(Nanowerk Spotlight) The interactions between water and surfaces create small electrical charges – a phenomenon that could power the next generation of wearable technology. Small-scale devices that generate electricity from water movements and evaporation have attracted significant research interest as a potential power source for low-power electronics like health monitors and environmental sensors.
Creating flexible versions of these water-powered generators has proven challenging. Early devices were rigid and would break when bent or stretched, making them impractical for wearable applications. While researchers developed more flexible versions using materials like fabric or sponges, these designs typically produced too little power for practical use or lost functionality when deformed.
Scientists from Northwestern Polytechnical University in China have developed a novel solution by combining paper’s natural water-absorbing properties with precisely placed cuts that allow stretching. Their device, measuring 2.0 centimeters by 3.0 centimeters, generates electricity as water moves through treated paper fibers.
The researchers created their generator by coating paper with carbon black particles – microscopic carbon spheres that conduct electricity – held in place by a small amount of polyvinyl alcohol binder. When water contacts one end of this treated paper, it naturally flows through the paper’s fiber network. The movement of water carries positively charged hydrogen ions (H3O+) through these channels, creating an electrical potential difference between the wet and dry regions.
Characterizations of paper with CBs. a–c) SEM images of paper fibers before and after loading CBs. d) Zeta potentials of CBs@paper (red line) and CBs/PVA@paper (blue line). e) Photographs of CBs shedding before and after PVA modification. f) Photograph of flexible CBs/PVA@paper. g) Schematic of hydrophobic treatment on a paper surface. h,i) The contact angle characterization of the hydrophilic and hydrophobic parts of CBs/PVA@paper. j) Photographs of wet and dry regions of CBs/PVA@paper. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The key innovation lies in the cutting pattern, inspired by kirigami – a variation of origami that involves cutting rather than just folding. This precise pattern of cuts allows the paper to stretch to twice its original length while maintaining its ability to transport water and generate electricity. The device produces an open-circuit voltage of 1.2 volts and a short-circuit current of 6.0 microamperes, with a maximum power density of 0.25 microwatts per square centimeter.
The team identified three distinct stages in the device’s stretching behavior. From 0% to 200% stretch, the cuts allow the paper to deform while experiencing minimal stress, maintaining stable power output. Between 200% and 280% stretch, stress begins concentrating at certain points, affecting performance. Beyond 280% stretch, the paper structure begins to fail, though the device can still reach 300% elongation before breaking.
Multiple units can be connected in series or parallel to increase output. Three devices connected in series produced 3.0 volts, while parallel connection of three units increased the current from 6.0 to 15.0 microamperes. The team demonstrated practical applications by powering LED lights and small electronic displays.
A particularly promising application emerged in sweat monitoring. When attached to skin during exercise, the device generates electricity from evaporating sweat while simultaneously measuring salt concentration. The electrical output changes with sweat composition, enabling continuous health monitoring without batteries. Tests with different concentrations of sodium chloride in artificial sweat showed a clear correlation between salt levels and electrical output.
The device maintained consistent performance through 500 stretching cycles and functioned under various environmental conditions. Temperature increases improved power generation by increasing evaporation rates, while higher humidity slightly decreased output by slowing evaporation. These variations remained within 10% of standard performance.
This technology offers potential advantages for powering wearable electronics. Unlike batteries, it doesn’t require charging or replacement. It generates power continuously from natural evaporation, though current power levels limit applications to low-power devices like simple sensors and displays.
The researchers are working to increase power output and adapt the design for manufacturing. Challenges include protecting the paper from damage while maintaining its water absorption properties, and scaling up production while preserving the precise cutting patterns needed for proper function. While the current version successfully powers basic sensors, advancing to more demanding applications would require improved efficiency or larger units.
As wearable health monitoring technology becomes more prevalent, the need for flexible, maintenance-free power sources continues to grow. This paper-based generator demonstrates a promising approach that harnesses everyday water evaporation to generate electricity while maintaining the flexibility needed for wearable applications.
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