## 3 Results and Discussion
The strong Bragg reflection peaks (2\(\theta\) = 42.3, 49.2, 71.5, and 86.4\({}^{\circ}\)) reflected their Miller indices ((111), (200), (220), and (311) lattice planes), respectively. The positions and relative intensities of all diffraction peaks corresponded well with the Fm3m crystal structure of the Pd-Ni alloy, which was in good agreement with the values reported in the literature (JCPDS Card No. 65-9444). The strong and sharp peaks suggested that the Pd-Ni alloy crystals were highly crystalline. In addition, this result indicated that the electroplating method clearly formed a Pd-Ni alloy structure without any annealing process. The average crystallite size (D) of the electrodeposited Pd-Ni alloy crystals was estimated by fitting the diffraction peak profiles with a convolution of Lorentzian functions (inset of Figure 5a), and the extent of line broadening was estimated using the Scherrer equation:  
\[\mathrm{D}=\frac{0.9\lambda}{\beta\cdot cos\theta},\]  
where \(\lambda\) is the wavelength, \(\beta\) is the full width at half-maximum (fwhm) measured in radians on the 2\(\theta\) scale, and \(\theta\) is the Bragg angle for the measured hkl peak. The mean crystallite size of Pd-Ni alloy was 11.69 nm, which was calculated using the most intense peak (111) on the XRD pattern. Figure 4: FESEM images of the (**a**) cross-sectional and (**b**) surface morphologies, and (**c**) result of the energy dispersive X-ray spectroscopy (EDS) elemental analysis of the micro-porous membrane filter. Figure 5: Analysis of micro-porous membrane filter: (**a**) X-ray powder diffraction (XRD) and (**b**) thermal gravimetric analysis (TGA). The inset is fitting curve of the (111) main peak. Thermogravimetric analysis (TGA) was performed to study the oxidation behavior of the Pd-Ni alloy filter in the temperature range 20 \({}^{\circ}\)C\(\sim\)900 \({}^{\circ}\)C in air (Figure 5b). At low temperatures, i.e., below 300 \({}^{\circ}\)C, the membrane filter was hardly oxidized as there was inadequate energy to overcome the activation energy required for oxidation reactions to occur. As the ambient temperature was increased to 900 \({}^{\circ}\)C, the oxidation rate of the Pd-Ni alloy filter increased almost exponentially. The initial oxidation rate was extremely low at lower temperatures, because the reaction was in the chemical rate regime. This oxidation property of the Pd-Ni alloy filter confirmed that oxidation corrosion during piezoelectric vibration rarely occurred. The micro-hardness of the Pd-Ni alloy membrane filter is a very important factor to measure mechanical surface hardness. The electroplating Pd-Ni alloy membrane filter had a hardness close to 553.5 Hv (average value of 10 measurements). This result indicated that the Pd-Ni alloy membrane filter is unlikely to fracture when it undergoes high-frequency vibration at 100 kHz, as previously reported [32]. Therefore, we can confirm that the Pd-Ni alloy membrane filter with a hardness approximating 553.5 Hv was more suitable than pure nickel with a hardness below 300 Hv. In vitro cytotoxicity testing to evaluate the biocompatibility of the fabricated micro-porous membrane filter is essential to assess, not only the filter itself, but also the safety and cytotoxic potential of the material passing through the filter. Therefore, cytotoxicity tests on the L-929 (mouse fibroblast) and A-549 (human alveolar basal epithelial cell) cells were conducted using two recommended methods, a direct contact method and an extract dilution method, according to the ISO 10993-5 standard; the methods were modified as described previously [28; 29]. First, the cytotoxicities of the membrane filter were confirmed with fibroblast and alveolar basal epithelial cells, using the direct contact method, as the qualitative evaluation of the biocompatibility by using morphological change.