Effect of Ag/AgCl Paste Composition on the Performance of Screen-Printed Flexible Ag/AgCl Biomedical Electrodes

Abstract

This study focused on the flexible Ag/AgCl biomedical electrode fabricated by screen printing. It systematically investigated the influence of the Ag/AgCl paste on its performance. By adjusting the type of silver powder and the mass ratio of Ag to AgCl, multiple groups of Ag/AgCl pastes were prepared, and their conductivity, microstructure, electrochemical properties, and mechanical stability were systematically characterized. The research results indicated that the specific surface area of the silver powder and the ratio of Ag to AgCl significantly affected the resistivity of the paste and the interface structure of the electrode. When using high specific surface area sheet-like silver powder H and an Ag:AgCl ratio of 5:5, the prepared electrode exhibited the best comprehensive performance: a lower resistivity (2.16 × 10⁻⁷ Ω·m), stable open-circuit voltage, good redox reversibility, and an impedance lower than that of commercial electrocardiogram electrodes. Further verification through an electrocardiogram detection system confirmed that this electrode could clearly and stably collect human electrocardiogram signals, meeting the practical requirements of electrocardiogram detection. This study provided an important theoretical and experimental basis for the development of high-performance and low-cost screen-printed Ag/AgCl flexible electrodes.

1. Introduction

With the continuous development of chemical sensors, flexible biosensors and biomedical electrodes in the fields of health and medical treatment, food and soil quality, climate change and environmental protection, higher requirements are put forward for the corresponding reference electrodes in terms of reliable performance, low cost, designability and flexibility [1,2,3,4]. Ag/AgCl electrode has the advantages of low polarization, low impedance, simple manufacture and sensitivity to a low-frequency electric field, which makes it surpass the standard hydrogen electrode and saturated calomel electrode in application scope. Furthermore, with its minimal and stable half-cell potential, the non-polarized Ag/AgCl electrode satisfies the requirements for biomedical electrodes, including good conductivity, robust stability, minimal potential fluctuation, and low polarization potential [5,6,7]. Its widespread use in clinical monitoring and biomedical measurements underscores its significance [8,9,10]. Therefore, Ag/AgCl electrode materials have aroused extensive research interest.

Various methods exist for preparing Ag/AgCl electrodes. The classical structure of the Ag/AgCl electrode is a glass shell structure, prepared by electrolytic chlorination, which has excellent stability [11]. However, due to the extremely brittle, rigid and bulky structure of traditional glass electricity, its further application is limited [12,13]. The silver/silver chloride electrode prepared by the powder tablet method can be obtained with excellent performance, because it enhances the close contact and dispersion of Ag/AgCl particles at a certain temperature and pressure. Nevertheless, this method has complicated procedures and high cost, and it is not easy for mass production [14]. In addition, Ag/Ag electrodes can also be prepared by thermoelectrolysis and other methods [15,16], but they can not meet the requirements of flexible, low cost, good flexibility and so on. Transforming Ag and AgCl powder into a low-temperature curable paste for screen printing on a special substrate offers significant advantages [17,18]. This method boasts flexible design, compact size, simple structure, low cost, excellent repeatability, high consistency, and suitability for industrial production [19,20]. In biomedicine, this electrode offers significant research value and a vast potential market by meeting the low-cost, customizable, and flexible needs of medical and reference electrodes [21,22,23].

