Property evolution and secondary microplastic release of plastic film MPs in simulated digestive fluids

Microplastics (MPs) derived from agricultural or commercial films may enter the human digestive system through ingestion. However, the surface characteristics of MPs within the digestive system and the behavior of secondary MP release necessitate further investigation. This study focuses on polyethylene (PE) and poly butylene adipate-co-terephthalate (PBAT) as typical traditional and biodegradable films. Employing static in vitro digestion simulation, the surface characteristic evolution and secondary MP release behavior of both fresh and aged PE and PBAT-MPs in vitro simulated saliva fluid (SSF), gastric fluid (SGF) and intestinal fluid (SIF) were evaluated. Results indicated that aging induced micro-cracking on PBAT and enhanced its hydrophilicity, while PE maintained surface stability and high hydrophobicity due to its chemical inertness. None of the three simulated digestive fluids significantly affected the structure of the MPs, but influenced surface characteristics, including water contact angle, elemental composition and roughness. SGF notably reduced the water contact angle of aged PBAT-MP from 89.95° to 52.03° within 48 h. PBAT-MP showed a higher potential for secondary MP release (0-0.92 items cm^-²) compared to PE-MP (0-0.12 items cm^-²). This study provides empirical data to support the evaluation of the biosafety of film MPs and green applications of biodegradable materials.