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Jan et al. Soft Sci 2024;4:10 https://dx.doi.org/10.20517/ss.2023.54 Page 3 of 12

Furthermore, the device was utilized for harvesting the waste energy from everyday activities, such as
walking and jogging, producing peak-to-peak voltages of 18.3 and 57.4 V during these motions.

EXPERIMENTAL
Materials
The triboelectric materials employed for this study included PMMA (950PMMA A7, K1 solution, Korea)
and commercial PTFE films of thickness 100 μm (Marhynchus, China). PDMS (Sylgard 184, Dow Corning
Corp) served as the backbone material for the TENG due to its flexibility and biocompatibility, providing
the necessary structure for the device. The sputter targets, Cu (diameter = 57 mm, thickness = 1.5 mm) and
ITO (diameter = 57 mm, thickness = 1 mm), were purchased from Vacuum Thin Film Materials, Korea.
Pressure-sensitive tape (Scotch™ Magic Tape 810) was used as a mold on a glass substrate to control the
thickness of PDMS. The commercial silver paste (ELCOAT P-100) was used to connect the tin-plated
copper tape terminals (3M 1183) to the electrodes.

Characterization and measurement
The surface modification of PDMS was obtained by reactive ion etching RIE (VITA, Femto Science),
allowing for targeted modification of surface properties. A Sputter Coater (Q300T D Plus, Quorum Tech)
was used to deposit Cu and ITO to create ITO/Cu electrodes. The surface morphology analysis was
conducted utilizing advanced electron microscopy techniques, specifically scanning electron microscopy
coupled with energy dispersive X-ray spectroscopy (SEM-EDX), using a Hitachi SU-6600 instrument at an
operating voltage of 15 kV. To investigate the electrical response of TENGs as a function of load, frequency,
and the gap between upper and lower surfaces, we used a custom-designed tapping device. Tapping was
then conducted by adjusting displacement and frequency through a Z-axis motorized stage, controlled by
modifying the amplitude and period of sinusoidal waves. Current measurements were meticulously
obtained using a high-precision parameter analyzer (Keithley, 4200A). Additionally, capacitor charging
voltage measurements were performed by monitoring voltage fluctuations during mechanical tapping. The
data was accurately recorded and analyzed using an oscilloscope (Keysight, DSOX1202G). All the electrical
measurements were taken at ~60% relative humidity and ~25 °C. Following the acquisition of voltage data
through the oscilloscope, a crucial step in our methodology involved post-processing to ensure data
accuracy only for the applications involving subtle voltages such as finger bending and wrist bending. To
enhance the clarity and precision of the obtained data and discern differences more accurately, a 60 Hz low-
pass filter was meticulously applied to filter out electrical noise, refining the figures and aiding in a more
nuanced analysis.

Fabrication of LF-TENG
The presented LF-TENG consists of two triboelectric layers. A 100 μm thick PTFE film was selected as the
[34]
tribo-negative layer owing to its high electronegativity stemming from the presence of fluorine atoms .
Meanwhile, a ~1.2 μm thick PMMA layer was utilized as the tribo-positive layer due to its excellent
[35]
mechanical and wear resistance properties . A ~300 μm thick flexible PDMS layer served as the structural
backbone for the tribo-positive PMMA layer.

The fabrication process of LF-TENGs is illustrated in Figure 1A. First, the PDMS base (Sylgard 184A, Dow
Corning Corp) and curing agent (Sylgard 184B, Dow Corning Corp) were mixed at a weight ratio of 10:1.
The PDMS mixture was degassed and cured at 65 °C for 2.5 h on a glass mold made with a pressure-
sensitive tape (Scotch™ Magic Tape 810) to control the thickness of the PDMS film. A 300 μm thick
20 × 20 mm PDMS film was peeled off from the glass mold and treated with oxygen (O ) plasma in a
2
2
reactive ion etching (100 W, 20 sccm) chamber for 50 s to obtain the wrinkle-like nanostructures on the
PDMS surface. These wrinkle-like nanostructures enhanced the adhesion of electrodes on the PDMS