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Page 10 of 21 Zhang et al. Soft Sci 2024;4:23 https://dx.doi.org/10.20517/ss.2023.58

Deposition
To maintain the original mobility of LM and avoid the generation of other intermetallic compounds (e.g.,
[88]
AgGa ), Park et al. prepared an LM microscale network film using the deposition method . The
2
preparation process is presented in Figure 3. First, a PI shadow mask was placed on a PDMS substrate. After
cleaning the surface of the prepared PDMS substrate with isopropyl alcohol and air plasma, the substrate
was placed in the vacuum chamber of a thermal evaporator. In was deposited once to form a solid metal
layer followed by the deposition of Ga on a layer-by-layer basis. After setting the vacuum level and
deposition rate, LM microdroplets were introduced into the chamber by introducing ambient air into the
chamber for 60 s with air pressures > 1.0 kPa. By applying tensile strain to the resulting LM microstructures,
LM microscale network (LMMN) films were obtained with a thickness of a few micrometers. The thickness
of the LM electrodes can be adjusted by varying the deposition time and deposition rate. The width and
thickness of the electrodes can be a few micrometers. The resolution of the electrodes prepared by this
method depends on the precision of the pattern structure in the mask.

Neuro-electrodes have a wider range of applications than neural conduits and are currently used in brain-
computer interfaces, precision medicine and neuroscience. There are more methods for preparing LM
neuro-electrodes than those for LM nerve conduits. Currently, all studies on the LM nerve conduit used the
injection method. In 2014, Zhang et al. proposed the fundamental strategy of using LM conduct to bridge
the disconnected nerves and, thus, reconstruct their signal transmission functions for the first time .
[10]
Subsequently, Liu et al. investigated the injection of LMs as feasible agents to repair the function of
peripheral nerves, using stainless steel wire to plug the stumps of the fragments . The LM-based NEI
[8]
prepared using this method was demonstrated to connect the injured sciatic nerve in mice. In contrast, LM
nerve electrodes can be prepared using processes such as printing, injection, selective wetting and
deposition. Actually, these methods are not limited to the fabrication of electrodes but also can be used to
prepare nerve conduits. Future attempts should be made to prepare LM nerve conduits using different
preparation processes.

APPLICATIONS OF LM NEIS
LM-based neural electrodes
Throughout the development of the LM nerve electrodes, the researchers first conducted a series of in vitro
testing experiments, including electrical stimulation and electrical signal acquisition of isolated cells, tissues,
and organs. These experiments laid the foundation for conducting in vivo implantation experiments.
Currently, the animals commonly used for in vivo implantation experiments are mice, rats, and nonhuman
primates.

Hallfors et al. conducted electrical stimulation tests on isolated neuronal cells . They combined pure Ga
[89]
and EGaIn with their previously developed microfluidic culture platform to obtain a neurostimulation
platform that achieved stimulation of neurons with subcellular precision. As shown in Figure 4A, Jin et al.
prepared implantable bioelectrodes (cylinders with diameters of 1 mm) by constructing electrode molds in
gelatin followed by LM injection . The in vivo animal experiments showed that LM-based electrodes can
[90]
be used for the acquisition of electrical signals and the administration of electrical stimulation.
Subsequently, in 2017, the present group designed an LM-based neural electrode capable of being used in
the PNS [Figure 4B]. The mechanical and electrical properties of the electrodes were tested in detail and
included tensile, fatigue resistance, and electrochemical tests. The experimental results showed that the LM-
based electrodes were suitable for recording electrical signals and for electrical stimulations during
prolonged periods . The conductive material in direct contact with tissues in the studies listed above was
[73]
LM. However, LM exposed to physiological aqueous environments may undergo oxidative behavior that
affects the electrical performance of the electrodes. To address this issue, Lim et al. electrochemically