*3.3. Electrode Material Analysis*

We selected conductive silica gel, conductive rubber, and a conductive brush as the electrode materials for the experiments and found that the egg cracks could be identified with all three materials. The resistivity of the conductive rubber was large, and the current change was not obvious enough when it was used as an electrode. When a conductive brush was used as the electrode, the conductive brushes would fuse after discharge and cause a great loss of electrode material. In contrast, the resistivity of the conductive silica gel was small and could produce an obvious current change when passing the cracked area. Therefore, conductive silica gel was selected as the electrode material in this paper.

To sum up, the current is not only related to the resistivity of electrode materials but also closely related to the contact area of the conductive materials. However, it is not a case of "the larger the better" for the contact area, as too large a contact area will lead to a large current for non-cracked eggs. The more ideal form of contact is line contact, which is made according to the outline of the egg so as to fit the eggshell perfectly. The actual structure of the electrode is shown in Figure 9.

**Figure 9.** Real figure of electrode.

#### *3.4. The Importance of Multi-Layer Flexible Electrodes*

Since eggs vary somewhat in size and shape, the design of a flexible electrode can better fit the eggshell and achieve full coverage of an effective detection area by dynamically adjusting the angle according to the eggs. Although a single-layer flexible electrode can effectively detect cracks, their coverage area is limited. When detecting larger eggs, gaps between the electrode strips may cause omissions during the egg rotation if the cracks are just perpendicular to the gaps. The use of multi-layer flexible electrodes can reduce the chances of missed detection of egg cracks, which plays a significant role in improving the overall detection accuracy and can also further reduce the detection voltage.

#### *3.5. Lab Environment*

We selected 10 eggs randomly and put 5 eggs in a group to test the electrical characteristics under different humidity environments. The mean current curve is shown in Figure 10. The experiment found that the measured current value in the environment with a humidity of 72% and voltage of 1500 V was equivalent to that in the environment with a humidity of 54% and voltage of 1800 V, which further proved the conclusion of Section 2 that the detection of egg cracks based on current signals was greatly affected by environmental humidity. Therefore, during the data collection, the humidity and temperature of the experimental environment should be stabilized within a certain range to reduce the influence of the environment on the experimental data.

**Figure 10.** Standard deviation of the current signal of eggs at different voltages.

The voltage value used in HVLD is generally high, even reaching up to tens of thousands of volts at certain times. If it is directly used for the detection of egg cracks, the protein may be denatured. In order to avoid this, we had to choose an appropriate voltage range. All things considered, we finally determined that the experimental environment was perfect at an average temperature of 18.5 ◦C, a relative humidity of 40%, and a voltage of 1500 V. We strictly controlled the current size, and the system current protection mechanism would be triggered to cut off the power when the current was greater than 1 mA so it would not cause damage to the eggs.

In order to quickly obtain a sufficient number of egg samples with microcracks and avoid the instability of manual striking, we designed an egg crack striking machine to control the size of the artificial cracks and prevent the egg contents from leaking. The machine is shown in Figure 11a. Eggs are fixed at the bottom of the track, and the rollers are released from different heights and strike the egg at the equatorial part to generate controllable microcracks. The width of the artificial microcracks is generally less than 3 microns, which is usually not easy to observe with the human eye. Microcracks are

mainly located in the central area between two ends of the egg and only present in the effective detection area. Egg samples with cracks at the tip or blunt end will be discarded. In actual production, there are not only large cracks caused by strong striking but also a large number of microcracks of several microns, which are difficult to detect by traditional methods. The structure of a microcracked egg under an industrial microscope is shown in Figure 11b,c.

**Figure 11.** Egg crack and its generating device. (**a**) Egg crack striking machine. (**b**,**c**) Pictures of cracks of different sizes under the industrial microscope.
