The amplitude of electrocardiogram (ECG) signals is only in the millivolt range, and during the conduction process from the heart to the body surface, the stratum corneum of the skin forms the largest signal attenuation barrier. The task of the electrode is to establish a stable, low-noise electron ion conversion channel on this barrier.

Ag/AgCl gel wet electrode is widely used in traditional clinical ECG monitoring. The conductive gel fills the micro gap between the electrode and the skin, reducing the interface impedance to a sufficiently low level. In static and short-term monitoring, the signal-to-noise ratio and signal fidelity of wet electrodes perform well. However, the gel electrode faces three insurmountable engineering bottlenecks in long-term wear: the gel dries up over time, resulting in a continuous rise in impedance, and the signal quality begins to deteriorate after a few hours; The ingredients of gel may cause skin allergy and contact dermatitis; Poor breathability leads to sweat accumulation, further deteriorating the interface conditions.

2、 Four major engineering propositions for dry electrodes

The flexible dry electrode does not need conductive gel, and can directly contact with the skin to collect signals. But in order to achieve a leap from the laboratory to large-scale applications, it must systematically address the following four core engineering propositions:

3、 Coating material system: performance comparison of gold, platinum, silver/silver chloride

The performance of electrocardiogram electrodes is rooted in the selection of their conductive coating materials. The three precious metals of gold (Au), platinum (Pt), and silver (Ag) each have their own strengths in conductivity, chemical stability, biocompatibility, and electrochemical activity.

Gold (Au)It has the advantages of better biocompatibility, strong chemical inertness, and easy self-assembly and functional modification, and is widely used in electrochemical sensors and immune detection electrodes. The chemical inertness of gold prevents it from corroding and releasing toxic ions in physiological environments, making it suitable for long-term skin contact scenarios. The disadvantage is that the conductivity is slightly inferior to platinum and silver, and the impedance is relatively high.

Platinum (Pt)It has the strongest charge transfer ability and excellent catalytic activity, making it the preferred material for electrical stimulation electrodes, neural regulation, and biosensors. The resistivity of platinum electrodes can be as low as 1.65 μ Ω· cm. The main challenge is the high cost and the need to optimize the adhesion between the coating and the substrate.

Silver/Silver Chloride (Ag/AgCl)It is the most classic material system in biopotential electrodes. The silver chloride layer can reduce polarization effects, stabilize signal transmission, and avoid DC drift. Silver has the best conductivity among the three and possesses natural antibacterial properties. However, silver is prone to oxidation and discoloration, and its long-term stability is poor. It is usually used in the form of an Ag/AgCl composite layer to balance performance and stability.

4、 From wet electrode to dry electrode: the diverse evolution of technological routes

The technological route of dry electrodes is rapidly differentiating, with different schemes making different trade-offs between impedance, flexibility, and wearing comfort.

Conductive fabric electrodeDeveloped using textile materials and techniques, it has the characteristics of flexibility, breathability, and washability. Research has shown that fabric electrodes prepared using embroidery techniques can capture high-quality electrocardiogram signals both dynamically and statically.

Conductive Polymer and Nanomaterial ElectrodeNew functional materials such as liquid metal, nano materials and conductive hydrogel are used to improve electrical performance and wear comfort. Some new hydrogel electrodes have shown better performance than traditional Ag/AgCl electrodes in signal-to-noise ratio.

Microneedle array electrodeThe use of microneedle structures to penetrate the stratum corneum of the skin, bypass the high impedance barrier of the stratum corneum, and collect signals in the active epidermal layer, combining the low impedance of wet electrodes with the non-invasive convenience of dry electrodes, is considered the mainstream solution for the next generation of wearable electrodes.

5、 Coating process: a key link in converting material properties into electrode reliability

The coating process is the core step in translating the theoretical advantages of conductive materials into the actual performance of electrodes. Regarding the four major engineering propositions of electrocardiogram electrodes,Advanced Institute of TechnologyConstructed a complete technical system from materials to processes.

