the description of the tribological processes in the contact region, with the objective of preparing the basis for further necessary surface treatments improving wear resistance. The CoCrMo alloy was employed as a reference, enabling results comparison. The articulating surface was then augmented with a range of micro textures, which were subsequently modified by electrochemical machining. This allowed for the investigation of the impact of these modifications on the lubrication processes in the contact area. The rationale behind modifying the surface in this manner is to identify optimal conditions for the formation of a film capable of separating the articulating surfaces, thereby reducing the quantity of loose abrasive particles.
Surface topography
In the initial stage of the study, the articulating surfaces of the samples were evaluated without texture, and subsequently, after texture production and electrochemical treatment with DLyte. The initial surface was polished to achieve a roughness value below 10 nm (without texture), which meets the ISO 7206-2 standard for articulating surfaces of metallic implants [55]. The surface itself was also evaluated in terms of its overall geometry, with respect to the possible influence on the contact area and, thus the contact pressure to be induced [56]. Overall, a deviation of up to 10% of the required nominal radius R100 was accepted. This deviation was based on the accuracy of the measurements themselves and the manufacturing capabilities. It was also considered that the specimens were manufactured using the same method as commonly used implants, which guaranteed the same manufacturing deviations and surface quality achieved in endoprosthetics. However, an issue arose in the evaluation of the overall surface waviness, where irregularities were more frequent, which influenced the shape of the contact area. Based on these conditions, the specimens that demonstrated the most favorable results in the analysis of the contact area were selected. The samples were then textured with five different texture depths of the same distribution and diameter using a picosecond laser. Laser surface texturing process has a thermal ablation process, due to these high temperatures are encountered around the dimple and cause microstructural changes, residual thermal stresses, rim formation in the laser-irradiated zone and substrate [57]. As the depth of the texture increased, the rim of dimples relative to the reference surface also increased. This was caused by the greater material removal and material melting at the rim of the texture. Some larger rims (270–380 nm) around the dimples (2–6 µm) were observed for the Ti6Al4V alloy. (Fig. 4- C, D). For the CoCrMo, the rims were significantly lower, the maximum rim heights were up to 270 nm. To eliminate these inequalities, the electrochemical method – DLyte has proven and led to a relative decrease in the rims for both materials. However, despite the removal of the rims, this method proved to be unsuitable for the CoCrMo alloy. Even a short exposure with the DLyte method (0.4 s) resulted in the removal of rims, at the same time it led to damage to the rest of the surface, severely damaging the observed area and making further evaluation of the results impossible. This phenomenon can be observed in the plot of total roughness (Fig. 4- A, B), where notable discrepancies were observed in the CoCrMo alloy. Several studies found that Co-Cr-Mo alloys have a dendritic surface structure with a 1 to 4 nm thick oxide layer, providing a corrosion barrier in the human body. Corrosion barrier can affect processes during electrochemical machining, which prevents us from creating an ideal geometry with a uniform contact surface. For this reason, the results from colorimetric interferometry of the CoCrMo alloy after application of the DLyte method could not be evaluated [58, 59]. In contrast, the Ti6Al4V material showed a significant reduction in the rim without a significant damage to the rest of the surface. The method was applied for textures with more pronounced rims where the rim reduction from 53–80% occurred without a damage to the surface geometry. The surface quality achieved may be influenced by the chloride ion content of the electrolyte, indicating a suitable reaction for anodic dissolution. At the same time, the electrolyte may alter the surface tension, which could lead to the formation of a stable TiO2 film and thus increase corrosion resistance and alter the affinity of the proteins in the contact area [60].
