The authors have declared that no competing interests exist.
The cartilage surface was characterized using wettability test fresh and depleted AC samples. In this work, we demonstrated experimentally that the cartilage smart biomaterial at varies pH is sensitive to friction and introduces a novel concept in joint lubrication on charged surfaces.The surface charge density of the articular cartilage surface is related to the amphoteric character of phospholipids, PLs functional groups (-NH3+) and (-PO4-). The maximum surface energy of AC was found to occur at pH for isoelectric point ~4.5 (H3N+(CH2)n PO4- -R1R2) and with a wide range minimum of pH 6.5 to 9.5 of the phospholipidic membrane covering biological pH ~7.4 lubrication condition. The hydrophilic and hydrophobic character of cartilage was determined.
The chemical and physical nature of the biological surfaces is seen in an entirely different light than that of engineering surfaces immersed in water. Author
The lamellar-repulsive joint lubrication mechanism is not so well-known in biotribology and therefore deserves to be published. The determination of the biophysical and biochemical parameters of articular cartilage and introduction a new mechanism of lubrication in natural joints was an idea of “Articular cartilage: lamellar-repulsive lubrication of natural joints” publication
The articular cartilage (AC) surface charge density is related to the amphoteric character of phospholipids (PLs) since they contain both (-NH3+) and (-PO4-) group. Previous research mainly focused on examining the contribution of macromolecules to joint lubrication. While in this paper, the authors attempt to explain a joint lubrication mechanism in such a way that the AC surface phospholipids can act significantly as a lubricant. To prove the hypothesis, two experiments were used. In the first one, the authors attempt to determine that AC surface with phospholipids undergo the conformational change in air-dry conditions. The second experiment attempts to determine the correlation between the AC surfaces' friction and the tissue charge density.
In this paper, we examine wettability of bovine cartilage (BC) surface in wet and dry condition and the influence of pH on the surface charge density on friction on (cartilage/cartilage) surfaces.
The articular cartilage specimens were collected from bovine knees aged ~1.5 years. Osteochondral plugs, of 5 and 10 mm in diameter, were harvested from lateral and medial femoral condyles. The cartilage discs were cut into 3-mm plugs with full attachment to the underlying bone. The specimens were stored at 253 K in 0.155M NaCl (pH = 6.9) and fully defrosted prior to testing. The discs were then glued to the disc and pin’s stainless steel surfaces, and friction tests were conducted in the universal buffer solution.
A KSV CAM100 computerized tensiometer was used to measure the contact angle of cartilage samples. A drop of the 0.155M saline solution was deposited on the air-dry cartilage surface. The tests on the normal, partial and completely depleted cartilage samples were repeated five times.
The measurements were performed using a sliding pin-on-disc tribotester T-11 manufactured by the NISTR, Poland. The tests were conducted at room temperature, at a speed of 1 mm/s during 5 minutes, and under a load of 15 N (1.2 MPa) which corresponded to thephysiological lubrication condition.
Prior to the friction tests, the lubricants were prepared using the Britton-Robinson universal buffer solution and its pH values were measured. The samples were equilibrated with each buffer solution under a load for 5 minutes and the results of
The wetted surfaces (pH~7.3) of the phospholipid membranes are negatively charged (–PO4-). Biosurface wettability can be measured relative to differences in the charge density of the functional phosphate (–PO4-) groups. In this regard, the wettability of a hydrated surface is characterized by the concentration of charged anionic phosphate (–PO4-) groups that become deactivated when the surface is dehydrated. The dehydration of the phospholipid bilayer surface activates hydrophobic groups, R (CH2)n- due to the formation of a hydrophobic monolayer
-NH3+ + OH- → -NH2 + H2O; -PO4H + OH- → -PO4- + H2O (1)
Curve part (a) friction increase, pH 2 → 4.5: -NH3+ → -NH2(surface losingcharge)
The maximum point, IEP, pH 4.5, no net electrical charge, H2N(CH2)nPO4H-R1R2 ⇔
H3N+(CH2)n PO4- -R1R2 (2)
Curve part (b), friction decrease, pH 4.5 → 6.8: -PO4H → -PO4- (gaining negative charge)
Curve part (c), constantfriction, pH ~ 7 to 9.0 (-PO4-, surface is negatively charged)
By varying the charge density, the friction coefficient can be varied by about one order of magnitude over the experimental range studied. Cartilage’s extremely low friction is significantly dependent on the electrostatic interaction between two cartilage surfaces
The amphoteric PLs are the main solid-phase components on the surface of articular cartilage (AC). Cartilage phospholipidic membrane has been shown to undergo conformational reorientation when wettability changed from the wet (~ 0°) super hydrophilic to dry-air (103°) hydrophobic state (named smart material),
It has been well established that low friction is supported by the PLs bilayers mechanism which essentially consists of a surface amorphous layer (SAL) surrounded by a 0.155 M electrolyte synovial fluid (SF) of pH ~7.4 with high-molecular-weight charged biomacromolecules. The cartilage implication from osteoarthritis disease by a gradual erosion of the surface amorphous layer has shown anincreased friction coefficient
The surfaces, coated with PL bilayers and a lamellar structure negatively charged on the articular surface with synovial fluid, have been referred to as a “lamellar-repulsive” mechanism. The role, played by hydration or structural force, is believed to arise from a strongly bound and oriented first layer of the water molecules on charged surfaces. The water molecule allows participating in strong polar (electrostatic charge - dipole or hydrogen - bonding) interactions. The short-range repulsion often observed between biological surfaces is not due to the layered structure of water but to entropic repulsion. The cartilage surfaces experience weak van der Waals attractive forces and much stronger short-range repulsive forces due to hydration repulsion. Hydration repulsion dominants the interaction between charged cartilage surfaces at nanometer separations and ultimately prevents the sticking together of cartilage surfaces, even at high load. A hydration water layer strongly binds to the negative charge cartilage surface, when in contact with synovial fluid components (charged biomacromolecules, PL lamellar aggregates, and liposomes), acts to reduce the friction between cartilage surfaces
In this work, we demonstrated experimentally that the smart cartilage biomaterial at varies pH is sensitive to friction and introduces a novel concept in joint lubrication on charged surfaces. The cartilage surface was characterized using wettability test fresh and depleted AC samples.