coatings Article Understanding the Unique Role of Phospholipids in the Lubrication of Natural Joints: An Interfacial Tension Study Aneta D. Petelska 1,*, Katarzyna Kazimierska-Drobny 2, Katarzyna Janicka 1, Tomasz Majewski 3 and Wiesław Urbaniak 2,* 1 Institute of Chemistry, University of Bialystok, Ciolkowskiego 1K, 15-245 Bialystok, Poland; [email protected] 2 Faculty of Mathematics, Physics and Technical Sciences, Kazimierz Wielki University, J.K. Chodkiewicza 30, 85-867 Bydgoszcz, Poland; [email protected] 3 Institute of Armament Technology, Faculty of Mechatronics and Aerospace, Military University of Technology, W. Urbanowicza 2, 00-908 Warsaw, Poland; [email protected] * Correspondence: [email protected] (A.D.P.); [email protected] (W.U.); Tel.: +48-85-73-88-261 (A.D.P.); +48-52-32-57-641 (W.U.) Received: 28 February 2019; Accepted: 17 April 2019; Published: 19 April 2019 Abstract: Some solid lubricants are characterized by a layered structure with weak (van der Waals) inter-interlayer forces which allow for easy, low-strength shearing. Solid lubricants in natural lubrication are characterized by phospholipid bilayers in the articular joints and phospholipid lamellar phases in synovial fluid. The influence of the acid–base properties of the phospholipid bilayer on the wettability and properties of the surface have been explained by studying the interfacial tension of spherical lipid bilayers based on a model membrane. In this paper, we show that the phospholipid multi-bilayer can act as an effective solid lubricant in every aspect, ranging from a ‘corrosion inhibitor’ in the stomach to a load-bearing lubricant in bovine joints. We present evidence of the outstanding performance of phospholipids and argue that this is due to their chemical inertness and hydrophilic–hydrophobic structure, which makes them amphoteric and provides them with the ability to form lamellar structures that can facilitate functional sliding. Moreover, the friction coefficient can significantly change for a given phospholipid bilayer so it leads to a lamellar-repulsive mechanism under highly charged conditions. After this, it is quickly transformed to result in stable low-friction conditions. Keywords: amphoteric articular cartilage; friction coefficient; interfacial tension; deactivation of surface-active phospholipid; β2-Glycoprotein 1 (β2-GP-1) 1. Introduction In the study of aqueous solutions and phospholipid chemistry, it has been established that molecular self-assembly is the key to their functionality as the noncovalent interactions of hydrophobic forces facilitate the assemblage of molecules. The typical examples of these physicochemical representatives with functional surfaces include surfactant molecules that are assembled as liposomes, bilayers, and membranes. The surfaces formed by these low-friction lubricating agents in the body system are comprised of an interface and colloid aggregates. The surface physics are determined by the interrelationships between interfacial tension, wettability, and friction coefficient. On the other hand, the surface chemistry (e.g., that of the surface of articular cartilage) depends on the rate of adsorption, hydration, liposomes, and bilayer formation as well as the nature of the lamellar phospholipid phases. Coatings 2019, 9, 264; doi:10.3390/coatings9040264 www.mdpi.com/journal/coatings Coatings 2019, 9, x FOR PEER REVIEW 2 of 12 Coatings 2019, 9, 264 2 of 12 Ultimately, the behavior of a solution-based interface is affected by the surface charge, dipoles and theirUltimately, distribution the behavior within ofthe a solution-basedelectric double interface layers, which is affected are formed by the surface by the charge,present dipoles structural and moleculartheir distribution species. within Numerous the electric types double of lamellar layers, bodies/structures which are formed built by theby presentthe molecular structural species molecular have beenspecies. identified Numerous in different types of lamellar cells and bodies tissues/structures and ar builte classified by the molecular according species to their have specific been identified tissue or in cellulardifferent function, cells and including tissues and (a) are hydrophilic classified according surface coating to their specificand protectives tissue or cellular(e.g., in function, lung alveoli), including (b) hydrophobic(a) hydrophilic protective surface coating barriers and (e protectives.g., in skin), (e.g., (c) in lunghydrophobic alveoli), (b) pr hydrophobicotective linings protective (e.g., barriersin the stomach),(e.g., in skin), and (c) hydrophobic(d) hydrophilic protective sliding linings (e.g., (e.g., in in thethe stomach), articulating and (d)joints). hydrophilic These sliding multilamellar (e.g., in the