Friction on charged and neutral soft natural matter surfaces

Objective: In the paper, the authors examine the surfaces of the bovine articular cartilage at varying pH solutions by measuring the friction coefficient. Design: Cartilage pairs with the same sign of charges have been investigated: positively (+/+), negatively (-/-) and neutral (IEP) cartilage surfaces under aqueous buffer solution. Results: We present evidence that the (cartilage/cartilage) pair charged with the identical positive or negative charge density exhibited the same value of the friction coefficient. Interfacial energy the “bell-shaped curve” of spherical lipid bilayers a model membrane described regarding the electrostatic repulsion of the cartilage surfaces is supporting friction curve interpretation. Conclusion: We found that charged surfaces had a smaller friction coefficient than neutral (cartilage/cartilage) pair. It was found that the friction was mostly dependent on the charge densities of the cartilage surface and the friction between charged surfaces is lower than that of neutral cartilage surfaces. *Correspondence to: Zenon Pawlak, Tribochemistry Consulting, Salt Lake City, UT 84117, USA, E-mail: zpawlak@xmission.com


Introduction
The two natural surfaces charged with the same sign and sliding against one another require better understanding [1][2][3][4]. Due to this, using the lamellar repulsion model, the strong dependence of the friction from both the cartilage surface charge density and the surface wettability [1,5] is supporting (cartilage/cartilage) friction. The natural cartilage of animal joints has very low friction for the sliding velocity -a few centimeters per second under a load of 18 MPa [6]. The lipid bilayers on the cartilage surface (surface active phospholipid, SAL) of the healthy joint cartilage contain mainly phosphatidylcholine (41%), phosphatidylethanolamine (27%), and sphingomyelin (32%) of total phospholipids [3,[7][8][9]. Liposomes and lamellar phases are composite structures made of phospholipids in synovial fluid and bilayers adsorbed on cartilage surface. The two models for natural cartilage surfaces boundary lubrication at different pH imparted by phospholipids bilayers on cartilage surface are given by Figure 1.
The paper presents the application of the lamellar-repulsion model of the charge density of the interaction between amphoteric surfaces to depict the friction of the sliding surfaces made of cartilage [1][2][3]. By this model, the low friction may be observed while repulsive articular cartilage surfaces are sliding against one another with the layer of the hydration water, which is located between the interface gap and is operating as a lubricant. Because of that, the friction may be lowered by the bilayers lamellar slippage and repulsion over short distances.
The negatively charged surface of cartilage with synovial fluid at pH 7.4 is supported under load by lamellar slippage of bilayers, thus resulting in low friction. Phospholipids which are in the synovial fluid or form the AC surface bilayers were experimentally proven to play a crucial role as a lubricant [7][8][9].
In this paper, we studied the friction of the amphoteric soft matter (cartilage/cartilage) pair surfaces vs. pH buffer aqueous solution.. The amphoteric cartilage surface in pH range 1 to 6.5 change charge from positive (-NH 3 + → -NH 2 ) to uncharged state, or approaching neutral, IEP, and turns to negative (-PO 4 H → -PO 4 -) with pH increasing. The friction coefficient of (cartilage/cartilage) pair was measured in the following cases: both cartilages were positively charged (-NH ) and finally neutral surface and it was shown that charge the cartilage surface is a significant factor of the frictional force. Studying the friction in (cartilage/cartilage) pairs of bovine origin over the pH of buffer solutions is fascinating to observe the electrostatic mechanism of joint lubrication. . Such radical change is prescribed to material with amphoteric properties. With increase of pH friction approached minimum value, and with increase pH friction run unchanged.
The different values of friction for various cartilage pH as shown in Figure 2 (B) should be associated with their charge density. The friction coefficient of the (cartilage/cartilage) tribological pair surfaces with positively charged surface (+/+) at pH 2.5 and 3.5, with negatively charged surface (-/-) at pH 6.0, 7.4, and 9.0 and at isoelectric point, IEP (±/±) at a pH 4.5, it was found that the friction was largely dependent on the charge densities of the cartilage surface.

