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Chen et al. Soft Sci. 2026, 6, 48 Page 7 of 19

Statistical analysis
Quantitative data are expressed as the mean ± standard deviation (SD). Unless otherwise stated, experimental
data were obtained from three independent replicates (n = 3). For the branching-channel navigation tests,
ten repeated trials were conducted for each representative path. For comparisons between two groups,
statistical significance was evaluated using a two-tailed independent Student’s t-test. A P-value of less than
0.05 (P <​ 0.05) was considered statistically significant.

RESULTS AND DISCUSSION
Design, fabrication, and working principle of the MBF
Incorporating the navigation of magnetic guidewires and the volumetric expansion of the phase-change
balloon, we developed a MBF with a diameter of 1 mm. The MBF consists of three main components: a
magnetic tip at the distal end (length 2.5 cm), a phase-change balloon (length 2 cm), and a non-magnetized
fiber (Polydimethylsiloxane, PDMS; length 1.5 m) at the proximal end. These three components are
connected using a single, continuous nitinol core (diameter: 0.15 mm), which acts as a backbone, with their
interfaces secured by cured silicone [Figure 1B and Supplementary Figure 2]. Notably, the core consists of a
superelastic nitinol core with a phase transition temperature of approximately -10 °C. This ensures the wire
remains in a stable austenite phase across operational temperatures. Consequently, microwave actuation
does not induce any undesired shape changes or bending via the shape memory effect, allowing the design to
maintain its intended flexibility and configuration. The magnetic tip is fabricated by uniformly dispersing
NdFeB microparticles in the PDMS matrix. This composite is then injected into a tubular mold and cured,
and subsequently axially magnetized by an impulse magnetic field (~4 T), enabling the tip to bend when an
external actuation field B is applied, thus facilitating navigation through tortuous vasculature.

The phase-change balloon is constructed by affixing a pre-stretched Ecoflex tube to the anchor points on the
nitinol core. It is then injected and sealed with 15 μL Novec engineered fluid, which serves as the inflation
medium. This liquid has an intrinsic liquid-to-gas phase transition temperature (boiling point) of 42 °C
[Supplementary Figure 3]. Consequently, when the actual internal temperature of the fluid reaches this 42 °C
threshold, it triggers the phase transition required for balloon expansion. This specific thermal property
ensures that the balloon remains unexpanded at normal body temperature [Supplementary Figure 4]. For
potential biomedical use, the balloon was operated within a safety-guided temperature window of 42-50 °C,
where the lower bound corresponds to the onset of phase transition and the upper bound serves as a
conservative control limit [35-38] . To enable wireless heating, the balloon is uniformly coated with CNTs via a
spraying technique [Supplementary Figure 5]. A scanning electron microscope (SEM) image of the
cross-section is provided in Figure 1C. When exposed to microwave energy, the CNTs absorb the energy,
triggering the liquid-to-gas phase transition that inflates the balloon [Figure 1D].

The inflation process was further visualized through optical and infrared imaging [Figure 1E and
Supplementary Video 1]. In the absence of biological tissue attenuation, the MBF reaches a baseline
temperature of approximately 80 °C when positioned 15 cm from a 50 W microwave irradiator (Baoxing,
China). We note that this value was obtained under a barrier-free in-air condition and is reported only to
characterize the maximum wireless heating capability of the device, rather than its intended operating
temperature in biological applications. Under this condition, the balloon region remains the primary
functional heating zone, while limited heating beyond the balloon may also occur, likely owing to microwave
coupling and heat conduction associated with the embedded nitinol core. The microwave operates at
2.45 GHz (wavelength ~12.4 cm), a frequency widely used in microwave thermotherapy-related
applications [39,40] .