Xiaoxing Xing

Interdigitated 3-D Silicon Ring Microelectrodes for DEP-Based Particle Manipulation

This paper describes the design, the fabrication, and the characterization of an interdigitated 3-D silicon (Si) ring microelectrodes for dielectrophoretic (DEP) manipulation of particles. The 3-D microelectrodes are derived from a high-aspect-ratio comb structure etched in a doped single crystal Si on an insulating dielectric (silicon-on-insulator). Fingers of the comb are evolved into ring microelectrodes once perforated with a linear array of well-defined round lateral constrictions. This is achieved by the segmented finger layout and the Si dry release strategy borrowed from inertial microelectromechanical systems. The fingers and their interspaces are sealed with a cover layer forming a microfluidic flow chamber surrounded by 3-D microelectrodes and accessible via single inlet/outlet. The functionality of the device has been verified on 2- and 10-μm polystyrene microspheres in pressure-driven flow through the ring microelectrodes at 3.3 μL/min effectively focusing them into streams or trapping them around the fingers at moderate voltage levels (20-40 Vpp).

Label-free enumeration of colorectal cancer cells from lymphocytes performed at a high cell-loading density by using interdigitated ring-array microelectrodes.

We report the label-free enumeration of human colorectal-carcinoma cells from blood lymphocytes by using interdigitated ring-array microelectrodes; this enumeration was based on the dielectrophoretic selection of cells. Because of the novel design of the device, a continuous flow of cells is uniformly distributed into parallel streams through 300 rings (~40 μm in diameter each) that are integrated into the electrode digits. Using this array, 82% of cancer cells were recovered and 99% of blood lymphocytes were removed. Most of the cancer cells recovered were viable (94%) and could be cultivated for >8 days, during which period they retained their normal cell morphology and proliferation rates. The recovery rate correlated closely with cancer-cell loadings in spiked samples and this relationship was linear over a range of at least 2 orders of magnitude. Importantly, because of the 3D structure of the rings, these results were obtained at a high cell-loading concentration (10(7)cells/mL). The rings could be further optimized for use in accurate label-free identification and measurement of circulating tumor cells in cancer research and disease management.

Continuous-Flow Electrokinetic-Assisted Plasmapheresis by Using Three-Dimensional Microelectrodes Featuring Sidewall Undercuts

We present a novel plasmapheresis device designed for a fully integrated point-of-care blood analysis microsystem. In the device, fluidic microchannels exhibit a characteristic cross-sectional profile arising from distinct three-dimensional (3D) microelectrodes featuring sidewall undercuts readily integrated through a single-mask process. The structure leverages mainly electrothermal convective rolls that efficiently manifest themselves in physiological fluids and yet have received inadequate attention for the application of plasmapheresis due to concerns over Joule heating. Using this device, we show that such convective rolls not only lead to plasma extraction at a high yield and purity but also deliver plasma at an acceptable quality with no evidence of hemolytic stress or protein denaturation. Specifically, plasma from 1.5 μL of whole blood diluted to 4% hematocrit in a high-conductivity buffer (1.5 S/m) is extracted in a continuous flow at a fraction of 70% by using a peak voltage of ±10 Vp applied at 650 kHz; the extracted plasma is nearly 99% pure, as shown by a rigorous assessment using flow cytometry. The plasmas obtained using this device and using conventional centrifugation and sedimentation are of comparable quality as revealed by absorbance and circular dichroism spectra despite thermal gradients; however, these gradients effectively drive electrothermal bulk flows, as assessed using the microparticle image velocity technique. The device achieves high target molecule recovery efficiency, delivering about 97% of the proteins detected in the plasma obtained using sedimentation. The utility of the extracted plasma is further validated based on the detection of prostate-specific antigen at clinically relevant levels.

