Nano-Ru-HKUST-1 enhanced PVA hydrogels: Laying the foundation for MOF-based cartilage tissue scaffolds

Metal-organic frameworks (MOFs) gather a unique set of properties, namely tunable porosity, large internal surface area, and modifiable chemical functionality, that make them particularly suitable for biomedical applications, specially as delivery agents. However, many pristine MOFs suffer from poor chemical stability, limited conductivity and mechanical fragility, which restrict their practical use. An emerging strategy for biomedical applications is to combine the chemical versatility of MOFs with the flexibility, biocompatibility, and water-retentive nature of hydrogels. We herein report the development of a novel hydrogel composite incorporating a nano-sized MOF with the skeleton of HKUST-1 ([Cu 3 (BTC) 2 ], a copper-based MOF originally developed at the Hong Kong University of Science and Technology), in which ruthenium replaces copper as the metal center. Nano-Ru-HKUST-1, obtained by microwave-assisted synthesis, was directly dispersed into poly(vinyl alcohol) (PVA) solutions at three different concentrations (0.06, 0.3, and 1 wt.-%). The resulting mixtures were physically crosslinked into hydrogels using a freeze–thaw process. Composite hydrogels with concentrations of 0.06 and 0.3 wt.-% of nano-Ru-HKUST-1 exhibited enhanced water uptake, improved stiffness, energy dissipation, and stable viscoelastic behavior. They also showed significantly reduced friction coefficients compared to a metallic reference, which supports their suitability for load-bearing, lubricated environments. In contrast, composites with 1 wt% of nano-Ru-HKUST-1 had lower mechanical performance, likely due to particle aggregation. Cytocompatibility studies revealed that nano-Ru-HKUST-1 can support cell viability. Biological tolerance was cell-type-specific: primary human chondrocytes remained viable up to 0.3 wt.-%, whereas L929 fibroblasts showed signs of cytotoxicity at the same concentration. Overall, incorporating Ru-HKUST-1 into PVA hydrogels enables tunable mechanical and biological performance, with an optimal concentration range likely between 0.06 wt.-% and 0.3 wt.-%. These findings support the potential of Ru-HKUST-1-PVA composites for use in soft-tissue biomedical applications that require both structural integrity and biocompatibility.