Interphase coordination design in carbamate-siloxane/vaterite composite microparticles towards tuning ion-releasing properties

Siloxane-containing vaterite (SiV) microparticles were prepared with controlling the degree of aminopropyl-functionalization in the siloxane; they are aiming for applications as bone regenerative devices. The aim of this work was to evaluate the structure at siloxane/vaterite interphase and to control the solubility of particles by the structural tuning of siloxane. The particles were spherical with average diameters of 1.1–1.4 μm. Differential infrared spectrometry revealed the transformation of aminopropyl terminals in the siloxane into carbamate (NH-COO^?) groups. Moreover, the vaterite crystallites in the particles were slightly oriented towards the (0 0 1) plane. These results describe the interphase structure, with the carbamate groups coordinating on the Ca²⁺ ion face in the (0 0 1) plane of neighbouring vaterite. Upon soaking in buffer solution, the particles exhibited a rapid initial release of Ca²⁺ ions within 30 min and of soluble silica within 2 h. The vaterite in this particle survived for more than 6 h. The chemical stability of the siloxane was enhanced by incorporating tetraethoxysilane-derived siloxane with fractions of 24 mol% or 50 mol%. This enhancement controlled the initial release of not only soluble silica but also Ca²⁺ ions.