3D-Printed Magnetic Soft Robots Program Magnetism While They Print

A team of researchers has developed a 3D printing method that magnetizes soft materials during the printing process itself. The technique, described in Advanced Science, uses direct ink writing with a moving external magnet to program complex magnetic profiles directly into elastomer structures. The result is soft robots and sensors that can be controlled with external magnetic fields, without needing post-processing magnetization steps.

Why In-Process Magnetization Is Hard

Soft magnetic actuators have been around for years, but making them with arbitrary 3D magnetization patterns has been a bottleneck. Post-process magnetization with strong impulse fields only produces one-dimensional profiles. Deforming a structure before magnetizing restricts you to flat, simple shapes. Printing with the magnet at the nozzle confines magnetization to the print path, making it impossible to magnetize out-of-plane regions on curved shells.

The new approach places the magnet above the print bed and moves it in three dimensions while rotating it about two axes. After each layer cures, the magnet repositions and reorients to apply the next magnetization pattern. Because the magnet stays close to the material throughout fabrication, the magnetic field stays strong even at the top of tall builds. The researchers found that cylindrical samples magnetized from above had top-surface fields about 6.2 times stronger than those magnetized conventionally from below.

Tuning the Material

The team used neodymium-iron-boron microparticles suspended in silicone elastomer. They tested particle loadings from 10 to 50 weight percent, measuring coercivity, saturation magnetization, and mechanical flexibility at each level. A 40 weight percent formulation gave the best balance: strong magnetic output with print resolution around 50 to 100 micrometers and material flexibility near 10 megapascals.

Curing kinetics turned out to be critical. The researchers discovered that magnetizing the composite before full curing produces stronger net magnetization than magnetizing after cross-linking. Magnetic domains align more readily in low-viscosity, uncured material. A bed temperature of 60 degrees Celsius, yielding a nine-minute cure, best synchronized cross-linking with the magnetization step. They also measured the minimum safe distance between the magnet and uncured droplets; after 150 seconds at 50 degrees Celsius, the jumping distance fell by 76 percent, making the process much safer to run.

What They Demonstrated

The researchers printed several functional devices. Thin strips with two different magnetization regions bent in distinct patterns under the same external field. A cubic shell with each face magnetized normal to its surface deformed in competing ways depending on field orientation. Flexible mesh cubes acted as strain sensors: compression, tension, and bending each changed the stray magnetic field detected by a Hall sensor, with stable responses over 2000 cycles. A flexible keycard with sixteen independently magnetized pixels generated unique field signatures that triggered distinct LED patterns.

Continuous magnetization profiles were achieved by sweeping a rotating magnet along strips. A single full rotation created a strip with one central attractive bulge, while 1.5 rotations produced simultaneous attraction and repulsion zones. A planar flower with quarter-rotation magnetization per petal folded into S-shaped patterns unattainable with uniform magnetization.

Bio-Inspired Demonstrators

The most striking demonstrations were biologically inspired. A two-centimeter dragonfly with oppositely magnetized wings flapped near a 7-hertz resonant frequency. A neutrally buoyant robotic octopus with eight continuously magnetized arms swam vertically at up to 10.4 millimeters per second and glided laterally at 17.4 millimeters per second. A hollow serpentine catheter, 20 centimeters long with six discrete magnetic nodes, steered through a mock bifurcated tissue environment while self-lubricating and delivering a dye payload, mimicking minimally invasive drug delivery.

What It Means

This is not just an academic proof of concept. The catheter demonstration, in particular, shows a path toward magnetically navigable surgical tools that could be printed on demand for specific patient anatomies. The strain-sensing meshes suggest wearable or embedded sensors that report damage or deformation without wires. And the ability to program arbitrary 3D magnetization profiles opens design space that was previously inaccessible to soft robotics engineers.