Cell-assisted assembly of colloidal crystallites

Active osmoticlike pressure on permeable inclusions

We use a standard minimal active Brownian model to investigate the osmotic-like effective pressure generated by active fluids on fixed hollow inclusions. These inclusions are enclosed by a permeable (albeit nonflexible) membrane, and the interior and exterior regions of the inclusions have different particle motility strengths. We consider both rectangular and disklike inclusions and analyze the effects of various system parameters, such as excluded volume interaction between active particles, hardness of membrane, and active particle density, on the effective pressure produced on the enclosing membrane. We focus on the range of intermediate to high motility strengths and analyze the effective pressure in the steady state. Our findings for the active pressure produced in the interior and exterior regions of the inclusion indicate that the pressure is higher in the region with lower motility due to the relatively stronger accumulation of active particles.

Alignment-mediated segregation in an active-passive mixture

We report segregation between the athermal active and passive particles mediated by the local alignment interaction in a confined space. The competition between the alignment interaction and self-propulsion force results in a transition between disordered and ordered phases. We show that as the coordination between the particles increases, they form an ordered mill, which helps the particles to aggregate into isotropic clusters. As a result, particles segregate into active core and passive shells. This segregation phenomenon is adversely affected by the packing fraction and the size dispersion between active and passive particles. We show that this adverse effect can be overcome by incorporating higher coordination in the system. We report that the monodispersed system is more desirable for segregation in a binary mixture than a bidispersed system, as the latter favors the mixed state.

Effective interactions mediated between two permeable disks in an active fluid

We study steady-state properties of a bath of active Brownian particles (ABPs) in two dimensions in the presence of two fixed, permeable (hollow) disklike inclusions, whose interior and exterior regions can exhibit mismatching motility (self-propulsion) strengths for the ABPs. We show that such a discontinuous motility field strongly affects spatial distribution of ABPs and thus also the effective interaction mediated between the inclusions through the active bath. Such net interactions arise from soft interfacial repulsions between ABPs that sterically interact with and/or pass through permeable membranes assumed to enclose the inclusions. Both regimes of repulsion and attractive (albeit with different mechanisms) are reported and summarized in overall phase diagrams.

Assembly structures and dynamics of active colloidal cells

Many types of active matter are deformable, such as epithelial cells and bacteria. To mimic the feature of deformability, we built a model called an active colloidal cell (ACC), i.e. a vesicle enclosed with self-propelled particles (SPPs), which as a whole can move actively. Based on the model, we then study the role of deformability in the assembly structures and dynamics of ACCs by Langevin dynamics simulation. We find that deformability weakens the self-trapping effect and hence suppresses the clustering and phase separation of the deformable soft ACCs (sACCs). Instead of forming a large compact cluster like ordinary SPPs, sACCs pack into a loose network or porous structure in the phase-separation region. The condensed phase is liquid-like, in which sACCs are strongly compressed and deformed but still keep high motility. The interface between the gas and the condensed phases is blurry and unstable, and the effective interfacial energy is very low. Our work gives new insights into the role of deformability in the assembly of active matter and also provides a reference for further studies on different types of deformable active matter.

Phagosomal transport depends strongly on phagosome size

Macrophages internalize pathogens for intracellular degradation. An important part of this process is the phagosomal transport from the cell periphery to the perinuclear region. Biochemical factors are known to influence the fate of phagosomes. Here, we show that the size of phagosomes also has a strong influence on their transport. We found that large phagosomes are transported persistently to the nucleus, whereas small phagosomes show strong bidirectional transport. We show that dynein motors play a larger role in the transport of large phagosomes, whereas actin filament-based motility plays a larger role in the transport of small phagosomes. Furthermore, we investigated the spatial distribution of dyneins and microtubules around phagosomes and hypothesize that dynein and microtubule density differences between the nucleus-facing side of phagosomes and the opposite side could explain part of the observed transport characteristics. Our findings suggest that a size-dependent cellular sorting mechanism might exist that supports macrophages in their immunological roles.

Biomaterial Strategies for Immunomodulation

Strategies to enhance, suppress, or qualitatively shape the immune response are of importance for diverse biomedical applications, such as the development of new vaccines, treatments for autoimmune diseases and allergies, strategies for regenerative medicine, and immunotherapies for cancer. However, the intricate cellular and molecular signals regulating the immune system are major hurdles to predictably manipulating the immune response and developing safe and effective therapies. To meet this challenge, biomaterials are being developed that control how, where, and when immune cells are stimulated in vivo, and that can finely control their differentiation in vitro. We review recent advances in the field of biomaterials for immunomodulation, focusing particularly on designing biomaterials to provide controlled immunostimulation, targeting drugs and vaccines to lymphoid organs, and serving as scaffolds to organize immune cells and emulate lymphoid tissues. These ongoing efforts highlight the many ways in which biomaterials can be brought to bear to engineer the immune system.

