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15 cm), the balloon expands safely within 42-50 °C. This efficient heating stems from a carbon-nanotube coating
that absorbs energy effectively, combined with the electromagnetic resonance of the optimized nitinol core. The
nitinol core thus acts as both a structural backbone and a microwave-coupling element. We validate MBF
performance in vitro using a 3D cerebrovascular phantom and ex vivo in porcine placenta models. With reduced
power demands, long-range actuation, and favorable biocompatibility, the MBF represents a promising catheter-free
strategy for minimally invasive treatment of blockages in small arteries.

INTRODUCTION
Arterial blockages due to the plaque buildup inside the artery severely restrict blood flow and cause critical
health issues. When these blockages occur in the brain, they can result in life-threatening conditions such as
stroke - one of the leading causes of mortality worldwide . Currently, the standard treatment for restoring
[1,2]
blood flow is catheter-assisted balloon angioplasty, a minimally invasive procedure where a mechanical
guidewire and a balloon catheter are sequentially threaded through the artery to the blocked lesion and the
balloon is inflated to widen the artery . However, this approach encounters major limitations when applied
[3]
to the complex vasculature in the brain. The pre-shaped guidewire struggles to navigate tortuous and narrow
arteries, especially in small arteries with diameters ≤ 2 mm . Notably, these small vessel blockages are
[4]
responsible for approximately 25% of stroke cases, underscoring the urgent need for more effective
techniques that can access these hard-to-reach areas .
[5]
Emerging magnetic guidewires with a magnetically bendable tip have shown promise in addressing this
challenge [6-10] . Distinct from the pre-shaped guidewire, magnetic guidewires can swiftly adjust the orientation
of their tip by responding to external magnetic fields, allowing enhanced navigation through complex
vascular branches. Pioneering works can be found in Prof. Nelson’s [11-13] , Zhao’s [14-16] , and Sitti’s groups. For
[17]
example, Dreyfus et al. utilized small integrated magnets, while Kim et al. dispersed ferromagnetic
[12]
[16]
particles to achieve magnetically controlled deflection. Tiryaki et al. developed a magnetic guidewire that
[17]
can operate in ultra-high magnetic fields produced by magnetic resonance imaging scanners. However,
despite the enhanced navigation capability of the magnetic guidewire, the subsequent advancement of the
mechanical catheter and deployment of the balloon in the angioplasty still remains challenging, because the
larger size and higher bending stiffness of the balloon catheter prevent them from effectively following the
guidewire through tortuous and small arteries (as illustrated in Supplementary Figure 1) .
[18]

Recent advances in functional miniature robots offer another potential solution to widen arteries, leveraging
wirelessly actuated body expansion without needing a traditional balloon catheter [19-23] . These miniature
robots expand their body in response to external stimuli at the targeted site [24-26] . Among these, miniature
robots that utilize liquid-to-gas phase transition have been extensively reported [27,28] . Most such systems rely
on radio frequency (RF) heating, in which alternating magnetic fields heat embedded magnetic
nanoparticles. For instance, Tang et al. developed a miniature magnetic balloon by combining the
[28]
biocompatible liquid of Novec 7000 with Fe O nanoparticles and demonstrated its large volumetric
3
4
expansion under alternating magnetic fields. While effective in generating localized heating, RF heating often
has limited actuation distances and prohibitively high power. They typically require the target to be
positioned within electromagnetic coils to achieve sufficient field strength and some even require up to
10 kW power to operate [29,30] . These requirements are impractical for the safe cerebrovascular application
where the actuation source is usually required to be placed at least 10 cm away from the target inside the
brain. In contrast to RF heating, microwave heating holds the promise for long actuation distance at
relatively low power [31,32] . Numerous studies have demonstrated the potential of microwaves to actuate
miniature robots recently [33,34] . However, achieving efficient heating within deep tissue such as brain yet
remains a grand challenge because microwaves are attenuated when passing through biological tissues,

thereby limiting the volumetric expansion of the miniature robots.