Mechanically Robust Lubricating Hydrogels Beyond the Natural Cartilage as Compliant Artificial Joint Coating

Abstract Natural cartilage exhibits superior lubricity as well as an ultra‐long service lifetime, which is related to its surface hydration, load‐bearing, and deformation recovery feature. Until now, it is of great challenge to develop reliable cartilage lubricating materials or coatings with persistent robustness. Inspired by the unique biochemical structure and mechanics of natural cartilage, the study reports a novel cartilage‐hydrogel composed of top composite lubrication layer and bottom mechanical load‐bearing layer, by covalently manufacturing thick polyelectrolyte brush phase through sub‐surface of tough hydrogel matrix with multi‐level crystallization phase. Due to multiple network dissipation mechanisms of matrix, this hydrogel can achieve a high compression modulus of 11.8 MPa, a reversible creep recovery (creep strain: ≈2%), along with excellent anti‐swelling feature in physiological medium (v/v0 < 5%). Using low‐viscosity PBS as lubricant, this hydrogel demonstrates persistent lubricity (average COF: ≈0.027) under a high contact pressure of 2.06 MPa with encountering 100k reciprocating sliding cycles, negligible wear and a deformation recovery of collapse pit in testing area. The extraordinary lubrication performance of this hydrogel is comparable to but beyond the natural animal cartilage, and can be used as compliant coating for implantable articular material of UHMWPE to present, offering more robust lubricity than current commercial system.


Introduction
Articular cartilage exhibits lasting and recoverable lubrication performance in the decades of human body service. [1]Its internal matrix consists of multiple, orderly arranged layers of collagen, [2] providing the cartilage with excellent tensile stiffness and compressive strength. [3]dditionally, a large number of brushedbiomacromolecules and lipid molecules arranged on its superficial zone endow cartilage with strong hydration effect. [4]1c,5] Among them, the anti-creep feature, fatigue-resistance and deformation recovery after encountering dynamic loading/shearing are crucial for maintaining the durable lubricity of the cartilage layer. [6]However, due to the poor physiological recovery ability resulting from eldering, disease, and other factors, cartilage systems always fail to lubricate and can inevitably be damaged.It is essential to develop artificial cartilage lubricating materials for solving this problem.
1c,7] Even though recent researches for synthetic cartilage hydrogels based on doublecrosslinked network (DN) system have achieved significant progress, [7c-f] the inherent anti-swelling performance and biocompatibility in physiological media is still challenging.Among them, poly (hydroxyethyl methacrylate) (pHEMA) and polyvinyl alcohol (PVA) hydrogels are the most representative materials due to their excellent biocompatibility and medium stability. [8]8a] However, the lubrication lifetime may not be ideal due to the limitation of lubricant doping content.In fact, the inherent poor mechanical properties of pure pHEMA hydrogels such as low compression strength, irreversible creep recovery, and serve contact deformation under large-loads condition, limit its lubrication performance in bioengineering applications. [9]7c,10] However, their low moduli commonly lead to poor load-bearing capacity and obvious deformation contribution in the friction.7b,15] Until now, developing a mechanically robust cartilage-like hydrogel system suitable for biological medium with low COF, high load-bearing capacity, and deformation recovery simultaneously is still challenging.
Here, we developed a novel cartilage-inspired layered lubrication hydrogel (H PVA/CS-PSPMA ) by covalently manufacturing thick (several tens of micrometers) poly(3-sulfopropyl methacrylate potassium) (PSPMA) polyelectrolyte brushes (PB) phase through the sub-surface of a robust polyvinyl alcohol/chitosan (PVA/CS) hydrogel matrix (H PVA/CS ) with multi-level crystallization networks.The top lubrication layer shows excellent lubricity and wear resistance, whereas the nanocrystalline domain-enhanced H PVA/CS matrix exhibits good load-bearing, anti-swelling, and deformation recovery properties.The lubrication performance of H PVA/CS-PSPMA is even better than that of the natural cartilage under the same test condition and could be used as a robust and compliant coating for implantable material of UHMWPE (ultrahigh molecular weight polyethylene), which presents a great advantage for replacing the traditional thin polyelectrolyte brushes lubrication layer.Based on these results, this strategy provides a new design routine for cartilage-like hydrogels where surface lubrication and bulk mechanical strength merge well.

Design Concept and Preparation of Cartilage Hydrogels (H PVA/CS-PSPMA )
Natural articular cartilage exhibits low COF (0.001-0.03), durable lubricity (several decades), and dynamically mechanical recovery under high contact stresses (1-20 MPa), [16] which are highly related to its typical layered structure, good mechanical property and efficient surface hydration (Figure 1a(i)).Among them, the highly hydrated hydrophilic macromolecules anchored onto the cartilage surface are mainly responsible for friction-reduction and wear resistance, while the reversibly mechanical recovery feature of the deformable cartilage layer under cyclically loading and shearing process, [5a] is the prerequisite for resisting fatigue and maintaining its ultra-long lifetime (Figure 1a(ii)).Inspired by these mechanisms, a layered cartilage hydrogel (H PVA/CS-PSPMA ) composed of a top soft lubrication layer and a bottom loadbearing matrix, was prepared by covalently manufacturing hydrophilic PSPMA polyelectrolyte brushes phase through the sub-surface of mechanically robust PVA/CS hydrogel (H PVA/CS ) (Figure 1a(iii)).The detailed preparation process of H PVA/CS-PSPMA was shown in Figures S1 and S2 (Supporting Information).In typical case, H PVA/CS matrix surface was covalently anchored with thick atom transfer radical polymerization (ATRP) initia-tor layer to obtain H PVA/CS-Br , [17] and then the composite lubrication layer was generated by employing an in-situ permeationaccelerated polymerization strategy for spatially grafting of hydrophilic PSPMA brushes chains through its sub-surface.
