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Chen et al. Soft Sci. 2026, 6, 9 Page 17 of 36

The acoustic propulsion mechanism of LM nanoswimmers is driven by asymmetric acoustic radiation forces
generated under ultrasound field excitation [Figure 8B] . When exposed to continuous ultrasonic waves,
[56]
the rodlike LMs with geometrically asymmetric ends experience differential acoustic pressure distributions
along their longitudinal axis. This pressure gradient creates a net acoustic radiation force in the levitation
plane that propels the nanomachines forward, with the narrower tip serving as the leading edge due to its
lower hydrodynamic resistance. Simultaneously, secondary acoustic streaming flows develop near the
substrate surface, generating viscous drag that partially counteracts the primary propulsion force. The
dominance of acoustic radiation forces enables directional motion with velocities up to 23 μm·s at an
-1
optimal frequency of 420 kHz and voltage of 10 V . The propulsion efficiency is further tunable through
[56]
ultrasound parameters, where higher voltages amplify the pressure gradient while maintaining the
characteristic rotational swimming trajectory.

Photothermal actuation of LM nano-swimmers is achievable through localized temperature gradients
generated by laser irradiation [102] . Continuous near-infrared irradiation generates an axial thermal gradient
since the nano-swimmers exhibited asymmetric geometries of a broad tail and a sharp tip. The tail region,
containing more metallic volume, absorbs more heat and reaches a higher temperature than the tip. This
anisotropic heating drives self-thermophoresis, where the induced thermophoretic force pushes the
nano-swimmer tip-forward along its longitudinal axis. At the same time, the temperature gradient also
induces thermoosmotic flow within the electrical double layer adjacent to the substrate, creating a
hydrodynamic resistance that opposes motion. Thermophoretic force dominates, enabling controlled
nano-swimmer propulsion with velocities up to 31.22 μm·s under 5 W·cm irradiation . The propulsion
[102]
-1
-2
speed is further modulated by substrate surface potential, since higher potentials enhance thermoosmotic
resistance, slowing the swimmer.
Photothermal and acoustic fields can achieve the effective actuation of many LM nano-swimmers. However,
the directional control and collective behavior of nano-swimmers are inherently difficult to regulate under a
single optical or acoustic field. By incorporating magnetic nanoparticles into their structure, external
magnetic fields may be introduced as an additional degree of control, enabling the combination of robust
propulsion and precise navigation.

APPLICATIONS OF MLMs FOR SOFT ROBOTS
The practical applications of MLMs for soft robots are systematically mapped onto four categories: droplet,
slurry, particle, and composite, based on their composition and morphology. For instance, the morphology
of MLMs changes from a droplet to a slurry as the concentration of magnetic particles increases. In addition,
dispersed MLM nano- and microparticles generated via ultrasonic treatment exhibit a much higher
surface-to-volume ratio than bulk MLMs. Furthermore, integrating MLMs with other soft materials enables
composite systems that offer greater design flexibility and functionality for soft robots.

MLM droplet-based soft robots
When the content of embedded magnetic microparticles is low (typically < 10 wt%), the MLMs showed good
fluidity. In this condition, a small volume of MLMs naturally exhibits a spherical droplet morphology due to
the dominant influence of surface tension over gravitational forces . This behavior can be attributed to the
[103]
minimization of surface free energy, as determined by the Young-Laplace equation:

ΔP = γ (1/R + 1/R )
1
2
where ΔP is the pressure difference across a curved fluid interface, γ is the surface tension, and R and R are
1
2
the principal radii of curvature of the interface at a specific point. Thus, high surface tension promotes
compact, low-surface-area morphologies. Given the inherently high surface tension of LMs (~500 mN/m),