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Bai et al. Soft Sci 2023;3:40 https://dx.doi.org/10.20517/ss.2023.38 Page 11 of 34

[155]
conduction law , whereas conventional Ag/Pd, on the other hand, tends to form Schottky junctions with
the sensor, which will indirectly affect the charge flow. In addition, it is also possible to directly cover the
surface of LMNPs with semiconductors and form good contacts. WO NPs on the surface of LMNPs can
3
break the oxide layer and make direct contact with the liquid core, thus achieving ohmic current-like
characteristics at the interface as well, and the sensitivity of the sensor is enhanced due to the better charge
[156]
transfer efficiency at the WO /EGaIn interface , as shown in Figure 4A iv.
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Furthermore, the good deformability of the LM as a conductive material for the sensor also allows for long-
term resistance stability [Figure 4A v]. The normalized resistance of conductive materials filled with EGaIn
microparticles can remain well stabilized during 2,500 stretches of 0% to 80% because of the 3D LM
[157]
conductive network formed . Another idea is to combine LM with some highly conductive nanomaterials.
For example, when EGaIn microdroplets come into contact with nano-Ag flakes, the Ag flakes will cover
the surface of the droplets and react to produce AgIn ; a strong connection makes the material stretch so
2
that it can cope with the deformation , thus achieving a stable resistance compatible with flexibility.
[158]
Besides, it is also possible to use the breakage of the LMNPs and use the fluidity of LMs to repair the circuit
[159]
to achieve the tensile stability of the material’s resistance [Figure 4A v] .

Chemical transformation
Sensor preparation
Regardless of the presence of oxygen in water, gallium can react with water to generate GaOOH
microcrystals [Eq. (1)] , such as LMNPs with surface-modified graphene quantum dots, which can
[160]
promote the generation of GaOOH on the surface under light control, and can control the morphological
transformation of LMNPs to realize the light-responsive LM “nano-transformer” [Figure 4B i]. In addition,
the surface generation of GaOOH microcrystalline film will slightly reduce the passivation response of GaO
generation. During the preparation and use of LM nanomaterials, the GaOOH on the LM surface should be
continuously destroyed and generated to obtain LM droplets with smaller particle sizes and prevent particle
fusion, while the storage of LMNPs can be optionally placed in organic solvents such as pure ethanol to
avoid the formation of GaOOH.

The GaOOH microcrystals generated from gallium will again react reversibly with water and ionize to form
[161]
Ga [Eq. (2)] , a metal cation usually cross-linked as a ligand through its surface charge. For example, Xu
3+
3+
et al. used gallium-based LMNPs to generate Ga ions in the presence of H O and O under the action of
2
2
3+
ultrasonic waves and formed a cross-linked PAA shell layer through the complexation of Ga and -COO-,
and this whole shell layer can be regarded as the “colloidal cross-linker” of the hydrogel [Figure 4B ii] . If
[161]
3+
the PAA network is disrupted, it will promote the production of free radicals and Ga , and the free radicals
and Ga will cross-link again to achieve self-healing.
3+

As discussed before, conductive pathways cannot be achieved for relatively independent individual LMNPs.
Except for physical sintering, a chemical approach could also be applied to eliminate the oxide barrier. e.g.,
Li et al. performed rapid removal of surface oxides in HCl fume, brief chemical etching in HCl fume to