Bioelectrical signals are invaluable in clinical practice, providing insights into physiological functions and aiding diagnosis and treatment. High-fidelity signal acquisition necessitates stillness, but breathing-induced dynamic noise inevitably introduces artifacts, compromising signal quality. Traditional signal processing techniques like bandpass filtering can lead to information loss and signal delay. Therefore, the use of damping materials as bioelectronic interfaces to absorb dynamic noise in real-time is crucial. While hydrogels offer reliable adhesion through mechanical interlocking, diffusion, and chemical bonding, current adhesive hydrogels exhibit isotropic adhesion, leading to residue buildup and compromised accuracy. Janus adhesive hydrogels, with their asymmetric adhesion, offer a potential solution. However, existing Janus hydrogels lack both selective noise damping and reliable bioelectrical signal transmission capabilities. This study aims to address these challenges by developing a Janus adhesive hydrogel (JAH) that combines selective frequency damping with asymmetric adhesion, enabling high-fidelity bioelectrical signal acquisition in clinical settings.

Previous research has explored hydrogels as bioelectronic interfaces, focusing on improving adhesion and conductivity. Park et al. demonstrated a phase-transition-based hydrogel damper effective for low-frequency noise damping. However, achieving high bioelectrical sensitivity requires strong adhesion to avoid void formation, reducing interfacial impedance and improving signal quality. Hydrogels achieve adhesion through various mechanisms, including mechanical interlocking, diffusion, and chemical bonding, leading to their use in high-resolution bioelectric signal transmission. Yet, isotropic adhesion in current hydrogels results in unwanted residue after repeated use, affecting device accuracy. Janus adhesive hydrogels, possessing asymmetric adhesion, have been proposed for applications like postoperative adhesion prevention and wearable motion sensing, but their application in selective noise damping for bioelectrical signal acquisition remains unexplored. The key challenges in developing such a hydrogel include balancing asymmetric adhesion with consistent damping, preventing signal distortion from asymmetric structural features, and reconciling the need for weak bonds for energy dissipation and strong bonds for adhesion.

A Janus adhesive hydrogel (JAH) was fabricated using a synergistic approach combining natural sedimentation-assisted fabrication and gelation-limited sedimentation. Acrylamide served as the hydrogel base, sodium chloride provided ionic conductivity, and ethylene glycol enhanced water retention. Silver nanoparticles were incorporated to improve electrical properties. The Janus structure was achieved by controlling the sedimentation of silver nanoparticles during gelation. Natural sedimentation, inhibition by free radicals, and the Trommsdorff-Norrish effect created an asymmetric gradient distribution of silver nanoparticles. The adhesive side of the JAH features free catechol groups enabling Michael addition and hydrogen bonding with skin tissue, while the non-adhesive side displays significantly reduced adhesion due to silver nanoparticle aggregation. Dynamic mechanical analysis characterized the selective damping properties, revealing a high loss factor in the respiratory frequency range (0.1–1 Hz), approximately 60 times greater than at other frequencies. The stability of the damping properties was confirmed over 60 days. Electrochemical characterization, including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), demonstrated the JAH's high current density, low impedance (<50 Ω at 100–1000 Hz), and excellent electrical endurance over 10,000 charge injection/ejection cycles. The enhanced electrical performance was attributed to the improved dielectric properties, extended Debye length, and densified electric field resulting from the silver nanoparticles. In-vivo studies on Sprague Dawley rats were conducted to assess the JAH's performance in otitis media diagnosis and clinical polysomnography (PSG) monitoring for obstructive sleep apnea (OSA). Auditory brainstem response (ABR) signals were recorded using the JAH as a non-invasive interface, demonstrating superior sensitivity compared to invasive probes, even in rats with otitis media. Clinical PSG monitoring using JAH showed effective noise damping in the respiratory frequency range, resulting in high-fidelity ECG, EEG, and EOG signals across different sleep stages, aiding in accurate OSA diagnosis.

The JAH demonstrated a superior damping effect at breathing frequencies (0.1-1 Hz), approximately 60 times greater than at other frequencies. The asymmetric adhesion difference between the two sides of the hydrogel reached up to 537 times. The JAH maintained excellent electrical properties, including low impedance (<50 Ω at 100–1000 Hz), and high stability over 10,000 charge injection/ejection cycles. In otitis media diagnosis, the JAH showed significantly higher sensitivity than invasive probes in detecting ABR signals, achieving a detection threshold as low as 25 dB compared to 35 dB for invasive probes. The JAH also demonstrated its ability to detect ABR signals in rats with otitis media at a lower threshold (35 dB) than the invasive probe (45 dB), showcasing its capability to mitigate signal distortion caused by respiratory noise. In clinical PSG monitoring, JAH effectively dampened dynamic noise in the 0.1-1 Hz range, leading to superior signal quality compared to commercial EEG gel. It successfully captured characteristic signals of different sleep stages, enabling accurate sleep stage classification and OSA diagnosis.

The JAH successfully addresses the challenges of dynamic noise and residue buildup in bioelectronic interfaces. The selective frequency damping and asymmetric adhesion significantly improve the quality and reliability of bioelectrical signal acquisition. The superior performance in both otitis media diagnosis and PSG monitoring highlights the JAH's potential to enhance various clinical applications. The enhanced electrical properties are attributed to the synergistic effects of the silver nanoparticles, which improve dielectric properties, extend the Debye length, and densify the electric field. These results suggest a promising avenue for developing high-performance bioelectronic interfaces for various applications. Future research could focus on optimizing hydrogel composition, exploring different conductive nanoparticles, and developing sophisticated integration techniques to further improve performance and expand clinical applicability.

This study successfully developed a Janus adhesive hydrogel with selective frequency damping for bioelectronic interfaces. Its superior performance in mitigating breathing noise, preventing residue buildup, and ensuring stable signal transmission over extended use makes it a valuable tool for enhancing various clinical applications. The JAH's superior sensitivity in otitis media diagnosis and its efficacy in clinical PSG monitoring demonstrate its potential to improve diagnostic accuracy and patient care. Future research should focus on improving integration with electronic devices and exploring new applications of this technology.

The study primarily focused on in-vivo experiments in rats. Further studies with larger human populations are needed to validate the clinical efficacy and generalizability of the JAH across diverse demographics and clinical scenarios. Long-term studies are necessary to evaluate the longevity of the JAH's properties and its potential long-term effects on the skin. Further investigation is needed to understand the precise mechanisms of the JAH's selective frequency damping and to explore the effects of various parameters on this mechanism.