Fork in the road: patterned surfaces direct microcapsules to make a decision

Computational fluid dynamics in the microcirculation and microfluidics: what role can the lattice Boltzmann method play?

Patient-specific simulations, efficient parametric analyses, and the study of complex processes that are otherwise experimentally intractable are facilitated through the use of Computational Fluid Dynamics (CFD) to study biological flows. This review discusses various CFD methodologies that have been applied across different biological scales, from cell to organ level. Through this discussion the lattice Boltzmann method (LBM) is highlighted as an emerging technique capable of efficiently simulating fluid problems across the midrange of scales; providing a practical analytical tool compared to methods more attuned to the extremities of scale. Furthermore, the merits of the LBM are highlighted through examples of previous applications and suggestions for future research are made. The review focusses on applications in the midrange bracket, such as cell-cell interactions, the microcirculation, and microfluidic devices; wherein the inherent mesoscale nature of the LBM renders it well suited to the incorporation of fluid-structure interaction effects, molecular/particle interactions and interfacial dynamics. The review demonstrates that the LBM has the potential to become a valuable tool across a range of emerging areas in bio-CFD, such as understanding and predicting disease, designing lab-on-a-chip devices, and elucidating complex biological processes.

Microcapsule mechanics: from stability to function

Microcapsules are reviewed with special emphasis on the relevance of controlled mechanical properties for functional aspects. At first, assembly strategies are presented that allow control over the decisive geometrical parameters, diameter and wall thickness, which both influence the capsule's mechanical performance. As one of the most powerful approaches the layer-by-layer technique is identified. Subsequently, ensemble and, in particular, single-capsule deformation techniques are discussed. The latter generally provide more in-depth information and cover the complete range of applicable forces from smaller than pN to N. In a theory chapter, we illustrate the physics of capsule deformation. The main focus is on thin shell theory, which provides a useful approximation for many deformation scenarios. Finally, we give an overview of applications and future perspectives where the specific design of mechanical properties turns microcapsules into (multi-)functional devices, enriching especially life sciences and material sciences.

Harnessing fluid-driven vesicles to pick up and drop off Janus particles

Using dissipative particle dynamics (DPD) simulations, we model the interaction between nanoscopic lipid vesicles and Janus nanoparticles in the presence of an imposed flow. Both the vesicle and Janus nanoparticles are localized on a hydrophilic substrate and immersed in a hydrophilic solution. The fluid-driven vesicle successfully picks up Janus particles on the substrate and transports these particles as cargo along the surface. The vesicle can carry up to four particles as its payload. Hence, the vesicles can act as nanoscopic "vacuum cleaners", collecting nanoscopic debris localized on the floors of the fluidic devices. Importantly, these studies reveal how an imposed flow can facilitate the incorporation of nanoparticles into nanoscale vesicles. With the introduction of a "sticky" domain on the substrate, the vesicles can also robustly drop off and deposit the particles on the surface. The controlled pickup and delivery of nanoparticles via lipid vesicles can play an important step in the bottom-up assembly of these nanoparticles within small-scale fluidic devices.

pH-dependent control of particle motion through surface interactions with patterned polymer brush surfaces

In this Article, we show that inclined silicon surfaces patterned with poly(methacrylic acid) brushes are able to control the position and movement of 20 μm silica particles, which are propelled across the patterned surface by sedimentation forces. Three different types of behavior were observed depending on the angle between the direction in which a particle sedimented and the orientation of the polymer-brush silicon interface. At small angles, particles were found to sediment to the brush interface and then sediment following the direction of the brush interface. At larger angles, particles sedimented to the interface and then followed the direction of the brush interface, but then after a certain distance changed direction to pass over the interface. At the largest angles where the brush interface was approximately perpendicular to the motion of the particle, particles were found to travel over the interface unperturbed. This behavior was also found to be pH dependent, allowing the formation of pH responsive "gates", which allow particles to pass at low pH but not at high pH. It was also found that if patterned polymer brush surfaces were oriented in the correct way, they were able to control the number of particles present at specific locations.

