Biomimetic Layered Hydrogel Coating for Enhanced Lubrication and Load-Bearing Capacity

Abstract

Biomimetic hydrogel lubrication coatings with high wettability and low friction show great promise in tissue engineering, wound dressing, drug delivery, and intelligent sensing. Inspired by the hierarchical structure of natural cartilage, a layered hydrogel coating was constructed to functionalize rigid polyetheretherketone (PEEK). The layered hydrogel coating features a structural design comprising a top soft layer and a middle robust layer. The porous structure of the top soft hydrogel layer stores water molecules, providing surface lubrication, while the dense structure of the middle robust hydrogel layer offers load-bearing capacity. These synergistic effects of the gradient hydrogel layer endow the PEEK substrate with an ultra-low coefficient of friction (COF~0.010 at 5 N load), good load-bearing capacity (COF~0.031 at 10 N load), and excellent wear resistance (COF < 0.05 at 5 N load after 20,000 sliding cycles). This study introduces a novel design paradigm for robust hydrogel coatings with exceptional lubricity, displaying the potential application in cartilage replacement materials.

1. Introduction

Hydrogels, as soft and elastic materials, exhibit extraordinary performance that mimics the functions of living tissues [1,2]. Synthetic hydrogels have garnered significant attention due to their potential applications in tissue engineering, wound dressing, drug delivery, and intelligent sensing [3,4,5]. However, most chemically cross-linked hydrogels, which have a random distribution of cross-linking points, often experience low mechanical strength and limited functionality, limiting their widespread use [6,7]. Inspired by the well-ordered structures of natural organisms, advanced hydrogels with desirable structures and enhanced performance have attracted much interest [8,9,10]. For example, Gong et al. were the first to report double-network hydrogels produced by an interpenetrating polymer network (IPN) or semi-IPN, which exhibited extraordinary mechanical features [11]. Yang et al. developed cellulose nanofiber/poly(vinyl alcohol) nanocomposite hydrogels with high fracture strength and an elastic modulus via forming strong hydrogen bonds between the cellulose nanofibers and poly(vinyl alcohol) [12]. Although numerous high-strength hydrogels have been proposed as cartilage substitutes, their tribological properties and mechanical strength still differ somewhat from those of natural cartilage. Recently, gradient-layered hydrogels have been extensively studied and achieved a combination of super-lubricity and outstanding load-bearing capacity, significantly advancing the development of bio-lubrication materials [13,14].

More recently, functional hydrogels have been successfully grafted onto various solid substrates to enable new functions and applications [15,16]. These hydrogel-coated substrates combine the advantages of both the substrate and the hydrogel, playing key roles in medical applications such as artificial joint implants and anti-biofouling medical devices [17,18,19,20,21]. For example, with the rise in the aging population, osteoarthritis has become a severe threat, making artificial joint implants a promising therapy for patients [22,23,24]. Currently, ultrahigh-molecular-weight polyethylene (UHMWPE) and polyetheretherketone (PEEK) have been employed as artificial joints for several decades due to their high strength, wear resistance, and excellent biocompatibility [25,26,27]. However, the direct insertion of uncoated polymers can generate wear debris from high friction with the surrounding tissues, potentially causing trauma during clinical use [15]. Lubricious hydrogel coatings provide rigid polymers with lubricity, anti-biofouling properties, and resistance to prolonged shear forces [28,29,30]. For instance, Wei et al. developed a polymer brush-hydrogel bilayer coating to functionalize rigid PEEK using C–C covalent bonds [31]. The lubricating polymer brush layer and load-bearing hydrogel layer endowed the rigid PEEK with outstanding lubrication and high load-bearing capability. Similarly, Chen and co-workers constructed multilayer hydrogel material grafted onto the UHMWPE substrate through physical bonding [32]. The gradient structure of these multilayer hydrogels exhibited high strength and low friction features, substantially improving the tribological properties of hard substrates. Despite a number of studies focusing on fabricating bio-inspired hydrogel coatings with low friction, their load-bearing and anti-wear properties are still not ideal [13]. Therefore, developing facile and efficient strategies to construct hydrogel coatings with super-lubricity and high load-bearing on rigid substrates is urgently essential.

