Research Highlights

Polyelectrolyte diodes on a microchip

Rectification in a polyelectrolyte solution was theoretically described and experimentally demonstrated about 50 years ago, and several interesting applications such as the creation of nonlinear electronic systems and model systems for the asymmetric functions of cell membranes have been proposed. However, only minor advances have been made in the past decades concerning fundamental understanding and useful applications. The miniaturisation of polyelectrolyte systems that can be integrated in more complex circuits would be a great step toward advanced systems with a fast response to external electric input. In a recent work, Taek Dong Chung and co-workers from Seoul National University (Korea) have presented a microchip based polyelectrolyte diode, which consists of two microchannels filled with oppositely charged polyelectrolyte solution (Fig 1).¹ At a small junction, these polyelectrolyte solutions are in contact. Upon application of a forward bias, counter ions migrate towards this junction, whereas a reverse bias drives them to the bulk solution. Hence, the polyelectrolyte solutions are similar to holes and electrons in a solid-state diode, and this junction system acts as a rectifier like a silicon-based electronic p–n junction. In this miniaturised system, the rectification is exclusively controlled by the ionic conductance across the polyelectrolyte junction. A significant breakthrough for the fundamental understanding of such diodes is the feasibility of real-time monitoring of the mass transport and dynamic distribution of the ions when forward or reversed bias is applied, which is achieved by use of fluorescent ions. Furthermore, the researchers fabricated microchips with two integrated diodes that operate as logic circuits (AND, OR, NAND). In future, the system may be part of intelligent chip devices that mimic nonlinear biological functions, e.g. signalling across cell membranes.

Fig. 1 Scheme of a microfluidic polyelectrolyte diode. The ions of the electrolyte solution are shown in bright and dark spheres. Similar to holes and electrons in a solid-state diode, the counterions in each polyelectrolyte solution carry charge toward the junction, or away from it. (Reprinted with permission from ref. 1. Copyright 2009 Wiley-VCH).

Combining thermoplastics and elastomeric polymers for microchip fabrication

The material of choice for the fabrication of microfluidic devices is most often polydimethylsiloxane (PDMS), which is casted from a master mould and afterwards bonded on a rigid glass plate. Besides the simplicity of fabrication, PDMS has a lot of advantages such as the low cost of the material, the transparency for visible light, and the elasticity that allows integration of deformation-based valves and pumping components. However, there are some drawbacks of this material as well, particularly the permeability of PDMS to water and gas. In consequence, concentrations of chemical compounds, osmolarity and pH of the solutions inside the microchannels change over time, which obstructs the use of PDMS devices for some biological and cell biological applications. Researchers from the University of Michigan (Ann Arbor) report in a current article the use of alternative hard–soft hybrid materials that provides the positive properties of flexible materials, while the evaporation of water and permeation of oxygen is significantly reduced.² In these devices, the channel features are created on rigid polymers such as polyethylene terephthalate glycol (PETG), cyclic olefin copolymer (COC), and polystyrene (PS) by hot embossing. Afterwards, the channels are covered with a soft membrane made of polyurethane, or PDMS coated with parylene C. Leakage-free and stable bonding was achieved by combination of oxygen and argon plasma followed by heating. The soft membrane enabled the construction of peristaltic valves, which is realised by interfacing the pins of a Braille tactile array to the microchannel. The devices are employed for culturing of different cell lines without the use of an incubator. Due to reduced evaporation of water, the cells are viable for a longer time (100% viability for primary human dermal microvascular cells after 1–2 days). Furthermore, due to the low oxygen permeability, it is possible to achieve a hypoxic environment (oxygen concentration down to 1%), which is needed for many cell studies (e.g. embryonic and adult stem cells, cancer cells).

Clustering of vesicles and cells in microcapillaries

When blood is flowing through narrow veins, the red blood cells (RBCs) change their shape thereby reducing the flow resistance. Furthermore, the shape deformation of RBCs plays a key role in the regulation of oxygen delivery. Abnormalities in the deformability of lipid membranes are involved in diseases such as sickle cell anaemia and diabetes mellitus. Hence, it is of fundamental interest to understand the relation between elasticity and deformability of RBCs and their flow-induced morphology. It is known that already individual cells in diluted suspensions exhibit a complex behaviour in a microflow. In a recent study, researchers from the Forschungszentrum Jülich (Germany) have taken a step forward, and simulated the collective behaviour of several RBCs and vesicles in narrow capillaries.³ They combined a particle-based mesoscale simulation technique for the fluid hydrodynamics with a triangulated-membrane model. By variations of the flow rate and the concentration of RBCs (i.e. the so-called hematocrit) they found different phases, in which cells are present in different shapes such as biconcave disk-like forms and parachute-like forms. Furthermore, the simulation predicts a flow-induced cluster formation above a threshold flow velocity (Fig. 2). These findings can be of great importance to predict flow properties of deformable particles that are potential drug carriers.

Fig. 2 Simulation of the collective flow-induced behaviour of elastic red blood cells and vesicles. Clustering of vesicles is predicted already for low concentrated suspensions as shown in these snapshots. (Reprinted with permission from ref. 3. Copyright 2009 National Academy of Science).

Petra S. Dittrich

ETH Zürich, Switzerland

dittrich@org.chem.ethz.ch

References

1. J. H. Han, K. B. Kim, H. C. Kim and T. D. Chung, Ionic Circuits Based on Polyelectrolyte Diodes on a Microchip, Angew. Chem. Int. Ed., 2009, 48, 3830–3833.
2. G. Mehta, J. Lee, W. Cha, Y.-C. Tung, J. J. Lindermann and S. Takayama, Hard Top soft Bottom Microfluidic Devices for Cell Culture and Chemical Analysis, Anal. Chem., 2009 DOI:10.1021/ac802178u.
3. J. L. McWhirter, H. Noguchi and G. Gompper, Flow-induced clustering and alignment of vesicles and red blood cells in microcapillaries, Proc. Natl. Acad. Sci. USA, 2009, 106, 6039–6043.

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