Traditional Ag/AgCl wet electrodes exhibit excellent performance in clinical static monitoring, with low contact impedance and reliable signals. However, when it is used in wearable devices, especially in scenes requiring long-term wear, problems begin to emerge: the conductive gel will dry up in a few hours, and the interface impedance will rise; The skin continues to deform during daily activities (with a stretching rate of over 30%), and rigid electrodes cannot follow, resulting in motion artifacts; Long term attachment can lead to skin impermeability, sweat accumulation, irritation, and even dermatitis.

The root of the problem lies in a fundamental fact that has been long overlooked: the skin is alive - it moves, sweats, and breathes. To enable the electrode to coexist peacefully with the skin, it cannot just be 'attached', but must 'grow' - mechanically matching the softness and elasticity of the skin, allowing the skin to breathe freely in heat and moisture management, and establishing a stable, low-noise signal channel at the interface.

2、 Two types of stretchable paths: "structural stretchability" and "intrinsic stretchability of materials"

There are two different technological paths to achieve electrode stretchability, and understanding the differences between the two is the key to correctly understanding the technological evolution in this field.

• Structure is stretchableIt is the earliest explored solution - designing geometric structures such as serpentines, island bridges, wrinkles, etc. on elastic substrates, absorbing strain through structural deformation, and protecting the conductive layer from damage. The Kirigami (Paper Cuttings) structure is a representative development of this path - by cutting specific patterns on the film, the original non stretchable material can be stretched. The PEDOT: PSS/PVA/AgNWs/PU epidermal electrode based on Kirigami structure achieved a skin conformal stretching rate of>100%, with a low square resistance of about 3.9 Ω/□ and an interface impedance of about 27.41k Ω at 100Hz.
• The material is inherently stretchableThis is the breakthrough direction in recent years - to make conductive materials themselves stretchable, rather than relying on structural design to "absorb" strain. Liquid metal, intrinsically stretchable conductive polymer, ionic conductive hydrogel and other material systems can make the electrode bear deformation at the molecular level. The advantage of this path is that it does not require complex structural design, the material itself can conform to the skin in a conformal manner, and the resistance change is smaller under large deformation.

3、 The Four Horsehorses of Material Systems: Ionic Conductivity, Liquid Metals, Conductive Polymers, and Nanocomposites

4、 Three major engineering challenges: impedance, artifacts, and breathability

The stretchable flexible electrode on the skin surface is moving from the laboratory to practical applications, facing three core engineering challenges:

• Interface impedance:The electrode skin interface impedance is the primary parameter that determines signal quality. The interface impedance of the advanced epidermal dry electrode can be as low as about 27k Ω at 100Hz, which is equivalent to or even better than the commercial Ag/AgCl gel electrode. The interface impedance of a liquid metal skin electrode with a thickness of only about 200nm is as low as 8.5k Ω at 1kHz.
• Motion artifact:This is the biggest challenge that distinguishes dry wearable electrodes from clinical static monitoring. The self-adhesive conductive polymer film adheres to both the skin and metal electrode through strong hydrogen bonding and coordination bonding, fundamentally reducing the artifacts introduced by cable movement. The fluid properties of liquid metal enable it to maintain a continuous conductive network during skin deformation, further suppressing motion artifacts.
• Breathability and long-term wear:The skin needs to breathe. The porous structure of the ion nanomembrane ensures high moisture and air permeability. The biomimetic flexible epidermal electronic device showed excellent breathability and stability in tests of continuous sweating for 5 hours and continuous wearing for 7 days. The signal-to-noise ratio of some epidermal dry electrodes still reached 35.2 ± 1.7dB during continuous 4-day electrocardiogram monitoring.

5、 From laboratory to clinical: performance is surpassing the 'gold standard'

In the past five years, the performance of skin surface stretchable flexible electrode has evolved from "laboratory curiosities" to "high-performance biological interface" - in some dimensions, it has begun to rival or even surpass the clinical Ag/AgCl gel electrode. The signal to noise ratio of the interface electrode of soft skin can reach up to 38dB, and the interface impedance is one order of magnitude lower than the gel standard, which is still stable even in the state of exercise or sweating.

At the application level, stretchable electrodes on the skin have covered multimodal signal acquisition from electrocardiogram (ECG), electromyography (EMG) to electroencephalography (EEG). The electrode based on stretchable dry adhesive has been integrated into wearable devices for overnight sleep staging analysis, and its results are comparable to traditional clinical multi-channel sleep monitoring. The signal to noise ratio (SNR) of ECG based on hydrogel electrode was 35.2 ± 1.7dB for four consecutive days.

6、 Advanced Institute Technology's Flexible Electrode Solution

Advanced Institute (Shenzhen) Technology Co., Ltd. (referred to as "Advanced Institute Technology") was established in 2016 and is a national high-tech enterprise specializing in flexible substrate coating, shielding materials, absorbing materials, and precious metal pastes. The company 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 various flexible films and elastic surfaces such as PI, PET, FEP, LCP, PEEK, PDMS, etc.

In the field of stretchable flexible electrodes for skin surfaces, Advanced Institute Technology provides the following core capabilities:

• Substrate selection:Various flexible and stretchable substrates such as PI, PET, PDMS, TPU, etc., support different stretching rates and mechanical performance requirements.
• Coating scheme:High purity precious metal coatings such as gold, platinum, silver, as well as Ag/AgCl biopotential electrode coatings.
• Process platform:Composite processes such as magnetron sputtering, vacuum evaporation, and pulse electroplating.
• Core Technology:Plasma pretreatment activation, gradient coating design (achieving stress transition from rigid substrate to metal layer), cyanide free gold plating.
• Quality assurance:Coating adhesion level 5B (ASTM D3359 highest level), ISO9001 certification, and compliance with RoHS environmental requirements.
• Customized services:Support electrode patterning customization, and adjust substrate thickness and width as needed.

7、 Conclusion

The technological evolution of stretchable flexible electrodes on the skin is essentially a paradigm shift from "making devices stretchable" to "making devices like skin". The early implementation of "stretchable structure" through snake like and Kirigami structural designs solved the problem of whether it could bend or not; Nowadays, breakthroughs in material systems such as ion conduction, liquid metal, and intrinsic stretchable conductive polymers are solving the problem of whether they can breathe, adhere, and feel like skin.

From 3.55 μ m skin like ion electrodes to 1.1 μ m liquid metal "tattoos", from 4m Ω· cm MXene composite films to 110S/cm self-adhesive conductive polymers - the core driving force of this technological revolution has always been clear: allowing electronic devices to infinitely approach the mechanical, thermal, and electrical properties of the skin. Understanding the evolution logic from "structurally stretchable" to "material intrinsic stretchable", and the trade-offs between impedance, artifact, and breathability in different material systems - these are the key to upgrading skin stretchable flexible electrodes from "cutting-edge materials" to "engineering choices".