Article pubs.acs.org/Langmuir
Insight into the Tribological Behavior of Liposomes in Artificial Joints Yiqin Duan,† Yuhong Liu,*,† Caixia Zhang,‡ Zhe Chen,† and Shizhu Wen† †
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China Beijing Key Laboratory of Advanced Manufacturing Technology, Beijing University of Technology, Beijing 100124, China
‡
S Supporting Information *
ABSTRACT: Liposomes are widely used in drug delivery and gene therapy, and their new role as boundary lubricant in natural/artificial joints has been found in recent years. In this study, the tribological properties of liposomes on titanium alloy (Ti6Al4 V)/UHMWPE interface were studied by a ballon-disc tribometer. The efficient reduction of friction coefficient and wear on both surfaces under various velocities and loads is found. A multilayer structure of physically adsorbed liposomes on Ti6Al4 V surface was also observed by atomic force microscope (AFM). Except for the hydration mechanism by phosphatidylcholine (PC) groups, the well-performed tribological properties by liposomes is also attributed to the existence of adsorbed liposome layers on both surfaces, which could reduce asperities contact and show great bearing capacity. This work enriches the research on liposomes for lubrication improvement on artificial surface and shows their value in clinical application.
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INTRODUCTION Osteoarthritis and cervical spondylosis, known as degenerative disease, affect many adults all over the world.1,2 Although artificial joint replacement has been introduced as a possible treatment, problems such as infection, prosthesis loosening, and lubrication failure are still remained.3,4 Improving the tribological properties of artificial implants will greatly increase their service life. Thus, research on surface modification by coatings5−7 and lubrication improvement by biolubricants8−10 has been studied for many years. It is well-known that natural joints reveal an excellent tribological performance, with friction coefficient of less than 0.00111,12 and bearing capacity of 5 MPa at physiological condition in hips or even 20 MPa in some areas.13 To be the widely recognized mechanism in natural joints lubrication, boundary lubrication14−23 emphasizes the role of biological macromolecules in synovial fluid, where sliding surfaces are lubricated primarily by a molecularly thin film attached onto surfaces. The energy dissipating mechanisms among these molecules have been often described as breaking of bonds24 or viscous losses in the subnanometer hydration shells.25 Common macromolecules in synovial fluid include hyaluronic acid (HA),26−33 lubricin28−36 and phospholipids.24−26,37,38 In fact, HA and lubricin have been found to bear high load and work as antiadhesion components to reduce the wear of cartilage, rather than lubricants in articular cartilage.27−29,34−36 Moreover, direct measurements with HA, lubricin, and their combination indicate that none of these can explain the low friction (0.001) of the cartilage surface at the high pressures (at least 10 MPa).30−33 However, in surface force balance (SFB) measurement, it was observed that when adding phospholipids, the lubrication and © XXXX American Chemical Society
bearing capability could be greatly enhanced to reach the physiological level in human joints.19 Phospholipids are amphiphilic with highly hydrated PC head groups and hydrophobic hydrocarbon chains and are easily selfassembled to form different structures, like bilayer and vesicle (liposome). They have been considered as the boundary lubricant in recent years.20−26,37−39 As for the lubrication mechanism by lipid bilayers, an oligolamellar theory was proposed.18 It is pointed out that shear slip occurs between layers, like graphite, to obtain a very low friction of 0.01,17 whereas they could only bear pressure of 1−2 MPa.13 To the contrary, liposomes, which were found to exist on the outer surface of cartilage,40,41 show the ability to reduce friction