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Page 8 of 19 Chen et al. Soft Sci. 2026, 6, 48

The balloon’s superior wireless heating efficiency is attributed to the excellent heat absorption capability of
CNTs and the electromagnetic resonance effect of the nitinol core (detailed in the next section). This
efficiency is quantified by the temperature rise per heating power, ∆T / P, and plotted against the heating
distance in Figure 1F. The data in Figure 1F were compared under broadly similar barrier-free, in-air
conditions, thereby reducing variability introduced by media attenuation. Under these conditions, the MBF
shows improved wireless heating performance relative to previous techniques utilizing RF heating [41-46] and
microwaves [33,34,47,48] (see Comparison Supplementary Table 2). Note that, the data in Figure 1F compares the
wireless heating performance under barrier-free, in-air conditions, ensuring a fair comparison across
different approaches without interference from biological tissues. This comparison is intended as a
representative reference, since ∆T / P may still show some dependence on factors such as ambient
temperature.

Wireless heating performance of the phase-change balloon
CNTs exhibit excellent electrical conductivity and electromagnetic wave absorption properties, making them
effective for microwave-induced wireless heating [49-52] . When exposed to a microwave electromagnetic field,
free electrons in CNTs oscillate rapidly, converting the absorbed energy into thermal energy through lattice
collisions. To evaluate the impact of CNT content on the heating performance, we tested balloons coated
with varying CNT volume fractions of 10%, 20%, and 30%. As shown in Figure 2A, higher CNT content leads
to a more pronounced temperature increase, with the temperature rising by 15 °C within 30 s at a 15 cm
heating distance and a 50 W power.

To further enhance heating efficiency, we introduced a nitinol core into the balloon. As seen in Figure 2B,
under the same conditions, the balloon with the nitinol core achieved a significantly higher temperature (P <​
0.05, compared to the balloon without the core), and the one with 30% CNT content underwent a
temperature increase of ∆T = 55.8 °C. We then investigated the effect of nitinol core length on heating
performance. The optimal temperature enhancement occurred when the length of the nitinol core was close
to approximately half of the free-space microwave wavelength [~6 cm; Figure 2C] under the present
configuration. This enhancement is attributed to the electromagnetic resonance effect that maximizes the
core’s ability to absorb and convert microwave energy into thermal energy, remarkably amplifying the
heating efficiency [Figure 2D]. Electromagnetic thermal simulations conducted in COMSOL Multiphysics
further validate this effect [Figure 2E]. Introduction of the nitinol core dramatically enhances the
temperature within the balloon, demonstrating the critical role of resonance in optimizing wireless heating
performance. We note that this ~6 cm value was identified as an empirical optimum in the current
experimental setup, and the corresponding behavior under tissue-loaded conditions should be further
examined in more physiologically relevant environments.

When considering small-vessel intervention scenarios such as cerebrovascular applications, the microwaves
need to penetrate biological tissues and the skull to heat the balloon. To approximate this tissue-penetration
scenario, we examined the effectiveness of microwave heating on the MBF after passing through porcine
tissues with bone. The porcine tissues used in this experiment had a total thickness of approximately 15 mm
(including both bone and attached soft tissue). This fan-bone specimen was selected because it provided a
relatively large and intact tissue-bone area, which facilitated stable placement and repeatable
microwave-penetration testing. The porcine tissues, placed 7 cm away from the MBF, approximate
controlled ex vivo tissue-barrier condition [Supplementary Figure 6]. As shown in Figure 2F, the balloon’s
steady-state temperature reached 49.3 °C at a 15 cm distance and 50 W power after penetrating porcine
tissue with bone. This temperature is sufficient to induce the liquid-to-gas phase transition for effective
balloon expansion. Specifically, we have defined the operational temperature window of 42-50 °C as a
safety-guided protocol. The lower bound of 42 °C is set to correspond with the initiation threshold of the