Freeze-Dried Porous Composites Based on Sodium Alginate and Powders of Calcium Salts Synthesized from Mixed-Anionic Solutions Prepared from (NH4)2CO3 and (NH4)2H2P2O7 Taken in Different Proportions

The creation of new biocompatible and resorbable composite materials with an alginate matrix and fine synthetic calcium salts as a filler for potential applications in regenerative medicine is an important area of materials science. Freeze-dried porous (0.05–0.06 g/cm3) composite materials were prepared from homogeneous molding compositions containing distilled water (95 wt. %), fine inorganic powders of calcium salts (2.5 wt. %), and sodium alginate (2.5 wt. %). An aqueous solution of calcium chloride (CaCl2) and an aqueous mixed-anionic solution, which were prepared by mixing solutions of (NH4)2CO3 and (NH4)2H2P2O7 taken in different proportions, were used for syntheses of fine inorganic powders. The molar ratios (NH4)2H2P2O7)/((NH4)2CO3+(NH4)2H2P2O7) of salts used for mixed-anionic solutions preparation were established as 0 (reference powder), 0.02, 0.04, 0.08 and 0.16. Quantities of aqueous solutions of calcium chloride (CaCl2) were taken to provide a molar ratio CaCl2/((NH4)2CO3+(NH4)2H2P2O7) equal to 1.00 (reference powder), 1.02, 1.04, 1.08 and 1.16. The phase composition of synthesized inorganic powders included calcium carbonate (CaCO3) (mostly vaterite) and a quasi-amorphous phase of calcium phosphates (primarily hydrated calcium pyrophosphate (Ca2P2O7 xH2O)). All synthesized powders contained adsorbed and occluded salts present in the mother liquor after synthesis was completed, before separation of the precipitate. The mother liquors were aqueous solutions of ammonium chloride (NH4Cl) (all syntheses) and calcium chloride (CaCl2) (syntheses from mixed-anionic solutions). As components of inorganic powders used as fillers in the composite materials, ammonium chloride (NH4Cl) and calcium chloride (CaCl2) provoked a partial cross-linking of sodium alginate in the stages of preparation of molding compositions, composite samples molding, freezing, and freeze-drying. All freeze-dried porous composite materials demonstrated a high water absorption capacity of 4600–9200%. The most pronounced cross-linked effect was observed for alginate matrices loaded with powders synthesized at molar ratios CaCl2/((NH4)2CO3+(NH4)2H2P2O7) equal to 1.04 and 1.08, demonstrating the lowest water absorption and dry mass loss after dipping in distilled water for 12 h. Prepared freeze-dried porous composite materials with an alginate matrix and calcium carbonate and/or quasi-amorphous phase of calcium phosphates (primarily hydrated calcium pyrophosphate) powders, used as a filler, can be recommended for subsequent testing as an extracellular matrix for soft/hard tissue repair, as wound dressings or bone defect fillers.