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This paper presents a contactless droplet manipulation system that relies on thermally generated Marangoni flows. Programmable 2-D control of aqueous microdroplets suspended in an oil film on a plain featureless glass substrate is achieved using a 128-pixel heater array suspended 100-500 μm above the oil layer. The heaters generate surface temperature perturbations (< 25 °C), resulting in local Marangoni flows that can move droplets in either a push or a pull mode. Programmed movement is achieved by the sequential activation of the heaters, with digital control circuitry and a graphical interface providing addressable control of each heater. Droplets with diameters of 300-1000 μm are manipulated and merged at speeds up to 140 μm/s. Evaporation rates can be reduced by almost two orders of magnitude by using a two-layer-oil medium, and the choice of an optimum carrier fluid can achieve fluid velocities over 17000 μm/s. The system provides a contactless platform for parallel droplet-based assays. As such, it circumvents the challenges of sample contamination and loss that occur when a droplet interacts with a solid surface [ABSTRACT FROM AUTHOR]

The authors comment on a study which investigated the use of thermochemical lithography to pattern an electroluminescent polymer with nanoscale resolution, conducted by Franco Cacialli and colleagues at the London Centre for Nanotechnology in England. They also note that the study highlights the potential for high resolution and the ability to pattern a wide range of materials, two of the most promising aspects of thermochemical lithography.

Remote patient monitoring (RPM) relies on low-cost, low-power, wearable sensors for continuous physiological assessment. Photoplethysmographic (PPG) sensors generally require >10 components, occupy an area >300 mm(2), consume >10 mW power, and cost >$20 USD. Although the principle of PPG sensing is straightforward, in practice, a robust implementation requires a careful design including optical alignment, analog circuits, ambient light cancellation, and power management. This paper reports the first use of digital optical proximity sensors (OPS) for "plug-and-play" PPG. OPS have traditionally been used for distance sensing in smartphones and factory automation. Here we show that a digital OPS can perform PPG functions in a single 4×4 mm package which also provides a direct digital interface to a microcontroller. By exploiting its key features, a digital OPS can provide substantial performance advantages over existing state-of-the-art PPGs, including: i) 10X lower power consumption (200 µW) due to pulse operation; ii) high signal to noise ratio (>90), as a result of built-in optical barriers, filters, and ambient light cancellation; iii) 10X lower cost ($2 USD); and iv) 12X smaller area. We show single wavelength PPG measurements in multiple anatomical locations, including fingertips and earlobes. The results suggest that a digital OPS can provide an elegant solution for battery-powered, wearable physiological monitors. To the authors' knowledge, this is the smallest and lowest power PPG sensor reported to date.

Understanding the hydrodynamics of liquid-liquid slug flow is important in the emerging field of plug-based microfluidics; however, the subtle aspects of the vortex geometry are still not comprehensively understood. This paper discusses the hydrodynamics of deformation dependent vortices that develop inside a water-in-oil slug as it flows through a channel. In contrast to prior studies, our simulations and experiments on slug flow reveal multiple vortices inside the moving slug, caused by the deformation of the hemispherical caps by Laplace pressures. These vortices appear in the front and rear of the plug at capillary number between 10(-4) and 10(-2). A theoretical and simulation model shows the cause of asymmetry in slug deformation and the resulting vortices. Understanding the relevant parameters helps in optimizing slug flow for mixing and particle manipulation, which is important for plug-based microreactors and bead based assays.

Biological systems respond not only to chemical stimuli (drugs, proteins) but also to physical stimuli (light, heat, stress). Though there are many high throughput tools for screening chemical stimuli, no such tool exists for screening of physical stimuli. This paper presents a novel instrument for photonic high throughput screening of photosynthesis, a light-driven bioprocess. The optical microplate has a footprint identical to a standard 96 well plate, and it provides temporal and intensity control of light in each individual well. Intensity control provides 128 dimming levels (7-bit resolution), with maximum intensity 120 mE/cm(2). Temporal modulation, used for studying dynamics and regulation of photosynthesis, can be as low as 10 μs. We used photonic screening for high throughput studies of algal growth rates and photosynthetic efficiency, using the model organism Dunaliella tertiolecta, a lipid producing algae of interest in biofuel production. Due to the ability to conduct 96 studies in parallel, experiments that would require 2 years using conventional tools can be completed in 1 week. This instrument opens up novel high throughput protocols for photobiology and the growing field of phenomics.

Biochemical samples are complex mixtures containing 1000's of components which often must be fractionated prior to analysis. Conventional fraction collectors, which can only accommodate 10's of fractions, are not well suited for high throughput analysis. This paper describes microfractionation in droplets (μFD), a scalable microfluidic technique for generating thousands of fractions. A drop generator, placed downstream from a high performance liquid chromatography (HPLC) column, encapsulates the separated components into a serial array of monodisperse droplets. The droplets can be stored in a capillary or immediately used in subsequent assays. Using μFD, a mixture of 3 dyes separated in a C18 column was fractionated into 2,160 droplets in <6 min. The volume and frequency of the droplet fractions are governed by the capillary number (Ca), which depends on the viscosity of the carrier fluid, flow rate, and interfacial tension. With HPLC-compatible flow rates of 0.38-0.7 mL/min, in a 1.5 mm Teflon capillary, fractions contain volumes of 1-6 μL and are generated at 2-10 drops/s. Droplet fractions can be mixed with a subsequent reagent using a downstream tee junction. In theory, μFD can be coupled to a wide variety of separation processes, enabling high throughput fractionation and screening of complex mixtures in μL to sub-nL volumes.

Multispectral photometry is often required to distinguish samples in flow injection analysis and flow cytometry; however, the cost of multiple light detectors, filters, and optical paths contribute to the high cost of multicolor and spectral detection systems. This paper describes frequency division multiplexing (FDM), a simple approach for performing multi-wavelength absorbance photometry with a single light detector and a single interrogation window. In previous efforts, modulation frequencies were <10 KHz, resulting in a detector bandwidth of <20 Hz. This paper presents a high frequency FDM circuit which can increase the oscillation frequencies to several 100 KHz, improving the detection bandwidth by a factor of 10 while still maintaining low cost. Light from 3 different LED sources are encoded into unique frequency channels, passed through the detection cell, and later demodulated using phase-sensitive electronics. Electronic multiplexing couples all light sources into a single optical train without spectral filters. Theory and high frequency considerations are demonstrated. Simultaneous three color absorbance detection is demonstrated in solutions and in flowing droplet microreactors. This technique can potentially reduce the cost of multicolor photometry by replacing expensive optical components with low-cost electronics.

Microdroplet systems can drastically reduce costs and increase throughput in high throughput screening (HTS) assays. While droplets are well suited for biomolecular screening, cell-based screens are more problematic because eukaryotes typically require attachment to solid supports to maintain viability and function. This paper describes an economical, off-the-shelf microfluidic system which encapsulates eukaryotic cells in gelatinous alginate capsules for the purpose of HTS. The flow-through system consists of i) a cross junction, which forms monodisperse droplets of alginate and cell suspension in an immiscible carrier fluid, followed by ii) a T junction which delivers BaCl(2) to crosslink and solidify each droplet. With an appropriate carrier fluid, the system is self-synchronized and can produce cell-alginate-BaCl(2) capsules with virtually 100% reliability. Droplet volumes and frequency are determined by flow rates and the diameter of the cross junction. The present implementation, which utilizes 1.5 mm Teflon tubing and plastic junctions, can generate 0.4-1.4 microL droplets at frequencies >10 droplets/sec. Cell viability is >80% at 4 hours post-encapsulation. With low recurring cost (