Double Emulsion Generation Using a Polydimethylsiloxane (PDMS) Co-axial Flow Focus Device

Summary

Microfluidic double emulsions generation typically involves devices with patterned wettability or custom-fabricated glass components. Here we describe the fabrication and testing of an all polydimethylsiloxane (PDMS) double emulsion generator that does not require surface treatment or complicated fabrication processes, and is capable of producing double emulsions down to 14 µm.

Abstract

Double emulsions are useful in a number of biological and industrial applications in which it is important to have an aqueous carrier fluid. This paper presents a polydimethylsiloxane (PDMS) microfluidic device capable of generating water/oil/water double emulsions using a coaxial flow focusing geometry that can be fabricated entirely using soft lithography. Similar to emulsion devices using glass capillaries, double emulsions can be formed in channels with uniform wettability and with dimensions much smaller than the channel sizes. Three dimensional flow focusing geometry is achieved by casting a pair of PDMS slabs using two layer soft lithography, then mating the slabs together in a clamshell configuration. Complementary locking features molded into the PDMS slabs enable the accurate registration of features on each of the slab surfaces. Device testing demonstrates formation of double emulsions from 14 µm to 50 µm in diameter while using large channels that are robust against fouling and clogging.

Introduction

Double emulsions consist of droplets separated from a carrier phase by an intermediate, immiscible fluid layer, and are of particular interest due to their potential uses in industrial, pharmaceutical, and biological applications¹. In some cases, the ability to encapsulate high value compounds in a double emulsion's core enables material to be protected and released in a controlled manner. For example, drugs may be encapsulated under solubility conditions not appropriate for the external carrier fluid². Additionally, the intermediate oil layer can be used as a capsule template for the encapsulation and delivery of drugs, cosmetics, and nutrie....

Protocol

1. SU8 Master Fabrication

1. Design the microfluidic structures for two layer fabrication using AutoCAD software and have the designs printed by a vendor on circuit board film with 10 µm resolution. The details of device design are given in an attached reference¹¹ and the channel geometries are shown in Figure 1. The layers should include alignment marks to help collocate features from each fabrication layer¹².
2. Place a pre-cleaned 3 inch diameter silicon wafer on a spin coater and turn on the vacuum to affix it to the chuck. Apply 1 ml of SU8-3035 in the center of the wafer and spin for 20 sec at 500 rpm, ....

Results

The double emulsion generator consists of a co-axial flow focusing device created using 3D PDMS fabrication (Figure 1A). The geometry enables that formation of a three-phase co-axial jet to be sheared into a square, 50 µm x 50 µm orifice, allowing the formation of water / oil / water double emulsions (Figure 1B, Figure 1C). The inner aqueous phase and the middle oil phase are brought together at a junction with channel dimensions of 10 µm.......

Discussion

The double emulsion generating geometry described here is designed to mimic the physics of glass capillary devices⁸. In these, aligned cylindrical glass capillaries are used to create a three phase coaxial jet that is sheared into uniform double emulsion droplets. The function of our 3D PDMS device is dependent on the central alignment of small features formed with 50 µm tall fabrication with carrier phase channels that are 320 µm in total height. There is a significant potential for to misaligning t.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Photomasks	CadArt Servcies
3" silicon wafers, P type, virgin test grade	University Wafers	447
SU-8 3035	Microchem	Y311074
SU-8 2050	Microchem	Y111072
Sylgard 184 silicone elastomer kit	Krayden	4019862
1 ml syringes	BD	309628
10 ml syringes	BD	309604
27 gaugue needles	BD	305109
PE 2 polyethylene tubing	Scientific Commodities, Inc.	B31695-PE/2
Novec 7500	Fisher Scientific	98-0212-2928-5	Commonly knowns as HFE 7500
Biocompatable surfactant	Ran Biotechnologies	008-FluoroSurfactant
35,000 MW PEG	Sigma Aldrich	1546660
Tween 20	Sigma Aldrich	P1369
Sodium dodecyl sulfate	Sigma Aldrich	L3771