## 2 Materials and Methods
### Measurement Setup  
Four commercial drugs were used in this experimental study. The EIS method was used to obtain a description of the impedance variations caused by the conductive properties of different compounds over the liquid test fixture operational frequency range (20 Hz to 30 MHz). The experimental setup is shown in Figure 6, which consists of three main components: (1) an impedance analyser with an operational frequency range from 20 Hz to 50 MHz (Keysight E4990A), (2) a liquid test fixture with an operational frequency range of 20 Hz to 30 MHz (Keysight 16452A) and (3) a 4-wire connection (Keysight 16048A).  
Figure 5: Modelling electrochemical cell using an array of Voigt elements plus a single resistor.  
The specifications of the liquid test fixture (Figure 7) are:  
1. Electrode size: \(38\pm 0.5\) (mm) (circular geometry).
2. Casing dimensions: 85 height \(\times\) 85 width \(\times\) 37 depth (mm).
3. Materials (electrodes, spacers, liquid inlet and outlet): nickel-plated cobal (Fe 54%, Co 17%, Ni 29%).
4. Insulator: ceramic insulator (alumina Al\({}_{2}\)O\({}_{3}\)).
5. O-ring: Viton (Fluoro rubber).
6. Insulator soldering: silver-copper and gold copper.  
The measurement equipment selected is an auto-balancing bridge, which offers the advantage of high accuracy over a wide impedance measurement range (20 Hz to 30 MHz), limited by the liquid test fixture operational frequency range [30]. This is well within the frequency range required to analyse the dielectric properties of the electrochemical cell filled with test samples. For every test, 200 impedance and phase data points, logarithmically separated over the measurement range, were obtained. The excitation voltage/current values were adjusted to 10 mV\({}_{\text{RMS}}\)/200\(\upmu\)\({}_{\text{RMS}}\).  
Figure 6: Experimental array for measurements of electrical impedance of commercial drugs. Impedance analyser and Liquid Test fixture schematic.  
Figure 7: Keysight 16452A liquid test fixture. (**A**) Assembly detail (Short compensation) and (**B**) Assembled liquid test fixture. Reproduced with permission, courtesy of ©Keysight Technologies, Inc., 1994–2019.  
Commercial drugs are often highly lipophilic and insoluble in aqueous media, materials frequently require the use of a solvent or carrier system to ensure adequate bioavailability. Thus, liquid crystals are a mixture of compounds in a state between liquid and solid forms [31]. Here, we used liquid-based drug suspensions, syrup and injectable solution (C\({}_{14}\)H\({}_{18}\)N\({}_{4}\)O\({}_{3}\)-C\({}_{10}\)H\({}_{11}\)N\({}_{3}\)O\({}_{3}\) Liomont Lab), (C\({}_{13}\)H\({}_{18}\)Br\({}_{2}\)N\({}_{2}\)O.HCl Bruuart Lab), (C\({}_{13}\)H\({}_{16}\)N\({}_{3}\)NaO\({}_{4}\)S SON'S Lab), and (C\({}_{13}\)H\({}_{22}\)N\({}_{4}\)O\({}_{3}\)S.HCl PiSA Lab) of commercial-grade, respectively as listed in Table 2.  
The drugs were introduced in the test device using a syringe of 5 mL for each as shown in Figure 8.  
The measurement setup was 40 points per decade in a frequency range of 20 Hz to 30 MHz controlled signal. All tests were repeated twice to decrease the margin of measurement error for each test trial. The impedance measurement was made at 27 \({}^{\circ}\)C.