Effect of pH and precursor salts on in situ formation of calcium phosphate nanoparticles in methylcellulose hydrogel

Thermo-reversible hydrogels have been widely studied in biomedical research fields, because of their potential performance in drug delivery and tissue engineering. In this study, a novel thermo-sensitive methylcellulose (MC) hydrogel based on calcium phosphate nanoparticles (CaP NPs) was prepared by in situ formation process in which precursor salts induced a salt-out effect in the MC solution. Based on the Hofmeister series, CaCl₂, Na₂HPO₄, and NaH₂PO₄ precursor salts with a good salt-out effect were used as the precursor salts for CaP NPs. The gelation of MC solution was accelerated by the salt addition, and its effect varied with the salt type. When CaP precursor salts with two different Ca/P ratios (0.8/1 and 1.6/1) were added into the MC solution, more elastic MC hydrogel was formed at 1.6/1 Ca/P ratio. The CaP NPs were rectangular in shape (40-50 nm), and evenly distributed in the MC gel. The X-ray diffraction (XRD) observation indicates that dicalcium phosphate dehydrate (DCPD) and octacalcium phosphate (OCP) were synthesized at 0.8/1 Ca/P ratio, whereas DCPD was synthesized at 1.6/1 Ca/P ratio. This indicates that the CaP NPs with different crystalline phases were in situ synthesized in accordance with the Ca/P ratio and pH condition (pH 7.4, 9.0). Moreover, the DCPD and OCP crystalline phases were converted to the hydroxyapatite (HAP) crystalline phase, as the pH was changed to alkaline region. Therefore, the resulting MC composite hydrogel containing HAP NPs will be suitable for bone regeneration as an injectable hydrogel.