Carbon fiber cannot always reduce the wear of PEEK for orthopedic implants under DPPC lubrication

Excellent wear resistance is an important feature of orthopedic implants. However, although pure polyetheretherketone (PEEK) is outperformed by carbon fiber-reinforced PEEK (CF-PEEK) for stability and durability under laboratory conditions, it is not clear whether CF-PEEK should be preferred in all real-world applications. Results indicate that, under dipalmitoylphosphatidylcholine (DPPC) lubrication, the wear rates of PEEK are 35%—80% lower than the wear rates of CF-PEEK for different implant materials, speeds, loadings, and DPPC concentrations. Molecular dynamics calculations confirm that DPPC self-assembles on the PEEK surface to form an easily adsorbed continuous phospholipid lubricating film. In contrast, the carbon fibers on the CF-PEEK surface hinder the formation of the protective DPPC film and the CF-PEEK surface is thus subject to faster wear.


Introduction
Polyetheretherketone (PEEK) is a fully aromatic, semicrystalline thermoplastic with high wear resistance, rigidity, strength, thermal stability, chemical resistance, and radiation damage resistance. PEEK and enhanced PEEK are widely used in the manufacture of aircraft parts and other equipment used under extreme conditions [1][2][3]. In the field of orthopedic implants, PEEK is a high-wear-resistance orthopedic biomaterial with an elastic modulus close to that of human cortical bone, good biocompatibility, and high transmittance [4][5][6]. Recently, PEEK has been successfully applied in spinal interbody fusion and other orthopedic applications. Given its excellent wear resistance, PEEK has thus attracted considerable attention as an implant material [7][8][9] and possible replacement for polyethylene polymer implant materials [10]. For example, Brockett et al. [5] found that PEEK reduced the wear rate by 60% compared to ultrahigh-molecular-weight polyethylene (UHMWPE).
In contrast, carbon fiber-reinforced PEEK (CF-PEEK) has demonstrated even better mechanical properties and stability than pure PEEK, while maintaining good biocompatibility. In particular, numerous preliminary orthopedic implant studies indicate that the wear resistance of CF-PEEK is superior to that of PEEK under laboratory conditions [11][12][13]. For example, in the early 1990s, Maharaj et al. [14] experimentally determined the tribological properties of CF-PEEK and ZrO 2 under bovine serum and water lubrication and found that the wear rate of CF-PEEK was only one tenth that of UHMWPE. Later, Wang et al. [15] explored the applicability of medical-grade CF-PEEK and UHMWPE as load-bearing surfaces in total hip arthroplasty; they found that the wear rate of the CF-PEEK articulating against Al 2 O 3 , ZrO 2 , and CoCr under bovine serum lubrication was lower than that of the UHMWPE friction pair. Given these highly promising results, it is unsurprising that CF-PEEK is widely considered to be an ideal polymer material for the next generation of orthopedic implants.
Parallel breakthrough discoveries are also being made in the lubrication materials that facilitate the high-performance operation of these artificial implants. In the process of friction and lubrication within natural joints, the cartilage covering the joint surfaces and the synovial fluid in the joint play vital roles. In a healthy joint, the cartilage tissue effectively prevents direct contact between joint materials, greatly reducing wear [16]. In artificial joints, the lubricants between the friction surfaces mainly include deionized water, calf serum, and sodium hyaluronate [17][18][19]. In recent years, phosphatidylcholine liposomes have emerged as high-quality biomimetic joint lubricants and biomimetic articular cartilage materials [20][21][22][23]. Therefore, it is important to study the tribological properties of artificial joint friction pairs under phospholipid lubrication. Common phospholipids include 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-snglycero-3-phosphocholine (DMPC), 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), 1,2distearoyl-sn-glycero-3-phosphocholine (DSPC), and 1-palmitoyl-2-stearoyl-sn-glycero-3-phosphocholine (HSPC). Especially, DPPC has been successfully used as a drug carrier for treating human organ diseases [24,25]. Therefore, DPPC has great potential as the first high-efficiency lubricant injected into human joints [26,27]. In 2016, Sorkin et al. [28] investigated the hydration lubrication of lipid bilayer boundary lubricants in dipalmitoyl phosphatidylcholine (DPPC) dispersions and reported self-healing induced by phosphatidylcholine shearing. Four years later, Angayarkanni et al. [29] studied the lubrication of phospholipids at the boundaries of artificial joints and observed that phospholipids self-assemble during the lubrication process. These two phenomena (selfassembly and self-healing) allow the phospholipids to continuously provide ultra-low friction and ultra-small wear during the lubrication process.
These dramatic improvements in the artificial synovial fluid replacement materials have been further matched by ceramic materials discoveries. Ceramic artificial joint materials such as silicon nitride (Si 3 N 4 ), alumina (Al 2 O 3 ), and zirconia (ZrO 2 ) have attracted considerable research attention in recent years. Compared to metals, these ceramic materials have very low friction coefficients, very low wear rates, and excellent biological compatibility [30][31][32][33]. In recent studies, ceramic materials and PEEK have been combined to form new friction pairs. For example, in studying the performance of biocompatible Si 3 N 4 and PEEK in a DPPC lubrication condition, Wang et al. [34] found that the friction coefficient between PEEK and Si 3 N 4 reached 0.04. However, no comparative study with CF-PEEK and DPPC was conducted.
To better evaluate the combination of next-generation high-performance artificial implant materials, the present study determined whether or not carbon fibers can reduce the wear rate of PEEK articulating against ceramics under DPPC lubrication. The wear rates of PEEK and CF-PEEK articulating against ceramics under DPPC lubrication were systematically studied at different friction speeds and loads, and the friction mechanisms of PEEK and CF-PEEK under DPPC lubrication were analyzed.

