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AFM study on superlubricity between Ti6Al4V/ polymer surfaces achieved with liposomes Yiqin Duan, Yuhong Liu, Jinjin Li, Shaofei Feng, and Shizhu Wen Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01683 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 10, 2019

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AFM study on superlubricity between Ti6Al4V/polymer surfaces achieved with liposomes Yiqin Duan, Yuhong Liu*, Jinjin Li*, Shaofei Feng, Shizhu Wen State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

ABSTRACT ： Liposomes have been considered as the boundary lubricant in natural joints. They are also the main component of bionic lubricant. In this study, the tribological properties of liposomes on Ti6Al4V/polymer surface were studied by atomic force microscope (AFM) at the nanoscale. The superlubricity with a friction coefficient of 0.007 was achieved under the maximal pressure of 15 MPa, consisting with the lubrication condition of natural joints. Especially, when the AFM probe was hydrophilically modified and pre-adsorbed, the friction coefficient and load bearing capacity could be further improved. In addition, probe with large radius could maintain the stable lubrication of liposomes in the contact zone. Finally, an optimal lubrication model of liposomes was established and the critical force for superlubricity was also proposed. It was the boundary between elastic deformation and plastic deformation for vesicles. It was also the indicator of plough effect appearing on the adsorbed layer. This work reveals the interfacial

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behavior of liposomes and realizes the controllable superlubricity system, providing more guidance for clinical application.

KEYWORDS:

liposome,

superlubricity,

critical

force,

titanium

alloy,

AFM


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INTRODUCTION Arthrosis affects many adults all over the world, and artificial joint placement has become an effective treatment

1-3.

However, in practical applications, lubrication failure

often occurs among these prostheses

4, 5,

resulting in the tissue inflammation and

reduction of prosthesis life. The surface modification 6-9 is often used to increase the wear resistance of surface. In addition, choosing a suitable bionic lubricant can also improve the joint lubrication and reduce the wear of surface

10.

It is well known that natural joints

have an excellent tribological performance, with friction coefficient less than 0.001 11, and average bearing capacity of 5 MPa (20 MPa in some areas)

12.

The macromolecules in

synovial fluid, such as hyaluronic acid (HA), lubricin, aggrecan and phospholipids

13, 14,

play an important role. It was reported that the HA had poor lubrication 15, but it contributed greatly to the bearing capacity and the wear prevention of cartilage

16, 17.

Lubricin, as a

glycoprotein, was found on the surface of cartilage 13. It had a strong anti-adhesive effect and could reduce the wear of cartilage

18-20.

However, either lubricin or aggrecan, its

bearing capacity was less than 4 MPa 20-22, which is far away from the natural joints. In fact, phospholipids have been considered as the boundary lubricant in recent years 23, 24.

They are amphiphilic with hydrophilic head groups and hydrophobic hydrocarbon

chains, and thus they are easily self-assembled to form bilayer, micelle or vesicle (liposome). Liposomes are widely used in drug delivery systems in medicine or additive


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in cosmetics due to their great biocompatibility proposed by Bangham et al.

27.

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25, 26.

Their role as lubricant was first

Afterwards, Sivan et al.

28

found that the multilamellar

vesicles were more likely to remain on the cartilage surface and obtained a low friction coefficient of 0.01. However, due to the face-to-face contact mode, the contact pressure in this experiment was only 2.4 MPa. Veselack et al.

29, 30

found that the combination of

DPPC and HA could effectively improve the lubricity of cartilage surface, but the problem was the low bearing capacity (only 2 MPa). With the development of microscopic measurement technologies, the explorations have also begun to move from the macro field to the micro field. Surface force apparatuses (SFA) is commonly used in the tribological study 31-35. Liposomes are firmly adsorbed on two opposite mica surfaces by electrostatic force. Goldberg and Klein et al.

23, 31

found

that the hydrogenated soybean phospholipids (HSPC) liposomes could achieve such a low friction coefficient of 2×10−5 in water at the maximal pressure of 10 MPa. The superlubricity of liposomes under high load were explained as the highly hydrated PC head-groups and the robust hydration shells

35, 36.

