Digital microfluidics (DMF) is an emerging liquid-handling technology that enables individual control over droplets on an open array of electrodes. These picoliter- to microliter-sized droplets, each serving as an isolated vessel for chemical processes, can be made to move, merge, split, and dispense from reservoirs. Because of its unique advantages, including simple instrumentation, flexible device geometry, and easy coupling with other technologies, DMF is being applied to a wide range of fields. In this review, we summarize the state of the art of DMF technology from the perspective of analytical chemistry in sections describing the theory of droplet actuation, device fabrication and integration, and applications.

We introduce DropBot: an open-source instrument for digital microfluidics (http://microfluidics. utoronto.ca/dropbot). DropBot features two key functionalities for digital microfluidics: (1) real-time monitoring of instantaneous drop velocity (which we propose is a proxy for resistive forces), and (2) application of constant electrostatic driving forces through compensation for amplifier-loading and device capacitance. We anticipate that this system will enhance insight into failure modes and lead to new strategies for improved device reliability, and will be useful for the growing number of users who are adopting digital microfluidics for automated, miniaturized laboratory operation.

We introduce an automated digital microfluidic (DMF) platform capable of performing immunoassays from sample to analysis with minimal manual intervention. This platform features (a) a 90 Pogo pin interface for digital microfluidic control, (b) an integrated (and motorized) photomultiplier tube for chemiluminescent detection, and (c) a magnetic lens assembly which focuses magnetic fields into a narrow region on the surface of the DMF device, facilitating up to eight simultaneous digital microfluidic magnetic separations. The new platform was used to implement a three-level full factorial design of experiments (DOE) optimization for thyroid-stimulating hormone immunoassays, varying (1) the analyte concentration, (2) the sample incubation time, and (3) the sample volume, resulting in an optimized protocol that reduced the detection limit and sample incubation time by up to 5-fold and 2-fold, respectively, relative to those from previous work. To our knowledge, this is the first report of a DOE optimization for immunoassays in a microfluidic system of any format. We propose that this new platform paves the way for a benchtop tool that is useful for implementing immunoassays in near-patient settings, including community hospitals, physicians' offices, and small clinical laboratories.

The first example of so-called "digital microfluidics" (DMF) implemented on paper by inkjet printing is reported. A sandwich enzyme-linked immunosorbent assay (ELISA) is demonstrated as an example of a complex, multistep protocol that would be difficult to achieve with capillary-driven paper microfluidics. Furthermore, it is shown that paper-based DMF devices have comparable performance to traditional photolithographically patterned DMF devices at a fraction of the cost.

Dried blood spot (DBS) samples on filter paper are surging in popularity as a sampling and storage vehicle for a wide range of clinical and pharmaceutical applications. For example, a DBS sample is collected from every baby born in the province of Ontario, Canada, for quantification of approximately one hundred analytes that are used to screen for 28 conditions, including succinylacetone (SA), a marker for hepatorenal tyrosinemia. Unfortunately, the conventional methods used to evaluate DBS samples for newborn screening and other applications are tedious and slow, with limited options for automated analysis. In response to this challenge, we have developed a method to couple digital microfluidics (DMF) to nanoelectrospray ionization mass spectrometry (nESI-MS) for SA quantification in DBS samples. The new system is formed by sandwiching a pulled glass capillary emitter between the two DMF substrates such that the capillary emitter is immobilized without external seals or gaskets. Moreover, we introduce a new feedback control system that enables high-fidelity droplet manipulation across DBS samples without manual intervention. The system was validated by application to on-chip extraction, derivatization, and analysis of SA and other analytes from DBS samples, with comparable performance to gold-standard methods. We propose that the new methods described here can potentially contribute to a new generation of analytical techniques for quantifying analytes in DBS samples for a wide range of applications.

Portable digital microfluidic serological immunoassays for measles and rubella were developed and evaluated in a remote setting.

Digital microfluidics (DMF) is a technique in which discrete droplets are manipulated by applying electrical fields to an array of electrodes. In an ideal DMF system, each application of driving potential would cause a targeted droplet to move onto an energized electrode (i.e., perfect fidelity between driving voltage and actuation); however, in real systems, droplets are sometimes observed to resist movement onto particular electrodes. Here, we implement a sensing and feedback control system in which all droplet movements are monitored, such that when a movement failure is observed, additional driving voltages can be applied until the droplet completes the desired operation. The new system was evaluated for a series of liquids including water, methanol, and cell culture medium containing fetal bovine serum, and feedback control was observed to result in dramatic improvements in droplet actuation fidelity and velocity. The utility of the new system was validated by implementing an enzyme kinetics assay with continuous mixing. The new platform for digital microfluidics is simple and inexpensive and thus should be useful for scientists and engineers who are developing automated analysis platforms.

The diagnosis of infectious disease is typically carried out at the point-of-care (POC) using the lateral flow assay (LFA). While cost-effective and portable, LFAs often lack the clinical sensitivity and specificity required for accurate diagnoses. In response to this challenge, we introduce a new digital microfluidic (DMF) platform fabricated using a custom inkjet printing and roll-coating process that is scalable to mass production. The performance of the new devices is on par with that of traditional DMF devices fabricated in a cleanroom, with a materials cost for the new devices of only US $0.63 per device. To evaluate the usefulness of the new platform, we performed a 13-step rubella virus (RV) IgG immunoassay on the inkjet printed, roll-coated devices, which yielded a limit of detection of 0.02 IU mL, well below the diagnostic cut-off of 10 IU mL −1 for RV infection and immunity. We propose that this represents a breakthrough for DMF, lowering the costs to a level such that the new platforms will be an attractive alternative to LFAs for the diagnosis of infectious disease at the POC.