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Yu et al. Soft Sci. 2026, 6, 19 Page 9 of 24

Figure 2F compares the normalized surface friction coefficients of samples with and without micro-needles
under a normal load of 35 g (0.343 N) during linear sliding (100 mm/min) against a PLA plate. The friction
coefficient directly influences grasping force, which in turn affects the payload capacity and success rate of
grasping tasks. Results show that the micro-needle structures significantly enhanced surface friction.
Specifically, for the 60 wt% samples, the friction coefficient without micro-needles is 0.72 (normalized), while
the presence of micro-needles increased it to 0.96, representing a 25% enhancement.

Figure 2G illustrates the mass fraction of magnetic powder inside the gripper and the influence of the
electromagnetic coil’s magnetic field on the gripper’s retraction speed. Using the time required for the
second section of the gripper to bend and retract to 90° as a criterion, the retraction speed of the gripper
significantly increases with the gradual increase of the electromagnetic coil current. Furthermore, the content
of magnetic powder inside the gripper also significantly affects the retraction speed. When the current
reaches 6 A, the retraction time is less than 0.5 s. The retraction performance demonstrates that the
magnetically driven flexible gripper in this study possesses rapid response capabilities. Therefore, it can
quickly wrap around the target and complete the gripping task during operation, exhibiting superior
performance compared to flexible grippers driven by traditional methods.

Taken together, the characterization results indicate that the 60 wt% samples offered an optimal balance:
Comparing samples with magnetic-powder mass fractions of 50 wt%, 60 wt%, and 70 wt%, we measured
Young’s modulus and surface remanence and obtained the following trends: relative to the 50 wt% sample,
the 60 wt% sample exhibited an increase in Young’s modulus of approximately 17% and an increase in
surface remanence of approximately 25.3%; compared with the 60 wt% sample, the 70 wt% sample showed an
increase in Young’s modulus of approximately 65% and an increase in surface remanence of approximately
29.9%. These results indicate that increasing the magnetic-powder content from 50 wt% to 60 wt% yields a
relatively pronounced gain in magnetic properties, resulting in greater magnetic responsiveness; however,
further increasing the filler content to 70 wt% leads to a substantially larger rise in material stiffness than in
surface remanence. The above findings suggest that, in the design of MSGs, a trade-off between magnetic
responsiveness and compliance must be considered in order to determine an optimal magnetic-powder mass
fraction that balances actuation performance and deformability. Thus the 60 wt% composition successfully
balanced magnetic responsiveness, structural flexibility, and surface friction, establishing a solid material
basis for subsequent performance validation. Furthermore, the effectiveness of the study and the feasibility of
the gripper were verified by testing the gripper’s contraction and bending angle, the frictional properties of
the gripper surface, and the gripper’s response speed. In the following section, flexible grippers fabricated
with 60 wt% NdFeB will be systematically compared with conventional flexible grippers to further highlight
the engineering value of this design.

Key characteristics of the MSG and stability of the MSG performance
Building on a 60 wt% NdFeB composite formulation, this section highlights the MSG’s advantages in
dynamic response, target adaptability and environmental compatibility. Through systematic experiments and
comparison with literature data, we validate the MSG’s comprehensive performance in complex underwater
scenarios.

The principal advantage of the proposed MSG over conventional soft grippers is its rapid actuation. Even
under water, the MSG’s response speed substantially exceeds that of traditional driving methods. Figure 3A
compares the device response in air and in water under a coil current of 6 A. The test protocol comprised an
expansion phase followed by a contraction phase: from t = 0 the gripper expanded and held the open
configuration for 3 s, power was then removed, and after 2 s the current polarity was reversed to trigger
contraction, which was maintained for 2 s. In air the gripper completes the response in 0.2 s; viscous