To obtain softer biocompatible

surfaces, Klein et al. 33 pre-adsorbed a soft polymer layer (alginate-on-chitosan) onto mica before the friction test. It was found that the adsorption of HSPC liposomes on the surface was more stable, and thus a friction coefficient of 10-4 under the high load of 35 MPa was realized.


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Moreover, AFM is also used in the lubrication study

37-41.

Park et al

40

measured the

friction coefficient between AFM tip (radius < 10 nm) and cobalt-chromium alloy surface immersed in DPPC suspension with different concentrations. They found that the lowest friction coefficient was 0.026. In order to simulate the interaction between two flat surfaces, Raj et al.

38

glued a silica particle with a diameter of 19 μm on top of the cantilever, and

measured the friction coefficient between two silica surfaces adsorbed with DPPC and HA. The friction coefficient was 0.006 under the pressure of 23 MPa. The above studies show that liposomes can achieve the same lubrication effect as the natural joints by AFM. Although liposomes have been shown to achieve the superlubricity under the high load (more than 20 MPa) on silica/mica surfaces. For an applicable purpose, their tribological behavior on titanium alloy (Ti6Al4V) surface is more important, which is the common material for prosthesis due to its excellent biocompatibility and mechanical properties 42.

Furthermore, in our previous work

43,

9,

we found that liposomes adsorbed on

Ti6Al4V/UHMWPE surfaces could efficiently reduce the wear of two surfaces and realize the friction coefficient of 0.015 under pressure of 31 MPa. However, due to the large roughness of UHMWPE surface, the superlubricity of liposomes was not realized. In order to reduce the influence of roughness and to learn more about the interfacial behavior of liposomes, the experiment conducted at the nanoscale is needed. Therefore, in the present study, AFM was used to investigate the lubrication of liposomes on Ti6Al4V/polymer surfaces. The superlubricity was achieved under the


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critical pressure of 15 MPa. Afterwards, the influences of adsorption type, probe modification and contact area on the critical load were studied to further improve the lubrication. Finally, an optimal lubrication model was established to explain the mechanical and frictional behavior of liposomes.

EXPERIMENTAL SECTION Materials.

1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine

(DPPC)

lipid

was

purchased from Avanti (Alabama, USA) in powder form. The monodispersed spherical polystyrene (PS) particles were purchased from Nano-Micro, Co., Ltd (Ra=5.1 nm). The purified water used in sample preparation and measurement procedures had a resistivity of 18.2 MΩ at 25 ℃. Ti6Al4V foils (10×10 mm, 1 mm thickness) were purchased from Goodfellow, Inc. and polished to achieve flat and smooth surfaces (Ra=2.1 nm). Preparation of liposomes and adsorbed surfaces. Lipid powders were mixed into water and bathed for 1 h at a temperature of 60 °C, which is above the Tm of DPPC (41 °C). Then, the hydrated lipid sheets were detached by oscillation to form self-closed multilamellar vesicles (MLVs)

31.

Afterwards the MLVs were downsized to form small

unilamellar vesicles (SUVs, 20–200 nm) by stepwise extrusion through polycarbonate membranes (Whatman, Inc.), starting with 400 nm, then 200 nm, and ending with 100 nm, using an extruder (Avanti, USA). The final size of the SUVs was 121 ± 20 nm as determined by dynamic light scattering (DLS) using a zetasizer (Malvern Nano-ZS). As


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with all procedures for extrusion, the experiments were performed at a temperature of 60 °C. Liposome-adsorbed surfaces were used during AFM experiment. Polished Ti6Al4V surfaces were placed in a 1 mg/mL DPPC-SUV suspension for no less than 2 h at a room temperature. After incubation, the surfaces were rinsed by placing the adsorbed surfaces into a container of pure water, and shook to remove excess, non-adsorbed liposomes.

