Biomimetic synthesis and tensile properties of nanostructured high volume fraction hydroxyapatite and chitosan biocomposite films

Chitosan–Hydroxyapatite (HA) nanostructured composite films have been prepared by solvent casting their hybrid suspensions. The synthesis of suspensions involves mediating the crystallization of HA by introducing chitosan solution into the reaction mixture. Both formaldehyde-treated chitosan and untreated chitosan solutions were used to study the effect of formaldehyde on chitosan–HA interactions and subsequent effective load transfer in biocomposites. The nanostructure and phase purity of HA (mildly carbonated HA) in the films were verified using SEM, XRD and FTIR. Tensile testing of the films showed significant increases in both the Young's modulus (E) and ultimate tensile strength (UTS) with HA content, reaching up to 17.3 GPa and 222 MPa, respectively, for films containing 66 wt.% (or 47 vol.%) HA in formaldehyde-treated chitosan films. In comparison to untreated chitosan composite films (<40 wt.% HA), formaldehyde-treated chitosan composite films resulted in higher E and UTS values, despite the observation of similar dispersion levels of HA particles. It is argued that apart from the uniform dispersion of HA particles in the chitosan matrix, the chemical binding between the constituents in the formaldehyde-treated chitosan composite films played a major role in the observed high tensile properties.