Particle trapping using dielectrophoretically patterned carbon nanotubes

Assembly of long carbon nanotube bridges across transparent electrodes using novel thickness-controlled dielectrophoresis

The assembly of carbon nanotubes (CNTs) across planner electrodes using dielectrophoresis (DEP) is one of the standard methods used to fabricate CNT-based devices such as sensors. The medium drag velocity caused by electrokinetic phenomena such as electrothermal and electroosmotic might drive CNTs away from the deposition area. This problem becomes critical at large-scale electrode structures due to the high attenuation of the DEP force. Herein, we simulated and experimentally validated a novel DEP setup that uses a top glass cover to minimize the medium drag velocity. The simulation results showed that the drag velocity can be reduced by 2-3 orders of magnitude compared with the basic DEP setup. The simulation also showed that the optimum channel height to result in a significant drag velocity reduction was between 100 μm and 240 μm. We experimentally report, for the first time, the assembly and alignment of CNT bridges across indium tin oxide (ITO) electrodes with spacing up to 125 μm. We also derived an equation to optimize the CNT's concentration in suspensions based on the electrode gap width and channel height. The deposition of long CNTs across ITO electrodes has potential use in transparent electronics and microfluidic systems.

Dielectrophoretic Crossover Frequency of Single Particles: Quantifying the Effect of Surface Functional Groups and Electrohydrodynamic Flow Drag Force

We present a comprehensive comparison of dielectrophoretic (DEP) crossover frequency of single particles determined by various experimental methods and theoretical models under the same conditions, and ensure that discrepancy due to uncertain or inconsistent material properties and electrode design can be minimized. Our experiment shows that sulfate- and carboxyl-functionalized particles have higher crossover frequencies than non-functionalized ones, which is attributed to the electric double layer (EDL). To better understand the formation of the EDL, we performed simulations to study the relationship between initial surface charge density, surface ion adsorption, effective surface conductance, and functional groups of both functionalized and nonfunctionalized particles in media with various conductivities. We also conducted detailed simulations to quantify how much error may be introduced if concurrent electrohydrodynamic forces, such as electrothermal and electro-osmotic forces, are not properly avoided during the crossover frequency measurement.

Self-contained microfluidic systems: a review

Microfluidic systems enable rapid diagnosis, screening and monitoring of diseases and health conditions using small amounts of biological samples and reagents. Despite these remarkable features, conventional microfluidic systems rely on bulky expensive external equipment, which hinders their utility as powerful analysis tools outside of research laboratories. 'Self-contained' microfluidic systems, which contain all necessary components to facilitate a complete assay, have been developed to address this limitation. In this review, we provide an in-depth overview of self-contained microfluidic systems. We categorise these systems based on their operating mechanisms into three major groups: passive, hand-powered and active. Several examples are provided to discuss the structure, capabilities and shortcomings of each group. In particular, we discuss the self-contained microfluidic systems enabled by active mechanisms, due to their unique capability for running multi-step and highly controllable diagnostic assays. Integration of self-contained microfluidic systems with the image acquisition and processing capabilities of smartphones, especially those equipped with accessory optical components, enables highly sensitive and quantitative assays, which are discussed. Finally, the future trends and possible solutions to expand the versatility of self-contained, stand-alone microfluidic platforms are outlined.

All-optical microfluidic chips for reconfigurable dielectrophoretic trapping through SLM light induced patterning

We explore a novel approach for fabricating polymeric microfluidic-channelled dielectrophoretic (DEP) chips by direct laser projection through a holographic Spatial-Light-Modulator (SLM) onto photorefractive crystal substrates. As the first step, an all-optical mould-free approach was used to fabricate the PDMS microfluidic channel, by exploiting the light induced space charge field in Fe-doped lithium niobate crystals, with the aim of integrating a microfluidic channel directly onto the functionalized substrate. Subsequently, as the second step, geometrical flexible DEP traps can be created onto the substrate by the same SLM holographic projection system. The experimental verification shows the trapping of flowing carbon nanotubes (CNTs) and the formation of chaining effects with graphite nanofibers. The main feature of the SLM is the ability to display an arbitrary light intensity pattern that is used here for fabricating the channels. Moreover, the reconfigurable trapping of CNTs is possible simply by the optical writing/erasing of various light intensity patterns projected by the SLM.

Electric field guided assembly of one-dimensional nanostructures for high performance sensors

Various nanowire or nanotube-based devices have been demonstrated to fulfill the anticipated future demands on sensors. To fabricate such devices, electric field-based methods have demonstrated a great potential to integrate one-dimensional nanostructures into various forms. This review paper discusses theoretical and experimental aspects of the working principles, the assembled structures, and the unique functions associated with electric field-based assembly. The challenges and opportunities of the assembly methods are addressed in conjunction with future directions toward high performance sensors.

Cascade and staggered dielectrophoretic cell sorters

We have developed two new microfluidic cell sorters based on conventional negative dielectrophoresis (DEP) for continuous flow operations. The first is a cascade configuration sorter designed to increase purity of isolated target cell. The second has two staggered side channels in opposite side walls to increase sample throughput without compromising enrichment factor. Particles (carboxylate microspheres) of different sizes were first used to demonstrate the feasibility of the present DEP sorters for cell isolation. Then biological cells, i.e. human prostate cancer cell line LNCaP and human colorectal cancer cell line HCT116 were used to test the performance of the DEP sorters. In the present work, applied voltage was in the range of 0-20 V(p-p) , and frequency was from 0 to 10 MHz. Comparing to a single side channel DEP cell sorter, the isolation purity was improved from 80 to 96% by a single cascade sorter and the sample throughput was increased from 0.2 to 0.65 μL/min by a single staggered side channel sorter. In this article, we report the cell sorter designs, cell separation and enrichment factors.

Passive optical separation and enrichment of cells by size difference

A size-selective cell sorting microfluidic device that utilizes optical force is developed. The device consists of a three-dimensional polydimethylsiloxane microstructure comprised of two crossed microchannels in a three-dimensional configuration. A line shaped focused laser beam is used for automatic size-selective cell sorting in a continuous flow environment. As yeast cells in an aqueous medium are fed continuously into a lower channel, the line shaped focused laser beam is applied (perpendicular to the direction of flow) at the junction of the two crossed channels. The scattering force of the laser beam was employed to push cells matching specific criteria upward from one channel to another. The force depends on the size of the cells, the laser power, and the fluid flow speed. The variation in size of yeast cells causes them to follow different routes at the intersection. For flow speeds below 30 μm∕s, all yeast cells larger than 3 μm were removed from the main stream. As a result, a high purity sample of small cells can be collected at the outlet of bottom channel.