Calcium phosphate biomineralization in peptide hydrogels for injectable bone-filling materials

Abstract

We performed amorphous calcium phosphate (ACP) and hydroxyapatite (HAp) mineralization in peptide hydrogels for the formation of a novel bone-filling material. We prepared two kinds of β-sheet peptides, (LE)₈ and (VEVSVKVS)₂, respectively, using hydrophilic glutamic acid (E), serine (S) and lysine (K), and hydrophobic leucine (L) and valine (V). Both peptides hierarchically self-assembled as nanofibers and formed hydrogels in the presence of calcium ions. The formation of the hydrogel was due to the ionic cross-linkage between carboxyl groups in the glutamic acid side chains of the peptide nanofibers and the calcium ion. (LE)₈ formed a clear hydrogel above a calcium ion concentration of 4.0 × 10⁻³ M and the hydrogel collapsed at 1.0 × 10⁻² M, owing to excess ionic cross-linkage. On the other hand, (VEVSVKVS)₂ containing two glutamic acid residues per molecule retained the hydrogel structure at a higher concentration of calcium ions in the hydrogel where the (LE)₈ hydrogel collapsed. The viscoelastic property of both peptide hydrogels was increased by increasing the calcium ion concentration, showing adequate strength as a bone-filling material. When phosphate ion was added into the (LE)₈ hydrogel containing calcium ion, ACP was mineralized along the peptide nanofiber in the hydrogel under neutral pH. The (VEVSVKVS)₂ hydrogel, on the other hand, produced HAp under basic pH. In addition, the peptide hydrogel smoothly recovered its original moduli just after the shear deformation of the hydrogel. It was also clarified that calcium ions are not only a source of mineralization but also induce an increase in the mechanical strength of the hydrogels as a reinforcing agent. It was shown, moreover, that the crystallinity of the HAp was slightly dependent on the modulus of the peptide hydrogels. Therefore, it may be said that peptide hydrogels are applicable for tailor-made injectable bone-filling materials.