Synergies in Biolubrication

Sammanfattning: The principal objective of this thesis was to advance the level of understanding in the field of biolubrication, finding inspiration from the human synovial joints. This was addressed specifically by investigating the synergistic association of key biolubricants and the resulting lubrication performance. A range of techniques was employed during the course of this thesis work. Atomic force microscopy (AFM) was used to carry out force and friction measurements. Further information on the topography, nature, and structure of the adsorbed layers of biolubricants was obtained by using instruments such as Quartz crystal microbalance with dissipation monitoring (QCM-D), X-ray reflectivity (XRR), and AFM imaging.Key synovial fluid and cartilage components have been used as biolubricants in the investigations, namely dipalmitoylphosphatidylcholine (DPPC), hyaluronan (HA), lubricin, and cartilage oligomeric matrix protein (COMP). Focus was directed towards two lubrication couples, i.e. DPPC-hyaluronan and COMP- lubricin. DPPC-hyaluronan mixtures were probed on hydrophilic silica model surfaces whereas COMP-lubricin association structures were explored on weakly hydrophobic poly (methyl methacrylate) (PMMA) model surfaces. Both the systems included salt solution at a concentration of ≈ 150 mM.Investigations of the COMP-lubricin pair revealed that individually these components are unable to reach the desired level of lubrication. However, when they associate synergistically, COMP facilitates firm attachment of lubricin to the PMMA surface in a favourable confirmation that imparts low friction coefficient (≈ 0.06).The interplay between DPPC and hyaluronan imparts a lubrication advantage over lone DPPC bilayers, wherein hyaluronan provides a reservoir of DPPC on the model surface and thereby imparts self-healing of the lubricating layer. The system resulted in very low friction coefficient (< 0.01).Other factors such as temperature, presence of calcium ions, molecular weight of hyaluronan, and pressure were also explored. DPPC bilayers at higher temperature (liquid disordered phase) had higher load bearing capacity due to higher flexibility (as the chains were flexible enough to patch defects and self- heal). Association between DPPC Langmuir layers and hyaluronan was enhanced in the presence of calcium ions. Further, lower molecular weight hyaluronan had a stronger tendency to bind to DPPC, and it thereby affects the packing and organisation more strongly. Finally by subjecting the model system to high pressures, it was found that DPPC-hyaluronan composite layers were more stable and robust compared to lone DPPC bilayers.