Meanwhile, the H PVA/CS matrix was constructed by employing nanocrystalline domain-enhancement strategy with multiple physically crosslinking networks (Figure S3, Supporting Information), including the first hydrogen bonding network of PVA/CS or CS/CS, the second ion coordination network of sodium citrate (Na 3 Cit)/CS with additional salting out effect, and the third hydrogen bonding network between chains of PVA molecules by freeze-thaw and annealing crystallization.The typical multiple physically crosslinking feature of the H PVA/CS matrix endowed it with multi-level crystallization networks and good mechanical robustness, as well as excellent load-bearing and dynamically deformation recovery feature as natural cartilage layer.As a result, the negatively charged PSPMA chains in the top composite lubrication layer can mimic the brush-like proteoglycan chains on the superficial layer of cartilage, acting as the efficient lubrication phase, while the bottom H PVA/CS matrix possessed excellent load-bearing and anti-fatigue functionalities.Therefore, the H PVA/CS-PSPMA , a novel cartilage hydrogel, would present reversibly mechanical deformation-recovery feature under dynamic loading/unloading processes (Figure 1a(iii)), which enables to achieve lasting lubrication.
Fluorescence photograph of H PVA/CS-PSPMA hydrogels sample was shown in Figure 1b(i).7c] ) weight, compared to the traditional PVA/CS hydrogels (control sample) presenting large mechanical deformation (Figure 1b(ii,iii)).This implied that H PVA/CS-PSPMA hydrogels possessed considerable load-bearing capacity as cartilage-like matrix.Overall, based on novel bionic design concept, our current H PVA/CS-PSPMA exhibited excellent anti-swelling feature in physiological medium, good mechanical properties (load-bearing, anti-creep, and self-recovery), and excellent lubrication performance (low friction, wear-resistance, and long service life).

Characterizations of Structure and Anti-Swelling Properties of H PVA/CS Matrix
The mechanisms responsible for mechanical robustness of H PVA/CS matrix were investigated.Figure 2a showed the corresponding surface morphology and micro-structure characterizations of the as-prepared hydrogels samples in different stages.By jointly deploying multi-physical interactions and subsequent annealing treatment, networks of hydrogel samples showed different chains stacking densities.Traditional PVA/CS hydrogels matrix was loose and porous, while the multi-step treated H PVA/CS was dense (Figure 2a(i-iii)).Dense entanglement of polymer chains and effective dissipation mechanisms due to multiple physical networks contributed to the overall high stiffness and toughness of the H PVA/CS hydrogel. [6,12]The multiple physically crosslinking networks within H PVA/CS matrix could be confirmed by Fourier Transform infrared spectroscopy (FT-IR) (Figure S4, Supporting Information).Compared to the PVA/CS hydrogels and PVA/CS-Cit3 − hydrogels (without annealing treatment), the obvious shift of stretching vibration peak of hydroxyl groups (-OH) toward low wavenumber (cm −1 ) for H PVA/CS indicated the successful network enhancement by hydrogen bonding interactions.The signal appeared at 1142 cm −1 belongs to the typical crystalline phase, [10c] indicating that crystallinity could effectively be improved by employing salting out and annealing means.The crystallinities of different samples were characterized by X-Ray Diffraction (XRD) and Differential Scanning Calorimeter (DSC) (Figures S5 and S6, Supporting Information).Compared to the untreated PVA/CS, the gradual increase of signal strength at 10 1 peak (2 = 20°) of PVA chains for PVA/CS-Cit3 − hydrogels and H PVA/CS was observed, indicating the existence of a crystalline micro-domain from synergism of ions coordination, salting out and annealing treatments.Besides, further annealing treatment for strengthening the hydrogen bonding interaction between PVA chains resulted in the continuous increase of the network crystallinity as well as the shrinkage of porosity.
