Tunable Magnetic Trap: Using Passive Elements to Control Magnetic Microrobots

Magnetic microrobots have the ability to navigate through difficult-to-reach environments, thereby holding promise for applications in the fields of micromanipulation and biomedicine. The actuation of these microrobots requires the generation and control of magnetic fields and gradients. However, the decay of gradients with the distance the (
$d$
) from the magnetic source (as
$1/d^{4}$
), poses challenges in generating gradients using remote sources. A possible solution is to use miniaturized magnetic sources that can be positioned closer to the microrobot. However, the small size of miniature magnetic sources limits the fields that can be generated. In this study, we propose a tunable magnetic trap—a system that combines remote magnetic field generation and local passive gradient generation. The remote field generation is achieved by a traditional Helmholtz system while gradients are generated locally by an arrangement of ferrite rods. The system benefits from strong external electromagnets and local gradient generation. The magnitude and direction of the magnetic gradient can be regulated with the external field, enabling the actuation of particles in the magnetic trap. The proposed system is validated experimentally through the control of a magnetic particle floating on the air-water interface. Our results show that relatively low magnetic fields (<5 mT) are necessary to displace a
$500 \,\mathrm{\mu }\mathrm{m}$
metallic particle against the meniscus capillary force.