Bio-based nanomaterials are emerging as key enablers for Safe- and Sustainable-by-Design (SSbD) innovation, offering renewable, high-performance alternatives to conventional, fossil fuel-based materials for use in cosmetics, food, and other consumer products. Among them, cellulose nanomaterials, particularly cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs), have gained increasing attention due to their biodegradability, low environmental footprint, and tunable physicochemical properties (1). A comprehensive physicochemical characterization of these materials is crucial, but presents several challenges, including their inherent polydispersity, complex morphology, and the dynamic transformations they undergo in biological matrices (2, 3). In this study, we present an in-depth physicochemical characterization of CNCs and CNFs under pristine conditions and within representative biological fluids. Key parameters such as particle size distribution, aspect ratio, elemental composition, and surface zeta potential are evaluated using a multi-analytical approach combining dynamic and electrophoretic light scattering, particle tracking analysis, multi-detector field-flow fractionation, atomic force microscopy and focused ion beam microscopy coupled to secondary ion mass spectrometry. Our results highlight the complex morphology of CNCs and CNFs and give insight into their behavior in biological fluids. This study thus provides essential physicochemical information for their reliable toxicological assessment supporting future regulatory acceptance and SSbD-compliant product development. References: 1) L. Johnston, Nanoscale, 2024, 16, 18767, https://doi.org/10.1039/D4NR02276A 2) V. Grachev et al. Nanomaterials, 2024, 14(5), 455, https://doi.org/10.3390/nano14050455 3) E. Johan Foster et al. Chem. Soc. Rev., 2018, 47, 2609-2679, https://doi.org/10.1039/C6CS00895J