A Low-Backpressure Single-Cell Point Constriction for Cytosolic Delivery Based on Rapid Membrane Deformations

Mechanically deforming biological cells through microfluidic constrictions is a recently introduced technique for the intracellular delivery of macromolecules possibly through transient membrane pores induced in the process. The technique is attractive for research and clinical applications mainly because it is simple, fast, and effective while being free of adverse effects often associated with well-known techniques that rely on field- or vector-based delivery. In this nascent approach, an utmost and crucial role is played by the constriction, often in rectangular profile, and it squeezes cells only in one dimension. The results achieved suggest that the longer the constriction is the higher the delivery performance. Contrary to this view, we demonstrate here a unique constriction profile that is highly localized (point) and yet returns comparably effective delivery. Point constrictions are of a semiround geometry, forcing cells in both dimensions while introducing very little backpressure to the system, which is a silicon-glass platform wherein constrictions are arranged in series along an array of channels. The influence of the constriction size and count as well as treatment pressure on delivery performance is presented on the basis of the flow-cytometric analyses of HCT116 cells treated using dextran as model molecules. Delivery performance is also presented for common mammalian cell lines including NIH 3T3, HEK293, and MDCK. Moreover, the versatility of the platform is demonstrated in gene knockdown experiments using synthetic siRNA as well as on the delivery of proteins. Target proteins in some cells exhibit nondiffusive distribution profile raising the plausibility of mechanisms other than transient membrane pores.

Nanofluidic diode biosensor featuring a single nanoslit for label-free detection of cardiac troponin biomarker

This paper presents an integrated nanofluidic diode biosensor based on a single asymmetric nanoslit with a nominal width of 30 nm. The nanoslit dimension is fine-tuned by the highly conformal atomic layer deposition (ALD) of Al2O3. The surface of the nanoslit is further modified with an ultra-thin layer of SiO2 that is placed onto the Al2O3 layer for improved sensing performance. The device has been demonstrated for electrical label-free detection of human cardiac troponin biomarker at clinically relevant concentrations across a range over four orders of magnitudes. The precise control of the slit dimension and its robust sensing performance could pave the way for the single-slit nanofluidic diode biosensor as a promising diagnostic tool for acute myocardial infarction.

A single-cell impedance flow cytometry microsystem based on 3D silicon microelectrodes

This is the first account of a single-cell impedance flow cytometry microsystem based on 3D silicon microelectrodes. Such microelectrodes emerge being readily aligned during lithographic patterning of silicon and thus relieve the burden of alignment that comes with planar microelectrodes and their opposing arrangement for a maximum sensitivity. The opposing arrangement of the microelectrodes generate homogeneous electric field and thus maintain maximum sensitivity while individual cells passing by. The microsystem has been showcased here for the flow impedance measurement of polystyrene microspheres and cells at 10 MHz. Successful classification of human colorectal cancer cells and red blood cells has also been demonstrated using this device.

Continuous-flow dielectrophoretic sorting of particles via 3D silicon electrodes featuring castellated sidewalls

Continuous-flow dielectrophoretic sorting of particles has been demonstrated using a simple microfluidic design incorporating 3D electrodes with castellated sidewalls. Two variations of the design have been fabricated, slightly differing in their sidewall and separation junction profiles, through a single-mask process on silicon-based platforms. These 3D silicon electrodes have shown the capacity to segregate polystyrene beads into distinct flow layers along the channel depth while delivering them to separate outlets through a downstream junction of a specific design. The utility of either structure has been showcased by sorting a mixture of 1 and 10 μm beads based on their size continuously at a velocity of 1.5 mm/s.

Dielectrophoretic (DEP) separation of live/dead cells on a glass slide functionalized with interdigitated 3D silicon ring microelectrodes

This paper describes a microfluidic device which offers a dielectrophoretic solution to cell sorting via interdigitated 3D silicon (Si) electrodes on a transparent glass substrate. The Si electrodes with a self-aligned array of lateral rings were fabricated through a single mask process on an anodically bonded Si-glass wafer. The device has been characterized for separation of live/dead human colorectal carcinoma cells at select flow rates and voltages. The live cell capture and dead cell removal efficiencies both reached 90% at an AC activation of 5Vp 450kHz applied against a flow rate up to 0.25ml/hr. The device disclosed here, with successful demonstration of continuous live/dead cell separation, offers great potential as a bioparticle sorting front-end module for a lab-on-a chip system.