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Page 8 of 20 Li et al. Soft Sci 2023;3:37 https://dx.doi.org/10.20517/ss.2023.30

composites and flexible substrates could be stronger. For example, circuits prepared by LM-
Polydimethylsiloxane (PDMS) composites can be integrated into PDMS substrates after curing, which can
significantly improve the working reliability. Since LMs are dispersed in the elastomer in the form of
microparticles, the risk of bulk LM leakage can be reduced. Similar to LMNP inks, LM-elastomer
interconnects should be activated by additional processes such as pressing or freezing [85,87] . By adding some
[91]
rigid materials, such as copper [88,89] , silver [86,90] , and nickel particles, in an LM-elastomer composite, the
interconnects can be actively activated through the extrusion effect of rigid particles when it is subjected to a
stretching state. Attributed to the non-leaking performance and excellent biocompatibility of LM-elastomer
composites, we believe that interconnects prepared using LM-elastomer composites have broader prospects
in wearable electronics.

Wearable sensors
Wearable sensors affixed to garments or directly onto the human skin have emerged as crucial tools for real-
time activity monitoring. Wearable devices necessitate highly reliable sensors that can conform to the
curvilinear surface of the human body with minimal discomfort. LM-based materials are outstanding
candidates for preparing wearable sensors due to their fluidic nature and biocompatibility. Due to the
fluidity of LMs and low elastic modulus of encapsulating polymers, LM channels inside the polymer can be
deformed upon the application of external forces, inducing the change of electrical signals. LM-based
flexible and wearable sensors have recently demonstrated immense potential for various applications,
[93]
including healthcare monitoring [23,92] , disease early warning , motion detection [26,62,94,95] , and soft robotics .
[96]
These sensors, particularly stretchable and skin-mountable variants, can function as pressure, strain, optical,
and temperature sensors, as elaborated in detail below.

Pressure Sensors: Wearable LM-based pressure sensors commonly use resistance or capacitance-based
detection methods to sense pressure changes. Most LM pressure sensors detect the change of resistance. The
LM encapsulated within a fluid channel deforms when an external force is applied to it
[Figure 5A and B] [79,97] , leading to the change in cross-sectional area. Consequently, the resistance of the
sensor changes. For capacitive-based pressure sensors, the sensing mechanism mainly depends on detecting
the change in the thickness of the dielectric layer or the overlapping area between the LM and the
underlying electrode [98-101] . The utilization of LM pressure sensors in devices for detecting pulse rate,
footsteps, and tactile feedback has shown great potential in biomedical applications [Figure 5C] [23,25,102-105] .

The sensitivity of a sensor is a critical parameter for sensing applications, and research has demonstrated
that the Poisson’s ratio of the material, the thickness of the top layer, and the dimension of microchannels
all play significant roles in the sensitivity of microchannel-based pressure sensors [106,107] . The pressure
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-3
sensitivity of previously reported LM-based pressure sensors is within the range of 0.2-80 × 10 kPa , and
such sensitivity is not optimal due to the relatively thick elastomer dielectric layer . Therefore, strategies,
[103]
such as microchannel cross-sectional geometry design for enhancing the penetrating effect of channel
base , ‘S’-shaped microfluidic structure design with a higher and sharper deformation profile , and
[102]
[108]
[23]
Wheatstone bridge circuit design , have been proposed to improve the sensitivity. Kim et al. introduced
3D-printed rigid microbumps on the top of an LM microchannel that can offer extremely high sensitivity
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(0.158 kPa ) compared with traditional straight channel LM pressure sensors (0.2-80 × 10 kPa ), as shown
-1
in Figure 5D . Furthermore, a low-cost and highly-sensitive LM pressure sensor for gastrointestinal
[103]
manometry was prepared to refer to the quipu-knotted strings, as shown in Figure 5E . By simply typing
[109]
knots on the EGaIn-filled fiber, a small pressure of less than 50 kPa can be detected, while no signal can be
detected when the same pressure is applied to the unknotted region. The enhanced sensitivity is attributed
to the amplification of the effective total pressure induced by the folded and stacked channel layers. For
application demonstration, the designed ribbon-like manometry device with eight knots can clearly record