Application Cases

Experiment Name: Application of Voltage Amplifiers in Microfluidic Chip Research with Microelectrodes

Research Direction: Microfluidic Biochips

Experiment Objective:
Microelectrodes play a crucial role in microfluidic chips. They not only transmit external electrical signals into the chip to enable functions such as electrophoresis, dielectrophoresis, electroporation, and electrofusion but also serve as sensors that convert internal environmental parameters (e.g., pH, pressure, concentration, temperature, impedance) into electrical signals for external detection. Based on thickness, microelectrodes are classified into nanoscale two-dimensional (2D) electrodes and microscale three-dimensional (3D) electrodes. 3D electrodes offer advantages such as uniform spatial electric fields, larger effective surface areas, higher current tolerance, and improved robustness. Based on material composition, microelectrodes can be categorized as ITO electrodes, metal electrodes, carbon-based electrodes, and composite electrodes. Among these, composite electrodes are advantageous due to their low equipment dependency, simple fabrication, and low cost. 3D composite microelectrodes combine the benefits of both 3D structures and composite materials, demonstrating significant potential for large-scale production and industrial application of microfluidic chips.

Testing Equipment: ATA-2042 High-Voltage Amplifier, Function Signal Generator, Conductivity Meter, Multi-Channel Syringe Pump, Laptop, Research-Grade Camera, Inverted Fluorescence Microscope, etc.

Experimental Procedure:

Figure: Experimental Platform for Dielectrophoretic Separation of Polystyrene Microspheres

The experiment required the setup of three subsystems: the sample injection system, the observation system, and the excitation signal system.

• The sample injection system consisted of a dual-channel syringe pump with adjustable flow rates and two 1 mL disposable syringes.

• The observation system employed electronic monitoring and. The microfluidic chip was placed on the microscope stage. The coarse focus knob was used to locate the microchannel, followed by the fine focus knob to achieve a clear field of view.

• The excitation signal system utilized a function signal generator as the signal source. A voltage amplifier was added after the signal output to enhance system power and load capacity. Since signal amplification may alter the waveform, the final output signal was monitored using an oscilloscope to ensure the required amplitude and frequency were achieved. Flat clamps, matching the dimensions of the microelectrode interface on the microfluidic chip, were used. These clamps, connected to wires, were tightly attached to the electrode interface. The signal from the voltage amplifier was transmitted through these wires to the 3D composite microelectrodes, forming a closed circuit.

① Sample Preparation:
Non-biological monodisperse polystyrene microspheres of sizes 5 μm, 10 μm, and 20 μm were selected as separation targets. The microspheres were retrieved from refrigeration. Using a pipette, 5 μL, 10 μL, and 20 μL of each size were transferred into a centrifuge tube. Subsequently, 2 mL of anhydrous ethanol was added for dispersion. Finally, 5 mL of 1 mmol/L PBS buffer (10% concentration) was added. To prevent aggregation among microspheres, between microspheres and microchannels, and between microspheres and 3D microelectrodes, 0.1% (v/v) Tween 20 was added as a surfactant. The final concentration of the three types of microspheres was approximately 10⁴–10⁵ particles/mL, with a liquid conductivity of 0.17 S/m.

② Sample Loading:
Syringe 1 of the injection system was filled with 10% PBS buffer, and Syringe 2 was filled with the mixed solution containing all three microsphere sizes. The syringes were installed, and flow rates were set: Syringe 1 at 3 μL/min and Syringe 2 at 1 μL/min.

③ Chip Pretreatment:
Before the experiment, the chip was checked for leaks. The channels were wetted, and air bubbles were expelled to prevent interference with microsphere trajectories and potential electrode electrolysis caused by bubbles during separation. The chip was placed under the microscope. Inlet 1 and Inlet 2 were connected to Syringes 1 and 2, respectively. Syringe 1 was started first to inject 10% PBS buffer, verifying no leakage and ensuring all bubbles were removed.

④ Experiment Commencement:
After complete bubble removal, Syringe 2 was activated to inject the mixed microsphere solution. Simultaneously, all instruments of the excitation signal system were turned on. The separation process was controlled by adjusting the flow rates and their ratio of Syringes 1 and 2, as well as the voltage amplitude of the excitation signal. The separation process was recorded using the observation system.

Experimental Results:

Figure: Simulation results of negative dielectrophoretic force generated by Ag-PDMS composite microelectrodes. Red lines represent electric field contours; blue arrows indicate the direction of FDEP at specific points.

In 1 mM PBS buffer, due to its relatively high conductivity, the polarization of the buffer exceeded that of the microspheres across the 1–10 MHz frequency range. Consequently, Re[K(ω)] remained negative, subjecting the microspheres to negative dielectrophoretic force (nDEP), causing them to move away from the electrodes. The figure above shows Comsol simulations of the nDEP generated by Ag-PDMS 3D microelectrodes. The red contour lines represent the electric field generated by the 3D electrodes. Results indicate that FDEP magnitude increases closer to the electrodes, where electric field lines are denser. Polystyrene microspheres migrate from regions of dense electric field lines to regions of sparser field lines.

Figure: Dielectrophoretic response of 10 μm polystyrene microspheres in 0.1 mM PBS solution under negative DEP. A 20 Vpp, 1 MHz sinusoidal signal was applied for 60 seconds. Microspheres were pushed away from the electrodes.

To verify the dielectrophoretic response of polystyrene microspheres, a pair of 3D composite microelectrodes was fabricated and placed in a groove. A mixture containing 10 μm polystyrene microspheres and PBS was added to the groove. Without any applied electric field, microspheres were distributed relatively uniformly near the electrodes and within the groove. Upon applying a 20 Vpp sinusoidal signal and varying the frequency from 1 MHz to 10 MHz, all polystyrene microspheres moved away from the electrodes. The figure above shows the distribution of polystyrene microspheres after applying a 20 Vpp, 1 MHz sinusoidal signal for 60 seconds. The experimental results confirm that within the 1–10 MHz frequency range, the polarization of polystyrene microspheres is consistently lower than that of the 0.1 mM PBS buffer, resulting in negative DEP and motion away from the electrodes.

Figure: ATA-2042 High-Voltage Amplifier Specifications and Parameters