The Ag/AgCl reference electrode requires an internal electrolyte to achieve ideal performance. In order to achieve stable and low-cost Ag/AgCl electrode preparation and make industrial production possible, many researchers mainly focus on the improvement of internal electrolytes. The development of electrolytes such as poly(vinyl acetate) ink containing KCl and paste of sodium alginate containing KCl improved the potential stability and extended the maintenance time effectively [12,24]. However, in recent years, the preparation of low-cost and disposable Ag/AgCl reference electrodes has aroused great interest among researchers. Therefore, it is also a popular research direction to successfully prepare Ag/AgCl pseudo-reference electrode with stable performance by optimizing printing method, electrode treatment and preparation process [25,26,27]. Among them, Musa et al. developed a potentiometric system composed of an Ag/AgCl QRE and a pH-electrode (ISM deposited on a graphite electrode) that can be used continuously for a period of not less than 7 days in aqueous solutions. At the same time, an average pH sensitivity of (−60.8 ± 1.7) mV/pH per pH unit is achieved in the pH range of 7.00~7.63 [27]. Notably, Silva et al. fabricated an ink-jet printed solid-state Ag/AgCl pseudo-reference electrode on PET or chromatographic paper substrates. This preparation technique is suitable for mass-producing solid Ag/AgCl pseudo-reference electrodes in various structures and sizes. The Ag/AgCl pseudo-reference electrode demonstrated excellent reproducibility and stability within 30 min, with no significant change observed after 30 days of dark storage [25]. These approaches primarily optimize the external electrolyte, printing methods, or post-treatment processes. However, the Ag/AgCl paste itself—a core component determining the electrode’s fundamental electrical properties—has received comparatively little systematic investigation. Specifically, the effects of key paste parameters, such as the specific surface area of the Ag powder and the Ag:AgCl mass ratio, on the conductivity, interfacial structure, and overall electrochemical performance of screen-printed electrodes remain unclear. Therefore, this paper focuses on elucidating the influence of these paste material characteristics on electrode performance, bridging this knowledge gap and providing guidance for developing high-performance, printed flexible Ag/AgCl electrodes.

With the advancement of biomedical electrodes, increasingly stringent demands are placed on the potential stability and impedance characteristics of Ag/AgCl reference electrodes. As a critical component determining these properties, the formulation of the Ag/AgCl paste itself—particularly the characteristics of the Ag powder and the Ag:AgCl ratio—is paramount, yet systematic studies focusing on these fundamental paste parameters are scarce. To address this gap, this study aims to systematically investigate the influence of the specific surface area of Ag powder and the Ag:AgCl mass ratio on key performance metrics of screen-printed electrodes, including paste resistivity, electrode hydration kinetics, long-term potential stability, and electrochemical impedance. The performance of the optimized electrodes will be evaluated and compared with commercial Ag/AgCl electrodes through impedance spectroscopy. Furthermore, their practical applicability will be validated using an electrocardiogram (ECG) detection system. This work seeks to establish crucial composition-structure-property relationships, providing a theoretical foundation and practical guidance for the development of high-performance, low-cost, screen-printed flexible Ag/AgCl biomedical electrodes.

2. Experiment

2.1. Materials and Experimental Equipment

This study utilized the following chemicals: low diameter/thickness ratio flaky Ag powder L (specific surface area: 0.57 m²/g, from Sino-Platinum Metals Co., Ltd., Kunming City, Yunnan Province, China), high diameter/thickness ratio flaky Ag powder H (specific surface area: 1.24 m²/g, from Sino-Platinum Metals Co., Ltd., Kunming City, Yunnan Province, China), NaCl (≥99.5%, from Sinopharm Chemical Reagent Co., Ltd., Kunming City, Yunnan Province, China), AgNO₃ (≥99.8%, from Sinopharm Chemical Reagent Co., Ltd., Kunming City, Yunnan Province, China), and modified bisphenol A epoxy (from Sino-Platinum Metals Co., Ltd., Kunming City, Yunnan Province, China).

Three-roll grinder (MTR-50EP, WINNER, Suzhou, China) was used for Ag/AgCl paste rolling; high-resolution screen printing machine (TY-CP6090B, CHNTOP, Shenzhen, China) was used for electrode printing; cold-field emission scanning electron microscope (SU8010, HITACHI, Tokyo, Japan) was used for electrode micro-morphology observation; electrochemical workstation (600E, CH Instruments, Austin, TX, USA) was used for electrode potential and cyclic voltammetric curve testing; and impedance analyzer (E4990A, KEYSIGHT, Santa Rosa, CA, USA) was used for impedance testing, with the measurement frequency range being 20 Hz–10 kHz and the amplitude being 10 mV. The ECG detection system was used for ECG testing.

