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Page 16 of 27 Kim et al. Soft Sci 2024;4:24 https://dx.doi.org/10.20517/ss.2024.09

electrodes repeatedly contacted and separated to generate a maximum output power of 33.2 mW
[Figure 7F].

With the increasing requirement of wearable energy harvesters for daily use, Liu et al. demonstrated an all-
[72]
in-one power-generating and sensing system (A-PGSS) based on TENG and flexible solar cell (f-SC) .
Figure 7G shows a structural design of the A-PGSS with two rabbit fur-based TENGs and one amorphous
Si-based SC. When the rabbit fur with good tribo-positivity and low friction resistance moves by external
force, electrons flow to the opposite electrode through an external circuit, maintaining electrostatic balance
between the two electrodes and generating a brief current [Figure 7H (i) and (ii)]. Negative charges
accumulate on the fluorinated ethylene propylene (FEP) membrane surface, while the rabbit fur surface
accumulates an equal number of positive charges. When the rabbit fur slider moves back, electrostatic
equilibrium is disrupted in the fourth stage, causing positive charges on electrode-2 to flow in the opposite
direction, towards electrode-1, resulting in a reverse transient current in the external circuit [Figure 7H (iii)
2
and (iv)]. The fabricated A-PGSS produced a maximum power density of 0.072 W/m (load resistance:
3 MΩ) at a 2.5 Hz contact frequency. Finally, the authors demonstrated the usability of A-PGSS with low-
power conventional signal conditioning circuit (LP-SCC), low-power microcontroller unit (LP-MCU) and
Bluetooth low energy (BLE) under daily life conditions such as daytime, nighttime and emergencies
[Figure 7I].

Biodegradable sensors
E-skins are expected to be mainly used as disposable products. However, considering the environmental
destruction caused by electronic waste, which is a severe issue due to the exponential development of
electronic devices, the development of biodegradability is an essential factor for e-skins in healthcare
[73]
applications . Figure 8A shows biodegradable materials for substrates, electrodes, semiconductors,
dielectrics, encapsulants, and adhesives according to their applications in implantable and non-invasive
[74]
sensors . The biodegradable properties of these materials are suited to monitor temporary phenomena
such as wound healing, aftereffects, and side effects. Veeralingam et al. reported a NiSe -grown cellulose
2
paper with silver paste for a low-cost and disposable sensing platform that can be applied to pH meters,
breath analysis, and strain sensors . The cellulose paper-based flexible substrate was dipped in a seed
[75]
solution with selenium powder, sodium borohydride, and NiCl in deionized (DI) water for about one hour.
2
The seed-coated cellulose paper was transferred to a Teflon autoclave for a hydrothermal reaction at 200 °C
for 20 h. Thereafter, the NiSe /Cellulose paper was dried at 70 °C and integrated with an Ag electrode to
2
realize a breath sensor [Figure 8B]. Figure 8C presents the breath sensing results for one minute. The
current of the sensor increased during exhalation, whereas the device current decreased during inhalation.

Wang et al. developed a highly strain-sensitive, fast-responding, stable, and biodegradable e-skin using silk
fibroin (SF) as a structural basis, combined with a hydrogel and a flexible conductive film electrode . The
[76]
biodegradable e-skin was fabricated with a SF/polyvinyl alcohol (PVA) hybrid film and a SF-based hydrogel.
The SF/PVA film was manufactured with SF/PVA mixed solution using the solution casting method, and
the hydrogel was obtained with horseradish peroxidase (HRP)-blended. Figure 8D illustrates the fabrication
process of the SF-based pressure sensor using SF/PVA hybrid film and SF hydrogel with an enzymatic
crosslinking method. The hierarchical structure of SF enabled the design of elastic hydrogels with resilience
and flexible films with a porous structure with high deformability. The fabricated SF sensor could accurately
sense a wide surface area of pressure due to its variable capacitance depending on pressure. Figure 8E shows
excellent properties of the biocompatible and biodegradable sensor such as a high strain sensitivity of 4.78.
The SF sensor also demonstrates fast responsiveness of < 0.1 s and superior durability for 20,000 cycles of
20% stretching. Figure 8F and G presents an 8 × 8 array of SF-based sensors for mapping the pressure by
gathering the capacitance change of each unit sensor. The detailed vertical and horizontal pressure data was
used to generate a mapping image of the pressure distribution in 3D [Figure 8G].