Detailed crystallization enhancement mechanisms within hydrogel networks were explored by using Wide Angle X-Ray Scattering (WAXS) and Small Angle X-Ray Scattering (SAXS).Through X-ray scattering tests, the details of the internal microstructures within the hydrogel matrix can be detected.As shown in Figure 2a(iv-vi), compared to the PVA/CS sample, the H PVA/CS sample showed significant diffraction rings, and the intensity data was further plotted and analyzed (Figure 2b).The 2D WAXS patterns showed a clear Debye scattering ring with maximum intensity at q = 1.33 Å −1 , indicating the existence of higher crystallinity.Moreover, the other weaker scattering ring corresponded to q = 1.57Å −1 .The distinct peak at q = 1.33 Å −1 revealed the lattice spacing (L 1 ) of 0.47 nm, which may be correlated to the ordered hydrogen bonding network structure between the hydroxyl groups (-OH) of PVA molecules.An additional peak appeared at q = 1.57Å −1 with a lattice spacing of 0.4 nm, which was associated with the C-C spacing of the hydrophobic domains of the CH 2 -CH 2 backbone. [18]Such a lattice (0.4 nm) might originate from the hydrogen bond-stabilized spacing between two CH 2 -CH 2 backbones.At the same time, the SAXS results showed that the inter-crystal spacing within the hydrogels networks became smaller after the multi-step crystallization treatment (Figure 2a(vii-ix),c).After salting out and annealing treatment, the crystal dimension (D) increased from 1.27 to 3.47 nm, while inter-crystal spacing (L 2 ) decreased from 16.89 to 13.99 nm (Figure 2d).This increase in the size of the nanocrystalline domain and a decrease in the distance between them led to the crystallinity increasing (Figure 2e).
As a result, the multiple physically crosslinking and crystallinity networks of H PVA/CS could effectively improve its anti-swelling property.Especially, the anti-swelling property of cartilage-like hydrogels materials was crucial due to their fluidencountered application environment.In addition, as long-term potential implant material and coating, it was essential to maintain structure stability of hydrogels in the physiological medium over time.Therefore, the media-stability of H PVA/CS was investigated in PBS buffer solution (pH 7.4), while the classical double crosslinking networks (DN) hydrogels system (Fe-P(AAm/AAc)) was used as comparison. [19]The swelling process was real-time recorded by monitoring the mass and volume change ratio (swelling ratio) of samples within 168 h (7 days) (Figure 2f,g).The Fe-P(AAm/AAc) hydrogels swelled rapidly within 48 h and then gradually achieving equilibrium after 72 h, and the volume swelling ratio was obviously more than 1600%.15c] The crystallinity enhancement of H PVA/CS enabled the obvious increase of density for the sample interior network, while the decrease of porosity made the whole material more hydrophobic and stable (Figure 2h).These results indicated the successful preparation of the mechanically robust H PVA/CS , suggesting that it can be used as a load-bearing matrix for cartilage-like hydrogels.

Evaluation of the Mechanical Properties of H PVA/CS Matrix
In order to validate our design concept for achieving mechanically robust H PVA/CS , a series of mechanical tests were performed on PVA/CS samples (the hydrogel without salting out and annealing treatment) and H PVA/CS samples.All of the tests were performed after the samples encountering dynamic swelling equilibrium in PBS solution.First of all, the mechanical properties of the samples were systematically investigated by employing macro-mechanical tests mean.The tensile stress-strain curves of the samples were shown in Figure 3a.For PVA/CS, the elongation and tensile strength at break were 372% and 1.67 MPa, while those of H PVA/CS were 503% and 6.7 MPa, separately.Correspondingly, the elastic modulus and the tensile toughness (Γ) were 0.07 MPa and 2.41 MJ m −3 for the PVA/CS sample, while those of H PVA/CS were 1.32 MPa and 18.12 MJ m −3 , separately (Figure S7, Supporting Information).The compressive property was crucial for lubrication materials in friction process, because it reflected the ability to resist mechanical deformation.The compressive modulus of the H PVA/CS is 11.8 MPa that was nearly two orders of magnitude higher than that of PVA/CS sample (0.19 MPa) (Figure 3b).At the same compression strain of 70%, the compression strength of PVA/CS sample was only 1.09 MPa (toughness: 0.09 MJ m −3 ), while that of H PVA/CS was as high as 15.25 MPa (toughness: 3.38 MJ m −3 ).These results indicate that H PVA/CS possesses excellent load-bearing capacity after employing salting out and annealing treatment means.However, the short-time compression test could not fully reflect the real loading-condition during friction process.Hydrogel materials commonly tend to produce creep under continuous loading process, and then irreversible deformation and failure were occurred.The anti-creeping ability of the hydrogel was a key factor to dominate its load-bearing capacity and lubrication performance.Through a single creep test (5 N, 3600 s), it was found that the creeping strain of the PVA/CS sample was about twice that of the H PVA/CS sample, confirming that our multi-level crystallization strategy could well suppress the creep (Figure 3c).Both the compression strength and compression modulus of H PVA/CS were superior to most reported hydrogels systems, and comparable to that of natural cartilage (Figure 3d).The excellent loadbearing capacity of the H PVA/CS was attributed to its high mechanical strength from nanocrystalline domains by dense polymer chains and effective dissipation mechanism from multiple physically crosslinking networks, for which could effectively improve the overall stiffness and toughness to resist deformation.Obviously, both the compression strength and modulus of H PVA/CS were superior to most reported hydrogel systems and comparable to that of natural cartilage. [6,12]urthermore, creep-resistance and mechanical recovery features were also of paramount importance for achieving lasting high load-bearing capacity.Correspondingly, the ability to maintain minor compression strain and recover to the original state after encountering deformation of samples was evaluated.In order to simulate the situation of human's joint in real life (Figure 3e), a 16 h creep versus 8 h recovery experiment was conducted to analogize the process of daytime loading and nighttime recovery of articular cartilage.7c] ) at room temperature for 96 h, were shown in Figure 3f.In the creep process, the viscoelasticity of the samples caused the nonlinear variation of mechanical deformation with time.The creep deformation increased rapidly after being subjected to instantaneously loading at the beginning, then the creep rate gradually slowed down due to the slow deformation of polymer chains over time.By comparing the normal strain values from creep-recovery curves of PVA/CS and H PVA/CS , it was found that the creep deformation of H PVA/CS was smaller and remained unchanged after reaching equilibrium.After four creeprecovery cycles, the net deformation strain value of H PVA/CS was only 2%.As an intuitive contrast, the net deformation strain value of the PVA/CS sample was 45% after encountering four cycles.The mechanism responsible for this could be attributed to that the salting out and annealing treatment enabled the densification of the PVA/CS hydrogels networks, which largely restricted molecular chains movement and improved the ability of H PVA/CS to resist creep deformation.
