Bioinspired Bottlebrush Polymers for Aqueous Boundary Lubrication

Abstract

An extremely efficient lubrication system is achieved in synovial joints by means of bio-lubricants and sophisticated nanostructured surfaces that work together. Molecular bottlebrush structures play crucial roles for this superior tribosystem. For example, lubricin is an important bio-lubricant, and aggrecan associated with hyaluronan is important for the mechanical response of cartilage. Inspired by nature, synthetic bottlebrush polymers have been developed and excellent aqueous boundary lubrication has been achieved. In this review, we summarize recent experimental investigations of the interfacial lubrication properties of surfaces coated with bottlebrush bio-lubricants and bioinspired bottlebrush polymers. We also discuss recent advances in understanding intermolecular synergy in aqueous lubrication including natural and synthetic polymers. Finally, opportunities and challenges in developing efficient aqueous boundary lubrication systems are outlined.

Keywords: aqueous boundary lubrication; bioinspired bottlebrush polymers; friction; wear resistance.

Figure 1

Lubricin: containing the N-terminal 2-somatomedin B (SMB)-like domains and the C-terminal hemopexin (PEX)-like domain. Lubricin also contains a chondroitin sulfate (CS) side chain, and in the middle region, it has a densely glycosylated and mucin-like domain. Aggrecan: containing three globular domains (G1, G2, and G3), and in the middle, it has a large extended domain heavily modified with glycosaminoglycans. Hyaluronan (HA): the anionic disaccharide building unit of hyaluronan.

Figure 2

An illustration of three main synthetic approaches of bottlebrush polymer synthesis: grafting-from, grafting-to, and grafting-through.

Figure 3

The theoretical surface excess (Γ_exe) as a function of the percentage of charged segments, X, on the mica (a) and silica (b) surfaces with random (squares), regular (diamonds), and diblock (circles) distributions of the uncharged and charged main-chain segments. In panel a, the fluctuations of Γ_exe given as twice the standard deviation for 100 realizations of random distributions with the fixed fraction of charged segments are given as error bars. The figures were adopted with permission from [13]. Copyright © 2022 American Chemical Society.

Figure 4

The effective friction coefficient (μ_eff) of PEO₄₅MEMA:METAC-X systems (filled squares), μ_eff of the bare mica–silica surface pair (unfilled square). The figures were adopted with permission from [12]. Copyright © 2022 American Chemical Society.

Figure 5

(Left panel) Dissipation change (ΔD) as a function of frequency change (−Δf) upon adsorption of (METAC)_m-b-(PEO₄₅MEMA)_n (upper curve) and (PEO₄₅MEMA)_n (lower curve) on silica. The inset shows the data in the range of −Δf up to 40 Hz in more detail. The figures were adopted with permission from [29]. Copyright © 2022 American Chemical Society. (Middle panel) F_n/R as a function of separation between the silica surfaces coated with (METAC)_m-b-(PEO₄₅MEMA)_n. Fitted DLVO forces were obtained by using constant charge (upper line) and constant potential (lower line) boundary conditions. The inset shows the forces between the polymer layers prior to (black squares) and after (red circles) rinsing with water. (Right panel) Friction force (F_f) as a function of load (F_n/R and F_n) of the bare silica surfaces in water (triangles) and after the adsorption of (METAC)_m-b-(PEO₄₅MEMA)_n in a 50 ppm polymer solution (the first cycle (squares) and the subsequent one (circles)). The straight lines were fitted to the data points obtained at low loads. The error bars corresponded to multiple friction force measurements. Filled and unfilled symbols represent the data points obtained on loading and unloading, respectively. The figures were adopted with permission from [30]. Copyright © 2022 The Royal Society of the Chemistry. In all cases, the polymer concentration of the aqueous solution was 50 ppm and the surface was silica.

Figure 6

(Left panel) Schematic representation of the ABA triblock bottlebrush copolymer and the molecular structure of the triblock copolymer. (Right panel) Experimental results of the friction force vs. the normal force for the triblock bottlebrush adsorbed layer. The figures were adopted with permission from the study by Banquy et al. [116]. Copyright © 2022 American Chemical Society. The friction coefficient values of lubricin (LUB) in PBS in the right panel were adopted with permission from [119]. Copyright © 2022 Elsevier Inc.

Figure 7

(Top row) The molecular structure of the catechol-based polymers. (Bottom row) Surface stiffness maps show the worn area in the middle of the AFM image. From left to right, at high loads (top of the worn area), wear was readily seen for electrostatic anchoring, less clearly seen when catechol groups were used, and not observable when both electrostatic anchoring and catechol groups were used. The figures were adopted with permission from [124]. Copyright © 2022 American Chemical Society.