Schemes for the fragmentation and merging of droplets resting on a solid substrate using a patterned wettability gradient

A methodology has been proposed for the breakup and joining of water drops over a solid surface by varying its wettability gradient. A computational scheme using diffuse interface based smoothed particle hydrodynamics has been employed to simulate the droplet dynamics as the proof of the concept. For both the joining and breakup of drops, Y shaped patterns with the same or different wettability gradient along its different arms have been proposed. The breakup of a drop at the junction of the Y joint may be achieved if it is subjected to pulling forces along both of the branch arms. The pulling force can be brought into existence by providing an appropriate wettability gradient along the branches. By a proper selection of the branch orientation and the wettability gradient along them, it is possible to break a drop into two droplets of equal or unequal size. It has also been demonstrated that a drop can be broken into three small droplets by a specially designed four junction pattern of wettability on the solid surface. A Y junction has also been used for the merger of two drops. For this, two water drops placed at the end of the two branches are made to move toward the junction due to the gradient of contact angle. At the junction, they collide and merge into a single drop. A scheme has also been described where a combination of Y patterns can be exploited to generate a multitude of drops of different size.

Using nanoparticle-filled microcapsules for site-specific healing of damaged Substrates: creating a "repair-and-go" system

Using a hybrid computational approach, we simulate the behavior of nanoparticle-filled microcapsules that are propelled by an imposed shear to move over a substrate, which encompasses a microscopic crack. When the microcapsules become localized in the crack, the nanoparticles can penetrate the capsule's shell to bind to and fill the damaged region. Initially focusing on a simple shear flow, we isolate conditions where the microcapsules become arrested in the cracks and those where the capsules enter the cracks for a finite time but are driven to leave this region by the imposed flow. We also characterize the particle deposition process for these two scenarios, showing that the deposition is greater for the arrested capsules. We then determine the effect of utilizing a pulsatile shear flow and show that this flow field can lead to an effective "repair-and-go" system where the microcarriers not only deliver a high volume fraction of particles into the crack but also leave the fissure and, thus, can potentially repair additional damage within the system.

Designing oscillating cilia that capture or release microscopic particles

We use computational modeling to capture the three-dimensional interactions between oscillating, synthetic cilia and a microscopic particle in a fluid-filled microchannel. The synthetic cilia are elastic filaments that are tethered to a substrate and are actuated by a sinusoidal force, which is applied to their free ends. The cilia are arranged in a square pattern, and a neutrally buoyant particle is initially located between these filaments. Our computational studies reveal that, depending on frequency of the beating cilia, the particle can be either driven downward toward the substrate or driven upward and expelled into the fluid above the cilial layer. This behavior mimics the performance of biological cilia used by certain marine animals to extract suspended food particles. The findings uncover a new route for controlling the deposition of microscopic particles in microfluidic devices.

Roles of cell and microvillus deformation and receptor-ligand binding kinetics in cell rolling

Polymorphonuclear leukocyte (PMN) recruitment to sites of inflammation is initiated by selectin-mediated PMN tethering and rolling on activated endothelium under flow. Cell rolling is modulated by bulk cell deformation (mesoscale), microvillus deformability (microscale), and receptor-ligand binding kinetics (nanoscale). Selectin-ligand bonds exhibit a catch-slip bond behavior, and their dissociation is governed not only by the force but also by the force history. Whereas previous theoretical models have studied the significance of these three "length scales" in isolation, how their interplay affects cell rolling has yet to be resolved. We therefore developed a three-dimensional computational model that integrates the aforementioned length scales to delineate their relative contributions to PMN rolling. Our simulations predict that the catch-slip bond behavior and to a lesser extent bulk cell deformation are responsible for the shear threshold phenomenon. Cells bearing deformable rather than rigid microvilli roll slower only at high P-selectin site densities and elevated levels of shear (>or=400 s(-1)). The more compliant cells (membrane stiffness=1.2 dyn/cm) rolled slower than cells with a membrane stiffness of 3.0 dyn/cm at shear rates >50 s(-1). In summary, our model demonstrates that cell rolling over a ligand-coated surface is a highly coordinated process characterized by a complex interplay between forces acting on three distinct length scales.