coefficient to extremely low value of 2 × 10−5 in pure water and 6 × 10−4 in 150 mM salt system under pressures at least 6 MPa by SFB.38 The low friction under high pressures is attributed to the hydration lubrication mechanism by highly hydrated PC headgroup exposed at the outer surface.14,24 Such PC groups are highly hydrated with up to 15 water molecules reported in the primary hydration shell in the gel state42−44 to form a hydration layer surrounding the PC groups at the outer surface of vesicles. The closed nature of vesicles themselves and repulsion force from hydration layers can even sustain a large pressure of 10 MPa measured in SFB between mica surfaces.38 The rapid exchange between water molecules in the hydration shell and free water in the surrounding enables the fluidity of shear plane,25,45 leading to low friction. Meanwhile, the mechanical properties of liposomes were also examined by AFM.20,37 Combined with SFB results, it Received: July 28, 2016 Revised: September 21, 2016
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thermally equilibrate for 30 min before imaging. TEM Images were collected at 80 kV at pixel sizes of 0.2 nm. The contact angle was measured by an optical equipment (Powereach JC2000A). The characteristic of Ti6Al4 V surface before and after lubrication was detected by XPS (ESCALAB 250 XI, Thermo Scientific Instrument, USA) and Raman Spectrometer (LabRAM HR800, Horiba). XPS measurements were performed with a monochromatized Al Kα X-ray source. All binding energy values were calibrated against a C 1s peak at 285 eV. Raman spectrometer used a polarized 514 nm argon-ion laser source. Each sample was measured from at least three points on the surface. Universal Microtribometer (UMT) for Friction Testing. Friction test with liposomes suspension under 37 °C was carried out using a ballon-disc tribometer (UMT-3, CETR). Polished Ti6Al4 V foils and UHMWPE balls were chosen as friction pairs under a reciprocating movement. The two materials are immersed by lubricant. To simulate a range of physiological movements, the average sliding speed was in the range of 0.03 to 48 mm/s, and the normal loads were 1 to 4 N, equivalent to maximum Hertz pressure of 24.50 to 38.89 MPa, which are large enough to make sure that artificial joints are used safely in human body (physiological pressure of joints is range from 5 to 30 MPa). All reported average friction coefficient is obtained from stable stage of curves and calculated from at least five repetitions. Wear scars on UHMWPE balls and tracks on Ti6Al4 V were characterized by the optical microscope SZX12 (OLYMPUS, Japan).
is shown that the lubrication by liposomes can be improved with better mechanical stability. Another study shows that, in an ex vivo cartilage experiment, liposomes could also reduce the friction coefficient to 0.01 under 2 MPa.39 However, there has been little evidence to state that liposomes are also effective under pressure of 20 MPa or more, which is common in the human body.13 Furthermore, titanium alloys are widely used as the bearing materials for artificial joints, due to its excellent biocompatibility, corrosion resistance and mechanical properties.7,46,47 Ti6Al4 V and UHMWPE have been considered as one of the most important biomaterials system for total joint replacement prosthesis.6,47 To the best of our knowledge, the role of liposomes on lubrication of Ti6Al4 V/UHMWPE pairs is yet to be elucidated. This work will enrich the theory about how liposomes work under higher pressure in joints, and also give more guidance to design bionic lubricant for treatment of joints disease. In our work, we examined the effects of velocity, normal load, lipid concentration and vesicle size on friction coefficient using a ball-on-disk tribometer. Raman and X-ray photoelectron spectroscopy (XPS) were used to analyze the reaction between liposomes and substrate. We also measure the adsorption morphology using AFM to establish the relationship between adsorption behavior and tribological properties.