2.2. Preparation of AgCl Powder

A 0.02 mol/L AgNO₃ solution was gradually added to an excess 0.02 mol/L NaCl solution, with continuous stirring, to form an AgCl suspension. Among them, the amount of NaCl solution used was 100 mL, and it was continuously stirred for 30 min to ensure complete precipitation. Following repeated centrifugation and washing, excess chloride ions were removed until the supernatant showed no turbidity upon titration with AgNO₃ solution. The sediment was then washed three times with alcohol through centrifugation and subsequently placed in a constant-temperature drying oven. AgCl powder was prepared by drying at 70 °C for 6 h. Since AgCl is easily decomposed under visible light, the whole preparation process must be carried out in a dark room or under dark conditions.

2.3. Preparation of Ag/AgCl Pastes

Prepared AgCl powder, flaky Ag powder, and modified bisphenol A epoxy were mixed in the agate mortar according to a specific ratio. Subsequently, a three-roll mill is used for secondary mixing and rolling until the fineness is reduced to less than 10 μm, at which point the Ag/AgCl paste was ready. The mass ratio of Ag/AgCl powder to resin used in this paper was 3:2. Two types of silver powder were used: flaky Ag powder L with low diameter-thickness ratio (approximately 15:1) and flaky Ag powder H with high diameter-thickness ratio (approximately 50:1). The microstructure of different types of silver powders is shown in Figure 1. Two types of flaky Ag powder and AgCl powder were combined into 10 paste groups based on various mass ratios: (L1, H1) Ag:AgCl = 7:3, (L2, H2) Ag:AgCl = 6:4, (L3, H3) Ag:AgCl = 5:5, (L4, H4) Ag:AgCl = 4:6, and (L5, H5) Ag:AgCl = 3:7.

Figure 1. SEM images of flaky Ag powder: (a) flaky Ag powder L, (b) flaky Ag powder H.

2.4. Ag/AgCl Electrode Design

Figure 2 shows the structure diagram of the Ag/AgCl electrode. The preparation process is as follows: firstly, a self-made silver paste (composed of flaky silver powder, a thermoplastic resin vehicle, and organic solvents), which can be cured at a low temperature (150 °C), was printed on flexible PET film. This was followed by oven curing at 150 °C for 15 min to form the base silver electrode. Subsequently, the Ag/AgCl paste was printed onto the silver electrode and oven-cured at 150 °C for 15 min to solidify the Ag/AgCl electrode layer. Finally, a protective isolation resin was applied to the center of the electrode assembly.

Figure 2. Structure of Ag/AgCl electrode.

2.5. Conductivity Characterization of Ag/AgCl Paste

Ten groups of Ag/AgCl electrode pastes were printed onto PET film, with ten samples in each group. Following curing, a multifunctional four-point probe tester was used to measure the sheet resistance of the Ag/AgCl electrodes. Subsequently, the thickness of the Ag/AgCl electrodes was determined using a film thickness gauge, allowing for the calculation of their resistivity.

2.6. Electrochemical Characterization of Ag/AgCl Electrode

The self-made Ag/AgCl electrode served as the working electrode, with a commercial saturated potassium chloride Ag/AgCl reference electrode and a platinum auxiliary electrode comprising the three-electrode system. Electrochemical workstations were used to characterize the electrode potential stability, hydration period, cyclic voltammetry, and consistency across different solution environments. In this study, the self-made Ag/AgCl electrode was used as the working electrode, and its exposed active surface area in the electrolyte was 0.785 cm². This area was determined by the geometric area of the printed pattern on the electrode. The electrolyte used in all electrochemical tests was 100 mL in volume. This volume was sufficient to ensure that the ion concentration near the electrode surface remained stable during the test, preventing concentration polarization caused by the consumption of the electrolyte.

2.7. Impedance Test of Ag/AgCl Electrode

The impedance of the self-made Ag/AgCl electrode was measured with an impedance analyzer, using a hydrogel layer between two identical electrodes as the sample, and then compared to the impedance of a commercial Ag/AgCl electrode.

2.8. Ag/AgCl Electrode ECG Test

Through electrochemical and impedance tests, the sample with the best performance is selected, and ECG test was observed by ECG detection system.

