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Bai et al. Soft Sci 2023;3:40 https://dx.doi.org/10.20517/ss.2023.38 Page 17 of 34

regulated by controlling the content of the NP. When the content of LMs is low, a complete conductive
network cannot be formed in the elastomer. However, the distance between the LMNPs changes after
applying stress, which causes a change in the resistance of the material. Based on this mechanism, LMEE
can be used for strain-resistance type sensors [Figure 5D ii] [154,193,194] . In addition, the good flexibility and
electrical conductivity of the composite material also facilitate the detection of bioelectrical signals from the
[192]
human body , and the abundance of polar groups allows LMEE to form rich hydrogen and coordination
bonds with human skin and thus obtain self-adhesive properties [Figure 5D iii] [190,192,195] . LMEE could also
[196]
exhibit good self-healing properties by choosing appropriate substrates , e.g., LMNP/poly(3,4-
ethylenedioxythiophene):sulfonated bacterial cellulose nanofiber (LMNP/PEDOT:BCNF) suspensions with
AA can re-establish ion and electron transport channels within 0.16 s [Figure 5D iv]. A new conductive
[197]
sensor using LMNPs/MWCNTs/CNFs as multiphase hybrid fillers has also been prepared by incorporating
a variety of highly conductive materials at the slight expense of the stretchable properties of the sensor ,
[159]
combining the high aspect ratio of CNTs with the fluid properties of LMs [Figure 5D v], which can
reconstitute new connections for CNT/CNFs when subjected to large stresses within the oxide layer. In
addition, the material exhibits excellent stress-strain-resistance sensitivity under low-strain conditions.

APPLICATION CATEGORIES
Motion monitoring
Movements of limbs are routinely adopted by hospital patients, especially for elderly individuals in
recovery. The use of sensors for long-term monitoring of these behaviors can save significant medical
resources and facilitate research data for patient recovery. The thin and sensitive nano-LM sensors can
achieve monitoring and sensing without hindering human movement, which will greatly ameliorate the
user experience. For instance, Zhang et al. produced an LM@CNC Janus film through a facile
ultrasonication-deposition process and activated conductive paths on the LM side to realize a capacitive
sensor . The capacitive sensor was sensitive enough to detect the bending of the palm [Figures 6A and B],
[172]
and the sensor showed good stability during 150 cycles of bending tests [Figure 6B]. The LM Janus
membrane was made by using the sink of LM droplets, while LMs tend to be stably distributed in the
suspension in more studies. For example, Feng et al. used a variety of interactions (chemical bonding,
hydrogen bonding), the fabricated double cross-linked network cellulose-based LM hydrogel (LMCNF), as
shown in Figure 6C, could be used to monitor the movement of human limbs , the relative resistance
[198]
increased as the joint flexed. In addition, the sensor also has good sensitivity with a sensitivity factor of
2.92 over a strain range of 0% to 400% [Figure 6D]. Apart from direct monitoring of muscle
movement, Liu et al. also reported a sensor capable of monitoring human breathing based on a
polycaprolactone@LMNP thin film . The film was placed onto an N95 mask along with a Bluetooth
[199]
device to demonstrate the breath-monitoring function. The film was deformed by the airflow inhaled or
exhaled by a volunteer, thus causing fluctuation in its resistance [Figure 6E]. To improve the
biocompatibility of the sensors, using substances from human tissues as a matrix is a feasible solution.
The researchers mixed gluten, which mainly consists of proteins, with EGaIn to make an EGaIn/gluten-
based e-skin (E-GES) , which helped the matrix to form more β-sheets due to the fact that EGaIn
[200]
induces new coordination and hydrogen bonds. The β-sheets are the most solid confirmation of gluten;
thus, the increase of their content made the composite highly viscoelastic [Figure 6F]. Moreover, the E-
GES could also be used as a strain gauge to monitor human movement, as bending during movement
increases the resistance of the E-GES, and its superior stability allows the parameters to remain stable
after more than 100 cycles. To further expand the application scenario of the sensor, Chen et al.
realized a non-contact sensor at a longer distance (> 1 m) by simply adding ultrasonically treated EGaIn
to a 3,4-ethylenedioxythiophene (EDOT) monomer solution containing EDOT , polystyrene sulfonate
[201]
and ammonium persulphate to obtain a Ga-PP solution and immersing the textile in the solution to get a
Ga-PP-based conductive textile (Ga-PP-CT), which uses the non-contact interaction between different
charges to change the resistance of the sample [Figure 6G], thus allowing the monitoring of human
movement such as jumping rope or playing tennis.