• Breakthrough in interface integration - plasma pretreatment activation.In response to the industry problem of insufficient bonding strength between metal and polymer substrates, the plasma pretreatment process developed by Advanced Institute Technology effectively improves the bonding strength between the substrate and the metal layer by bombarding the substrate surface with plasma. In conjunction with gradient coating design, stress transition from substrate to metal layer is achieved, effectively avoiding interface delamination caused by modulus differences. The coating has a 100 grid adhesion of 5B level, a peel strength exceeding 0.8 N/mm, and can withstand hundreds of thousands of dynamic bending and multiple uses without significant drift in electrode impedance.
• Low impedance and signal fidelity.By precisely controlling the coating thickness (continuously adjustable from nanometer to micrometer level) and microstructure, the product exhibits extremely low and stable electrode skin contact impedance in the frequency range of 10Hz-10kHz. Low impedance means higher signal-to-noise ratio and more accurate signal acquisition, which can significantly reduce common mode interference and motion artifacts, especially suitable for dynamic electrocardiogram monitoring.
• Biocompatibility and sweat corrosion resistance.The conductive coating is made of metal materials that have undergone biological safety evaluation. The full vacuum dry process eliminates chemical solvent residues, and the finished product meets the ISO 10993 and GB/T 16886 series biocompatibility standards. For high humidity and sweat environments, the chemical inertness of gold endows the coating with excellent corrosion resistance, making it suitable for long-term wearing scenarios.
• Customizable coating solution.In addition to standard Ag/AgCl electrodes, Advanced Institute Technology can also provide solutions such as titanium plating, gold plating, platinum plating, as well as carbon based and conductive polymer coatings such as carbon nanotubes, graphene, PEDOT: PSS, etc. Titanium layer can be used for flexible dry electrodes, combining conductivity and biological inertness; Gold plating is suitable for high-precision EEG recording; Graphic technology can manufacture multi-channel electrode arrays, supporting customer differentiated design.

Advanced Institute Technology has independently built a magnetron sputtering and vacuum evaporation production line, using roll to roll continuous production technology, which can achieve precise deposition of metal layers on flexible polymer film substrates such as PI, PET, FEP, LCP, PEEK, etc. The thickness of the substrate can be adjusted from a few micrometers to several hundred micrometers, and can be coated with a full range of metals including gold, silver, copper, aluminum, tin, nickel, titanium, platinum, etc.

6、 Selection considerations: from application scenarios to coating solutions

When engineers choose ECG monitoring electrodes, the following dimensions deserve special attention:

• The application scenario determines the material path.The Ag/AgCl system can be chosen for short-term clinical diagnosis, and its low polarization potential and mature manufacturing process ensure reliable signal quality. Long term wearable monitoring should prioritize gold or platinum plating solutions to achieve more stable long-term impedance and better biocompatibility. Electrodes that require electrical stimulation function (such as neuromuscular electrical stimulation) should choose platinum or platinum/conductive polymer composite solutions.
• Contact impedance is the first threshold for signal quality.When selecting, attention should be paid to the impedance characteristics of the electrode in the target frequency band (where the effective electrocardiogram signal is concentrated between 0.05-100Hz), rather than just focusing on the nominal value at a single frequency. Low impedance means higher signal-to-noise ratio and more accurate feature recognition.
• The adhesion and deformation stability of the coating determine long-term reliability.5B level adhesion (ASTM D3359 highest grade) is the process threshold to ensure that the electrode does not detach during repeated bending, application, and sweat infiltration. Traditional coating or printing conductive layers are prone to detachment during repeated deformation, severely limiting the service life of electrodes.
• The processing accuracy meets the requirements of multi-channel array.High density electrocardiogram mapping arrays require micrometer level patterning accuracy. Suppliers that support roll to roll continuous production and precision coating processes should be selected to ensure batch consistency and traceability.

7、 Conclusion

The essence of electrocardiogram monitoring electrodes is to establish a stable, low-noise, and biologically friendly signal channel between a rigid electronic system and soft, dynamic, and sweaty skin. It matches the mechanical properties of the skin with a flexible substrate, ensures signal quality with a gold/platinum/Ag/AgCl conductive layer, and converts the chemical inertness of the material into reliable long-term performance through plasma pretreatment and gradient coating processes - all three work together to answer a core engineering question: how to make electrodes as soft as skin while also understanding the language of the heart.

The paradigm shift from wet electrodes to dry electrodes reflects the upgrading of electrocardiogram monitoring from "clinical short-term detection" to "daily long-term monitoring". Understanding the mutual constraints between the four major engineering propositions, the performance boundaries of different coating material systems, and the decisive role of coating processes in the long-term reliability of electrodes - these are the key to upgrading electrocardiogram monitoring electrodes from "a sensor device" to "engineering decision-making".