A new class of soft, dry electrodes conform to the complex geometry of the human ear, improving comfort and reliability in non-invasive nerve stimulation therapies.
Study: Profiled Wet Spinning of Polyurethane Composites for Soft Dry Electrodes in Transcutaneous Stimulation Applications. Image Credit: H_Ko/Shutterstock.com
Reported in the journal Materials, the work describes a manufacturing approach based on profiled wet spinning of conductive polyurethane composites for use in transcutaneous auricular vagus nerve stimulation (taVNS) – a technique increasingly explored for neurological and rehabilitation applications.
The study shows how controlling structure and geometry can significantly improve electrical performance at the skin interface, a persistent limitation in wearable neurostimulation devices.
Get all the details: Grab your PDF here!
In taVNS, electrical pulses are delivered through the skin of the outer ear to stimulate branches of the vagus nerve. The approach avoids surgery, but its effectiveness depends on stable, low-impedance contact between electrode and skin.
The ear makes this difficult. Its curved, irregular anatomy varies widely between individuals, which can lead to discomfort and inconsistent electrical coupling. Gel-based electrodes improve contact but dry out and require replacement, while rigid metal electrodes lack the compliance needed for extended wear.
Finding a Materials-Driven Alternative
To address this, the researchers designed soft conductive elastomer composites using polyurethane filled with carbon black, a low-cost and skin-compatible conductive filler.
Polyurethane provides elasticity and mechanical resilience, while carbon black forms conductive pathways once its concentration exceeds a percolation threshold.
The material was processed using a solution-based wet-spinning method. Polyurethane was dissolved in dimethylformamide and mixed with carbon black, then extruded into a water bath, where solvent exchange caused the polymer to solidify.
This process naturally produced a porous internal structure that proved central to the electrode’s performance.
While wet spinning is typically used to produce fibres, the team extended the approach with custom 3D-printed nozzles to create hollow cylindrical electrodes with more complex cross-sections. These shapes reduce weight, allow internal wiring, and better match the dimensions of in-ear devices.
After extrusion, the electrodes remained in the coagulation bath long enough to ensure solvent removal and were then air-dried at room temperature. Heating was deliberately avoided, as elevated temperatures caused the porous structure to collapse and reduced mechanical compliance.
Balancing Conductivity And Softness
By varying the carbon black content, the researchers mapped how electrical and mechanical properties changed together. Low filler concentrations produced highly stretchable but electrically resistive materials, whereas higher loadings improved conductivity but made the composite stiffer and more brittle.
An intermediate formulation containing 6 % carbon black provided the best balance. Electrodes made from this material showed resistance in the kΩ cm-1 range, tolerated strains above 50 %, and remained stable under repeated mechanical cycling.
Microscopy revealed a core-shell structure, with a dense outer skin surrounding a porous interior made up of micron-scale pores. This architecture reduced the effective elastic modulus to below 1 MPa, allowing the electrodes to compress easily under light pressure.
Compression Improves Electrical Contact
Electromechanical testing showed that compression was critical to performance. Gentle contact was sufficient to establish conductivity, while moderate forces comparable to those experienced in ear-worn devices reduced resistance by several orders of magnitude.
Under these conditions, electrode impedance dropped to around 1 kΩ, well within the range required for taVNS.
Repeated compression caused little mechanical degradation, indicating that the electrodes could be reused without significant loss of performance, provided forces remained within comfortable limits for dry, non-invasive wear.
Testing On Human Skin
To assess near-clinical performance, the researchers measured impedance on human skin using frequency-domain spectroscopy and current-controlled stimulation pulses. For consistency and reproducibility, testing was conducted on the forearm rather than the ear, allowing controlled electrode placement and pressure.
Under dry conditions, the composite electrodes matched or outperformed conventional silver electrodes and remained within the voltage compliance limits of standard taVNS stimulators.
Adding a small amount of electrolyte improved performance across all electrodes, with wet electrodes still achieving the lowest absolute impedance.
The results show that improvements in dry operation were driven primarily by mechanical compliance and porous geometry, rather than simply by increasing intrinsic conductivity.
Not Yet a Finished Device
The authors stress that the work represents an early-stage materials platform rather than a clinically validated device. While the electrodes performed well on planar skin surfaces, the ear's complex anatomy introduces additional challenges that will require dedicated testing.
Long-term wear, comfort, and ear-specific performance remain topics for future study. Even so, the approach offers a scalable way to produce soft, dry electrodes with tunable shapes and properties.
The same strategy could be applied to wearable sensors, soft robotic interfaces, and other bioelectronic systems where comfort and reliable skin contact are essential.
Journal Reference
Shokurov, A. V. et al. (2026). Profiled Wet Spinning of Polyurethane Composites for Soft Dry Electrodes in Transcutaneous Stimulation Applications. Materials 19(3), 557. DOI: 10.3390/ma19030557
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.