Digital microfluidics (DMF) is a powerful technique for sample preparation and analysis for a broad range of biological and chemical applications. In many cases, it is desirable to carry out DMF on an open surface, such that the matrix surrounding the droplets is ambient air. However, the utility of the air-matrix DMF format has been severely limited by problems with droplet evaporation, especially when the droplet-based biochemical reactions require high temperatures for long periods of time. We present a simple solution for managing evaporation in air-matrix DMF: just-in-time replenishment of the reaction volume using droplets of solvent. We demonstrate that this solution enables DMF-mediated execution of several different biochemical reactions (RNA fragmentation, first-strand cDNA synthesis, and PCR) over a range of temperatures (4-95 °C) and incubation times (up to 1 h or more) without use of oil, humidifying chambers, or off-chip heating modules. Reaction volumes and temperatures were maintained roughly constant over the course of each experiment, such that the reaction kinetics and products generated by the air-matrix DMF device were comparable to those of conventional benchscale reactions. This simple yet effective solution for evaporation management is an important advance in developing air-matrix DMF for a wide variety of new, high-impact applications, particularly in the biomedical sciences.

Digital microfluidics (DMF) is a powerful technique for simple and precise manipulation of microscale droplets of fluid. This technique enables processing and analysis of a wide variety of samples and reagents and has proven useful in a broad range of chemical, biological, and medical applications. Handling of "real-world" samples has been a challenge, however, because typically their volumes are greater than those easily accommodated by DMF devices and contain analytes of interest at low concentration. To address this challenge, we have developed a novel "world-to-DMF" interface in which an integrated companion module drives the large-volume sample through a 10 μL droplet region on the DMF device, enabling magnet-mediated recovery of bead-bound analytes onto the device as they pass through the region. To demonstrate its utility, we use this system for extraction of RNA from human whole blood lysates (110-380 μL) and further purification in microscale volumes (5-15 μL) on the DMF device itself. Processing by the system was >2-fold faster and consumed 12-fold less reagents, yet produced RNA yields and quality fully comparable to conventional preparations and supporting qRT-PCR and RNA-Seq analyses. The world-to-DMF system is designed for flexibility in accommodating different sample types and volumes, as well as for facile integration of additional modules to enable execution of more complex protocols for sample processing and analysis. As the first technology of its kind, this innovation represents an important step forward for DMF, further enhancing its utility for a wide range of applications.

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Laser-induced breakdown spectroscopy (LIBS) is an atomic-emission spectroscopy technique that employs a focused laser beam to produce microplasma. Although LIBS was designed for applications in the field of materials science, it has lately been proposed as a method for the compositional analysis of agricultural goods. We deployed commercial handheld LIBS equipment to illustrate the performance of this promising optical technology in the context of food authentication, as the growing incidence of food fraud necessitates the development of novel portable methods for detection. We focused on regional agricultural commodities such as European Alpine-style cheeses, coffee, spices, balsamic vinegar, and vanilla extracts. Liquid examples, including seven balsamic vinegar products and six representatives of vanilla extract, were measured on a nitrocellulose membrane. No sample preparation was required for solid foods, which consisted of seven brands of coffee beans, sixteen varieties of Alpine-style cheeses, and eight different spices. The pre-processed and standardized LIBS spectra were used to train and test the elastic net-regularized multinomial classifier. The performance of the portable and benchtop LIBS systems was compared and described. The results indicate that field-deployable, portable LIBS devices provide a robust, accurate, and simple-to-use platform for agricultural product verification that requires minimal sample preparation, if any.

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