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Zhang et al. Soft Sci 2024;4:23 https://dx.doi.org/10.20517/ss.2023.58 Page 13 of 21

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For most cells in the human body, potassium ions (K ) predominantly exist within the cell, and sodium ions
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(Na ) predominantly exist outside the cell in the resting state. As the cell membrane is more permeable to K
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than to Na , the resting state is dominated by the efflux of K (from the cytoplasm to the extracellular space).
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However, as the number of K leaving the cell increases, the electric field force on both sides of the
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membrane that prevents K from being transported extracellularly will also increase. Finally, the
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concentration gradient difference and the electric potential difference on both sides of the membrane are
opposite in direction but equal in magnitude, and the net movement of K becomes zero. The potential
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difference at this state, which defines the extramembrane potential as zero, is called the equilibrium
potential of K or the resting potential (-70 mV) . The process of action potential generation is shown in
[96]
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Figure 5A, and the threshold for a nerve cell to generate an action potential is -55 mV (intracellular
potential). An action potential is triggered when a stimulus can depolarize the cell membrane from the
resting potential to the threshold potential. Therefore, one of the keys to neural connectivity and nerve
repair is to realize the transmission of action potentials. When LM is used for nerve connection and repair,
a double electric layer is formed on the surface of the LM connecting the two ends of the nerve [Figure 5B].
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When the right side of the nerve is stimulated to generate an action potential, Na flows from the outside to
the inside of the cell, decreasing the potential on the right surface of the LM, resulting in a potential
difference between the left and right ends of the LM. As shown in Figure 5C, the movement of electrons
within the LM will affect the intracellular potential of the cell in the left portion. The action potential can be
triggered when the intracellular potential of the left cell part increases to -55 mV. At this state, the LM then
realizes the action potential transmission effect.

In addition, the LM material has a broad liquid–phase temperature range, which makes its use easier for
fabrication, encapsulation, and surgical manipulation. Not only is the stiffness of LM close to zero, but its
electrical conductivity is several orders of magnitude higher than those of nonmetallic materials. LM can be
directly printed on a broad range of materials, including polymers, textiles, and hydrogels [97,98] . These
properties make it suitable for applications such as reconnecting transected nerves.

In 2014, Zhang et al. clarified that LM (EGaInSn) could be used as a connecting or functional restoration
channel to repair PNI [Figure 5D] . In vitro experiments showed that LM effectively reconnected the
[10]
transected sciatic nerves and allowed the conduction of electrical signals. In addition, the visualization of the
LM under flat X-ray films showed its convenient use in performing secondary surgery. It was revealed that
the electrical nerve signals (including amplitude and frequency) recorded after the electrical stimulation of
the bullfrog sciatic nerve reconnected by the LM were close to those of the intact sciatic nerve. Control
experiments using conventional Rigel solutions in place of EGaInSn showed that the performance of Rigel
solutions as functional recovery channels could not be compared with that of LM. In addition, by evaluating
the basic electrical properties, the EGaInSn material appears more suitable for conducting weak electrical
nerve signals because its impedance was several orders of magnitude lower than that of the well-known
Rieger’s solution. Using more test animals, Liu et al. demonstrated the adoption of liquid Ga to reconnect
the transected sciatic nerve in mice [Figure 5E] . Their experiments showed that the electrical signals
[8]
detected in the sciatic nerve after the LM connection were almost identical to those of the intact nerve.
Moreover, there was no negative burst firing caused by PNI on the nerve discharge curve after surgery.
According to the pathological examination, the tendency of atrophy of the gastrocnemius muscle was
delayed considerably, and the fibrillatory potentials appeared immediately in the PNI mice; by contrast, the
mice that underwent nerve connection surgery did not generate fibrillatory potentials until the third month.
The findings of this study confirm the stability of Ga and its potential for use in future clinical applications.
It is expected that this technique will work well in treating nerve injuries (including CNS injuries) in future
[99]
clinical procedures. In 2024, Chung et al. prepared a three-dimensional LM-based microelectrode array .