Reversible Swelling and Shrinking of Paramagnetic Nanoparticle Swarms in Biofluids With High Ionic Strength

Abstract

A microrobotic swarm has drawn extensive attention recently, due to its potential advantages in real-time tracking and targeted delivery in vivo. Herein, we use paramagnetic nanoparticles with a diameter of 500 $\mu \text{m}$ as building blocks to investigate the reversible pattern transformation of a swarm, i.e., swelling and shrinking, in biofluids by simulation and experiments. Three kinds of biofluids, i.e., phosphate buffered saline with Tween-20, simulated body fluid (SBF), and fetal bovine serum, are used, which have much higher ionic strength (up to 0.212 mol/L) and different viscosity compared with deionized water. Our results indicate that, except for the magnetic dipole interactions and hydrodynamic effects, the electrostatic interaction also plays an important role for swarm generation and reconfiguration in biofluids. The equivalent electrostatic torques exerted between nanoparticles, induced by ions, are quantified by an analytical model and are successfully compensated by tuning applied magnetic fields. The input magnetic field has a maximum field strength of 10 mT and a maximum frequency of 40 Hz. The experimental results validate the simulation, and meanwhile, the magnetic particle swarm is capable of performing significant swelling in the biofluids. In addition, by applying rotating magnetic fields, the swelled patterns can be reversibly shrunk into more concentrated regions.