Tannic acid-crosslinked layer-by-layer nanocomposite coatings enhance mechanical stability and pre-osteoblast response in porous polymer-based scaffolds for bone tissue engineering

This study investigated tannic acid crosslinking as a strategy to improve the hydrated mechanical stability, physicochemical properties, degradation behavior, and biocompatibility of highly porous polyurethane scaffolds modified with layer-by-layer nanocomposite coatings for bone tissue engineering. Open-cell polyurethane foams were coated with 10 quadlayers of poly-L-lysine, poly-L-glutamic acid, poly(diallyldimethylammonium chloride), and montmorillonite, followed by crosslinking in 1 wt% tannic acid. Layer-by-layer coating substantially increased scaffold compressive modulus under dry conditions, from 85.1 ± 5.7 kPa for uncoated polyurethane to 562.3 ± 15.2 kPa for coated scaffolds. Hydration in PBS at 37 °C caused substantial softening of uncrosslinked coatings, reducing modulus to 130.1 ± 12.4 kPa. Tannic acid crosslinking mitigated this water-induced plasticisation, producing a hydrated compressive modulus of 423 ± 18.9 kPa and allowing the scaffolds to retain approximately 75% of their dry-state mechanical properties. Crosslinking also increased surface hydrophilicity, reducing water contact angle from 138.8 ± 2.85° for uncoated polyurethane to 70.3 ± 3.10°, while preserving high interconnected porosity. During 56-day enzymatic degradation testing, tannic acid-crosslinked scaffolds showed the lowest mass loss at 10.5%, compared with 14.6% for uncrosslinked coated scaffolds and 19.2% for uncoated scaffolds. Biological testing with MC3T3-E1 pre-osteoblasts showed no cytotoxicity, high cell viability, 97.3 ± 2.2% cell attachment after 24 hours, increasing metabolic activity over 14 days, and extensive cell spreading throughout the porous structure. Overall, tannic acid crosslinking stabilized the nanocomposite coating against hydration-induced softening while improving degradation resistance, wettability, and cellular response, supporting the scaffolds’ potential for non-load-bearing bone tissue regeneration.