The nanocrystalline domains also act as strong crosslinking sites within H PVA/CS networks to resist the irreversible deformation as well as enhance the deformation recovery.Such typical recovery feature was essential for achieving ultra-long lubricity and load-bearing during friction process.Fracture resistance was also one of the most important properties of the synthetic cartilage hydrogels.7c] ).e) Schematic diagram showing the cyclic process of daytime loading (pink area) and nighttime recovering (blue area) of the natural cartilage layer.f) Four cyclic strain-time curves of PVA/CS and H PVA/CS under dynamic creep loading (pink area, 16 h)/unloading (blue area, 8 h) process.g) Schematic diagram of the tensile tearing test of the sample and anti-fracture mechanism.h) Tensile stress-strain curves for the tearing test of PVA/CS and H PVA/CS .Snapshots of i) PVA/CS sample and j) H PVA/CS in real-time fracture process (Scale bar: 5 mm).
<4.2 fracture strain ratio.Correspondingly, the fracture toughness (Γ) of samples was calculated.The Γ of the PVA/CS was only 5.30 kJ m −2 , while that of H PVA/CS can achieve as high as 26.02 kJ m −2 .Figure 3i,j showed the snapshots of the PVA/CS and H PVA/CS in during the real-time fracture testing process.The crack of the PVA/CS expanded rapidly and then completely broke in less than 10 s, while the crack of H PVA/CS expanded slowly.The mechanism responsible for this was that nanocrystalline domains within H PVA/CS could protect the network from crack propagation.This meant that the H PVA/CS could effectively inhibit the microscopic defects induced by fracture during deformation.

Characterizations of Layered Cartilage Hydrogels (H PVA/CS-PSPMA )
Using multiple physically crosslinking strategy to improve the density of the polymer networks indeed could enhance the mechanical strength of bulk hydrogels, but it would weaken the surface hydration degree to induce poor lubrication ability.In nature, the collagen matrix in cartilage layer gives it good load-bearing capacity while the hydrated bottle-brush polysaccharide macromolecules on the superficial layer can maintain its lasting lubricity.This layered strategy inspired us to modify the surface of mechanically robust hydrogels for achieving good lubrication.Grafting hydrophilic PSPMA polyelectrolyte brush chains on the surface of the H PVA/CS matrix with excellent load-bearing and mechanical recovery properties could vividly simulate the biochemical structure of cartilage, for which would generate extraordinary lubrication performance.
As shown in Figure 4a and Figure S8 (Supporting Information), the morphologies of H PVA/CS and H PVA/CS-PSPMA were characterized by Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS).The surface of H PVA/CS was dense and flat (Figure 4a(iv)), while it became rough after anchoring the ATRP initiator layer (Figure 4a(v)), and generated much of the Br signals (Figure S9, Supporting Information).The surface of H PVA/CS-PSPMA appeared a large number of uniform micro-pores after grafting PSPMA polyelectrolyte brushes (PB) chains (Figure 4a(vi)), and these micro-pores would facilitate the considerable absorption of water molecules for providing rich surface hydration.As can be seen from the cross-section images, the thickness of the ATRP initiator layer (H PVA/CS-Br ) on the surface of the H PVA/CS matrix was 50-75 μm, while the thickness of PSPMA chains-embedded composite lubrication layer on the surface of the H PVA/CS-Br was 60-80 μm (Figure 4a(vii,ix)).The hydrated and lubricating PB chains on the surface were covalently entangled with the sub-surface of the nanocrystalline domain-enhanced H PVA/CS matrix, resulting in a continuous transition from the hydrated surface layer to the underlying bulk matrix.Correspondingly, a typical layered structure of H PVA/CS-PSPMA was obtained successfully.In addition, the successful preparation of H PVA/CS-PSPMA was confirmed by X-ray Photoelectron Spectroscopy (XPS) and FT-IR.Compared to the control H PVA/CS sample (Figure 4b,c), the appearance of carbonyl (-C═O) vibration peak at 1737 cm −1 (Figure 4b) and Br 3d signal peak at 70 eV (Figure 4c) on the surface of H PVA/CS-Br , indicated the successful modification of the ATRP initiator.Furthermore, the appearance of carbonyl (-C═O) vibration peak at 1730 cm −1 and characteristic absorption peak of the sulfonic group (S═O) at 1050 cm −1 (Figure 4b), as well as the generation of K2p and S2p signal peaks at 293 and 168 eV (Figure 4c), all well confirmed the successful formation of top composite lubrication layer after grafting PB chains.