Herein, we propose a facile and efficient approach to fabricate robust lubricating hydrogel coatings by structuring a layered hydrogel on a rigid PEEK surface. Vertically, the hydrogel coating features a structural design comprising a soft hydrogel layer, a robust hydrogel layer, and a hard PEEK substrate. The soft porous layer can adsorb large amounts of water to swell, while the highly hydrated layer prevents the removal of lubricants under high loads. The underlying layer of the hydrogel has high mechanical strength and a rigid polymer skeleton that resists swelling in water, providing a strong foundation for high load-bearing. These gradient soft-hydrogel materials endow the rigid PEEK substrate with ultra-low friction, good load-bearing capacity, and excellent wear resistance. The cartilage-inspired hydrogel design opens new avenues for developing bio-lubrication materials with performance that surpasses the natural counterparts.

2. Materials and Methods

2.1. Materials

Acrylamide (AM, 99%) was purchased from Macklin (Shanghai, China). Acrylic acid (AA, 99%) was purchased from TCI (Shanghai, China). N,N’-methylenebis acrylamide (MBA) was purchased from Energy Chemical (Shanghai, China). α-ketone glutaric acid (KA, 98%) was purchased from Aladdin (Shanghai, China). Sodium citrate (SC) was purchased from Tianjin Kemio Chemical Reagent Co., Ltd. (Tianjin, China). Iron chloride hexahydrate (FeCl₃⋅6H₂O) was purchased from Tianjin Damao Chemical Reagent Co., Ltd. (Tianjin, China). All chemicals were used without any purification. Deionized water was self-prepared. PEEK plates (20 mm × 20 mm × 3 mm) were provided by Jiuyi Plastic Material Co., Ltd. (Shenzhen, China).

2.2. Synthesis of PEEK Coated with Hydrogel Sample (HP)

The PEEK coated with hydrogel sample (named as HP) was prepared by the following steps. AM (4.20 g), AA (1.44 g), cross-linker MBAA (0.0184 g), and photo-initiator KA (0.0117 g) were added to 20 mL deionized water to prepare monomer solution. After degassing with vacuum chamber for 2 min, the monomer solution was poured into the template composed of the PEEK and glass plate and exposed to ultraviolet light (365 nm) for 1 h. Next, the PAA/PAM hydrogel sample was immersed in the FeCl₃ solution (0.25 mol/L) for 8 h to allow Fe³⁺ loading. Finally, the Fe³⁺-loaded hydrogel sample was transferred to the deionized water for 12 h in order to remove the residual Fe³⁺ to form a high-strength PAA-Fe/PAM hydrogel sample.

2.3. Synthesis of PEEK Coated with Layered Hydrogel Sample (LHP)

The fabrication of PEEK coated with layered hydrogel sample (named as LHP) was followed by two steps. First, the PAA-Fe/PAM hydrogel sample (HP) was immersed in the sodium citrate (SC) solution (0.3 mol/L) with different times (1, 2, and 5 min). Then, the hydrogel sample was soaked in deionized water for 12 h to remove any unreacted components, and the PEEK coated with layered hydrogel sample (LHP) was successfully obtained. The LHP samples with different SC treatment times were named as LHP-X (X refers to treatment time).

2.4. Characterization

Surface and cross-sectional photographs of samples were obtained with an optical microscope (MAZ7, OX-LZ, Suzhou, China). Scanning electron microscopy (SEM, Merlin, Zeiss, Oberkochen, Germany) was utilized to examine the surfaces and cross-sectional morphologies of samples. Prior to SEM analysis, samples were treated with liquid nitrogen to freeze, and then dried at −80 °C for 48 h in a freeze dryer. After freeze-drying treatment, the hydrogel coating literally fell off the PEEK substrate and then was adhered to the conductive tape of the base for SEM testing. X-ray photoelectron spectroscopy (XPS, Nexsa, Thermo Fisher Scientific, Waltham, MA, USA) was conducted on the top layer of hydrogel coatings, in order to analyze the surface elemental compositions of coatings, with the binding energy referenced to C1s at 284.8 eV. The static contact angle (CA) values were measured using an optical contact angle meter (JC2000C1, POWERE, Shanghai, China) at ambient temperature (∼20 °C). A droplet of 2 μL deionized water was placed on the sample, and the average CA value was calculated by taking measurements at three diverse positions. The electric universal test machine with a 500 N load cell (MTS, E44.304, Minneapolis, MN, USA) was used to access the interfacial adhesion between the PEEK substrate and the hydrogel layer by employing a 90° peeling test. The crosshead velocity was maintained at 5 mm/min, and the sample width was set to 20 mm.

