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Page 6 of 26 Chen et al. Soft Sci. 2026, 6, 3

physiological environments characterized by fluctuating pH, ionic strength, and protein-rich media can
compromise colloidal stability, promoting aggregation, sedimentation, or magnetic shielding effects.
Although ferrofluids offer excellent deformability and magnetic responsiveness, additional investigations are
required to develop biocompatible oil-based ferrofluids suitable for in vivo use. One promising direction is
the use of biologically compatible carrier oils to mitigate toxicity and improve physiological stability. Recent
studies have demonstrated that ferrofluids formulated with corn oil [125] or silicone oil [128] and Fe O 4
3
nanoparticles exhibit reduced cytotoxicity and improved colloidal stability, with successful in vivo validation
in mouse and rabbit models, respectively.

Magnetically responsive active composites
Magnetic soft composites deform elastically under external magnetic fields and revert upon field removal,
which limits their use in applications requiring semipermanent shape retention. To retain deformation or
desired shapes without continuous external stimuli, thermally responsive polymers, including shape memory
polymers and liquid crystal elastomers, have been widely integrated into magnetic soft matter.

Shape memory polymers typically remain stiff below their thermal transition temperature, becoming soft and
deformable upon heating [129-134] . Incorporation of magnetic particles allows shape modulation through remote
magnetic actuation . At elevated temperatures, the composite can be reconfigured into a temporary shape,
[134]
which is fixed upon cooling. Subsequent reheating induces shape recovery in the absence of magnetic fields,
while heating under magnetic actuation enables deformation into alternative geometries. Such capabilities
have been exploited in applications including deployable scaffolds, steerable catheters, and shape-adaptive
implants. Liquid crystal elastomers [135-138] , composed of loosely cross-linked polymer networks bearing
anisotropic mesogens, exhibit programmable, anisotropic deformation through nematic-isotropic phase
transitions. Above the critical temperature, contraction along aligned mesogens facilitates reversible motion.
Patterned mesogen alignment further enables complex actuation modes such as bending and folding. Recent
advances have incorporated hard magnetic particles into liquid crystal elastomers to create untethered soft
robotic systems with reprogrammable magnetization profiles. These composites enable multimodal
locomotion, such as walking on solid substrates and swimming via thermally induced shape changes and
magnetic steering [135,136] .

The multifunctional responsiveness and structural programmability of magnetically responsive active
composites present a promising pathway for developing intelligent soft robotic platforms for biomedical
intervention. However, their clinical translation critically depends on a deeper understanding of long-term
biocompatibility, immunogenic responses, and degradation mechanisms under physiological conditions.

Magnetization programming strategies
Recent progress in fabrication technologies has enabled increasingly sophisticated magnetization control in
small-scale magnetic soft robots. These approaches can be broadly classified into four categories: mold-
assisted programming [57,69,134,139-141] , spray-assisted programming [72] , 3D-printing-assisted
programming [58,59,142-145] , and microassembly-assisted programming [60-62] . Each method supports distinct levels
of geometric complexity, spatial magnetization resolution, and functional integration.

Mold-assisted programming represents the most established strategy, particularly for robots with simple
geometries and deformation modes. A soft composite sheet containing hard-magnetic particles is first
mechanically constrained into a temporary 3D shape and then magnetized to saturation under a uniform
magnetic field [57,141] . After release, the planarized structure retains a programmed, spatially varying
magnetization profile that drives predictable 3D deformation under external fields. Such methods have
enabled millimeter-scale swimmers exhibiting travelling-wave propulsion and multimodal locomotion,
including rolling, crawling, and jumping. However, the approach offers limited freedom in magnetization
distribution and internal architecture.