During the process of contraction and relaxation, human muscles produce weak electrophysiological signals - electromyographic signals. Surface electromyography (sEMG) plays an increasingly important role in rehabilitation medicine, sports science, human-computer interaction, and neuroengineering by capturing these signals through electrodes attached to the skin surface.

However, sEMG signal acquisition faces two natural challenges. Firstly, the amplitude of surface electromyographic signals is only at the microvolt level (μ V), which is easily overwhelmed by environmental noise and motion artifacts. Secondly, human skin is a soft, stretchable, non planar biological tissue, and traditional rigid electrodes are difficult to achieve mechanical matching with it. As the first gateway for signal acquisition, the performance of electrodes directly determines the upper limit of the quality of subsequent signal processing.

At present, the silver/silver chloride (Ag/AgCl) wet electrode widely used in clinical practice is regarded as the "gold standard" of biological signal acquisition, but there are three major congenital defects: the gel dries up over time, resulting in the rise of contact impedance and continuous attenuation of signal quality; The ingredients of gel may cause skin allergy, and poor air permeability may cause skin redness and swelling; Skin preparation is time-consuming and the application process is complex, making it unsuitable for daily wearable scenarios. Under these limitations, flexible dry electrodes have become a recognized direction for technological evolution in the industry.

2、 Four major engineering propositions: the technical threshold of electromyographic flexible electrodes from laboratory to application

The flexible dry electrode does not need conductive gel and adhesive, and can directly contact with the skin to collect signals. It has the core advantages of non irritation, long-term wear, and reuse. But to achieve a leap from laboratory to large-scale application, it is necessary to systematically address the following four core engineering propositions.

3、 Coating material system: diverse choices from precious metals to carbon based materials

The performance of electromyographic flexible electrodes is rooted in their conductive material system. Different materials have their own trade-offs in terms of conductivity, chemical stability, biocompatibility, and cost.

Gold (Au)Due to its excellent conductivity, chemical stability, and biocompatibility, it has become the preferred material for flexible sEMG electrode coating. The chemical inertness of gold prevents it from corroding and releasing toxic ions in physiological environments, making it suitable for long-term skin contact.Silver/Silver Chloride (Ag/AgCl)It is the most common material system in biopotential electrodes, with low polarization potential and excellent electrochemical stability, usually achieved by electrochemically depositing silver chloride on a silver electrode. In recent years, stretchable 3D printed Ag/AgCl dry electrode ink has provided a new approach for flexible electrode manufacturing.

carbon-based materialsCarbon nanotubes and graphene have attracted much attention due to their excellent conductivity, mechanical flexibility, and lightweight properties. The ultra-thin conductive biological electrode developed by Tokyo University of Technology forms a fiber network structure by coating a single-layer carbon nanotube fibrous conductive material on the elastomer film, and its signal-to-noise ratio is equivalent to the commercial gel electrode. The average impedance of the multi walled carbon nanotube/polydimethylsiloxane (MWCNT-PDMS) composite dry electrode at 10Hz is 490.75 ± 71.57k Ω. When integrated into a wearable armband, the signal-to-noise ratio can reach 34.23 ± 1.74dB (40-160Hz), which is better than the control results of Ag/AgCl wet electrode.

conductive polymerPEDOT: PSS is currently one of the most extensively researched materials. PEDOT: PSS coating can reduce the contact impedance of textile electrodes by over 85% while suppressing metal corrosion. A dry sEMG patch based on graphene silver hybrid electrode was prepared by screen printing, with a signal-to-noise ratio of 11.5dB, comparable to Ag/AgCl electrode (11.6dB), and a 4 × 2 circular electrode grid achieved spatially resolved electromyographic recording.

4、 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 electromyographic flexible 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. Cooperating with gradient coating design to achieve stress transition from substrate to metal layer, effectively avoiding interface peeling caused by modulus differences. The coating has a hundred grid adhesion of 5B level and a peeling strength exceeding 0.8N/mm.
• Breakthrough in Coating Quality - Pulse Electroplating.Introducing pulse electroplating technology, by precisely controlling the switching time and frequency of the current, the porosity of the coating is effectively reduced, and the density and uniformity of the coating are improved.
• Breakthrough in Environmental Protection and Safety - Cyanide free Gold Plating.The use of cyanide free gold plating technology in the gold plating process has significant advantages in reducing environmental pollution and improving operational safety.

5、 Key performance and selection considerations

Comprehensive industry public information andAdvanced Institute of TechnologyThe key selection parameters for electromyographic flexible electrodes based on technical data are as follows:

Engineers should focus on the following dimensions when selecting electromyographic flexible electrodes:

• The application scenario determines the material path.The Ag/AgCl system can be chosen for short-term clinical diagnosis; Long term wearable monitoring should prioritize the use of gold-plated or carbon based material systems to achieve more stable long-term impedance and better biocompatibility.
• 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 (sEMG effective signal concentrated in 20-500Hz), rather than just focusing on the nominal value at a single frequency.
• 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.
• The processing accuracy meets the requirements of high-density arrays.High channel sEMG arrays require micrometer level patterning accuracy, and suppliers that support roll to roll continuous production and precision coating processes should be selected.

6. Conclusion

The essence of electromyographic flexible electrodes is to establish a stable, low-noise, and biologically friendly signal channel between rigid electronic devices and soft, dynamic skin. It uses flexible substrates to match the mechanical properties of the skin, gold/silver/carbon based conductive layers to ensure signal quality, and plasma pretreatment and pulse electroplating processes to convert the chemical inertness of the material into reliable long-term performance - the four work together to answer a core engineering question: how to make the electrode as soft as skin while still understanding the language of muscles.

The paradigm shift from wet electrodes to flexible dry electrodes reflects the upgrading of sEMG technology 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 electromyographic flexible electrodes from a "sensor device" to an "engineering decision".