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Chen et al. Soft Sci. 2026, 6, 48 Page 5 of 19

of the motor (Gelvwei, China), set the speed of the motor to 300 rpm. The silicone solution was then injected
into the cup of a spray gun (Ustar, China), with the spraying pressure set to 500 kPa, and the distance
between the spray gun and the needle set to 15 cm. After spraying, the needles, which were uniformly
sprayed with silicone solution, were placed in an oven at 60 °C to cure for 2 h.

In addition, CNTs were added to the anhydrous ethanol with different volume ratios. The CNT solution was
placed in an ultrasonic cleaner and shaken for 5 min to evenly distribute CNTs in anhydrous ethanol. This
CNT solution was then sprayed using another spray gun (CNYQ, Japan) onto the needle with the cured
silicone fabricated in the first step to create a CNT layer. After spraying, the needles uniformly coated with
CNT solution were placed in the oven at 60 °C again to evaporate the anhydrous ethanol as a solvent.

Finally, the silicone solution was sprayed again to seal the CNT layer. The needles, now coated with both the
base silicone layer and the CNT layer, were placed in the oven at 60 °C for a final curing time of 2 h. The
preparation of the functionalized Ecoflex tube was completed after demolding from the needle.

The phase-change liquid was prepared by mixing Novec 7000 (boiling point: 34 °C) and Novec 649 (boiling
point: 49 °C) in a ratio of 1:3.

To assemble the phase-change balloon, the prepared functionalized Ecoflex tube was first positioned onto
the nitinol core, and both ends of the balloon were then tightly secured using medical-grade PVDF
heat-shrinkable tubing. Afterward, the phase-change liquid was injected into the annular cavity between the
nitinol core and the balloon wall using a 34G ultrafine needle, and the injection site was finally sealed with
silicone. Through this process, the phase-change balloon was formed as an integrated structure around the
nitinol core, which helps maintain mechanical continuity during actuation and reduce the risk of fluid
leakage.

Fabrication of the PDMS fiber and MBF assembly
The PDMS fiber was fabricated by injecting uncured PDMS into a tubular mold. To facilitate the central
alignment of the nitinol core (diameter: 0.15 mm), a guiding needle was inserted at one end of the mold.
After the PDMS was injected, a 0.12 mm metal wire was inserted through the same needle into the uncured
PDMS. The inner diameter of the needle confined the metal wire to a relatively central region of the mold,
keeping the positional deviation within the limits of the needle’s inner diameter. Following curing at 60 °C
for 2 h, the metal wire was extracted, leaving a longitudinal hole with a diameter of 0.12 mm. Finally, the
nitinol core was inserted into this slightly smaller hole and glued by uncured silicone, creating an
interference fit that ensured a tight and secure integration between the nitinol core and the PDMS fiber.

Finite element simulation of the MBF
Electromagnetic thermal simulation of the MBF
COMSOL Multiphysics was used to study the thermal effects of phase-change balloon. Firstly, Microwave
Heating was selected in the Select Physics tree, and a Frequency-Transient, One-Way Electromagnetic
Heating study sequence was added. After completing the modelling of the CNT-coated balloon, nitinol core,
and air domain, set the corresponding material parameters for each part separately. For CNT, the relative
permeability was defined as 1, the density was defined as 1,350 kg/m , the heat capacity at constant pressure
3
was defined as 100 J/(kg·K), the electrical conductivity was defined as 1,000 S/m, the thermal conductivity
was defined as 3,000 W/(m·K). The nitinol core and air were sourced from the built-in material library in
COMSOL. Secondly, in the Electromagnetic Waves, Frequency Domain (emw) section, the air domain
boundary located directly above the CNT-coated balloon was selected to serve as the microwave emission