2.5. Friction Characterization

Friction measurements of different samples were carried out using a ball-on-disk reciprocating tribometer (CFT-1, Lanzhou, China). The friction coefficient between the sample and the stainless steel (diameter: 6 mm) was measured at different sliding frequencies (0.5, 1, 3, and 5 Hz) and different loads (2, 5, 8, and 10 N). Deionized water, phosphate buffer solution (PBS, pH = 7.4), 2 wt% sodium alginate solution (SA), and physiological saline solution (0.9 wt% NaCl) were used as lubricants. During the friction test, the samples were completely immersed in sufficient lubricant. The distance of one sliding cycle was 10 mm. Unless specified, each measurement was performed with 300 sliding cycles, and the friction coefficient was calculated from the average value during the test. Three friction tests were repeated for each sample to obtain the average friction coefficient. The wear morphology of hydrogel layer after friction test was obtained by an optical microscope and SEM.

3. Results and Discussion

3.1. Fabrication of PEEK Coated with Layered Hydrogel (LHP)

Figure 1 illustrates the fabrication process of the PEEK coated with layered hydrogel (LHP). Firstly, a pre-polymerization solution consisting of acrylic acid (AA) and acrylamide (AM) monomers, cross-linker N,N’-methylenebis acrylamide (MBA), photo-initiator α-ketone glutaric acid (KA), and deionized water underwent ultraviolet (UV) irradiation for 1 h after bubble removal. The structural units on the PEEK surface could generate free radicals under UV irradiation, which initiated the polymerization of AA/AM monomers to form a hydrogel layer on it [27]. This resulted in the chemical grafting of a soft PAA/PAM hydrogel layer onto the PEEK substrate through C-C covalent bonds. To enhance the strength of the hydrogel layers, the obtained sample was immersed in FeCl₃ solution for 8 h and then in deionized water for 12 h, forming a dual cross-linked PAA-Fe/PAM hydrogel. In this hydrogel, the Fe³⁺–carboxylate coordinates can dissipate energy efficiently, resulting in a high-strength hydrogel layer (HP) [33]. Subsequently, the HP sample was immersed in the sodium citrate (SC) solution, causing Fe³⁺ to dissociate from the hydrogel network, which led to the top layer of the hydrogel developing a loose porous structure, termed a layered hydrogel (LHP-X, where X refers to SC treatment time) [34]. Specifically, the physical cross-linked network of Fe³⁺–carboxylate was broken through SC solution dissociation, resulting in the formation of a porous structure. As a result, a layered hydrogel was grafted on the PEEK substrate to create a load-bearing and lubrication interface. The bottom PEEK served as a hard substrate for bone implants, while the middle hydrogel layer with high strength could bear normal loads, and the top layer, with its soft features, functioned as the lubrication layer.

3.2. Surface Morphologies and Chemical Compositions

The optical microscope was employed to capture the surface and cross-sectional images of each sample in its wet state. As illustrated in Figure 2a, prominent traces were visible on the surface of the bare PEEK plate, whereas the surface became smooth following the production of the hydrogel layer. We compared the surface images of the samples before and after the SC treatments for 1, 2, and 5 min (Figure 2b–e). The untreated hydrogel exhibited a dense, flat, and smooth surface. After the SC solution treatment, due to the dissociation of the Fe³⁺–carboxylate complex, the hydrogel’s surface layer developed an obvious folded structure with increased roughness. As the treatment time increased, the size of the folds gradually enlarged. The occurrence of these folds can be attributed to the different elastic properties between the two layers of hydrogel. The cross-sectional image clearly identified the three-layered structure of the LHP-2 sample, with distinct boundaries (Figure 2f).

The dry morphologies of the LHP samples with different SC treatment times were characterized by a scanning electron microscope (SEM) after undergoing the freeze-drying treatment. As shown in Figure 3, the soft and porous structure of the top hydrogel layer formed by the SC treatment can be well observed. The surface layer of the hydrogel after the SC treatment showed a highly porous structure, while the lower basal layer showed a dense structure with a clear boundary. Then, we compared the surface and cross-section morphologies of the hydrogels undergoing the SC treatment for 1, 2, and 5 min. With the prolongation of the treatment time, the pore size gradually increased. As for the LHP-1 sample, the surface was relatively flat with evenly distributed folds. In contrast, the surfaces of the LHP-2 and LHP-5 samples exhibited a uniform pore structure, with the mean pore size increasing from 20 to 210 µm. The pore size of the LHP-5 sample was significantly larger than that of LHP-2, and their features depend on the degree of surface dissociation of the hydrogel. The EDS analysis was performed to detect the element distribution of the hydrogel coating, as shown in Figure S1. The decreased concentration of element Fe in the top layer further proved the dissociation of the Fe³⁺ ions from the hydrogel network. The thickness of the top layer was increased from 7 to 74 and 97 µm with the treatment time increasing from 1 to 2 and 5 min. Therefore, the pore size and thickness of the porous layer were both positively correlated with the treatment time, which would have important effects on the lubrication properties of the hydrogel samples.