The interfacial energy bell-shaped curve of the phospholipid membrane vs. pH
The interfacial energy ( γ ) of spherical lipid bilayer formed from phospholipids (PLs) vs. pH "bell-shaped curve", Figure 2(C) with the isoelectric point, IEP, 4.0 ± 0.2 is based on literature data [1] and [4] (quoted from Table 1).
The isoelectric point, IEP, is at a pH of ~ 4 when a PL molecule carries none net electrical charge H 2 N (CH 2 ) n PO 4  The relation between the buffer capacity (β) and components ratio is presented in Figure 2 It was experimentally proved that the charged surface of cartilage resulted from lower friction than that measured at pH ~ 4 (IEP) for no net charges. The low friction characterizes low interlayer (contact angle ~ 0 o ), lamellar slippage (Figure 3b) of bilayers and short-range repulsion. The interfaces of AC surfaces negatively charged (-PO 4 -) and contribution of the hyaluronan, proteoglycans (PGs), a glycoprotein termed lubricin and lamellar PLs phases [2,11] support the concept of the lamellar-repulsive mechanism.

Material and methods
The samples of articular cartilage were taken from the knees of an aged-one-to-two-years-old ox. Osteochondral plugs, which diameters were 5 and 10 mm long, were collected from lateral and medial femoral condyles with the use of a circular stainless steel cutter. Then, the articular cartilage discs were cut in such a way to form 3-mm plugs with underlying bone. Then, the samples were stored at 253 K in 0.15 M NaCl (pH = 6.9) and, before testing, they were fully defrosted. Next, the discs of articular cartilage were glued to the disc and attached to the surface made of stainless steel. Finally, friction tests were carried out.
To prepare the buffer solutions 0.2 M sodium hydroxide was added to 100 mL of a solution made of 0.04 M acids: acetic (80% of the solution), phosphoric and boric acid. A sodium hydroxide solution was used at the temperature 22 o C to adjust a suitable buffer pH [10]. The electrolyte pH was controlled using a pH-meter in the process of the measurements.

Friction test
The friction coefficient (f) was measured with the usage of the ITeR sliding friction tester pin-on-disc tribotester at room temperature. The tribotester measured the friction between two samples of articular cartilage which were soaked in the buffor solution playing a role of lubricating fluid and exposed to a load with sliding velocities for some time. The speed of sliding discs was very low (1mm/s in 2 and 5 min), and the load was 15 N (1.2 MPa), what is relevant to the natural physiological conditions. Before running the test, the specimen of articular cartilage was put to buffer solution for 60 min. Then, five tests were carried out with the usage of fresh articular cartilage discs for each experiment which is described in [5].

Results and discussion
The friction coefficient of (cartilage/cartilage) pair vs. pH Following Amonton's law, the frictional force (F) occurring between two sliding solids is proportional to the load (W) and the coefficient of the proportionality, (f), called the friction coefficient, The friction tests were conducted before the IEP using pairs of positively charged cartilage surfaces (-NH 3 + /-NH 3 + ) (curves no 1 and 2). Tests for curves 3, 4 and 5 were carried out after the IEP using pairs of negatively charged cartilage surfaces (-PO 4 -/-PO 4 -), while curve 6 was obtained at pH ~ 4.5 using cartilage surfaces at IEP with no net electrical charges (NH 3 + (CH 2 ) n PO 4 -R 1 R 2 ) (Figure 2A).   Figure (3a) shows transformation of hydrophilic bilayer to hydrophobic monolayer during drying process at room condition. A tree of 'lamellar-repulsive mechanism' of joints lubrication shown in Figure (3b). Surface amorphous multilayer (SAL) membrane provides the boundary lamellar-repulsive hydration lubrication. The lamellarrepulsive mechanism is supported by PL lamellar phases and charged macromolecules present in the synovial fluid between two charged cartilage surfaces.

Conclusion
In this article, we have examined the influence of pH on the friction lipid bilayers on the cartilage surface. The friction between (cartilage/cartilage) pair carrying the same sign of charges with positively charged surface (+/+) at pH 2.5 and 3.5, with negatively charged surface (-/-) at pH 6.0, 7.4, and 9.0 and at isoelectric point, IEP (±/±) at a pH 4.5 has been investigated in buffer aqueous solution. It is demonstrated that the friction was largely dependent on the electrostatic interaction between two charged of the cartilage surfaces. Due to these observations the negatively charged range 7.4 ± 1 covering natural lubrication draw most our attention. At pH 7.4, the negatively charged bilayers of PLs demonstrated very high buffer effectiveness (β) transmitted by a phospholipid, (ΔH + ) /ΔpH = β. It suggests that the lamellar-repulsive hydration mechanism can explain lubrication in joints. In the paper, there is shown that the curve of the friction coefficient Figure 2A and interfacial energy curve Figure  2C have an isoelectric point, IEP, and the "bell-shaped curve" or the Gaussian curve. Moreover, slippage of lamellar bilayers and shortrange repulsion between the interfaces of the negatively charged cartilage surfaces are the main determinants of the low frictional properties of the joint.