# Presentation: 170382

## Manipulation of microparticles by shaping the applied electric field in microelectrodes

**Jorge Quijano, Shalini Prasad  **
**Department of Electrical Engineering, Portland State University **

**ABSTRACT**
-   A microscale platform consisting of a planar array of microelectrodes has been optimized for the electrical manipulation of microparticles. This platform was used to observe and characterize the effects of dielectrophoresis and electrorotation in microparticles. By making use of selected sets of voltage waveforms, the electric field gradient induced in the microelectrode array was shaped. Variations in the voltage waveform were used as the controlling parameter for positioning of microparticles in specific areas of the device, without changing its geometry. This waveform-oriented design of a particle positioning system resulted in the development of a multipurpose device as the same platform was used to accomplish several electric field micromanipulation functions. The development of this device is targeted to build a cost effective platform using simple electronic components such as resistor-capacitor (RC) networks and a commercial digital microcontroller which can be employed to generate excitation waveforms in a wide range of frequencies. The eventual goal is the construction of a closed loop system for tracking/positioning of single particles.

-   The 3x3 microelectrode array (MEA) composed of platinum electrodes (80 μm diameter, with spatial separation of 200 μm between elements) on a silicon/silicon nitride substrate has been used to generate non-uniform electric fields with different distributions (Figure 1). Trapping of microparticles in broad areas of the MEA was achieved by exciting specific array elements.
-   A silicon chamber was placed over the MEA to localize the liquid media over the electrode platform and to maintain a stable microenvironment.
-   A function generator was used to vary the applied input signal strength and shape. Sinusoid and rectangular pulses were utilized as excitation sources for electrical manipulation of particles. A resistor-capacitor filter was used to generate a third waveform based on the signal from the generator. This waveform has an exponential shape and is associated with a faster decay/rise time compared to a sinusoid. The combination of sinusoid, square and exponential signals were used to demonstrate and characterize particle manipulation. The movement of the particles was optically monitored with a CCD camera attached to a compound microscope.

**MATERIALS AND METHODS**

**Figure 2**. Experimental set up for positioning of microparticles using dielectrophoresis forces. By changing the applied voltage waveform and by selecting the excited microelectrode, the electric field distribution can be shaped to produce different effects in the microparticles, like translation and rotation.

-   Two sinusoid waveforms with a phase delay were applied to the microelectrode array as shown in figure 3. The resulting electric field was a vector with an angular frequency equal to the frequency of the applied signal. The dielectrophoresis process was more evident in the vicinity of the microelectrodes were the excitation voltage was applied.

**Figure 4. ****(C)** The pattern followed by the polystyrene beads is similar to the pattern shown in figure 3, but the dielectrophoretic force is clearly more intense, yielding a more defined pattern.

**ACKNOWLEDGEMENTS**

**SUMMARY AND CONCLUSIONS**

-   We have experimentally demonstrated the hypothesis that by selecting a set of waveforms and by controlling the timing and the terminals of the MEA where the voltage signals are applied, the electric field gradient can be shaped and manipulated. This results in a versatile platform that performs several functions like centrifugation, sorting, linear movement and positioning of particles in selected areas without any variations to the MEA geometry. The use of exponential waveforms instead of the traditional sinusoids has in general two advantages:
- Since the voltage decay of the exponential waveforms is faster than that of sinusoids, the resulting electric field for a given configuration presents steeper gradients, which translates in faster dielectrophoresis processes.
- Generation of exponential waveforms with controllable frequency and phase can be easily achieved by using the digital outputs of any commercial microcontroller in combination with RC.
-   The performance of such a  device with multifunctional capabilities can be improved by:
- Optimizing the design of the microelectrode array
- Variation in the MEA geometry for coarse positioning, fine movements, electrorotation and trapping.

- [1]** **Madou, Marc**. Fundamental of microfabrication. **pp570.

**REFERENCES**

**Figure 5 (A).** Four delayed unipolar exponential waveforms with 2 Vpp were applied symmetrically to four specific points of the microelectrode array. This example proved the potential use of  multiple digital signals for particle manipulation. **(B)**  The effect of the dielectrophoretic force after 80 seconds produced patterns not only around the excited microelectrode, but also in the vicinity of the metal track that interconnects the microelectrode with the pad**. **The size of the microelectrode is still too big compared with the particles, which results in good control of regions but deficient control of single particles.

