Continuous electrocardiogram monitoring devices often perform well on the first day of wear - with clear signals and stable baselines. But on the third day, the waveform began to drift; One week later, the signal quality was no longer suitable for clinical interpretation.

The electrodes are not 'broken', and the skin is not 'changed' either. The problem lies at the microscopic interface between the electrode and the skin.

The human skin is constantly undergoing evaporation and exchange of moisture. The transdermal water loss rate in adults is usually in the range of 300-500 g/(m ² · day), and is higher when sweating. If the electrode is an airtight dense film, water vapor cannot escape and sweat accumulates below the electrode. The stratum corneum of the skin gradually softens under continuous hydration, the interface impedance increases, and the signal quality continues to deteriorate. What's even more tricky is that the salt and metabolites in sweat can also cause skin irritation and even contact dermatitis.

2、 Two directions of water management: engineering balance between breathability and water retention

The ideal long-term wearing of electrodes requires meeting two seemingly contradictory requirements: breathability to the outside, allowing the normal evaporation of moisture from the skin to dissipate; For internal water retention, the conductive medium (water in the hydrogel or ionic liquid) of the electrode itself cannot be lost quickly.

The "water retention" ability of the traditional hydrogel electrode depends on the sealed packaging, but the packaging sacrifices the permeability. The breathable conductive hydrogel network (BCHN) proposed by the team of Suzhou Nano Institute in 2026 provides a different idea. The material embeds PVA hydrogel containing NaCl into the three-dimensional porous PLA fiber skeleton, and forms a continuous ion path while retaining open pores. Its water vapor transmission rate reaches 1.85 kg · m ⁻² · day ⁻¹, far higher than the natural evaporation demand of the skin; At the same time, the water loss was only about 6% after 14 days.

This means that the BCHN electrode maintains interfacial hydration balance through a dynamic mechanism of "sweat capture water permeation evaporation release" - the water evaporated from the skin escapes through the electrode, and the electrode's own water is anchored by the fiber skeleton and hydrogen bond network, without rapid loss. Even in a dry environment with a relative humidity of 20%, the electrode can still maintain a low impedance of approximately 55 Ω.

3、 Permeability map of material system: from hydrogel to nanofiber

There are significant differences in breathability, water retention, and electrical properties among different material systems. Understanding these differences is a prerequisite for selecting long-term wearing applications.

hydrogel networkThe advantage lies in ion conductivity and skin affinity. The 1 kHz impedance of the BCHN electrode is as low as 19.93 Ω, with an ion conductivity of 1.43 S · m ⁻¹ and a bending stiffness of approximately 10 ⁻¹⁰ N · m ². Its charge injection capacity reaches 0.535 mC · cm ⁻ ² and remains stable after 1000 cycles. The air permeability comes from the open porous structure of PLA fiber skeleton, rather than the water permeability of hydrogel itself.

nanofiber membraneThe fiber network prepared by electrospinning achieves breathability. The PVA/WBPU blend nano mesh electrode only increased its resistance by 1.02 times under 24-hour continuous water flow, and maintained conductivity under 80% strain and 1000 tensile cycles. In palm applications, the electrode maintains a stable resistance (<50 Ω) for at least 4 hours, while pure PVA electrodes frequently experience resistance increases above 1 k Ω or open circuits. Breathability comes from the pores between fibers and does not rely on water channels.

Liquid metal filmAchieve conformal bonding with extreme thinness. The total thickness of the epidermal electrode prepared with a 200 nm thick liquid metal conductive film is only 1.1 µ m, and the interface impedance at 1 kHz is as low as 8.5 k Ω. It can collect medical grade electromyographic signals under dynamic conditions. But its breathability is limited by the continuous structure of the film, making it more suitable for short-term dynamic monitoring rather than continuous wearing for several days.

4、 How to convert breathability into signal stability

The impact of breathability on signal quality can be understood through a specific comparison. After wearing the dense thin film electrode, the water evaporated from the skin is trapped under the electrode, and the hydration level of the stratum corneum continues to increase. The ion mobility of the hydrated stratum corneum increases, and the interface impedance first decreases and then increases - in the initial stage, the impedance decreases due to improved contact caused by hydration, and then rebounds due to sweat salt accumulation and skin softening. The fluctuation of impedance is directly transformed into the drift of signal baseline and the increase of noise.

Breathable electrodes maintain a dynamically balanced moisture environment. The BCHN electrode maintains a relatively stable interface moisture content through a mechanism of "sweat capture water permeation evaporation release". The direct engineering result of this dynamic balance is that the signal-to-noise ratio remains above 25 dB after continuous use for 30 days, and it can be used for cognitive state recognition and quantitative assessment of driving fatigue.

5、 Selection considerations: from wearing time to environmental humidity

When engineers choose flexible wearable electrodes, the following dimensions deserve special attention:

• The duration of wearing determines the material path.Dynamic monitoring within a few hours (such as electromyography and short-term electrocardiogram screening) can be performed using both liquid metal film and nanofiber film, with a focus on motion artifact suppression and dynamic impedance stability. Continuous monitoring for days to weeks (such as chronic ECG monitoring, sleep EEG), hydrogel network and breathable nanofiber membrane are more appropriate options, and the core focus is water management capability and long-term impedance drift.
• Environmental humidity is a variable that is easily overlooked.In dry environments, water retention capacity becomes the dominant factor; In high humidity or sweaty environments, water resistance and sweat compatibility become the dominant factors. The BCHN electrode maintains a 55 Ω impedance at 20% relative humidity, indicating that its water retention design is effective in dry environments; The stability of PVA/WBPU nanomesh electrodes under continuous water flow indicates that their water-resistant design is effective in high humidity environments.
• Skin impedance monitoring requires different design logics.Unlike ECG/EMG signal acquisition, skin impedance monitoring itself relies on the moisture state of the electrode skin interface. Breathable nano mesh electrodes have unique advantages in this application - their open structure allows for normal evaporation of skin moisture, while the electrode's own resistance changes can be decoupled, accurately reflecting changes in skin barrier function.

6、 Conclusion

The core obstacle to the transition of flexible wearable electrodes from "being able to stick on" to "being able to stick on indefinitely" is not the conductive material itself, but the invisible moisture interface between the electrode and the skin. Breathability is not an additional function, but a fundamental engineering constraint that determines long-term signal stability - it directly affects the hydration state of the stratum corneum, the drift rate of interface impedance, and the biological response of the skin to the electrode.

Hydrogel network, nanofiber film and liquid metal film represent three different paths of breathability technology, and each makes different trade-offs between impedance, mechanical properties and long-term stability. Understanding the physical mechanism of moisture balance at the skin interface, as well as the breathability and water retention strategies of different material systems, are key to upgrading flexible wearable electrodes from "short-term demonstration products" to "long-term monitoring tools".