The effects of lipid deactivation on repulsive-slippage lubrication: Macroscopic phenomena governed by quantum-level...

Purpose: This study aimed to determine how the integrity and charge state of superficial phospholipid PL bilayers govern the boundary lamellar-repulsive-slippage BLRSL lubrication mechanism in articular cartilage. The work quantified how progressive lipid deactivation affects wettability, electrostatic surface potential ESP, and friction, and explored whether nanoscale proton-dynamics concepts may complement classical interpretations. Methods: Bovine articular cartilage samples were tested in their native state or after controlled lipid extraction using the Folch method. Measurements included: (i) surface wettability, (ii) friction in a cartilage-cartilage pair under boundary-lubrication conditions (1 mm/s, 15 N), (iii) pH-dependent friction, and (iv) ESP of a DPPE phospholipid calculated via MD simulations (AMBER14) and the APBS method. Delipidated samples served as a model of early osteoarthritic degradation. Results: Lipid depletion lowered the contact angle from ~100° to ~35° and increased the friction coefficient from ultra-low values ( f ≈ 0.002-0.006) to ~0.02-0.023. Friction displayed a bell-shaped dependence on pH, peaking near the isoelectric point (~pH 4.5), consistent with protonation-state changes of PL headgroups. ESP calculations confirmed minimal interfacial stability around the IEP. Interaction of PLs with β ₂-Glycoprotein I produced deactivated species unable to form lamellar structures, eliminating the slippage plane. Conclusions: The transition from ultra-low friction to high boundary friction is governed by the number and integrity of PL bilayers, not by wettability alone. Loss or deactivation of PLs collapses the repulsive-slippage mechanism despite increased hydrophilicity. These findings reveal that macroscopic cartilage lubrication is controlled by molecular-scale PL organization, while proposed quantum-level effects remain a complementary hypothesis requiring further validation.