Bottlebrush Polymers for Articular Joint Lubrication: Influence of Anchoring Group Chemistry on Lubrication Properties

Abstract

The role of carboxylic, aldehyde, or epoxide groups incorporated into bottlebrush macromolecules as anchoring blocks (or cartilage-binding blocks) is investigated by measuring their lubricating properties and cartilage-binding effectiveness. Mica modified with amine groups is used to mimic the cartilage surface, while bottlebrush polymers functionalized with carboxylic, aldehyde, or epoxide groups played the role of the lubricant interacting with the cartilage surface. We demonstrate that bottlebrushes with anchoring blocks effectively reduce the friction coefficient on modified surfaces by 75-95% compared to unmodified mica. The most efficient polymer appears to be the one with epoxide groups, which can react spontaneously with amines at room temperature. In this case, the value of the friction coefficient is the lowest and equals 0.009 ± 0.001, representing a 95% reduction compared to measurements on nonmodified mica. These results show that the presence of the functional groups within the anchoring blocks has a significant influence on interactions between the bottlebrush polymer and cartilage surface. All synthesized bottlebrush polymers are also used in the preliminary lubrication tests carried out on animal cartilage surfaces. The developed materials are very promising for future in vivo studies to be used in osteoarthritis treatment.

Keywords: articular cartilage; biolubrication; bottlebrushes; friction coefficient; osteoarthritis.

Figure 1

Scheme of natural lubricin (a) and bottlebrush polymer (b).

Scheme 1. Synthetic Procedure for the Preparation of Bottlebrush Polymers with Anchoring Groups. Step 1: Copolymerization of MMA and Monomer X for the Synthesis of Monoblock P(X-co-MMA) via RAFT Polymerization. Step 2: Synthesis of a Diblock Copolymer of P(X-co-MMA)-b-P(BIBEMA-co-MMA) via RAFT Polymerization. Step 3: Grafting of MPC on the Polymeric Backbone for the Synthesis of the P(X-co-MMA)-b-(P(BIBEMA-co-MMA)-g-PMPC) Bottlebrush via ATRP

Figure 2

¹H NMR spectra of P(FBMA₅₉-co-MMA₄₉)-b-P(BIBEMA₂₅₀-co-MMA₂₄₀) (a), P(tBMA₅₀-co-MMA₄₈)-_b-P(BIBEMA₂₁₀-co-MMA₂₅₀) (b), and P(GMA₄₉-co-MMA₄₆)-b-P(BIBEMA₂₄₅-co-MMA₂₅₀) (c) in chloroform-d₃.

Figure 3

SEC chromatograms of P(FBMA₅₉-co-MMA₄₉)-b-P(BIBEMA₂₅₀-co-MMA₂₄₀) (a), P(tBMA₅₀-co-MMA₄₈)-b-P(BIBEMA₂₁₀-co-MMA₂₅₀) (b), and P(GMA₄₉-co-MMA₄₆)-b-P(BIBEMA₂₄₅-co-MMA₂₅₀) (c).

Figure 4

SEC-MALS results for bottlebrush polymers P(BIBEMA₂₄₆-co-MMA₂₄₉)-g-PMPC₅₂ (a), P(FBMA₅₉-co-MMA₄₉)-b-(P(BIBEMA₂₅₀-co-MMA₂₄₀)-g-PMPC₄₈) (b), P(MAA₅₀-co-MMA₄₈)-b-(P(BIBEMA₂₁₀-co-MMA₂₅₀)-g-PMPC₆₁) (c), and P(GMA₄₉-co-MMA₄₆)-b-(P(BIBEMA₂₄₅-co-MMA₂₅₀)-g-PMPC₅₀) (d) using PBS buffer (pH 7.4) and 100 mM sodium phosphate (pH 2.5) with 0.2 vol % of trifluoroacetic acid as an eluent.

Figure 5

Hydrodynamic diameter (D_h) of P(BIBEMA₂₄₆-co-MMA₂₄₉)-g-PMPC₅₂ (a), PGMA₄₉-co-PMMA₄₆-b-PBIBEMA₂₄₅-g-PMPC₅₀-co-PMMA₂₅₀ (b), PMAA₅₀-co-PMMA₄₈-b-PBIBEMA₂₁₀-g-PMPC₆₁-co-PMMA₂₅₀ (c), and PFBMA₅₉-co-PMMA₄₉-b-PBIBEMA₂₅₀-g-PMPC₄₈-co-PMMA₂₄₀ (d) determined by dynamic light scattering in water at 25 °C.

Figure 6

Atomic force microscopy images of bottlebrush polymers with different binding blocks: carboxylic (a), aldehyde (b), epoxide (c), and no anchoring group (d).

Figure 7

Results of dynamic friction force measurements performed on mica surfaces, unmodified (a) and modified with APTES (b). Friction coefficient (defined as friction force/load) for studied systems (mean value: height of the colorful bars; standard deviation: vertical bars; median: horizontal bars) (c).

Scheme 2. Anchoring of the Bottlebrush Polymers to the APTES-Modified Silicon Surface

Figure 8

Fringes of equal chromatic order (FECO) in neat mica in air, mica modified with APTES, and mica with the polymer (BBs) deposited on the APTES layer (a), exemplary results of the thickness of the evolution of the polymer film and friction force under applied load for the polymer with a carboxylic anchoring group (b), film thickness evolution for all polymers on the mica surface (c), and film thickness evolution for all polymers on the APTES layer (d).

Figure 9

Lubrication test of bottlebrush polymers on cartilage tissue scheme (a) and comparison of friction coefficient reduction on chicken cartilage tissue and friction coefficient on APTES-modified mica (b).