Inspired by the collective intelligence in natural swarms, microrobotic agents have been controlled to form artificial swarms for targeted drug delivery, enhanced imaging, and hyperthermia. Different from these well-investigated tasks, this work aims to develop microrobotic swarms for embolization, which is a clinical technique used to block blood vessels for treating tumors, fistulas, and arteriovenous malformations. Magnetic particle swarms were formed for selective embolization to address the low selectivity of the present embolization technique that is prone to cause complications such as stroke and blindness. We established an analytical model that describes the relationships between fluid viscosity, flow rate, branching angle, magnetic field strength, and swarm integrity, based on which an actuation strategy was developed to maintain the swarm integrity inside a targeted region under fluidic flow conditions. Experiments in microfluidic channels, ex vivo tissues, and in vivo porcine kidneys validated the efficacy of the proposed strategy for selective embolization.

Self-assembly of dynamic colloidal structures along the vertical direction has been challenging because of gravity and the complexity in controlling agent-agent interactions. Here, we present a strategy that enables the self-growing of gravity-resisting colloidal collectives. By designing a unique dual-axis oscillating magnetic field, time-varying interparticle interactions are induced to assemble magnetic particles against gravity into vertical collectives, with the structures continuing to grow until reaching dynamic equilibrium. The collectives have swarm behavior and are capable of height reconfiguration and adaptive locomotion, such as moving along a tilted substrate and under nonzero fluidic flow condition, gap and obstacle crossing, and stair climbing.

Sensing of key parameters in fluidic environments has attracted extensive attention because the physical features of body fluids could be used for point-of-care disease diagnosis. Although various sensing methods have been investigated, effective sensing strategies of microenvironments remains a major challenge. In this paper, we propose an approach that can realize sensing of fluidic viscosity and ionic strength using microswarms formed by simple colloidal nanoparticles. The influences of fluidic ionic strength and viscosity on two swarm behaviors are analyzed (i.e., the spreading of circular vortex-like swarms and the elongation of elliptical swarms). The data models for quantifying the fluidic viscosity and ionic strength are obtained from experiments, and the fluidic features can be sensed successfully using the swarm behaviors. Furthermore, we demonstrate that the microswarms have the capability of passing through tortuous and narrow microchannels for sensing. Continuous sensing of different fluidic environments using swarms is also realized. Finally, the sensing of viscosity and ionic strength of porcine whole blood is presented, which also validates the feasibility of the sensing strategy.