Investigation of the lubrication properties and synergistic interaction of biocompatible liposome-polymer complexes applicable to artificial joints
Graphical abstract
Introduction
In recent years, continuing efforts have been expended in the field of biotribology, including biolubrication and biomaterials for many medical applications, such as synovial joints, dental, and heart valve prostheses [1]. The synovial joints, involving the hip, knee, and elbow in mammals, consist of two interacting bones covered by a thin layer of cartilage tissue, and synovial fluid (SF) between them [1]. Many studies have been carried out to reveal the compositions of the cartilage tissue [2] and SF, as well as the load bearing and lubrication mechanisms for the treatment of joint diseases [[3], [4], [5], [6]]. Some models, such as fluid film theories [1], and boundary lubrication theories [4,5], have been proposed to explain the lubrication properties of the adsorbed or grafted layer on the joint surfaces. However, little reliable information is available about the composition and mechanism of the sliding layers that keep the joint surfaces well lubricated [1]. Recently, phosphatidylcholine (PC) liposomes and bilayers were extensively considered as efficient biological lubricants in physiological joint lubrication [[7], [8], [9], [10]]. Sorkin et al. found that liposomes have more stable and significantly efficient lubricant capabilities compared to bilayers on the surface force balance (SFB) measurements [11]. Tribological interactions between the layers of surface-attached small unilamellar vesicles (SUVs) or liposomes have also been examined showing that such layers can provide an efficient boundary lubrication [12], but also depend on the PC used and the presence of SUVs in the surrounding dispersions [8,10,13,14]. Seror et al. confirmed that the efficient hydration lubrication capability of phospholipids could aid the reduction of physiological coefficient of friction (COF) under physiological pressures, thereby suggesting a potential role for the boundary lubricant of synovial joints [1,3,7,8,15]. Some reports have also shown that the lubrication ability of the phospholipids is mainly related to the lipid stability on the biosurfaces as surface-attached molecules provide lubrication when the sliding surfaces come into contact [10]. Biocompatible polymers have already been used in biomedical applications [[16], [17], [18], [19]], owing to the considerable potential of their mechanical and biological properties [[20], [21], [22], [23]]. Many studies reported that PC lipids complexed with biopolymers, such as hyaluronic acid and lubricin, could provide efficient boundary lubrication between two biomaterial surfaces [[24], [25], [26], [27], [28]]. A complex of hyaluronan and PC lipids between mica surfaces could yield low-friction coefficients (μ ≈ 10−3) following nanotribological measurements [17]. It is thus tempting to evaluate a synergistic effect between cartilage macromolecules and complexes with PC lipids, providing efficient lubrication of synovial joints at the macroscale [1,24,25,29].
Many materials, such as metal alloys, ceramic and polymers have been increasingly used for knee and hip joint replacements owing to their excellent wear and abrasion resistance [30,31]. Some ceramics, such zirconia (ZrO2) and alumina (Al2O3), have already been reported to be used as implant materials [32]. Si3N4 is an advanced ceramic material that might also play a prominent position as composite ceramics used in the artificial joints for biomedical applications. PEEK has been extensively considered as a suitable polymer material for use in the replacement of metal components in orthopaedics, trauma, and spinal implants. It is characterized by increased strength and stiffness, good fracture toughness, resistance to corrosion, and comparable elastic modulus and density to those of human tissue [33,34]. However, most studies on friction of PEEK materials focused on water, phosphate buffered solution (PBS), or calf serum (CS) as the lubricants, few of these have studied the interaction between lipids and PEEK. Therefore, in the present work, we focus on the synergistic interaction between lipid complexes and PEEK for potential applications in artificial joints and biolubrications. Moreover, the synergistic lubrications between lipids and biocompatible polymer were also studied at the macroscale.
Section snippets
Materials
The lipids used in this study were (using the notation X : Y to indicate the number of carbon in the fatty acid chains and the degree of unsaturation, respectively) [35], 1, 2-dilauroyl-sn-glycero-3-phosphocholine (DLPC 12:0, Mw = 621.8 g mol−1); 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC 14:0, Mw = 677.933 g mol−1); 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC 16:0, Mw = 734.039 g mol−1); 1, 2-distearoyl-sn-glycero-3-phosphocholine (DSPC 18:0, Mw = 790.2 g mol−1); and
Tribological behavior of PC–SUV liposomes
Fig. 1a depicts the evolution of COFs with the lubrication of five different PC liposomes, showing that all the COFs were always maintained at a stable level with time until the end of the test. The liposomes of HSPC and DSPC both exhibited the lowest COF at approximately 0.045, while DLPC exhibited the highest COF at 0.15. An obvious phenomenon was observed whereby the COFs experienced an abrupt decline and a minor decrease from a value higher than 0.15 to a value of 0.045 approximately, in
Conclusions
In summary, we have revealed that the lubrication effects of liposomes at the macroscale are determined by their carbon chain lengths. The COFs between the PEEK and Si3N4 surfaces show a decreasing tendency for liposomes with increasing carbon chain length. Friction experiments are less sensitive to sliding speed effects but are obviously dependent on normal loads. The surface topography of the adsorbed PC liposomes also demonstrated that liposomes and bilayers are significant parameters for
Acknowledgement
This work is financially supported by the National Natural Science Foundation of China (51775295, 51522504).
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