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Orthopedic and Rehabilitation Engineering

Orthopedic and Rehabilitation Engineering - Poster Session E

Y15 - Characterization of Super-Lubricious Copolymer Coated Hydrogel Microspheres for the Treatment of Knee Osteoarthritis

Saturday, October 11, 2025
10:00 AM - 11:00 AM PDT
Location: Exhibit Hall F, G & H

Introduction: : Knee osteoarthritis (OA) affects over 32 million individuals in the United States alone, causing pain, stiffness, and reduced joint mobility due to cartilage degradation and inadequate joint lubrication. This loss of lubrication is associated with reduced concentrations of hyaluronic acid (HA) and lubricin that are catabolized in the inflammatory joint. Current treatments to rescue this joint lubrication focus on HA injections, though the relief is often temporary and requires multiple injections. Here, we aim to develop hydrogel microspheres coated with a super-lubricious copolymer to reduce friction in the knee for prolonged periods. These microspheres are fabricated from polyethylene glycol (PEG), which has been demonstrated to be biocompatible and biodegradable with tunable mechanical and physical properties. These PEG microspheres are coated with a custom-synthesized copolymer consisting of mussel glue-like dopamine methacrylate (DMA) for adhesion to PEG and zwitterionic sulfobetaine methacrylate (SBMA) to impart enhanced lubrication. This study aims to optimize the PEG microsphere and copolymer formulation with regards to copolymer adsorption to PEG microspheres, as well as lubrication performance. Relative DMA and SBMA ratios were examined in both random- and block-copolymer formats. The successful fabrication of these super-lubricious hydrogel microspheres is expected to reduce friction and enhance the natural lubrication within the knee.

Materials and

Methods: : Poly(DMA-co-SBMA) was synthesized using a RAFT-mediated alkene polymerization (Fig. 1A). The copolymer was synthesized with the fluorophore Rhodamine B (RhoB) to track copolymer binding to PEG microspheres. The product was isolated by dialysis against water. FTIR and 1H NMR were used to confirm the presence of and quantify DMA and SBMA. Size exclusion chromatography (SEC) was used to identify polymer molecular weight and polydispersity. UV-vis spectroscopy was used to confirm the incorporation of RhoB into the copolymer. Hydrogel microspheres were prepared by reacting 4-arm PEG-acrylate and PEG-dithiol via Michael-type addition to form a chemically crosslinked mesh network. The PEG solution was then put through a modified electrospraying setup to form microspheres (Fig. 1B). After gelation, microspheres were dip-coated in copolymer dissolved in de-ionized water. To confirm copolymer absorption onto the hydrogel microspheres, fluorescent microscopy and confocal imaging were used. The mechanical properties of copolymer-coated hydrogel microspheres were analyzed, with rheology evaluating the stiffness of copolymer-coated versus uncoated spheres. Additionally, a custom tribo-rheology setup was used to determine the coefficient of friction (COF) of coated spheres in comparison to simulated healthy and osteoarthritic synovial fluid. The injection force of copolymer-coated microspheres was also determined and compared to uncoated spheres, simulated healthy synovial fluid, and simulated osteoarthritic synovial fluid. To optimize the relative ratios of DMA and SBMA and the copolymer formats (random- versus block-copolymers), various properties of coated spheres were compared between the different copolymer configurations, including copolymer absorption, release, and mechanical properties.

Results, Conclusions, and Discussions:: We synthesized 5 different poly(DMA-co-SBMA) copolymers with incorporated RhoB: random and block DMA-rich, random and block SBMA-rich, and random copolymer with DMA:SBMA ratio of 1:1. The presence of DMA and SBMA was confirmed with FTIR and NMR, while spectroscopy demonstrated the incorporation of RhoB into the copolymer. SEC revealed copolymer molecular weights of ~40 kDa for all polymers. Stable PEG hydrogel microspheres were fabricated via electrospraying with a mean diameter of ~100 µm prior to washing, which swelled to ~150 µm following washing.

Adsorption of poly(DMA-co-SBMA) to the microspheres was confirmed via fluorescent microscopy, as microspheres exhibited bright fluorescence that was localized near the surface of the microspheres. We also showed that longer coating times of the microspheres in copolymer solution resulted in spheres with higher fluorescence, suggesting greater copolymer adsorption to the microspheres. The fluorescence of copolymer-coated microspheres was maintained for over 21 days (Fig. 1C), highlighting the coating stability over time.

Comparing different variations of the copolymer (i.e., different ratios of DMA:SBMA and random- versus block-copolymers), preliminary data revealed that DMA-rich and random-copolymer coated microspheres had better copolymer absorbance. Tribological analysis of the copolymers revealed that coated microspheres achieved a lower COF than did uncoated PEG microspheres and simulated healthy or OA synovial fluid (Fig. 1D), indicating enhanced lubrication. Injection testing showed that coated microspheres were capable of being extruded through a 31 G syringe needle (Fig. 1E). Current tests are underway to compare the developed copolymer-coated microspheres with clinically used HA treatments meant to improve SF lubrication.

Together, these findings demonstrate the development of an optimized super-lubricious poly(DMA-co-SBMA) copolymer and the successful fabrication of injectable poly(DMA-co-SBMA)-coated PEG microspheres to be used as lubricants in the treatment of knee OA. It is hoped that by reducing the friction within the knee, pain may be alleviated for patients and the progression of OA may be slowed.

Acknowledgements and/or References (Optional): :