Preparation of Organic–Inorganic Hybrid (Sr, Ca)CO₃ Capsules Based on Thermoresponsive Degradable Coacervation

Recent advances in bone regeneration materials have focused on the development of artificial bone scaffolds incorporating bioactive ions, such as strontium ions, that promote bone formation. Incorporating drug retention and release capabilities into these materials is expected to not only improve bone regeneration efficiency but also provide additional drug-derived benefits. The aim of this study is to synthesize organic-inorganic hybrid capsules with a shell containing strontium salts that can retain and release therapeutic agents. The synthesized temperature-responsive polymer formed coacervate droplets that could encapsulate hydrophobic model drugs at temperatures above the lower critical solution concentration (LCST). After preparing Pickering emulsions by mixing calcium carbonate powder and coacervate droplets in an aqueous solution, the calcium carbonate on the surface was allowed to grow crystals under various solvent conditions to produce (Ca,Sr)CO₃ capsules. The (Ca,Sr)CO₃ capsules released Sr²⁺ ions from the shell phase and also released the encapsulated hydrophobic drug from the inner coacervate phase. In vitro studies using MC3T3-E1 cells showed that exposure to these capsules increased the expression of osteogenic markers, such as alkaline phosphatase (ALP), osteopontin (OPN), and osteocalcin (OCN). Combining bone regenrative activity with controlled drug loading and release capabilities, the prepared biomaterials have potential as multifunctional scaffolds for bone regeneration strategies.