Chitosan with defined intrinsic viscosity enables physicochemical entrapment of microplastics under in vitro gastric conditions.

Microplastics (MPs) are increasingly detected in food and biological systems, raising concerns about their interaction with the gastrointestinal environment. Strategies capable of limiting their mobility and epithelial contact are therefore of growing interest. Here, we investigate the role of chitosan physicochemical properties in the entrapment of MPs under gastric conditions (pH = 3 and 37 °C). Using chitosan samples with a comparable degree of deacetylation (DDA) but different intrinsic viscosities, we identify a threshold behaviour governing MP capture. Only chitosan with intrinsic viscosity ≥90 cP (corresponding to 100 kDa and ≥310 nm contour length) forms semi-dilute entangled networks capable of effectively entrapping high-density polyethylene (HDPE), polyvinyl chloride (PVC) and polyethylene terephthalate (PET) MPs, achieving up to 87% topological entrapment. Lower-viscosity variants remain in a dilute regime and show negligible binding. Combined proton nuclear magnetic resonance (H-NMR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) demonstrate that this transition arises from the polymer architecture rather than the surface charge, enabling multipoint interactions and topological confinement within a continuous polymer network. These findings establish intrinsic viscosity as a key design parameter for polymer-based interception of MPs at the biointerface. While the biological fate of the resulting aggregates requires further investigation, this study provides a physicochemical framework for understanding and engineering polymer-MP interactions under gastric conditions. These findings suggest a new material-based approach for reducing the bioavailability of MPs by dietary intervention.