Lab on a Chip Blog

Electrowetting on dielectric-coated electrodes (EWOD) enables manipulation of individual droplets. Rather than direct current, AC-EWOD is used to additionally oscillate droplets for mixing and transport.

For wireless AC-EWOD, magnetic induction with a high frequency signal is needed, however this is above the dynamic range for droplet oscillation and an amplitude demodulator is currently needed in front of the EWOD chip to reduce this below 1 kHz.

In today’s HOT article, collaborators from Gangneung-Wonju National University, Republic of Korea, and University of Pittsburgh, USA, show theoretically and experimentally that an EWOD-actuated droplet actually has the ability to demodulate a high frequency amplitude modulation frequency itself due to the contact angle and does not require external artificial demodulation.

The team point out that this phenomenon is unique in that this is the first physical entity to show visible demodulation behaviour. They briefly show that this demodulating functionality is not applicable with frequency modulation signals. Further investigation will focus on the possible range of signal that can be applied and successfully transmitted to the droplet and recovered by the inherent demodulation.

Read the theoretical and experimental explanation in full, as this HOT article is free to access for four weeks*:

Inherent amplitude demodulation of an AC-EWOD (electrowetting on dielectric) droplet
Myung Gon Yoon, Sang Hyun Byun and Sung Kwon Cho
DOI: 10.1039/C2LC41043E

*Free access to individuals is provided through an RSC Publishing personal account. Registration is quick, free and simple

Here we have two HOT articles for you to read, both of which are free to access for the next 4 weeks*:

Researchers at North Carolina State University, USA, integrate microelectrodes into a neuron culture platform creating a single device with which cells can be electrically stimulated and their behaviour recorded simultaneously for scientific experiments. They do this using gallium and its alloys, which are liquid at room temperature and flow into microchannels to create the microelectrodes. This integrated biocompatible device avoids the complication of manually aligning the electrodes and provides subcellular accuracy.

Integration of pre-aligned liquid metal electrodes for neural stimulation within a user-friendly microfluidic platform
Nicholas Hallfors, Asif Khan, Michael D. Dickey and Anne Marion
DOI: 10.1039/C2LC40954B

As featured on the outside front cover of Issue 5, HOT article number two is from a team led by Peter Ertl at the Austrian Institute of Technology, Austria, and Siemens AG, Germany. They combine two methods of cell analysis into one miniaturised device for dual-parameter analysis of cell cultures. The first technique is electrical impedance spectroscopy, normally used in analysing adherent cell cultures. The second is optical light scattering, normally used in flow cytometry. The combination of the two to detect light scattering from adherent cells enables rapid identification of cell number variations during culture, assessment of cell adhesion and interactions in physiological conditions. Applications are envisaged in cell–drug interaction assays and cytotoxicity screening. It also identifies intracellular granularity, which indicates apoptosis.

Standardization of microfluidic cell cultures using integrated organic photodiodes and electrode arrays
Verena Charwat, Michaela Purtscher, Sandro F. Tedde, Oliver Hayden and Peter Ertl
DOI: 10.1039/C2LC40965H

*Free access to individuals is provided through an RSC Publishing personal account. Registration is quick, free and simple

Lab on a Chip editorial board member Aaron Wheeler and Andrea Kirby at the University of Toronto enable analysis on-chip with a new device format for digital microfluidics (DMF) based chemical synthesis that they are calling “microfluidic origami”.

The setup allows DMF to be integrated with mass spectrometry for in-line analysis. Offline analysis on the benchtop is particularly troublesome for mass spectrometry as it requires further handling steps and takes further time. There are three devices that have coupled DMF with electrospray ionisation before. However, these either have challenges associated with device fabrication or need external equipment and tricky alignment of device and ESI emitter.

This video from the Wheeler laboratory clearly shows how the device is folded, like origami, and explains how it works.

http://www.youtube.com/watch?v=-lpQhPBlu7M 

The “origami” aspect is down to a folded nanoelectrospray ionization (nanoESI) emitter on a single flexible polyimide film. This work combines previously known concept of folded ESI emitters and that of making DMF devices with flexible substrates to move droplets on non-planar surfaces to get an integrated device on one flexible substrate. They include a two-plate-to-one-plate DMF interface to give ease of droplet delivery from the mixing region to the nanoESI emitter.

They validate the device with a Morita-Baylis-Hilman reaction, monitoring it in-line in real time. This is the first time a digital microfluidic device has been used to monitor chemical synthesis in real time with in-line mass spectrometry.

