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affect microwave absorption, we further repeated the comparison using porcine tissue immersed in fresh
anticoagulated porcine whole blood [Supplementary Figure 7B]. In both tissue-only and blood-containing
conditions, the dominant hot spot remained localized at the balloon region, while the surrounding tissue or
blood-containing medium showed limited heating, supporting the preferential microwave-heating behavior
of the CNT-coated balloon under the tested static ex vivo conditions.

We further evaluated the thermal influence of the heated balloon on contacting porcine tissue
[Supplementary Figures 8 and 9]. Infrared imaging showed that, although the balloon itself reached the
actuation temperature, the temperature rise in the contacting tissue remained much smaller and no obvious
extensive overheating zone was observed [Supplementary Figure 8]. Tissue-level thermal influence was
further assessed by comparing the macroscopic appearance and microscopic surface morphology of porcine
tissue after different thermal treatments [Supplementary Figure 9]. The tissue contacted with the MBF
during 3 min of microwave heating retained a normal reddish appearance and showed no obvious
microscopic structural alteration compared with the room-temperature control. Together, these results
suggest that the balloon temperature should not be directly interpreted as the temperature of adjacent tissue,
and support limited local tissue-level thermal influence under the tested static ex vivo conditions.

Overall, these results show that the MBF can maintain effective balloon actuation after tissue penetration
while limiting excessive heating of adjacent tissue under the tested conditions. A limited local temperature
increase may still appear on the porcine tissue surface, likely due to partial microwave absorption by the
heterogeneous tissue sample and local variations in field distribution. Nevertheless, tissue attenuation,
together with active modulation of power and distance, helps confine the heating effect and supports
operation within the target 42-50 °C actuation window.

Expansion behavior of the phase-change balloon
For effective angioplasty, the output pressure to reopen the blocked artery mainly depends on the radial
expansion, while axial expansion is ineffective and undesired. Actually, minimizing axial expansion confines
the balloon’s volume, increasing its internal pressure during the liquid-to-gas phase change. To achieve this,
we implemented an axial pre-stretching strategy to control the balloon’s expansion behavior. As illustrated in
Figure 3A, the Ecoflex tube was stretched before being affixed to the anchor points, enabling desired radial
expansion while suppressing axial elongation. Let L 0, L, and L denote the length of the unstretched Ecoflex
1
tube, stretched balloon, and inflated balloon, respectively, and the d 0, d, and d denote the corresponding
1
diameters (Note d = 1 mm for the MBF). The pre-stretch, radial expansion, and axial expansion are defined
as L / L 0, d / d, and L / L, respectively.
1
1
To assess the impact of the pre-stretch, we investigated both radial and axial expansion in free space under
various internal pressures (denoted as p ) which were controlled by a pump. As shown in Figure 3B,
free
increasing the pre-stretch increases radial expansion while simultaneously reducing axial expansion. This is
because axial pre-stretching pre-consumes the axial elongation that would otherwise occur during inflation,
thereby shifting the subsequent deformation mainly to the radial direction. In addition, the reduced initial
diameter after pre-stretching further increases the apparent radial expansion ratio. Specifically, with a
pre-stretch L / L 0 = 1.6, the radial expansion reaches up to d / d = 300% at 20 kPa, whereas axial expansion
1
remains nearly unchanged, i.e., L / L = 1. Compared to the unstretched case, the radial expansion at 20 kPa
1
increases by 1.6 times while the axial expansion is eliminated [Figure 3C]. Based on these findings, the
pre-stretch of 1.6 is adopted for the MBF design to ensure optimal expansion performance.

Since it is difficult to directly measure the internal pressure of the balloon under microwave heating due to
its small dimensions, we adopt an indirect method by correlating the radial expansion of the heated balloon