Organic–Inorganic Multilayer Microcarriers with Superior Mechanical Properties for Potential Active Delivery in Fast-Moving Consumer Goods

This study introduces an eco-friendly approach to fabricating superstrong, core–shell, composite microcapsules, offering a sustainable alternative to traditional insoluble microplastic-based materials like melamine-formaldehyde. These microcapsules were engineered with a thick CaCO₃ shell formed via crystal ripening in the presence of water-soluble poly(acrylic acid), encasing a hexylsalicylate oil core armored by hydrophilic SiO₂ nanoparticles. An additional polydopamine layer was deposited via oxidative autopolymerization at pH 8.5 for improved structural and surface properties of the resulting microcapsules. These microcapsules (D_3,2 = 8.8 ± 0.3 μm) were spherical, with a relatively smooth surface, and exhibited unique mechanical properties, which are essential to broaden their applications in industry. Remarkably, compression tests showed a mean rupture stress of 73.5 ± 5.0 MPa, which dramatically surpasses any other inorganic/synthetic microcarrier reported in the literature. In addition, only 10–20% of the core active was released within 2 h into a mixed water-propanol medium used as an accelerated release test, where the solubility of the active oil is high, with full release over 3 days. Herein, we also propose a novel pathway-specific binding constant (PSBC) that describes the strong interaction between Ca²⁺ ions and poly(acrylic acid), in connection with their stoichiometric ratio. Overall, these microcapsules hold promise for multiple fast-moving consumer goods, where maximizing the mechanical strength of microcapsules for encapsulation of valuable functional actives is paramount; this includes but is not limited to energy storage, household, agrochemical, personal care, and healthcare applications.