Then, the surface and bulk moduli of H PVA/CS-PSPMA were measured by employing a soft matter nano-indentation instrument, and compared them with the natural cartilage layer.The results showed that our strategy of bonding a soft PB layer onto a highstrength H PVA/CS matrix well simulates the mechanics of natural cartilage (Figure 4d).The elastic modulus of the surface PB layer was ≈0.16 MPa, while that of the bottom H PVA/CS matix was 1.45 MPa, which was similar to the values of natural bovine articular cartilage (≈0.26 and 1.89 MPa).Correspondingly, grafting PSPMA on the surface of H PVA/CS resulted in a significant wettability change from a hydrophilic state to super-hydrophilic state (Figure 4e).The initial contact angles of water droplets were 29.5±8.76°on the surface of H PVA/CS and 53.3±8.66°on the surface of H PVA/CS-Br .By contrast, the surface of H PVA/CS-PSPMA was in a typical super-hydrophilic state and the spreading time was <1 s, indicating the extraordinary surface hydration capacity of the top PB layer.This hydration capacity would enable its good swelling and rehydration feature under dynamic loading/shearing condition, which was beneficial to achieve durable and reliable lubrication.As validation, the shear stress on the surface of different samples was measured by employing a rheometer under small stress (10 kPa) (Figure 4f).It was found that the initial shear force on the surface of PVA/CS sample was very low, but which increased gradually along with time-dominated dehydration.In contrast, the robust H PVA/CS matrix showed high shear stress due to the poor surface hydration, while the surface of H PVA/CS-PSPMA achieved persistent and low surface shear stress because of its strong surface hydration ability.

Evaluation of the Lubrication Performance of H PVA/CS -PSPMA
The lubricity of H PVA/CS-PSPMA was investigated systematically on a reciprocating ball-on-disk tribometer, where 316L steel balls with 6 mm diameter were used as friction pairs.To better simulate lubrication performances in the physiological environment, all friction tests were carried out in PBS buffer solution (pH 7.4) after achieving swelling equilibrium.Through the sphere-ball friction contact mode, the influence of the load-bearing capacity and surface lubrication of the sample on the COF of the whole system were investigated.The excellent load-bearing capacity of H PVA/CS enabled to achieve low COF by resisting mechanical deformation (F def ), while the strong hydration of PB layer reduced the COF by classical hydration lubrication mechanism for minimizing the interface contribution (F int ).By increasing the load-bearing capacity of the matrix and improving the hydration degree of the surface based on the classical friction model, F = F int + F def , the interface COFs can be significantly reduced (Figure 5a).The specific friction comparison data were shown in Figure 5b.The poor bearing capacity of PVA/CS hydrogels caused a large mechanical deformation in shearing process as well as a high COF, while it generated wear easily under a low normal load of 5 N.Although the H PVA/CS can resist mechanical deformation and present strong load-bearing capacity of 20 N, the low surface hydration also led to a high COF.In contrast, H PVA/CS-PSPMA was able to maintain a stable and low COF under a 20 N load.
The COFs under different normal loads were also investigated.The COFs of H PVA/CS-PSPMA were always low and stable in a wide range of normal loads (Figure 5c).Correspondingly, the interface average contact stresses of the samples at different normal loads were calculated.The local contact stress between H PVA/CS and steel ball increased obviously with the rising of normal loads from 1, 5, 10 to 20 N (5.51-7.90MPa), while it decreased significantly when sliding against H PVA/CS-PSPMA (1.13-4.37MPa) (Figure S10, Supporting Information).Therefore, the existence of the soft composite lubrication layer can effectively dissipate the contact stress under the same load condition.The interface sliding frequencies may also affect the lubrication behavior of hydrogels.The COFs of H PVA/CS-PSPMA increased gradually by extending the sliding frequencies from 0.2, 0.4, 1.0, 2.0 to 5.0 Hz, while that of the H PVA/CS remained almost constant below 5 Hz (Figure S11, Supporting Information).The mechanism responsible for this can be explained by noticing that the composite lubrication layer was affected by the speed at which lubricant replenishes the contact.At higher speeds, a different equilibrium of fluid flow was reached corresponding to less lubricant being available for weeping into the contact.Furthermore, it was found H PVA/CS-PSPMA demonstrated good lubricity in different physiological media (Figure S12, Supporting Information).
