Ion-bridged hydroxyapatite nanofiber bundles via supercritical CO2-mediated mesoscale nanoarchitectonic for robust hydrophobicity

Conventional hydrophobic ceramic coatings exhibit compromised mechanochemical robustness due to static templating and decoupled organic-inorganic assembly. Here we introduce a supercritical carbon dioxide (SC CO 2 ) strategy to engineer mesoscale self-assembly—where interactive inorganic-organic building blocks couple across time and space—to enable hierarchical structured hydroxyapatite (HAP) nanofiber bundles with embedded hydrophobicity. Our findings demonstrate that SC CO 2 generates dynamic microcompartments and a mildly acidic environment that not only facilitate the rapid growth of HAP nanofibers with restricted c -axis orientation but also establish directional Ca 2 + –PO 4 3− ionic bridges that interconnect nanofibers into a cohesive inorganic network, in which oleate anions (OL − ) in situ bind at ionic junctions via bidentate chelation, forming a robust hydrophobic layer. This green and scalable process yields advanced hydrophobic ceramic coatings, which have been successfully used to construct HAP-based fire-resistant paper with superior self-cleaning and oil-water separation capability, as well as sustainable Chinese Xuan paper with enhanced writing performance. Such mesoscale engineering, which unifies synthesis, assembly, and function, represents a new paradigm for creating robust, multifunctional biomimetic materials. Supercritical CO 2 unifies HAP nanofiber growth and mesoscale self-assembly, creating an organic-inorganic hybrid structure with inherent hydrophobicity. • Supercritical CO 2 enables the spatiotemporally coupled inorganic-organic assembly. • Hydroxyapatite hybrid nanofiber bundles with intrinsic hydrophobicity were constructed. • Such hydrophobic nanofiber bundles advance two sustainable paper products.