Vat Photopolymerisation AM enables magnetic microscale robotics

Researchers from the Massachusetts Institute of Technology (MIT), USA, together with collaborators from the École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, and the University of Cincinnati, Ohio, have detailed a new approach for fabricating magnetically responsive microscale structures using Additive Manufacturing. The work was published in Matter and focuses on overcoming long-standing limitations in the production of soft magnetic microdevices.

Researchers demonstrated the technology using a series of micron-scale soft robotic structures, including a magnetic gripper and a bistable switch. The team stated that the approach could support future developments in minimally invasive medical devices, microfluidics and programmable metamaterials.

The fabrication process combines two-photon polymerisation — a high-resolution Vat Photopolymerisation Additive Manufacturing process — with a post-processing co-precipitation method that forms iron oxide nanoparticles directly within an additively manufactured hydrogel structure.

Conventional attempts to fabricate magnetic microscale structures by directly mixing magnetic particles into photopolymer resins have faced challenges due to particle-induced light scattering and agglomeration, which can reduce print fidelity and structural integrity. The MIT-led team instead produced the structures in two stages.

First, researchers used two-photon lithography to additively manufacture polymer hydrogel microstructures without magnetic additives. The resultant parts were then immersed in an iron-ion solution, followed by a hydroxide-ion bath. This secondary treatment generated magnetic iron oxide nanoparticles within the hydrogel matrix itself.

By adjusting laser exposure during the AM process, the researchers could locally vary the hydrogel cross-link density, allowing selective control over how many nanoparticles formed in different regions of a structure. This enabled the production of components with spatially tuned magnetic and mechanical behaviour within a single part.

“We can now make a soft, intricate 3D architecture with components that can move and deform in complex ways within the same microscopic structure,” stated Carlos Portela, the Robert N Noyce Career Development Associate Professor of Mechanical Engineering at MIT.

The team demonstrated the concept using lollipop-shaped microstructures measuring less than 1 mm in height. When exposed to an external magnetic field, the structures bent and snapped together in a gripping motion similar to a Venus flytrap.

Researchers also produced a bistable magnetic switch consisting of a small hydrogel beam with four magnetic paddle-like structures attached. Applying a magnetic field caused the paddles to rotate and lock the beam into different stable positions, suggesting potential applications in microfluidic valves and microscale actuators.

According to the paper, the method enables “precise control of mechanical and magnetic properties toward microscale metamaterial and robotics applications.”

Rachel Sun, co-lead author of the study, stated, “This provides unprecedented design freedom to print multifunctional structures and materials at the microscale.”

The researchers stated that this technique could support the development of remotely actuated soft microrobots capable of performing tasks such as targeted drug delivery or minimally invasive biopsy collection.

‘Magnetically responsive microprintable soft nanocomposites with tunable nanoparticle loading’ is available here.

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