Shrinkable Hydrogel-Based Magnetic Microrobots for Interventions in the Vascular Network

We previously showed that microrobots containing ferromagnetic or superparamagnetic material can be propelled in the vascular network while being tracked for navigation control purposes using magnetic gradients generated by a clinical magnetic resonance imaging (MRI) scanner. Here, we show that it is possible to synthesize such microrobots to allow them to change size in response to heat while maintaining the same gradient-based propulsion and MRI-based tracking characteristics of the previous versions. These microrobots are made of magnetic nanoparticles (MNPs) encapsulated in thermo-sensitive hydrogels (poly(N-isopropylacrylamide)). This configuration allows them to shrink in response to temperature elevation caused by the embedded MNPs when exposed to an AC magnetic field. In this paper, spherical PNIPA–MNP microrobots were synthesized and propelled using magnetic gradients of 400 mT/m inside a clinical MRI scanner. The same MRI scanner was used for imaging and tracking of the microrobots before the same microrobots were heated by an AC magnetic field of 4 kA/m at 160 kHz, resulting in a 25% volume reduction of the microrobots. These results suggest the possibility of implementing advanced polymorphic microrobots to accomplish complex tasks in the human body.