-   The distribution of microparticles in the microelectrode array varied according to the nature of the input waveform applied to the array. Figure 4 shows the dielectrophoretic process when a combination square wave/exponential wave was applied to the MEA. It was observed that after 40 s of the application of the input signal, the average distribution of the particles remained the same either for sinusoid input or square/exponential input, but the speed of the dielectrophoretic process was improved when the asymmetric square/exponential wave was applied to the microelectrode array (approximately two times faster). This state was identified as a “steady state”, because after long periods (>2 min), the microparticles distribution did not undergo significant variations under visual inspection

**Figure 1.** The net force experienced by spherical particles in suspension due to the gradient of the electric field is known as dielectrophoretic force, which is proportional to the radius (r) of the particle, its permittivity, the Clausius-Mosoti factor (Re(α)) and the gradient of the squared magnitude of the electric field(▼E2).

**RESULTS**

**Waveform dependent particle manipulation**

**Figure 3 ****(C)** The effect of the dielectrophoretic force after 40 seconds produced patterns, which evidence the regions were the electric field gradient is more intense. The prototype model comprised of polystyrene particles of 15 μm diameter (permittivity of 2.56 and conductivity of 10-16 S/m) suspended in de-ionized  water.

**II) Exponential waveforms in combination with square waveforms**

**Figure 7 **(A)** **Microelectrode geometry for the development of a multipurpose platform. The features in (B), (C), and (D) are intended for coarse positioning of groups of particles. (E) The small features  allow the manipulation of particles as small as 10 μm. Once a group of particles is trapped into the region of interest, a constant voltage can be applied to the four “region microelectrodes”, isolating the particles for further manipulation with the fine adjustment microelectrodes.**  **

**III) Delayed exponential waveforms**

**Prototype system for the positioning of bioparticles over microelectrodes**

**I) Sinusoid waveforms**

-   Microelectrode arrays have proved to be a convenient method for the recording of electrical activity in cells to be used in biosensors, but a reliable technique for positioning the bioparticles onto the recording microelectrodes has to be implemented.
-   The effects of dielectrophoresis and electrorotation can be applied to achieve electrical self assembly of microparticles in the device. By controlling the temporization of a set of input signals applied to the microelectrode array, the induced electric field gradient can be shaped for coarse and fine positioning of bioparticles. This waveform-oriented design of particle positioning systems results in a multipurpose device, as the same platform can be used to accomplish different effects. The development of this device is targeted to build a cost effective platform using simple electronic components such as RC networks and a commercial digital microcontroller which can be used to generate excitation waveforms in a wide range of frequencies and phases. The eventual goal is the construction of a closed loop system for tracking/positioning of single particles.

**(A)**

**(B)**

**(B)**

**(C)**

**(A)**

**(D)**

**(E)**

-   The advantage of using delayed exponential waveforms for the manipulation of microparticles by dielectrophoresis is that multiple signals can be generated by a combination of digital pulses and a resistor-capacitor network.
-   In this experiment, four exponential signals were generated with a 90 ̊ phase between each other.
-   The dielectrophoresis process was slower because of the smaller amplitude used in the excitation signals(2 Vpp), but this could be an advantage when dealing with temperature sensitive microparticles such as cells, because the heat transfer to the liquid media is reduced by the limited power of the excitation signals.

- (A) Particle experiencing zero dielectrophoretic force. In this case, the density of electric field lines in both sides of the particle is equal; the net force experienced is zero.

- (B) Particle experiencing positive dielectrophoretic force. Due to asymmetry of the  density of electric field lines in both sides of the particle a net force is produced. The strength of this force is given by:

-   We would like to thank Portland State University Faculty Enhancement Grant and ONAMI.
-   The author would like to thank the Fulbright scholarships program for the sponsorship and support given.

**Figure 3 (A).** Two delayed sinusoidal waveforms were applied asymmetrically in four specific points of the microelectrode array.

**Figure 3 ****(B)** Theoretical representation of the rotating nature of the electric field in two dimensions by solving the partial differential equation using Matlab®.

**Figure 4. (A).** A combination of a bipolar square wave and a bipolar exponential wave was applied asymmetrically in four specific points of the microelectrode array.

**Figure 4. ****(B) **Theoretical representation of the rotating nature of the electric field in two dimensions by solving the partial differential equation using Matlab®.