Ionogels and eutectogels for stable and long-term EEG and EMG signal acquisition

doi: 10.1088/2752-5724/ad5c84

Department of Chemical and Biological Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong Special Administrative Region of China, People’s Republic of China

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Corresponding author: E-mail: hnyein@ust.hk; E-mail: kehsing@ust.hk

摘要

Abstract: AbstractNeurological injuries and disorders have a significant impact on individuals’ quality of life, often resulting in motor and sensory loss. To assess motor performance and monitor neurological disorders, non-invasive techniques such as electroencephalography (EEG) and electromyography (EMG) are commonly used. Traditionally employed wet electrodes with conductive gels are limited by lengthy skin preparation time and allergic reactions. Although dry electrodes and hydrogel-based electrodes can mitigate these issues, their applicability for long-term monitoring is limited. Dry electrodes are susceptible to motion artifacts, whereas hydrogel-based electrodes face challenges related to water-induced instability. Recently, ionogels and eutectogels derived from ionic liquids and deep eutectic solvents have gained immense popularity due to their non-volatility, ionic conductivity, thermal stability, and tunability. Eutectogels, in particular, exhibit superior biocompatibility. These characteristics make them suitable alternatives for the development of safer, robust, and reliable EEG and EMG electrodes. However, research specifically focused on their application for EEG and EMG signal acquisition remains limited. This article explores the electrode requirements and material advancements in EEG and EMG sensing, with a focus on highlighting the benefits that ionogels and eutectogels offer over conventional materials. It sheds light on the current limitations of these materials and proposes areas for further improvement in this field. The potential of these gel-based materials to achieve a seamless interface for high-quality and long-term electrophysiological signal acquisition is emphasized. Leveraging the unique properties of ionogels and eutectogels holds promise for future advancements in EEG and EMG electrode materials, leading to improved monitoring systems and enhanced patient outcomes.
- ionogel /
- eutectogel /
- electrode /
- electroencephalography /
- electromyography
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Figure 1. Schematic illustrating the key characteristics of ionogel and eutectogel suitable for EEG and EMG signal recording.

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Figure 2. (a) Commercially available wet Ag/AgCl EMG and EEG electrodes. Adapted from [5] with permission from the Royal Society of Chemistry. Reprinted from [16], © 2019 Elsevier B.V. All rights reserved. Reproduced from [17]. CC BY 4.0. (b) An example of a titanium-based dry electrode for surface EMG recording. Reproduced from [15]. CC BY 4.0. (c) An example of conducting polymer-based adhesive dry electrode for EMG and EEG signal acquisition. Reproduced from [18]. CC BY 4.0. (d) An example of conformal and adhesive MXene hydrogel-based electrodes for EMG signal acquisition. Reprinted from [6], © 2023 Elsevier Inc.

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Figure 3. (a) Common cations and anions in ILs. Reproduced with permission from [9]. CC BY-NC 3.0. (b) Common hydrogen bond acceptors and donors in DESs. Adapted from [10], with permission from Springer Nature. (c) Ionogel-based electrodes for EMG and blood oxygen monitoring. [11] John Wiley & Sons. © 2023 Wiley-VCH GmbH. (d) Eutectogel electrodes based hydrophobic deep eutectic solvents for underwater EMG recording. Reproduced from [29]. CC BY 4.0.

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Table 1. Characteristics of various electrode materials employed for EEG and EMG signal acquisition.

	Materials	Biocompatibility	Mechanical characteristics	Impedance	SNR	Long-term stability	Adhesiveness	References
Traditional Wet electrodes	Ag/AgCl	Inflammatory and cytotoxicity risk	Not flexible	2.8-130 kΩ	24.9 dB	Lacks long-term stability	Use electrolyte gel for adhesion	[4]
Metal-based Based Dry electrodes	Titanium coated on Stainless steel/Polyurethane	Improved biocompatibility during in-vivo tests	Not flexible	10 kΩ (SS) at 10-100 Hz & 200-250 kΩ (TPU) at >100 Hz	18.3 dB (SS), 19.2 dB(TPU) v/s 18.3 dB commercial Ag/AgCl	—	Not adhesive	[15]
Metal-based Based Dry electrodes	Gold plated pin electrodes	Improved biocompatibility, no skin abrasion or irritation	Not flexible	66.7 kΩ at 50 Hz	—	Stable for 60 d	Not adhesive	[44]
Soft Dry Electrodes	Carbon fiber and polyurethane(PU)/carbon nanotube(CNT)	Biocompatible	Reduced rigidity compared to Ag/AgCl	133 kΩ	—	Suitable for long-term use	Not adhesive	[45]
Soft Dry Electrodes	PEDOT:PSS, WPU, D-sorbitol	Biocompatible. No skin irritation observed on prolonged use of 16 h	Stretchable with elongation at break ∼ 43% strain	82 kΩ cm² at 10 Hz 148 kΩ cm² for Ag/AgCl	—	Stable impedance for 1 h	Adhesive on dry and wet skin	[18]
Hydrogel based electrodes	Polyvinyl alcohol, Polyvinylpyrrolidone, Polydopamine nanoparticles	Highly biocompatible. No erythema, eschar or oedema observed on rat skin for 6 h	Stretchable with no obvious hysteresis up to 800% strain	3-4 kΩ (1-100 Hz)	—	Stable impedance for 7 d	Adhesive on dry and wet skin	[46]
Hydrogel based electrodes	Carboxymethyl chitosan, Alginate, MXene	Exhibited cytocompatibility	Self-healing. Stretchable > 500% strain	20-30 kΩ (100 Hz)	29.7 dB v/s 16.7 dB commercial Ag/AgCl	—	Adhesive	[6]
Ionogel based electrodes	Sulfobetaine methacrylate, Poly(ethylene glycol) diacrylate IL: 1-butyl-1-methylpyrrolidinium bistrifluoromethanesulfonylimide	Good biocompatibility with no significant damage after 72 h attachment	Stretchable up to 500% strain	99.1 kΩ cm² at 100 Hz after 24 h Several hundred kΩ cm² to 1 MΩ cm² for Ag/AgCl hydrogel electrode	Initial and after 2 weeks: 13.3 dB to 13.5 dB v/s 16.3 dB to 9.2 dB commercial Ag/AgCl	Stable signal acquisition for 2 weeks	Adhesive on dry and wet skin	[11]
Eutectogel based electrodes	Graphene foam, poly(acrylic acid) (PAAc) DES: choline chloride and urea	Biocompatible	Stretchable up to 500%-600% strain	99 Ω	37 dB	Stable EMG recording even after 2 months	Adhesive on dry and sweaty skin	[34]

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