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RESULTS AND DISCUSSION Characterization of Liposomes and Adsorption Morphology on Surfaces. After preparation, liposomes were characterized by DLS to obtain the size distribution. Table 1
EXPERIMENTAL SECTION
Materials. The lipid, 1,2-dipalmitoyl-sn-glycero-3 phosphatidylcholine (DPPC 16:0) was supplied by Genzyme (Massachusetts, America). The lipid phase transition temperature (Tm) is 41 °C. Both in sample preparation and measurement procedures, the water is highly purified with a resistivity of 18.2 MΩ. Ti6Al4 V foils (10 × 10 mm, 1 mm thickness) were purchased from Goodfellow, Inc. and polished to achieve flat and smooth surfaces (Ra ≈ 2.1 nm). The diameter of ultrahigh molecular weight polyethylene (UHMWPE) balls is about 6.2 mm (Ra ≈ 500.2 nm). The thickness of UHMWPE foil is 1 mm and was polished to make surface roughness close to the ball. They were bought from AoTe, Inc. Preparation of Liposomes and Coated Surfaces. Lipid powders were mixed into water, bathed for 1 h at the temperature above the lipid phase transition temperature (Tm). Then the hydrated lipid sheets were detached by oscillation to form a self-closed multilamellar vesicles (MLVs).38 Next, the MLVs were downsized to form small unilamellar vesicles (SUVs, 20−200 nm) by stepwise extrusion through polycarbonate membranes (Whatman, Inc.), starting orderly with 400 nm, 200 nm, and ending with 100 nm, using an extruder (Avanti, USA). The size of SUVs was characterized by dynamic light scattering (DLS) using a zetasizer (Malvern Nano-ZS). As with all procedures for extrusion, the experiments should be done at a temperature above Tm. The liposome-coated surfaces were used in subsequent adsorption morphology measurement by AFM and TEM (transmission electron microscope). Surfaces prepared for AFM were done as follows. First, polished Ti6Al4 V surfaces were covered by enough DPPC-SUVs suspension at room temperature. After several hours, the surfaces were rinsed by placing the adsorbed surfaces into a container full of water with shaking to remove excess, nonadsorbed liposomes. For TEM, liposomes were adsorbed for 1 min by floating copper grids on a 10 μL drop of DPPC-SUVs suspension. Then the samples were negatively stained for 1 min by floating previous grids on a 10 mL drop of uranyl acetate. Finally, excess uranyl acetate was absorbed by filter paper to obtain a dry liposome-coated surface. Characterizations of Morphology and Surface Analysis. Morphology experiments were conducted using MFP-3D AFM (Asylum Research, USA) and H-7650B TEM (Hitachi, Japan). AFM Images were recorded in tapping mode in pure water using a silicon nitride tips mounted on V-shaped cantilevers with a nominal spring constant of 0.35 N/m, length of 120 μm, width of 25 μm, and a nominal tip radius of 10 nm (NP-S, Bruker). The microscope was allowed to
Table 1. Average Diameter of Liposomes with Different Membrane Pore Size membrane pore size/nm
400
200
100
average diameter/nm PDI
330 ± 150 0.279
160 ± 80 0.103
122 ± 35 0.042
describes the diameter of liposomes extruding through three different membrane pore sizes in pure water. It is found that the final vesicle size is determined by the membrane pore size. Sometimes, the obtained vesicles are a little larger than the membrane pore size due to the softness and liquidity of lipid membrane after passing through pores. When extruding though 400 nm membrane pore size, it is common that an obvious peak appears in 450 nm, with two small peaks appearing in 80 nm and 5 μm, while the distribution of vesicles under smaller pore size (200 or 100 nm) is unimodal (see Figure 1). Meanwhile, the polydispersity index (PDI) below 0.1 indicates that the samples are spherical and have narrow distribution. Thus, liposomes extruding from 100 nm pore size are most homogeneous. This is why we choose liposomes prepared by 100 nm pore size to be the lubricant in following experiments. Surfaces with adsorbed DPPC liposomes were characterized by TEM and AFM. The TEM image is shown in Figure 2. Images show that the DPPC liposomes could stably adsorb onto the surface and are 120 ± 40 nm in diameter, which agree with results from DLS. Some liposomes collapse in the center (arrow). It is most likely due to TEM preparation procedure: in such a high concentration solution of uranyl acetate, the great pressure difference makes liposome lose its inner water. Meanwhile, AFM images were obtained and shown in Figure 3. The adsorption amount and morphology rely on lipid concentration. Figure 3a shows a high dense layer of spherical structures on Ti6Al4 V in pure water, with a lipid concentration B
