In this study, we demonstrated that hydroxyapatite (HAp) biomaterial can be synthesized entirely from biowastes, i.e., eggshells and urine. Eggshells were cleaned, crushed, and calcined at 900 °C and then the powder was dissolved in water to form an aqueous calcium hydroxide solution. The aqueous solution was mixed with synthetic urine (SU) in stoichiometric amounts corresponding to HAp (Ca/P ratio ∼1.67). Calcium phosphate (CaP) was potentiostatically synthesized on magnesium or stainless steel electrode at cathodic potentials (−2 to −4 V). CaP particles formed on the metal surface at −2 V. The growth of the particles increased as the potential was increased from −2 to −3 V. However, at −4 V, the particles formed on the metal surface decreased as a result of excessive hydrogen evolution on the metal surface disrupting particle adhesion. An increase in the electrolyte concentration by 3-fold enhanced the particle growth, but further increase in the concentration by 5-fold did not show any additional improvement. X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR) analysis confirmed that the powder formed on the metal sample was an amorphous CaP. Alkaline treatment at 80 °C for 2 h converted the amorphous CaP into crystalline HAp.

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