Bioinspired polymers for lubrication and wear resistance
Graphical abstract
Introduction
To function properly, advanced mechanical and biomedical devices often require complex surface modifications to resist severe and specific friction and wear [1]. Improper lubrication not only reduces the long-term stability of medical implants but also may trigger friction-induced inflammation, tissue damage and infection, leading to severe or chronic pain [2,3]. Thanks to the recent technological advances in nanoscale characterization of natural systems, they have become a great source of inspiration for material scientists in designing effective and sustainable lubricants and wear resistant materials. For example, the mucilage of the aquatic plant Brasenia Schreberi, which is secreted by the plant to protect it from herbivores, is composed of polysaccharide gel nanosheets with thickness of 75 nm and a coefficient of friction (CoF) as low as 0.005 when adsorbed on glass substrates [4,5]. This mucilage is of great interest in the development of lubricating pill coatings for oral drug administration [4]. In human body, healthy joints can provide CoF in the range 10−3-10−1 depending on the measurement method, the tribopairs, the lubrication regime, the location of the cartilage sample and the preparation of cartilage explants [6]. Tribopairs are facing solid substrates that are subject to sliding motion during a tribology experiment, i. e., the experiment to evaluate lubrication and wear resistant properties of materials and surfaces. The synovial fluid (a dialysate of blood plasma including molecules from the synovium) is the joints lubricating fluid, which is composed of high molecular weight (Mw) hyaluronic acid (HA), a lubricating protein known as proteoglycan 4 (PRG4) or lubricin, and phospholipids, which work in synergy to provide excellent lubrication [7,8]. Essentially, the HA-phospholipids complexes at the outer surface of the cartilage, which are immobilized on the surface via HA-lubricin interactions, form robust boundary layers that provide very low friction through hydration lubrication mechanism [7], [8], [9], [10]. Among these, phospholipids have been reported to be the main component responsible for the boundary lubrication and excellent hydration of joints [11]. Similarly to the human joints, friction reduction in artificial systems is generally achieved by the presence of a lubricating fluid at the rubbing contact.
This review mainly focuses on lubricating polymeric fluids, since they have been found to be responsible for excellent lubrication in natural media mainly due to non-specific interactions [12]. For example, natural polymers glycoproteins (mucins), proteoglycans and lipids are the main components controlling the lubrication in the joints. Both glycoproteins and proteoglycans have a unique architecture, which is referred to as bottlebrush (BB) [13], with chemical groups that can be firmly anchored to substrates and provide an extremely low CoF due to high steric hindrance generated by the pendant chains. The excellent lubrication property also originates in the highly hydrated moieties (charged sialic acid or glycosaminoglycans (GAG)) [14], [15], [16]. Nevertheless, for practical applications, synthetic polymers are preferred due to their lower cost and ease of production at large scales.
In addition to lubrication, wear protection is equally important for the effective and sustainable function of different systems. In natural systems, such as in teeth, claws, shells, exoskeletons, bones, nacre and enamel, the anti-wear structures are often organic materials with special architectures, yielding elastic moduli with GPa magnitudes. For instance, the inner shells of seashells and nacre−used as a protective armor against predators−have a brick-and-mortar architecture composed of 95% calcium carbonate glued by 5% chitin and fibroin, producing an elastic moduli in the range of 1-100 GPa [17]. For a complete map of wear-resistance properties of biological materials, readers are encouraged to refer to the review article written by Amini and Miserez [18]. Compared to friction, wear is more scarcely studied mainly because of the difficulty to quantify it in deformable poroelastic structures, such as in hydrogels. Wear is usually probed using qualitative (visual) [19], [20], [21], [22], [23], [24], [25] or quantitative techniques, such as gravimetric (mass loss) [26], [27], [28], [29], [30], volumetric (volume, width and depth of wear tracks) [31], [32], [33] or dosage (release of wear debris) [34], [35], [36] approaches.
Bioinspired polymers have been extensively discussed in several review articles [6,37]. However, the design and lubrication mechanism of polymeric lubricants, despite being crucial parts of recent studies on bioinspired polymers, have been overlooked by recent reviews. Especially, the wear protection properties of these polymers have been a major research topic recently, requiring further survey and comparative analyses. Here, the concept of bioinspired lubrication using novel polymeric structures, which has led to the production of a myriad of new systems with stronger and more sustainable anti-friction and wear-protective properties, is described. In addition, new insights on different methodologies for developing and implementing the new biomimetic lubricating polymeric materials will be provided. The emphasis will be on the lubrication and wear protection properties of polymeric materials, as both are equally influential in real-life applications. Lubrication and wear resistance are evaluated for (i) thin polymeric coatings (thin films of nanometer-micrometer thickness, grafted or physisorbed on the surface), which are used to lubricate solid substrates (e. g., mica, silicon, steel), as well as (ii) soft viscoelastic solid polymeric substrates (e. g., gels, cartilage and elastomers) with superior intrinsic lubrication and wear resistant properties. We also evaluate the use of naturally-occurring polymers, commonly known as natural polymeric lubricants, which have been identified as effective lubricants and wear resistant coatings and, in recent years, have been uniquely modified to improve their lubrication performance. Nevertheless, the main focus will be on synthetic polymers, most of which owe their excellent lubrication and wear resistance to their biomimetic structures. Scheme 1 represents some of the polymeric systems reviewed in this article for their attractive lubrication or wear resistant properties.
