[0034] Figure 1 presents the formation mechanism of (hybrid alginate-silica) beads of the invention. This formation relies on a coarcevation process in which the addition of a polycationic organic polymer (e.g., PDADMAC) decreases the solubility of a hybrid solution containing silica precursor(s) and sodium alginate. In fact, the alginate acts as a template and the PDADMAC plays the role of a silica concentrator. The core part of the beads contains a hybrid sodium alginate-silica 1 , the intermediate layer 3 is composed of hybrid calcium alginate-silica and the external layer (shell) 2 comprises silica and the silica concentrator PDADMAC. The PDADMAC-containing layer reduces or prevents any leakage of silica species outside the beads