A simple method to control morphology of hydroxyapatite nano- and microcrystals by altering phase transition route

Hydroxyapatite (HAp) particles with various morphologies such as sphere, rod, whisker, and platelet have attracted a great deal of scientific and technological interest for their broad utilization as reinforcing agents in bone cement, bone fillers, drug carriers, and adsorbents for chromatography. In this Article, a simple method to control the morphology of HAp particles by adjusting the initial pH of precursors and the amount of gelatin and urea additions is introduced. Initially formed calcium phosphate products such as octacalcium phosphate (OCP), hydroxyapatite (HAp), and amorphous calcium phosphate (ACP) are found to be altered by changing the pH of solutions, which induces variation of HAp morphology as well as phase transformation route to HAp. From the observation of HAp formation behavior, the addition of gelatin is revealed to retard HAp formation as well as to change the aspect ratio of HAp particles, which is ascribed to strong adsorption of gelatin on the surface of calcium phosphate. Also, urea is observed to boost HAp formation rate by enhancing hydrolysis reaction. Through the understanding of the influence of the aforementioned variables, the morphology of pure HAp particles is successfully controlled, and this enables the promotion of the applicability of HAp particles in various fields.