Lubrication is defined as minimizing the friction force between surfaces sliding on each other. Lubrication is often quantified in terms of the CoF, defined as the ratio of the friction force to the normal force or as the derivative of the friction force with respect to the load. The static CoF is assessed at the point of sliding, whereas the kinetic CoF is measured when objects are in motion. Lubrication can occur in different tribological conditions called tribological regimes which depend on the applied load and sliding speed. The fluid film lubrication, which occurs at separation distances larger than the surface roughness (low applied force or high sliding speed), involves dissipating the applied load through hydrostatic, hydrodynamic and elastohydrodynamic forces [42]. Therefore, the fluid film viscoelasticity, geometry and shear rate play important roles in this regime. However, at separation distances close to a few molecular layers thickness, which typically occur at high loads, boundary lubrication is responsible for lubrication properties. In this regime, dissipation arises from molecular rearrangements and deformations, including hysteretic breakage of molecular bonds or passage of the opposing molecules over local repulsive energy maxima [6]. A widely discussed concept in boundary lubrication in aqueous systems is the hydration lubrication, that is, water molecules forming the hydration shells associated with charged moieties, dissipating energy by their fluidlike (resembling molecular ball bearings) response to the applied sliding motion [6,15]. In synovial joints, lubrication typically occurs in a mixed regime where both fluid film and boundary lubrication are in effect. Different theories have been proposed to understand the complex mechanism involving the synergistic action of synovial fluid molecules and cartilage structure [43]. The early weeping lubrication theory proposes that compression of asperities on opposite cartilage interfaces lead to local fluid exudation and interstitial fluid flow, preventing cartilage on cartilage contacts [44]. The boosted or ultrafiltration lubrication mechanism on the other hand postulates that under loading, synovial fluid is pressurized towards the inside of cartilage leading to an enrichment of lubricating macromolecules at the interfaces by sieving effect of the cartilage porosity. The enrichment of macromolecular species at the interface at the stressed area, once again prevents direct cartilage-cartilage contact [45]. For more detailed discussion on lubrication mechanisms readers are encouraged to see available comprehensive reviews by McNary et al. [42] and Jahn et al. [6].
Another pathway of frictional energy dissipation is through surface wear, where molecular bonds are broken while producing wear debris. Theoretically, Reye-Archard-Khrushchov wear criterion is often used to evaluate wear on solid surfaces, stating that the wear volume is directly proportional to the dissipated energy [46]. On soft elastic materials, the instabilities during frictional sliding is often modeled and discussed in terms of Schallamach waves [47]. On polymer coated surfaces, two types of wear can occur: (i) adhesive wear, which occurs when the lubricating material is removed and solid-solid contact causes severe deformation and material exchange between the worn surfaces, and (ii) abrasive wear, which occurs when the solid surface is scratched as a result of polymeric film failure and deformation. Theories describing frictional wear of polymeric coatings and hydrogels are scarce. Therefore, the extent of wear characterization and analysis in these systems is often limited to surface topography analysis and quantitative wear debris mass measurement, although these do not provide a mechanistic view to the wear phenomenon.
Section snippets
Modified naturally-occurring polymeric coatings
Lubrication of tissues or biomaterial implants is essential to ensure the sustainable mobility of body parts. For instance, healthy mammalian synovial joints are remarkable lubricated systems able to provide excellent mobility over an individual's lifetime. The in vivo whole joint CoF has been reported to range from ~0.001 to 0.03 [6], even though it is likely that these values are higher than actual physiological cartilage on cartilage CoF due to challenges in measuring the friction in intact,
Bioinspired wear protection using polymeric materials
Materials capability to resist wear typically emerges from their mechanical properties and surface characteristics, as highly stiff and organized structures with well-lubricated surface are able to sustain coercions. Also, polymeric materials with self-repair properties, which is defined as the ability to repeatedly reorganize polymeric bonds and structures under external stress, typically show superior wear resistance. Incorporating additives, which improve the materials mechanical properties
Conclusions and outlook
Polymeric fluids with excellent friction and wear protection abilities are abundant in nature and have long been an inspiration in designing new biomimetic lubricating systems. In recent years, several naturally-occurring polymers have been modified to not only benefit from their extraordinary natural coefficient of frictions (CoF) but also to address various technical shortcomings. For example, crosslinked or chemically modified hyaluronic acid (HA) has been shown to provide CoF as low as 0.05
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Support from CRC (Canada Research Chairs program) and NSERC (Natural Sciences and Engineering Research Council of Canada) to X. B., and FRQNT and TransMedTech postdoctoral research scholarships to M. M. and V. A. is gratefully acknowledged. J.F. is grateful for the financial support from the Arthritis Society. D.W.Lee is supported by National Research Foundation (NRF) of Korea (NRF-2019R1A2C2005854) and Basic Science Institute Research Fund (1.200049.01) of UNIST (Ulsan National Institute of
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These authors contributed equaly to this work.