Singling Out Cells

Randomness is a part of life, even at the cellular level. For example, a spontaneous change in protein expression can increase a tumor cell’s mobility and virulence, leading to metastasis. But biologists don’t fully understand these so-called stochastic processes, because they lack an efficient way to monitor rare events in a large population of cells. Now researchers have developed a microfluidic chip that allows scientists to monitor thousands of cells individually by trapping them in tiny pits (Anal. Chem., DOI: 10.1021/ac2011153).

Currently scientists rely on flow cytometry and microscopy to study cells individually, but neither technique is ideal for studying stochastic events, says Hang Lu of the Georgia Institute of Technology. Flow cytometry can sort through large numbers of cells, but can only provide a snapshot of cell behavior. Standard microscopy provides more continuous data, but can’t handle large numbers of cells at one time. In addition, cells on a microscope slide refuse to hold still while researchers snap pictures, making individual cells difficult to track.

To solve these problems, Lu and her team used soft lithography to design a microfluidic chip that serves as a microscope slide for thousands of cells. After the scientists add cells to the chip, the channel design and fluidics force the cells to flow single file into its main chamber. This chamber consists of a channel snaking back and forth with cell-sized pits along its bottom. While the fluid flow carries the cells through the chamber, tiny fluid jets near the pits push individual cells into the traps. The researchers optimized the chip’s fluid dynamics and the channel’s geometry so that within 30 seconds, 4,000 cells land in a trap and 95% of the pits contain only one cell.

To test the device, the researchers watched as the compound ionomycin triggered lymphoma cells to absorb calcium, a well-studied phenomenon. They washed the cells with a fluorescent calcium indicator and then loaded them onto the microfluidic chip. Next, they added ionomycin at increasing concentrations. As expected, even though each lymphoma cell received the same stimuli, each one had a unique response to the compound. Lu says that at any given time, “only a fraction of the cells responded at all,” and more cells responded as the ionomycin concentration increased.

Denis Wirtz of Johns Hopkins University says the device has a clear application in cancer research. In addition to studying spontaneous metastasis, he says random cellular events may explain why a cancer treatment works in one population of cells, but not in others. Researchers could use this device to profile a patient’s cancer cells and then tailor specific, more effective treatments for the patient, he adds.