3. Results and Discussion

3.1. Structural Characterization and the Effect of Different Ag Powder and Ag:AgCl Ratio on the Conductivity of Ag/AgCl Pastes

Table 1 displays the resistivity test results for ten groups of Ag/AgCl pastes. Note: The resistance of Ag/AgCl pastes with a 3:7 Ag powder L ratio exceeded the equipment’s range, and, thus, resistivity was not measured. According to the table, for Ag powder H, resistivity increases rapidly when the Ag:AgCl ratio is below 4:6. Similarly, for Ag powder L, resistivity surges when the ratio is below 5:5. Beyond a specific Ag:AgCl ratio, the increase in resistivity becomes negligible.

Table 1. Resistivity test results of silver/silver chloride pastes.

Ag:AgCl	High Diameter/Thickness Ratio Flaky Ag Powder H			Lower Diameter/Thickness Ratio Flaky Ag Powder L
	Ag/AgCl Pastes	Resistivity (Ω·m)	Standard Error	Ag/AgCl Pastes	Resistivity (Ω·m)	Standard Error
7:3	H1	1.63 × 10−7	0.027	L1	1.77 × 10−7	0.029
6:4	H2	1.87 × 10−7	0.032	L2	2.42 × 10−7	0.025
5:5	H3	2.16 × 10−7	0.035	L3	5.97 × 10−6	0.043
4:6	H4	1.15 × 10−6	0.048	L4	1.76 × 10−2	0.272
3:7	H5	3.24 × 10−3	0.327	L5	-	-

Note: The resistance of the Ag/AgCl paste with Ag powder L at a 3:7 ratio (L5) exceeded the measurement range of the equipment; thus, its resistivity could not be determined.

Figure 3 presents SEM images of the Ag/AgCl electrode’s surface morphology. Electron beam irradiation was used to remove the resin layer and expose AgCl particles, as they are wrapped underneath it in the Ag/AgCl electrode. Consequently, resin removal was incomplete in some images, allowing only partial observation of AgCl areas. Figure 3a,b show that nano-AgCl particles are dispersed on the flaky silver powder surface without forming agglomerates upon solidification of the L2 and H2 Ag/AgCl pastes. Despite the identical Ag:AgCl ratios in H2 and L2 pastes, the larger specific surface area of Ag powder H results in fewer nano-AgCl particles on its surface. These particles tend to preferentially accumulate at the edges of flaky Ag powder with high surface energy.

Figure 3. SEM images of Ag/AgCl electrode surface: (a) H2 paste, (b) L2 paste, (c,d) H3 paste, (e) L3 paste, (f) H4 paste, (g) L4 paste, (h) H5 paste.

Figure 3c,d reveal that nano-AgCl particles are uniformly distributed on flaky Ag powder H’s surface after H3 pastes solidify, with fewer particles near the interfaces where flakes contact. Consequently, this distribution minimally affects the conductive contacts between flaky silver powders. This observation aligns with the phenomenon where the Ag:AgCl ratio in H3 pastes decreases slightly, yet there is a minor increase in paste resistivity. Figure 3e,f show that, after L3 and H4 pastes solidify, nano-AgCl particles on the flaky Ag powder surface begin to cluster together. The resulting AgCl layer on the flaky Ag powder surface thickens, leading to an increase in electrode resistivity. Figure 3g,h illustrate that, after L4 and H5 pastes solidify, AgCl particles agglomerate into non-conductive clusters, disrupting flaky Ag powder contact and impeding conductive network formation, thus sharply increasing resistivity.

The observed phenomena may result from the lower Ag/Ag interface energy within silver powder mixtures compared to the AgCl/silver and AgCl/AgCl interfaces, and a lower Ag/resin interface energy compared to AgCl powder and resin. At low AgCl powder concentrations, nanoparticles preferentially adsorb on non-contacting surfaces of flaky silver powders. As AgCl powder content increases and nanoparticles fully cover flaky silver powder surfaces, further AgCl nanoparticles cannot form additional Ag-AgCl interfaces. Instead, they tend to form AgCl-AgCl interfaces, causing Ag/AgCl particles to aggregate rather than disperse uniformly in the resin. Increased AgCl agglomeration disrupts contact among flaky silver powders, destroying electrical conduction paths within the electrode and raising resistivity. The larger specific surface area of silver powder H than L allows for more Ag-AgCl interfaces, necessitating a higher proportion of AgCl to increase the Ag/AgCl electrode’s resistivity.

