Effects of Air, Vacuum, and Nitrogen Annealing on Structure, Bone Regeneration, and Antimicrobial Activity of Zinc-Doped Ca-P-O Microfibers

Calcium phosphate (CaP) is a family of materials closely resembling the mineral phase of natural bone and is widely used in bone tissue engineering for its biocompatibility and bioactivity. In this work, the starting material, Zn-doped calcium phosphate (Zn-CaP), was synthesized using the sol-gel method. Then, microfibers were fabricated using polyvinylpyrrolidone/Zn-CaP and the electrospinning technique. The samples were annealed under various conditions (air, vacuum, and nitrogen) at 800°C. The results indicated that the average fiber diameter ranged from 300 to 2000 microns, as observed by scanning electron microscopy (SEM). X-ray diffraction (XRD) analysis demonstrated that the main phase in air-annealed samples was calcium carbonate, with hydroxyapatite as the second phase. For vacuum annealing, the main phase was calcium carbonate, and the second phases were calcium oxide and hydroxyapatite, while nitrogen annealing resulted in an amorphous phase. Fourier transform infrared spectroscopy (FTIR) results were consistent with the XRD analysis. In addition, the local structure of Zn was investigated using X-ray absorption spectroscopy (XAS), which indicated the presence of a ZnO phase in air-annealed fibers and a ZnS phase in vacuum-annealed fibers. Nitrogen annealing resulted in an amorphous phase. Moreover, the simulated body fluid (SBF) immersion test results showed that the most significant formation of apatite layers occurred in nitrogen-annealed samples, enhancing the in vivo bone bioactivity of the microfibers. The antimicrobial activity results showed that the air and vacuum-annealed fibers demonstrated 100% effective against Staphylococcus epidermidis and Pseudomonas aeruginosa.