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while the smaller vesicles are flattened with diameter about 160 nm and height of 10 nm. This height value is equal to a double value of one bilayer, which is the minimum of a flatted vesicle. When the lipid concentration continues to decrease (Figure 3c,d), the thickness of adsorbed film is decreasing, and attraction force from the substrate becomes stronger, leading liposomes to become larger and flatter. The diameter is 180 ± 20 nm in Figure 3c and 230 ± 20 nm in Figure 3d, and the corresponding height is 30 ± 10 nm and 55 ± 10 nm. The liposomes in the lower layer (insets) are easily ruptured to bilayers due to the stronger force from the substrate. It was reported that a characteristic bilayer thickness of 4 nm had been measured in mica,37 in agreement with the distance between the PC hydrophilic headgroup (∼4 nm). In our experiment, the bilayer thickness is 5−6 nm. This discrepancy can be explained by the fact that the thickness of water layer between bilayer and Ti6Al4 V substrate is larger than that with mica, due to different wetting ability for two substrates, where water is easier to spread on mica to form a thinner water layer. According to the above AFM results, the sequential adsorption process and mechanisms of liposome vesicles are proposed, as shown in the scheme of Figure 3e−g. Previous studies show that vesicles could adsorb on mica either as a vesicle or as a disk,48 depending on the vesicle size, lipid concentration49 and acyl chain length.20 In this study, the stability of liposome vesicles adsorbed on surface is determined by internal and external forces (Figure 3e). The former come from strong chain−chain hydrophobic attractions,20 which tend to keep vesicles intact. The external forces arise from the substrate, AFM tip37 and nearby vesicles. The closed (vesicular) structure, as well as lateral constraints arising from neighboring vesicles, enables liposomes to keep their structural integrity and robustness, but the forces coming from substrate and AFM tip tend to rupture the bilayer membrane. Thus, adsorbed liposomes are metastable. When liposomes first adsorb onto Ti6Al4 V substrate, the attraction force from substrate and tip compression overcome the hydrophobic attraction, so that liposomes rupture to bilayers spontaneously with thicknesses of 5−6 nm (Figure 3f). For subsequent vesicles, they would retain their closed shape, due to decreased attraction forces and increased lateral extrusion forces from nearby vesicles. For liposomes on top layers, the shape and robustness are more like unperturbed vesicles. As a result, a multilayer structure of liposomes adsorbed on Ti6Al4 V substrate is proposed as Figure 3g. As the counter surface of Ti6Al4 V, the surface activities of liposomes on UHMWPE are also essential to determine the tribological properties. Compared to Ti6Al4 V (see Figure 3a), the AFM result in Figure 4a shows a similar dense distribution of liposomes on UHMWPE with lipid concentration of 1 mg/mL. This adsorption phenomenon is also supported by others.50 So far, the liposomes are found on both sliding surfaces with a high lipid concentration. It is believable that this adsorbed layers could work on lubrication in the following friction test. Note that the adsorption strength between liposomes and two surfaces is not the same. It is obvious to see that liposomes cannot stay on the UHMWPE surface with low lipid concentration in Figure 4b, while liposomes form a coating of bilayers on Ti6Al4 V in Figure 3d. The strength is a complex result determined by surface properties, such as the existence of charge on surface and surface energy (wetting ability), since UHMWPE is known as a hydrophobic polymer.51 Friction Test. The tribological behaviors of water-based DPPC liposomes on Ti6Al4 V against UHMWPE were evaluated
Figure 1. Size distribution for vesicles extruded though 100 nm pore size membrane. The measured vesicle diameter is 122 ± 35 nm.
Figure 2. TEM images of DPPC liposomes adsorbed on copper grids. Liposomes were extruded from 100 nm membrane pore size.
of 1 mg/mL. This coating is composed of a close-packed layer of liposomes, with a sparse, loose coating of excess liposomes on top of this layer (either separate or in small clusters), which are not fully removed during rinsing before measurement. When scanning repetitively the same area, AFM tip gradually moves away top liposomes, and close-packed liposomes in lower layer appear (inset). They are flattened with height of 50 ± 10 nm. This value is likely to be much lower than the unperturbed liposome height from DLS (calculated as 122 ± 35 nm), due to attraction force from substrate and compression by AFM tip. In addition, the diameter measured by AFM is 115 ± 10 nm, corresponding well to the images by TEM and DLS. The main difference is that vesicles are distorted arising from AFM tip and substrate. By decreasing the lipid concentration to 0.1 mg/mL (Figure 3b), there are fewer and flatter liposomes on top layer, with diameter of 130 ± 10 nm and height of 40 ± 10 nm (mark a). It is notable that, in lower layer (see insets), liposomes are weak to keep intact due to stronger attraction force from substrate (mark b). The larger vesicles rupture to bilayers with thickness of 5 nm, C
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Figure 3. AFM images of liposomes absorbed onto Ti6Al4 V with lipid concentrations of (a) 1 mg/mL, (b) 0.1 mg/mL, (c) 0.05 mg/mL, and (d) 0.01 mg/mL. Insets in the bottom left corner are images below the top layers. (e) Mechanical analysis of liposome vesicles. (f−g) Schemes of continuous adsorption process of vesicles and final structure of liposomes adsorbed on Ti6Al4 V substrate. Scale bars are 500 nm.
