Surface Modification of Magnetoactive Elastomers by Laser Micromachining.

It has been recently demonstrated that laser micromachining of magnetoactive elastomers is a very convenient method for fabricating dynamic surface microstructures with magnetically tunable properties, such as wettability and surface reflectivity. In this study, we investigate the impact of the micromachining process on the fabricated material's structural properties and its chemical composition. By employing scanning electron microscopy, we investigate changes in size distribution and spatial arrangement of carbonyl iron microparticles dispersed in the polydimethylsiloxane (PDMS) matrix as a function of laser irradiation. Based on the images obtained by a low vacuum secondary electron detector, we analyze modifications of the surface topography. The results show that most profound modifications occur during the low-exposure (8 J/cm2) treatment of the surface with the laser beam. Our findings provide important insights for developing theoretical models of functional properties of laser-sculptured microstructures from magnetoactive elastomers.

Locally induced shockwaves for selective perforation of cargo loaded lipid vesicles with temporal and spatial control.

Controlled poration of lipid membranes is crucial for numerous biomimetic applications such as targeted drug delivery. Although several chemical and physical mechanisms have been proposed for the poration of synthetic membranes, achieving good temporal and spatial control remains a challenge. In this study, we introduce a novel method for membrane poration that utilizes the mechanical shockwave generated by the photo-acoustic effect, which occurs when an optically opaque microparticle is illuminated by a near-infrared laser of optical tweezers. We show that the shockwave effectively porates membranes of giant unilamellar vesicles in close proximity to the microparticle without damaging nearby cells, which is a desirable outcome for potential targeted drug delivery. The poration effect is nonspecific and operates on both liquid and gel phase membranes. Since the photo-acoustic effect can be triggered by standard optical tweezers, this method holds broad applicability in various experimental settings within the field of soft matter research.

Dynamically tunable lamellar surface structures from magnetoactive elastomers driven by a uniform magnetic field.

Stimuli responsive materials are key ingredients for any application that requires dynamically tunable or on-demand responses. In this work we report experimental and theoretical investigation of magnetic-field driven modifications of soft-magnetic elastomers whose surface was processed by laser ablation into lamellar microstructures that can be manipulated by a uniform magnetic field. We present a minimal hybrid model that elucidates the associated deflection process of the lamellae and explains the lamellar structure frustration in terms of dipolar magnetic forces arising from the neighbouring lamellae. We experimentally determine the magnitude of the deflection as a function of magnetic flux density and explore the dynamic response of lamellae to fast changes in a magnetic field. A relationship between the deflection of lamellae and modifications of the optical reflectance of the lamellar structures is resolved.

Adhesion and Stiffness of Detached Breast Cancer Cells In Vitro: Co-Treatment with Metformin and 2-Deoxy-d-glucose Induces Changes Related to Increased Metastatic Potential.

Metastatic cancer cells can overcome detachment-induced cell death and can proliferate in anchorage-independent conditions. A recent study revealed that a co-treatment with two drugs that interfere with cell metabolism, metformin and 2-deoxy-D-glucose, promotes detachment of viable MDA-MB-231 breast cancer cells. In the present study, we analyzed if these detached viable MDA-MB-231 cells also exhibit other features related to cancer metastatic potential, i.e., if they are softer and more prone to adhere to epithelial cells. The cell mechanics of attached cells and floating cells were analyzed by optical tweezers and cell deformability cytometry, respectively. The adhesion was assessed on a confluent monolayer of HUVEC cells, with MDA-MB-231 cells either in static conditions or in a microfluidic flow. Additionally, to test if adhesion was affected by the state of the epithelial glycocalyx, HUVEC cells were treated with neuraminidase and tunicamycin. It was found that the treated MDA-MB-231 cells were more prone to adhere to HUVEC cells and that they were softer than the control, both in the floating state and after re-seeding to a substrate. The changes in the HUVEC glycocalyx, however, did not increase the adhesion potential of MDA-MB-231.

Emergence of self-organized multivortex states in flocks of active rollers.

