Recent advances in superlubricity of liposomes for biomedical applications.

Achieving superlubricity, a state of lubrication where friction nearly vanishes, has become one of the most promising approaches to combat friction-induced energy dissipation and medical device failure. Phospholipids are amphiphilic molecules comprising highly hydrophilic phosphatidylcholine head groups as well as hydrophobic hydrocarbon chains, When solubilized, phospholipids can readily self-assemble to form different structures such as bilayers and vesicles (liposomes). Recently, liposomes have been identified as excellent lubricants, especially in the boundary lubrication regime the most common lubrication status in the field of biotribology. In this review, we summarize recent progress in employing liposomes as key players for employing superlubricity in biomedical applications. The relationship between lipids and liposomes, manufacturing approaches, lubrication regimes, and regulation mechanisms of liposomes are discussed. Finally, we indicate possible future directions for the use of liposome-mediated superlubricity in biomedical applications.

Insight into the Lubrication Behavior of Phospholipids Pre-adsorbed on Silica Surfaces at Different Adsorption Temperatures.

Phospholipids, as essential components in joint synovial fluid, play a dominant role in joint lubrication. In this study, atomic force microscopy was used to evaluate the normal and shear forces between two surfaces bearing three types of phospholipids with different acyl chain lengths, which were pre-adsorbed onto silica surfaces at different temperatures (25, 45, and 60 °C). When the pre-adsorption temperature was below the phospholipid phase transition temperature (Tm), a super-low friction coefficient [1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC): 0.002; 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC): 0.007] between two opposing silica surfaces in water was achieved because of the super-low shear strength of the hydration shell and robustness of the vesicle when the load was less than the critical value (DSPC: 500 nN; DPPC: 85 nN). However, when the pre-adsorption temperature exceeded Tm, the silica surface was covered by a bilayer structure with many defects, which exhibited poor adsorption density and low bearing capacity, resulting in a relatively high friction coefficient. This study gains insights into the influence of structure and temperature on the lubrication mechanism of phospholipids as biolubricants, providing guidance for the application of artificial joint synovial fluid.

AFM Study on Superlubricity between Ti6Al4V/Polymer Surfaces Achieved with Liposomes.

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 preadsorbed, the friction coefficient and load bearing capacity could be further improved. In addition, the probe with a 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 the plough effect appearing on the adsorbed layer. This work reveals the interfacial behavior of liposomes and realizes the controllable superlubricity system, providing more guidance for clinical application.