*2.1. Synthesis and Characterization of Porous Hydroxyapatite Nanofibers*

Mesoporous hydroxyapatite nanofibers were synthesized using poly(ethylene glycol)-block-poly (propylene glycol)-block-poly(ethylene glycol) (P123, Sigma-Aldrich, St. Louis, MO, USA) as the structure-directing agent. Briefly, 1.0 g of P123 and 6.172 mL of triethyl phosphite (TEP, Merck, Germany) were mixed in a 10 mL ethanol aqueous solution (50% *v*/*v*), which was then continuously stirred to form a clear solution. Ca(NO3)2 (14 g, Sigma-Aldrich, St. Louis, MO, USA) was dissolved in 10 mL of absolute ethanol under magnetic stirring at room temperature. The calcium nitrate solution was slowly added under stirring to the above P123/TEP solution to form a precursor solution. The precursor solution was tightly capped and placed in an oven at 60 ◦C for 12 h. Then, 1.5 g poly(vinyl pyrrolidone) (PVP, Sigma-Aldrich, St. Louis, MO, USA), 0.45 g P123 and 7 mL absolute ethanol were prepared and incorporated into the 3 mL precursor solution to obtain a transparent mixture solution. The mixture solution was placed inside a plastic syringe fitted with a stainless needle (18 G, inner diameter = 0.838 mm) and then inserted into a syringe pump to supply a steady flow rate (1.27 mL/h). An electrical field (1.3 kV/cm) was applied between the needle and an aluminum substrate (grounded collector) using a high-voltage power supply (SL 60, Spellman, New York, NY, USA). The polymer solution, which formed a Taylor cone upon exit, was collected on an aluminum substrate in the form of nonwoven nanofiber structures and was then calcined at different temperatures (600 ◦C, 800 ◦C and 1000 ◦C) and environments (air and nitrogen) to obtain the calcined p-HApFs nonwoven structures.
