Ling Sheng Jang

Single-cell trapping utilizing negative dielectrophoretic quadrupole and microwell electrodes

The handling of individual cells, which has attracted increasing attention, is a key technique in cell engineering such as gene introduction, drug injection, and cloning technology. Alternating current (AC) electrokinetics has shown great potential for microfluidic functions such as pumping, mixing, and concentrating particles. The non-uniform electric field gives rise to Joule heating and dielectrophoresis (DEP). The motion of particles suspended in the medium can be influenced directly, by means of dielectrophoretic effects, and indirectly, via fluid flow through a viscous drag force that affects the particles. Thus alternating current electrothermal effect (ACET) induced flow and DEP force can be combined to manipulate and trap single particles and cells. This study presents a microfluidic device which is capable of specifically guiding and capturing single particles and cells by ACET fluid flow and the negative dielectrophoretic (nDEP) trap, respectively. The experiment was operated at high frequencies (5-12 MHz) and in a culture medium whose high conductivity (sigma=1.25S/m) is of interest to biochemical analysis and environmental monitoring, which are both prone to producing ACET and nDEP. Manipulation of particle motion using ACET-induced fluid flow to the target trap is modeled numerically and is in good agreement with the experimental results.

Glycated hemoglobin (HbA1c) affinity biosensors with ring-shaped interdigital electrodes on impedance measurement.

Glycated hemoglobin (HbA1c) is one of the most important diagnostic assays for the long-term mark of glycaemic control in diabetes. This study presents an affinity biosensor for HbA1c detection which is label-free based on the impedance measurement, and it features low cost, low sample volume, and requires no additional reagent in experiments. The ring-shaped interdigital electrodes (RSIDEs) are designed to promote the distribution uniformity and immobilization efficiency of HbA1c, and are further employed to characterize the impedance change and identify various concentrations of HbA1c. The self-assembled monolayer (SAM) of thiophene-3-boronic acid (T3BA) is provided to modify the gold electrode surface. Afterwards, the esterification reaction between HbA1c and T3BA produces a relative change of electrical property on the electrode surface. The RSIDEs with SAM of T3BA exhibit a wide range from 100 to 10 ng/µL producing an approximate logarithmic decrease of impedance, a low detection limit of 1 ng/µL, a good selectivity and short-term stability for HbA1c determination. The remarkable advantages (miniaturization and low-cost) fill the bill of point-care diagnostics for portable sensor development.

Integration of single-cell trapping and impedance measurement utilizing microwell electrodes

The ability to research individual cells has been seen as important in many kinds of biological studies. In the present study, cell impedance analysis is integrated into a single-cell trapping structure. For the purpose of precise positioning, a cell manipulation and measurement microchip, which uses an alternating current electrothermal effect (ACET) and a negative dielectrophoresis (nDEP) force to move a particle and cell on measurement electrodes, is developed. An ACET and an nDEP can be easily combined with subsequent analyses based on electric fields. A microwell presented in a previous study is separated into two parts, which are regarded as the measurement electrodes. The original structure is modified for precise positioning. Numerical simulations and analyses are conducted to compute and analyze the effects of the structural parameters. The results of simulations and analyses are used to obtain the optimum structure for the cell. The capture range of the microwell can be designed for cells of various sizes. In order to demonstrate the precision of the positioning, a particle is captured, measured, and released twice. The results show that the impedance error of the particle is about 3%. Finally, the developed structure is applied to trap and measure the impedance of a HeLa cell.

A systematic investigation into the electrical properties of single HeLa cells via impedance measurements and COMSOL simulations.

