Iatrogenic facial nerve injury often occurs during parotidectomy owing to the difficulty associated with accurately locating the facial nerve from adjacent tissues (parotid gland and muscle). This injury causes postoperative facial nerve dysfunction, leading to functional and/or cosmetic problems. In this study, we propose an in-vivo detection method to distinguish the facial nerve from the adjacent tissues using electrochemical impedance spectroscopy (EIS). The electrical impedance of the facial nerve, parotid gland, and muscle via in-vivo experiments using guinea pigs (n = 3) was measured using microelectrodes on polyimide film (MoP), consisting of detection and connection parts. The connection part was selectively coated with a photoresist that exhibits high resistivity to attenuate a distortion of the sensor output. Statistically significant differences in impedance of the real and imaginary parts were observed between the facial nerve and the adjacent tissues in the frequency range of 0.99 kHz to 1 MHz (p-value < 0.05, t-test). Optimal frequency, where the difference between adjacent tissues and facial nerve had the most distinction, were 13.64 kHz in the imaginary part of impedance. Additionally, the estimated electrical properties (conductivity and permittivity) of the facial nerve and adjacent tissues were statistically distinguishable (p-value < 0.05, t-test). These findings indicate that MoP with EIS could enable surgeons to easily locate the facial nerve during parotidectomy.

ENN (electrical impedance spectroscopy on-a-needle with non-passivation) can be used to discriminate bio-tissues based on depth-profiling. Since the ENN electrodes are comprised of a detection and connection parts, electrical impedance on the connection part tends to cause a distortion in the sensor output depending on the penetration depth. This distortion leads to an inaccurate evaluation of the electrical properties (conductivity and permittivity). In order to attenuate the distortion, two methods were proposed in previous studies as follows: ENS (electrical impedance spectroscopy on-aneedle with selective-passivation on the connection part) and ENNI (ENN with incremental compensation method). In this study, we compared the detection error between the two proposed methods in terms of the depth-profiling for threelayered porcine tissues. To evaluate the detection error of the aforementioned methods, the electrical properties obtained by the two methods were compared against reference values obtained directly from the side of the tissues penetrating only detection part. In the case of ENS, the detection errors of conductivity and permittivity were 3.86 % and 3.25 %, respectively, while for ENNI, the detection errors were 10.50 % and 4.57 %, respectively. Thus, despite involving a complicated fabrication process, ENS is expected to more useful than ENNI for the characterization of biotissues.