Picoinjection of Microfluidic Drops Without Metal Electrodes

Summary

We have developed a technique for picoinjecting microfluidic drops that does not require metal electrodes. As such, devices incorporating our technique are simpler to fabricate and to use.

Abstract

Existing methods for picoinjecting reagents into microfluidic drops require metal electrodes integrated into the microfluidic chip. The integration of these electrodes adds cumbersome and error-prone steps to the device fabrication process. We have developed a technique that obviates the needs for metal electrodes during picoinjection. Instead, it uses the injection fluid itself as an electrode, since most biological reagents contain dissolved electrolytes and are conductive. By eliminating the electrodes, we reduce device fabrication time and complexity, and make the devices more robust. In addition, with our approach, the injection volume depends on the voltage applied to the picoinjection solution; this allows us to rapidly adjust the volume injected by modulating the applied voltage. We demonstrate that our technique is compatible with reagents incorporating common biological compounds, including buffers, enzymes, and nucleic acids.

Introduction

In droplet-based microfluidics, micron-scale aqueous droplets are used as "test tubes" for biological reactions. The advantage to performing reactions in the tiny droplets is that each drop uses only a few pl of reagent and, with microfluidics, the drops can be formed and processed at kilohertz rates¹. Combined, these properties allow millions of reactions with individual cells, nucleic acid molecules, or compounds to be performed in a matter of min with μl of total material.

To use drops for applications like these, techniques are needed for adding controlled volumes of reagents to the drops; such ope....

Protocol

1. Design Device Dimensions and Topologies Based on Experimental Needs Using Computer Aided Design (CAD) Software

Note: Select emulsion channel diameters smaller than those of the spherical droplets. This forces the droplets into a cylindrical or "sausage" shape and allows for more effective picoinjection. For our purposes, we designed 30 x 30 μm channels for droplets that were 50 μm in diameter.

1. Model picoinjection site(s) after those described by Abate et al.³ with the exception that the channels for the metal electrodes are removed, as they are unnecessary.
2. Add channels to serve....

Results

Microscopic images taken at the picoinjection site show that electrification of the picoinjection fluid is enough to trigger injection (Figure 2). The injected volume can be controlled by modulating the amplitude of the applied voltage, with higher voltages allowing for higher injection volumes. We plot the injection volume versus the magnitude of the applied voltage for three representative molarities of injection fluid in (Figure 3). To demonstrate the speed our method, we selectively .......

Discussion

The relationship between injection volume and applied voltage is dependent on many factors including device dimensions, length of the tubing carrying the picoinjection fluid to the device, molarity of picoinjection fluid, and the velocity of the droplets as they pass they injector. For this reason we recommend that the volume/voltage relationship be characterized before each run of picoinjection by measuring injection volumes at the edges of the working ranges of voltage and molarity. Additionally, at higher voltages and.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1 ml Luer-Lok™ syringes	BD Medical	309628
LocTite UV-cured adhesive	Henkel	35241
PE-2 tubing	Scientific Commodities	BB31695-PE/2
Novec HFE-7500	3M	98-0212-2928-5
NaCl	Sigma Aldrich	S9888
1.5 ml centrifuge tubes	Eppendorf	22363531
BD Falcon 15 ml tube	BD Biosciences	352097
Air pressure control pump	Control Air Inc.		We recommend one under the control of DAQ and control software
Syringe pumps	New Era		Must be capable of holding 1 ml syringes and flowing at rates as low as 100 μl/hr
HV-amplfier			Must be capable of 1,000x amplification of signals between 0.01 and 10 V
Plasma bonder/cleaner	Harrick Plasma
3” silicon wafers	Sigma Aldrich	647535
PDMS	Dow Corning		Sylgard 184 with curing agent should be included
SU-8 photoresist	MicroChem		Viscocity depends on device dimensions