Materials
Medical-grade PEEK and CF-PEEK were obtained from the Invibio Company (UK) and their respective physical parameters are shown in Table 1. CF-PEEK with randomly distributed carbon fibers was selected to prevent the influence of carbon fiber orientations. CF-PEEK contains 30% weight fraction of pitch-based carbon fibers and the length of these carbon fibers is 25 μm [35]. Commercial balls of four different materials commonly used to mimic bone in artificial implants were obtained from the China National Machinery Industry Corporation, (China): Si 3 N 4 , Al 2 O 3 , ZrO 2 , and stainless steel. The diameter of all balls was 9.525 mm and their physical parameters are shown in Table 2. DPPC 16:0 (Mw = 734.04 g·mol −1 , purity ≥ 99%) was obtained from Macklin Biochemical Technology Co., Ltd. (Shanghai, China). The main transformation temperature (T m ) of the DPPC is 41.6 °C [34].
In the experiment, 10 g/L DPPC aqueous solution was prepared according to standard approaches presented in the literature [36][37][38][39]. All experimental processes were carried out at temperatures above the main phase transition temperature (T > T m ) of DPPC. DPPC was hydrated with deionized water and subjected to ultrasonic processing (JP-010-t, SKYMEN, China) for 1,800 s to obtain 10 g/L DPPC aqueous www.Springer.com/journal/40544 | Friction

Friction experiments
Friction reciprocating tests were carried out using a ball-on-disc tribological testing machine (UMT-TriboLab, Bruker, USA). Before tribological testing, all samples were subjected to ultrasonic treatment with alcohol and pure water to ensure accurate experimental data. The tribological testing temperature was 22±3 °C. The loads applied in the friction experiment were 5, 10, and 15 N (Hertz stress = 90-200 MPa). The tribological test time was 3,600 s. These loads are in line with the Hertz stress limit required for human joints [34,40] and these loads meet the compressive strength of all materials. The reciprocating frequencies were 1, 1.5, and 2 Hz (sliding speed = 20-40 mm·s −1 ). Figure 1 depicts the experimental friction process.

Characterization and test methods
The friction coefficients of the different experimental materials were recorded and output by a UMT-TriboLab tribological properties testing machine in real time. A non-contact profilometer (ST400, Nanovea Ltd., USA) was used to measure the wear scars on the PEEK discs. Wear rate (w) of the PEEK and CF-PEEK discs was determined by Eq. (1) (according to the ASTM-G99 Standard [41]): where V d (mm 3 ) is volume of the worn track. D (m) and F (N) stand for sliding distance and normal load, respectively.

