## 3 Results  
### Physicochemical Evaluation  
The X-ray diffraction (XRD) and GIXRD patterns of the HA powder and disc displayed characteristic peaks of the high crystalline HA phase (JCPDS-00-009-0432) (Figure 2). The change in the relative intensities of the 002, 211, and 300 peaks of the GIXRD pattern was attributed to surface stress caused by disc pressing, followed by sintering. Chemical analysis and the FTIR spectra confirmed the hydroxyapatite chemical composition. The HA powder displayed a calcium phosphate ratio of 1.68, and the FTIR spectrum consisted of (PO4)\({}^{3-}\) bands at 1091 cm\({}^{-1}\) (v3a), 1044 cm\({}^{-1}\) (v3b), 959 cm\({}^{-1}\) (v1), 601 cm\({}^{-1}\) (v4), and 569 cm\({}^{-1}\) (v4), with OH- bands at 3570 cm\({}^{-1}\) and 632 cm\({}^{-1}\). The complete results from the Chemical analyses by XRF and FTIR are presented in Table S1 (Supplementary Materials) and Figure S1 (Supplementary Materials), respectively. The ZP of the HA powder nanoparticles immersed in sodium chloride (NaCl) solutions was assessed at pH values of between 3 and 11 before and after RGD immobilization. Two  
Figure 2: X-ray diffraction (XRD) patterns of the HA powder and the grazing-incidence X-ray diffraction (GIXRD) of the discs after the heat treatment and RGD functionalization. APTES molarities (20 and 40 mM) were analyzed for the silanization step. However, due to the distinct surface areas, we continued with 40 mM to treat HA powders and 20 mM to treat HA discs. It was found that the untreated HA surface displayed a ZP of \(-\)20.6 \(\pm\)1.7 mV at pH = 7.0 and an isoelectric point (IP) of 4.99 (Table 2). After the RGD physical adsorption, the IP of p-RGD/HA increased to 6.76, with a ZP of +1.0 \(\pm\)1.9 mV at pH = 7.7, confirming the RGD bond to the HA surface. Removing the weakly immobilized RGD peptides through sonication did not significantly change p-RGD/HA's IP (Table 2). In the group submitted to chemical immobilization, the samples that were silanized in the first stage (c-HA1) with 20 mM showed a positive charge at pH = 4 (+18.7 \(\pm\) 1.3 mV) and +2.3 \(\pm\) 0.5 mV around pH = 7, considered low compared to the 40 mM sample, which presented a charge above +30 (+43.8) at pH 4 and +10.6 \(\pm\) 1.3 mV at pH = 7. In the subsequent stages, spacer (EDC and c-HA2) insertion and the immobilization of RGD (c-RGD/HA) markedly reduced the ZP to \(-\)5.7 mV in a 40 mM sample at pH around 7. The charges were not affected by sonication. During the first two steps of RGD chemical immobilization, the high IP values reflected the silanization process and the subsequent coating with EDC. By the end of the chemical RGD immobilization process, the IP value of 5.34 was close to the control (4.99) and similar to p-RGD/HA, indicating a surface with a negative charge. Contact angle (CA) measurements were used to assess the effect of RGD on the wettability of each HA disc surface following either the physical or chemical addition of RGD, as summarized in Table 2. The untreated HA surface exhibited a CA of 61\({}^{\circ}\)\(\pm\) 2, indicating its expected hydrophilic nature. After RGD adsorption, the CA of the surface physically adsorbed with RGD (p-RGD/HA) immediately increased to 91\({}^{\circ}\)\(\pm\) 1 (_p_ < 0.05), but it later decreased to 83\({}^{\circ}\)\(\pm\) 2 after sonication was performed to remove weakly bound peptides. Regarding the chemically bound RGD material (c-RGD/HA), the CA values varied significantly during its production steps, from 75\({}^{\circ}\)\(\pm\) 1 following the APTES treatment to 68\({}^{\circ}\)\(\pm\) 3 after the subsequent carbodiimide treatment, remaining at similar levels (69\({}^{\circ}\)\(\pm\) 4) in the final step, where the RGD peptide was finally associated with the HA/APTES/Carbodiimide surface, resulting in a material with a wettability more similar to the untreated HA than p-RGD/HA.