Eco‑Friendly Starch‑Paper Triboelectric Nanogenerator for Real‑Time Human Sweat Sensing

Introduction

Flexible electronics that are stretchable, compact, and biodegradable are rapidly emerging as the backbone of disposable bio‑sensors, smart packaging, and wearable devices. Their biocompatibility and environmental friendliness have attracted significant attention, prompting research into substrates that can be safely degraded after use. Traditional power supplies for wearables rely on non‑biodegradable batteries, creating a bottleneck for truly sustainable systems. Triboelectric nanogenerators (TENGs) convert ambient mechanical motion into electrical energy through contact electrification and electrostatic induction, offering a self‑powered alternative. While many TENGs use polymers that resist decomposition, starch—a cheap, abundant, renewable carbohydrate—provides a promising, fully degradable substrate. In this study, we present a simple, disposable S‑TENG constructed from starch paper that can harvest mechanical energy and simultaneously monitor human sweat, demonstrating its potential for eco‑friendly wearable electronics.

Method

Assembly of the S‑TENG

The starch paper (thickness ~1 mm) was sourced from GILRO Corp. (Israel). One face of the sheet was bonded to a metallic wire and then exposed to water vapor, forming the active triboelectric layer. Figure 1 illustrates the rapid, cost‑effective fabrication sequence.

Schematic of the S‑TENG assembly process

Measurements and Human Demonstration

Electrical characteristics were recorded on a DSOX6004A digital oscilloscope. A 4.4 × 4.4 cm² S‑TENG was positioned on a human elbow, with the metal electrode facing the skin. Output signals were captured during various periods of elbow motion.

Results and Discussion

The S‑TENG operates by coupling the human hand with the starch paper. When contact occurs, the paper acquires negative surface charges while the hand becomes positively charged. Upon separation, the imbalance drives electrons toward the grounded electrode, generating a measurable voltage. Figure 2 illustrates this mechanism.

a A water film starts to form around the electrode side of the starch paper, b a water network is formed, c the working mechanism of the S‑TENG

Two operating regimes emerge based on the moisture level. In the first regime, a thin water film partially traps charges, creating a potential barrier that limits carrier movement. In the second regime, a continuous water network reduces the internal resistance dramatically, boosting the output voltage. The output voltage rises with increasing water vapor until it plateaus once the network fully forms.

A photograph of the fabricated S‑TENG is shown in Figure 3a. Using an adjustable resistor as a load, we recorded the voltage response for different water‑spray intervals. Figure 3b demonstrates that as the load resistance increases from 100 kΩ to 100 MΩ, the open‑circuit voltage climbs steadily, peaking at 11.2 V. The maximum power is achieved at a load of ~15 MΩ, indicating an internal resistance of roughly the same value. Stability tests (Figure 4) reveal only a marginal voltage drop under repeated vertical loading.

a Photograph and b electronic output of the fabricated S‑TENG

Vertical force test of fabricated S‑TENG. The output voltage decreases only slightly under a loading resistance of 100 MΩ.

The sheet resistance of the starch paper decreased from 19 MΩ (dry) to 130 kΩ after seven water sprays, reflecting the progressive formation of conductive pathways. Figure 5 shows the corresponding voltage evolution: the output rises during the first three sprays (regime 1) and saturates thereafter (regime 2). This clear correlation between moisture content and electrical response confirms the device’s suitability for sweat detection.

Dependence of the output voltage on the number of water spray steps

To validate sweat sensing, the S‑TENG was attached to the elbow after different durations of motion. The skin was cleaned with a dry towel before each test to remove residual sweat. The voltage recorded under a 100 MΩ load mirrored the trend observed in Figure 5, demonstrating that the device can quantify perspiration and monitor motion time in real‑time.

a, b working pattern for harvesting human elbow motion energy, c electronic output (load 100 MΩ) versus human motion time

Degradability was assessed by immersing the starch paper in tap water under gentle agitation. As shown in Figure 7, the paper fully disintegrated within four minutes, confirming its fully biodegradable nature.

a Degradability tests performed by dipping starch paper in water, b immediately, and after c 2, d 3, and e 4 min

Conclusion

We have introduced a rapid, low‑cost method for fabricating a disposable, biodegradable TENG from starch paper. The device not only harvests mechanical energy but also provides a reliable, real‑time readout of human sweat, making it an attractive platform for sustainable wearable electronics. Its complete dissolution in water within four minutes further emphasizes its eco‑friendly profile.

Abbreviations

TENG:

Triboelectric nanogenerator