## 2 Materials and Methods  
### Experimental Setup  
Complex bipolar impedance data recording of the electrodes during the insertion process was carried out by a system comprising a three-degree-of-freedom actuation system, a saline-filled plastic cochlea model, an impedance meter, and an electrode array attached to the actuation system.  
The electrode array is Oticon Medical's Evo with 20 platinum-iridium electrodes. The EA has an active length of 24 mm. As shown in Figure 1A, electrodes are individually wired and connected to the 20-pin header socket. Electrodes are evenly spaced but their size reduces from basal to apical electrode, as shown in Figure 1B,C.  
An actuation system shown in Figure 2 was used for controlled insertion of EA in the ST model. This system has three DoF, where it can be moved horizontally, vertically and rotationally. The vertical stage was connected on the horizontal stage and the rotational stage was placed on the vertical stage. A holder was placed on the rotational stage to place EA for insertion. The EA was inserted from three different directions to form different trajectories during insertion.  
The overall configuration chain of impedance measurement and its detailed block diagram is shown in Figure 3. The EA was connected with the impedance meter through the female header connector. Bipolar impedances of electrode pairs were recorded using a voltage divider circuit, where DUT is an electrode pair (EP) in series with a known resistance, as shown in Figure 4. A voltage signal 1 V p-p at 1 kHz frequency is applied to the series circuit through an NI DAQ 6211 device's analogue output port, and voltages are recorded across both electrode pair and known resistance using the same device's analogue input ports. The current through the circuit is calculated using the known resistance (\(R\)) and voltage (\(V_{R}\)) across it using [PERSON]'s law, as shown in Figure 3B. Impedance magnitude of the electrode pair is calculated using the relation \(|Z|=V_{EP}/I_{c}\). The impedance phase  
Figure 1: **(A)** Electrode array with associated wiring (wraped in silicon carrier) soldered and glued to a female connector, **(B)** close-up version of 20 electrode contacts, **(C)** part of electrode array under microscope.  
is calculated by subtracting the current phase from the phase of voltage across EP. In our previous study, it was shown that impedance phase and its resistive and reactive parts are also useful for classification of different trajectories [28]. Using geometrical representation, we can extract Cartesian coordinates (R and X) with the help of polar coordinates (\(\mid\)Z\(\mid\) and \(\theta\)), as shown in Figure 3B. We collected bipolar impedance magnitude (\(\mid\)Z\(\mid\)), phase (\(\theta\)), resistance (\(R\)), and reactance (X) of eight electrode pairs. The setup configuration chain of the impedance meter is shown in Figure 3A. EA is connected to the multiplexer that selects a certain EP to act as a DUT in the voltage divider circuit. NI DAQ devices (6211 and 6009) are responsible for all analogue input/output voltage signals and digital control signals to the multiplexer (\(C_{1-3}\) and \(C_{4-6}\)), respectively. These control signals select particular electrodes to make an EP. A custom MATLAB GUI application handles all the data input/output to the DAQ devices and processing to spit out the required features (\(\mid\)Z\(\mid\), \(\theta\), \(R\), and \(X\)). These features will further be used for our machine-learning-based prediction of trajectory and position of electrode array during the insertion.  
Figure 3: (**A**) Impedance measurement setup configuration chain, (**B**) detailed block/circuit diagram for the complex impedance measurement system. The diagram is colour-coded according to the sections of the system.  
Figure 2: Three-DoF actuation system for electrode array insertion in ST model. Graphical representation of insertion from three different directions are in the inset.