Characterizing Nanoparticle Swarms with Tuneable Concentrations for Enhanced Imaging Contrast

Microrobots capable of performing targeted delivery are promising for biomedical applications. Due to the restriction of their small size and volume, however, the real-time in vivo imaging strategy for microrobots meets limitations at the current stage. Performing swarm actuation and control may be a promising candidate to increase the contrast of the imaging feedback signal. Herein, we use paramagnetic nanoparticles as tiny agents to systematically investigate the relationship between the imaging intensity and the areal concentration of the nanoparticles using three different bio-imaging approaches. Magnetic actuation strategies are applied to effectively tune the nanoparticle concentration via spread and gathering processes. Three imaging modalities are applied, including fluorescent imaging (FI), ultrasound imaging (UI), and photoacoustic imaging (PAI) for tracking the nanoparticles. The results of spreading and vortex-like swarm generation (gathering) are first validated using optical microscope, and then the processes are performed under the monitoring of FI, UI, and PAI, respectively. The change of the imaging intensity with the particle areal concentration is analyzed. This work supports that forming a high-concentrated swarm enhances the intensity of feedback signals with multiple imaging methods.