Finally, the lubrication persistence of the H PVA/CS-PSPMA was tested by applying long-period friction experiment (100k cycles) at a normal load of 5 N (2.06 MPa) with PBS as lubricant, while the fresh bovine cartilage was used for comparison.As shown in Figure 5d, the COF of the H PVA/CS-PSPMA was extremely stable and remained at ≈0.027 during the entire testing period.By contrast, even though the natural bovine cartilage also exhibited a stable lubrication state, the COF was higher than that of H PVA/CS-PSPMA .Subsequently, the surface morphologies of the samples after friction tests were evaluated.Compared to morphologies before testing, slight surface scars at the sliding area were both observed for natural bovine cartilage and H PVA/CS-PSPMA after encountering 100k sliding cycles (Figure 5e).Meanwhile, the cumulative width of the wear scar of the H PVA/CS-PSPMA sample (≈2500 μm) was smaller than that of natural bovine cartilage (≈3200 μm), implying its stress-dissipation capability to increase the degree of conformity with the rigid indenter surface.However, after encountering a long-period friction test (100k cycles), it was observed that both surfaces at the sliding contact area deformed heavily.The deformation depth of natural bovine cartilage was 0.83 mm while that of H PVA/CS-PSPMA was only 0.65 mm (Figure 5f).Due to the good creep recovery property of H PVA/CS-PSPMA , the deformation depth could recover to ≈0.62 mm after undergoing <24 h free recovery, while that of natural bovine cartilage was still as high as ≈0.72 mm.Even though H PVA/CS-PSPMA appeared to surface wear scars and mechanical deformation, its load-bearing and lubricity were still better than that of natural bovine cartilage.These results indicate the excellent lubrication persistence, robustness, wear-resistance, and mechanical recovery feature of H PVA/CS-PSPMA are comparable to but beyond the natural cartilage.
Furthermore, as proof of application concept in cartilage replacement on implant device, H PVA/CS-PSPMA material was tried to be used as lubrication coating on the surface of traditional UHMWPE, and then its lubricity was compared with the reported lubricating system that is surface-grafted poly(2methacryloyloxyethyl phosphorylcholine) (PMPC) polyelectrolyte brushes coating. [20]To achieve this purpose, H PVA/CS-PMPC was successfully prepared by grafting PMPC polyelectrolyte brushes onto the sub-surface of the mechanically robust hydrogel (H PVA/CS ) matrix. [21]Then, H PVA/CS-PMPC was decorated onto the surface of UHMWPE as lubrication coating by applying a commercial biological adhesive as the bonding layer, [10a] and UHMWPE-H PVA/CS-PMPC was obtained successfully (Figure 5g).Correspondingly, the UHMWPE-PMPC sample was also prepared by traditional photo-initiated grafting polymerization technique. [22]In order to evaluate the lubrication performances of UHMWPE-H PVA/CS-PMPC and UHMWPE-PMPC, the friction tests were investigated on a reciprocating ball-on-disk tribometer, where 316L steel balls with 6 mm diameter were also used as friction pairs and PBS buffer solution (pH 7.4) was used as lubricant.Compared to UHMWPE-PMPC, UHMWPE-H PVA/CS-PMPC showed lower COFs in a wide range of normal loads (1, 2, 5, 10, 20 N) (Figure S13, Supporting Information).This may be attributed to the grafting density and thickness (≈dozens of microns) of hydrated PMPC polyelectrolyte brush chains on the surface of UHMWPE-H PVA/CS-PMPC are obviously higher than that of UHMWPE-PMPC (≈dozens of nanometer).The lubrication persistence of bare UHMWPE, UHMWPE-PMPC, and UHMWPE-H PVA/CS-PMPC were further evaluated by applying a long-period friction experiment (20k cycles, 5 N) in PBS.The COF of bare UHMWPE increased continuously from 0.06 to 0.12 within 15 000 cycles and then kept stable, resulting from poor surface hydration lubrication.For UHMWPE-PMPC, the sliding interface presented an ultralow COF (≈0.01) at the start-up stage, but soon rose to a relatively high level (COF: ≈0.04) in the 20th second, and then gradually increased with extending the sliding cycles.When the sliding cycles reached 20k, the surface COF of UHMWPE-PMPC increased to 0.09.By contrast, the COF of UHMWPE-H PVA/CS-PMPC always remained stable and low (COF: ≈0.028) during the entire sliding period (Figure 5h).
Subsequently, the surface wear states of these three kinds of samples were investigated by observing the morphologies of the sliding test areas.The wear scar width was ≈523 μm for bare UHMWPE (Figure 5i(i)), while it increased to ≈664 μm for UHMWPE-PMPC (Figure 5i(ii)).As a stark contrast, the wear scar width of UHMWPE-H PVA/CS-PMPC was as large as ≈1435 μm (Figure 5i(iii)).Under the same loading and testing conditions, the difference in wear scar width well reflects the contact stress situation at the sliding interface.The local interface contact stress of bare UHMWPE was as high as 47.80 MPa, while it decreased slightly after grafting PMPC brushes chains as the lubrication layer (25.74 MPa).In this case, due to the thin thickness of the PMPC brushes, the interface was still in a typical stiff contact state, so the lubrication layer was easily worn out.By comparison, our layered H PVA/CS-PMPC coating on the surface of UHMWPE could not only provide lubrication but also effectively dissipate the interface contact stress (5.55 MPa).As a robust/low-friction coating, H PVA/CS-PMPC exhibited stronger load-bearing capacity and more lasting lubricity than that of traditional surface-grafted PMPC polyelectrolyte brushes coating.

