Electrokinetics with Soft and Charged Interfaces

Recently, we published a review on theory and experiment for electrokinetics with soft and charged interfaces. Our paper discusses the governing physics, modeling, and experimental verifications of theories for electrokinetics on soft and charged interfaces. To codifiy this understanding, we connect the cognate electrokinetic phenomena using the electrokinetic coupling matrix.

A soft and charged interface between a rigid wall and pure fluid phase. Effects of the soft and charged interface on fluid velocity profiles for pressure (solid line) and electric field (dashed line) are shown at left, and effects on the local potential distribution are shown at right. (Enlarge by clicking on the image.)

Descriptions of soft and charge interfaces deviate from classical theories of electrokinetics by postulating a porous charge layer at the interface between the completely impermeable wall and pure fluid. In this interfacial region, a large amount of charge may be attached to the porous layer. Depending upon the permeability of the porous layer, immense amplifications of electroosmosis, streaming potential, and/or conductance may be realized. Changes in these phenomena are not explained using rigid-surface electrokinetic theories.

Descriptions of soft and charged layers are essential to understand and predict the behavior of systems with non-rigid interfaces bearing fixed charge. Soft and charged interfaces are present in a variety of systems, both synthetic (e.g., polymer exchange membrane, grafted polymer layers) and natural (e.g., cartilage, lipid membranes). We hope the theories and experiments discussed in the review provide researchers with a framework to interpret and plan results and experiments.

Engineering Microfluidic Devices for Neural Culture

We recently communicated our efforts toward the development and implementation of a novel neural cell culture device with spatiotemporal solute delivery. Our work, in collaboration with Gary Banker and Cheng Fang at Oregon Health and Sciences University, details the construction and culture of nerve cells within a microfluidic device. We demonstrate that neurons seeded in our device grow predictably and survive for several days within the closed device. We additionally present an analytical framework to engineer solute interactions in nerve cell culture, which can be generalized to other device geometries.

This work is motivated by the need to screen and evaluate solutes for their effects on intracellular transport within the neuron. Several researchers have proposed that the cause of neurodegeneration is due to transport defects within individual nerve cells. Our platform can impose solutes on neurons at specific locations. These solutes can be toxins, meant to stimulate a transport defect within the cell, or a therapeutic agent, to protect against transport failures. Additionally, our device layout is both multiplexed and directional; these design aspects can lead to high inference studies, as a large number of cells can be probed on one device with minimal time spent determining cell polarity/orientation.

Our design approach of the neural cell culture system considers both the effects of geometry on the cells (via shear stress) as well as the solute distribution (via concentration profiles in space and time), in addition to other effects such as materials compatibility. Toward generalizing our approach to different types of cells and solutes, we simplify the output of the governing convection-diffusion equations to parametrized plots and algebraic relations so that designs may easily be evaluated by predicting device performance.

Link to PDF.

86th ACS Colloids and Surface Science Symposium

Recently I attended the 86th ACS Colloids and Surface Science Symposium at Johns Hopkins University in Baltimore, Maryland, where I presented a talk on our recent work examining the electrokinetic properties of thin charge-carrying films. I discussed our method of film fabrication, chemical and physical characterization of the films, as well as our custom-built electrokinetic analysis device and results that we collected with our device.

These surfaces occur in a variety of natural (e.g., cartilage) and industrial (e.g., fuel cells) systems, and physicochemical models of these systems are necessary to inform engineering decisions regarding design and optimization. Our particular interests are investigations of well-understood chemical systems to test and inform current and proposed electrokinetic models of diffuse charge interfaces.

The current study explores interfaces which are non-rigid and contain charge distributed over a fluid-permeable volume, in contrast to our previous studies on silica and hydrophobic microfluidic substrates, both of which exhibit rigid charge-carrying surfaces in contact with liquid. Fluid permeable diffuse-charge interfaces introduce interesting physics involving the charging and permeability of the film layer. We hope to describe these processes in greater detail in future communications.