A printable polymer that traps water molecules flips humidity from a performance-killer into a performance-booster for motion-powered generators, enabling wireless charging of implantable electronics through tissue.
(Nanowerk Spotlight) Static electricity is fragile. A charged surface in dry air can hold its charge for minutes, but moisture in the air drains it away within seconds, forming thin films of adsorbed water that conduct accumulated electricity before it can do useful work. That ordinary property of water has frustrated a class of small generators designed to turn motion into electricity.
Triboelectric nanogenerators (TENGs), produce current when two dissimilar materials touch and separate, leaving one briefly positive and the other briefly negative. They are promising candidates for powering wearable health monitors and implantable medical devices. Their output typically collapses once the relative humidity climbs above 60 or 70 percent, and human skin, body cavities, and tropical climates all push past that threshold.
Research teams have tried to defend TENG surfaces by making them water-repellent or by sealing devices inside impermeable packaging. Both strategies trade robustness for complexity, and neither scales well to the intricate three-dimensional geometries that wearable and implantable electronics increasingly require.
A different approach inverts the logic: make the tribolayer eagerly hydrophilic so that bound water contributes to charge generation rather than stealing it. Turning that idea into a printable, high-performance material has remained the unsolved step.
Schematic of LCD printing and photoinitiated polymerization: AA, HEA, and HEAA monomers crosslinked with PEGDA to form dense polymer networks for tribolayer generation. (Image: Adapted from DOI:10.1002/adfm.75354, CC BY)
The team began by comparing three acrylic monomers crosslinked with polyethylene glycol diacrylate to form thin films roughly 200 micrometers thick. Carboxyl, hydroxyl, and amide groups decorate these networks and form hydrogen bonds with ambient water. Of the three, the amide-bearing formulation produced the strongest response, with output that rose rather than fell as the chamber humidity climbed to 90 percent.
Hydrogen bonding alone did not exhaust the chemistry available. The team then added a zwitterionic monomer, sulfobetaine methacrylate, which carries a permanent positive ammonium charge and a permanent negative sulfonate charge within the same molecule. These built-in dipoles provide an extra polarization mechanism and an unusually strong affinity for water.
At a 5 wt% loading, the formulation reached 45.6 microamperes, 802 volts, and a peak power density of 48.4 watts per square meter at 90 percent relative humidity. Compared with a previously reported moisture-tolerant TENG based on lithium chloride and MXene in polyvinyl alcohol, the new device doubles the power density while using no inorganic fillers and remaining fully printable.
Pushing the zwitterionic content to 10 percent reversed the gains. Humidity-dependent dielectric measurements showed that higher loadings create ionic clusters that increase conductivity and dielectric loss, allowing charge to leak through the film before it can drive an external circuit. The optimum sits at the point where polarization gain outpaces conductive loss.
Density functional theory and molecular dynamics simulations clarified why water amplifies rather than degrades performance. Binding-energy calculations ranked the sulfonate-ammonium pair as the strongest water anchor, with the amide, hydroxyl, and carboxyl groups following in that order. When water molecules attached to the polymer, calculated dipole moments rose substantially, and radial distribution functions placed water in tight, structured shells around the polar groups.
Raman spectroscopy supported this picture experimentally. As relative humidity climbed from 35 to 90 percent, the fraction of free water in the film barely changed, while strongly and weakly hydrogen-bonded water dominated the hydration layer. Additional moisture settled into anchored positions rather than pooling into mobile films, which explains why the output scales cleanly with humidity instead of collapsing.
Surface-potential measurements tied the molecular story to device behavior. The zwitterion-containing film showed the most positive surface potential and the lowest work function of any formulation tested, meaning electrons escape more easily from its surface during contact with the negatively charging counter material, polytetrafluoroethylene. Electrons flow more readily, and the resulting charge transfer is larger.
The authors then pushed the platform into application. Using the optimized resin, they printed complex lattice structures, twisted hexagonal networks, and flexible wearable forms. A finger sleeve transmitted Morse code through tap patterns. An insole distinguished walking from running by the period and amplitude of its output signal. Recessed features down to 80 micrometers were cleanly resolved, adequate for most wearable sensor geometries.
The most ambitious demonstration ties the generator to a wireless communication scheme designed for powering implanted medical electronics without batteries. A rectifier converted the TENG’s alternating current and stored it in capacitors, which then drove a backscatter communication reader. The reader transmitted a radio-frequency signal through a layer of pig skin to a semi-passive tag, simulating transmission through human tissue.
The tag harvested roughly 11 to 17 milliwatts. Pig skin attenuated the signal by 36 to 59 percent without disabling the link. Arrays of capacitors extended reader operation to tens of seconds per charging cycle, enough for meaningful energy transfer in a realistic biomedical setting.
Recasting a limitation as an asset carries weight beyond this one device. Humid environments, including the interior of the human body, are exactly where implantable and wearable electronics must operate, and they are exactly where conventional TENGs perform worst. A printable resin that converts that humidity into a performance boost narrows the gap between laboratory triboelectric devices and practical self-powered implantable triboelectric systems for biomedical use. Demonstrating wireless charging through tissue, even at modest distances and durations, marks a concrete step toward implants that a patient could replenish through ordinary body motion.
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