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The use of global magnetic fields—typically produced by pairs of Maxwell and Helmholtz coils—offers the advantage of generating uniform magnetic fields and magnitude uniform gradient distributions over extensive volumes, while ensuring easy control. However, achieving selective control of multiple magnetic microrobots under such global fields remains problematic. In this study, we utilize a magnetic focus field generated by a pair of Maxwell coils to impede the magnetic torque-based movement of magnetic robots confined within channels. Although the magnetic field is null at the focus field’s center, it increases linearly in all directions. This exerts magnetic forces that push robots, which are situated away from the center, toward the channel walls. This not only restricts their positioning but also reduces their responsiveness to additional fields. By introducing uniform dc magnetic fields to the focus field, we can change the controlled robot. This mechanism for selective locomotion has been empirically validated with the selective movement of two helical magnetic robots and swarms of magnetic microparticles. Furthermore, leveraging the same focus field, we present a novel separation mechanism for magnetic particle swarms, enhancing the aforementioned selective locomotion mechanism. The practical implications of our proposed locomotion mechanism have been showcased in targeted delivery, drilling, and improved magnetic heating experiments.

Heat can be induced within a tissue enriched with magnetic nanoparticles by exposing it to an alternating magnetic field. In this paper we examine the evaluation of the heat distribution and therefore the temperature development around such a heat source which can be suitable for magnetic heating treatment. We study the heat transfer from tissue enriched with magnetic nanoparticles to regions of no or minor enrichment of nanoparticles, particularly. The evaluation of the temperature distribution took place with the help of a tissue phantom. The phantom is composed of two concentric cylinders. The inner cylinder consists of a defined mixture of polyurethane gel and magnetic fluid. This cylinder represents tissue enriched with nanoparticles. The outer cylinder, which stands for pure tissue consists of polyurethane only. This tissue phantom has been exposed to an alternating magnetic field according to the protocol of the magnetic heating treatment. The temperature measurements were performed by thermocouples which are placed on defined positions. The experimentally obtained temperature data is the basis for a finite element method (FEM) simulation model. The FEM model allows the determination of heat transition from regions enriched with magnetic nanoparticles to regions with no or minor nanoparticle accumulation. [ABSTRACT FROM AUTHOR]

Herein, we have synthesized citric acid functionalized monodisperse (6–12 nm) octahedral-shaped Fe3O4 nanoparticles (NPs). The as-prepared octahedral-shaped Fe3O4 NPs exhibit a bulk-like magnetization ( \sim 92 emu/g) value due to lesser surface spin disorder. In addition, compared with spherical morphology, these octahedral NPs (Oct-NPs) exhibit much higher specific absorption rate (SAR) value as a potential candidate for hyperthermia treatment and higher R_{2} value as an efficient contrast agent for magnetic resonance imaging. The enhanced R_{2} relaxivity and SAR values are attributed to the higher saturation magnetization ( MS ) value and a larger surface area of Oct-NPs. The R2 value of 12 nm-sized Oct-NPs suspension is 353 ${{\rm {mM}}^{-1}}{{\rm {s}}^{-1}} , which is 3.5 times higher than that of the commercial contrast agent. With increase in size (6–12 nm), the R2 value gradually increases from 198 to 353 {{mM}^{-1}}{{{s}}^{-1}} ; this increase is a consequence of the MS enhancement as well as the surface area. [ABSTRACT FROM AUTHOR]