Active matter, both synthetic and biological, demonstrates complex spatiotemporal self-organization and the emergence of collective behavior. A coherent rotational motion, the vortex phase, is of great interest because of its ability to orchestrate well-organized motion of self-propelled particles over large distances. However, its generation without geometrical confinement has been a challenge. Here, we show by experiments and computational modeling that concentrated magnetic rollers self-organize into multivortex states in an unconfined environment. We find that the neighboring vortices more likely occur with the opposite sense of rotation. Our studies provide insights into the mechanism for the emergence of coherent collective motion on the macroscale from the coupling between microscale rotation and translation of individual active elements. These results may stimulate design strategies for self-assembled dynamic materials and microrobotics.

Reconfigurable structure and tunable transport in synchronized active spinner materials.

Ensembles of actuated colloids are excellent model systems to explore emergent out-of-equilibrium structures, complex collective dynamics, and design rules for the next generation materials. Here, we demonstrate that ferromagnetic microparticles suspended at an air-water interface and energized by an external rotating magnetic field spontaneously form dynamic ensembles of synchronized spinners in a certain range of the excitation field parameters. Each spinner generates strong hydrodynamic flows, and collective interactions of the multiple spinners promote a formation of dynamic lattices. On the basis of experiments and simulations, we reveal structural transitions from liquid to nearly crystalline states in this novel active spinner material and demonstrate that dynamic spinner lattices are reconfigurable, capable of self-healing behavior and that the transport of embedded inert cargo particles can be remotely tuned by the parameters of the external excitation field. Our findings provide insights into the behavior of active spinner materials with reconfigurable structural order and tunable functionalities.

Guided Self-Assembly and Control of Vortices in Ensembles of Active Magnetic Rollers.

Active magnetic colloids are capable of rich collective behavior and complex self-organization. The interplay between short- and long-range interactions taking place away from equilibrium often results in a spontaneous formation of localized dynamic microstructures. Here we report a method for guided self-assembly and control of self-organized colloidal vortices emerging in a ferromagnetic particle ensemble energized by a uniaxial alternating (ac) magnetic field. The structure of a vortex composed of rolling magnetic particles can be stabilized and manipulated by means of an additional strongly localized alternating magnetic field provided by a minicoil. By tuning the parameters of the localized field, we effectively control the dimensions and particle number density in the vortex. We find that the roller vortex self-organization is assisted by field-induced magnetic "steering" rather than magnetic field gradients and is only possible while the system is in the active (magnetic rollers) state. We demonstrate that parameters of the emergent vortex are efficiently tuned by a phase shift between alternating magnetic fields. The method for assisted self-organization of rolling magnetic colloids into a vortex with on-demand characteristics suggests a new tool for active matter control and manipulation that may lead to a development of new approaches toward the guided microscopic transport in active particle systems.

Diffusive ferromagnetic roller gas.

An ensemble of actively rotating ferromagnetic particles is used to realize an active roller gas. Here, we investigate the diffusive properties of such a gas in experiments and simulations. We reveal that ferromagnetic rollers demonstrate a normal (Fickian) diffusion with a characteristic linear growth of the mean-squared displacement, while statistics of displacements stay non-Gaussian. At short times the system has a bimodal distribution of the displacements that transitions with time to a quasi-Gaussian distribution (Gaussian core with overpopulated tails) for a range of studied particle number densities. Inert particles introduced into the active roller gas exhibit similar diffusive behavior. The results provide insights into diffusive properties of active colloidal systems with activity originating from spinning degrees of freedom.

Manipulation of emergent vortices in swarms of magnetic rollers.

Active colloids are an emergent class of out-of-equilibrium materials demonstrating complex collective phases and tunable functionalities. Microscopic particles energized by external fields exhibit a plethora of fascinating collective phenomena, yet mechanisms of control and manipulation of active phases often remains lacking. Here we report the emergence of unconfined macroscopic vortices in a system of ferromagnetic rollers energized by a vertical alternating magnetic field and elucidate the complex nature of a magnetic roller-vortex interactions with inert scatterers. We demonstrate that active self-organized vortices have an ability to spontaneously switch the direction of rotation and move across the surface. We reveal the capability of certain non-active particles to pin the vortex and manipulate its dynamics. Building on our findings, we demonstrate the potential of magnetic roller vortices to effectively capture and transport inert particles at the microscale.

Active turbulence in a gas of self-assembled spinners.

