Lab on a Chip Blog

These HOT articles published in May 2015 were recommended by our referees and are free* to access for 4 weeks

High-resolution CMOS MEA platform to study neurons at subcellular, cellular, and network levels
Jan Müller, Marco Ballini, Paolo Livi, Yihui Chen, Milos Radivojevic, Amir Shadmani, Vijay Viswam, Ian L. Jones, Michele Fiscella, Roland Diggelmann, Alexander Stettler, Urs Frey, Douglas J. Bakkum and Andreas Hierlemann
Lab Chip, 2015,15, 2767-2780
DOI: 10.1039/C5LC00133A

Fast size-determination of intact bacterial plasmids using nanofluidic channels
K. Frykholm, L. K. Nyberg, E. Lagerstedt, C. Noble, J. Fritzsche, N. Karami, T. Ambjörnsson, L. Sandegren and F. Westerlund
Lab Chip, 2015,15, 2739-2743
DOI: 10.1039/C5LC00378D

Gecko gaskets for self-sealing and high-strength reversible bonding of microfluidics
A. Wasay and D. Sameoto
Lab Chip, 2015,15, 2749-2753
DOI: 10.1039/C5LC00342C

Take a look at our Lab on a Chip Recent HOT Articles Collection!

*Access is free until 24.07.15 through a publishing personal account. It’s quick, easy and free to register

A group of scientists at IMTEK, University of Freiburg have developed a new method for the production of monodisperse droplets. Previous methods, such as T-junctions and flow focusing require several channels, containing either the disperse phase (which will form the droplet) or the continuous phase (which will surround the droplet), with the droplets forming at a constriction point in the tubing. Extremely precise control of flow rate is therefore required in order to achieve consistent droplet diameters. These methods also have substantial dead volumes due to sample material remaining in the tubing at the end of the process.

An alternative method is step emulsification (as highlighted recently in a Lab on a Chip HOT article). This only requires one channel, containing both phases, and the droplet formation is caused by a change in capillary pressure. The droplet size depends on the nozzle, rather than on pressure and flow rate, so this method is less sensitive to fluctuations than the methods mentioned previously. The main limitation of step emulsification is the relatively low throughput due to droplet accumulation at the nozzle. This publication reports the use of centrifugal force in order to solve this problem.

By spinning the whole system, the disperse phase (water) is overpressured relative to the continuous phase (oil), resulting in the droplet being forced away from the nozzle by the centrifugal gravitational field (step 3 to 4 above). In order to avoid sample material being wasted as dead volume, an additional aliquot of oil is added in order to push the last few droplets of water out of the nozzle (step 5 to 6 above).

Medium throughput monodisperse droplet formation

The authors found that droplet diameter is controlled by the nozzle geometry, while rate of formation is controlled by spinning frequency. In light of these findings, they were able to increase droplet production rate from less than 1 droplet per second, to greater than 500 droplets per second, while maintaining monodispersity. They were also able to set up multiple nozzles in parallel (as seen in the microscopic image), all feed by a larger channel, to further increase throughput.

In order to demonstrate the potential applications of this new method, the authors performed digital droplet recombinase polymerase amplification (ddRPA) of L. monocytogenes (a potential contaminant during food production). They found that the number of copies measured with ddRPA was consistent with those measured with digital droplet PCR, and the overall processing was 30 minutes, compared with 2 hourlis for ddPCR.

ddRPA is just one small example of how this new technique can be used – there are a huge range of potential applications where formation of monodisperse particles are a requirement and hopefully we will see this new method being adopted!

To download the full article for free click the link below:

Centrifugal step emulsification applied for absolute quantification of nucleic acids by digital droplet RPA
Friedrich Schuler, Frank Schwemmer, Martin Trotter, Simon Wadle, Roland Zengerle, Felix von Stetten and Nils Paust
DOI: 10.1039/C5LC00291E
Open Access

These HOT articles published in April 2015 were recommended by our referees and are free* to access for 4 weeks

Active pneumatic control of centrifugal microfluidic flows for lab-on-a-chip applications
Liviu Clime, Daniel Brassard, Matthias Geissler and Teodor Veres
Lab Chip, 2015,15, 2400-2411
DOI: 10.1039/C4LC01490A

Exosome isolation: A microfluidic road-map
A. Liga, A. D. B. Vliegenthart, W. Oosthuyzen, J. W. Dear and M. Kersaudy-Kerhoas
Lab Chip, 2015,15, 2388-2394
DOI: 10.1039/C5LC00240K

Micromilling: A method for ultra-rapid prototyping of plastic microfluidic devices
David J. Guckenberger, Theodorus E. de Groot, Alwin M. D. Wan, David J. Beebe and Edmond W. K. Young
Lab Chip, 2015,15, 2364-2378
DOI: 10.1039/C5LC00234F

Take a look at our Lab on a Chip 2015 HOT Articles Collection!

*Access is free until 29.06.15 through a publishing personal account. It’s quick, easy and free to register

As children, you may remember being fascinated by pond skaters and their ability to walk on water. This is due to water’s high surface tension and there are numerous other ways in which this property is vital to many biological functions. It is also an important factor to take into account when it comes to engineering, and therefore it is essential that there is an accurate and straight forward method for measuring surface tension.

To measure this property, the pressure has to be reduced to such an extent that it causes the water to rupture and form vapour cavities. These vapour cavities must be a result of homogeneous nucleation alone and not heterogeneous nucleation (so should occur spontaneously and randomly, rather than due to nucleation sites). The pressure at which cavitation occurs in termed the tensile strength.

Previous methods have shown a large discrepancy in results, due to the requirement of large volumes of water leading to heterogeneous nucleation. More recently, the mineral inclusion method has overcome this, however has other limitations, such as the requirement of an autoclave. Alternatively, microfluidics allows the use small volumes of water to ensure homogenous nucleation, but this method is limited to low-viscosity liquids.

Dr. Liu Ai Lin and co-workers, at NTU, Singapore, have reported the direct measurement of water’s tensile strength using an optofluidic chip. Their method relies on an infrared laser that is focussed into a microchannel partially filled with water. The laser pulse results in the formation and recombination of plasma, which in turn produces a bubble, causing a spherical shock wave. The reflection of the shock wave on the air-water interface generates a negative pressure and, if this is larger than the tensile strength, the water ruptures, causing nucleation of vapour bubbles near the interface. The pressure value can be attained by both measuring the spreading of the shock wave over time and the displacement of the water-air interface.

By imaging the microchannel at increasing standoff distances (defined in the diagram above), the distance and pressure at which water no longer ruptures can be found. This can be directly converted to a value for the tensile strength.

This work provides a simple, low-cost method for calculating tensile strength that can easily allow rapid testing of a wide range of samples. In order to demonstrate this, the authors also measured the tensile strength of glycerol, a highly viscous fluid.

To download the full article for free* click the link below:

Water’s tensile strength measured using an optofluidic chip
Z. G. Li,  S. Xiong,  L. K. Chin,  K. Ando,  J. B. Zhang and  A. Q. Liu
DOI: 10.1039/ C5LC00048C

*Access is free through a registered RSC personal publishing account