Electrospinning Machine | Temporary Tattoo-Inspired, Skin-Adaptable Epidermal Electrode from an Ultrathin PU−PVA Film

In the fields of wearable health monitoring and neural rehabilitation, high-fidelity, long-term monitoring of physiological electrical signals such as electromyography (EMG), electroencephalography (EEG), and electrocardiography (ECG) is crucial. However, existing epidermal electrodes face significant challenges in balancing wearing comfort, signal stability, and the scalability of the fabrication process. Although traditional dry metal electrodes offer excellent conductivity, they are prone to detachment in sweaty environments; while wet hydrogel electrodes have good skin conformity, their millimeter-level thickness results in poor breathability and a tendency to dehydrate and dry out, making it difficult to support long-term continuous monitoring. Furthermore, many electrode design schemes based on advanced materials (such as gold and graphene) often rely on expensive raw materials and complex preparation processes, severely limiting their large-scale application and promotion. Therefore, developing an electrode system that combines an ultra-thin structure, high breathability, strong interfacial adhesion, low contact impedance, and can be prepared via a simple, scalable process has become a core research goal for current epidermal electronic devices.

Recently, a research team led by Associate Professor Yan Wang from Guangdong Technion - Israel Institute of Technology published their latest findings, titled "Temporary Tattoo-Inspired, Skin-Adaptable Epidermal Electrode from an Ultrathin PU−PVA Film," in the journal ACS Sensors. The researchers developed an ultra-thin, breathable composite electrode via electrospinning and dip-coating processes. This electrode consists of a polyvinyl alcohol (PVA) film merely 5.2 μm thick combined with an electrospun polyurethane (PU) nanomesh reinforcement layer. It exhibits excellent mechanical extensibility (withstanding 1000 cycles of 100% stretching), high breathability (air permeability 0.94 cm³ cm⁻² s⁻¹ cmHg⁻¹), and moisture permeability (water vapor transmission rate 1856.5 ± 36.9 g m⁻² day⁻¹).

Its core innovation lies in a hydrogen bond adhesion mechanism activated by a NaCl/glycerol/water solution, enabling the formation of an ultra-thin, highly conformal interface with the skin without the need for external adhesives. This design endows the electrode with extremely low skin-electrode contact impedance (21.0 kΩ at 100 Hz) and mechanical stability during intense exercise and long-term use, truly achieving "unperceivable wearing" and "invisibility" for electrophysiological signal monitoring devices.

Figure 1. The 5.2 μm thick, skin-adaptable PU–PVA tattoo electrode. (A) Schematic diagram of the design principle, comparing the structural concepts of the PU–PVA tattoo electrode and commercial temporary tattoo stickers. Scale bar: 5 mm. (B) Cross-sectional scanning electron microscope (SEM) image of the PU–PVA tattoo electrode, showing its ultra-thin structure. Scale bar: 10 μm. (C) Optical micrograph of the dry-state PU–PVA electrode, revealing its internal structure. Scale bar: 10 μm. (D) Photo of the PU–PVA tattoo electrode supporting 16 g of suspended liquid metal (EGaIn), demonstrating its excellent mechanical strength and stability. Scale bar: 5 mm. (E) Comparison of infrared and optical images of the PU–PVA electrode and commercial gel attached to human skin before exercise and after 40 minutes of exercise. Scale bar: 10 mm. (F) Application of the PU–PVA tattoo electrode in wireless electromyography (EMG) monitoring, used to quantify muscle activation patterns during different push-up forms and rock climbing fingertip pulls. 