Coefficient of friction
Another parameter evaluated for all articulating surfaces was the friction coefficient in the pin-on-plate configuration. Results shows the average CoF value after the first cycle when the values stabilized (Fig. 5). Initially, the CoF values were observed to be the lowest, and then they increased with the accumulation of proteins in the contact region. This finding is partially corroborated by studies that have examined the shear stresses within the protein layers in the contact region.[18, 25, 61, 62]. This phenomenon was also described in relation to the individual proteins in the contact area: albumin, γ-Globulin [63]. The polished CoCrMo alloy, commonly used for the articulating surfaces in implantology, showed the CoF value of 0.5 (SD 0.02). The use of textures resulted in an increase in the CoF value for the texture depths of 1–4 µm. The CoF increase was about 0.02 to 0.09. There was a slight decrease for the texture depths of 0.4 and 6 µm which may be due to a more pronounced deviation from the ε ratio which showed the lowest results in terms of the coefficient of friction [39]. The subsequent utilisation of the DLyte method resulted in significant damage to the geometry of the contact area, thereby distorting the results due to local deformations that also altered the contact pressure distribution. In certain instances, the friction coefficient decreased, which can be attributed to differing protein aggregation processes on the surface due to the change in pressure. In this case, it is essential to consider the specific circumstances when interpreting the results. Further research is necessary to ascertain the viability of the DLyte method in CoCrMo alloy. In particular, future research should focus on optimising the electrolyte exposure time, the type of electrolyte, and optimising the technology to remove irregularities while avoiding any disturbance to the overall geometry.
Prior to the DLyte treatment, the Ti6Al4V alloy exhibited a markedly lower friction coefficient value of 0.32 (SD 0.01) compared to the CoCrMo alloy. Concurrently, the CoF values demonstrated a notable acceleration in stabilization with the advancement of continuous development. The incorporation of textures in the remaining samples resulted in an increase in friction compared to the reference, suggesting a heightened concentration of proteins in contact. The increase in coefficient of friction (CoF) was observed to be within the range of 28–118%. This increase can be attributed to protein friction, which is the response of proteins to shear friction. Previous studies have reported a decrease in friction with textured surfaces; however, it is often the case that phosphate-buffered saline (PBS) or Ringer's solution leads to a different lubrication mechanism [64]. The texture with a depth of 6 µm achieved the highest average coefficient of friction (0.71, SD 0.02). The significant deviation may be due not only to protein aggregation but also to a combination with adhesive wear in the contact region, as described by the study of Sonekar et al.[65]. In the case of the Ti6Al4V alloy, the DLyte method did not damage the overall geometry and the contact area remained in its original dimension, but there was a partial increase in the overall surface roughness compared to the original polished specimen. Nevertheless, the roughness of 30 nm was not exceeded for any of the textured samples. The DLyte method reduced the texture rims. The reduction was to the extent that there was a full separation by the protein lubricating film, eliminating the risk of a significant abrasive and adhesive wear. With regard to the coefficient of friction, the DLyte-treated Ti6Al4V alloy surface exhibited a notable increase from 0.32 to 0.47 for the reference sample. Concurrently, there was a slight increase from 3 to 20% for all textures, with the exception of the 6 µm depth, where there was a decrease of 13%. These results are corroborated by the lubricating film itself, which effectively decouples the surfaces at a given geometry. This means that the frictional forces are solely generated by shear friction in the synovial fluid. In the case of the deeper 6 µm texture, it was also possible to observe a slight decrease in the thickness of the lubricating film. This, together with the deeper texture and the different contact pressure distribution, partly explains the change in the coefficient of friction behavior [66].