The electrical properties of single cells provide fundamental insights into their pathological condition and are therefore of immense interest to medical practitioners. Accordingly, this study captures single HeLa cells using a microfluidic device and then measures their impedance properties using a commercial impedance spectroscopy system. The experimental system is modeled by an equivalent electrical circuit and COMSOL simulations are then performed to establish the conductivity, permittivity and impedance of single HeLa cells under various operational frequencies and voltages. At an operational voltage of 0.2 V, the maximum deviation between the experimental and simulation results for the magnitude and phase of the HeLa cell impedance is found to be 9.5% and 4.2%, respectively. In general, both sets of results show that the conductivity and permittivity of single HeLa cells increase with an increasing operational voltage. Moreover, an increasing frequency is found to increase the conductivity of HeLa cells at all values of the operational voltage, but to reduce the permittivity for operational voltages in the range 0.6-1.0 V. Based upon the simulation and experimental results, empirical equations are constructed to predict the conductivity and permittivity of single HeLa cells under specified values of the operational voltage and frequency, respectively. The maximum discrepancy between the predicted results and the simulation results for the permittivity and conductivity of the HeLa cells at an operational voltage of 0.2 V is found to be just 0.5% and 4.5%, respectively.

Effects of electrode geometry and cell location on single-cell impedance measurement

Measurements on single cells provide more accurate and in-depth information about electrical properties than those on pathological tissues. The relationship between electrode geometry and the location of a cell on microfluidic devices greatly affects the accuracy of single-cell impedance measurement. Accordingly, this study presents numerical solutions from the FEM simulation of the COMSOL multiphysics package and experimental measurements to analyze the effects of electrode geometry and cell location on microfluidic devices. An equivalent electrical circuit model is developed to obtain the impedance and sensitivity of various cell locations on various electrode geometries using FEM simulation. According to the simulation results, the parallel electrodes have the largest sensing area (39 microm(2)) and the highest sensitivity (0.976) at a voltage of 0.1 V and a frequency of 100 kHz. Increasing the width of electrodes provides a large sensing area but reduces sensitivity, whereas decreasing the gap between electrodes increases both sensing area and sensitivity. In experiments, the results demonstrate that the magnitude is inversely proportional to the overlap area of the cell and electrodes. Moreover, the impedance of single HeLa cells measured at various cell locations can be modified using equations determined from the modeling and experimental results.

Adjustable trapping position for single cells using voltage phase-controlled method

We present an advanced technique improving upon the micron-sized particle trap integrated in biochip systems using a planar structure to generate an adjustable trapping position by utilizing voltage phase-controlled (VPC) method and negative dielectrophoresis (nDEP) theory in high conductivity physiological media. The designed planar and split structure is composed of independent components of measuring and trapping micro-electrodes. Through different voltage configurations on the device, the trapped position of single particles/cells was selected and adjusted in vertical and horizontal directions. The numerical simulations verify our theoretical predictions of the effects at the various voltages. It shows that the trapped position can be adjusted in the vertical (0 to 26 μm) and horizontal (0 to 74 μm) directions. In experiments, the single particles/cells is captured, measured, and then released, with the same process being repeated twice to demonstrate the precision of the positioning. The measurement results determined that particles at various heights result in different magnitude values, while the impedance error is less than 5% for the proposed electrode layout. Finally, the experiments are performed to verify that a particle/cell can be precisely trapped on the selected site in both the vertical and horizontal directions.

Single HeLa and MCF-7 cell measurement using minimized impedance spectroscopy and microfluidic device

This study presents an impedance measurement system for single-cell capture and measurement. The microwell structure which utilizes nDEP force is used to single-cell capture and a minimized impedance spectroscopy which includes a power supply chip, an impedance measurement chip and a USB microcontroller chip is used to single-cell impedance measurement. To improve the measurement accuracy of the proposed system, Biquadratic fitting is used in this study. The measurement accuracy and reliability of the proposed system are compared to those of a conventional precision impedance analyzer. Moreover, a stable material, latex beads, is used to study the impedance measurement using the minimized impedance spectroscopy with cell-trapping device. Finally, the proposed system is used to measure the impedance of HeLa cells and MCF-7 cells. The impedance of single HeLa cells decreased from 9.55 × 10(3) to 3.36 × 10(3) Ω and the impedance of single MCF-7 cells decreased from 3.48 × 10(3) to 1.45 × 10(3) Ω at an operate voltage of 0.5 V when the excitation frequency was increased from 11 to 101 kHz. The results demonstrate that the proposed impedance measurement system successfully distinguishes HeLa cells and MCF-7 cells.

