Effect of Surface Texturing on Friction and Lubrication of Ti6Al4V Biomaterials for Joint Implants

Vol.:(0123456789) Tribology Letters (2025) 73:15 https://doi.org/10.1007/s11249-024-01950-5 ORIGINAL PAPER Effect ofSurface Texturing onFriction andLubrication ofTi6Al4V Biomaterials forJoint Implants MatúšRanuša1· LukášOdehnal1· OndřejKučera1· DavidNečas1· MartinHartl1· IvanKřupka1· MartinVrbka1 Received: 6 August 2024 / Accepted: 4 December 2024 / Published online: 19 December 2024 © The Author(s) 2024 Abstract 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. The Ti6Al4V and CoCrMo alloys, renowned for their biocompatibility and osseointegration properties, represent promising printable materials, although they are 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 colorimetric 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. * Matúš Ranuša [email protected] 1 Biotribology Research Group, Faculty ofMechanical Engineering, Brno University ofTechnology, Technická 2896/2, 61669Brno, CzechRepublic Tribology Letters (2025) 73:1515 Page 2 of 18 Graphical Abstract Glass plate Light source Ti6Al4V alloy & CoCrMo alloy Materials and Methods Film thickness &Coefficient of friction Keywords Ti6Al4V· Micro-texture· Implant· Optical interferometry· Friction· Film thickness 1 Introduction 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. A number of studies in this field demonstrate the interplay between individual surface materials and the complexity of the molecular components of the synovial fluid, which change with patient age [4]. The basic materials that combine the advantages of biocompatibility and resistance, while allowing the use of 3D printing by selective laser melting (SLM), are represented by the CoCrMo and Ti6Al4V alloys [5]. The possibility of 3D printing of both materials offers the advantages of customization and osteointegration, but brings new challenges at the same time. One of the challenges is the behavior of the printed articulating surface compared to conventional production. One of the differences is the tensile strength. The SLM-produced CoCrMo alloys exhibit a higher ultimate tensile strength and hardness compared to those made via investment casting. For instance, the SLM samples can achieve yield strengths around 1.4 GPa, but at the same time, they tend to have a higher wear rate compared to the cast samples, which exhibit lower wear rates due to their denser microstructure [6, 7]. However, in both cases, the wear resistance is relatively high, and both production technologies exhibit sufficient mechanical properties and corrosion resistance [8]. 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 [9]. This oxide layer is mainly composed of cobalt, Tribology Letters (2025) 73:15 Page 3 of 18 15 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 [10, 11]. In the case of the Ti6Al4V alloys, the conventional methods produce coarser grains, which can hinder dislocation movement, leading to a lower strength compared to the 3D-printed surfaces. The 3D-printed Ti6Al4V alloys can achieve strengths up to 1492.89MPa, although with a reduced elongation (5.76%) due to microstructural defects [12]. 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 [13]. 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. It is, therefore, a challenging task to determine the most appropriate manufacturing technique for surface treatment and, at the same time, to design the optimal texture geometry or multi-level texture with respect to the kinematics of the selected joint. Micro-texturing has the potential to modify the behavior of the lubrication layer, thereby enhancing the sustainability of the performance, while allowing the use of different materials [14, 15]. Modified materials frequently exhibit not only enhanced durability but also the presence of highly cross-linked polyethylenes, which are often doped with a range of substances, including the E vitamin. This doping process augments the material's frictional resistance while simultaneously stabilizing the free radicals within the structural matrix. This dual effect results in a notable enhancement in the material's resistance to the oxidation and delamination at the surface [16, 17]. The texturing of surfaces has been demonstrated to exert a beneficial influence on the tribological properties, resulting in an augmented hydrodynamic pressure and diminished surface wear. However, the efficacy of this approach remains a topic of contention, particularly in the context of non-conformal contacts [18]. In contrast, for the conformal contacts, it results in the separation of the contact surfaces by a thicker layer of protein-containing lubricant, which alters the lubrication regime and reduces the wear. Simultaneously, the textures serve as lubricant reservoirs, facilitating the desired separation of the articulating surfaces [19, 20]. 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 [21, 22]. Several studies have investigated the effect of the microtexturing on the coefficient of friction [23–25]. 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 [26–28]. 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 [29]. 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 [30]. 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. [31] 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°, with approximately 76° of dorsal flexion and 34° 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. [32] 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 [33], the contact pressures were Tribology Letters (2025) 73:1515 Page 4 of 18 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 [25, 34]. 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. [35] 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 [36, 37], which can be resolved by surface modifications or modifications that create a lubricating film to separate the joint surfaces. 1.1 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 [36, 38–40]. 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. Conversely, alternative texture shapes have been identified that demonstrate the potential to reduce the coefficient of friction (CoF) even further. However, the question of the efficiency of the production of these shapes and their applicability in the context of more complex kinematics and in combination with different materials remains unanswered [41]. The basic texture parameter is the ratio of depth (hp) to diameter (dp), which is critical to the wear performance. Correct adjustment of this ratio (ε) can result in a reduction in the CoF of up to 30% due to locally varying hydrodynamic pressures [42]. In general, small depth textures (around 1μm) can increase the CoF and wear at low loads due to insufficient hydrodynamic pressure generation. However, this statement does not apply to the use of a protein lubricating film due to the formation of protein clusters during the contact passage, and therefore, an increase in the CoF does not necessarily imply worse results in terms of the long-term wear. In the synovial joint environment, deeper textures (1–15μm) appear optimal as they both increase the hydrodynamic pressure and act as lubricant reservoirs [43]. Despite the positive effect of the texture on the lubricity, a negative effect is possible as well, especially if the load exceeds optimum thresholds and the lubrication limit is reached [44]. Another critical parameter is the combination of the ε ratio with the texture, surface coverage density (Sp), and shape, which determine the effectiveness in the different lubrication regimes during the implant cycle [45, 46]. It is expected that the friction reduction will be more pronounced for shallow dimples (3–10μm) having smaller diameters (100–200μm) [47]. 