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Li et al. Soft Sci 2023;3:37 https://dx.doi.org/10.20517/ss.2023.30 Page 3 of 20

state metals, such as mercury (Hg), rubidium (Rb), cesium (Cs), and francium (Fr). This is due to their
unique physical properties, such as relatively stable chemical properties in air and water, low melting
[15]
[30]
point , high electrical/thermal conductivities [28,29] , low viscosity [28,29] , and low toxicity . Pure Ga alloys with
a melting point of 29.8 °C are solid-state at room temperature. The addition of In and Sn alloys through
alloying can reduce the melting point (e.g., the melting point of Galinstan with 20.5 wt.% In and 12.5 wt.%
Sn is 11 °C). One important characteristic of Ga-based LMs is their low crystallization temperature, which is
significantly lower than their melting point. This phenomenon is known as the supercooling effect [31,32] . The
supercooling effect enables Ga-based LMs to remain in a liquid state at temperatures far below their melting
point, thereby expanding the working temperature range for LM-based flexible and wearable electronics.
This feature is critical for ensuring the working stability of these devices. In addition, viscosity is one of the
essential parameters for patterning bulk LM into specific 2D/3D structures. The viscosity of Ga-based LMs
[33]
is comparably low, approximately twice that of water . The viscosity of LMs can be adjusted through the
alloying process. For instance, the viscosity of eutectic Ga-In (EGaIn) is 1.99 × 10 kg/m/s, which is lower
-3
-3
than pure liquid Ga (2.04 × 10 kg/m/s).
Toxicity: Unlike mercury, Ga-based LMs can remain stable at room temperature, attributed to their
negligible vapor pressure and solubility in water . Although the toxicity of LMs remains controversial,
[19]
more recent reports suggest that LMs are relatively safe for biological applications. Chitambar and
White et al. have summarized the medical applications, toxicities, and health impacts of Ga and its
compounds [34,35] . Ga-based LMs are relatively safe to be applied to medical treatment. For example, LM-
printed electronics applied to the skin for tumor-treating field therapy exhibit no obvious side effects .
[36]
EGaIn/calcium alginate hydrogel can be used as a candidate for endovascular embolization and tumor
embolotherapy with low toxicity . Furthermore, Ga-based LM electrodes showed no adverse effects on the
[37]
growth of neurons within the culture platform and were able to effectively simulate target neurons . The
[38]
mechanism of the low toxicity of LMs remains elusive, and it might relate to the following factors. On the
one hand, Ga-based LMs can be degraded in body fluid, and the metabolites of LMs can be excreted to the
outside of the body in the manner of both fecal and renal excretions . On the other hand, for wearable
[39]
flexible electronics, LMs are ordinarily encapsulated within biocompatible elastomers. Indirect contact
between LMs and tissue further ensures the safety of biological applications. However, studies about the
toxicity of LMs are unsystematic, and more rigorous research should be applied to understand the influence
of LMs on human health. Nevertheless, it is essential to continue studying the toxicity of Ga-based LMs and
to exercise caution during their fabrication and application processes [15,40] .

Oxidation and Wettability: Similar to other metals such as aluminum, magnesium, and copper alloys, a
self-limiting growth Ga O oxide layer immediately forms on the surface of LMs when the oxygen
2
3
[41]
concentration is higher than the ppm level . The thickness of the oxide layer is associated with the oxygen
concentration (~0.7 nm under vacuum conditions and much thicker at ambient conditions) [42-44] . It is elastic
and stable under critical yield surface stress (~0.5-0.6 N/m). Only when the stress is above the yield strength,
[44]
the oxide layer will rupture and the fresh LM flows out . Attributed to the oxide layer, the wettability and
surface tension of LM droplets are significantly altered. Furthermore, oxidizing LMs is an important
method to control the wettability for patterning circuits on different substrates [45-47] .

LMNP ink
Although Ga-based LMs possess tremendous advantages for the fabrication of flexible and wearable
biosensors, the ability to fabricate biosensors using high-efficiency preparation methods remains
constrained by their high surface tension [44,47] . Besides, directly using raw LMs as the patterning material
leads to high resource consumption, which will increase the costs and limit their market competitiveness .
[48]