Fig. 1 Experimental friction method and process.
| https://mc03.manuscriptcentral.com/friction where S n (n = 1, 2, 3) is cross-section area of the worn track, which got directly from non-contact profilometer (ST400, Nanovea Ltd., USA). l stands for the length of scratch (l = 10 mm). After the friction experiment, the friction scratches, friction surface film, and wear interface on the PEEK disc were observed with a super-depth-of-field microscope (VHX-7000, KEYENCE, Japan). X-ray photoelectron spectroscopy (XPS; ESCALAB Xi+, Thermo Fisher, USA) was used to study the film in the friction scratches of the PEEK disc. The morphology and film thickness in the PEEK disc scratches were evaluated by atomic force microscopy (AFM; Dimension Icon, Bruker, Germany). Figure 2 shows the average friction coefficients ( Fig. 2(a)) and wear rates ( Fig. 2(b)) of PEEK and CF-PEEK with stainless steel, Si 3 N 4 , Al 2 O 3 , and ZrO 2 , under water lubrication. It is clear that, in each case, CF-PEEK had a dramatically lower friction coefficient and a commensurately lower wear rate under water lubrication. For example, the wear rate of the PEEK/ stainless steel friction pair was 21.5×10 −6 mm 3 /(N·m), while that of the CF-PEEK/stainless steel pair was only 2.2×10 −6 mm 3 /(N·m) (i.e., the wear rate of CF-PEEK was only 10% that of PEEK). Figure 2 also shows the average friction coefficients (Fig. 2(c)) and wear rates (Fig. 2(d)) of PEEK and CF-PEEK under DPPC lubrication. PEEK and CF-PEEK showed similar friction coefficients under DPPC lubrication, as shown in Fig. 2(c). Nevertheless, the friction coefficient was lowest between Si 3 N 4 and PEEK (a reduction of 67% compared to water lubrication), which indicates that Si 3 N 4 may be preferable for use in artificial joints with DPPC lubrication [31,42]. Surprisingly, the wear rate of the PEEK/Si 3 N 4 friction pair was 0.3×10 −6 mm 3 /(N·m), which is only 21% of the wear rate of the CF-PEEK/Si 3 N 4 friction pair (1.4×10 −6 mm 3 /(N·m)). In sum, CF-PEEK dramatically outperforms PEEK under water lubrication, but PEEK dramatically outperforms CF-PEEK under DPPC lubrication.

Tribological behaviors of different materials
The fact that the wear rate of CF-PEEK is strongly dependent on the type of lubricant contradicts the current predictions that CF-PEEK will outperform PEEK in biomedical applications [43,44]. Given the advantages of using DPPC-related phospholipids in www.Springer.com/journal/40544 | Friction biomimetic cartilage and biomimetic joint lubricants [27,38,45], it is therefore necessary to understand the tribological properties of artificial implant materials under DPPC lubrication. Because the wear rates were lowest for the PEEK/Si 3 N 4 and CF-PEEK/Si 3 N 4 friction pairs under DPPC lubrication. Hence, we systematically studied the tribological properties and tribological mechanism of PEEK/Si 3 N 4 and CF-PEEK/Si 3 N 4 under DPPC lubrication.

