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We demonstrate the generation of highly accurate nanoliter droplets with a predefined composition. This composition control over a single droplet is achieved by merging two droplets with known concentrations and defined volumes. A forced coalescence is accomplished by synchronizing two piezoelectric-based active droplet generators. A microscope-mounted CCD camera is used to record, quantify and monitor the process to assure its high fidelity. The device is disposable, surfactant free, simple to operate and does not require microelectrode fabrication. It delivers a single on-demand droplet with adjustable high resolution mixing ratios up to 9 at a volume range of 1–10 nanoliters. The presented platform offers, for the first time, a means to perform droplet-based high-throughput screening in the nanoliter range. [ABSTRACT FROM AUTHOR]

Droplet based microfluidic systems have been shown to be most valuable in biology and chemistry research. However droplet modulation and manipulation requires still further improvement in order to make this technology feasible particularly for biological applications. On demand generation of droplets and droplet synchronization, which is crucial for coalescence, remain largely unanswered. The present study describes a simple and robust droplet generator based on a piezoelectric actuator which is integrated into a microfluidic device. The droplet generator is able to independently control the droplet size, rate of formation and distance between droplets. Moreover, the droplet uniformity is especially high, deviating from the mean value by less than 0.3%. The cross flow and T-junction configurations are tested and show no significant differences, yet the inlet to main channel ratio is found to be important. As this ratio increases, droplets tend to be generated in bursts instead of individually. The physical mechanisms involved are discussed, providing insight into optimized design of such systems. [ABSTRACT FROM AUTHOR]

The culture of cells in a microbioreactor can be highly beneficial for cell biology studies and tissue engineering applications. The present work provides new insights into the relationship between cell growth, cell morphology, perfusion rate, and design parameters in microchannel bioreactors. We demonstrate the long-term culture of mammalian (human foreskin fibroblasts, HFF) cells in a microbioreactor under constant perfusion in a straightforward simple manner. A perfusion system was used to culture human cells for more than two weeks in a plain microchannel (130 µm × 1 mm × 2 cm). At static conditions and at high flow rates (>0.3 ml h−1), the cells did not grow in the microchannel for more than a few days. For low flow rates (<0.2 ml h−1), the cells grew well and a confluent layer was obtained. We show that the culture of cells in microchannels under perfusion, even at low rates, affects cell growth kinetics as well as cell morphology. The oxygen level in the microchannel was evaluated using a mass transport model and the maximum cell density measured in the microchannel at steady state. The maximum shear stress, which corresponds to the maximum flow rate used for long term culture, was 20 mPa, which is significantly lower than the shear stress cells may endure under physiological conditions. The effect of channel size and cell type on long term cell culture were also examined and were found to be significant. The presented results demonstrate the importance of understanding the relationship between design parameters and cell behavior in microscale culture system, which vary from physiological and traditional culture conditions. [ABSTRACT FROM AUTHOR]