Millirobots with tunable adhesive microstructures grip and release solids and liquids in both air and underwater environments using magnetic control.
(Nanowerk Spotlight) Magnetic soft robots have attracted growing attention for their potential in manipulating objects across miniature and hard-to-reach environments. Unlike traditional machines that rely on mechanical parts or onboard power, these robots respond to external magnetic fields, making them safer, more flexible, and easier to control remotely. But a central challenge remains unresolved: how to reliably attach to and release a variety of targets—solid or liquid, small or large—in both dry and submerged environments.
Many past attempts to solve this problem have relied on rigid mechanical grippers or wrapping mechanisms, which are often bulky, slow, or only work for objects with predictable shapes. Other designs have used chemical adhesives or tapes that stick well but are difficult to remove quickly or cleanly, especially in liquid environments. Some robots apply soluble adhesives that dissolve to release a payload, but these require waiting for chemical reactions and don’t allow for dynamic re-use. These limitations have made it hard to build a universal robotic gripper that works across materials, shapes, and environments.
Efforts to improve adhesion have drawn on inspiration from biology. Micropatterned surfaces—such as mushroom-shaped tips or reentrant structures—can mimic the dry adhesion seen in insect feet or gecko toes. These geometries allow for increased surface contact without requiring glue, relying instead on intermolecular forces. But integrating such surfaces into soft robotic systems, especially ones that can switch adhesion states on demand, has remained difficult. Without active control, these structures grip but cannot easily let go. Some rely on mechanical peeling to detach, which limits precision and versatility.
A recent study by researchers in China, published in Advanced Materials (“Magnetically Switchable Adhesive Millirobots for Universal Manipulation in both Air and Water”), presents a solution to this problem using magnetically switchable adhesive surfaces built into soft millimeter-scale robots. These robots combine flexible magnetic bodies with carefully engineered surface structures called double-reentrant micropillar arrays (DRMAs). Using a process called two-photon lithography-assisted molding, the researchers created fine-scale microstructures that respond to magnetic fields by changing shape, enabling fast and reversible adhesion to targets of different types.
Each micropillar has a cap-like top that overhangs slightly, forming a reentrant edge that improves grip by maximizing surface contact. These structures trap air at the interface and generate strong van der Waals forces—weak, short-range attractions between molecules. When a magnetic field is applied, the micropillars bend, reducing contact and allowing the robot to let go of the object. Removing the field causes them to straighten and reattach. This switch between “on” and “off” adhesion states takes less than a second, making it practical for repeated use.
Concept and fabrication of the DRMA-integrated adhesive millirobot. a) Schematic illustration of a live snail whose behavior is taken over by a worm, Leucochloridium paradoxum. b) Schematic illustration of a typical adhesive millirobot. c) Principle of the magnetic-responsive switchable adhesion property of DRMA. The horizontal dashed lines in the diagram represent the direction of the applied magnetic field. d) Essential steps in the TPLAM process for creating DRMA with magnetic-responsiveness. e) Structural design and scanning electron microscopy (SEM) images of the fabricated DRMA. f) Schematic illustration of complex target manipulation tasks performed by the DRMA-integrated adhesive millirobots, including adhesion, alignment, assembly, and actuation in both air and underwater conditions. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
To ensure the system was both flexible and strong, the researchers tested different concentrations of magnetic particles in the silicone used for the micropillars. At high concentrations, the material became too stiff and brittle. At lower concentrations, it lost magnetic responsiveness. They found that a 50% mass ratio offered the best balance, allowing enough deformation for actuation while maintaining mechanical integrity.
The robots were tested on various materials, including glass, quartz, and common plastics, as well as on liquid droplets. In each case, they could pick up and release targets using only magnetic fields, without leaving residue or causing damage. For liquid manipulation, a common failure point is infiltration—when water seeps into the microstructure and breaks the adhesion. To prevent this, the researchers added microstructures along the sides of the pillars that repel water, helping to maintain a trapped air layer even when submerged.
This design enabled the robots to function reliably underwater. Experiments showed that adhesion remained strong after more than 100 cycles of switching between attachment and release, even when the robots operated in moving water or vibrating environments. In one test, a small nut was picked up and transported underwater using a vibrating robot body. The robot maintained grip even while deforming rapidly, demonstrating the strength of the adhesion system under mechanical disturbance.
To increase versatility, the researchers developed three different locomotion modes. The first used a rolling design, with a magnetization pattern that allowed the robot to curl and move in a wheel-like motion. This version was used to navigate tight spaces and remove obstructions from a damaged electrical circuit. The second mode used a vibrating gait that mimicked crawling. This allowed the robot to carry objects underwater, resisting the effects of turbulence and flow.
The third mode focused on precision. It involved a waddling motion, enabled by a magnetization profile aligned along the robot’s body. By adjusting the magnetic field in a controlled way, the robot swayed from side to side, walking forward in small steps. This configuration offered more precise control and was used for an assembly task in a confined environment. The robot successfully picked up a turbine blade, placed it onto a spindle, and then detached without displacing the assembly. Importantly, the magnetic field used to move the robot was weaker than the one used to switch adhesion, allowing independent control of locomotion and grip.
These adhesive robots showed the ability to maintain control even in flowing water. Tests using syringe pumps and mechanical stirrers demonstrated that targets remained attached for over an hour in dynamic conditions. In simulated blood-vessel-like environments, the robot could navigate to designated locations while carrying a target, indicating the potential for biomedical or environmental applications where flow resistance and clean detachment are critical.
The significance of this work lies in the combination of three capabilities: reversible adhesion, amphibious operation, and magnetic actuation. Each has been demonstrated before, but rarely in a single system. The use of programmable microstructures gives the robots the flexibility to grip many different types of targets without mechanical grippers or chemical adhesives. The control provided by magnetic fields means that motion and adhesion can be tuned separately, enabling complex tasks like assembly or navigation in fluid environments. And because the system operates at the millimeter scale, it opens up possibilities for applications in confined or sensitive spaces, such as inside machinery or the human body.
By demonstrating consistent performance across solid and liquid targets, in dry and submerged environments, the researchers show a path toward general-purpose manipulation at small scales. The combination of magnetic control and microengineered adhesion may allow soft robots to take on a wider range of tasks without needing to be reconfigured for each one. While further work will be needed to scale production and validate performance in real-world conditions, this study lays important groundwork for multipurpose, reconfigurable soft robotic tools.
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