by liposomes, wear scars of both surfaces were characterized. The surface of UHMWPE ball without friction is observed in Figure 5b. There are uniformly distributed islands and valleys formed by
using ball-on-disc equipment. Results in Figure 5a indicate that the liposomes can efficiently reduce friction coefficient from 0.035 (by water) to nearly 0.015. To evaluate the wear resistance D
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Figure 4. AFM images of liposomes absorbed onto UHMWPE foil with lipid concentrations of (a) 1 mg/mL and (b) 0.01 mg/mL. Insets in the bottom left corner are images below top layers. Scale bars are 500 nm.
Figure 5. (a) Friction reduction by DPPC liposomes under sliding velocity of 0.3 mm/s. (b−d) Wear scars on UHMWPE balls. (e−f) Wear tracks on Ti6Al4 V. Experiments were conducted under lipid concentration of 1 mg/mL and normal load of 2N.
mechanical semifinish turning. Because of poor lubrication by water, the islands are flattened and form a scar in center of ball in friction test, as shown in Figure 5c. But for DPPC, fewer flattened islands are found in Figure 5d, showing a higher wear resistance. A same conclusion is found from Ti6Al4 V. As shown in Figure 5e,f, the grooves are less and shallower on Ti6Al4 V surface lubricated with DPPC, rather than water. Therefore, the wear improvement both on Ti6Al4 V and UHMWPE surfaces by DPPC give an indication that adsorbed liposome layers successfully reduce asperities contact, and therefore the wear and friction coefficient are reduced. (In fact, if any, nearly all the wears on Ti6Al4 V surface in our study is negligible, this is why we only consider wear on UHMWPE balls in following experiments.) Because the velocity used in our study is low and the surface roughness for UHMWPE ball is large enough, it is predicable that friction may happen in the regime of boundary lubrication. It is also confirmed by calculations according to Hamrock−Dowson theory (details of calculations and references are shown in the Supporting Information). Therefore, both the properties of boundary layers and mechanical effect of asperities determine the tribological properties of the whole system. Raman was used to determine whether there is chemical reaction happening between boundary layer and Ti6Al4 V
surface during shearing process at 37 °C. As shown in Figure 6, compared to the spectrum of DPPC polymer/powder (black line), there is no distinct peak displacement or peak generation
Figure 6. Raman spectroscopy of surface analysis. E
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Figure 7. (a) Effect of sliding velocity on average friction coefficient. (b) Friction coefficient of DPPC under sliding velocity of 24 mm/s. Wear scars on UHMWPE balls under sliding velocity of (c) 0.3 mm/s and (d) 24 mm/s. The lipid concentration is 1 mg/mL and normal load is 2N (P = ca. 30.87 MPa).
Figure 8. (a) Effect of normal load on average friction coefficient. (b) Friction coefficient of DPPC under load of 4N. Wear scars on UHMWPE balls under load of (c) 2N and (d) 4N. The lipid concentration is 1 mg/mL, and sliding velocity is 0.3 mm/s.
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Figure 9. (a) Friction coefficient versus time with different membrane pore sizes and their mixture with equal lipid concentration. (b) Average friction coefficient with three membrane pore sizes and their mixture. AFM images of liposomes extruding from (c) 400 nm membrane pore size and (d) 200 nm membrane pore size. All experiments were carried out under load of 2 N (P = ca. 30.87 MPa), sliding velocity of 0.3 mm/s and lipid concentration of 1 mg/mL. Scale bars are 500 nm.
