Abstract
In two-phase flow microfluidics, there is an increasing interest in technologies which enable the encapsulation of biological cells into aqueous drops and the subsequent study of their molecular (excretion or lysis) products. One not yet available but very promising analysis method is the use of biospecific surface patches embedded in the wall of microfluidic channels. In this paper, we tackle some technological challenges encountered in the development of such applications. In the detection protocol, each drop must be enabled to wet the designated patch, be held in contact long enough for biomolecular detection and subsequently be released. This is engineered via a combination of well-defined chemical sites in the walls of the flow channel and insulated microelectrodes. The tunability of the local electric field allows to modify the competition between chemical (pinning) forces which tend to immobilize the drop and hydrodynamic forces which oppose this process. We developed a prototype microfluidic device which offers this functionality. A channel structure is sandwiched between an actuation surface with electrowetting (EW) electrodes on one side and a detector surface with a hydrophilic patch amidst a hydrophobic environment on the other. Two pairs of carefully aligned EW electrodes are used: one for drop adherence and another one for the subsequent release. We demonstrate these operations and discuss the required voltage signals in terms of the forces on the drop. Finally, we discuss possible steps for further improvement in the device.
Highlights
Microfluidics—in particular droplet-based microfluidics—has become a very popular tool since its advent in the 1980s
The actuation chip half contains the electrode geometries aligned to a thin slab of PDMS, which forms the microchannel and simultaneously functions as the insulating layer required for electrowetting
In the first set of experiments, we use a microfluidic device containing the actuation surface but not the patch surface to study the electrostatic trapping of droplets, against the drag force
Summary
Microfluidics—in particular droplet-based microfluidics—has become a very popular tool since its advent in the 1980s. Advantages include miniaturization, parallelization and compartmentalization, which—together with high speeds and the ability of upscaling—result in cost reduction and enable the use of statistical analysis (Casadevall i Solvas and deMello 2011; Seemann et al 2012). One of the key application areas in microfluidics is biomedical diagnostics (Guo et al 2012; Song et al 2006; Theberge et al 2010). In this field, challenges arise in combining material properties with biochemical molecules and in the detection of small quantities of analyte in complex samples. The analysis of individual cells has become a topic of interest (Takinoue and Takeuchi 2011)
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