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Bai et al. Soft Sci 2023;3:40 https://dx.doi.org/10.20517/ss.2023.38 Page 19 of 34

EMG signal detection
Unlike the monitoring of human muscle movement through strain, EMG sensors have been used to
monitor the electrical signals of muscles using sensors that generate, conduct, and transmit potentials when
the motor muscle is excited, which results in potential changes and triggered EMG signals. It could assist in
determining the degree of muscle fatigue and other conditions, thus providing a credible evaluation of
muscles. Lee et al. added LM droplets and polystyrene sulfonate to a solution containing acetic acid and
formed stable polyelectrolyte-attached LM microgranular-particles (PaLMPs) conductive ink under
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ultrasonic treatment [Figure 7A] . The composite of the layer and substrate exhibited excellent
mechanical and electrical properties and could continuously work for 10,000 cycles with a stable response.
The sensor exhibited a high degree of fidelity when applied to monitoring EMG signals [Figure 7B]. In
addition, based on their original study, they improved the conductivity of the sensor by using Pt-modified
CNTs, namely Pt-decorated CNTs on gallium-based liquid-metal particles (CMP) with high electrical
conductivity. Covered with LM droplets, the resulting LM ink was manifested with excellent wettability and
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high vapor pressure, which could conform and assemble the particles directly on the skin within 10 s .
The prepared myoelectric sensor was used to monitor the muscle electrical signals in the thighs of mice
[Figure 7C]. After amplification of the signal by an external voltage, the signal of the CMP electrode was
clear, whereas the normal LMNP electrode failed to detect the signal of muscle activation [Figure 7C],
indicating that the sensor made of CMP had better electrical contact with the skin [Figure 7D]. The good
biocompatibility of LMs makes in vivo biosensors possible. Based on EGaIn NPs, a three-layer structure
bioelectrodes, as shown in Figure 7E, could detect EMG signals from 1 to 100,000 Hz in living organisms
[Figure 7F], and this flexible electrode was simple to prepare and had a good performance, which was
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prospective for application .

Electrocardiographic (ECG) signal monitoring
Long-term monitoring of cardiac activity can help diagnose arrhythmias by identifying abnormal
heartbeats. To this end, in addition to monitoring bioelectricity in biological muscles, nano-LMs have also
been made into electrocardiographic (ECG) sensors. For example, Cao et al. first used thermoplastic PU-
hexafluoroisopropanol to form fine and strong nanofibers, which were then combined with an isopropanol
solution suspended with LMNPs by electrostatic spinning to form a nano-LM-based high-robust stretchable
electrode (NHSE) with a thickness of 50 μm , where the LM conductive layer was several hundred
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nanometers in thickness. After 330,000 cycles from 0% to 100%, its resistance only increased by 5%, so
NHSE shows excellent mechanical and electrical conductivity. When used as an electrode for monitoring
human ECG signals, the contact impedance with the skin was comparable to that of commercial AgCl gel
electrodes [Figure 8A]. In addition, it had higher sensitivity, as the signal measured by the NHSE remains
almost constant during wrist movements. The NHSE also showed impressive waterproof capability, with
little impact on sensing performance after being poured by water [Figure 8B]. In addition, the researchers
also found that the ECG electrodes made of PaLMPs also had higher fidelity compared with commercial
electrodes. They monitored in three places: left leg, left arm, and right arm, respectively, and the measured
electrical signal data displayed more obvious peaks compared with those measured by commercial
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electrodes . LM hydrogel complexes could also be used as ECG electrodes. The LM-SA (sodium alginate)-
PAA hydrogel formed by mixing LMNP, SA, and AA had good self-adhesive properties and could be used
as an ECG electrode material to detect human ECG signals in close proximity to the human epidermis
[Figure 8C]. As shown in Figure 8D, the quality and stability of the measured ECG signals are better than
those of commercial ECG electrodes .
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Gas sensors
Gas concentration sensors
Not only can certain gas abnormalities be used in respiratory medicine as evidence of disease, but also