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 [48]. 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 [49]. Qiu etal. [50] 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. [51] 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. [52] 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. [53] 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. [54] 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. [55], 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. [48] 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-machining tilling method [56], 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 Tribology Letters (2025) 73:15 Page 5 of 18 15 abrasive wear, where the sharp rims cause micro-scratches on the counter-surface, increasing the coefficient of friction. 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 [57, 58]. 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 [59–61]. 1.2 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 [62]. 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. 2 Materials andMethods 2.1 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 [63, 64]. The device was equipped with a Fig. 1 Schematic of the measuring apparatus in a pin-on-plate configuration Tribology Letters (2025) 73:1515 Page 6 of 18 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 [65]. 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: (1) 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. (2) Interferograms of a fully loaded contact during the translation of the carriage were captured using a highspeed camera (Phantom V710, Vision Research, USA). (3) The thickness at any arbitrary location of the contact could be determined by matching the captured interferograms with the calibration curves. 2.2 Samples A sample of an optical glass BK270 with dimensions of 155 × 44 × 4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 × 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 42.7µm. The shape of the micro-texture itself was circular, while the bottom of the texture was flat as far as possible (Fig.2). This texture shape Fig. 2 Distribution and geometry of the textures on the surface of the samples Tribology Letters (2025) 73:15 Page 7 of 18 15 was chosen because it is easy to fabricate with a picosecond laser while maintaining the geometric repeatability at each pit. The circular shape also gives us the advantage of the consistency of the film behavior even with changes in the direction of the motion, which is expected due to its use in an articulating implant. A schematic of the arrangement and shape of the texture is shown in Fig.2. Based on the results of the literature search, the most effective coverage density Sp of 15% was chosen in terms of the lubricant film formation, considering the number of textures in contact with the materials at full load. The condition was the participation of at least 5 textures in the contact area, which is ensured in the given configuration. The chosen variable parameter is the ratio ε. In order to vary the value in the range of 0.01–0.2, which is given in the literature as the most effective value for the given coverage, the only variable parameter left is the texture depth dp. For this reason, 5 texture parameters were chosen (dp 0.4–6μm). The distance between the centers of the 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. 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. 2.3 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) [66]. 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. 2.4 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 [67], 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. The overall design of the experiment was devised with the objective of observing phenomena in the contact region, to gain some fundamental insights and comparisons. In this context, several simplifications were made to the invitro conditions in comparison to the real invivo conditions in the MTP joint. The primary limitation is the non-conformal surface and the associated increase in contact pressure to a higher range of values that occur at the joints. Concurrently, the alteration in contact pressure results in the replacement of the articulating pair with a transparent glass with a lower Young's modulus than the Ti6Al4V alloy. These differences can affect the thickness of the lubricating film, which can ultimately have a negative effect on the overall wear. Therefore, these limitations must be considered in any wear evaluation and long-term wear tests [68, 69]. 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. 2.5 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. Tribology Letters (2025) 73:1515 Page 8 of 18 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s. 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/s. 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 [65]. 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. 3 Results andDiscussion 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. Table 1 Experimental conditions (A) sample characteristics, (B) kinematic conditions (A) Samples Material Semi-finished product Young’s modulus (GPa) Poisson’s ratios Contact pressure pin on plate (MPa) Pin (radii 100mm) Ti6Al4V alloy Bar 114 0.34 26.3 CoCrMo alloy Bar 230 0.28 29.6 Plate Borosilicate glass B270 62 0.22 – Albumin γ-Globulin Phospholipids Hyaluronic acid 26.3 8.2 0.35 0.82 (B) Load (N) Velocity (mm/s) Stroke (mm) Total distance (mm) Number of cycles Duration (s) Temperature (°C) 0.5 20 20 2400 60 120 37 Fig. 3 Experimental set-up Tribology Letters (2025) 73:15 Page 9 of 18 15 3.1 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 [70]. 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 [71]. 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 [72]. 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.4C, 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.4A, 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 Fig. 4 Average surface roughness with masked texture: A Ti6Al4V, B CoCrMo, rim height of textures for Ti6Al4V: C before DLyte, D after DLyte Tribology Letters (2025) 73:1515 Page 16 of 18 and analyzed the experiments and wrote the original draft of the manuscript. D. Nečas, M. Hartl and I. Křupka supervised the study. M. Vrbka supervised and financed the study. Funding This research was carried out under the Project “Friction and lubrication of small joint implants produced by 3D metal printing additive technology” funded by the Czech Science Foundation, No. 22-02154S and by the Project “Mechanical Engineering of Biological and Bio-inspired Systems,” funded as Project No. CZ.02.01.01/00/22_008/0004634 by Programme Johannes Amos Commenius, Call Excellent Research, administered by the Ministry of Education, Sports and Youth. Also, thanks to ProSpon, spol. s r. o. company for the preparation of the samples. Data Availability The data that support the findings of this study are openly available in repository Zenodo at http:// doi. org/https:// doi. org/ 10. 5281/ zenodo. 13235 495. Declarations Conflict of interest The authors declare no conflict of interest. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. 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