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Page 16 of 34 Bai et al. Soft Sci 2023;3:40 https://dx.doi.org/10.20517/ss.2023.38

[182]
by peeling off the upper and lower cover layers . Apart from spraying, there are other methods to prepare
[183]
planar structures, such as precipitation , filtration [155,180] , etc. [Figure 5C i]. However, with the presence of
oxide films, these methods still require sintering to create the conductive pathways [155,180,183,184] . However,
most sintering methods have certain disadvantages, such as mechanical sintering, which still carries the risk
of leakage, and laser sintering is difficult to scale up . Therefore, it is ideal to avoid the sintering process
[184]
and achieve self-sintering. Chiu et al. used conjugated thiol molecules attached to the Ga surface to form
gallium-sulfur bonds to provide a pathway for electron flow between Ga particles, which inhibited the
[163]
formation of oxide films and avoided the sintering process . Park et al. used HCl and
polyvinylpyrrolidone solution to remove the LM oxide film and stabilize the LM droplets, resulting in a thin
[185]
(10-25 μm) and flat LM layer , as seen in Figure 5C ii.

The flexibility and electrical conductivity of LMs allow them to be used in many sensing devices. As
bioelectrodes, LMs can be closely fitted to the human body and are able to work stably under conditions of
intense movement and high stretching of the electrode material [Figure 5C iii] . The resistance between
[184]
the LMNPs in the plane also changes due to deformation when subjected to stress , which can be used in
[183]
sensors that monitor the movement of human muscles. In addition, surfactants also impart unique surface
properties to the LM. Firstly, some surfactants can react with the targeted substance when combined with
the LM. For example, 4-mercaptopyridine interacts non-covalently with NO on the functionalized Ga
2
metal surface , allowing the detection of NO levels [Figure 5C iv]. Secondly, surfactants also contribute to
[163]
2
the attachment of other substances to the surface of LM droplets, such as biological enzymes or
[186]
semiconductors [155,187] , which allows LMs to form a planar reaction zone on the surface of the substrate with
sweat secreted by the body, gases in the environment, etc. This is certainly an infusive feature for the
functional enrichment of LM sensors. Furthermore, for self-powered sensors with multiple layers of
friction, Yang et al. used a conventional sintering process to achieve material formation and prepared a
composite film of LMNPs by ultrasonic dispersion and mechanical sintering, which was used as a friction
layer with an encapsulation layer of PDMS . The excellent flexibility and conductivity were achieved by
[188]
combining the film as a friction layer with the encapsulation layer of PDMS, and the friction nanogenerator
could achieve up to 268 V open-circuit voltage, 12.06 μA short-circuit current, and 103.59 nC transferred
charge by replicating the PDMS friction layer into the microstructure using different master templates,
which significantly enhanced the frictional charge density.

Nanometer LM embedded elastomer structure (3D)
In recent years, hydrogels have gained increasing interest in many biomedical applications because of their
ability to convert stimuli during bioassay processes into electrical signals [189,190] . Other elastomers, such as
[151]
rubbers, plastic polymers , etc., are also being used more and more widely. To fabricate functional
healthcare sensors, liquid-state precursors of these elastomers usually serve as base solvents to disperse
conductive micro or nanoparticles. Under the effect of post-dispersion treatment, conductive fillers
embedded elastomers are formed.

When LM micro/nanoparticles are embedded into composites with elastomer, the formed structure is
always denoted as LM embedded elastomer (LMEE), where the oxide film on the surface of the droplet
interacts with the polar groups (-OH, -NH , etc. ) of the organic matter, resulting in a stable and dispersed
[191]
2
suspension distribution in the organic matter. Meanwhile, an extremely complex cross-linked network is
formed in the LMEE [Figure 5D i]. For example, LM droplets mixed with acrylic acid (AA) form a double
cross-linked network, both physically and chemically cross-linked [Figure 5D i] . It has also been shown
[192]
that the addition of LM droplets accelerated the crosslinking process . The good flexibility of both the
[191]
matrix and the filler allows LMEE to have excellent tensile properties, even stretching to 2,200% of its length
and still not breaking . The mechanical properties and electrical conductivity of the composite can be
[192]