Exosomes, the smallest sized extracellular vesicles (30 ~ 150 nm) packaged with lipids, proteins, functional messenger RNAs and microRNAs, and double-stranded DNA from their cells of origin, have emerged as key players in intercellular communication. Their presence in bodily fluids, where they protect their cargo from degradation, makes them attractive candidates for clinical application as innovative diagnostic and therapeutic tools. But routine isolation and analysis of high purity exosomes in clinical settings is challenging, with conventional methods facing a number of drawbacks including low yield and/or purity, long processing times, high cost, and difficulties in standardization. Here we review a promising solution, microfluidic-based technologies that have incorporated a host of separation and sensing capabilities for exosome isolation, detection, and analysis, with emphasis on point of care and clinical applications. These new capabilities promise to advance fundamental research while paving the way toward routine exosome-based liquid biopsy for personalized medicine.

The ability to mix liquids in microchannel networks is fundamentally important in the design of nearly every miniaturized chemical and biochemical analysis system. Here, we show that enhanced micromixing can be achieved in topologically simple and easily fabricated planar 2D microchannels by simply introducing curvature and changes in width in a prescribed manner. This goal is accomplished by harnessing a synergistic combination of (i) Dean vortices that arise in the vertical plane of curved channels as a consequence of an interplay between inertial, centrifugal, and viscous effects, and (ii) expansion vortices that arise in the horizontal plane due to an abrupt increase in a conduit's cross-sectional area. We characterize these effects by using confocal microscopy of aqueous fluorescent dye streams and by observing binding interactions between an intercalating dye and double-stranded DNA. These mixing approaches are versatile and scalable and can be straightforwardly integrated as generic components in a variety of lab-on-a-chip systems.Dean flow ͉ expansion vortex ͉ microfluidics ͉ lab on a chip A lthough microf luidic mixing is a key process in a host of miniaturized analysis systems (1-7), it continues to pose challenges owing to constraints associated with operating in an unfavorable laminar f low regime dominated by molecular diffusion and characterized by a combination of low Reynolds numbers (Re ϭ Vd͞v Ͻ Ͻ 100, where V is the f low velocity, d is a length scale associated with the channel diameter, and v is the f luid kinematic viscosity) and high Péclet numbers (Pe ϭ Vd͞D Ͼ 100, where D is the molecular diffusivity). The relatively large discrepancy between convective and diffusive timescales implies that in a straight smooth-walled microchannel, the downstream distances over which liquids must travel to become fully intermixed (⌬y m ϳ Vd 2 ͞D ϭ Pe ϫ d) can be on the order of several centimeters. These mixing lengths are generally prohibitively long and often negate many of the benefits of miniaturization.A wide variety of micromixing approaches have been explored (8, 9), most of which can be broadly classified as either ''active'' (involving input of external energy) or ''passive'' (harnessing the inherent hydrodynamic structure of specific flow fields to mix fluids in the absence of external forces). Passive designs are often desirable in applications involving sensitive species (e.g., biological samples) because they do not impose strong mechanical, electrical, or thermal agitation. Examples of passive micromixing approaches that have been widely investigated include the following: (i) ''split-and-recombine'' strategies where the streams to be mixed are divided or split into multiple channels and redirected along trajectories that allow them to be subsequently reassembled as alternating lamellae yielding exponential reductions in interspecies diffusion length and time scales (4, 10-12); and (ii) ''chaotic'' strategies where transverse flows are passively generated that continuously expand interfacial...

An integrated microfluidic device capable of performing a variety of genetic assays has been developed as a step towards building systems for widespread dissemination. The device integrates fluidic and thermal components such as heaters, temperature sensors, and addressable valves to control two nanoliter reactors in series followed by an electrophoretic separation. This combination of components is suitable for a variety of genetic analyses. As an example, we have successfully identified sequence-specific hemagglutinin A subtype for the A/LA/1/87 strain of influenza virus. The device uses a compact design and mass production technologies, making it an attractive platform for a variety of widely disseminated applications.

Mixing of fluids at the microscale poses a variety of challenges, many of which arise from the fact that molecular diffusion is the dominant transport mechanism in the laminar flow regime. While considerable progress has been made toward developing strategies to achieve improved mixing in microfluidic systems, many of these techniques introduce additional complexity to device fabrication and/or operation processes. In this work, we explore the use of compact spiral-shaped flow geometries designed to achieve efficient mixing in a format that can be constructed using a single planar soft lithography step without the need for multilayer alignment. A series of 150 microm-wide by 29 microm-tall channels were constructed, each of which incorporated a series of spiral shaped sections arrayed along the flow path. Five spiral designs with varying channel lengths were investigated, and mixing studies were carried out at flow rates corresponding to Reynolds numbers ranging from 0.02 to 18.6. Under appropriate conditions, transverse Dean flows are induced that augment diffusive transport and promote enhanced mixing in considerably shorter downstream distances as compared with conventional planar straight channel designs. Mixing efficiency can be further enhanced by incorporating expansion vortex effects via abrupt changes in cross-sectional area along the flow path.