Colloidal particles subject to an external periodic forcing exhibit complex collective behavior and self-assembled patterns. A dispersion of magnetic microparticles confined at the air-liquid interface and energized by a uniform uniaxial alternating magnetic field exhibits dynamic arrays of self-assembled spinners rotating in either direction. Here, we report on experimental and simulation studies of active turbulence and transport in a gas of self-assembled spinners. We show that the spinners, emerging as a result of spontaneous symmetry breaking of clock/counterclockwise rotation of self-assembled particle chains, generate vigorous vortical flows at the interface. An ensemble of spinners exhibits chaotic dynamics due to self-generated advection flows. The same-chirality spinners (clockwise or counterclockwise) show a tendency to aggregate and form dynamic clusters. Emergent self-induced interface currents promote active diffusion that could be tuned by the parameters of the external excitation field. Furthermore, the erratic motion of spinners at the interface generates chaotic fluid flow reminiscent of 2D turbulence. Our work provides insight into fundamental aspects of collective transport in active spinner materials and yields rules for particle manipulation at the microscale.

Dynamic self-assembly and self-organized transport of magnetic micro-swimmers.

We demonstrate experimentally and in computer simulations that magnetic microfloaters can self-organize into various functional structures while energized by an external alternating (ac) magnetic field. The structures exhibit self-propelled motion and an ability to carry a cargo along a pre-defined path. The morphology of the self-assembled swimmers is controlled by the frequency and amplitude of the magnetic field.

Dynamic Assembly of Magnetic Colloidal Vortices.

Magnetic colloids in external time-dependent fields are subject to complex induced many-body interactions governing their self-assembly into a variety of equilibrium and out-of-equilibrium structures such as chains, networks, suspended membranes, and colloidal foams. Here, we report experiments, simulations, and theory probing the dynamic assembly of superparamagnetic colloids in precessing external magnetic fields. Within a range of field frequencies, we observe dynamic large-scale structures such as ordered phases composed of precessing chains, ribbons, and rotating fluidic vortices. We show that the structure formation is inherently coupled to the buildup of torque, which originates from internal relaxation of induced dipoles and from transient correlations among the particles as a result of short-lived chain formation. We discuss in detail the physical properties of the vortex phase and demonstrate its potential in particle-coating applications.

A novel strain energy relationship for red blood cell membrane skeleton based on spectrin stiffness and its application to micropipette deformation.

Red blood cell (RBC) membrane skeleton is a closed two-dimensional elastic network of spectrin tetramers with nodes formed by short actin filaments. Its three-dimensional shape conforms to the shape of the bilayer, to which it is connected through vertical linkages to integral membrane proteins. Numerous methods have been devised over the years to predict the response of the RBC membrane to applied forces and determine the corresponding increase in the skeleton elastic energy arising either directly from continuum descriptions of its deformation, or seeking to relate the macroscopic behavior of the membrane to its molecular constituents. In the current work, we present a novel continuum formulation rooted in the molecular structure of the membrane and apply it to analyze model deformations similar to those that occur during aspiration of RBCs into micropipettes. The microscopic elastic properties of the skeleton are derived by treating spectrin tetramers as simple linear springs. For a given local deformation of the skeleton, we determine the average bond energy and define the corresponding strain energy function and stress-strain relationships. The lateral redistribution of the skeleton is determined variationally to correspond to the minimum of its total energy. The predicted dependence of the length of the aspirated tongue on the aspiration pressure is shown to describe the experimentally observed system behavior in a quantitative manner by taking into account in addition to the skeleton energy an energy of attraction between RBC membrane and the micropipette surface.

Magnetically Self-Assembled Colloidal Three-Dimensional Structures as Cell Growth Scaffold.

Understanding the chemical and physical conditions for cell growth is important from biological and medical aspects. Many tissues and cell types (e.g., epithelial cells and neurons) naturally grow on surfaces that span in three-dimensions and offer structural or mechanical support. The scaffold surface has to promote adhesion and cell proliferation as well as support their weight and retain its structural integrity. Here, we present a flexible method that uses self-assembly of micrometer superparamagnetic particles to produce appropriate scaffold surfaces with controllable general appearance in three dimensions, such as oriented membranes, branched structure, or void network. As a proof of principle, the Chinese hamster ovary epithelial cell line was successfully grown for several days on inclined membranes. Robustness of the oriented membrane architecture was probed with optical tweezers. We measured the magnetic force holding one particle in a self-assembled upright hexagonal sheet and modeled it as a sum of pair interaction forces between spatially arrested static dipoles.