phospholipidarticulating joints). membranes These multilamellar (a–d) are co phospholipidmmonly classified membranes as lamellar (a–d) are bo commonlydies [1]. According classified as to lamellar Hills’ ‘biologicalbodies [1]. lubrication According to model’ Hills’ ‘biological[2] and our lubrication lamellar-r model’epulsive [2 ]mechanism and our lamellar-repulsive [3], phospholipids mechanism constitute [3], thephospholipids main solid-phase constitute component the main in solid-phase the diarthrodial component jointin tribosystem. the diarthrodial joint tribosystem. ManyMany scientists scientists have have been been able able to to apply apply knowle knowledgedge about about natural natural surfaces surfaces in in relative relative motion, motion, andand their their friction friction or or lubrication lubrication and and wear, wear, to to the creation of new lubricants, such as surface-grafted polymerpolymer brushes brushes [4–12]. [4–12]. Bayer Bayer [13] [13] has has presented presented information information about about new new materials materials and and modified modified lubricinlubricin structures structures for for low low friction. friction. This This review review presents presents the the latest latest advances advances in in understanding understanding the the functionfunction of of lubricin lubricin in in joint joint lubrication lubrication and and docu documentsments previous previous achievements in in transforming transforming this this biomedicalbiomedical knowledge knowledge into into a a new new polymer polymer design for advanced engineering tribology. InIn this this paper, paper, we we study study the the physics physics and and chemistr chemistryy of of the the articular articular surface surface of of natural natural bovine bovine knee knee cartilage.cartilage. TheseThese include include the the measured measured key parameters key parameters/characteristics,/characteristics, such as hydrophilicsuch as /hydrophilic/hydrophobic hydrophobicand amphoteric and characteristics, amphoteric characteristics, interfacial tension, interfacial charge tension, density, andcharge friction density, coeffi andcients friction of the coefficientssurface under of the varying surface pH under and varying friction pH loading and friction conditions loading in bu conditionsffer solutions. in buffer We solutions. introduce We the introducemechanism the of mechanism degenerative of degenerative joint diseases joint and diseases the destruction and the ofdestruction phospholipid of phospholipid bilayers. The bilayers. surface Thecartilage surface deterioration cartilage deterioration with the deactivation with the ofdeactivation surface-active of surface-active phospholipid bilayersphospholipid is seen bilayers to activate is seen to activate β2-Glycoprotein 1 through interactions. The interaction of the β2-Glycoprotein+ 1 (– β2-Glycoprotein 1 through interactions. The interaction of the β2-Glycoprotein 1 (–NH3 ) group + − + − + − NHand3 the) group phospholipid and the phospholipid (–PO ) group (–PO of (–NH4 ) group+) + of(–PO (–NH)3 ) + (–NH(–PO4 +) PO (–NH–) is3 strongPO4 –) enoughis strong to 4− 3 4− ! 3 4− enoughdeactivate to deactivate the bilayer the surface bilaye ofr thesurface phospholipids. of the phospholipids. 1.1.Hydrophilic Hydrophilic and and Hydrophobic Hydrophobic Character Character of the of Phospholipidthe Phospholipid Membrane Membrane − InIn phospholipid phospholipid membranes, membranes, the the wetted wetted surfac surfaceses (pH (pH of 6.5) of 6.5) are arenegatively negatively charged charged (–PO (–PO4 ). The4−). − differencesThe differences in the in thecharge charge density density of ofthe the phosphoric phosphoric groups groups (–PO (–PO4 )4 −allow) allow us us to to determine determine the the wettabilitywettability of of the the natural natural surface. surface. Therefore, Therefore, the the wettability wettability of ofthe the hydrated hydrated surface surface is described is described by − theby thechanges changes in inthe the concentration concentration of ofne negativelygatively charged charged phosphate phosphate groups groups (–PO 44−),), which which are are deactivateddeactivated when thethe surfacesurface is is dehydrated. dehydrated. Under Under such such conditions, conditions, the the hydrophobic hydrophobic groups groups that formthat forma hydrophobic a hydrophobic monolayer monolayer are activated, are activa whichted, which is shown is shown in Figure in Figure1. 1. (a) (b) FigureFigure 1.1.The The intelligent intelligent building building of the of surface the surface of an articular of an cartilagearticular phospholipid cartilage phospholipid bilayer under bilayer(a) wet andunder (b) the(a) wet air-dry and conditions. (b) the air-dry conditions. AA change change in in the the