Battery‐powered portable instrument system for single‐cell trapping, impedance measurements, and modeling analyses

A battery‐powered portable instrument system for the single‐HeLa‐cell trapping and analyses is developed. A method of alternating current electrothermal (ACET) and DEP are employed for the cell trapping and the method of impedance spectroscopy is employed for cell characterizations. The proposed instrument (160 mm × 170 mm × 110 mm, 1269 g) equips with a highly efficient energy‐saving design that promises approximately 120 h of use. It includes an impedance analyzer performing an excitation voltage of 0.2–2 Vpp and a frequency sweep of 11–101 kHz, function generator with the sine wave output at an operating voltage of 1–50 Vpp with a frequency of 4–12 MHz, cell‐trapping biochip, microscope, and input/output interface. The biochip for the single cell trapping is designed and simulated based on a combination of ACET and DEP forces. In order to improve measurement accuracy, the curve fitting method is adopted to calibrate the proposed impedance spectroscopy. Measurement results from the proposed system are compared with results from a precision impedance analyzer. The trapped cell can be modeled for numerical analyses. Many advantages are offered in the proposed instrument such as the small volume, real‐time monitoring, rapid analysis, low cost, low‐power consumption, and portable application.

Experimental study of dielectrophoresis and liquid dielectrophoresis mechanisms for particle capture in a droplet

This work presents a microfluidic system that can transport, concentrate, and capture particles in a controllable droplet. Dielectrophoresis (DEP), a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non‐uniform electric field, is used to manipulate particles. Liquid dielectrophoresis (LDEP), a phenomenon in which a liquid moves toward regions of high electric field strength under a non‐uniform electric field, is used to manipulate the fluid. In this study, a mechanism of droplet creation presented in a previous work that uses DEP and LDEP is improved. A driving electrode with a DEP gap is used to prevent beads from getting stuck at the interface between air and liquid, which is actuated with an AC signal of 200 Vpp at a frequency of 100 kHz. DEP theory is used to calculate the DEP force in the liquid, and LDEP theory is used to analyze the influence of the DEP gap. The increment of the actuation voltage due to the electrode with a DEP gap is calculated. A set of microwell electrodes is used to capture a bead using DEP force, which is actuated with an AC signal of 20 Vpp at a frequency of 5 MHz. A simulation is carried out to investigate the dimensions of the DEP gap and microwell electrodes. Experiments are performed to demonstrate the creation of a 100‐nL droplet and the capture of individual 10‐μm polystyrene latex beads in the droplet.

The Effect of Particles on Performance of Fixed-Valve Micropumps

Although numerous designs for micropumps have been reported in the literature, very few studies have addressed pumping of anything but pure fluids. In a previous study done in our laboratory, the ability of fixed-valve micropumps to directly transport particle-laden fluids was demonstrated with suspensions of polystyrene microspheres ranging from approximately 3 to 20 µm in diameter without clogging at densities as high as 9000 particles/µl of suspension. In the present study, degassing procedures and a wide range of particle concentration were utilized to better understand characteristics associated with maximum pump performance, defined as the point at which higher driving voltage causes cavitation. Our findings indicate that compared to pure water degassing procedures profoundly affected pumping characteristics for particle suspensions composed of either charged or uncharged polystyrene particles. In addition, degassing durations after which pumping characteristics did not change were found to be approximately 250% longer for particle-laden fluid compared to pure water. Lastly, maximum pump performance was inversely related to particle concentration for low concentrations, to 70 µ/ml for 3 µm diameter microspheres, which, when combined with previous results indicated that higher concentrations result in a concentration threshold beyond which maximum pump performance is not affected. The results of this study serve as a guideline for transport of particle-laden fluids directly through and design of fixed-valve and other types of micropumps.