Recent Advances in Conductive Hydrogels for Electronic Skin and Healthcare Monitoring

Abstract

In recent decades, flexible electronics have witnessed remarkable advancements in multiple fields, encompassing wearable electronics, human-machine interfaces (HMI), clinical diagnosis, and treatment, etc. Nevertheless, conventional rigid electronic devices are fundamentally constrained by their inherent non-stretchability and poor conformability, limitations that substantially impede their practical applications. In contrast, conductive hydrogels (CHs) for electronic skin (E-skin) and healthcare monitoring have attracted substantial interest owing to outstanding features, including adjustable mechanical properties, intrinsic flexibility, stretchability, transparency, and diverse functional and structural designs. Considerable efforts focus on developing CHs incorporating various conductive materials to enable multifunctional wearable sensors and flexible electrodes, such as metals, carbon, ionic liquids (ILs), MXene, etc. This review presents a comprehensive summary of the recent advancements in CHs, focusing on their classifications and practical applications. Firstly, CHs are categorized into five groups based on the nature of the conductive materials employed. These categories include polymer-based, carbon-based, metal-based, MXene-based, and ionic CHs. Secondly, the promising applications of CHs for electrophysiological signals and healthcare monitoring are discussed in detail, including electroencephalogram (EEG), electrocardiogram (ECG), electromyogram (EMG), respiratory monitoring, and motion monitoring. Finally, this review concludes with a comprehensive summary of current research progress and prospects regarding CHs in the fields of electronic skin and health monitoring applications.

Keywords: E-skin; conductive hydrogels; electrophysiological signal; healthcare monitoring.

Figure 1

The categories and recent application advancements of CHs. The categories of CHs are mainly summarized into five parts, namely ionic CHs, polymer-based CHs, carbon-based CHs, metal-based CHs, and MXene-based CHs. Practical applications of CHs are presented, including their use in electrophysiological signal monitoring (ECG, EMG, EEG), motion detection, and respiratory monitoring [90,91,92,93,94,95,96,97,98,99]. EMG monitoring: Reproduced with permission [97]. Copyright 2025, Wiley. Reproduced with permission [95]. Copyright 2024, Wiley. ECG monitoring: Reproduced with permission [99]. Copyright 2024, Wiley. Reproduced with permission [90]. Copyright 2025, Elsevier. EEG monitoring: Reproduced with permission [92]. Copyright 2023, Wiley. Reproduced with permission [93]. Respiratory monitoring: Copyright 2024 Springer Nature. Reproduced with permission [94]. Copyright 2022, Wiley. Reproduced with permission [98]. Copyright 2025, Elsevier. Motion detection: Reproduced with permission [96]. Copyright 2025, Wiley. Reproduced with permission [91]. Copyright 2024, Wiley.

Figure 2

Ionic conductive hydrogels. (a) Illustration depicting the designed microarchitecture of cellulose/BT composite hydrogels. (b) Images displaying cellulose solutions and corresponding hydrogels [100]. (c) Diagram illustrating the fabrication methodology for ionic liquid/polyvinyl alcohol composite hydrogels [101]. (d) Hydrophilic deep eutectic solvent components and hydrophobic DES components. (e) Optical photograph of Li₅₀Ch₅₀ [102]. (f) SEM image of PSDIC-gels. All pictures have been adopted with permission.

Figure 3

Polymer-based conductive hydrogels. (a) Diagrams demonstrating molecular-scale interactions within PANI/P(AAM-co-AAC) organo-hydrogels [103]. (b) Network deformation during stretching. (c) The corresponding snapshots of tensile loading-unloading cycles. (d) Schematics of structure and corresponding photographs of four types of hydrogels [104]. (e) The isotropic PVA-PPy@CNF hydrogel was prepared from a PVA/PPy@CNF mixture via the freezing-thawing method. (f) Schematic of anisotropic hydrogel morphology characterization. (g) Process diagram depicting the synthesis methodology for PPS-based organo-hydrogel [105]. (h) Optical image of PPS organo-hydrogel. All pictures have been adopted with permission.

Figure 4

Carbon-based conductive hydrogels. (a) Fabrication steps of hydrogel [106]. (b) AFM images of fCNT/TA/PVA and fCNT/TA/PVA/PAA. (c) Photographs of a hydrogel under stretching. (d) Schematic illustrations showing the fabrication of SGP hydrogels by the three-step method [107]. (e) Under UV excitation, all gel groups completed the sol–gel process within 30 s. (f) Strategy for fabricating GTHs with highly tough, stretchable, and biocompatible properties [108]. (g) Photographs of GTHs under stretching. (h) The GTHs can be expanded up to 3000% of their original area under biaxial stretching. All pictures have been adopted with permission.

Figure 5

Metal-based conductive hydrogels. (a) Preparation of the composite hydrogel [140]. (b) SEM images of the composite hydrogel. (c) Preparation process of PAL hydrogels [109]. (d) Interaction between the oxide layer and PVA molecular chains. (e) Digital photographs of the tensile process of PAL hydrogel from 0% to 5000%. (f) Preparation of precursor solution and (g) hydrogel composite [110]. (h) Hydrogel as wearable sensors attached in (i) human finger and (ii) finger joint. All pictures have been adopted with permission.

