Integrated liquid and droplet dielectrophoresis for biochemical assays

Abstract

Surface microfluidic systems have emerged as an attractive alternative to conventional closed-channel microfluidic devices. In many such systems, electric fields are leveraged for the manipulation and transport of discrete nanoliter droplets on open planar surfaces. The present research work discusses dielectrophoretic liquid and droplet actuations, which provide an attractive methodology for dispensing and manipulating nanoliter and picoliter droplets on planar surfaces. We demonstrate the integration of two independent sample actuation schemes, namely liquid dielectrophoresis (L-DEP) and droplet dielectrophoresis, and furthermore validate its applicability through model biochemical assays (DNA-PicoGreen^® assay and DNA FRET assay). We also describe and present ‘tapering L-DEP’ actuation scheme, whereby we demonstrate how to simultaneously create multiple droplets of different sizes and volumes in the range of nanoliter and picoliters, from a given larger parent sample droplet.

APPENDIX: Droplet DEP: theory and actuation methodology

The D-DEP actuation scheme utilizes a pair of coplanar metal electrodes, patterned on an insulated substrate. Figure 10 shows a schematic representation of the planar D-DEP electrode geometry, which consists of an array of diagonal fishbone-shaped electrodes. The electrodes are electrically insulated from the sample by depositing thin films of dielectric materials, while the top surface is made hydrophobic by spin-coating a thin layer of amorphous Teflon^®. Teflon^® coatings impart a high initial contact angle (θ~110°) to the sample droplet, which has been found to be critical for droplet motion. A sample droplet (~1–2 μL, DI water) is manually dispensed at one end of the electrode structure. On application of a low-frequency AC voltage (typically in the range 10–110 Hz) across the electrodes, the sample droplet undergoes periodical deformations (spreading and restoration of the spherical shape, induced by AC electric field) and the entire droplet is transported along the electrodes toward the opposite end. Selected video frames captured through a high-speed camera during D-DEP actuation, illustrating this behavior is shown in Fig. 11.

Fig. 10

Schematic representation of D-DEP electrode structure. Note the fishbone-shaped electrode array and the pitch illustrated in the magnified view on the left. A sample droplet is manually pipetted at one end of the structure and, on application of low-frequency AC voltage, the droplet is transported to the other end of the structure

Fig. 11

Video frames illustrating D-DEP actuation, captured by a high-speed camera (recorded at 1,000 frames/s). Note the unique shape of the droplet in frame ‘b’ and ‘c’, which is induced by electric field patterns and shape of the electrodes. A 1 μL DI water sample droplet was used for the experiment and an AC voltage of 110 V_RMS at 50 Hz was applied across the electrodes

The D-DEP actuation principle and droplet motion can be explained as a combinatorial effect of DEP and electrowetting. The geometrically asymmetric fishbone-shaped electrodes are used to configure spatially non-uniform electric fields, while the low-frequency electric fields are simultaneously utilized for modulating the solid–liquid interfacial energy of the sample droplet, thereby increasing the sample’s surface wetting ability.

The droplet DEP motion is further detailed in Fig. 12. As shown in Fig. 12, when the applied AC voltage increases from 0 to V _MAX, the electric field intensity increases, which causes the droplet to spread over the electrodes. Figure 13 shows the equipotential contours and electric field strength computed along the gap of D-DEP electrodes. Since the electric field intensity is maximum in the gap between electrodes (Fig. 13), the droplet meniscus remains dielectrophoretically pinned along the electrode gap, while electrowetting proceeds to spread the droplet asymmetrically along the electrodes. At this point, the shape of the droplet can be attributed to the electrode geometry. Subsequently, as the voltage decreases from V _MAX to 0, the droplet re-configures its spherical shape and its center of mass shifts by a distance p, which is equal to the pitch of the fishbone-shaped electrode array. The same cycle is repeated again, this time the voltage decreases from 0 to −V _MAX and subsequently increases from −V _MAX to 0, transporting the droplet again by a distance p. Since droplet spreading occurs both at the positive and negative peak of the AC signal, the frequency of the droplet deformations (f _DROP) is twice that of the applied AC voltage (i.e., f _DROP = 2f _AC) and the sample droplet moves by two pitches in one complete sine cycle.

Fig. 12

Droplet dielectrophoresis actuation at f _AC = 50 Hz. The micrographs illustrate the resonance between applied AC voltage and mechanical oscillations of the droplet. The arrowheads act as a pointer in order to determine the distance covered by the droplet. In one complete sine cycle, the droplet covers two pitches. Droplet volume = 1 μL, pitch = 240 μm, gap = 50 μm

Fig. 13

Equipotential contours and electric field strength calculated along the center axis. A color gradient scheme (red to blue) or white to black shading indicates the maximum and minimum voltage, i.e., 100 to 0 V across DEP electrodes (shown in white). Electrode pitch = 20 μm, gap = 5 μm

The pitch of the D-DEP electrodes is an important design feature and needs to be optimized for reliable droplet actuation. The electrode pitch depends on several parameters, such as size of the droplets being actuated, sample properties (surface tension and viscosity), hydrophobicity of the surface coatings, and frequency and magnitude of the applied voltage. These parameters together affect the spreading and relaxation of droplets, while they are being manipulated by electric fields. Optimization of the pitch size for D-DEP electrodes requires a detailed study of the dynamics of droplet motion (electric field-induced droplet deformation), which is beyond the scope of the present study. However, as a design guideline, the pitch (p) of the fishbone-shaped DEP electrodes should be considerably smaller compared to droplet size (p ~ 1/6th of droplet diameter). If the droplet size is comparable to pitch (i.e., p ~ droplet diameter), the droplet will spread to cover regions of maximum field strength; however, this may not produce a sufficient displacement of the center of mass of the droplet.