Nature provides numerous examples of organisms that self-assemble into collectives. These include bird flocks and bacteria colonies, which carry out complex tasks that surpass individual capabilities. Scientists draw inspiration from these natural life forms in developing collective systems of artificial active matter.
Self-Assembled Colloidal Collectives
For decades, scientists have been trying to emulate the collective behavior of natural systems in creating synthetic microscale robots. These microrobots can perform various tasks, from drug delivery to environmental remediation. However, enabling robust 3D motion in these synthetic swarms without dispersion is challenging.
In a collaborative study entitled "Bioinspired self-assembled colloidal collectives drifting in three dimensions underwater," researchers from the Chinese University of Hong Kong and the Max Planck Institute for Intelligent Systems have developed a bimodal actuation strategy for making bioinspired robots. They combined magnetic and optical fields to achieve controlled 3D movement of colloidal robot collectives.
Most existing colloidal microrobots depend on physical boundaries to move around. For instance, ultrasound-activated colloidal particles use the walls of their container to "walk" along surfaces. This strong dependence hinders maneuverability since the microrobots cannot detach from substrates or overcome barriers that are larger than them. Although some 3D control has been demonstrated with magnetically-driven helical swimmers, these systems struggle to maintain cohesion during 3D movement.
To make this innovation possible, scientists take inspiration from nature, specifically the migration mechanism of plankton. To adapt this mechanism to synthetic microrobot collectives, experts need to address three main challenges: enabling a smooth transition across the air-water interface, achieving underwater 3D locomotion against gravity, and preventing dispersion of the swarm during drifting.
The research team used magnetic and optical fields to achieve these goals, a combination of modalities that allowed smooth 3D locomotion underwater and at the air-water interface. The team pioneered collectives entirely detached from any boundaries, which can demonstrate horizontal drifting, hovering, and vertical ascent. The swarm also remained cohesive as a single stable entity throughout the drifting movement. Further studies confirmed that the collective's flip-over maneuvers and posture adjustments can be carried out mid-ascent or descent by adjusting the magnetic field orientation.
This adaptive locomotion can enable microrobot collectives to traverse terrains impassable to other microscopic locomotive systems. The proposed actuation strategy and 3D capabilities can inspire future mobile microsystems across micromedicine, environmental remediation, and materials science.
Migration Mechanism of Planktons
Plankton refers to a diver collection of organisms that live at and beneath the surface of oceans, ponds, rivers, and lakes. This term is taken from the Greek word for "drifter" or "wanderer." An organism is classified as plankton if currents and tides carry it without the ability to swim well enough to move against such forces.
It was initially thought that plankton are exclusive water inhabitants, but it was found that there are also airborne species that live part of their lives drifting in the air. These organisms travel long distances by drifting with currents and tides instead of swimming against them.
Most plankton are denser than water, so they rely on buoyancy and locomotion to stay suspended in the water column. They move through the action of cilia, antennae, jointed appendages, muscle contractions in the body wall, and jet propulsion.
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