Mimicking biological size, structure, and functionality with synthetic analogs is a longstanding scientific and engineering goal. While linear polymers of a few million daltons (MDa) exist, synthetic globular polymers of several MDa are largely unknown. Such structures are predicted to exhibit unique properties due to their size, compactness, and low inter-chain interactions. This paper addresses the significant synthetic challenge of creating these mega-macromolecules, focusing on the synthesis of hyperbranched polyglycerols (HPGs) in the MDa range. The choice of glycerol backbone is motivated by its biodegradability, biocompatibility, and chemical tunability, making it suitable for biomedical and environmentally friendly applications. The study aims to synthesize mega HPGs and characterize their properties, particularly focusing on their potential as lubricants.

The synthesis of large, complex macromolecules, especially dendrimers, has seen significant advancements. Linear polymers reaching several MDa have been synthesized using techniques like reversible deactivation radical polymerization, atom transfer radical polymerization, and ring-opening metathesis polymerization. However, the synthesis of globular polymers of comparable size has remained a challenge, primarily due to the increasing viscosity of the polymerization medium which leads to limitations in controlling molecular weight and polydispersity. Previous research on dendrimers, like PAMAM, has shown interesting viscosity behaviors, but the synthesis and properties of mega-macromolecules with million-Dalton molecular weight remain largely unexplored. Existing studies on cartilage lubrication have explored linear and brush polymers, demonstrating the potential for synthetic polymers to improve lubrication in joints. However, there's a need for novel materials with improved efficiency and biocompatibility.

Mega HPGs were synthesized using a homogenous ring-opening multibranching polymerization (ROMBP) method. A high molecular weight HPG (840 kDa) was employed as a macroinitiator, with a partially deprotonated form used to initiate polymer growth from the macroinitiator. Glycidol was slowly added to the reaction mixture under dry conditions, enabling controlled molecular weight synthesis and minimizing polydispersity. Three different molecular weights (1.3 MDa, 2.9 MDa, and 9.3 MDa; mega HPG-1, -2, and -3, respectively) were achieved by varying the glycidol-to-macroinitiator ratio. The resulting mega HPGs were characterized using various techniques, including gel permeation chromatography with multi-angle light scattering (GPC-MALS), nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), dynamic light scattering (DLS), and cryo-scanning electron microscopy (cryo-SEM). Hydrodynamic diameter and intrinsic viscosity were determined. Lubrication properties were evaluated using two experimental setups. First, Stribeck curves were generated using a DHR-2 rheometer, measuring the coefficient of friction (COF) on stainless steel surfaces under varying loads and speeds, comparing the mega HPGs to motor oil, Synvisc One (hyaluronic acid solution), and bovine synovial fluid (BSF). Second, cartilage-on-cartilage friction tests were performed on bovine osteochondral plug pairs using a Bose Electroforce 3200. Cartilage pairs were incubated in saline, BSF, human osteoarthritic synovial fluid, and different concentrations of mega HPGs before the friction test was conducted. Cell viability (cytocompatibility) was measured using an MTT assay on human chondrocytes and fibroblasts. Finally, atomic force microscopy (AFM) measured the Young's Modulus of mega HPG-3.

The ROMBP method successfully yielded gram quantities of mega HPGs (up to 9.3 MDa) with low polydispersity (1.2-1.4). GPC-MALS confirmed their monomodal molecular weight distribution. NMR analysis confirmed the structure and branching density (53-57%). DSC showed high hydration, with mega HPG-3 possessing ~389,300 bound water molecules per polymer. The mega HPGs were highly water-soluble (>380 mg/mL). DLS and cryo-SEM revealed their compact, nanoscale, single-particle nature, with hydrodynamic diameters ranging from 21 nm (1.3 MDa) to 43 nm (9.3 MDa). Importantly, intrinsic viscosity was surprisingly low and nearly independent of molecular weight, contrasting sharply with linear polymers and following the Einstein viscosity theory for hard globular polymers. Stribeck curve analysis on stainless steel showed that mega HPGs, at both 7 and 23 w/v%, exhibited boundary mode lubrication initially and transitioned to mixed-mode lubrication at higher Hersey numbers, with performance comparable to BSF and Synvisc but with significantly lower viscosities. Cartilage-on-cartilage friction tests demonstrated that mega HPGs reduced the COF to levels comparable to healthy BSF and significantly lower than osteoarthritic synovial fluid (p<0.0001). The 9 MDa mega HPG-3 showed the most consistent performance. The mega HPGs exhibited high cytocompatibility (>80% cell viability). AFM analysis revealed a Young's Modulus of 7.9 kPa for mega HPG-3.

The findings demonstrate that mega HPGs function effectively as single-molecule lubricants for both hard and soft surfaces. Their unique combination of high molecular weight, nanoscale size, low intrinsic viscosity, and high hydration contributes to their lubrication mechanism. The near-independence of intrinsic viscosity on molecular weight suggests a compact globular conformation limiting entanglement. The lubrication mechanism likely involves a combination of factors, including hydration shell lubrication (maintaining a water film between surfaces) and a potential mechanical trapping mechanism similar to hyaluronic acid where the polymers aggregate at the contact leading edge. The high water solubility and cytocompatibility highlight their biocompatibility. The low viscosity compared to Synvisc suggests potential advantages in ease of administration. The study's success in synthesizing high-molecular weight globular polymers with controlled properties paves the way for designing new materials with tailored functionalities.

This research successfully synthesized gram-scale quantities of mega HPGs with million-Dalton molecular weights, demonstrating a significant advance in polymer chemistry. These mega-macromolecules exhibited unexpected lubrication properties, acting as single-molecule ball bearings to reduce friction on both hard and soft surfaces. Future research should focus on exploring the precise mechanisms of lubrication, investigating various surface chemistries, evaluating long-term performance, and exploring applications in biomedicine and other fields requiring high-performance lubricants.

The study utilized bovine cartilage, and extrapolation to human cartilage requires further investigation. While the cytocompatibility was promising, long-term in vivo studies are needed to fully assess biocompatibility and potential degradation. The lubrication mechanism requires further investigation to fully elucidate the interplay of hydration, surface interactions, and polymer conformation. The study concentrated on a specific polymer architecture; investigating other architectures with comparable properties could further expand the potential applications.