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Page 14 of 27 Kim et al. Soft Sci 2024;4:24 https://dx.doi.org/10.20517/ss.2024.09

Electrical power sources
The ultimate objective of innovative wearable systems is to emulate real human skin, achieving
imperceptibility and high comfort levels. However, the electrical wiring between the device and the external
[65]
power supply is still regarded as a significant restriction for user movements . To resolve this issue, several
types of energy harvesters have been reported to attain self-powered electronic systems without any external
power sources such as solar cells, thermoelectric power generators, and piezoelectric nanogenerators
[66]
(NGs) .
In particular, triboelectric NGs (TENGs) have been spotlighted as a novel energy-harvesting technology due
to their simple device structure, easy/fast fabrication processes, inexpensive materials, and high-power
generation . Figure 7A shows four fundamental TENG working modes: contact-separation, contact-
[67]
[68]
sliding, single-electrode, and freestanding triboelectric layer modes . The TENG, consisting of two types of
materials, generates electrical power through the charge redistribution, temporarily or continuously creating
a potential difference through contact and subsequent separation (contact-separation mode) or sliding
motion (contact-sliding mode), respectively. Conversely, the TENG with single triboelectric material makes
a potential difference with a grounded or oppositely charged object (single-electrode mode). In the
freestanding triboelectric layer mode, the device generates electricity by utilizing a generated electric
potential difference across the device with deformation. The TENG working mechanism combines contact
electrification and electrostatic induction. The polarized triboelectric charges contribute to the voltage
V (x), which depends on distance x. The transferred charges Q will also influence the electric potential
OC
difference. If the device has no triboelectric charges, it acts as a typical capacitor which is affected by
capacitance and charges between the two electrodes, denoted by -Q/C(x). Thus, the total voltage difference
V is determined by:

which is the fundamental relationship among Q , C, and V OC [69] .
SC
Under the short circuit (SC) conditions, the transferred charges (Q ) are completely generated from
SC
polarized triboelectric charges where V is 0:

Gai et al. reported a self-powered wearable sweat analysis system (SWSAS) by hybridizing TENG and an
electromagnetic generator (EMG), which harvested electrical energy from human motion through two
hybrid NG modules (HNGMs) . The SWSAS selectively monitored biomarkers of Na and K in human
[70]
+
+
sweat, wirelessly transmitting the sensing data via a Bluetooth module. By applying external force, the
polytetrafluoroethylene (PTFE) film at the TENG center contacts and makes sliding friction with both sides
of the nylon films, resulting in negative charge accumulation due to the difference in triboelectric polarity.
On the other hand, the surface of the nylon film accumulates an equivalent number of positive charges
[Figure 7B (i-iii)]. The repetitive, vertical movement of the PTFE film generates an electron flow due to the
potential difference, resulting in a continuous alternating current (AC) output [Figure 7B (iv)]. Figure 7C
shows the working principle of the EMG. Initially, the magnet is fixed to the upper coil, showing no current