Aggregation and segregation of confined active particles

We simulate a model of self-propelled disks with soft repulsive interactions confined to a box in two dimensions. For small rotational diffusion rates, monodisperse disks spontaneously accumulate at the walls. At low densities, interaction forces between particles are strongly inhomogeneous, and a simple model predicts how these inhomogeneities alter the equation of state. At higher densities, collective effects become important. We observe signatures of a jamming transition at a packing fraction ϕ ∼ 0.88, which is also the jamming point for non-active athermal monodisperse disks. At this ϕ, the system develops a critical finite active speed necessary for wall aggregation. At packing fractions above ϕ ∼ 0.6, the pressure decreases with increasing density, suggesting that strong interactions between particles are affecting the equation of state well below the jamming transition. A mixture of bidisperse disks segregates in the absence of any adhesion, identifying a new mechanism that could contribute to cell sorting in embryonic development.

Mixture of nonionic/ionic surfactants for the formulation of nanostructured lipid carriers: effects on physical properties

The objective of the present work was to investigate the effects of the mixture of nonionic/ionic surfactants on nanostructured lipid carriers (NLCs). Nonionic surfactant (polyethylene-poly(propylene glycol), Pluronic F68) and ionic surfactant (octenylsuccinic acid modified gum arabic, GA-OSA) were chosen as emulsifier for NLCs. The NLCs systems, which were composed of lipid matrix, modified 4-dedimethylaminosancycline (CMT-8), and various emulsifier agents, were characterized with dynamic light scattering (DLS), high performance liquid chromatography (HPLC), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), in vitro release, and phagocytosis assay. This mixture of nonionic/ionic surfactants showed significant effects on physical properties including particle size, polydispersity index (PDI), entrapment efficiency, and particle morphology. Compared with single stabilizer, this mixed nonionic/ionic surfactant system provided NLCs with better drug carrier properties including prolonged release profile and low phagocytosis by phagocyte. We expect that these explorations can provide a new strategy for the development of lipid nanoparticles as drug delivery.

Adaptive micro and nanoparticles: temporal control over carrier properties to facilitate drug delivery

Recent studies have led to significant advances in understanding the impact of key drug carrier properties such as size, surface chemistry and shape on their performance. Converting this knowledge into improved therapeutic outcomes, however, has proved challenging. This owes to the fact that successful drug delivery carriers have to navigate through multiple physiological hurdles including reticuloendothelial system (RES) clearance, target accumulation, intracellular uptake and endosomal escape. Each of these processes may require unique, and often conflicting, design parameters, thus making it difficult to choose a design that addresses all these hurdles. This challenge can be addressed by designing carriers whose properties can be changed in time so as to successfully navigate them through various biological hurdles. Several carriers have been reported that implement this strategy. This review will discuss the current status and future prospects of this emerging field of "adaptive micro and nanoparticles".

Optical chromatography using a photonic crystal fiber with on-chip fluorescence excitation

We describe the realization of integrated optical chromatography, in conjunction with on-chip fluorescence excitation, in a monolithically fabricated poly-dimethylsiloxane (PDMS) microfluidic chip. The unique endlessly-single-mode guiding property of the Photonic Crystal Fiber (PCF) facilitates simultaneous on-chip delivery of beams to perform optical sorting in conjunction with fluorescence excitation. We use soft lithography to define the chip and insert the specially capped PCF into it through a predefined fiber channel that is intrinsically aligned with the sorting channel. We compare the performance of the system to a standard ray optics model and use the system to demonstrate both size-driven and refractive index-driven separations of colloids. Finally we demonstrate a new technique of enhanced optofluidic separation of biological particles, by sorting of human kidney embryonic cells (HEK-293), internally tagged with fluorescing microspheres through phagocytocis, from those without microspheres and the separation purity is monitored using fluorescence imaging.

In-vitro sensing of biomechanical forces in live cells by a whispering gallery mode biosensor

Direct measurement of the biomechanical stress induced by a live cell during endocytosis is reported. Fluorescent dye-doped polystyrene microspheres were used as microscopic remote optical sensors applying whispering gallery modes (WGMs) as transducer mechanism. Monitoring of the WGMs throughout the incorporation of the microsphere into the cell enabled the determination of the deformation experienced by the microsphere, characterized by both a broadening and a blue shift of the resonances, and consequently the stress induced by the cell. The results reveal an unexpectedly high stress with a magnitude of up to five times that of the passive cortical tension, which can be only explained by a so far undetermined active stress component induced by the cytoskeletal machinery during particle incorporation. The method is adaptable to the study of any other kind of phagocyte and thus provides a novel research tool of high interest for cell biology.

Rapid and dynamic intracellular patterning of cell-internalized magnetic fluorescent nanoparticles

Conjugated magnetic nanoparticles have recently demonstrated potential in activating unique and specific activity within cells. Leveraging microfabrication, we have developed a technique of localizing nanoparticles to specific, subcellular locations by a micropatterned ferromagnetic substrate. Controlled patterns of nanoparticles were assembled and dynamically controlled with submicrometer precision within live cells. We anticipate that the technique will be useful as a compact, simple method of generating localizable, subcellular chemical and mechanical signals, compatible with standard microscopy.

Optical redistribution of microparticles and cells between microwells

The shaping of laser beams has developed into a powerful tool for optical micromanipulation. In this context, Airy and parabolic laser beams which follow curved trajectories have drawn considerable attention. These beams may allow clearing of microparticles through particle transport along curved paths, a concept termed "optically mediated particle clearing (OMPC)." In this communication we apply this concept to microparticles and cells within specially designed microwells. Our results open novel perspectives for the redistribution of cells between different media within a microfluidic environment.