fig3

Figure 3. Contact impedance of the sponge electrode on hairless skin (A-C) and hairy skin (D-F). (A) Cross-sectional schematic and equivalent circuit model of the sponge electrode on hairless skin; (B) Electrode-skin impedance spectrum of the sponge electrode and solid gel electrode on a total of nine hairless sites of three different subjects. The markers are average values, and the error bars indicate the standard deviation; (C) A comparison of the contact impedance between the dry and wet sponge electrodes on hairless skin (n = 6). The inset plots the loading-unloading stress- strain curves of dry (black) and wet (red) sponges; (D) Cross-sectional schematic and equivalent circuit model of the sponge electrode on hairy skin; (E) Electrode-skin impedance spectrum of the sponge electrode and solid gel electrode on the hairy scalp; (F) A comparison of the contact impedance between the dry and wet sponge electrodes on the hairy sites (n = 6); (G) Electrode-skin contact impedance spectrum of the sponge electrode before (black) and after (red) being compressed 100 times (three measurements at each frequency); (H) The contact impedance of the sponge electrodes on different locations is stable after 60 min of continuous wear; (I) A radar chart that compares the sponge electrodes with other EEG electrodes in the literature based on the impedance, softness, hair compatibility, and reusability criteria. Grading is based on the following criteria. Impedance lower than 10 kΩ·cm² is good, 10 kΩ·cm²-100 kΩ·cm² is medium, and higher than 100 kΩ·cm² is poor. Softness with Young’s modulus lower than 50 kPa is good, 50 kPa-1 MPa is medium, and higher than 1 MPa is poor. Hair compatibility, if the electrode cannot deform to contact the hairy scalp, it is considered poor; if the electrode can deform to reach the hairy scalp but has medium impedance, it is defined as medium; if the electrode can form very good contact with the hairy scalp with good impedance, it is considered high. Reusability: if the electrode can be reused without being destroyed after use, it is considered good; otherwise, it is considered poor. EEG: electroencephalography.