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Page 18 of 33 Ren et al. Soft Sci. 2026, 6, 6

Magnetic drive is noncontact, deeply penetrating, rapidly switchable, and effective in opaque or sealed
media. It requires no onboard power supply and integrates naturally with microfluidic and biomedical
environments. Its main challenges include the force availability scaling proportionally with part volume and
magnetic field or gradient, step-out, hysteresis or heat generation at high actuation frequencies, possible
demagnetization, particle agglomeration or leaching, matrix stiffening at high filler loadings, and
manufacturing issues such as sedimentation, rheology control, and print fidelity.

Electric field
The electric actuation in 4D-printed soft microrobots relies on field-charge interactions that convert
electrical inputs into fluid flow, pressure gradients, or electrostatic stress. A representative mechanism is
electro-osmosis , where an applied electric field drives cations toward the negative electrode, establishing
[126]
an internal-external ion concentration gradient. This induces solvent diffusion and osmotic pressure, thereby
generating programmable expansion and bending [Figure 6E] [127] . By tuning electric field magnitude,
frequency, phase, and electrode geometry, deformation amplitude and direction can be controlled to achieve
motions such as rolling, walking, or crawling. Quantitatively, when electrostatic stress is the dominant driver,
the effective Maxwell pressure P follows a compact scaling relation p = εE , where ε is permittivity and E is
2
the electric field, highlighting that achievable stress increases with permittivity and scales with the square of
the field strength.

Wang et al. fabricated electrically activated reversible composite actuators via 4D printing that combined a
conductive ink with shape memory poly(ether ether ketone) (PEEK) [128] . Under electrical excitation, the
electrothermal sintering of the ink and the phase transition of PEEK produced controlled and repeatable
deformation, which the author validated under varied current amplitudes, circuit designs, and printing
conditions [Figure 6F]. Building on electric actuation in soft systems, Xia et al. integrated 4D-printed LCE
actuators with associated electronics to realize an untethered robot . A modified LCE paired with a
[32]
polyimide heating film and a silicone adhesive delivered tunable transition temperature and modulus and
sufficient propulsive force, so the compact robot grasped objects and traversed obstacles on challenging
terrains [Figure 6G]. Extending electrothermal control to higher stiffness, Morales Ferrer et al. introduced
multiscale heterogeneous polymer composites with tunable electrical conductivity for Joule heating and self-
sensing [129] . Electrically controllable bilayers morphed from flat sheets into a self-standing lifting robot with
record weight-normalized load and actuation stress, and a printed lattice demonstrated multigait crawling
while carrying up to 144 times its own weight [Figure 6H].

In summary, electric actuation offers fast response, precise spatiotemporal addressability, easy
programmability, and straightforward integration with printed electrodes and conductive pathways, which is
attractive for on-chip manipulation and compact soft robots. Limitations include rapid field attenuation in
conductive or ionic media, electrolysis and Faradaic reactions at low frequencies, electrode fouling and
delamination, safety constraints for in vivo use, and limited penetration depth compared with magnetic or
ultrasound fields.

Ultrasound
Ultrasonic actuation has become an effective stimulus for inducing shape changes in smart materials. By
converting acoustic energy into mechanical forces, fluid flows, or localized heating, it drives the deformation
and locomotion of 4D-printed soft microrobots [Figure 7A] [130] . Ultrasound-related parameters such as
amplitude, frequency, and duty cycle collectively determine force output, response speed, and motion
patterns. Consequently, the approach has found broad application in controlled drug release, soft robotics,
and mechanosensing .
[131]