Automated system isolates rare cells

Silicon Biosystems was challenged with designing, developing and producing a tool capable of detecting and isolating circulating tumour cells (CTCs) with the purpose of studying personalized care in oncology or fetal cells in maternal blood to allow for non-invasive prenatal diagnosis.

Its solution was to develop proprietary “lab-on-a-chip” technology that exploits the microelectronic properties of active silicon substrates and produces a miniature biological laboratory capable of individually manipulating suspension cells. The resulting system, called DEPArray, was developed with the help of a National Instruments embedded controller.

Bologna, Italy-based Silicon Biosystems’ technology is founded on the ability of electric fields to exert force upon neutral, polarizable particles, such as cells, suspended in a liquid. In accordance with this electrokinetic principle, which is called dielectrophoresis (DEP), a neutral particle subjected to non-uniform electric fields is under a force directed towards positions in space with increasing (positive) DEP (pDEP), or decreasing (negative) DEP (nDEP) field strength. More specifically, a particle can be subjected to a pDEP or nDEP force according to its electrical properties, which depend on the frequency, as well as to the properties of the medium in which it is suspended.

In the DEPArray system, the electric field is generated on the surface of a silicon chip directly interfaced with a microfluidic chamber containing the suspended cells. The microfluidic chamber is confined between the surface of the chip and a transparent cover that is located a few dozen micrometers away from the chip. The surface of the active chip implements a 2D array of microlocations, each consisting of a planar electrode and integrated logical circuits. Each electrode can be programmed to create a potential well, or dielectrophoretic cage, when placed in the area corresponding to the electrode. Inside each of these dielectrophoretic cages, a particle can be trapped in stable levitation and then analyzed individually. Because each cell is analyzed individually, the system can perform sophisticated analysis based on fluorescence, through which it is possible to identify the peculiar characteristics that distinguish a target cell from dozens of thousands of other contaminating cells. The target cells can be moved independently, but simultaneously, to an area of the chip where they can be retrieved automatically through microfluidic control.

DEPArray system

DEPArray is a technologically advanced system that is flexible and easy to use. The core of the system consists of a microchip that integrates an array of 300,000 electrodes within a microfluidic circuit.

The system uses National Instruments hardware and software to manage high-precision mechanics, microfluidics, commercial off-the-shelf (COTS) electronic and custom tools, and vision and image processing. It allows the user to perform the work-flow summarized by the following basic steps:

• Loading samples through microfluidic control
• Acquiring images in bright field and fluorescence
• Analyzing images
• Identifying and selecting target cells through a GUI
• Sorting the identified target cells automatically
• Retrieving target cells through microfluidic control

Software/hardware system control

Sample loading is a very delicate process. NI LabVIEW software is used to control the pump assembly that creates the pressure gradients needed to flow the sample from the entrance tank to the chip within the microfluidic chamber. The system uses algorithms implemented with the NI Vision Development Module vision libraries to automatically monitor and control the loading process.

Once the sample is loaded onto the chip, LabVIEW controls all the I/O lines needed to configure the array of electrodes that cage the cells and keep them suspended during all stages of the process, thus ensuring strong and reliable system control.

Sample analysis is conducted by optically scanning the entire chip surface with multiple filters in fluorescence, as well as in a bright field. LabVIEW controls the movimentation system where the chip is positioned with micrometer precision and manages the acquisition, image processing, and visualization of the high-resolution digital images from the microscope.

When target cells are selected, the DEPArray provides the user with a powerful human-machine interface (HMI), developed in LabVIEW and integrated with the Microsoft .NET framework, to classify and select the target cells. The cells can be analyzed in different ways to validate their nature. The HMI displays scatter plots or histograms of the analysis measurements and provides a tabular representation of all the measurements on the images. For each selected cell, the images captured during the analysis are also displayed to allow the user to integrate the measurements taken by the computer with a morphological assessment.

For automatic sorting, depending on cell maps and obstacles, LabVIEW dynamically configures the array of the chip electrodes capable of moving each cell of interest, individually and simultaneously, from the initial position to the retrieval point. Digitally controlling the movement carried out on each cell of interest allows the system to achieve high sorting purity.

In the retrieval step, LabVIEW interacts with the peristaltic pump assembly to create the pressure gradients needed for the down flow of the buffer portion containing the selected cell(s) on the retrieval medium, such as a well or a glass slide, within the microfluidic chamber. The process of sorting and retrieving can be repeated to collect multiple cells separately or groups of purified cells that will be genetically analyzed using traditional molecular biology techniques.

Conclusion

The technology that Silicon Biosystems developed by taking advantage of National Instruments hardware and software, as well as Sky Technology’s skills, opened the way for a series of research activities that aim to isolate CTCs to study personalized care in oncology, as well as identify fetal cells in maternal blood, to allow for non-invasive prenatal diagnosis.

This article originally ran in the February 2012 issue of Lab Product News.