Carbonated apatite coating on chitosan with gradient distribution via ions assembly

Carbonated apatite (CHA) coating on chitosan with gradient distribution was achieved by ions assembly at ambient condition in a short period time. Chitosan hydrogel can directly chelate with calcium via amino groups without inducing carboxyl or hydroxyl group previously. The complexation of chitosan with calcium ions subsequently absorbed phosphate ions via electrostatic interaction. Calcium phosphate (CaP) minerals dispersed in chitosan with the thickness of 0.5 mm is exactly CHA with Ca/P molar ratio of 2.01. The transformation route of CaP salts in chitosan hydrogel started from amorphous calcium phosphate with Ca/P molar ratio of 1.19 after ions assembly, and ended at CHA with help of alkali treatment. The content of calcium and phosphorous element decreased with increasing the depth toward the core according to the energy dispersive spectrometer (EDS) results. The gradient distribution of CHA in chitosan and Vickers hardness of chitosan with CHA coating were similar to the distribution of hydroxyapatite in bone and hardness of weight-bearing bovine bone, respectively.