Atomic Force Microscope (AFM). All AFM measurements were performed using an Asylum Research MFP-3D AFM. The friction measurements were obtained in contact mode and the normal force measurements were obtained during the probe approach and retrace process. Rectangular tipless cantilevers (TL-CONT) with dimensions of 450 μm in length and 50 μm in width were used, and the polystyrene particle with a diameter of 5-20 μm was glued on top of the cantilever using UV glue. The normal spring constant (kN) was determined by the thermal noise method

44.

The normal force between the

polystyrene particle and Ti6Al4V substrate was measured as a function of the vertical expansion of the Z-sensor, and then it was converted into normal force versus surface separation curve by the method described elsewhere

45.

The duration of a single force

measurement (both loading and unloading) was 1 s and the approach/retract velocity was 400 nm/s. The lateral scanning area was 1 μm ×1 μm and the scanning rate was 0.5-4 Hz, corresponding to the scanning velocity of 1.25-10 μm/s. Except for the velocity


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experiment, the commonly used velocity was 2.5 μm/s. The friction force was determined by the half width of the lateral force loop and the lateral detector sensitivity. The lateral detector sensitivity was calibrated by an improved wedge calibration method 46. AFM Images were obtained in tapping mode in pure water using silicon nitride tips mounted on V-shaped cantilevers with a nominal spring constant of 0.35 N/m and a nominal tip radius of 2 nm (SNL, Bruker). All experiments were executed in a fluid cell at a room temperature around 25 °C.

RESULTS AND DISCUSSIONS The SEM image of the probe glued with polystyrene particle is shown in Figure 1(a). Then the friction experiments were carried out under two adsorption conditions: the single-sided adsorption and the double-sided adsorption, as shown in Figure 1(b). In the single-sided adsorption condition, only Ti6Al4V surface was pre-adsorbed by liposomes before the friction experiment. The relationship between the normal load FN and the lateral force FL is shown in Figure 2 (black curve). It is found that the load and friction show a good linear relationship, and the friction coefficient can be obtained by linear fitting to FL-FN points. At first, it is necessary to define the different lubrication region, which is distinguished by the magnitude of the friction coefficient. The orders of magnitude of 0.001, 0.01, and 0.1 correspond to the first, second, and third lubrication regions, respectively. In Figure 2, the friction coefficient is not unique, and the lubrication state is


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divided into two regions by the critical force (Fcrit). In the first lubrication region, the friction coefficient is 0.007 when the load is less than 168 nN, thus the lubrication system achieves the superlubricity. In the second lubrication region, the friction coefficient increases to 0.049 when the load exceeds 168 nN, which is 7 times higher than the original value. Here the critical force is 168 nN, corresponding to a critical pressure of 15.09 MPa (using Hertz model). It is known that the physiological pressure in human joints is 5-20 MPa and the friction coefficient is 0.001 or even lower

11, 12.

Therefore, the high

bearing capacity and the low friction are achieved in the first lubrication region, which is similar to the natural joints.

Figure 1. (A) SEM images of polystyrene particles with diameters of 5 μm and 20 μm glued on top of the cantilever; (B) the schematic illustrations of single-sided adsorption and double-sided adsorption.


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Furthermore, the double-sided adsorption 23, 47 is often considered as an effective way to achieve a lower friction coefficient and a higher bearing capacity, because the shear plane often occurs between the adsorption layers and the influence of the roughness from substrate can be reduced. For this reason, the double-sided adsorption experiment was conducted (red curve). Compared with the single-sided adsorption, the critical force and friction coefficient under double-sided adsorption are not changed significantly. The friction coefficients in the first lubrication region and the second lubrication region are 0.008 and 0.048, respectively. Although the PS probe was pre-adsorbed with liposomes, the lubricating effect is not improved, indicating that liposomes are not successfully adsorbed on the probe.


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Figure 2. Lateral force as a function of normal load between PS probe and Ti6Al4V surface under two adsorption conditions. Experiments were conducted under lipid concentration of 1 mg/mL and scanning velocity of 2.5 μm/s.

