Design of an injectable, self-adhesive, and highly stable hydrogel electrode for sleep recording

SUMMARY High-quality and continuous electroencephalogram (EEG) monitoring is desirable for sleep research, sleep monitoring, and the evaluation and treatment of sleep disorders. Existing continuous EEG monitoring technologies suffer from fragile connections, long-term stability, and complex preparation for electrodes under real-life conditions. Here, we report an injectable and spontaneously cross-linked hydrogel electrode for long-term EEG applications. Specifically, our electrodes have a long-term low impedance on hairy scalp regions of 17.53 kΩ for more than 8 h of recording, high adhesiveness on the skin of 0.92 N cm−1 with repeated attachment capability, and long-term wearability during daily activities and overnight sleep. In addition, our electrodes demonstrate a superior signal-to-noise-ratio of 23.97 decibels (dB) in comparison with commercial wet electrodes of 17.98 dB and share a high agreement of sleep stage classification with commercial wet electrodes during multichannel recording. These results exhibit the potential of our on-site-formed electrodes for high-quality, prolonged EEG monitoring in various scenarios.


Figure S1
. The properties of the AIRTrode hydrogel compared with those of previously reported hydrogels in terms of adhesiveness, continuous recording time, and convenience of use.This figure is related to Table S1.

Figure S3 .
Figure S3.(a) Direct comparison of electrical impedance spectroscopy results between commercial EEG gel and the AIRTrodes with different AMPS loadings.(b) The influence of the presence of glycerol on the electrical impedance of AIRTrodes.Glycerol loading remained consistent at a 10 % weight ratio relative to PEDOT:PSS while adjusting AMPS loadings, and AMPS loading stayed consistent at a 100 % weight ratio relative to PEDOT:PSS while adjusting glycerol loadings.This figure is related to Figure 1.

Figure S4 .
Figure S4.Stability of adhesion energy across 20 attaching/detaching cycles.This figure is related to Figure 2c.

Figure S5 .
Figure S5.Illustration of the superior on-skin adhesion force and cohesion properties of AIRTrode.The AIRTrodes after pulling (left) could be reattached (right).This figure is related to Figure 2.

Figure S6 .
Figure S6.On-skin tensile adhesion force of AIRTrode.This figure is related to Figure 2.

Figure S7 .
Figure S7.Schematic illustration of the electrical impedance measurement setup under prolonged open-air conditions.This figure is related to Figure 3.

Figure S8 .
Figure S8.Impedance variation (1 Hz to 1000 Hz) of the AIRTrode electrodes under different relative humidity (RH) conditions.This figure is related to Figure 3.

Figure S9 .
Figure S9.Stability on the sweating skin.AIRTrode electrodes were worn on the forearm when the subject went for an hour of outdoor walking at a high environmental temperature (32 ºC).AIRTrode electrodes can remain adhered to the skin before (left) and after (right) the experiment.The scale bar is 1 cm.This figure is related to Figure3

Figure S11 .
Figure S11.(a) Schematic illustration of standard three-electrode method of skin-electrode interfacial impedance measurement.(b) Averaged skin-electrode impedance from a cohort of participants (n = 6) across 5 days.This figure is related to Figure 3.

Figure S12 .
Figure S12.Zoom-in images depicting the state before (left) and after (right) electrode removal.This figure is related to Figure 3.

Figure S13 .
Figure S13.Image of a volunteer wearing an EEG cap with AIRTrode filled in during daytime (around 8 hours) while maintaining daily activities.This figure is related to Figure 3.

Figure S14 .
Figure S14.Time series alpha band ([8, 13] Hz) power from both electrodes (a) before overnight sleep and (b) after overnight sleep.This figure is related to Figure 4.

Figure S15 .
Figure S15.(a) Overnight sleep EEG montage setup.AIRTrode and commercial EEG gel electrodes for each recorded channel were placed at the proximity location to ensure the similarity of the source of the EEG signal.(b) Schematic illustration of the placements of the facial electrode pairs (HEOG and EMG).Figure S15b is created with BioRender.com.This figure is related to Figure 5.

Figure
Figure S15.(a) Overnight sleep EEG montage setup.AIRTrode and commercial EEG gel electrodes for each recorded channel were placed at the proximity location to ensure the similarity of the source of the EEG signal.(b) Schematic illustration of the placements of the facial electrode pairs (HEOG and EMG).Figure S15b is created with BioRender.com.This figure is related to Figure 5.

Figure S16 .
Figure S16.(a, b) Hypnograms and (c, d) the corresponding confusion matrices of the overnight sleep EEGs for two of the three participants.This figure is related to Figure 5.

Table S1 .
Comparison between AIRTrode and other hydrogel-based electrodes for long-term EEG applications.