Enhancing the stability of urea-formaldehyde microcapsules for self-healing polymers through additive modulation.

Over the past decades, self-healing poly(urea-formaldehyde) (PUF) microcapsules have been extensively utilized across diverse fields, including textiles, construction, aerospace, and biomedical systems. However, concerns remain regarding their mechanical stability and potential formaldehyde toxicity. Here, we investigate the incorporation of melamine and the high-toughness acrylamide monomer N,N-Dimethylacrylamide (DMAM) to enhance the thermomechanical stability and cytocompatibility of PUF microcapsules containing either triethylene glycol dimethacrylate (TEGDMA) or a TEGDMA-DMAM blend at 20 wt%. Melamine was incorporated into the shell precursor up to 10 wt%, and the resulting microcapsules were characterized for morphology, mechanical, thermal, and biological properties. In general, melamine increased shell roughness and physicochemical stability, with 5 wt% providing the best balance. DMAM improved shell elasticity, and the combined use of DMAM and melamine yielded the highest elasticity (57 %). Formaldehyde release, quantified via a Purpald assay, increased with melamine content (up to 21 μM after one week), yet dental pulp stem cell (DPSC) viability remained ≥95 % across all tested concentrations. Direct exposure to microcapsules maintained DPSC viability above 85 % for all formulations. Healing efficiency was ≥82 % in all tested materials, with DMAM-containing systems showing a ∼ 20 % improvement compared to TEGDMA-only formulations. These results demonstrate that melamine and DMAM enhance microcapsule durability while preserving cytocompatibility, supporting their potential use in biomedical applications such as self-healing dental composites.