Evaluation of the Cytocompatibility and Anti-Proteins Property of H PVA/CS-PSPMA
The Cell Counting Kit-8 (CCK-8) cell viability assays were carried out to investigate the cytotoxicity of H PVA/CS-PSPMA .Specifically, mouse embryonic osteoblast precursor cells (MC3T3-E1) (Density: 1×10 4 cells/well) were cultured on the surface of H PVA/CS-PSPMA for 1, 3, and 5 days in MEM- medium, while the same cells on TCP plates were used as a control group.Correspondingly, 100 μL of complete medium containing 10% CCK-8 was added into the medium to incubate for 2 h, and then the absorbance was recorded at 450 nm in a Microplate Reader.The experiments were repeated at least three times.As shown in Figure S14 (Supporting Information), the cell viability remained all above 80% for 1, 3, and 5 days, indicating the low toxicity of the H PVA/CS-PSPMA .Furthermore, the anti-protein capacity is also an essential performance for implant material.After immersing into fluorescein isothiocyanate labeled bovine serum proteins (FITC-BSA) (0.5 mg mL −1 ) for 3 h, the samples were taken out for fluorescence imaging under a confocal laser scanning microscope (CLSM).Compared to the strong BSA proteins attachment of the control sample without the composite lubrication layer (H PVA/CS ), the protein adhesion on the surface of H PVA/CS-PSPMA decreased significantly (Figure S15, Supporting Information).The fluorescence intensities on each image were analyzed by the ImageJ software (NIH, USA).The mean fluorescence intensities of H PVA/CS-PSPMA decreased to ∼13.8%, relative to that of H PVA/CS is 67.9% (Figure S16, Supporting Information).This could be well explained as the thick surface hydration layer of H PVA/CS-PSPMA could effectively resist the approach of protein molecules.Combined with low cytotoxicity and good anti-protein properties, our H PVA/CS-PSPMA presents great application potential as cartilage replacement or implant coatings in vivo.

Conclusion
Natural cartilage as can achieve ultra-long lubricity lifetime at physiological media under high contact pressure condition, developing mechanically robust artificial cartilage materials is still challenging.In this work, we engineered one kind of novel cartilage-like hydrogels (H PVA/CS-PSPMA ) with high load-bearing, low-friction, wear-resistance, and deformation recovery properties by covalently manufacturing thick composite lubrication layer through the sub-surface of the mechanically robust hydrogels matix (H PVA/CS ).The H PVA/CS-PSPMA exhibited high mechanical strength (compression modulus: 11.8 MPa, compression strength: 15.25 MPa), good anti-swelling ability in the physiological medium, and typical creep recovery characteristics.The H PVA/CS-PSPMA could present good lubricity (COF: 0.02-0.03) in PBS under high contact pressure condition (2.0-3.0MPa).Surprisingly, under harsh friction test condition (100k cycles, P ≈2.5 MPa), the H PVA/CS-PSPMA could maintain lower and more stable COF (≈0.027) than that of natural bovine cartilage, as well as ignorable surface wear.More importantly, the mechanical deformation at the sliding test area of H PVA/CS-PSPMA could recover considerably after the friction test.The excellent lubrication persistence, robustness, wear-resistance, and mechanical recovery feature of H PVA/CS-PSPMA are comparable to but beyond the natural animal cartilage.Finally, as proof of concept, H PVA/CS-PMPC was prepared and then decorated onto the surface of implantable UHMWPE as a robust/low-friction coating, which exhibited higher load-bearing capacity and more lasting lubricity than that of traditional surface-grafted PMPC polyelectrolyte brushes coating.However, the experiments conducted in current research were in vitro friction tests, further validation of coating application value requires big animal experiments and long-term clinical trials, which is still a long way to go.The basic design concept of this work provides a new route for developing wearresistant cartilage replacement and artificial joint coatings.

Figure 1 .
Figure 1.Design concept of bionic cartilage hydrogel H PVA/CS-PSPMA .a) Schematic diagrams of the human articular cartilage system (i), reversible mechanical deformation and recovery behavior of natural cartilage layer composed of hydrated superficial layer and robust load-bearing matrix under loading/unloading process (ii), and reversible mechanical deformation and recovery feature of bionic layered cartilage hydrogel (H PVA/CS-PSPMA ) composed of top composite lubrication layer and bottom mechanical support layer under loading/unloading process (iii).b) (i) Fluorescence photograph of H PVA/CS-PSPMA .(Scale bar: 5 mm) (ii) H PVA/CS-PSPMA bears the weight of a 40 kg dumbbell.iii) Photographs of H PVA/CS-PSPMA and control hydrogel during compression process with 40 kg.

Figure 2 .