Rayleigh-Benard convection is caused by buoyancy-driven instability in a confined fluid layer heated from below (1). The dimensionless Rayleigh number Ra = ga(T -T)h3/VK expresses the interplay between buoyant forces driving the instability and diffusive restoring forces acting in opposition. Here, a is the coefficient of thermal expansion of the fluid, g is the acceleration due to gravity, T, and T2 are the temperatures of the top and bottom surfaces of the cavity, respectively, h is the height of the cavity, v is the kinematic viscosity, and K is the thermal diffusivity.The inherent structure of Rayleigh-Benard convection-steady circulatory flow between surfaces maintained at two fixed temperaturesis ideally suited for performing thermally activated chemical reactions that require temperature cycling. We have developed a device that uses Rayleigh-Benard convection to perform polymerase chain reaction (PCR) amplification of DNA inside a 35-p1l cylindrical cavity. Instead of the external temperature control of conventional thermocyclers, temperature cycling is achieved as the flow field continually shuttles fluid packets vertically through the temperature zones associated with denaturation (-95?C) and annealing/extension (60? to 70?C). The steady circulatory flow field must engage the entire reaction volume yet be slow enough to allow the reaction within each temperature zone to reach completion. The parameters available to control the fluid motion are the Rayleigh number and the aspect ratio hid, where d is the diameter of the cavity. In the case of PCR, the required reaction efficiency constrains the reaction solution and the temperature difference; thus, Ra can only be changed by varying the height of the cavity, leaving geometry as the primary flow control parameter.

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Recent studies of molecular orientation in sheared liquid crystalline polymers have often yielded contradictory results. To check the self-consistency of methods for quantitative measurements of molecular orientation, liquid crystalline solutions of (hydroxypropyl)cellulose [HPC] and poly(benzyl glutamate) [PBG] have been studied using flow birefringence, X-ray scattering, and neutron scattering. HPC X-ray scattering patterns show an arclike pattern with a distinct peak as a function of scattering vector, while PBG patterns show a more diffuse equitorial streak. These differences are attributed to more strongly correlated lateral packing in HPC solutions due to their higher concentration. Measurements of orientation in steady shear flow agree well among the three techniques. Lyotropic HPC and PBG solutions differ in orientation at low shear rates. HPC solutions exhibit near zero orientation at low rates, while X-ray and neutron scattering measurements confirm previous birefringence data showing a low shear rate plateau of moderate orientation in PBG. Differences with recent neutron scattering measurements on PBG solutions that show low orientation at low shear rate are attributed to choice of solvent, rather than choice of technique. X-ray and optical data are consistent in showing decreasing orientation in HPC solutions during relaxation, but discrepancies are found in relaxation of PBG solutions. Large increases in flow birefringence suggest substantial orientation enhancement. X-ray data on one PBG solution confirm increasing orientation, but X-ray and neutron scattering data on a more concentrated solution show only modest changes in orientation. It is suggested that flow birefringence fails in this case due to texture coarsening to the point where there is no longer effective averaging over the distribution of director orientations along the light path.

We introduce a portable biochemical analysis platform for rapid field deployment of nucleic acid-based diagnostics using consumer-class quadcopter drones. This approach exploits the ability to isothermally perform the polymerase chain reaction (PCR) with a single heater, enabling the system to be operated using standard 5 V USB sources that power mobile devices (via battery, solar, or hand crank action). Time-resolved fluorescence detection and quantification is achieved using a smartphone camera and integrated image analysis app. Standard sample preparation is enabled by leveraging the drone’s motors as centrifuges via 3D printed snap-on attachments. These advancements make it possible to build a complete DNA/RNA analysis system at a cost of ∼$50 ($US). Our instrument is rugged and versatile, enabling pinpoint deployment of sophisticated diagnostics to distributed field sites. This capability is demonstrated by successful in-flight replication of Staphylococcus aureus and λ-phage DNA targets in under 20 min. The ability to perform rapid in-flight assays with smartphone connectivity eliminates delays between sample collection and analysis so that test results can be delivered in minutes, suggesting new possibilities for drone-based systems to function in broader and more sophisticated roles beyond cargo transport and imaging.