Assessment of physicochemical properties and biological responses of chemically surface-modified microcapsules for self-healing dental restorative materials

• Melamine increased formaldehyde release, shown by a newly refined Purpald assay. • Melamine improved capsule strength and integrity under functionalization procedures. • Capsules functionalization improved thermal, mechanical, and biological performance. • Formaldehyde levels were safe; cytotoxicity came from early core release • Future work should target capsule stability over reducing formaldehyde. Approximately 50% of resin-based dental restorations fail within ten years, mainly due to secondary caries and material fractures. Incorporating self-healing microcapsules containing resin-based agents that polymerize upon release offers a promising strategy to repair microcracks and extend restoration longevity. Poly(urea–formaldehyde) (PUF) microcapsules are commonly used, but formaldehyde cytotoxicity remains a concern. This study developed alternative microcapsules by integrating melamine into the polymeric shell (PUMF) and functionalizing surfaces with methacrylate (TMSPM) or thiol (MPTMS) silanes, with or without a TEOS pre-coating, to reduce formaldehyde release and enhance biocompatibility. Microcapsules were synthesized via double-emulsion, encapsulating triethylene glycol dimethacrylate (TEGDMA), and characterized for morphology, mechanical stability, functionalization efficiency, formaldehyde release, and cytotoxicity. Melamine increased mechanical and morphological stability, surface roughness, and core retention, especially under functionalization, but also raised formaldehyde release in unmodified capsules. Functionalization with TMSPM and MPTMS significantly reduced formaldehyde release from both PUF and PUMF capsules. Metabolic assays showed dental pulp stem cells were more sensitive to TEGDMA, while fibroblasts were affected by TEGDMA combined with prolonged formaldehyde exposure. Overall, melamine-reinforced functionalized microcapsules exhibit improved stability, controlled formaldehyde release, and enhanced biocompatibility, supporting their potential as next-generation self-healing dental materials.