This HOT article has been made free to access for the next 4 weeks*, so after watching the video you can read the scientific detail of how they fabricated and validated the device:

Microfluidic origami: a new device format for in-line reaction monitoring by nanoelectrospray ionization mass spectrometry
Andrea E. Kirby and Aaron R. Wheeler
DOI: 10.1039/C3LC41431K

Counting of cells and even smaller particles, such as single molecules of DNA and viruses, can be carried out using resistive-pulse sensing (RPS). The particles move through a pore set up with a flow of current across it. When a particle goes through, it changes the resistance, which can be measured and used to count the particles and even size them. The difficulty is that small molecules in a heterogeneous mixture get lost amongst the large molecules and remain undetected as the pore must be large enough for the largest particle. The signal-to-noise ratio is the limiting factor on sensitivity.

Researchers at the University of California, Berkeley, USA, led by Lydia Sohn present solutions to these limitations in this HOT article with a method they title node-pore sensing (NPS). Instead of one uniform pore width, the microfluidic device has a pore with a sequence of nodes down the length of the pore channel. The nodes in their device are larger than the rest of the channel.  The presence of these nodes means that the device sensitivity is three orders above that of other RPS sensors. The simple method enables the spacing, number and size of these nodes to be changed.

The number and spacing of the nodes changes the normal pulse signal output accordingly, producing a very identifiable corresponding rise and fall pattern. Using this signature, the particle can be detected even with usually problematic signal-to-noise ratios.  By changing the flow rate and specifically designing the node arrangement, this signature pattern can be controlled to distinguish even very small particles through a large pore. The size range of particles that can be measure with the one pore is therefore considerable.

This article is well worth a read, offering numerous advantages over current devices and offering the potential for point-of-care analysis. It’s free to access for the next four weeks*, read it by clicking the link below:

Node-pore sensing: a robust, high-dynamic range method for detecting biological species
Karthik Balakrishnan, George Anwar, Matthew R. Chapman, Trongtuong Nguyen,  Anand Kesavarajud and Lydia L. Sohn
DOI:10.1039/C3LC41286E

*Free access to individuals is provided through an RSC Publishing personal account. Registration is quick, free and simple

In the past researchers developed many different microrobots for the mechanical studies of single cells. Some of them require complicated micromanipulators; others can be controlled with optical tweezers, but are limited in the force that they exert and so are inapplicable to cells with stiff cell walls.

A team from institutes in the USA and Japan, including MIT and RIKEN, describes in a recent Lab Chip article a new type of microrobot that can be installed in a microfluidic chip and used for mechanical stimulation of single cells. In this device a robotic arm with a mechanical probe at its end is built from a magnetic material and can be moved within the structure of the microchip by applying magnetic force. This design means that there are no complex mechanical assemblies necessary to move the arm. The probe itself consists of a microneedle placed on a flexible base. The force applied with the needle is measured by the extent of base deformation.

Tomohiro Kawahara and colleagues built a microchip with the microrobotic probe in its main chamber and use it to study the reaction of small single-cellular organisms, diatoms, to mechanical stimulus. They are able to adjust the applied force so that the cell itself isn’t damaged. Upon stimulation they observe agglomeration of chloroplasts into two separate groups. They also quantify the stimulus required for chloroplast assembly to occur.

Although the experiment described by Kawahara and colleagues is a simple proof-of-concept, it does mean that a wealth of phenomena can now be examined in more detail than ever before. Another example where mechanical stimulus elicits a response is in the small organism dinoflagellate, which bioluminesces when mechanically probed. The response of many cells to this sort of stimulus mimics their response to attack by pathogens and Miyawaki’s microrobot now opens the way to studying such responses.

This HOT article is free to access for the next 4 weeks*, just click on the link below:

On-chip microrobot for investigating the response of aquatic microorganisms to mechanical stimulation
Tomohiro Kawahara, Masakuni Sugita, Masaya Hagiwara, Fumihito Arai, Hiroyuki Kawano, Ikuko Shihira-Ishikawa and Atsushi Miyawaki
DOI: 10.1039/C2LC41190C

 *Free access to individuals is provided through an RSC Publishing personal account. Registration is quick, free and simple

Published on behalf of Rafal Marszalek, Molecular BioSystems web science writer. Rafal is an Assistant Editor of Genome Biology at BioMed Central