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Zhang et al. Soft Sci 2024;4:23 https://dx.doi.org/10.20517/ss.2023.58 Page 7 of 21

tension. Liquid Ga has low viscosity and high-surface tension, and both parameters decrease as a function of
[55]
temperature .
The oxidizing treatment reduces the surface tension of the LM, thus improving its wettability to the surface
of the substrate material. The increased viscosity makes the LM printable. For example, the surface tension
of pure eutectic gallium-indium (EGaIn) liquid drop is approximately 435 mN/m, while that of the EGaIn
liquid drop-coated oxide films could be adjusted to 624 mN/m. Therefore, this method influences the
wettability of the LM. It has been demonstrated that as the oxide content in the LM increases, the wettability
of the LM gradually improves on different substrates, including paper, silica gel plates, and rubber sheets .
[64]
[68]
Additionally, given the application of extra pressure, the wettability could also be improved . In the future,
the flow capability of LM can be changed by controlling its oxidation level, resulting in the manufacturing
of NEI devices that are stable over time and free of LM leakage. Appropriate treatment will promote LM to
fill the entire flow channel during injection and thus better make flexible conducting wires.

Biological properties
As a class of materials directly applied to the human body, the biocompatibility of LM is an important factor
to consider. As early as 70 years ago, researchers tested the toxicity of Ga lactate and chloride in rats and
[69]
rabbits following inhalation, ingestion, and injection. The low toxicity of Ga ions was verified . In 2003, a
pharmaceutical formulation of citrate-buffered Ga(NO ) was approved by the United States Food and Drug
3 3
Administration (FDA) for treating malignancy-associated hypercalcemia . In 2014, Wang et al. performed
[70]
an in vitro toxicity test (mouse embryonic fibroblasts) using the Cell Counting Kit-8 method and flow
cytometry . Twenty-four hours later, the cell survival rate of the Ga dip was 100.6%. In 2019, Liu et al.
[71]
injected 0.2 mL of Ga into the stomachs of female BALB/C mice (The mice used in the studies were
approximately eight weeks of age with weight ranging around 20 ± 1 g) . Their experimental results
[72]
showed that the mice were very healthy after injection and demonstrated good food consumption and
normal defecation habits. These studies have laid the foundation for the extensive use of Ga-based alloys in
biomedical applications. In 2017, Guo and Liu performed an in vitro cytotoxicity test (mouse 3T3
fibroblasts) on EGaIn and determined cell viability using the Methyl thiazolyl tetrazolium (MTT) assay .
[73]
Their experimental results showed that the cell viability of mouse fibroblasts containing LM elements
exceeded that of cells without LM elements. In 2019, Wang et al. conducted in vitro and in vivo cellular
experiments (Cell Counting Kit-8 assay, human malignant C8161 cells, and normal HaCaT cells) and in
vivo injection experiments (direct subcutaneous injection of 100 μL of LM, observed for four weeks) against
EGaIn . Both experiments demonstrated the low toxicity of EGaIn.
[27]
Most studies support the conclusion that Ga is associated with very low toxicity outcomes . However, the
[74]
potential toxicity of LMs in specific environments remains to be confirmed, and the state of the material
and the effects of long-term cumulative effects need to be considered. The biocompatibility studies of LMs
acting on neurons and neural tissues are still relatively few and need further validation.

PREPARATION OF LM NEURO-ELECTRODE/CONDUIT
Currently, LMs can be easily fabricated into micrometer-sized neuro-electrodes through various
preparation methods. When performing microfabrication, attention needs to be paid to the resolution and
precision of the patterned structures based on two important criteria: the smallest size that can be fabricated
and the roughness of the line edges [75,76] . The following section describes the four main methods and the
precision of each electrode.