Tribological behaviors of PEEK and Si 3 N 4
We systematically changed the experimental load and speed to study the tribological properties of the PEEK/Si 3 N 4 and CF-PEEK/Si 3 N 4 pairs under water   Figure 3 shows the PEEK/Si 3 N 4 friction coefficients ( Fig. 3(a)) and wear rates (Fig. 3(d)) in comparison with the CF-PEEK/Si 3 N 4 friction coefficients ( Fig. 3(b)) and wear rates (Fig. 3(d)) during the friction process under water lubrication. As anticipated, CF-PEEK showed a much lower average friction coefficient (Fig. 3(c)) and wear rate (Fig. 3(d)) for all experimental conditions (5 N-1 Hz, 10 N-1 Hz, 15 N-1 Hz, 15 N-1.5 Hz, and 15 N-2 Hz). Microscopy and three-dimensional (3D) morphological scanning were then applied to observe the surfaces of PEEK (Fig. 3(e)) and CF-PEEK ( Fig. 3(f)), visually confirming that the pits on the surface of the CF-PEEK were shallower than those on the surface of the PEEK.
As our initial experiment indicated, the tribological properties of the PEEK/Si 3 N 4 and CF-PEEK/Si 3 N 4 pairs were dramatically different under DPPC lubrication (1 g/L). Figure 4 shows the PEEK/Si 3 N 4 friction www.Springer.com/journal/40544 | Friction coefficients ( Fig. 4(a)) and wear rates (Fig. 4(d)) in comparison with the CF-PEEK/Si 3 N 4 friction coefficients ( Fig. 4(b)) and wear rates (Fig. 4(d)) during the friction process under DPPC lubrication. For all experimental conditions (5 N-1 Hz, 10 N-1 Hz, 15 N-1 Hz, 15 N-1.5 Hz, and 15 N-2 Hz), the friction coefficients of the PEEK/Si 3 N 4 and CF-PEEK/Si 3 N 4 pairs remained in the range of 0.03-0.05. However, despite their very similar, low, and stable friction coefficients, the wear rates of PEEK are consistently 50%-75% lower than the wear rates of CF-PEEK (Fig. 4(d)). To understand this phenomenon, we used a microscope to observe the worn and cleaned surfaces of PEEK and CF-PEEK under the experimental condition of 10 N-1 Hz in DPPC (1 g/L), where the contrast was most dramatic (Figs. 4(e) and 4(f)). In sum, although DPPC shows great potential as a new biomimetic articular fluid and biomimetic articular cartilage material to reduce the wear of artificial bone materials [40], CF-PEEK may have significantly lower wear resistance than ordinary PEEK in DPPC.

Material characterization analysis
To more clearly understand the effect of carbon fiber on wear, the wear efficiency was calculated using the Eq. (3) [46]: An efficiency greater than zero means that carbon fiber reduces wear (CF-PEEK outperforms pure PEEK), while an efficiency less than zero means that carbon fiber increases wear (pure PEEK outperforms CF-PEEK). The wear efficiencies obtained in this study are compared with previously reported values in Fig. 5. Zhu et al. [47] reported that CF-PEEK has a wear rate 40% lower than PEEK's under dry friction. Wang et al. [48] found that varying the carbon fiber in CF-PEEK could reduce the wear rate of PEEK by as  | https://mc03.manuscriptcentral.com/friction much as 90% under calf serum lubrication. Regis et al. [49] found that CF-PEEK maintained its excellent wear resistance under calf serum lubrication. Under water or human serum lubrication, Borruto [50] found that CF-PEEK had a wear rate approximately 80% lower than pure PEEK. Yamamoto and Hashimoto [51] similarly reported that CF-PEEK reduced the wear rate of PEEK by 50% under water lubrication. Song et al. [6] found that the wear rate of CF-PEEK was only half that of PEEK under calf serum lubrication. The reason why carbon fiber reduces PEEK wear rate inconsistently in the above references may be under a variety of conditions, with different materials and lubrications, and even different ratios of carbon fibers. In stark contrast, the present study suggests that, under DPPC lubrication specifically, the wear efficiency was less than zero (Fig. 5). Figure 6(a) shows a microscopy image of the scratches on the PEEK surface after tribological testing under 1g/L DPPC lubrication, where DPPC clusters adsorbed on the PEEK surface then combined into a continuous large lubricating film [38,52,53], which is known to www.Springer.com/journal/40544 | Friction play an important role in lubrication and friction reduction on the PEEK surface [45,54]. The inset 30 μm image shows that large, continuous lubricating films covered the scratches on the PEEK surface. In the XPS spectra of the surface film ( Fig. 6(b)), N 1s and P 2p peaks are clearly observed, confirming that the film on the surface of PEEK was a DPPC phospholipid film [34]. The height difference between the PEEK surface and the DPPC-coated PEEK surface can be clearly observed (Fig. 6(c)), further confirming that DPPC was continuously adsorbed on the PEEK surface during the tribological testing of the PEEK/Si 3 N 4 friction pairs. This process can be described as the self-assembly of the DPPC film [28,29,55], and was previously demonstrated by Klein's group on the atomic scale [26,28,29,56]. Our research group has clearly shown that the self-assembly of DPPC affects the tribological properties of PEEK on the macroscale [28,29].
To better understand the anomalous results for CF-PEEK in a DPPC concentration, we conducted the friction experiment of CF-PEEK/Si 3 N 4 (10 N-1 Hz) under 1 g/L DPPC concentration (Fig. 7). Rather than forming a continuous film, DPPC formed small, irregular pieces on a relatively small percentage of the CF-PEEK surface. Rather than self-assembling [28,29], the carbon fiber matrix inhibited the formation of a continuous bulk DPPC phospholipid on the CF-PEEK surface. Because the DPPC was unable to establish a large positive pressure-bearing capacity, the CF-PEEK achieved relatively poor tribological properties and a much higher wear rate.
To better understand the role that DPPC plays in lubrication, and how the amount of DPPC impacts the reduction in PEEK wear, we first calculated he wear rates of pure PEEK/Si 3 N 4 (10 N-1 Hz) under different DPPC concentrations (0.1, 0.2, 0.5, 1.0, and 2.0 g/L) (Fig. 8). The wear rate of PEEK at a DPPC concentration of 0.1 g/L was like that under water lubrication. However, when the DPPC concentration exceeded 0.2 g/L, DPPC began to play a lubricating role in the friction process. We speculate that when the concentration of DPPC is too low, it is not adsorbed or coated on the PEEK surface, and no DPPC phospholipid lubrication film is formed [57]. However, as the DPPC concentration increased, the wear rate of PEEK decreased gradually, indicating that DPPC formed a phospholipid lubrication film on the PEEK surface, thereby reducing PEEK wear [29,36].

