In the laboratory, as long as an electrode detects a signal, it proves that it is "usable". But in clinical practice, cardiologists need electrocardiographic electrodes that can continuously monitor for several days without drifting, neurosurgeons need neural interface electrodes that can work stably in cerebrospinal fluid for several years, and endocrinologists need sensors that can be implanted subcutaneously to continuously monitor blood sugar.

These scenarios pose a core constraint that the laboratory does not care about for the electrodes:Durability.

The amplitude of electrocardiogram signals is only in the millivolt range, while EEG signals are in the microvolt range. In the operating frequency range of 10Hz to 10kHz, the electrode needs to exhibit extremely low and stable impedance characteristics. Once the electrode surface corrodes, the coating falls off, or the biofilm covers in the body fluid, the interface impedance will rapidly rise, and weak signals will be drowned out by noise, causing the monitoring data to lose clinical value. At the same time, electrode materials must not release toxic ions or trigger immune rejection reactions, and must comply with ISO 10993 and GB/T 16886 series biocompatibility standards.

2、 Performance differences and selection logic of three precious metal coatings

The conductive functional layer of biomedical electrodes is mainly composed of three precious metal systems: gold, platinum, and silver/silver chloride. The three have their own focuses on conductivity, chemical stability, electrocatalytic activity, and biocompatibility.

Gold (Au)The electrical resistivity is about 2.44 μ Ω· cm, and the chemical stability is extremely high. It does not corrode or release toxic ions in physiological environments. The surface of gold is easily modified by thiolation to immobilize antibodies or aptamers, making it suitable for integrated design of "electrode as sensor". In scenarios that require long-term implantation, the chemical inertness of gold makes it the most reliable interface material.Advanced Institute of TechnologyThe tested purity of the PEEK gold plating film is ≥ 99.99%, the thickness range is adjustable from 0.1 to 5 μ m, and the uniformity is ± 0.05 μ m.

Platinum (Pt)The resistivity can be as low as 1.65 μ Ω· cm, with the strongest charge transfer ability and excellent catalytic activity. Platinum exhibits outstanding performance in nerve stimulation and electrochemical sensing, especially suitable for electrodes that require electrical stimulation function. Platinum iridium alloy is commonly used to enhance the mechanical strength and corrosion resistance of electrodes, in order to produce long-life electrodes.

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. But silver is prone to oxidation and discoloration, and its long-term stability is poor. Related studies have shown that the performance of Ag/AgCl electrodes slowly deteriorates within about 200 days after manufacturing, after which the degradation rate and critical failure probability increase. Therefore, the Ag/AgCl system is more suitable for short-term clinical diagnosis and disposable use scenarios.

3、 Substrate selection: The "skeleton" dispute between PEEK, LCP, and PI

The substrate determines the mechanical properties, biocompatibility, and implantation feasibility of the electrode. The selection of substrate for biomedical electrodes is essentially a trade-off between mechanical adaptation and long-term chemical stability.

Polyether ether ketone (PEEK)Has excellent biocompatibility, chemical stability, and mechanical properties. The microstructure of PEEK film can achieve directional and ordered arrangement in topological structure, which has unique value in the design of composite neural electrodes. PEEK has low surface energy and high crystallinity, and its bonding strength with metals is the core challenge of the coating process. Advanced Institute Technology adopts a composite process of low-temperature pulse magnetron sputtering ion beam assisted deposition to achieve a 5B level coating adhesion test on PEEK surface, with no blistering or peeling after 48 hours of salt spray. After 100 thermal shock cycles from -40 ℃ to 180 ℃, the adhesion still maintains a 5B level and the resistance change rate of the gold layer is less than 2%.

Liquid Crystal Polymer (LCP)The water absorption rate is less than 0.04%, and the dielectric and insulation properties will not deteriorate due to water infiltration after long-term immersion in cerebrospinal fluid environment. The thin film electrode array based on LCP can achieve higher electrode density and channel number, with stiffness only 1/11 of traditional clinical electrodes. The LCP based 25 channel neural electrode array is prepared by electroplating gold or iridium oxide, and is superior to PI electrodes in terms of bending force and flexibility.

Polyimide (PI)It is one of the most widely used substrates in flexible electrodes. The PI gold-plated film substrate of Advanced Institute Technology can be as thin as 5 μ m, suitable for cortical EEG recording electrodes with extremely high flexibility requirements.

4、 Coating process: Transforming the chemical inertness of precious metals into reliable long-term performance

Transforming the theoretical advantages of precious metal coatings into reliable electrode products relies on precise control of core processes. Advanced Institute Technology has achieved triple process breakthroughs in the engineering proposition of biomedical electrodes.

• 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 hundred grid adhesion of 5B level and a delamination strength exceeding 0.8 N/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. Low porosity is crucial for long-term implantation: the micropores in the insulation layer and coating are channels for water vapor and ion penetration, which can lead to signal attenuation and electrical coupling.
• 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、 From Materials to Devices: Key Selection Points for Three Major Application Scenarios

6、 Selection considerations: Engineering decisions from implantation duration to environmental conditions

• The duration of implantation determines the material pathway.Short term diagnosis (several hours to several days) can choose Ag/AgCl system; Long term wearable monitoring (weeks to months) should choose a gold plating solution; Long term implantation (several years) requires the selection of LCP or PEEK substrate with gold plating, and attention should be paid to accelerated life data.
• The body fluid environment is a corrosive touchstone.The chemical composition of cerebrospinal fluid, sweat, and blood varies significantly, and the requirements for corrosion resistance of the coating are also different. The chemical inertness of gold is most prominent among the three precious metals, making it suitable for long-term service in bodily fluid environments.
• The adhesion and porosity of the coating determine long-term reliability.5B level adhesion (ASTM D3359 highest grade) is the process threshold to ensure that the coating of the electrode does not peel off during repeated bending, implantation, and long-term service. The adhesion of PEEK gold-plated film remains at 5B level after 180 ℃ thermal shock cycling, and the resistance change rate of the gold layer is less than 2%, which is a key reference data for evaluating long-term reliability.
• The processing accuracy meets the requirements of high-density arrays.High channel count neural probes require micrometer level patterning accuracy. Advanced Institute Technology supports a full chain process from surface activation, vacuum coating, to metal coating, and can customize the coating system and microstructure of electrode materials according to different application needs.

7、 Conclusion

The essence of biomedical electrodes is to establish a stable, low-noise, and biologically friendly signal channel between rigid electronic systems and soft, moist, and immunologically active biological tissues. It uses flexible substrates (PEEK, LCP, PI) to match the mechanical properties of the tissue, precious metal coatings (gold, platinum, Ag/AgCl) to ensure signal quality, and plasma pretreatment and pulse electroplating processes to convert the chemical inertness of precious metals into reliable long-term performance - the three work together to answer a core engineering question: how to make the electrode "stay" in the body for several years while still understanding the body's signals.

From clinical electrocardiogram diagnosis to brain computer interface, from continuous blood glucose monitoring to neural regulation - biomedical electrodes are pushing medical testing from "blood tests" to "continuous monitoring". In this technological leap from "in vitro" to "in vivo", the coating process has always been the core variable determining the long-term performance of electrodes.