Film thickness
The film thickness of both alloys was evaluated using colorimetric interferometry. In the case of Ti6Al4V, the results before and after treatment by electrochemical polishing were used. In the case of the CoCrMo alloy samples, only the results before electrochemical treatment were published, as there was significant surface damage, which would have led to misleading results. The surface evaluated was with a roughness commonly used in implantology, up to 12 nm (with the textures used being masked out). Conversely, the surface of the contact area exhibited deviations within 5% despite significant deviations from circularity. In the case of the CoCrMo alloy, measurements on the reference specimen demonstrated a film thickness of 39 nm (SD 11 nm), which only marginally separated the articulating surfaces. Conversely, the relatively high deviations were attributed to the passage of protein clusters across the contact surface, which locally increased the film thickness and thus affected the overall results. Subsequently, measurements were performed on textured samples. The data set could only be evaluated for textures with depths of 0.4 and 1 µm, where complete separation of the articulating surfaces was observed, thus minimizing abrasive wear. For deeper textures, significant abrasive damage to the glass significantly affected the results and prevented a proper evaluation. During the translational motion, the texture rims made contact with the glass surface, as the height of the rims exceeded the predicted thickness of the lubricant film. This resulted in the formation of scratches and damage to the chrome layer on the glass. The 0.4 µm deep texture exhibited a slight increase in film thickness compared to the reference sample (50 nm, SD 12 nm), with no significant change in the behaviour of the lubrication layer. This indicates that the texture itself exerts an influence. In contrast, the 1 µm texture exhibited aggregation of protein clusters at the beginning of the experiment, leading to a local increase in film thickness up to 200 nm. Subsequently, the thickness of the protein layer increased, resulting in a further increase up to above 200 nm. All values of the film thickness are confronted with the heights of irregularities (including rims on the textures and overall roughness), which is demonstrated in the results by red limits.
In comparison to the CoCrMo alloy, the Ti6Al4V samples exhibited a disparate behaviour in the contact area. The surface roughness of the Ti6Al4V alloy was comparable to that of the CoCrMo alloy, with a range of 9–30 nm. Concurrently, however, minor scratches and localised damage were observed during surface polishing of the Ti6Al4V alloy. Following electrochemical treatment with DLyte, the protrusions were removed, yet the overall roughness remained unaltered, and the local damage was retained. Furthermore, irregularities can also affect the formation of the lubricating film, for example, by accelerating the aggregation of proteins in the contact area. This phenomenon can be observed in the reference sample (Fig. 7), where the formation of a cluster of proteins occurred from the outset of the experiment, which subsequently washed away from the contact area once it reached a critical size. This phenomenon can be observed by the rapid reduction of the film thickness below 100 nm. The reduction was such that only partial separation of the protein lubricating film occurred, thereby creating a risk of abrasive wear. In subsequent, another objective is to enhance the film thickness while simultaneously improving the overall surface roughness, thus ensuring the separation of the surfaces. The untextured surface of the Ti-6Al-4V alloy is hydrophilic in nature, and this hydrophilicity can be supported by suitable texturing. Nevertheless, the improvement in wettability is a combined effect of the roughness factor resulting from surface texturing, texture geometry and, of course, the material itself [67].The subsequent results with a textured surface clearly demonstrate an increase in film thickness, which supports previous claims. Additionally, experimental studies on the effect of textures have demonstrated that, in addition to the change in wettability, the authors often describe the effect mainly as a change in hydrodynamic pressure. Also, the ability to capture proteins in contact using the geometry of textures acting as lubricant reservoirs has been observed [35, 42, 68]. In our work, for shallower textures of 0.4 and 1 µm, the separation is evident with a thickness of the lubricating film of 310–320 nm, which is significantly greater than surface irregularities. In the case of deeper textures, the separation of the surfaces is ductile, and the glass often collides with protrusions at the rim of the textures. Nevertheless, it is evident that film thicknesses above 230 nm can still be observed, which is many times the minimum roughness required for implants. Consequently, texturing is beneficial. However, this requires additional surface treatment to remove irregularities. In the case of the Ti6Al4V alloy, the DLyte method used showed more satisfactory results than in the case of the CoCrMo alloy. The objective is to eliminate significant irregularities while maintaining the original geometry of the surface, which implies that alterations in contact pressure are minimal.
