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Nam et al. Soft Sci 2023;3:28 https://dx.doi.org/10.20517/ss.2023.19 Page 19 of 35

19 wt% PEDOT:PSS), allowing for comfortable, long-term monitoring of biopotentials. For EEG recording,
Zhang et al. designed the electrode with a 3D-comb structure and applied it to the occipital region of human
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subjects [Figure 6A] . The electrode successfully recorded the EEG signal pattern, which was highly
activated in the occipital region during the open-eye period [Figure 6B]. The subject reported no notable
complications during and after the measurement.

Meanwhile, Lee et al. reported a device for wireless real-time monitoring of electrophysiological signals by
integrating a flexible conductive composite into a personal earphone [Figure 6C, left] . The researchers
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developed a flexible conductive composite comprising AgNWs, CNTs, and PDMS. The resulting material
featured high electrical conductivity, excellent mechanical deformability, and biocompatibility, making it
well-suited for use in wearable devices. The combination of AgNWs and CNTs contributed to lower
impedance without sacrificing electrical conductivity, ensuring a high SNR for EEG recording. The EEG
earphone was used to monitor the level of concentration of subjects and to wake them up when they
entered a drowsy state of consciousness [Figure 6C, right].

In addition to monitoring brain activity, similar devices can be used to monitor the heart activity. The ECG
signal provides valuable information about the electrical excitation tendencies of myocardial cells in the
heart. Accurate diagnosis of cardiovascular diseases relies on identifying abnormal morphologies and
[159]
temporal appearances of the characteristic peaks, including P, QRS, T, and U waves . However, to
precisely analyze these peaks amid motion noise, a soft electrode that can make conformal contact and
provide high SNRs is required.

For example, electronic ink, which comprises Ag flakes, PEDOT:PSS, poly(3-hexylthiophene-2,5-diyl)
(P3HT)-nanofibrils, and ion gels, can be drawn on the skin with a ballpoint pen to create such an electrode
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[Figure 6D] . The drawn electrode demonstrated a low sheet resistance of 1.2 Ω·sq under zero strain and
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9.9 Ω·sq under 30% strain and mechanical reliability under repeated stretching (1,000 cycles of 10% strain).
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The electronic ink was printed on both wrists of a human subject to record ECG, and the shape of the
electrode could deform along with the skin deformation (10%) with any visible damage [Figure 6E, left]. No
significant differences were observed in the recorded ECG signals before and after the mechanical
deformation [Figure 6E, right].

Recently, Zu et al. developed another electronic ink composed of Ag microflakes, EGaIn droplets, and a
[161]
styrenic elastomer . The Ag microflakes served not only as conductive fillers that increased the initial
conductivity to as high as 6,380 S·cm but also as bridging materials that prevented EGaIn droplets from
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being separated from each other under induced strain. This led to excellent electromechanical properties,
with a relative resistance (R/R ) of only 7.78 when stretched up to 1,000%. The electronic ink was printed on
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a TPU substrate as surface electrodes and directly contacted the human chest to acquire ECG signals. The
researchers recorded ECG signals for over 20 min, detecting R and T peaks with ease [Figure 6F].
Conformal contact with the skin was maintained during recording, and the electrode caused no skin
irritation. Additionally, the suitability of the electronic ink for stretchable digital circuits allowed it to be
applied to interconnects for a wireless ECG monitoring system.

Lastly, there are examples of measuring muscle activities. EMG sensors are used to measure and record the
electrical activity generated by skeletal muscles, providing valuable information on muscle function and the
diagnosis of neuromuscular disorders. Unlike EEG and ECG, the subject has to activate the target muscles
(i.e., move) to record EMG signals, which makes it challenging to record stable and high-quality EMG
signals due to motion artifacts. For this reason, rigid needle electrodes that contact the muscle directly