In order to measure the adsorption strength between the polystyrene surface and liposomes, the XPS spectrum of phosphorus element on the top of polystyrene surface under two adsorption conditions was obtained (Figure S1). In single-sided adsorption, there is no P element appearing on PS surface before and after the friction test, indicating the low concentration of lipids or the absence of lipids. In the double-sided adsorption experiment, a low-intensity peak with the binding energy of 133 eV appears before the friction test, demonstrating the existence of phospholipids

48.

However, after the friction

test, the peak intensity of phosphorus decreases, indicating that these adsorbed liposomes quickly fall off from the polystyrene surface during the rubbing process and no liposomes come to adsorb again onto the surface. The results show that the interaction between the polystyrene surface and liposomes is very weak. This weak interaction is resulted from the hydrophobicity of the polystyrene

49, 50.

Due to the difference between

hydrophilic phospholipids and hydrophobic polystyrene, the adhesion force between them is very small (Figure S2).


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To improve the hydrophilicity of polystyrene, carboxyl groups were modified onto the PS probe, as shown in Figure 3. However, in the single-sided adsorption condition, it is found that the critical force and the friction coefficient are not changed after the surface modification. Therefore, even if the polystyrene surface becomes more hydrophilic, the shear force is still insufficient to make liposomes adsorb on the polystyrene surface. On the contrary, when liposomes are pre-adsorbed on the modified PS probe and Ti6Al4V surface (double-sided adsorption), the critical force and the friction coefficient are improved. The critical force increases from the original 168 nN to 302 nN, and the corresponding critical pressure increases from 15.09 MPa to 18.35 MPa, which is very close to the maximum pressure of the human joint (20 MPa). At the same time, the friction coefficient decreases from 0.007 to 0.005 in the first lubrication region and decreases from 0.048 to 0.033 in the second lubrication region. It indicates that the liposomes could stably adsorb onto two surfaces and make the shear slip happen between these adsorbed layers even the applied load exceeds the critical force. Moreover, when the applied load further increases to 534 nN (corresponding to the pressure of 22.20 MPa), the friction coefficient would increase to 0.278 (Figure S3), which is close to the friction coefficient between two surfaces across water (Figure S4). The result shows that in the third lubrication region, liposomes are extruded out of the contact zone or rupture to bilayers so that the two surfaces are in direct contact and the friction coefficient increases quickly. The XPS experiment also demonstrates that the pre-


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adsorption operation allows the liposomes to adsorb firmly on the modified PS surface (Figure S5). In the double-sided adsorption condition, the peak belonging to phospholipids appears before and after the rubbing process. Therefore, pre-adsorption ensures that liposomes could adsorb stably onto the modified polystyrene surface even during the rubbing process. It is because that the modified PS ball is negatively charged, and the head groups of phospholipid molecules are zwitterions, therefore, the modified PS ball can adsorb phospholipid molecules through the electrostatic interaction to form the vesicles.

Figure 3. Lateral force as a function of normal load between modified PS probe and Ti6Al4V surface under different adsorption conditions. Experiments were conducted under lipid concentration of 1 mg/mL and scanning velocity of 2.5 μm/s.


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Another factor that would influence the lubrication is the contact area between two surfaces. The contact area was changed by reducing the diameter of the polystyrene particle from 20 μm to 5 μm. In Figure 4, it is found that the friction coefficient in the superlubricity region increases from 0.007 to 0.009 when the diameter decreases from 20 μm to 5 μm. Furthermore, it is notable that the critical pressure increases from 15.09 MPa to 24.34 MPa when the diameter decreases from 20 μm to 5 μm. In theory, when the structure of the adsorption layer is not changed, the critical pressure would keep constant and not be affected by the diameter of the probe. However, the inconsistency of the critical pressure in this experiment may come from many factors, including the different surface roughness of the probe in two diameters, and the different adsorption structure when the contact area is changed. In fact, the change in the contact area has the greatest influence on the width of the second lubrication region. When the diameter is 5 μm, the friction coefficient increases directly from 0.009 to 0.335 with the increasing load, indicating that the whole lubrication system directly enters the third lubrication region from the first lubrication region. This phenomenon was also observed in the micelle system

47.