Emergence of reconfigurable wires and spinners via dynamic self-assembly.

Dissipative colloidal materials use energy to generate and maintain structural complexity. The energy injection rate, and properties of the environment are important control parameters that influence the outcome of dynamic self-assembly. Here we demonstrate that dispersions of magnetic microparticles confined at the air-liquid interface, and energized by a uniaxial in-plane alternating magnetic field, self-assemble into a variety of structures that range from pulsating clusters and single-particle-thick wires to dynamic arrays of spinners (self-assembled short chains) rotating in either direction. The spinners emerge via spontaneous breaking of the uniaxial symmetry of the energizing magnetic field. Demonstration of the formation and disaggregation of particle assemblies suggests strategies to form new meso-scale structures with the potential to perform functions such as mixing and sensing.

The dynamics of melittin-induced membrane permeability.

The transport of co-encapsulated solutes through the melittin-induced pores in the membrane of giant phospholipid vesicles was studied, and the characteristics of the pore formation process were modeled. Molecules of two different sizes (dextran and the smaller, fluorescent marker Alexa Fluor) were encapsulated inside the vesicles. The chosen individual vesicles were then transferred by micromanipulation from the stock suspension to the environment with the melittin (MLT). The vesicles were observed optically with a phase-contrast microscope and by monitoring the fluorescence signal. Such an experimental setup enabled an analysis of a single vesicle's response to the MLT on the basis of simultaneous, separate measurements of the outflow of both types of encapsulated molecules through the MLT-induced pores in the membrane. The mechanisms of the MLT's action were suggested in a model for MLT pore formation, with oligomeric pores continuously assembling and dissociating in the membrane. Based on the model, the results of the experiments were explained as a consequence of the membrane's permeability dynamics, with a continuously changing distribution of pores in the membrane with regard to their size and number. The relatively stable "average MLT pore" characteristics can be deduced from the proposed model.

Analysis of fluid flow around a beating artificial cilium.

Biological cilia are found on surfaces of some microorganisms and on surfaces of many eukaryotic cells where they interact with the surrounding fluid. The periodic beating of the cilia is asymmetric, resulting in directed swimming of unicellular organisms or in generation of a fluid flow above a ciliated surface in multicellular ones. Following the biological example, externally driven artificial cilia have recently been successfully implemented as micropumps and mixers. However, biomimetic systems are useful not only in microfluidic applications, but can also serve as model systems for the study of fundamental hydrodynamic phenomena in biological samples. To gain insight into the basic principles governing propulsion and fluid pumping on a micron level, we investigated hydrodynamics around one beating artificial cilium. The cilium was composed of superparamagnetic particles and driven along a tilted cone by a varying external magnetic field. Nonmagnetic tracer particles were used for monitoring the fluid flow generated by the cilium. The average flow velocity in the pumping direction was obtained as a function of different parameters, such as the rotation frequency, the asymmetry of the beat pattern, and the cilium length. We also calculated the velocity field around the beating cilium by using the analytical far-field expansion. The measured average flow velocity and the theoretical prediction show an excellent agreement.

Measurement of fluid flow generated by artificial cilia.

We observed and measured the fluid flow that was generated by an artificial cilium. The cilium was composed of superparamagnetic microspheres, in which magnetic dipole moments were induced by an external magnetic field. The interaction between the dipole moments resulted in formation of long chains-cilia, and the same external magnetic field was also used to drive the cilia in a periodic manner. Asymmetric periodic motion of the cilium resulted in generation of fluid flow and net pumping of the surrounding fluid. The flow and pumping performance were closely monitored by introducing small fluorescent tracer particles into the system. By detecting their motion, the fluid flow around an individual cilium was mapped and the flow velocities measured. We confirm that symmetric periodic beating of one cilium results in vortical motion only, whereas asymmetry is required for additional translational motion. We determine the effect of asymmetry on the pumping performance of a cilium, verify the theoretically predicted optimal pumping conditions, and determine the fluid behaviour around a linear array of three neighbouring cilia. In this case, the contributions of neighbouring cilia enhance the maximal flow velocity compared with a single cilium and contribute to a more uniform translational flow above the surface.