## 3 Result and Discussion
### Impedance Characterization of Porcine Tissue Using the Needle  
The segmental layers of pork tissue used for the experiment, including the dermis, hypodermis, and muscle is shown in Figure 3. From the ultrasound image of the pork tissue, the structure and thickness of the tissue layers in the pork meat could be found. With the aid of ultrasonography, the tip of the monopolar injection needle was successfully inserted and positioned in the specific tissue layer. The electrical impedance data of the tissue layers were measured while the needle position was observed in real-time by ultrasonography. Figure 2: (**a**,**c**,**e**) Real and (**b**,**d**,**f**) imaginary part of the impedance spectra of the resistor–resistor–capacitor (RRC) circuit measured with respect to different (a,b) C1, (c,d) R1, or (e,f) R2 by the DFT-impedance measurement system (ADuCM350) in comparison to the commercialized product (SP-200). Figure 4 shows the real and imaginary part of the measured impedance in the tissue layers of the porcine, in the frequency range of 200 Hz to 50 kHz. The real part of the impedance in the case of the dermis layer was higher than that of muscle and hypodermis in the measurement frequency due to the presence of lipid components with high resistivity in the dermal tissue [22]. Whereas, the muscle tissue showed higher conductivity compared to other tissue layers [11]. The impedance of the saline (0.9% NaCl) solution, which has similar conductivity of the synovial fluid [23], was lower than other tissue layers. The imaginary part of the impedance measured in the tissue type could be discriminated at frequencies higher than 10 kHz at which the magnitude of the electrode impedance decreased enough to be ignored. The electrode impedance governed the total impedance magnitude at low frequencies  
Figure 4: (**a**) Real and (**b**) imaginary part of the impedance spectra measured in the pork tissue layer at frequencies from 200 Hz to 50 kHz together with the fitted lines using the equivalent circuit model. Figure 3: (**a**) Pork tissue used for the experiment, (**b**) ultrasound image of the pork tissue showing the needle location in muscle layer indicated by the red colored boundary, and (**c**) the different thickness of the tissue layers indicated by the red arrows. because of the capacitive reactance of the electrode interface, which can be described using the constant phase element (CPE) as follows:  
\[\text{CPE}_{\text{el}}=\frac{1}{\text{T}(j\omega)^{\text{P}}}, \tag{1}\]  
where \(j\) is the imaginary unit, \(\omega\) is the angular frequency, T and p are the parameters of the CPE. At enough high frequencies, the total impedance is contributed to by not only the extra cellular resistance but also the cell membrane and intracellular resistance [24]. The result of Figure 4b showed that the reactance magnitude of the muscle was higher than other tissues. The relative permittivity of the muscle was higher than the hypodermis [25]. The measured impedance spectra of muscle, hypodermis, and dermis were well fitted by the equivalent circuit model shown in the inset of Figure 4b. The equivalent circuit consists of R\({}_{\text{in}}\) and R\({}_{\text{ex}}\) for the intra- and extracellular resistance respectively, C\({}_{\text{m}}\) for the membrane capacitance, and CPE\({}_{\text{el}}\) for electrode impedance. From the fitting analysis, the extrapolated values of the circuit parameters for each tissue type are arranged in Table 1 with \(\chi^{2}\) showing the precision of the data fitting. The measured impedance of tissue using the monopolar needle was characterized by the total complex impedance characteristics of each tissue layer. When the position of the needle was close to the neighbored tissue, the impedance of the tissue was more affected by the electrical properties of the neighbored tissue. However, the impedance characteristics of the different tissue layer could be significantly separated.