On-demand double emulsification utilizing pneumatically actuated, selectively surface-modified PDMS micro-devices

Abstract

This article presents an active, two-step emulsification scheme that is capable of producing double emulsions with desired geometries and compositions on demand. Three-layer PDMS micro-devices with pneumatically actuated membrane-valves constructed on top of specially designed fluidic-channels are utilized to meter and shape immiscible fluids into double emulsions. By intermittently squeezing a fluid into another one, controlled emulsification is realized, and successive emulsification steps result in the formation of multiple emulsions. In the prototype demonstration, a three-layer PDMS molding and bonding process was employed to fabricate the proposed microfluidic devices, whose channel surfaces were selectively modified into hydrophilic by a photo-grafting process. A governing computer program cooperating with a set of control hardware was employed to coordinate the actuation of the prototype system. It has been demonstrated that: (1) both water-in-oil-in-oil and water-in-oil-in-water double emulsions can be produced; (2) the sizes of inner aqueous droplets and outer oil drops can be controlled independently; and (3) adjacent oil drops with varying overall sizes, and both diameters and numbers of inner aqueous droplets can be produced on demand. As such, the demonstrated emulsification scheme could potentially fulfill the real-time controllability on emulsion formation, which is desired for a variety of chemical and biological applications.

Surface modification for PDMS‐based microfluidic devices

“…Graft polymer coating often involves the creation of radical sites as initiators on the surface for subsequent surface‐initiated polymerization by means of UV exposure 35–39. This approach is referred to as “grafting‐from”.…”

“…BP is widely used as a photoinitiator in this technique 35–40 because its photochemically produced triplet state can abstract hydrogen atoms from almost all monomers, thus generating radical sites for further graft polymerization 41. Su and co‐workers 36, 37 and Fiddes et al 40 have hydrophilized a PDMS surface via a one‐step UV exposure with a mixture of BP and monomer solutions (AAc and acrylamide, respectively). The resulting hydrogel modified‐PDMS microchannel could be employed to generate both water‐in‐oil‐in‐oil (W/O/O) and water‐in‐oil‐in‐water (W/O/W) emulsions, which will be discussed in more detail further below 36, 37.…”

“…Su and co‐workers 36, 37 and Fiddes et al 40 have hydrophilized a PDMS surface via a one‐step UV exposure with a mixture of BP and monomer solutions (AAc and acrylamide, respectively). The resulting hydrogel modified‐PDMS microchannel could be employed to generate both water‐in‐oil‐in‐oil (W/O/O) and water‐in‐oil‐in‐water (W/O/W) emulsions, which will be discussed in more detail further below 36, 37. Patterned poly(acrylamide)‐modified PDMS microchannel was obtained through a photomask and then successfully patterned with trypsin using EDC/NHS coupling to produce proteolytic surface patches 40.…”

“…Soft lithography is undoubtedly the most common technique for introducing controlled topography into microfluidic device surfaces due to PDMS's elastomeric properties and ease of molding into micrometer features 27, 31, 36, 37, 45. The following highlights some additional methods of introducing topographical features.…”

“…Due to its simplicity, UV‐induced polymerization through a photomask has been the process of choice to produce a desirable chemically patterned PDMS surface within microchannels 35, 37, 40, 46, 80. More recently, Su and co‐workers used membrane valves to protect hydrophobic areas on native PDMS and to control exposure of microchannels to monomer solution 37 or ethyl acetate 36. Any non‐protected areas were then exposed to UV‐induced polymerization of AAc to produce patterned surfaces within a 3‐D microfluidic device 36, 37.…”