Reconsidering a familiar physical process
Researchers at the University of Coimbra reported a liquid-gas phase-transition soft actuator in Nature Communications on April 25, 2025. A flexible electrical coil heats a sealed working fluid, producing vapor pressure that deforms a compliant structure. Cooling condenses the vapor and lets the actuator return.
Boiling a fluid to create motion is not a new idea. Soft-robotics researchers often avoided the approach because heating and cooling can be slow, energy can be lost, and delayed pressure response complicates control. Diogo Fonseca and Pedro Neto redesigned the heater, chamber and soft body as separate modules and paired them with a nonlinear controller intended to reduce lag and unwanted vibration.
Measured force, speed and voltage
The paper reports strain rates above 16 percent per second and pressurization rates of 100 kilopascals per second. Blocked-force tests exceeded 50 newtons while the devices operated at voltages up to 24 volts. The authors describe this as roughly an order-of-magnitude improvement in strain and pressurization rate over earlier phase-transition attempts.
Different numbers measure different properties. Blocked force is the force produced when an actuator is prevented from moving; it is not the payload that a complete mobile robot can carry. Strain rate describes how quickly its length changes relative to its size, while system response also includes cooling, structure, load and control. The values are laboratory measurements under documented conditions, not a guarantee for every geometry.
Water was attractive because it is inexpensive, comparatively safe and diffuses slowly through the surrounding elastomer. Its high enthalpy of vaporization requires substantial heat, however. The researchers therefore provide a method for selecting other working fluids when an application needs a different balance of temperature, pressure, safety and response.
From actuator samples to robot demonstrations
The team built linear McKibben-style devices and bending actuators based on fast pneumatic-network geometry. It demonstrated them in a cable-driven biomimetic hand and in a soft quadruped that could move in unstructured outdoor settings. The paper also reports more than one thousand actuation cycles for tested devices.
Those demonstrations show that the actuator can be integrated into mechanisms rather than measured only on a bench. They do not establish industrial lifetime, weather resistance or medical compatibility. A thousand cycles can be enough to compare prototypes, but production equipment may need millions of reliable cycles, predictable seals and service procedures.
The central tradeoff is thermal
Low-voltage electrical input and compliant output are useful because they avoid an external compressed-air line and high-voltage electrostatic hardware. The phase transition can generate high pressure in a small chamber, and the modular construction uses common fabrication methods such as casting and 3D printing.
Heat remains the main constraint. Energy must enter the fluid before motion and leave it before full recovery. Repeated operation can raise the temperature of the robot and its surroundings. Applications near people, sensitive products or tissue would require insulation, temperature monitoring and failure analysis. Efficiency also has to be evaluated at system level, including the battery and thermal cycle.
The 2025 result expands the design space rather than identifying a universal artificial muscle. It is most promising where compliance, compact force and simple electrical control matter more than very high cycle frequency. The peer-reviewed measurements establish a stronger baseline for phase-change actuation; durability and efficiency in sustained field operation remain the tests that determine where it becomes practical.