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Ma et al. Soft Sci 2024;4:26 https://dx.doi.org/10.20517/ss.2024.20 Page 11 of 34

the peel stress during the transfer process, could fabricate an ultrathin device. The fabricated ultrathin strain
sensor (~30 μm in thickness) exhibited a high sensitivity of 2,425 (GF) in a wide strain range from 0% to
115%. Moreover, the developed sensor could monitor on-body strain and was suitable for broad, intelligent
healthcare, including pulse wave monitoring, vocal sound detection, body movements perception, and
translation of American sign language.

Meanwhile, to tackle the mechanical shortcomings in LIG’s transfer process to elastomers, Lu et al.
proposed a cryogenic transfer strategy (at -196 °C) with a super thin (thickness of 1.0-1.5 μm) and adhesive
[27]
polyvinyl alcohol-phytic acid-honey (PPH) hydrogel [Figure 6H]. The fast-cooling process could improve
the interfacial binding strength between porous LIG and crystallized water inside the hydrogel. The
interfacial PPH hydrogel layer could be an energy dissipation layer and out-of-plane conductive paths.
There were electrically consistent deflected cracks in the developed LIG composites, resulting in an
enhancement in stretchability from approximately 20% to 110%, with a further increase to approximately
220%.

Real-time monitoring of human temperature achieved by wearable soft skin electronics is vital in
personalized mobile health monitoring. Based on the thermal response property of LIG, researchers have
developed different kinds of soft temperature sensors [35,91,94] . For instance, as illustrated in Figure 6I, Gandla
et al. reported a LIG-based soft temperature sensor featuring high stability and linearity, realizing real-time
[35]
recording of skin temperature via the wireless platform for intelligent health management . The proposed
temperature sensor was a standalone system composed of LIG-based temperature, battery, signal processing
circuits, and wireless modules that could be easily attached to human skin surfaces. The fabricated device
showcased a negative temperature coefficient of resistance (TCR) of 0.00142 °C and stable performance
-1
even under complex environments. Benefiting from its excellent performance, the proposed temperature
sensor can manage human health, such as breathing, touching with a finger, and blowing by mouth.

As a crucial physical indicator influencing our daily lives, humidity means the quantity of water in the
atmosphere. It can be employed to monitor respiratory activities in a non-contact manner, facilitating the
prevention and treatment of infectious respiratory diseases. Soft humidity sensors can be easily attached to
curved surfaces, realizing various applications. The LIG has been widely utilized to develop humidity
sensors because of the simple fabrication process, porous structures, good conductivity, and strong chemical
stability [38,95,96] . For example, as illustrated in Figure 6J, Lan et al. reported a flexible and wearable humidity
sensor based on a LIG-based interdigital electrode prepared by a laser direct engraving technology, which
was convenient, effective, and robust . Besides, due to unique 2D structures and super permeability to
[38]
water molecules, GO was integrated as a humidity sensing element, which improved the humidity sensing
performance of the device. The fabricated humidity sensor exhibited remarkable sensitivity [3,215.25 pF/%
relative humidity (RH)], high stability (variations < ±1%), and low hysteresis. The noteworthy performance
of the humidity sensor realized various applications, including non-contact humidity detection and human
breath recording.

Electrophysiological monitoring
Benefiting from its electrical conductivity, the LIG can be used as a superficial electrode for
electrophysiological signal recording, including electrocardiography (ECG) and electromyography (EMG).
For instance, Dallinger et al. fabricated a LIG/MPU-based EMG sensor, which consisted of a pair of circular
electrodes, connecting wires, and vertical interconnect access (VIA) for external wiring [Figure 7A]. The
[34]
developed EMG sensor was ultrathin, realizing outstanding conformal integration on the skin surface,
excellent stretchability, and inelegant breathability. The subject wore the EMG sensor for three consecutive