contrast, for water, with velocity increasing, the average friction coefficient increases to a maximum at 24 mm/s and slightly decrease afterward (time for run-in period is shorter with increased velocity). Moreover, the wear on the UHMWPE surface is more severe under velocity of 24 mm/s (see Figure 7c,d), The lubrication by water could be explained as follows: during the run-in period, the plastic deformation of asperities from the UHMWPE surface is accelerated at high velocity, and more asperities break away from the surface or embed onto the rubbing surfaces, causing high friction and wear (suitable for all velocities); after a period of time, the UHMWPE surface becomes smooth, so friction coefficient decreases (for velocity that is no less than 24 mm/s), as shown in Figure 7b. As above, it can be conclude that the adsorbed liposome layers on both surfaces can reduce the asperities contact of the two surfaces and thus present a good bearing capacity. Not only that, the adsorbed layers are able to resist the lateral deformation cause by velocity. The slight increase in average friction coefficient of DPPC is considered as the rupture of a part of the liposomes. A summary of normal load on average friction coefficient with DPPC liposomes is shown in Figure 8a. There is a clear friction
between DPPC liposomes and Ti6Al4 V surface (red line). It can be concluded that no obvious chemical reaction takes place. Moreover, the signal detected from surface after ultrasonic cleaning (blue line) is almost the same as surface (pink line). This means that the binding interaction between residuals and surface results from physical adsorption. The XPS results also prove that no covalent bond (like P−O− Ti) appears between liposomes and Ti6Al4 V surface during friction, the same as Raman results (details about analysis of O 1s spectra are shown in the Supporting Information). Effect of Sliding Velocity and Normal Load on Lubrication. It is worth noting that the velocity and normal load are crucial to determine the friction coefficient according to Stribeck Curve.52 Furthermore, their dependence on tribological behavior is guidable for application of liposomes as biomimetic lubricant. The effect of sliding velocity on average friction coefficient is shown in Figure 7. Curves in Figure 7a show an excellent friction reduction by DPPC liposomes under all velocities. The stability of average friction coefficient (around 0.02) by DPPC is attribute to the same wear scars on UHMWPE surfaces under different velocities, as shown in Figure 7c,d. By G
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Langmuir reduction by DPPC under all loads, even down by 67% at load of 4 N (ca. 39 MPa), as shown in Figure 8b. The similar wear scars on UHMWPE surfaces under two loads, as shown in Figure 8c,d, may contribute to the stability of friction coefficient by DPPC. However, for water, an increase in load results in a significant increase in friction coefficient, because more asperities start to contact, causing more severe wear (see Figure 8c,d). Therefore, the adsorbed liposome layers on both surfaces show their excellent bearing capacity again (even under high load, some ruptured vesicles would form thick and dense boundary layers to main low friction). It is considered to have great application potential both on human and artificial joints. Effect of Vesicle Size and Lipid Concentration on Lubrication. Since the adsorbed liposome layers are proved to reduce friction and wear under various loads and velocities, the control of factors (such as vesicle size and lipid concentration), which directly influence the structure of boundary layers, is considerable. The vesicle size correlates closely with membrane pore size, as shown in Table 1. Note that the size distribution is narrower for the case of small vesicles. Then the effect of membrane pore size on friction coefficient and corresponding adsorption morphologies on Ti6Al4 V surface by AFM are shown in Figure 9. Figure 9a,b gives a clear sight that the lowest friction coefficient (0.020 ± 0.005) is achieved when membrane pore size is 100 nm. Moreover, during initial contact (0−400s), there is a higher friction when membrane pore size is 400 nm (see Figure 9a). It is plausible that the large and cluttered vesicles existing on the top of adsorbed layers could increase the friction in the beginning (Figure 9c). Additionally, when the top vesicles are gradually removed, the underlying vesicles appear (see insets) and would affect the following friction process. Therefore, the welldistributed and uniform-sized vesicles layers extruded from smaller membrane pore size can obtain lower friction (see Figure 3a). To better verify this idea, the mixture vesicles, which have intermediate size distribution, was chosen to compare with other three samples. The result that the intermediate average friction coefficient is obtained from mixture vesicles can support our idea in some level. Furthermore, the relationship between concentration-induced adsorbed structure and average friction coefficient is shown in Figure 10. The decrease in average friction coefficient indicates that higher lipid concentration could form a thicker adsorbed
layers, which can better separate asperities from both surfaces and reduce their mechanical action. Based on our discussions above, the lubrication model by liposomes is proposed in Figure 11. In fact, the rubbing of two
Figure 11. Schematic illustration of the lubrication model by liposomes between Ti6Al4 V−UHMWPE surfaces.
