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Paintable 'second skin' gel for wearable bioelectronic sensors
(Nanowerk Spotlight) Accurately monitoring the body's bioelectrical signals is crucial for cardiology research and clinical diagnosis of heart diseases. However, this requires customizable bioelectrodes that can adapt to the complex topography and movements of the skin. While electronic tattoos made of conductive materials like metals have been used for this purpose, their weak skin adhesion and lack of electrolytes lead to motion artifacts and high impedance at the skin interface.
Hydrogels, on the other hand, contain sufficient electrolytes and tissue-like softness that could provide reliable skin contact for bioelectrical sensing. Recent breakthroughs have aimed to create skin-compliant hydrogel bioelectrodes through approaches like ionically conductive tattoos, self-adaptive gels that rapidly cure on the skin, and reversibly phase-changing gels.
However, most hydrogel bioelectrodes to date suffer from degradation in their adhesion, conductivity and skin compliance when exposed to aqueous environments like sweating or rainfall, severely limiting their practical utility for long-term bioelectrical monitoring in real-world conditions.
Now, researchers from Donghua University in China have developed a new type of biohydrogel that can be directly painted onto the skin from a liquid precursor ink. They report their findings in Advanced Functional Materials ("On-Skin Paintable Water-Resistant Biohydrogel for Wearable Bioelectronics").
Mechanism of on-skin paintable waterproof biohydrogel
Mechanism of the on-skin paintable waterproof biohydrogel. a) This schematic shows the concept and ability of an on-skin paintable PVA-TA (PT) biohydrogel. b) Ultradepth-of-field microscopy images show the reproducibility of the skin’s surface topography after removing the PT biohydrogel film, indicating their conformal contact with the skin surface, while the gel of commercial ECG electrode does not have this feature. c) Schematic diagram of the hydrogen bond reconstruction in the PT biohydrogel after solvent volatilization. d) Changes in the transmitted light intensity of PT biohydrogel ink after different days of placement. The illustration shows a comparison of the turbiscan stability index (TSI) changes between PT biohydrogel ink and commercial conductive paste after storage for up to 30 days. e) Photos of PT biohydrogel ink before and after storage for 30 days. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The key to this innovation lies in the clever use of ethanol in the gel's formula. Ethanol temporarily disrupts the hydrogen bonds between the polyvinyl alcohol and tannic acid polymers, keeping the ink in a stable, homogeneous liquid state. When the biohydrogel ink is painted on skin, the ethanol rapidly evaporates, allowing the hydrogen bonds to reform and solidify the gel within less than two minutes. This liquid-to-solid phase transition enables the biohydrogel to perfectly conform and adhere to the complex micro-topography of the skin before curing.
Importantly, the dynamic hydrogen bonding not only enables the on-skin paintability but also imparts the cured biohydrogel with remarkable water resistance. When exposed to aqueous environments, the free tannic acid in the gel forms even denser hydrogen bonding crosslinks with the polyvinyl alcohol. As described in the paper, these reinforced hydrogen bonds prevent water infiltration into the gel network, preserve adhesion to skin, and maintain high ionic conductivity through the Grotthuss mechanism of proton hopping along the hydrogen-bonded network.
The researchers demonstrated that the adhesion, mechanical properties, and conductivity of the biohydrogel actually improve upon water immersion due to this hydrogen bond densification. The biohydrogel exhibits over 1000% stretchability and a low Young's modulus matching that of soft skin tissue, allowing it to maintain conformal, motion-artifact-free contact with skin even under large strains.
To showcase the biohydrogel's capabilities for long-term waterproof bioelectrical monitoring, the researchers recorded high-fidelity electrocardiograms on human skin. They applied the biohydrogel onto the wrists as a liquid ink which then solidified into gel electrodes. Compared to commercial ECG electrodes, the biohydrogel electrodes showed lower interfacial impedance and captured ECG signals with higher signal-to-noise ratio.
Notably, the biohydrogel electrodes maintained stable ECG signals with clearly visible characteristic waveforms even after 72 hours of continuous use in a sweaty environment, while conventional gel electrodes failed under the same aqueous conditions. The biohydrogel electrodes also exhibited superior dynamic performance, preserving signal quality and a high P/R ratio during wrist movements.
The researchers further demonstrated the biohydrogel's versatility by integrating it with ordinary cotton cloth to fabricate a 'smart' shirt for wearable ECG monitoring. The cotton/biohydrogel electrodes outperformed cotton coated with commercial conductive pastes, achieving an impressive signal-to-noise ratio of 37.8 dB which was 83.5% higher. They also successfully demonstrated multichannel ECG mapping using an array of nine biohydrogel electrodes around the wrist, opening up possibilities for spatiotemporal cardiac monitoring with richer diagnostic information.
While this study presents a significant step forward, more research is needed to establish the long-term reliability and biocompatibility of the paintable biohydrogel electrodes. Future studies should assess their performance under varied environmental conditions and over extended periods to fully validate their durability and safety for prolonged skin contact. Nonetheless, this on-skin paintable and waterproof biohydrogel represents an exciting advance towards practical wearable bioelectronics.
By enabling high-fidelity epidermal monitoring of bioelectrical signals like electrocardiograms, electromyograms and electroencephalograms, such ionically conductive skin-like hydrogels could find wide-ranging applications in clinical healthcare, fitness tracking, human-machine interfaces and beyond.
The unique paintable delivery and water-resistant operation expand the horizon for seamless integration of soft bioelectronic sensors with the human body in real-world environments. With further development, this innovative biohydrogel could open up new possibilities for comfortable, continuous, and reliable monitoring of vital physiological signals, potentially transforming personalized healthcare and wellness management.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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