Mega macromolecules as single molecule lubricants for hard and soft surfaces

The study addresses whether ultra-large, globular synthetic polymers (in the multi-megadalton range) can be synthesized with control and whether their unique size and compact architecture impart unexpected solution and tribological properties, including functioning as single-molecule lubricants on hard and soft surfaces. Prior work has produced high-MW linear polymers and dendrimers, but globular polymers at several MDa have not been realized due to viscosity build-up and control challenges during polymerization. The authors propose a homogeneous, macroinitiated ring-opening multibranching polymerization to overcome synthetic barriers and hypothesize that the resulting mega HPGs, due to compactness and hydration, will display low intrinsic viscosity and effective lubrication on stainless steel and articular cartilage.

- Dendrimers enable precise 3D globular architectures and unusual rheology (e.g., molecular ball bearing behavior) but at lower molecular weights than targeted here. - High-MW linear polymers can be synthesized via ATRP, RAFT, ROMP, and Lewis pair polymerizations; however, they exhibit large size scaling and high viscosities, unlike anticipated for globular macromolecules. - Hyperbranched polyglycerols (HPGs) synthesized by ROMBP are known for biocompatibility, degradability, and tunability; previous high-MW HPGs remained below the multi-MDa regime. - In cartilage lubrication, linear and bottle-brush polymers and hyaluronic-acid-based formulations have been explored; mechanisms include hydration lubrication, brush tilting/thinning, and mixed-mode lubrication within synovial joints. The present work situates mega HPGs within these contexts, aiming for Newtonian, highly hydrated globular lubricants with low viscosity.

Synthesis: - Employed a macroinitiator-assisted, homogeneous solution ring-opening multibranching polymerization (ROMBP) of glycidol in dry DMF at 95 °C using partially deprotonated high-MW HPG (Mw 840 kDa, Đ 1.2) as macroinitiator; KH (30% in oil) served as base. - Slow monomer feed (glycidol) under dry, well-stirred conditions (overhead stir) controlled growth. Glycidol-to-macroinitiator ratio tuned final Mw. - Three target polymers: mega HPG-1 (Mw 1.3 MDa, Đ 1.2), mega HPG-2 (Mw 2.9 MDa, Đ 1.2), mega HPG-3 (Mw 9.3 MDa, Đ 1.4). Yields ~74–85%; single batch isolation up to ~42 g for mega HPG-3; reproducible across batches. Purification and characterization: - Precipitation (MeOH/acetone), neutralization, dialysis (RC, MWCO 50 kDa), lyophilization/storage as aqueous solution. - GPC-MALS (Waters ultrahydrogel columns, 0.1 N NaNO3, dn/dc 0.12 mL/g) for absolute Mw and Đ. - NMR (1H, 13C IG) for structural features and degree of branching (DB = 2D/(2D+L)). - DLS/QELS for hydrodynamic diameter; cryo-SEM for morphology and size. - DSC to quantify bound water and hydration; solubility testing in water; intrinsic viscosity via online viscometer (0.1 N NaNO3). - AFM force spectroscopy to estimate Young’s modulus of polymer particles adhered to epoxide glass (contact mode, calibrated cantilever). Rheology and tribology on hard surfaces: - Viscosity on AR2000 rheometer (2° aluminum cone, 47 µm gap, 25 °C), averaged across shear rates 10–100 s−1. - Stribeck curves using DHR-2 rheometer with stainless steel ring-on-plate geometry: 300 µL lubricant, 5 N normal load, radial velocity ramp 0.001–50 rad/s; COF vs Hersey number (velocity × viscosity / load). - Tested mega HPGs (1, 3, 9 MDa) at 7 and 23 w/v% vs controls: Pennzoil 80W-90, Synvisc One, bovine synovial fluid (BSF). N=3 replicates per lubricant. Cartilage tribology and biocompatibility: - Cartilage-on-cartilage COF using Bose Electroforce 3200 with skeletally mature bovine osteochondral plugs (7 mm diameter), unconfined geometry. - Incubation in test solutions (PBS/saline, BSF, human osteoarthritic synovial fluid, mega HPGs at 7% and 23% w/v) overnight at RT. - Creep loading: 8 N (~200 kPa) for 3 h submerged; then rotation at 360°/s for 120 s; equilibrium COF computed as μ = (3/2) × (τ / N r). N ≥ 3 replicates per group; N = 6 pairs noted for harvested plug tests. - Shear thinning assessed via flow curves; Newtonian vs non-Newtonian behavior noted. - Cell viability (MTT) in Tc28a2 human chondrocytes and 3T3 fibroblasts after 48 h exposure to 1.25 mg/mL polymer; triplicate technical replicates, studies repeated in triplicate; absorbance at 570 nm vs saline controls.