The following results demonstrate the behaviour of synovial fluid in the contact area following electrochemical treatment. The removal of the roughness has significantly shifted the range of potential roughness on the surface (red area), significantly reducing the risk of abrasive wear. In the case of the reference sample, without the influence of the texture, it is possible to observe the limit values of the film thickness at a thickness of approximately 60 nm. The nominal roughness of the given sample is up to 10 nm, with the possible presence of grooves caused by polishing, which increases the roughness. Under these conditions, there is a risk of contact and wear of the articulating surface. The following results demonstrate a clear effect of the presented texture. For each texture, there was an increase in the thickness of the lubricating film above 180 nm. The highest values were achieved for shallower textures of 0.4–4 µm. These textures with parameter ε 0.013–0.133 created a lubricating film with a significant overlap of surface irregularities. The thicknesses ranged from 280 to 321 nm, with the highest local surface roughness being 107.5 nm. In the case of the 6 µm texture, the film thickness was reduced to an average value of 180 nm. However, these differences are already predictable. On the one hand, they may be associated with increasing depth and changes in hydrodynamic pressure, which is consistent with published studies [38, 39]. At the same time, the irregularity of the contact surface, which was evident on the observed interferogram, may also have an effect (Fig. 8). Such irregularities will affect both the contact pressure and may impede the accurate determination of the zero film thickness during calibration.
One of the limitations of the study is the pressure distribution in the contact region. In the future, it would be beneficial to consider a method that would allow for the production of a more accurate geometry, with the aim of more clearly separating the different processes in the contact region. Additionally, it is important to note that the specimens were fabricated using the same method as conventional implants, which provides a more realistic representation of implant function. To ensure the reliability of the results, it is essential to increase the number of tested samples with a uniform texture and to account for potential random protein transitions across the contact region.
The study provided a detailed description of the behavior of micro textures in hard pairs used in implantology. These are the commonly used CoCrMo alloy and a promising biocompatible Ti6Al4V alloy that is also suitable for 3D printing. The aim of this work was to evaluate the behavior of the lubricant film at the joint site for the smooth surfaces and, subsequently for the textured surfaces. The tests were carried out in a pin-on-plate configuration using a specially modified tribometer with the ability to use colorimetric interferometry. The coefficient of friction and the thickness of the lubricating film were evaluated, allowing us to assess the lubrication performance and to describe the behavior of proteins passing through the contact. Texturing the surface of the Ti6Al4V alloy gives us three main advantages:
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micro-hydrodynamic bearings,
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the complete surface separation of the articulating materials,
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potential lubricant reservoir during lubricant film formation.
In order to fully exploit the potential benefits of the textured surfaces, the laser-generated textures required further modification due to the appearance of significant irregularities at the rims of the textures. The DLyte electrochemical method was used to reduce the local irregularities and to create the conditions for the formation of a smooth lubricating film. At the same time, the aim was to prevent the adhesive wear of the articulating surfaces. The result was an increase in the protein lubricating layer that was many times greater than the unevenness of the joint surface. Overall, this result indicated that the DLyte process can be used as a finishing process for surface texturing. At the same time the results demonstrated the beneficial effects of the micro texturing. The following conclusions were made:
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Ti6Al4V alloys exhibit a higher surface affinity with aggregated proteins, which is manifested by a thicker lubricating film relative to the conventionally used CoCrMo alloy.
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Micro textures are a suitable tool to increase the film thickness and to minimize the possible abrasive wear of surfaces.
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The use of a picosecond laser causes the material to melt at the rim of the texture, which increases the risk of the abrasive particles being released upon contact and subsequently leads to an increase in wear. However, the electrochemical machining - DLyte method has proven to be a suitable solution for the Ti6Al4V alloy, effectively eliminating these issues.
Furthermore, the performance of the lubrication mechanisms varies depending on the wetting properties and surface energy of the material. The rate of the protein deposition, and therefore lubrication, is affected by the contact pressure, the relative velocity, and the contact temperature.
Declaration of competing interest
The authors declare no conflict of interest.
Author Contribution
M. R. and O. K. conceived the idea and designed the experiments. M. R. and L. O. performed and analysed the experiments and wrote the original draft of the manuscript. D. N., M. H. and I. K. supervised the study. M. V. supervised and financed the study.
Acknowledgement
Thanks to ProSpon, spol. s.r.o. company for the preparation of the samples.
Data Availability
10.5281/zenodo.13235496
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No competing interests reported.