*2.2. Characterization of the p-HApF*

The p-HApFs morphology was observed using scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan), which operated at an accelerating voltage of 15 kV The average fiber diameter of the p-HApFs was analyzed by image analysis software (Image-Pro Express Version 6.0, Media Cybernetics, Rockville, MD, USA) based on the SEM images. Moreover, the p-HApFs pore structure was observed using transmission electron microscopy (TEM, JEOL JEM-2100, Tokyo, Japan) at an accelerating voltage of 100 kV. Nitrogen adsorption–desorption measurements were used to obtain the Brunauer-Emmett-Teller (BET) specific surface area, and the pore size was also analyzed (Micromeritic ASAP 2020 instrument, Norcross, GA, USA). The phase of p-HApFs was characterized by X-ray diffraction (XRD, Bruker D2-Phaser, Madison, WI, USA) with a copper target. Powder diffraction patterns were acquired over 2-theta, ranging from 20 to 60◦ with a step size of 0.04◦. The functional groups of p-HApFs were analysed with Fourier transitioned infrared spectroscopy (FTIR, Bruker tensor II, Madison, WI, USA). The spectra were recorded from 4000 to 400 cm−<sup>1</sup> wave number with a resolution of 2 cm<sup>−</sup>1.
