How water vapor is powering the next generation of soft robots

Phase-change actuation has been revived for the era of untethered, electrically driven soft robots. Our team at the University of Coimbra have developed a phase transition soft actuator designed to power electric soft robots that require high force and precision. Our innovation leverages the liquid-to-gas phase transition of water to generate mechanical motion in a way that is simple, scalable, and remarkably powerful.

Unlike traditional soft actuators, which often rely on bulky pneumatics, exotic materials, or high voltages, our design exploits a well-known process: boiling. Using a tiny embedded heater, our actuator transforms water into steam, generating internal pressure that drives motion in soft, flexible structures. As a result, our actuator can operate at voltages as low as 24 V, deliver forces exceeding 50 N, and achieve pressurization rates of up to 100 kPa/s.

Our findings are published in Nature Communications.

Why water?

Past attempts at phase transition actuators have struggled with slow strain rates, performance degradation, and complex designs. Our team addressed these challenges by focusing on engineering fundamentals: fluid selection, heat transfer, and control.

We chose water as our working fluid for its safety, availability, and thermodynamic efficiency. Through a rigorous selection process, we demonstrated that water’s relatively high enthalpy of vaporization, once seen as a drawback, could actually serve as a reliable performance benchmark. The higher energy requirement simply means there’s room for improvement with lower enthalpy alternatives, provided safety is maintained.

In direct comparisons, our actuators made with silicone elastomers and water significantly outperformed previous designs, achieving strain rates of 16.6%/s while remaining robust across more than a thousand actuation cycles.

The benefits of modularity

At the heart of our design is modularity. By separating the heating coil, working fluid chamber, and outer soft structure, we created a flexible system that can be adapted for various types of motion: linear, bending, or a combination of both. Our linear actuator is based on a McKibben design, while the bending version follows a fast Pneu-Net (fPN) geometry.

We designed our actuators using off-the-shelf materials and accessible manufacturing methods like 3D printing and casting.

Despite its simplicity, our design incorporates sophisticated engineering principles. For example, we solved a significant issue present in earlier designs: mechanical vibrations caused by subcooled boiling. These instabilities, stemming from thermodynamic mismatches inside the actuator, were mitigated using nonlinear control algorithms that maintain a stable boiling state.

Out in the wild

What can these high-power phase transition actuators actually do? To showcase their potential, we developed three prototypes:

• A biomimetic hand, powered by three linear actuators that enable smooth object manipulation.
• A soft gripper, using three bending actuators to pick up objects of various shapes and weights, including fruit models.
• And Bixo, a quadruped robot designed to climb tubes and crawl across tree trunks.

Bixo’s cyclic movement (gripping, pulling, and releasing) is controlled through localized heating and cooling of its actuators. Even with the known limitation of slow cooling (i.e., slow depressurization), we achieved stable cycles in as little as 25 seconds.

These prototypes don’t just demonstrate feasibility, they hint at an emerging new class of soft robots: cost-effective, safe, and capable of operating in natural environments without tethered pumps or hazardous materials.

Boiling into the future

Looking ahead, potential improvements include miniaturization (to enhance heat dissipation), higher pressure operation, and advanced preloading mechanisms. Even in its current state, though, our actuator design offers a reliable foundation for researchers seeking to build soft robots without the complexities of pneumatics or the risks associated with high-voltage systems.

The takeaway? With a bit of heat and some innovative engineering, a water-filled tube might just be the most powerful actuator you’ve never considered.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.