Search

Targeted, on‐demand delivery has been of interest using materials responsive to environmental stimuli. A delivery technique based on precise release of aqueous microdroplets from a liquid crystal (LC) medium with contactless stimulation is presented. A nematic LC is doped with a photothermal dye that produces heat under near IR light exposure. The heat is used to overcome the elastic strains in the LC phase, promoting the release of initially entrapped water droplets to the neighboring aqueous solution. Designing the geometry of LC‐based emulsions and tuning the light intensity and position allows for manipulation of the release in two distinct modes defined as pulsated and continuous. In the pulsated mode, water droplets are released transiently from the casted water‐in‐LC emulsion layer based on sweeping by the moving isotropic‐nematic phase boundary controlled by light. In the continuous mode, water droplets are ejected continuously from a droplet‐shaped water‐in‐LC emulsion, due to a heating‐induced internal flow controlled by light. The droplet release by contactless stimulation is used for the on‐demand dosing of dopamine and its oxidizing reagent from isolated reservoirs to obtain an in situ reaction signal for a hydrogen peroxide assay. A new dual‐mode release system developed with photothermal LCs holds potential in drug release, controlled mixing, and photothermal therapy. [ABSTRACT FROM AUTHOR]

Droplet-based microfluidic systems offer tremendous benefits for high throughput biochemical assays. Despite the wide use of electrical detection for microfluidic systems, application of impedimetric sensing for droplet systems is very limited. This is mainly due to the insulating oil-based continuous phase used for most aqueous samples of interest. We present modelling and experimental verification of impedimetric detection of hemagglutination in microdroplets. We have detected agglutinated red blood cells in microdroplets and screened whole blood samples for multiple antibody sera using conventional microelectrodes. We were able to form antibody and whole blood microdroplets in PDMS microchannels without any tedious chemical surface treatment. Following the injection of a blood sample into antibody droplets, we have detected the agglutination-positive and negative droplets in an automated manner. In order to understand the characteristics of impedimetric detection inside microdroplets, we have developed the lumped electrical circuit equivalent of an impedimetric droplet content detection system. The empirical lumped element values are in accordance with similar models developed for single phase electrical impedance spectroscopy systems. The presented approach is of interest for label-free, quantitative analysis of droplets. In addition, the standard electronic equipment used for detection allows miniaturized detection circuitries that can be integrated with a fluidic system for a quantitative microdroplet-based hemagglutination assay that is conventionally performed in well plates. [ABSTRACT FROM AUTHOR]

Droplet microfluidic systems have evolved as fluidic platforms that use much less sample volume and provide high throughput for biochemical analysis compared to conventional microfluidic devices. The variety of droplet fluidic applications triggered several detection techniques to be applied for analysis of droplets. In this review, we focus on label-free droplet detection techniques that were adapted to various droplet microfluidic platforms. We provide a classification of most commonly used droplet platform technologies. Then we discuss the examples of various label-free droplet detection schemes implemented for these platforms. While providing the research landscape for label-free droplet detection methods, we aim to highlight the strengths and shortcomings of each droplet platform so that a more targeted approach can be taken by researchers when selecting a droplet platform and a detection scheme for any given application. [ABSTRACT FROM AUTHOR]

• Revealed the root cause for droplet volume variation in fluidic systems. • Presented a universal novel method to achieve ultra-high monodispersity of droplets. • The novel method implemented for the most common droplet generators and flow sources. • Easily applicable method with no additional cost for existing droplet systems. Despite the importance of droplet monodispersity, a universal methodology for high monodispersity droplet generation does not exist yet. We have recently demonstrated that unlike the conventional method of droplet generation, applying an identical pressure from a single source makes the microfluidic droplet system immune to the external fluctuations that originate from the imperfection of the flow source. In this work, we show that our method is universal and applicable to other common microfluidic devices and flow sources. We applied this method to flow-focusing and coflow devices that are commonly used for high-frequency microdroplet generation. In addition to the pressure pump, we used a syringe pump to show that our method is applicable to flow rate controllable systems as well. We compared the monodispersity of droplets formed by the conventional methods and the novel method explained in this work. [ABSTRACT FROM AUTHOR]

Microfluidic capacitive sensors have been used for detection of droplets, however they have been lacking the sensitivity required for detecting the content of droplets. In this study, we developed a scalable, portable, robust and high sensitivity capacitive microdroplet content detection system using coplanar electrodes with nanometer thick silicon dioxide (SiO 2 ) passivation layer and off-the-shelf capacitive sensors. The microfluidic chip we have designed provides easy and rapid modification of droplet content by mixing two aqueous liquids at any given ratio. The change in dielectric constant of the droplet content leads to the change in capacitive signal. The dielectric content of droplets was modified continuously while corresponding capacitance signal was measured. The resolution of the system was measured as 3 dielectric permittivity units. The results were verified using a semiconductor parameter analyzer. The application specific integrated circuit used in this work enables a portable, low-cost detection system and matches the performance of bench-top analyzers. Automated and precise measurement of dielectric content in droplets for biochemical assay monitoring is a major application of the presented system. [ABSTRACT FROM AUTHOR]

• Investigated the underlying reasons for droplet volume variation in fluidic systems. • Presented 4 design recommendations to improve monodisperse droplet generation. • Presented practical limit of achieving absolute droplet monodispersity, i.e. 0% CV. • Achieved ultra-high droplet monodispersity, less than 0.2% CV in droplet size. • Fluidic evaluation chip design to test parameters under same experimental conditions. Droplet microfluidic systems are becoming routine in advanced biochemical studies such as single cell gene expression, immuno profiling, precise nucleic acid quantification (dPCR) and particle synthesis. For all these applications, ensuring droplet monodispersity is critical to minimize the uncertainty due to droplet volume variation. Despite the wide usage of droplet-based microfluidic systems, the limit of monodispersity for droplet generation systems is still unknown. Here, we present an analytical approach that takes into account all the system dynamics and internal/external factors that disturb monodispersity. Interestingly, we are able to model the dynamics of a segmented two-phase flow system using a single-phase flow analogy, electron flow, in electrical circuits. We offer a unique solution and design guidelines to ensure ultra-monodisperse droplet generation. Our analytical conclusions are experimentally verified using a T-junction droplet generator. Equally importantly, we show the limiting experimental factors for reaching the theoretical maximum of monodispersity. [ABSTRACT FROM AUTHOR]