3.2. Electrochemical Characterization, Stability and Repeatability

Based on the research findings, pastes H1–H4 (with lower resistivity) and L1–L3 were used to prepare Ag/AgCl electrodes He1–He4 and Le1–Le3, respectively, using the method described in Section 2.4. A three-electrode system was established with the self-made Ag/AgCl electrodes as the working electrodes, commercial Ag/AgCl electrodes as reference electrodes, and platinum electrodes as auxiliary electrodes. The electrode potential stability, hydration period, and cyclic voltammetry curves of the working electrodes in various solutions were measured.

3.2.1. Open Circuit Voltage Test

The Ag/AgCl electrode’s sensitivity to chloride ion concentrations significantly impacts its performance in various environments. The open circuit voltage of the self-made Ag/AgCl electrode was evaluated using a 0.9% NaCl solution to simulate the sweat environment of the human body. Upon contact with the solution, two equilibrium reactions take place on the surface of the Ag/AgCl electrode, as described below:

$${\mathrm{AgCl}}_{\left(\mathrm{s}\right)}+{\mathrm{e}}^{-}\leftrightharpoons {\mathrm{Ag}}_{\left(\mathrm{s}\right)}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}$$

(1)

$${\mathrm{Ag}}_{\left(\mathrm{aq}\right)}^{+}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}\leftrightharpoons {\mathrm{AgCl}}_{\left(\mathrm{s}\right)}$$

(2)

The electrode potential of Ag/AgCl satisfies the Nernst equation:

$${\mathrm{E}}_{({\mathrm{Ag}}^{+}/\mathrm{Ag})}={\mathrm{E}}_{({\mathrm{Ag}}^{+}/\mathrm{Ag})}^0-{\displaystyle \frac{\mathrm{RT}}{\mathrm{nF}}}\ln \left(\right[{\mathrm{Cl}}^{-}\left]\right)$$

(3)

where E⁰ represents the standard electrode potential (V), n denotes the number of electrons involved in the electrochemical reaction, R is the gas constant (J K⁻¹ mol⁻¹), T stands for the absolute temperature (K), and F is the Faraday constant (C mol⁻¹). The concentration of free Ag⁺ ions is expressed in mol/L.

Given that the solubility product of AgCl in aqueous solution remains constant, the theoretical electrode potentials of AgCl can be calculated by input of different concentrations of chloride ions. In a 0.9% NaCl solution, the chloride ion concentration is 0.1540 mol L⁻¹, and the corresponding theoretical electrode potential of Ag/AgCl is 0.2704 V at room temperature. At room temperature, the chloride ion concentration in a saturated potassium chloride solution is 3.4030 mol L⁻¹, and the corresponding theoretical electrode potential of Ag/AgCl is 0.1908 V. Consequently, the saturated Ag/AgCl electrode serves as the reference electrode for determining Ag/AgCl, with the theoretical open-circuit voltage in a 0.9% NaCl solution estimated at 0.0796 V. Figure 4 displays the open-circuit voltage test results for Ag/AgCl electrodes He1–He4 and Le1–Le3.

Figure 4. Open-circuit voltage test of Ag/AgCl electrodes in 0.9% NaCl solution.

As can be seen from Figure 4, the open circuit voltage of the He2–He4 and Le2–Le3 groups of electrodes is very close to the theoretical voltage of 0.0796 V, with deviations under 0.0030 V. The hydration cycle of He series electrodes is shorter than that of Le series electrodes, likely due to the resin layer’s thickness on the electrode’s surface. Thinner resin layers allow for quicker equilibrium of electrode potential. Additionally, the larger specific surface area of silver powder H results in a thinner resin layer than that on electrodes using silver powder L, thereby shortening the He group’s hydration period relative to the Le group. Comparatively, within each group, electrodes with lower AgCl content exhibit shorter hydration periods than those with higher AgCl content, likely due to the different amounts of chloride ions required to achieve equilibrium. Regarding the hydration period, the three groups of Ag/AgCl electrodes, He2–He4, demonstrate satisfactory performance. In terms of long-term stability, the open-circuit voltage changes for electrodes in groups He3 and Le3 were below 0.001 V within 24 h of reaching stability. Additionally, the stable open-circuit voltage of the He1 electrode deviates significantly from the theoretical value, possibly due to the lower AgCl content in the paste potentially affecting the establishment of the initial surface equilibrium.

