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Page 10 of 20 Li et al. Soft Sci 2023;3:37 https://dx.doi.org/10.20517/ss.2023.30

[113]
transmission line as distributed probes for sensing multimodal deformations, as shown in Figure 5F . In
this sensor, soft transmission line probes were integrated into a piece of stretchable fabric, and a custom
pulse generator was used in conjunction with a standard laboratory oscilloscope to induce time-domain
reflection. When the transmission line is pressed, steps of signal appear in the waveform and peaks equal to
the number of pressed points emerge in the distributed resistance profile, the height of which corresponds
to the applied pressure. These points can then be accurately positioned on the spatial map.

Strain Sensors: Similar to LM pressure sensors, sensors based on the change of resistance [114,115] ,
[117]
[116]
capacitance , and resonant frequency have been employed to detect strain. For LM-filled fiber and
microchannel strain sensors, LM elongates along the strain direction, leading to an increase of resistance
2
that conforms to the equation R = R (1 + ε) (R : initial resistance, ε: strain, R: the resistance corresponds to
0
0
ε). For most applications, these types of strain sensors are integrated into wired/wireless gloves to detect
hand gestures by monitoring the output voltage or resistance in real-time [25,26,48,87,114] . Tang et al. developed a
layer-by-layer fabrication method for integrating strain sensors with a multilayer electronic transfer tattoo
that can be stretched up to 800% strain and conformably attached to the skin [Figure 6A] . This tattoo can
[118]
amplify the output signal of integrated strain sensors by three times and achieve the monitoring of hand
movements in real time [Figure 6B]. In addition, Dong et al. proposed a fiber strain sensor with a surface
[92]
texture and six LM electrodes to realize the breath monitoring based on self-powered sensing [Figure 6C] .
The strain sensor consisted of fiber extremities fixed to the two ends of a longer stretchable belt. During
respiration, the movement of the abdomen caused friction between the belt and the fiber, resulting in a
slight relative movement that generated an electrical signal. This signal can be detected to evaluate the
inward and outward movements of the upper chest and abdomen [Figure 6C iv].

The gauge factor (GF), which reflects the sensitivity of the LM-based straight channel strain sensor, is
relatively low (below 5 under 100% strain). Theoretically, the relatively low average GF of a straight-channel
strain sensor, as described by the formula GF = ε + 2, fails to meet the criteria set for commercial wearable
strain sensors. To increase the sensitivity, Kramer et al. introduced a curvature sensor with a hollow
structure, in which an embedded strut can exert pressure on the microchannel during bending, leading to a
[119]
greater change in resistance . Moreover, a nacre-inspired and LM-based ultrasensitive strain sensor by a
[94]
spatially regulated cracking strategy has been developed [Figure 6D i and ii] . The biphasic pattern (LM
with Cr/Cu underlayer) acts as “bricks”, and strain-sensitive Ag film acts as “mortar”. Compared to LM
conductors, this strategy allows the conductive pathways to form a certain number of cracks under strain
[Figure 6D iii]; thereby, the sensitivity was increased by two orders of magnitude [Figure 6D iv]. In
addition, Li et al. designed a highly sensitive LM strain sensor based on strain redistribution and 3D-
structured circuit strategies [Figure 6E] . In the middle of the strain sensor, a high-elastic-modulus cuboid
[120]
elastomer (E650) is winded with the Ga-10In solid wire [Figure 6E i]. When the sensor is stretched, the
cuboid elastomer squeezes the LM channel to reduce its cross section, further increasing the sensitivity
[Figure 6E ii]. The average GF of 100% strain is improved more than 400 times compared with the
previously reported 2D LM strain sensor [Figure 6E iii]. Generally, the channel structure design strategy and
strain redistribution strategy can lead to a more significant resistance change under a small strain.

Temperature sensors: Temperature is a critical and fundamental parameter to evaluate human health. The
volume expansion of fluid in a closed pipe correlated to a specific temperature value is a commonly used
principle in thermometers, as seen in the traditional mercury thermometer. In contrast to mercury-based
thermometers, the preeminent merit of Ga-based thermometers for wearable biosensors is their neglected
toxicity. This remarkable trait, coupled with their supercooling performance, empowers Ga-based
thermometers-founded upon the principle of volume expansion-to reliably operate in environments as