Express EIS Platform for Microfluidics
This blog is aimed for those interested in knowing more about setting up an electrical impedance spectroscopy (EIS) based on a microfluidic setup. In 2012 Zurich Instruments joined forces with Fluigent and Micronit by proposing an Electrical Impedance Spectrosocpy platform (EISP) for microfluidic researchers. I would like to offer a quick look on this ‘starter kit’ for impedance based flow cytometry.
Dynamic impedance measurement is a popular technique for doing cell detection and cell discrimination in microfluidics laboratories. Increasingly, the impedance technique is also being considered by the food industry for quality control on products like wine, cheese and even potatoes. Whether for simple blood analysis or for bacteria counting, EIS based cytometry is getting more attention. Researchers with a specific measurement application in mind are also looking for a more integrated and flexible alternative to optical based flow cytometry.
A basic EIS platform for flow cytometry consists of three components
- a microfluidic chip,
- a flow control device, and
- an impedance spectroscope.
Without being an expert in chip processing (i.e. cleanroom), in fluidic mechanics (i.e. microfluidics flow pump), and in measurement electronics (i.e. electrical impedance spectroscope), one can quickly and cost-effectively put together an EIS platform in a research laboratory as shown below.
The first component is the microfluidic chip and the chip holder design that is part of Micronit’s expertise. One can have a set of chips fabricated which are tailored to a specific microfluidics application. A chip holder can also be customized to fit the required electrical connectors and flow tubes configuration of the chips. An example of a chip design is shown below. The chip can be fabricated with an SU-8 photo-resist channel layer between two glass substrates. The double-sided, parallel electrode configuration on the channel is an advanced technique to reliable detect impedance changes when a specific cell type flows through the channel. The plastic sheathing is designed to fit into the chip holder to ensure proper mechanical and electrical contacts.
To allow a precise control of flow speed and volume, users can opt for the pressure actuated flow control device from Fluigent. This type of flow control has the advantage of a much more precise fluid manipulation (i.e. forward and backward) than a syringe pump, as well a much quieter flow than a mechanically actuated pump. The third component of this platform is the HF2IS Impedance Spectroscope. The multi-frequency of the HF2IS-MF capability allows users to detect dynamic impedance variations between the channel electrodes at up to 8 frequencies simultaneously. This enables real time impedance variation capture over a wide frequency range without resorting to the slow frequency sweeping technique. Below is a real measurement obtained using the MATLAB API which controls the HF2IS Impedance Spectroscope. Each peak represents a cell passing through the channel electrodes.
Since the hardware and software compatibility of the recommended system has been verified in the field, more time can be spent on the actual measurement instead of debugging problems of system integration. It is important to mention that the EIS platform is geared towards those who wish to apply the EIS technique and want to speed-up their learning curve. Hopefully, this little introduction will make EIS based flow cytometry less intimidating.