Commercial wet Silver and Silver Chloride electrodes are used to monitor electrocardiogram (ECG) signals in numerous bioimpedance applications. These electrodes are frequently single-use components that adhere to the skin through an adhesive surface. This sticky surface is infamous for generating skin irritations during the replacement and removal of electrodes, making the process uncomfortable for the patient. Because this type of electrodes is inappropriate in many measuring situations, the applicability of dry electrodes is investigated.

This study introduces the use of a copper-based filament (Electrifi) with the highest reported conductivity (0.006 Ω.cm) in biomedical applications, showcasing the process parameters of 3D printed, semi-flexible and wearable dry electrodes to monitor ECG signals. The effect of the printing-process parameters (bed and nozzle temperatures, surface infill pattern) on the electrical performance is thoroughly investigated (10 parameters and >100 electrode samples) to find the highest conductivity and lowest impedance of the printed electrodes. The influence of ten process parameters on the resistivity of printed electrode samples with three different surface structures (namely concentric, rough, and flat as shown in Fig. 1a) and different thicknesses have been experimented. The analyzed parameters play a significant role in the electrodes’ impedance and conductivity values. Choosing a proper setup of these parameters can enhance the bio-impedance measurements of dry electrodes similar to ranges of wet electrodes and even below.

The flow of this study was divided into two main stages. First, electrodes of 15 mm diameter and 2 mm thickness were 3D printed with three surface structures, each with six temperature settings, including two nozzle temperatures (140 and 150 °C) and three bed temperatures (40, 60, and 80 °C). At this point, non-optimized electrodes are recognized. Second, another optimization strategy was presented, which involves experimenting with electrodes of 15 mm diameter and 0.5 mm thickness with two surface structures (concentric and flat), each with two temperature settings, including two nozzle temperatures (140 and 150 °C) and one bed temperature (80 °C).

A 2D profilometry is provided, showcasing the effect of printing parameters on the electrodes' surface roughness. Keithley and Agilent semiconductor device analyzers were used to record the impedance measurements; both can acquire complex impedance spectra per second in a frequency range from 20 kHz to 400 kHz. The excitation current was set to 20 µA. This study investigates the behavior of 2 stacked electrodes. For this purpose, two binder clips were used to securely hold the two electrodes, resulting in uniform force distribution. The resistance measurements are performed using the same equipment under a fixed frequency at 400 kHz that is then converted into conductivity given the electrodes' cross-sectional area and thickness to be 177 mm² and 2 mm (or 0.5 mm), respectively.

The process parameters significantly affect the electrodes surface structures, specifically the bed temperature (T_bed). The roughness of the structured surfaces (concentric and rough) was observed to be increasing with the increase in bed temperature. However, the roughness of the flat surface remained unchanged under all temperature parameters. The impedance measurements of the 2 mm thick electrodes decreased significantly over frequency, showing a capacitive behavior (Fig. 1b). This is due to the air gap created between the structured electrodes, parasitic factors from the devices themselves, and external factors such as light, airflow, and movement that influenced the measurement. The conductivity measurements are depicted in Fig. 1c, unlike the rough surface electrode, which uses additional ironing parameters, the flat and concentric surface electrodes are printed with the same process parameters, resulting in equivalent resistance and conductivity values. Thus, the concentric and flat structures of T_bed = 80 °C and T_nozzle = 140 and 150 °C showed the best performance among all samples. Decreasing the thickness to 0.5 mm of the concentric and flat structures and using the optimum bed and nozzle temperatures generated better and stable results with higher sensitivity and lower impedance measurements.

In conclusion, this study confirms that different electrode structures and printing temperatures significantly influence the electrodes' functionality, conductivity, and impedance measurements. Defining the optimum printing parameters for the electrodes' material is fundamental to obtain stable and reliable measurements. With a new insight into the electrical behavior of the copper-based Electrifi filament, process optimization and new printing strategies can be studied for the single-process fused filament fabrication (FFF) 3D printing to create functional and sensitive electrodes.

Figure 1. A) Images of the three 3D printed electrodes surfaces: flat (left), concentric (middle), and rough (right). B) Impedance measurement of the concentric structure at different frequencies. C) Conductivity measurements of the electrodes with three different surfaces.