Artificial swarming: Towards radiofrequency control of reversible micro-particle aggregation and deposition Nina Sarvašová a , Pavel Ulbrich b , Viola Tokárová a , Aleš Zadražil a , František Štěpánek a, ⁎ a Department of Chemical Engineering, Institute of Chemical Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic b Department of Biochemistry Microbiology, Institute of Chemical Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic abstract article info Article history: Received 17 September 2014 Received in revised form 14 January 2015 Accepted 17 January 2015 Available online 26 January 2015 Keywords: Stimuli-responsive particles Silica Magnetite PNIPAM Radiofrequency heating MRI 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 di- ameter 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 SiO 2 /Fe 3 O 4 /PNIPAM microparticles with a stimuli-responsive porous shell that possess the fol- lowing functionalities: (i) RF-induced local particle heating, due to the presence of superparamegnetic nanopar- ticles 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 hydro- philic/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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction While the radiofrequency (RF) control of mechatronic systems is ubiquitous in daily life and technology, the ability to remotely control molecular or colloidal systems using RF signals has so far been exploited rather scarcely. To achieve remote control of a chemical system, at least two conditions must be met: (i) the system has to be stimuli-responsive in some way, and (ii) there must exist a means of executing the stimulus without direct physical intervention with the chemical envi- ronment. Stimuli-responsive chemical systems—i.e. systems able to dynamically respond to changes in the physico-chemical properties of their microenvironment—can be traced back to the concept of thermo-sensitive liposomes [1]. Since then, numerous other chemical systems responsive to various endogenous or exogenous stimuli have been described. Examples of endogenous stimuli include pH-sensitive drug-delivery systems [2], redox-sensitive systems [3] and enzyme- sensitive nanocarriers [4]. Exogenous stimuli such as ultrasound [5], light of a specific wavelength [6], or magnetic field [7–9] have been reported. Thermo-responsive systems can have the form of microgels, vesicles, polymer brushes or other structures containing a material exhibiting a phase transition at a given temperature [10]. The phase transition then leads to a step change in a macroscopically observable property such as solubility, permeability, conductivity or wettability of the sys- tem. For example, the polymer poly-N-isopropylacrylamide (PNIPAM) exhibits a lower critical solution temperature (LCST), meaning that it is hydrophilic at temperatures below approx. 32 °C but changes its char- acter to hydrophobic above this temperature. Temperature cycling can then lead to phenomena such as shrinking-swelling behaviour of a gel [11,12] or the change in activity of cell-penetrating peptides [13]. The remote control of thermoresponsive systems requires that a local tem- perature change can be achieved at some distance without physical con- tact with the system. This is in principle possible by including an additional component into the system (i.e., a susceptor), liable to re- mote triggering by energy in other forms than heat. For instance, stimuli realized by electromagnetic field in the form of near-infrared (NIR) photo-thermal energy [14], inductively coupled magnetic (ICM) field [15] or radiofrequency (RF) field [16] have been already demonstrated to induce heating of metallic nanoparticles and consequently deliver targeted hyperthermia. However, non-invasive NIR irradiation is often limited by its low penetration depth [17] and ICM field by the applicable magnetic field Powder Technology 278 (2015) 17–25 ⁎ Corresponding author. Tel.: +420 220 443 236; fax: +420 220 444 320. E-mail address: Frantisek.Stepanek@vscht.cz (F. Štěpánek). http://dx.doi.org/10.1016/j.powtec.2015.01.030 0032-5910/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec