Hydroxyapatite-containing gelatin/chitosan microspheres for controlled release of lysozyme and enhanced cytocompatibility†
A simple polymer microsphere was dramatically transformed into a novel protein delivery carrier by the incorporation of hydroxyapatite (HA) into the polymeric phase. In this study, nano-sized HA was uniformly mixed in a gelatin/chitosan solution to ultimately form composite microspheres with a well-defined spherical shape. The physicochemical properties of these microspheres were extensively characterized in terms of morphological analysis, crystalline structure, swelling properties, etc. The average sizes of the microspheres increased slightly due to a gradual increase in viscosity of the HA-containing gelatin/chitosan solution. The typical characteristics of HA used in this work were confirmed to result in the successful fabrication of composite microspheres with graded HA content (0 to 55 %). The HA-containing microspheres showed outstanding features with respect to in vitro calcification, high protein loading/controlled release, and cytocompatibility. First, the addition of HA made composite microspheres greatly facilitated in vitro calcium ion deposition while initiating nucleation in stimulated body fluid (SBF). Second, the HA-involved composite microspheres acted as a highly efficient delivery system when compared to bare polymer microspheres. These features were mostly a result of the relatively good swelling properties and protein adsorption capability of HA. Third, the HA-containing composite microspheres showed significantly improved initial cell adhesion/proliferation. The increase in HA content and surface roughness provided a favorable matrix for the attached cells, which grew extensively in elongated, spindle-like shapes. In summary, this study investigated a high protein loading/controlled release system consisting of nano-sized HA as a key component, which contributed superior features of in vitro calcification and cytocompatibility.