Stiffness-Independent Highly Efficient On-Chip Extraction of Cell-Laden Hydrogel Microcapsules from Oil Emulsion into Aqueous Solution by Dielectrophoresis

Abstract

A dielectrophoresis (DEP)-based method achieves highly efficient on-chip extraction of cell-laden microcapsules of any stiffness from oil into aqueous solution. The hydrogel microcapsules can be extracted into the aqueous solution by DEP and interfacial tension forces with no trapped oil, while the encapsulated cells are free from electrical damage due to the Faraday cage effect.

Keywords: dielectrophoresis; hydrogels, microcapsules; interfacial tension; liquid electrodes; on-chip extraction.

Figure 1

The microfluidic device and modeling of electric field and DEP force in the electrode region of the device. (a) A schematic overview of the microfluidic device showing the three inlets (I1, I2, and I3), flow-focusing junction (FFJ), and two outlets (O1 and O2) together with a zoom-in view of the electrode (E1 and E2) region with dimensions (unit: μm). Compositions of fluids flowing into inlets: I1, oil emulsion; I2, 2% (w/v) sodium alginate in saline with or without cells (1.5 million/ml); I3, 1.3% (w/v) medium-viscosity carboxymethyl cellulose (Sigma) in saline; E1, the same as I1; and E2, the same as I3. (b) The conductivity of oil emulsion as a function of the DC electric field showing a breakover at ~200 kV/m. The error bars represent the standard error of mean (SEM). (c) Modeling results of the electric field distribution in the electrode region in the absence of a hydrogel microcapsule and in the presence of a hydrogel microcapsule at three different locations. (d) The y component of the DEP force that a microcapsule experiences at various locations in the electrode region. The x and y coordinates are the same as that shown in (c).

Figure 2

Generation and extraction of hydrogel microcapsules of calcium alginate in the microfluidic device (moving from left to right). (a) Formation of microdroplet at the flow-focusing junction (FFJ). (b) A microdroplet or hydrogel microcapsules travelling at the centerline of the serpentine microchannel. (c) A hydrogel microcapsules extracted into the aqueous phase exiting via the O1 outlet. (d) A hydrogel microcapsule in the oil emulsion exiting via the O2 outlet. (e) Movement of an alginate (2%) hydrogel microcapsule in the extraction channel (i.e., electrode region) without electrical field. (f) Extraction of 2% alginate hydrogel microcapsules with electric field 1700 V. (g) Extraction of 0.5% alginate hydrogel microcapsules with electric field 1700 V. Scale bar: 200 μm. (h) The DEP extraction efficiency of 0.5 and 2% alginate hydrogel microcapsules under various electric fields. The number (N) of microcapsules used to calculate the extraction efficiency was ~250 for each voltage. Four independent runs were performed for each alginate concentration.

Figure 3

The effect of on-chip and off-chip extraction on the viability of microencapsulated C3H10T1/2 cells. (a) The viability of cells without microencapsulation or extraction (control), microencapsulated cells with on-chip DEP extraction (2000 V), and microencapsulated cells extracted into the aqueous phase using the conventional off-chip method (i.e., centrifuging and washing). (**): p < 0.01. The error bars represent SEM. (b) Typical phase and fluorescence images showing cell viability for the same three different groups. Scale bar: 200 μm