Mechanical Perspective on Increasing Brush Cytology Yield.

Brush cytology is a sampling technique extensively used for mucosal surfaces, particularly to identify malignancies. A sample is obtained by rubbing the brush bristles over the stricture or lesion several times until cells are trapped. Brush cytology detection rate varies, with malignancy confirmed in 15-65% of cases of adenocarcinoma-associated biliary strictures and 44-80% of cases of cholangiocarcinoma. Despite the widespread use of brush cytology, there is no consensus to date defining the optimal biliary brushing parameters for the collection of suspicious lesions, such as the number of passes, brushing rate, and force applied. The aim of this work is to increase the brush cytology diagnostic yield by elucidating the underlying mechanical phenomena. First, the mechanical interactions between the brush bristles and sampled tissue are analyzed. During brushing, mucus and detached cells are transferred to the space between the bristles through the capillary rise and flow eddies. These mass transfer mechanisms and their dependence on mucus rheology as a function of pH, brush displacement rate, and bristle geometry and configuration are examined. Lastly, results from ex vivo brushing experiments performed on porcine stomachs are presented. Clinical practitioners from a variety of disciplines can apply the findings of this study to outline clear procedures for cytological brushing to increase the sensitivity and specificity of the brushings.

A note about convected time derivatives for flows of complex fluids.

We present a direct derivation of the typical time derivatives used in a continuum description of complex fluid flows, harnessing the principles of the kinematics of line elements. The evolution of the microstructural conformation tensor in a flow and the physical interpretation of different derivatives then follow naturally.

Microscale Hydrodynamic Cloaking and Shielding via Electro-Osmosis.

We demonstrate theoretically and experimentally that injection of momentum in a region surrounding an object in microscale flow can yield both "cloaking" conditions, where the flow field outside the cloaking region is unaffected by the object, and "shielding" conditions, where the hydrodynamic forces on the object are eliminated. Using field-effect electro-osmosis as a mechanism for injection of momentum, we present a theoretical framework and analytical solutions for a range of geometrical shapes, validate these both numerically and experimentally, and demonstrate the ability to dynamically switch between the different states.

Mass Transfer Limitations of Porous Silicon-Based Biosensors for Protein Detection.

Porous silicon (PSi) thin films have been widely studied for biosensing applications, enabling label-free optical detection of numerous targets. The large surface area of these biosensors has been commonly recognized as one of the main advantages of the PSi nanostructure. However, in practice, without application of signal amplification strategies, PSi-based biosensors suffer from limited sensitivity, compared to planar counterparts. Using a theoretical model, which describes the complex mass transport phenomena and reaction kinetics in these porous nanomaterials, we reveal that the interrelated effect of bulk and hindered diffusion is the main limiting factor of PSi-based biosensors. Thus, without significantly accelerating the mass transport to and within the nanostructure, the target capture performance of these biosensors would be comparable, regardless of the nature of the capture probe-target pair. We use our model to investigate the effect of various structural and biosensor characteristics on the capture performance of such biosensors and suggest rules of thumb for their optimization.

How Electrical Heterogeneity Parameters of Ion-Exchange Membrane Surface Affect the Mass Transfer and Water Splitting Rate in Electrodialysis.

Electrodialysis (ED) has been demonstrated as an effective membrane method for desalination, concentration, and separation. Electroconvection (EC) is a phenomenon which can essentially increase the mass transfer rate and reduce the undesirable water splitting effect. Efforts by a number of researchers are ongoing to create conditions for developing EC, in particular, through the formation of electrical heterogeneity on the membrane surface. We attempt, for the first time, to optimize the parameters of surface electrical heterogeneity for ion-exchange membranes used in a laboratory ED cell. Thirteen different patterns on the surface of two Neosepta anion-exchange membranes, AMX and AMX-Sb, were tested. Low-conductive fluoropolymer spots were formed on the membrane surface using the electrospinning technique. Spots in the form of squares, rectangles, and circles with different sizes and distances between them were applied. We found that the spots' shape did not have a visible effect. The best effect, i.e., the maximum mass transfer rate and the minimum water splitting rate, was found when the spots' size was close to that of the diffusion layer thickness, δ (about 250 µm in the experimental conditions), and the distance between the spots was slightly larger than δ, such that the fraction of the screened surface was about 20%.

Nonuniform Electro-osmotic Flow Drives Fluid-Structure Instability.

We demonstrate the existence of a fluid-structure instability arising from the interaction of electro-osmotic flow with an elastic substrate. Considering the case of flow within a soft fluidic chamber, we show that above a certain electric field threshold, negative gauge pressure induced by electro-osmotic flow causes the collapse of its elastic walls. We combine experiments and theoretical analysis to elucidate the underlying mechanism for instability and identify several distinct dynamic regimes. The understanding of this instability is important for the design of electrokinetic systems containing soft elements.

Electroosmotic Flow Dipole: Experimental Observation and Flow Field Patterning.

We experimentally demonstrate the phenomenon of electroosmotic dipole flow that occurs around a localized surface charge region under the application of an external electric field in a Hele-Shaw cell. We use localized deposition of polyelectrolytes to create well-controlled surface charge variations, and show that, for a disk-shaped spot, the internal pressure distribution that arises results in uniform flow within the spot and dipole flow around it. We further demonstrate the superposition of surface charge spots to create complex flow patterns, without the use of physical walls.