Therefore, reduced contact

area would narrow the width of the second lubrication region, and sometimes it would disappear. Therefore, choosing a large sphere can achieve an efficient lubrication (μ=0.001~0.01) and reduce the direct contact of the rubbing surfaces.


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Figure 4. Effects of the diameter of the polystyrene particle on the tribological behavior of PS/Ti6Al4V surfaces.

From the above results (Figure 2 to Figure 4), an improved lubrication is achieved when three conditions are reached at the same time: double-sided adsorption, hydrophilic polystyrene probe and a large radius of PS spheres. The lubrication state can be divided into three regions (see Figure 3 and Figure S3). In the first lubrication region, the friction coefficient is 0.005 and the critical pressure is almost 18 MPa. In the second lubrication region, the friction coefficient is 0.033. Until the applied pressure exceeds 22 MPa, two rubbing surfaces are in direct contact and the friction coefficient increase to 0.278. It is noted that the following experiments were carried out based on the improved lubrication results.


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In order to further explore the lubrication mechanism of the first lubrication region (super-low friction), it is necessary to determine the lubrication type. For this reason, the friction coefficients of liposomes adsorbed on Ti6Al4V under three sliding velocities were measured, as shown in Figure 5. Results show that the friction coefficients between the three sliding velocities are very similar (0.007-0.01), so it can be considered that the friction coefficient is almost independent of the velocity. Therefore, it can be inferred that the lubrication state of liposomes is boundary lubrication in the superlubricity region. The physical and chemical properties of the adsorbed layer will directly determine the bearing capacity and the friction coefficient of the whole lubrication system.

Figure 5. Lateral force as a function of normal load between two liposomes-adsorbed surfaces under three different scanning velocities. The applied load is less than 168 nN so that the whole system is in the first lubrication region.


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Furthermore, the interaction between two liposomes-adsorbed surfaces is also important to explain the lubrication mechanism in different lubrication regions. The approach curves between two surfaces before and after the friction test were measured, as shown in Figure 6. It is found that there is a soft layer between the PS probe and Ti6Al4V surface before the friction test, which can be considered as the liposome layers. In the first lubrication region (FNFcrit), the thickness of adsorption layer increases after the friction test (by comparing the red and blue curves). It suggests that the probe destroys the adsorption layer during the shearing process, and then these fragments easily fall into the lubrication zone, resulting in a thicker adsorption layer and a higher friction. This is the typical plough effect.


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Figure 6. The approach curves between two liposomes-adsorbed surfaces before and after the friction test.

In addition, the adsorption morphology of the liposomes on the Ti6Al4V surface after the friction test was also measured by AFM, as shown in Figure 7. In the first lubrication region (Figure 7(a)), there is a dense layer filled with intact and globular vesicles, indicating that the vesicles are elastic in the shearing process and their deformation can recover quickly. At the same time, the dense and uniformly distributed vesicles can facilitate the forming of a strong and smooth adsorption layer. However, in the second lubrication region (Figure 7(b)), the layer presents a distorted morphology, indicating that the vesicles are in plastic deformation instead of elastic deformation. Due to the larger energy barrier between the upper and the lower vesicles at a high load 47, the energy loss between these distorted vesicles slipping across each other is very high. This is one of


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the important reason for the higher friction coefficient in the second lubrication region. Moreover, when the applied load is large enough (Figure S6), some vesicles are flattened by the probe or rupture to bilayers, and other vesicles are removed from the contact zone, resulting in a similar friction coefficient to the substrates.


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Figure 7. AFM images of liposomes-adsorbed Ti6Al4V surface after the friction test: (A) in the first lubrication region; (B) in the second lubrication region. The images were obtained in tapping mode in pure water.

