Lab-on-a-chip (LOC) devices, first made in the early 1990s, are intended to integrate multiple laboratory functions onto one single portable platform in which microfluidics is employed for sample processing and transport. The development of LOC technology has been driven by the demand from bio-/chemical researchers for devices that can exceed the performance of current macro-sized assays. Scaling down brings the dimension of such devices in line with the samples to be analyzed, allowing for a level of control over the samples which is unachievable with batch processing1–4. From the beginning, most LOC devices have been based on single phase systems consisting of numerous miscible aqueous phases. The well-defined nature of laminar flow allows for the formation of unique working environments; however, it limits uses requiring multi-step reactions or fast mixing. Over the last decade or so, droplet microfluidics has gained increasing attention from both academia and industry because of its capabilities such as making targeted coalescence between droplets, enabling fast and high throughput mixing, and providing a tool for multi-step reactions by coupling different functions in a particular platform. One of the key features of droplet-based microfluidics is that, under well-defined wetting conditions, generated droplets – oil-in-water/water-in-oil – are separated from the channel wall by a thin film of the carrier fluid. Therefore, cross-contamination between droplets and the effects of wall impurities are prevented. So far, droplet microfluidics has been applied widely in high throughput screenings, diagnostics, and chemical synthesis, and many other uses.
A droplet microfluidic platform, recently designed by X.Chen and C. Ren 5 of the University of Waterloo, consists of multiple functions: (1) generate, (2) pair, (3) trap, (4) merge, (5) mix and (6) release two trains of droplets with two different reagents. This is an example of a complex droplet microfluidics chip for screening drug compounds that inhibit the tau-peptide aggregation, associated with neurodegenerative disorders such as Alzheimer’s disease. The platform details include two main parts: the first part generates two streams of droplets, and the second part pairs and traps two or more droplets for screening purposes. After two droplets are trapped in trapping wells, these droplets merge; then, mix thoroughly with each other. A noteworthy aspect of this design is that the waste channel/channel for eliminating dust inside the chip. Not only does it remove dust from the main channels, it also removes undesired droplets which are unstably generated from the beginning of experiments. With the aim of limiting the coupling effects caused by the integration of multiple functions, design criteria are essential, such as the channel dimensions and network design. More details of the design are provided in  and the significant ones are summarized next. Firstly, to guarantee that the two droplet generators perform robustly without the coupling effect, a long channel containing a large number of droplets (>50) should be designed for each generator. Thus, the hydrodynamic resistance caused by a droplet leaving and entering the generator region is reduced. Secondly, the trapping well design is considered to be good if it meets 2 requirements: (i) a droplet must enter the trapping well when it reaches the entrance of the trapping well; (ii) a droplet should be kept inside a trapping well unless it has been purposely released. The authors utilized a 1D circuit analysis model in order to illustrate the design of the trapping well region and its parameters (i.e. hydrodynamic resistance of the main channel, hydrodynamic resistance of a trapping well, hydrodynamic resistance of a bypass channel, flow rates, and so on). Furthermore, the practical concerns such as the sizes of the micro-channel are also considered. For instance, the bypass channel length should be reduced so that the range of the working capillary number is expanded; however, the length should be larger than the minimum required value. Another example is that either increasing the channel height or reducing the gap width expands of the range of the capillary number.
The platform was used to test drug compounds known to inhibit tau-peptide aggregation. The trend results tend to agree well with that of the conventional 96-well-plate method 6. Remarkably, the aggregation happened on-chip is much faster (5 min vs. 2 hr). Thus, this complex droplet microfluidic platform advances the possibility of using droplet microfluidic device in drug screening. Additionally, by combining a series of trapping wells, a set of concentration gradients can be rapidly achieved on-chip instead of through batch processing. The entire chip operation relies on liquid flow control (pressure system) and requires no electrodes, magnets or any other external parts; hence, it should be convenient for standard laboratory fabrication techniques.
(2) Köhler, J. M.; Henkel, T. Chip Devices for Miniaturized Biotechnology. Appl. Microbiol. Biotechnol. 2005, 69, 113–125.
(3) Lagus, T. P.; Edd, J. F.; Davies, R. T.; Kim, D.; Park, J.; Urbanski, M.; Reyes, C. G.; Noh, J.; Seemann, R.; Brinkmann, M.; et al. Droplet Based Microfluidics. Reports Prog. Phys. 2012, 75, 016601.
(4) Teh, S.-Y.; Lin, R.; Hung, L.-H.; Lee, A. P. Droplet Microfluidics. Lab Chip 2008, 8, 198–220.
(6) Mohamed, T.; Hoang, T.; Jelokhani-Niaraki, M.; Rao, P. P. N. Tau-Derived-Hexapeptide aggregation Inhibitors: Nitrocatechol Moiety as a Pharmacophore in Drug Design. ACS Chem. Neurosci. 2013, 4, 1559–1570.