Microfluidics/Single-Cell Detection & Sorting
Figure 1: HF2LI & HF2TA measuring AC current changes as cells pass through differential electrodes in a microfluidics channel. The measured current is used to determine the size and speed of the cell, followed by real-time discrimination for cell sorting. Cells are sorted by an AC dielectrophoresis signal from Output 2 of the HF2LI, the frequency of which depends on the impedance signature of the cell.
Trends in biosystems engineering for cells and sub-cellular components (nucleus, RNA, DNA) have created a demand for electrical impedance spectroscopy with increased sensitivity over an ever broader range of frequencies. From the few MHz of the past, applications now extend into the high-frequency band. Furthermore, the ability to perform measurements at several frequencies simultaneously represents a clear advantage as it means that the impedance profile of a cell can be taken in real time.
There exist three common methods for observing the size and the speed of a cell in a microfluidic channel.
The first is known as optical flow cytometry, and requires labelling the cells before these pass through a channel where laser light illuminates them and the resulting scattered light or fluorescence is detected. In addition to time-consuming sample preparation with dyes that can be toxic or expensive, maintaining and setting up the laser and the detection system limits the portability and robustness of this technique.
The second method is image-based cytometry, which relies on the use of a high-speed camera. Image processing is required to judge the size of cells before sorting them into different channels with a separate instrument. Typical cameras are limited in detection speed by their frame rate: it may take up to 200 microseconds to record a frame.
The third alternative is impedance cytometry, which has a fast response time, is marker-free and allows for integrated sorting. The technique involves monitoring the change in the dielectric properties of the channel as the cell passes through it. One way to achieve this is by measuring the current change between electrodes integrated in the microfluidic channel with a lock-in amplifier such as the HF2LI Lock-in Amplifier and a matching current amplifier, as shown in Figure 1. The current variation is measured when a cell passes between the electrodes, and the signal from the fluid is rejected to a high degree thanks to the experiment's differential configuration. This provides a well-defined signal from which it is possible to infer both the size and the speed of the cell. In fact, the measured signal can be used directly to decide into which channel the cell should be diverted by means of an AC dielectrophoresis (ACD) signal. With the HF2LI, ACD and signal detection take place on the same instrument for a reduced complexity of the experimental apparatus.
Your Benefits Measuring with Zurich Instruments
- Take advantage of simultaneous multi-frequency operation at up to 6 frequencies to characterize cells.
- Perform measurements on short timescales that are unavailable to camera-based solutions.
- Make fast and automated decisions for cell sorting.
- Optimise your measurement sensitivity by varying the test signal frequency over a wide range of values.
- Work successfully in noisy fluid environments thanks to a differential measurement scheme.
- Simplify your setup with an integrated instrument that combines detection and sorting.
- Electrical Impedance Spectroscopy for Single-Cell Analysis
- Microfluidic Impedance Flow Cytometer for Stem Cell Discrimination
- Monitoring of Cell Membrane Permeabilization in a Electroporation Device