Furthermore, an X-ray photoelectron spectrometer (XPS) was utilized to analyze the surface chemistry compositions of each sample with various modifications. As displayed in Figure 4a, compared to the bare PEEK, the presence of multiple Fe 2p peaks at 726.1 and 711.1 eV in the HP sample verified the incorporation of Fe³⁺–carboxylate coordination within the hydrogel network. This result also demonstrated that the PEEK plate surface was entirely coated by the hydrogel. Additionally, for the LHP sample, the significantly weakened Fe 2p peak signal after the SC treatment indicated the occurrence of an interface dissociation process (Figure 4b). Specifically, the coordination of the Fe³⁺ ions with the carboxylate groups has been “cut off”, resulting in the dissociation of the Fe³⁺ ions in the hydrogel network. This finding was consistent with the surface modification of the layered hydrogel on PEEK.

3.3. Surface Wettability and Interfacial Adhesion

The wettability behavior of the sample is critical for friction reduction on the basis of the hydration lubrication mechanism. Hence, the water contact angles (CAs) of the bare PEEK, HP, and LHP samples were evaluated. Figure 5 shows the CAs for all the samples at different stages of the modification process. The surface of the bare PEEK plate was hydrophobic, with the CA stabilizing at 83.0° after 4 min. In contrast, the surface of the LHP-5 sample exhibited a superwetting state (CA~7.1°), with the water drop spreading completely within 5 s of contact, demonstrating excellent wetting properties. As shown in Figure 5b, the mean CAs for all the samples were 91.1°, 81.6°, 32.8°, 16.8°, and 6.6°, respectively. The wettability of the sample significantly improved after being coated with a layered hydrogel due to the hydrophilic nature of the top soft layer. In addition, the CAs reduced with increasing SC treatment times, indicating that the surface wettability was likely influenced by the porous structure and thickness of the top hydrogel layer. These data strongly suggest that the LHP samples with a layered structure possess excellent hydrophilic properties, which are also effective for aqueous lubrication.

In addition, strong interfacial bonding is crucial for the mechanical stability of the sample. A 90° peeling test was conducted to evaluate the interfacial adhesion between the PEEK substrate and the hydrogel layer (Figure 6a,b). For the LHP sample, the interfacial bonding force reached approximately 740 N/m, demonstrating strong adhesion between the substrate and the hydrogel layer through C-C covalent bonds (Figure 6c). Moreover, the high interfacial adhesion made it difficult to peel the hydrogel from the PEEK substrate, resulting in a gradually reduced peeling force. Thus, strong interfacial bonding in our system plays a significant role in reducing friction and wear, which is critical for bio-lubrication applications.

3.4. Evaluation of the Lubrication and Wear Resistance

Lubrication measurements were conducted to assess the water lubrication properties of the prepared samples. A ball-on-disk reciprocating tribometer was utilized to conduct the friction tests between the sample and the steel ball. As illustrated in Figure 7, the friction curve of the bare PEEK showed obvious fluctuation from the beginning to the end of the test, with an average coefficient of friction (COF) maintained at a high level of around 0.25. It is evident that the lubrication property of PEEK was not remarkably enhanced after coating with the high-strength hydrogel, which might be attributed to the low water content and dense polymer network of the PAA-Fe/PAM hydrogel. In contrast, all the LHP samples exhibited significantly reduced COFs and smoother friction curves. The average COF of LHP-1 (~0.030) was obviously decreased compared to HP (~0.18) due to a thin porous layer providing effective lubrication. Similarly, the LHP-2 and LHP-5 samples demonstrated highly effective lubrication with ultra-low COFs of 0.010 and 0.013. The significant decrease in COF was likely due to the hydration lubrication layer formed by the top porous structure. The substantial interactions between the hydrophilic polymer chains and water dipoles tightly bond water molecules under compression, resulting in stable COFs for the LHP samples. Notably, the friction curve of the LHP-5 sample displayed a slow increasing trend throughout the friction test, which may be due to the thicker porous layer commonly inducing severe elastic deformation and higher friction. Therefore, we speculate that the formation of a hydration layer with moderate thickness plays a crucial role in reducing friction.