3.2.2. Cyclic Voltammetry Test

The redox reversibility of the self-made Ag/AgCl electrode was evaluated using cyclic voltammetry in a saturated potassium chloride solution. Electrochemical workstation parameters were configured with a high potential of 0.8 V, a low potential of −0.8 V, a scanning rate of 0.05 V s⁻¹, one scanning cycle, a sampling interval of 0.001 V, and a sensitivity of 0.01 A V⁻¹. As shown in Figure 5, the oxidation and reduction peaks for the He2 and He3 electrode groups are nearly symmetrical, indicating good redox reversibility. The He3 electrode exhibits the most symmetrical peaks and the highest peak currents, representing the best performance. This can be attributed to its optimal composition, which facilitates the formation of a thin, uniform, and continuous AgCl layer on the conductive silver network. This structure ensures efficient electron transfer and rapid ion diffusion/insertion during the redox reaction.

Figure 5. Cyclic voltammetry of Ag/AgCl electrode.

In contrast, the He4 electrode shows less symmetrical peaks and diminished current response. This exceptional behavior is likely due to the increased AgCl content approaching a critical threshold. As suggested by the SEM image in Figure 3f, AgCl particles begin to agglomerate, potentially forming a thicker and less continuous AgCl layer. This may hinder electron conduction through the layer and impede the mass transport of ions, thereby degrading the kinetics and reversibility of the surface redox reaction.

Combining the test results of open circuit voltage and cyclic voltammetry, electrodes composed of sheet silver powder H with an Ag:AgCl ratio of 5:5 demonstrate better overall performance. They feature a short hydration period, stable open-circuit voltage, and reversible redox reactions.

3.3. Impedance Test

Impedance variations for the five self-made electrode groups (He2–He4, Le2–Le3) and commercial Ag/AgCl cardiac electrodes from Shanghai Junkang Medical Equipment Co., Ltd., Shanghai City, China, across a frequency range of 20–10,000 Hz, were assessed using the method outlined in Section 2.7. Results are depicted in Figure 6. The five self-made Ag/AgCl electrodes demonstrated lower impedance values compared to the commercial electrode. This may be due to the fact that commercial electrodes are coated with thin AgCl layers on Ag, forming a double-layer structure with a single Ag-AgCl interface. Conversely, the self-made Ag/AgCl pastes ensure a uniform dispersion of AgCl particles on silver powder, yielding a greater number of Ag-AgCl interfaces, resulting in more Ag-AgCl interfaces, thus making the AgCl layer thinner, which stabilizes the electrode structure, enhances electrical conductivity, and reduces impedance. Furthermore, Figure 6 shows that the impedance of the Ag/AgCl electrode using the same silver powder increases with increasing AgCl content. This change is particularly pronounced in the frequencies between 0 and 5000 Hz. However, from 5000 to 10,000 Hz, the impedance disparity diminishes, and impedance overall declines as frequency increases.

Figure 6. Comparison of impedance values of self-made Ag/AgCl electrode and commercial Ag/AgCl cardiac electrode.

3.4. Mechanical Properties

Ag/AgCl electrodes with an L3 ratio as an example demonstrate excellent mechanical durability, facilitating their long-term use in wearable medical devices. Changes in resistance (R) relative to the original resistance (R0) were investigated following bending, polishing, and adhesive separation, as shown in Figure 7. The flexible Ag/AgCl electrode exhibited a slight increase in the R/R₀ ratio from 1.0 to 1.18 after 1000 cycles under 180° bending. Following abrasion with sandpaper and repeated tape separation, the electrode maintained stable resistance. These findings demonstrate that the flexible Ag/AgCl electrode at this ratio tolerates various mechanical processes with excellent mechanical stability, providing robust assurance for its long-term practical application.

Figure 7. Sheet resistance changes in the L3 Ag/AgCl electrodes before (R₀) and after (R) mechanical processing. (a) Bending to 180°. (b) Sanding. (c) Detaching by 3 M tape.