Molecular dynamics simulation
Molecular dynamics (MD) simulation technology uses computers to simulate the movement of molecules and atoms [58] and describe the force motion between particles and the bonding reaction between various particles [59].This paper uses the commercial software Materials Studio to build a three-layer model of β-Si 3 N 4 (100) [58], a DPPC aqueous solution, and PEEK. The density of DPPC aqueous solution is set as 1 g/cm 3 , (i.e., one DPPC molecule and 246 H 2 O molecules) [60,61]. Because PEEK is a polymer model, the degree of polymerization of the PEEK model is 3-5 and the molecular chain is condensed [62,63], thus, the equilibrated PEEK chain had a density of 1.35 g/cm 3 , which matches the experimental value (1.3 g/cm 3 ). As shown in Fig. 9, the layers were "sandwiched" and the universal force field was applied. To minimize calculation time and enhance efficiency, the scale of the molecular friction model box was set at 3.1 nm × 2.0 nm × 3.8 nm.    For the calculation, Nosé-Hoover thermostat controls were used to maintain a temperature of 298 K [64], and a canonical NVT (number of particles, volume, temperature) ensemble was applied [65]. A frictional molecular model was used to control the simulation environment of the whole simulation system by using the open-source molecular simulation software LAMMPS. The velocity Verlet algorithm was used to integrate the position and velocity sets [66]. The time step was set to 1 fs and the total time was set to 600 ps. In the simulation system, periodic boundary conditions were used in both X and Y directions. To calculate the total force between materials, images were captured every 20 fs. The long-range interaction was calculated by a particle mesh solver [67]. The cut-off distance of the Lennard-Jones Coulombic interaction was 12 Å.
In the MD simulation, the friction process of Si 3 N 4 and PEEK was realized by using the two-process Friction command in the open-source molecular simulation software LAMMPS to account for extrusion in the z-axis negative direction and shearing in the x-axis direction. The positive pressure applied in the z-axis direction was 3 nN. To calculate the friction stability process of Si 3 N 4 /PEEK under DPPC lubrication the total time was set at 25 ps, and the time step at 5 ps. The friction mechanism of Si 3 N 4 and PEEK under DPPC lubrication was analyzed based on the visualized  Figure 11 shows the friction coefficient of Si 3 N 4 and PEEK under DPPC lubrication during the period of 0-25 ps, when the friction coefficient remains between 0.06-0.07. The friction coefficient simulated by LAMMPS is consistent with the experimental friction coefficient, which implies that the established model is reasonable. At 0 ps, Si 3 N 4 , PEEK, and DPPC lubrication were completely distinctive layers. At 10 ps, the water molecules in the DPPC aqueous solution began to diffuse into the PEEK structure, the DPPC molecules were gradually approaching the PEEK chains, and a molecular chain of DPPC had contacted the PEEK layer. That quickly, the DPPC molecules are already beginning to be entangled with the PEEK layer. Note that the friction coefficient of an MD simulation is slightly different from that of actual friction process because the pressure applied in the z-axis direction is limited by the molecular model and will be somewhat different from the actual pressure. On the other hand, the concentration of DPPC in aqueous solution is different from that in actual friction. Therefore, it is very reasonable to use LAMMPS program command for Si 3 N 4 and PEEK under DPPC aqueous solution. Figure 12 shows the mechanisms by which DPPC lubrication affects the tribological properties of PEEK and CF-PEEK. During tribological testing, small fragments of the DPPC phospholipid in solution will self-assemble to form a large, continuous DPPC phospholipid film on the PEEK surface ( Fig. 12(a)) [28,29]. The phospholipid film coated on the PEEK surface is relatively complete and can withstand the pressure generated by the friction process [68], thereby  protecting the PEEK surface and improving the tribological properties of PEEK. Similarly, for CF-PEEK, the small DPPC fragments in solution selfassemble to form the DPPC phospholipid film [28,29]; however, due to the presence of carbon fiber ends or the carbon fiber itself, the DPPC phospholipid film is discontinuous and incomplete on the CF-PEEK surface ( Fig. 12(b)). The carbon fiber prevents the selfassembly of the DPPC film on the CF-PEEK surface, resulting in a significantly higher wear rate of CF-PEEK compared to PEEK. But and both PEEK and CF-PEEK achieved lower friction coefficient with DPPC (1 g/L) through friction tests. This result may be because DPPC can reduce the friction coefficient by forming a lubricating film on the surface of PEEK and CF-PEEK. DPPC film on surface of CF-PEEK may play a role in establishing a positive pressure-bearing capacity to achieve lower friction coefficient.