Before establishing the lubrication model, the surface roughness is a parameter that cannot be ignored. In this experiment, the roughness of Ti6Al4V substrate and polystyrene probe are 2.1 nm and 5.1 nm, respectively. Therefore, it has little effect on the friction coefficient and the role of adsorption layer is dominated. Finally, based on the force-separation curve and the adsorption topography after the friction test, the optimal lubrication model of liposomes adsorbed on the Ti6Al4V substrate and the modified PS probe (the diameter of PS probe is 20 μm) is established in Figure 8 and Figure 9. Before the friction test, the liposomes are stably adsorbed on two surfaces, forming the multilayer adsorption structure during the pre-adsorption process (Scheme 1). In the first lubrication region (FNFcrit), the lubrication system enters the second lubrication region (Scheme 4 and Figure 9(b)). The biggest change is that the deformation of vesicles irreversibly changes from the elastic region to the plastic region (distortion). It means that, for two opposite vesicles, the difficulty to slide past each other is increased when the shape of vesicle is non-circular, leading to a higher energy loss (friction coefficient). At the same time, the plough effect on the adsorption layer caused by the probe is enhanced (vesicles are squeezed out or ruptured), and the effect of hydration lubrication is weakened. Due to the distortion of the vesicle and the enhanced plough effect, the friction coefficient in the second lubrication region increases to the order of 10-2. When the applied load continues to increase, vesicles are gradually extruded from the contact zone and two surfaces are in direct contact with the friction coefficient of 0.20.4 (Scheme 5).


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Figure 8. Schematic illustrations describing the interaction between two liposomesadsorbed surfaces when the applied load is increasing. Scheme 1 is the adsorption morphology of two liposomes-adsorbed surfaces. Schemes 2-3 are in the first lubrication region (FNFcrit). Scheme 5 is the direct contact of two surfaces (F>>Fcrit).


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Figure 9. Lubrication model between two liposomes-adsorbed surfaces when vesicles are in (A) elastic deformation and (B) distorted deformation.

In addition, it is found that either in the single-sided adsorption experiment or in the double-sided adsorption experiment (Figure 3), the lubrication system can achieve the superlubricy in the first lubrication region. It is explained that high concentration of vesicles could form a multi-layer structure onto surfaces so that the shear plane occurs between the vesicles. However, double-sided adsorption ensures the high adsorption strength between vesicles and the modified polystyrene surface, while loose vesicles are easily extruded by the probe in the single-sided adsorption experiment. Therefore, the bearing capacity (the critical force) is weaker than that in the double-sided adsorption experiment. Especially in the second lubrication region, the friction coefficient in the double-sided adsorption experiment is lower, indicating the stable adsorption of vesicles.


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CONCLUSIONS The super-low friction coefficient of 0.007 under the maximal pressure of 15 MPa is achieved when liposomes are pre-adsorbed onto the TiAl4V surface. When the load exceeds the critical force, the friction coefficient increase quickly to 0.049. Afterwards, carboxyl groups were modified onto the PS probe to improve its hydrophilicity. It is found that liposomes can firmly adsorb on two surfaces during the pre-adsorption process, and thus the friction coefficient and the bearing capacity are both improved. The superlubricity of liposomes is attributed to the smooth shear plane, the elastic vesicles and the hydrated PC groups. Once the applied load exceeds the critical force, the distorted vesicles and the obvious plough effect lead to the increased friction coefficient of 10-2. This work explores the molecular interface behavior of liposomes on the surface of artificial joints, and realizes the controllable construction of superlubricity system. It enriches the lubrication theory of liposomes and provides guidance for the development of bionic lubricants.


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ASSOCIATED CONTENT Supporting Information. Details about the XPS spectrum of P 2p on the top of the PS probe before and after the friction test; the retraction curves between PS probe and liposomes-adsorbed Ti6Al4V surface; the whole lubrication regions between modified PS probe and Ti6Al4V surfaces; the friction coefficient between PS probe and bare Ti6Al4V surface under pure water; the XPS spectrum of P 2p on the top of the modified PS probe before and after the friction test; AFM image of liposomes-adsorbed Ti6Al4V surface when two surfaces directly contact. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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We thank engineers from Bruker Inc. for the guidance about AFM. We also appreciate the support from the State Key Laboratory of Tribology. This research is financially supported by National Natural Science Foundation of China (Grant No. 51875303, 51775295, 51527901).


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