surfaces can be considered as the mechanical effect of asperities from opposite surfaces in microscale, as shown in Figure 11a. A mount of liposome vesicles exist between asperities, as shown in Figure 11b. As mentioned before, liposomes could physically adsorb on two surfaces, but the adsorption interaction is stronger on Ti6Al4 V surface, due to electrostatic attraction and hydrophilicity. The adsorbed layers on both surfaces show a typical multilayer structure (introduced in Figure 3), and shear slip happens within these layers, as shown in Figure 11c. Here the PC groups exposed in water are highly hydrated,24 and these hydrated liposome vesicles could move easily between each other to achieve a low friction. Except for the hydration lubrication, the main contribution of adsorbed liposome layers in our experiments is the excellent bearing capacity and wear resistance. It is attributed to the great elastic property37 of liposome vesicles and soft adsorbed films, which increase the contact area and reduce contact pressure.
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CONCLUSIONS In this paper, our study has revealed that DPPC liposomes can reduce the friction coefficient and wear on both surfaces (Ti6Al4 V/UHMWPE) and also show stability under various velocities and loads. The lubrication by liposomes can also be enhanced
Figure 10. Effect of lipid concentration on average friction coefficient. All experiments were carried out under load of 2 N (P = ca. 30.87 MPa), sliding velocity of 0.3 mm/s and vesicle size around 100 nm. H
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(8) Bell, J.; Tipper, J. L.; Ingham, E.; Stone, M. H.; Fisher, J. The Influence of Phospholipid Concentration in Protein-Containing Lubricants on the Wear of Ultra-High Molecular Weight Polyethylene in Artificial Hip Joints. Proc. Inst. Mech. Eng., Part H 2001, 215, 259−263. (9) Gale, L. R.; Coller, R.; Hargreaves, D. J.; Hills, B. A.; Crawford, R. The Role of SAPL as a Boundary Lubricant in Prosthetic Joints. Tribol. Int. 2007, 40, 601−606. (10) Park, J.; Duong, C.; Chang, H.; Sharma, A. R.; Thompson, M. S.; Park, S.; Kwak, B.; Kim, T.; Lee, S.; Park, S. Role of Hyaluronic Acid and Phospholipid in the Lubrication of a Cobalt-Chromium Head for Total Hip Arthroplasty. Biointerphases 2014, 9, 031007. (11) Forster, H.; Fisher, J. The Influence of Loading Time and Lubricant on the Friction of Articular Cartilage. Proc. Inst. Mech. Eng., Part H 1996, 210, 109−119. (12) Dowson, D.; Wright, V.; Longfield, M. D. Human Joint Lubrication. Biomed. Eng. 1969, 4, 160−165. (13) Sotres, J.; Arnebrant, T. Experimental Investigations of Biological Lubrication at the Nanoscale: The Cases of Synovial Joints and the Oral Cavity. Lubricants 2013, 1, 102−131. (14) Klein, J. Molecular Mechanisms of Synovial Joint Lubrication. Proc. Inst. Mech. Eng., Part J 2006, 220, 691−710. (15) Schwarz, I. M.; Hills, B. A. Surface-Active Phospholipid as the Lubricating Component of Lubricin. Rheumatology 1998, 37, 21−26. (16) Hills, B. A.; Crawford, R. W. Normal and Prosthetic Synovial Joints are Lubricated by Surface-Active Phospholipid. J. Arthroplasty. 2003, 18, 499−505. (17) Hills, B. A.; Butler, B. D. Surfactants Identified in Synovial Fluid and their Ability to Act as Boundary Lubricants. Ann. Rheum. Dis. 1984, 43, 641−648. (18) Hills, B. A. Oligolamellar Lubrication of Joints by Surface-Active Phospholipid. J. Rheumatol. 1989, 16, 82−91. (19) Seror, J.; Zhu, L.; Goldberg, R.; Day, A. J.; Klein, J. Supramolecular Synergy in the Boundary Lubrication of Synovial Joints. Nat. Commun. 