- Successful gram-scale synthesis of three mega HPGs: Mw 1.3 MDa (Đ 1.2), 2.9 MDa (Đ 1.2), and 9.3 MDa (Đ 1.4) with monomodal distributions; yields ~74–85%; mega HPG-3 average Mw across batches 9.3 ± 0.03 MDa, yield 74 ± 0.21%. - Degree of branching 53–57% (13C IG NMR), consistent with semi-dendritic hyperbranched architecture. - Particle sizes (DLS): 21.2 ± 0.4 nm (1.3 MDa), 30.6 ± 0.6 nm (2.9 MDa), 43.0 ± 0.4 nm (9.3 MDa). Cryo-SEM shows spherical single particles: average sizes ~28, 34, and 51 nm for 1.3, 2.9, and 9.3 MDa, respectively. - Extremely high hydration and functionality: mega HPG-3 bears >87,000 hydroxyl groups per polymer and ~389,300 bound water molecules per polymer (DSC). - High aqueous solubility >380 mg/mL; contrasted with similarly sized linear PEO and PVA forming gels at high concentration. - Very low intrinsic viscosities, nearly independent of Mw: [η] = 4.67, 5.26, 6.15 mL/g for 1.3, 2.9, and 9.3 MDa, respectively, far below linear polymers (e.g., PEG 11 MDa ~2600 mL/g). - Stainless steel tribology: At 7% and 23% w/v, mega HPGs show boundary lubrication with transitions to mixed mode at Hersey numbers increasing with Mw and concentration. COFs at transition comparable to BSF but at higher speeds; Stribeck curves resemble Synvisc but mega HPGs are ~100× less viscous. - Cartilage-on-cartilage: All mega HPGs significantly reduced COF compared to human osteoarthritic synovial fluid and were statistically equivalent to healthy BSF; one-way ANOVA with p < 0.0001 for differences vs OA SF. 9 MDa (mega HPG-3) exhibited slightly lower and more consistent COF (lower variance), though not statistically significantly lower than other HPGs. - Rheology: Mega HPG-1 (1.3 MDa) at 23% shows shear thinning (non-Newtonian), while 3 and 9 MDa behave as Newtonian fluids with constant viscosity across shear rate; viscosities similar to healthy BSF and far lower than Synvisc (1226 mPa·s). - Mechanical property: Mega HPGs are soft and compressible; mega HPG-3 exhibited Young’s modulus ~7.9 kPa (AFM). - Cytocompatibility: ~80% viability in human chondrocytes and fibroblasts after 48 h exposure, comparable to saline controls. - Manufacturing: Protocol enabled up to ~42 g isolated mega HPG-3 per batch, indicating scalability.

The findings confirm that ultra-large, globular mega HPGs can be synthesized with controlled branching, low dispersity, and gram-scale yields. Their compact, hydrated, single-particle nature imparts unusually low intrinsic viscosities nearly independent of molecular weight, divergent from linear polymers and consistent with globular macromolecular behavior. Tribologically, these polymers reduce COF on both stainless steel and articular cartilage despite low bulk viscosities, suggesting a lubrication mechanism not governed by viscosity alone. The authors propose that mega HPGs act as interposed molecular ball bearings in aqueous media for hard surfaces, and on cartilage likely leverage hydration lubrication—maintaining a water-rich layer capable of supporting loads—along with potential mechanical trapping or ultrafiltration at the tissue surface that retains 20–40 nm particles. The Newtonian behavior of higher-MW mega HPGs is advantageous, avoiding shear thinning and displacement under load, which could maintain a stable lubricating layer at interfaces. The comparable performance to healthy BSF and superiority over osteoarthritic synovial fluid in ex vivo cartilage tests, combined with low injection viscosity and cytocompatibility, highlight their promise as biomedical lubricants and as tribological additives.

This work demonstrates the first synthesis and thorough characterization of mega-scale hyperbranched polyglycerols (1–9.3 MDa) as compact, highly hydrated, low-intrinsic-viscosity single-particle polymers. These mega HPGs function as effective lubricants on both hard and soft biological surfaces, achieving COF reductions comparable to healthy synovial fluid while maintaining Newtonian behavior and low viscosity. The scalable synthesis, tunable size, and extensive functional hydroxyl groups suggest broad opportunities in lubrication engineering and biomedical applications. Future research should elucidate the precise lubrication mechanisms on cartilage (boundary vs mixed vs hydrodynamic), optimize particle size and surface chemistry for retention and performance, assess long-term biocompatibility and in vivo efficacy, and explore functionalization for targeted interactions with tissues or surfaces.

- The lubrication mechanism on cartilage is hypothesized (hydration shell lubrication, mechanical trapping, ultrafiltration) but not directly verified; further mechanistic studies are needed. - COF comparisons to literature are limited by differing geometries, protocols, and tissue types, complicating direct benchmarking. - Cartilage tests were ex vivo using bovine tissue; in vivo performance, retention, and safety were not assessed. - Only two concentrations (7% and 23% w/v) and three molecular weights were tested; broader parameter exploration could refine structure–function relationships. - Shear thinning behavior and entanglement effects were inferred from rheology; direct measurements of interfacial adsorption/retention were not reported.