Figure 2. Characterizations of microstructure and anti-swelling properties.a) SEM images, 2D WAXS, and SAXS debye scattering rings of the hydrogel matrix after different treatments: (i, iv, vii) PVA/CS, (ii, v, viii) PVA/CS-Cit, 3− (iii, vi,ix) H PVA/CS .b) The integrated spectra of WAXS profiles were plotted as a function of the scattering vector, intensity∼q.c) The integrated spectra of SAXS profiles were plotted as a function of the scattering vector, intensity*q 2 ∼q.d) The crystal dimension and inter-crystal spacing data of different hydrogel matrixes.e) Schematic diagram of changes in microscopic crystal regions of different hydrogels.f) The mass swelling ratio-immersing time curves in PBS buffer solution (pH 7.4) of H PVA/CS sample and classic DN hydrogel (Fe-P(AAm/AAc) hydrogel) system.g) The volume swelling ratios in PBS (pH 7.4) buffer solution of H PVA/CS sample and classic DN hydrogel.h) The swelling photographs of H PVA/CS-PSPMA (i) and Fe-P(AAm/AAc) hydrogel (ii) after immersing into PBS for different time (Scale bar: 10 mm).Data in (F) and (G) are means ± SD, n = 3.

Figure 3 .
Figure3.Characterizations of the mechanical properties.a) Tensile stress-strain curves of PVA/CS and H PVA/CS .b) Compression stress-strain curves of PVA/CS and H PVA/CS (the maximum strain was limited to 70%).c) Creeping curves of PVA/CS and H PVA/CS .d) Ashby diagram of compress strengths versus compress moduli of H PVA/CS and some reported hydrogels systems (i.e., freeze-thawed PVA hydrogel,[10c,23]  Human cartilage,[6,7c,24]  Fe-P(AAm/AAc) hydrogel,[19,25] DN hydrogel,[26]  Fiber Reinforced hydrogel.[7c]).e) Schematic diagram showing the cyclic process of daytime loading (pink area) and nighttime recovering (blue area) of the natural cartilage layer.f) Four cyclic strain-time curves of PVA/CS and H PVA/CS under dynamic creep loading (pink area, 16 h)/unloading (blue area, 8 h) process.g) Schematic diagram of the tensile tearing test of the sample and anti-fracture mechanism.h) Tensile stress-strain curves for the tearing test of PVA/CS and H PVA/CS .Snapshots of i) PVA/CS sample and j) H PVA/CS in real-time fracture process (Scale bar: 5 mm).

Figure 4 .
Figure 4. Characterizations of components, morphologies, and hydration states.a) Schematic diagram, surface images (i, ii, iii), and cross-section SEM images (iv, v, vi) of H PVA/CS , H PVA/CS-Br , and H PVA/CS-PSPMA .b)The surface ATR-FTIR spectra of H PVA/CS, H PVA/CS-Br , and H PVA/CS-PSPMA .c) The surface XPS spectra of H PVA/CS , H PVA/CS-Br , and H PVA/CS-PSPMA .d) The surface moduli of the H PVA/CS-PSPMA and natural bovine articular cartilage are in an equilibrium swelling state.e) Water contact angles on the surfaces of H PVA/CS, H PVA/CS-Br, and H PVA/CS-PSPMA .f) The surface shear force of different hydrogels under 10 N load.Data in (E) are means ± SD, n = 3.

Figure 5 .
Figure 5. Lubrication, the anti-wear performance of hydrogels, and corresponding demonstrations.a) Schematic diagram to show the friction shearing test process for H PVA/CS-PSPMA on a ball-on-disk contact mode.b) Representative COF curves and c) average COFs values of different hydrogels (frequency: 1 Hz, lubricant: PBS).d) The COF curves of H PVA/CS-PSPMA and natural bovine articular cartilage during the 100k sliding cycles (Lubricant: PBS, the normal load: 5 N, the frequency: 1 Hz).Fresh natural bovine articular cartilage was commercially purchased from a local market without treatment.e) Surface morphologies of the natural bovine cartilage and H PVA/CS-PSPMA after encountering 100k sliding cycles.f) The recovery process of deformation depth for bovine cartilage and H PVA/CS-PSPMA at sliding test area after 100k test cycles.g) Photographs of the blank UHMWPE, lubrication coating-modified UHMWPE (UHMWPE-PMPC) prepared by traditional photo-initiated surface grafting and UHMWPE-H PVA/CS-PMPC in this work.h) The real-time COF curves of UHMWPE, UHMWPE-PMPC, and UHMWPE-H PVA/CS-PMPC during the 20k sliding cycles (Lubricant: PBS, the normal load: 5 N, the frequency: 1 Hz).i) Schematic diagram to show the local interface contact stress states at the friction shearing process for the UHMWPE-PMPC (stiff contact) and UHMWPE-H PVA/CS-PMPC (stress dissipation) on the ball-on-disk contact mode, and surface morphologies of test areas for UHMWPE, UHMWPE-PMPC, and UHMWPE-H PVA/CS-PMPC after completing 20k sliding cycles.Data in (C) and (F) are means ± SD, n = 3.