Furthermore, the optical images of all the samples after the friction test were recorded, as shown in Figure 8. Clearly, the worn surface was characterized by deep and narrow grooves for pure PEEK. As for the HP sample, wide and smooth wear tracks appeared on the worn surface instead of grooves, which suggests that the high-strength hydrogel could not provide effective lubrication. Meanwhile, for the LHP samples, the wear tracks were relatively shallow and narrow, indicating that the formation of the lubrication layer played a key role in reducing friction and wear. It was worth noting that the fold structure on the worn surface was nearly intact for the LHP-2 sample, and, as the treatment time was prolonged, the hydrogel layer became thicker, which is consistent with the changing trend regarding the friction coefficient. So, low friction and wear are presented for LHP-2 sample.

To assess the load-bearing properties of the LHP-2 sample, the friction tests were conducted under varying loads (2–10 N), with water as the lubricant (Figure 9a). It was found that the COFs increased with the load. By increasing the load from 2 to 10 N, the COFs of LHP-2 gradually rose from 0.009 to 0.031, demonstrating the sample’s ability to maintain a low COF even at high contact pressures. The superior lubrication performance of the LHP-2 sample was attributed to the enhanced surface hydration and moderate thickness of the porous layer. Additionally, there is a positive correlation between the load and the average COFs of the layered hydrogel due to the elastic deformation of the flexible porous structure under higher loads [35]. The COF values of the LHP-2 sample were also affected by the sliding frequency (Figure 9b). Minor variations in the COF values were detected at lower frequencies, while the COF values significantly increased as the frequencies reached 5 Hz. The rapid increase in the COF values at higher sliding speeds can be attributed to the deformed porous structure under pressure not having sufficient time to rehydrate and reform the hydration film, leading to higher COF values at high reciprocation speeds [31].

To further evaluate the lubrication performance, the LHP-2 sample was lubricated with deionized water, phosphate buffer solution (PBS, pH = 7.4), 2 wt % sodium alginate solution (SA), and physiological saline solution (0.9 wt % NaCl). As shown in Figure 9c, the COFs of the above lubricants were all around 0.010, which indicated that the LHP-2 sample has a stable lubrication performance in different biological fluids.

Excellent lubrication properties and long-term wear resistance under high loads are essential for cartilage-inspired hydrogel coatings. Therefore, we further investigated the long-term friction cycles of the layered hydrogel sample (LHP-2) under a 5 N load. As shown in Figure 10a, during the 10,000 friction cycles, the LHP-2 sample maintained a relatively low COF, and then the COF increased gradually without exceeding 0.05 after 20,000 cycles, indicating that the LHP-2 sample had excellent anti-wear performance. The increase in COF was primarily induced by the wear of the interfacial lubrication layer, with a rapid rise in the COF especially after 22,500 cycles of friction. What is more, the wear morphology of the LHP-2 sample after long-term friction cycles was characterized by the optical microscopy and SEM. As shown in Figure 10b, the surface of the LHP-2 sample was marked with abrasions, with a wear width of 1.9 mm, which also explained the gradual increase in COF with the friction cycles. As observed by the SEM image, there was even no porous structure at all compared to the original surface, indicating that the lubricious layer had been worn out completely after the long-term friction test. Overall, with the synergistic effects of the soft lubrication layer and stiff load-bearing phase of the layered hydrogel, the wear-resistance property of the PEEK substrate significantly improved. This demonstrated that the layered hydrogel coating possessed excellent mechanical and lubrication properties, making it promising in the field of bioinspired lubrication coatings.

4. Conclusions

In this study, a biomimetic layered hydrogel was coated onto a rigid PEEK substrate to serve as a lubricating bearing layer. The PAA-Fe/PAM hydrogel with high strength was chemically anchored to the PEEK substrate through C-C covalent bonds formed via UV irradiation. Following surface dissociation induced by the SC treatment, the hydrogel coating with a layered structure was generated, featuring a soft porous layer and a robust load-bearing layer. The prepared hydrogel coating demonstrated superior hydrophilic properties compared to the substrate due to the formation of a hydration layer by the porous structure. Under the synergistic effects of the soft layer and the robust layer of the layered hydrogel, the optimized sample exhibited excellent lubrication (COF~0.010 at 5 N), high load-bearing capacity (2–10 N), and outstanding wear resistance (COF < 0.05 over 20,000 cycles). The gradient structure of the hydrogel coating endowed the rigid PEEK with effective lubrication and hydrophilic properties, showing great potential in the field of artificial joint implants.