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Cellulose nanocrystal is a rod-like crystalline nanoparticle obtained from cellulose, typically 3-20 nm in width and 100-500 nm in length, in which the chains are packed in a crystalline allomorph whose form depends on the source material and the hydrolysis route ^[1]. It is produced by hydrolysing native cellulose with mineral acid, most commonly sulfuric acid, which also esterifies part of the surface with sulfate half-esters and gives the particle a negative surface charge that stabilises aqueous suspensions ^[2]. The particles have a high aspect ratio, a density near 1.6 g cm-3, a Young's modulus on the order of 100-150 GPa and a low coefficient of thermal expansion, so they serve as a reinforcing nanofiller in polymers, films and hydrogels ^[3]. Above a critical concentration in water they self-assemble into a chiral nematic liquid crystal, and drying this phase leaves an iridescent film whose colour depends on the pitch and on the ionic strength of the medium ^[1]. The surface chemistry can be modified by oxidation, esterification or grafting to adjust compatibility with a given matrix, and carboxylated grades are prepared by TEMPO-mediated oxidation ^[4]. Limiting factors are the cost of the sulfuric-acid route and the sulfate-induced thermal instability it leaves behind, which restricts melt processing, together with the difficulty of drying the particles without irreversible aggregation ^[2]. Reported mechanical reinforcement depends on dispersion quality, and aggregation during drying or processing usually reduces the gain; the choice of polymorph has also been shown to affect processability and the properties of the resulting composite ^[5].