Artificial swarming: Towards radiofrequency control of reversible micro-particle aggregation and deposition

• Design and synthesis of mutlifunctional microparticles
• Reversible aggregation controlled by radiofrequency field
• Five-fold increase in release rate due to phase transition in composite shell
• Physical realization of artificial swarming system

The ability to undergo a transition between dispersed or single-cellular state and aggregated or multi-cellular state provides distinct evolutionary advantages to many natural organisms. Due to a change of hydrodynamic diameter over several orders of magnitude and associated change of fluid–particle interaction (settling velocity) or intra-particle transport phenomena (heat transfer and/or diffusion) that typically scales with the square of the particle size, radically different behaviour can be achieved in terms of transport in a fluid environment, sourcing nutrition, escaping predators or maintaining homeostasis. In this work we report on the implementation of an artificial system that is able to undergo a reversible transition between dispersed and aggregated state, using the principles of “remote control” by radiofrequency (RF) signals. The individual artificial cells are represented by hollow-core SiO2/Fe3O4/PNIPAM microparticles with a stimuli-responsive porous shell that possess the following functionalities: (i) RF-induced local particle heating, due to the presence of superparamegnetic nanoparticles in the structure; (ii) temperature switchable storage/release functionality due to a combination of hollow-core porous silica skeleton with a PNIPAM layer; and (iii) temperature switchable aggregation, due to the hydrophilic/hydrophobic transition of the PNIPAM layer. The combination of RF-switchable aggregation and temperature-responsive release kinetics of a lipophilic substance makes it possible to trigger particle aggregation and deposition remotely, and thus control the release kinetics of encapsulated payload in both time and space.

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