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Zhao et al. Soft Sci. 2026, 6, 4 Page 3 of 15

EXPERIMENTAL
Fabrication of the triboelectric fibers
Copper wires with a diameter of 200 μm (99.95% purity) served as the conductive core for both triboelectric
materials. For epoxy-coated fibers, the copper wires were uniformly coated with epoxy resin at a controlled
withdrawal speed of 2 mm/s, and then dried using a hot plate with a temperature of 380 °C. For
polyvinylidene fluoride (PVDF)-coated fibers, poly (vinylidene fluoride-co-hexafluoropropylene)
(PVDF-HFP) was dissolved in N, N-dimethylformamide (DMF) at a weight concentration of 25% under
magnetic stirring at 60 °C for 12 h. The copper wires were then dip-coated with the PVDF solution at the
same withdrawal speed and dried at 380 °C to ensure complete solvent evaporation.

Low-pressure plasma surface treatment
Low-pressure plasma treatment was performed using a radio-frequency plasma system (PTL-VM500, PTL
Plasma, Shandong, China) operated at 13.56 MHz. The textile samples were placed in the plasma chamber
and evacuated to achieve a vacuum level of 30-50 Pa. A gas mixture of argon and oxygen (1:1 ratio) was
introduced at a controlled flow rate of 6 mL·min to maintain the desired working pressure. The plasma
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power was set to 300 W, and treatment duration was varied from 15 to 75 min to optimize sensor
performance. All plasma treatments were conducted at room temperature.

Fabrication of the on-mask sensor
The coated wires were helically twisted together using a custom twisting apparatus to form flexible yarns and
subsequently woven into textile structures using a plain weave pattern. The final textile was tailored into a
circular shape with a diameter of 35 mm to fit within the airflow channel of standard pilot oxygen masks.

Surface morphology characterization
Surface morphology of the triboelectric fibers was characterized using a scanning electron microscope
(ZEISS SIGMA 360, Carl Zeiss AG, Oberkochen, Germany). Energy dispersive spectroscopy (EDS) mapping
was performed using the same scanning electron microscopy (SEM) system to analyze elemental
composition changes after plasma treatment. Chemical modifications induced by plasma treatment were
analyzed using Fourier Transform Infrared (FTIR) spectroscopy (PerkinElmer Spectrum 3, PerkinElmer,
Waltham, MA, USA) and X-ray photoelectron spectroscopy (XPS; PHI GENESIS 500, ULVAC Inc.,
Kanagawa, Japan). Spectra were recorded from 4,000 to 500 cm . Three-dimensional surface topography was
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measured using atomic force microscopy (SPM-9700HT, Shimadzu Corporation, Kyoto, Japan). Scan areas
of 5 μm × 5 μm were analyzed for each sample, and surface roughness parameters, including arithmetic mean
roughness (Ra) and root mean square roughness (Rq), were calculated using SPM-9700 Analysis software
(Shimadzu Corporation, Japan).

Electrical measurement and characterization
A low-noise preamplifier (SR560, Stanford Research Systems, Sunnyvale, CA, USA) was employed to
measure the output voltage of the triboelectric sensors. Applied pressure was monitored using a force sensor
(Z2S-DPU-MZ-50N, IMADA, Toyohashi, Japan), while the compression frequency was varied from 0.25 to
2 Hz. Durability testing was performed by applying pressure at a constant frequency of 2 Hz, driven by a
linear motor (WN500TA, Winner Optical Instruments, Beijing, China). Long-term stability was evaluated by
measuring output voltage monthly over a 90-day period under the same testing conditions. Moisture
resistance testing was accomplished by deploying the sensor within the oxygen mask and continuously
wearing it at room temperature for 12 h, during which the electrical performance was monitored.