Figure 6

MXene-based conductive hydrogels. (a) Schematic of MS-MP hydrogel fabrication through the salting-out method [147]. (b) Digital photographs of MS-MP3 hydrogels. (c) Schematic diagrams depicting the synthesis of conductive hydrogels [111]. (d) SEM images of conductive hydrogels. (e) Images of the hydrogel film in bending state. (f) Fabrication procedure of the PAM/(MXene-SF) hydrogel [112]. (g) Optical images of PAM/(MXene-SF) hydrogel’s mechanical properties, including stretching, knotting, compression, and bearing a 500 g load. (h) Optical images of PAM/(MXene-SF) hydrogel adhesion to substrates like human finger and (i) PVC. All pictures have been adopted with permission.

Figure 7

The EEG signals monitoring based on CHs electrode. (a) Schematic illustration of the SSVEP test [93]. (b) The photograph of BCI recording devices integrating PEDOT/PVA hydrogels. (c) Impedance after PEDOT/PVA hydrogel injection at the occipital lobe. (d) Power spectrum of EEG signals from Oz channel. (e) Comparison of accuracy and ITR between commercial electrodes and PEDOT/PVA electrodes. (f) Optical image of the subject viewing movie clips for EEG-based emotion induction [151]. (g) Schematic of EEG electrode positions on head model. (h) Schematic illustration of the emotion classification system. (i) EEG signals and corresponding energy spectra across various emotional states. (j) Photographs of the multichannel hydrogel electrode [92]. (k) Correlation of hydrogel and Ag/AgCl electrodes in EEG recording. (l) Motion artifact comparison of hydrogel and Ag/AgCl electrodes. All pictures have been adopted with permission.

Figure 8

The ECG signals monitoring is based on CHs electrode. (a) Schematic illustration of the portable flexible ECG monitoring patch based on LM@SF-PAA hydrogel electrodes [158]. (b) Optical images of the ECG monitoring patch, including Bluetooth module, ECG detection, and processing system-on-chip. (c) ECG signals are continuously recorded by the ECG monitoring patch. (d) A schematic illustrates WSPD-enabled ECG recording and pacing in a rat heart [99]. (e) ECG signals from the rat were recorded in vivo using the WSPD system. (f) ECG changes in a rat model during AV block, stimulation, and post-stimulation phases. (g) Changes in rabbit sinus heart rate in response to stimulation with varying pulse widths. All pictures have been adopted with permission.

Figure 9

EMG signals monitoring is based on CHs electrode. (a) Optical photo of three different electrodes [164]. (b) Comparisons of sEMG signals recorded by three different electrodes. (c) SNR values of electrodes in static (dry skin) and vibratory (dry/wet skin) states. (d) Schematic of optical hydrogel surface and moisture-resistant Janus hydrogel for epidermal electronics [95]. (e) The raw EMG signals when the maximum load is set to 30 kg. (f) Schematic of P(AA-co-AU)-SC-Ga-Al³⁺ ionogel bioelectrode patch for electrophysiological monitoring [97]. (g) EMG signals from ionogel electrodes, commercial electrodes, and ionogel electrodes after 20 days of storage during repetitive fist clenching. (h) EMG signals were recorded by the ionogel electrode in different gripping forces. (i) Respiratory signals recorded by ionogel electrodes and commercial electrodes. All pictures have been adopted with permission.

Figure 10

The respiratory monitoring is based on CHs electrode. (a) Schematic illustration of airway configurations during normal respiration and obstructive sleep apnea [94]. (b) Resistance fluctuations are recorded during nostril airflow monitoring; the inset presents infrared images captured throughout inhalation and exhalation. (c) Resistance variations from monitoring the nostril airflow, inset show the infrared images of the inhaling and exhaling process. (d) Schematic of ATH eutectogel structure, performance, and high-sensitivity nostril airflow sensing [170]. Capacitive variation curves representing respiratory activity monitored via nostril airflow (e) and chest movement (f). (g) The complete spectrum of electrical signals generated by abdominal movements [171]. (h) Multimodal hydrogel sensors were employed to monitor sleep apnea. (i) The full range of electrical signals generated by nasal airflow activity. All pictures have been adopted with permission.

Figure 11

The motion monitoring is based on CHs. (a) Breathing monitoring is located in the chest [91]. (b) Breathing monitoring is in the abdomen. (c) The signals of pulse monitoring by strain sensor. (d) Recorded signals from the strain sensor during movement detection. (e) Human motion monitoring at five exemplified locations [96]. (f) Resistance variation with finger bending. (g) Resistance variations when holding the finger at different angles. Resistance variations with motions of the (h) elbow, (i) wrist, and (j) knee joints of the human body. (k) Resistance variation with throat motion during pronunciation. All pictures have been adopted with permission.