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droplets, they have been developed as self-healing, printable, permeable, or rewritable stretchable
conductors. Despite their potential benefits, the field of LM-based composites remains relatively unexplored
compared to carbon and metal-based nanocomposites. This is due to several challenges, including
controlling the size of LM droplets, improving performance reliability, and preventing LM leakage. More
research is needed to address these challenges and realize the full potential of LM-based nanocomposites for
high-performance device applications.

BIO-INTEGRATED WEARABLE SENSORS BASED ON NANOCOMPOSITES
The conductive and stretchable nanocomposites mentioned above can be utilized in numerous applications,
such as stretchable interconnects [139,140] , wearable heaters , triboelectric nanogenerators (TENGs) , and
[141]
[142]
[143]
skin-mounted sensors . Most studies have focused on developing on-skin biosensing devices because the
nanocomposites exhibit low Young’s moduli, similar to that of human skin. These soft biosignal recording
devices can adhere to the target parts of the human body with minimal mechanical mismatch and effectively
record biosignals with high SNRs for long-term periods .
[144]
In this chapter, we review the application of nanocomposites in the fabrication of wearable biosignal
recording sensors. The sensors are categorized into four different types: electrophysiological sensors, strain
sensors, pressure sensors, and biochemical sensors. Electrophysiological sensors include EEG, ECG, and
EMG sensors that require high SNRs for accurate signal analysis [145,146] . Strain and pressure sensors are
applied to various parts of the body to read body motions or perceive external stimuli, of which their
performance, such as high sensitivity, should be stable even under repetitive mechanical
deformations [147-149] . Biochemical sensors, which typically utilize sweat to detect and quantify the
concentrations of target biomolecules, such as glucose, also need to maintain high selectivity and sensitivity
even under dynamically deformed situations . Table 2 summarizes some research in recent years on the
[150]
wearable sensors based on soft conductive nanocomposites, including their material components, sensing
mechanisms, and sensing performances.

Electrophysiological sensors
Electrophysiological signals are essential for controlling our body and maintaining our health, as they
represent the electrical activity of organs, such as the brain, heart, and muscles. To record these signals for
healthcare, clinical, and research purposes, various wearable electronic devices, such as EEG, ECG, and
EMG sensors, have been developed. Among them, soft conductive nanocomposites and their devices are
particularly promising to overcome the limitations and challenges of the traditional rigid wearable
electrophysiological sensors. Unlike rigid electrodes, devices made of soft nanocomposites can make
[151]
conformal contact with skin tissue, avoiding air gaps that could result in high electrical impedance and
poor signal quality (i.e., SNR). The amplitude of electrophysiological signals collected through the skin is
[55]
typically small, making them vulnerable to mechanical and electrical noise . Therefore, conformal contact
is crucial to reduce noise levels and improve the signal amplitude.

Researchers have explored different approaches to ensure conformal contact of devices with the skin. One
effective method involves utilizing intrinsically soft organic materials instead of rigid metal materials to
maximize softness . Another key technique is to utilize the ultrathin thickness of the sensor, which lowers
[152]
the stiffness and enables seamless integration with the curved skin . Additionally, the application of
[153]
conductive inks directly onto the skin enhances adhesion, avoiding air gaps .
[154]
Operational stability is another important factor to consider when fabricating electrophysiological sensors
for long-term use . Optimizing the mechanical properties so that the sensor can just withstand up to the
[155]