Figure 2. Optimization and performance characterization of the PU−PVA tattoo electrode. (A) Comparison of the thickness of PU−PVA films prepared under different PU nanomesh density conditions. (B) Tensile stress-strain curves of hydrated PVA film and PU−PVA composite films under different PU nanomesh densities. (C) Stretch-release cycle performance test of the PU0.3–PVA electrode under 100% strain conditions. (D) Schematic diagram of the adhesion separation experiment, showing the detachment process of the PU−PVA electrode from artificial skin. (E) Force-displacement curves comparing the adhesion performance on artificial skin of pure PU membrane, and PU−PVA electrodes treated with and without glycerol in the hydration solution. The inset shows electrode rupture during testing without glycerol treatment. Scale bar: 5 mm. (F) Change in adhesion energy per unit area of the PU−PVA electrode on artificial skin after 0, 2, and 24 hours of adhesion. (G) Comparison of gas permeability of the PU−PVA electrode and contrast materials (such as parylene film). Error bars represent standard deviation (n = 3). (H) Comparison of water vapor transmission rate (WVTR) after sealing bottles using different encapsulation materials (1000 μm thick PDMS membrane, approximately 5 μm thick PU−PVA electrode membrane, and no cover condition). Error bars represent standard deviation (n = 3). (I) Comparison of the contact impedance on skin between the PU−PVA electrode and commercial gel electrode, and the impedance change of the PU−PVA electrode after 24 hours of continuous wear. (J) Box plot analysis of the tensile performance, adhesion performance, and electrical performance of newly prepared PU−PVA electrodes and those stored at room temperature for 60 days. 

Figure 3. High-fidelity electrophysiological signal monitoring and motion artifact suppression performance based on the PU−PVA tattoo electrode. (A) Electromyography (EMG) signals recorded using commercial gel electrodes under different grip strength conditions (50, 100, and 200 N). (B) EMG signals recorded using the PU−PVA electrode under the same grip strength conditions. (C) Comparative analysis of the signal-to-noise ratio (SNR) of EMG signals recorded by the PU−PVA electrode and commercial gel electrodes under different grip strength conditions. (D) Schematic diagram of the experimental setup for electroencephalography (EEG) signal measurement with eyes open and closed. (E) EEG signals recorded using the PU−PVA electrode in the eyes-open and eyes-closed states (top), and their corresponding spectrograms showing enhanced alpha rhythm in the eyes-closed state (bottom). (F) Power spectral density (PSD) analysis of wireless EEG signals in the eyes-open and eyes-closed states. (G) Experimental setup for simulating motion artifacts and baseline EMG signal acquisition scheme, including three conditions: no vibration (top), skin vibration (middle), wire vibration (bottom). Scale bar: 2 cm. (H) Quantitative analysis of the amplitude of EMG baseline noise under the three test conditions using scaled median absolute deviation (scaled MAD). (I) Continuous 24-hour wireless electrocardiogram (ECG) daily monitoring using the PU−PVA electrode, showing raw ECG signals (top), corresponding heart rate (HR, middle), and heart rate variability (RMSSD, bottom); locally magnified plots show typical ECG waveform characteristics during daily activities including computer work, sleep, and walking.

Figure 4. Dynamic analysis of muscle recruitment capability during push-ups and rock climbing movements based on the PU−PVA tattoo electrode. (A) Schematic diagram of electrode attachment on target muscle groups during the push-up experiment. (B) Schematic diagram of electrode attachment on target muscle groups during the rock climbing fingertip pull experiment. (C) Raw EMG signals and their rectified linear envelope curves (top) for the pectoralis major (PM) and triceps brachii (TB) in three push-up variations; normalized EMG activation (bottom) for PM and TB in each posture. The insets show three hand support methods: wide, shoulder-width, and narrow. (D) Normalized EMG signals for the brachioradialis (BR), first dorsal interosseous (IN), flexor digitorum superficialis (FD), and biceps brachii (BB) during a complete rock climbing fingertip pull process. The inset shows typical body postures at various stages of the movement. (E) Average normalized EMG activation (left) and relative muscle contribution (right) of PM and TB in three push-up variations, error bars represent standard deviation (n = 3). (F) Average normalized EMG activation (left) and relative contribution (right) of BR, IN, FD, and BB during the hanging, pulling up, and lowering phases of the rock climbing fingertip pull movement, error bars represent standard deviation (n = 3).

The electrode's remarkable ability to suppress motion artifacts, combined with its high-resolution tracking capability of muscle recruitment dynamics during complex dynamic activities (such as push-ups and rock climbing), demonstrates outstanding advantages in applications including sports rehabilitation assessment, neuromuscular function training, and long-term ECG monitoring (e.g., heart rate variability). It offers a highly promising new-generation platform for flexible epidermal electronics and electrophysiological monitoring systems designed for real-world applications.

Paper link: https://doi.org/10.1021/acssensors.5c02018