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  • Cell-based assays become three-dimensional

BioPhotonics
Nov 2006
David Shenkenberg

Cell-based assays can enable pharmaceutical manufacturers to determine the efficacy and toxicity of a drug. In these assays, cells containing a fluorescent reporter molecule grow in a single two-dimensional layer. Currently, cells must be removed and counted using a fluorescence microscope, a time-consuming process. Using a plate reader would be rapid and convenient, but the fluorescence intensity is too weak and contains too much noise from reporter molecules that are released from dead cells.

Shang-Tian Yang and colleagues at Ohio State University in Columbus have developed a device that they call a fibrous bed-bioreactor. Within it, cells proliferate on a scaffold of fibrous matrices that allows cells to grow in three dimensions. On a conventional plate, cells cannot grow three-dimensionally because they do not have a base of support.


The investigators placed their three-dimensional tissue-engineering scaffolds — fibrous matrices in a disk shape — inside the wells of a 96-well plate, as shown. They also incorporated the scaffold into a microfluidic device (not pictured).


Why build a device that enables cells to grow in three dimensions? The investigators believed that the greater cell density would increase the fluorescence intensity, resulting in reduced noise from the background fluorescence. Because the cluster of cells requires more surrounding growth medium, they thought that the larger amount of medium would more greatly dilute the background fluorescence. In addition, they conjectured that the cells might direct light vertically, whereas cells growing two-dimensionally have appeared to disperse light. They have incorporated the bioreactor into two patent-pending instruments, in the form of a microfluidic device for high-throughput drug screening and a 96-well plate for online monitoring of fluorescence.

Because the microfluidic device is designed with a separable top and bottom, the fibrous matrices and cells can easily be implanted into each microfluidic well, which can be used as a bioreactor. Drugs can be pumped through the microfluidic channels and into those wells.

In an experiment, the scientists inserted fibrous matrices that contained mouse embryonic stem cells that were expressing GFP. They employed an electrically driven pump to perfuse the cells with various drugs. Then they used a Tecan plate reader to measure the fluorescence.

Likewise, the researchers performed an experiment with their 96-well plate. They placed the fibrous matrices into six wells of the plate. Each well that contained the matrices was surrounded by eight wells that contained only growth medium. The cells in the matrices drew growth nutrients from those eight wells. Thus, the researchers had to replenish the medium only every two weeks. They examined the effect of an anticancer agent on a colon cancer cell line that was growing in the bioreactors and was expressing GFP. They employed the same plate reader for this experiment.


Inside the investigators’ device, fibers support cells as they proliferate, enabling them to grow in three dimensions. As a result, the fluorescence signal is stronger and background fluorescence is reduced.


For each device, the detected fluorescence was three times greater than that for cells growing in conventional, two-dimensional plates. The background fluorescence was comparatively reduced by 88.9 percent. In September, the researchers presented these findings at the 232nd American Chemical Society National Meeting in San Francisco.

Currently, they are expanding the 96-well plate design to accommodate 384 wells. In the future, Yang said, a spectrometer may be integrated into the microfluidic device, using fiber optics.


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