3.5. ECG Test Validation

Ag and PET exhibit biocompatibility, making them suitable for wearable medical devices. The ECG signal detection capabilities of self-made Ag/AgCl paste electrodes He3 was validated using an ECG detection system. Figure 8 displays ECG monitoring results using self-made electrode pastes. As shown in Figure 8, the self-made Ag/AgCl electrode successfully detected ECG signals with clear and complete periodic variations. In group He3, there was minimal noise interference, making the five standard ECG waveforms (P, Q, R, S, T) distinctly visible. The findings indicate that the self-madeHe3 Ag/AgCl electrode is sufficient for medical ECG testing.

Figure 8. ECG test diagram: (a) fast heart rate: 123 beats/min, (b) slow heart rate: 45 beats/min, (c) normal heart rate: 83 beats/min.

3.6. Comparison and Discussion of Performance with the Existing Literature

To objectively evaluate the comprehensive performance of the screen-printed Ag/AgCl flexible electrodes fabricated in this study, we quantitatively compared their key parameters with those of similar electrodes reported recently. Regarding conductivity, the electrode employing high-specific-surface-area silver powder H with a 5:5 Ag:AgCl ratio exhibited a significantly lower resistivity than most reported values based on similar slurry systems [25,26,27]. This is directly attributed to the higher AgCl particle accommodation capacity of the high-specific-surface-area silver powder, which delays the disruption of the conductive network. Regarding electrochemical stability, this electrode exhibits a rapid hydration response and excellent potential stability in simulated body fluids, with performance comparable to or even surpassing some complex structured reference electrodes designed for long-term monitoring [13,28]. More importantly, for the core performance metric in bioelectric signal acquisition—interface impedance—this electrode exhibits significantly lower impedance magnitude than typical commercial wet electrodes and several recently reported flexible dry electrodes across the critical 20–100 Hz frequency range [21]. This ensures a high signal-to-noise ratio when capturing faint electrocardiographic signals. Furthermore, after 1000 cycles of 180° bending, the electrode resistance increased by only 18%, demonstrating mechanical reliability suitable for wearable applications [28]. In summary, this study successfully optimized the conductive network and interfacial properties within the paste by revealing and regulating the often-overlooked key parameters of silver powder specific surface area and AgCl content. This approach achieves high conductivity, low impedance, excellent electrochemical stability, and mechanical flexibility simultaneously within a single, low-cost printed layer without relying on complex multilayer structures or external electrolytes. This material-level design strategy provides a clear and industrializable pathway for developing next-generation high-performance, low-cost flexible biomedical electrodes.

4. Conclusions

In conclusion, this study systematically reveals the quantitative regulation mechanism of electrode performance by the specific surface area of silver powder and the AgCl mass ratio in screen-printed Ag/AgCl slurry. Experiments demonstrate that using high-specific-surface-area flake silver powder and controlling the Ag:AgCl ratio at 5:5 yields an electrode with excellent mechanical flexibility (resistance increase of only 18% after 1000 bends), low volume resistivity (2.16 × 10⁻⁷ Ω·m), and fast, stable electrical response. the resulting electrode maintains excellent mechanical flexibility (resistance increase of only 18% after 1000 flex cycles) while achieving low volume resistivity (2.16 × 10⁻⁷ Ω·m), rapid and stable electrochemical response (potential deviation < 3 mV in 0.9% NaCl, 24 h drift < 1 mV), and significantly reduced interfacial impedance (below commercial electrodes in key bio-signal frequency bands). Scanning electron microscopy analysis confirmed that high-specific-surface-area silver powder effectively delays the agglomeration of AgCl nanoparticles, thereby maintaining the integrity of the conductive network even at higher AgCl loading. Ultimately, flexible electrodes fabricated from this optimized slurry demonstrated outstanding performance in real ECG testing, enabling clear and stable acquisition of complete electrocardiographic waveforms. By elucidating the structure–property relationship between paste composition, microstructure, electrical performance, and ultimate biosensing functionality, this work provides crucial material design principles and practical solutions for developing low-cost, high-performance printed flexible bioelectronic devices.