Why squeezing a soft generator can produce more electricity than pulling it

A soft pouch filled with insulating liquid can act as an electrical generator. The surprising part of a study from Stuttgart, the Max Planck Institute for Intelligent Systems and the University of Colorado Boulder is that squeezing the pouch can recover more energy than pulling it with the same force.

The devices are HASEL transducers: thin sealed pouches with electrodes covering part of their opposing surfaces. An initial electrical charge pulls those electrodes together and displaces liquid within the pouch. Mechanical force then drives them apart.

The electrical mechanism follows a familiar capacitor relationship. With charge held approximately constant, reducing capacitance raises voltage. Separating the electrodes requires mechanical work against their attraction; that work increases the stored electrical energy. The initial charge is part of the operating cycle, not an energy source the experiment can dispense with.

### Geometry changes the useful movement

The researchers combined experiments with a quasi-static model to examine how pouch geometry and loading affect capacitance change. Compression allowed a greater change than tension at the same force, making more electrical energy available from the chosen structure.

Their optimised experiments reached maximum specific energy of 15J/kg, specific power of 43W/kg and conversion efficiency of 40%. These are separate reported maxima, not three simultaneous ratings for a finished power supply.

Frequency testing shows why that distinction matters. Across tests from 0.25Hz to 10Hz, specific energy peaked at 8.8J/kg at 4Hz, while specific power reached 43W/kg at 6Hz. Low-frequency losses included charge leakage. Raising the movement rate therefore did not increase every performance measure together.

The electrical circuit also shapes how much mechanical work becomes useful output. The study used two capacitor banks and diodes for passive switching between voltage levels. A wearable sensor or robot would additionally need an interface appropriate to its supply rail and load cycle.

The paper, first published in August and highlighted in new university coverage, provides a design model and laboratory results. It does not demonstrate an ocean-wave installation or a self-contained wearable power product. Those applications demand a match between available motion, pouch geometry and the electrical harvesting cycle.

University of Stuttgart research summary. Advanced Science paper, DOI 10.1002/advs.76834.