Conclusions
This study revealed that the presence of carbon fiber in CF-PEEK affects the formation of the phospholipid protective film under DPPC lubrication, causing the wear rate of CF-PEEK to be higher than that of PEEK. Based on our experimental and modeling results, the following conclusions can be drawn: 1) Under water lubrication, the wear rate of CF-PEEK is lower than pure PEEK in contact with different materials (stainless steel, Si 3 N 4 , Al 2 O 3 , and ZrO 2 ), such that the wear rate of CF-PEEK was 50% that of PEEK. For the friction pairs containing stainless steel, the wear rate of CF-PEEK was only 10% that of PEEK. In contrast, under DPPC lubrication, the wear rates of PEEK are 35%-80% lower than the wear rates of CF-PEEK.
2) For the PEEK/Si 3 N 4 friction pairs under water lubrication, the CF-PEEK had a lower wear rate than PEEK under all loads (5, 10, and 15 N) and speeds (1, 1.5, and 2 Hz). However, under DPPC lubrication, the wear rates of PEEK are 50%-75% lower than the wear rates of CF-PEEK.
3) During DPPC lubrication, the DPPC self-assembled to form phospholipid films on the PEEK surfaces, inhibiting the wear of PEEK. However, the presence of carbon fiber hindered the self-assembly process of DPPC on the surface of CF-PEEK, preventing the DPPC from forming a continuous phospholipid film. This explains the higher wear rate of CF-PEEK compared to PEEK under DPPC lubrication. In the absence of carbon fiber, a continuous phospholipid film formed on the surface of PEEK, resulting in better wear resistance compared to CF-PEEK.
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