2015, 6, 6497. (20) Sorkin, R.; Kampf, N.; Dror, Y.; Shimoni, E.; Klein, J. Origins of Extreme Boundary Lubrication by Phosphatidylcholine Liposomes. Biomaterials 2013, 34, 5465−5475. (21) Daniel, M. Boundary Cartilage Lubrication: Review of Current Concepts. Wien. Med. Wochenschr. 2014, 164, 88−94. (22) Briscoe, W. H.; Titmuss, S.; Tiberg, F.; Thomas, R. K.; McGillivray, D. J.; Klein, J. Boundary Lubrication Under Water. Nature 2006, 444, 191−194. (23) Seror, J.; Sorkin, R.; Klein, J. Boundary Lubrication by Macromolecular Layers and its Relevance to Synovial Joints. Polym. Adv. Technol. 2014, 25, 468−477. (24) Klein, J. Hydration Lubrication. Friction 2013, 1, 1−23. (25) Ma, L.; Gaisinskaya-Kipnis, A.; Kampf, N.; Klein, J. Origins of Hydration Lubrication. Nat. Commun. 2015, 6, 6060. (26) Jahn, S.; Klein, J. Hydration Lubrication: The Macromolecular Domain. Macromolecules 2015, 48, 5059−5075. (27) Benz, M.; Chen, N.; Israelachvili, J. Lubrication and Wear Properties of Grafted Polyelectrolytes, Hyaluronan and Hylan, Measured in the Surface Forces Apparatus. J. Biomed. Mater. Res. 2004, 71A, 6−15. (28) Das, S.; Banquy, X.; Zappone, B.; Greene, G. W.; Jay, G. D.; Israelachvili, J. N. Synergistic Interactions Between Grafted Hyaluronic Acid and Lubricin Provide Enhanced Wear Protection and Lubrication. Biomacromolecules 2013, 14, 1669−1677. (29) Ogston, A. G.; Stanier, J. E. The Physiological Function of Hyaluronic Acid in Synovial Fluid - Viscous, Elastic and Lubricant Properties. J. Physiol. 1953, 119, 244−252. (30) Seror, J.; Merkher, Y.; Kampf, N.; Collinson, L.; Day, A. J.; Maroudas, A.; Klein, J. Articular Cartilage Proteoglycans as Boundary Lubricants: Structure and Frictional Interaction of Surface-Attached Hyaluronan and Hyaluronan−Aggrecan Complexes. Biomacromolecules 2011, 12, 3432−3443. (31) Mirea, D. A.; Trunfio-Sfarghiu, A. M.; Matei, C. I.; Munteanu, B.; Piednoir, A.; Rieu, J. P.; Blanchin, M. G.; Berthier, Y. Role of the
with higher lipid concentration and smaller vesicle size. At the same time, a multilayer structure of liposomes adsorbed on Ti6Al4 V is observed by AFM, where intact and close-packed liposome vesicles exist on upper layers, and ruptured vesicles (bilayers) form the lower layer due to stronger attraction force from the substrate. We attribute low friction to the presence of physically adsorbed liposome layers on both surfaces, which could reduce asperities contact. The excellent bearing capacity by the adsorbed layers is due to the elastic liposome vesicles existing on the upper layer and soft film to reduce contact pressure. Meanwhile, the close-packed liposome layers in the shear plane can achieve low friction by hydration mechanism from PC groups. This work gives reference on how liposomes work on artificial surfaces and makes liposomes attractive for further clinical development both in mammalian and artificial joints.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02822. Details of calculation of minimum film thickness by Hamrock−Dowson formula and analysis of O 1s spectra by XPS (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Guoshun Pan, associate professor of Tsinghua University, for kindly providing polishing solution of Ti6Al4 V for us. We thank the funding provided by the National Natural Science Fund for Excellent Young Scholars program (Grant No. 51522504), State Key Laboratory of Tribology, and Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51321092).
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REFERENCES
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DOI: 10.1021/acs.langmuir.6b02822 Langmuir XXXX, XXX, XXX−XXX
