Trapping Cells and Organelles

ANALYTICAL CHEMISTRY

A new method enables individual cells or subcellularcomponents to be encapsulated in aqueous droplets surrounded by an immiscible phase in amicrofluidic device. Assays or other chemical reactions can then be performed within the droplet,according to Daniel T. Chiu andcoworkers at the University of Washington, Seattle (Anal. Chem.2005, 77, 1539). Chiu also reported the work at the Pittsburgh Conference in Orlando,Fla., held last week.

"We're trying to develop this droplet platforminto a nanolab for single-cell or single-organelle measurements so that we can do the kind ofchemical manipulation that you cannot do otherwise with a bulk macroscopic container," Chiusays.

Because the volume of the droplet can be carefullycontrolled, the volume can approach that of the cell or organelle, avoiding problems of diffusionand dilution that can happen in an open microchannel. Chiu has demonstrated that encapsulated cellscan be quickly opened with laser photolysis, essentially freezing the cell in its state at the timeof photolysis. In addition, he has performed enzymatic assays with lysed cells.

"Although the approach is probably notapplicable to analysis of every constituent in the cell, Chiu and his team beautifully demonstratedall steps needed to assay a model component in one cell," says Z. Hugh Fan, a microfluidics expert in the departmentof mechanical and aerospace engineering and the department of biomedical engineering at theUniversity of Florida. "The confined volume provides a significant advantage for studyingbiomarkers and signaling pathways in a single cell due to the fact that there is no diffusion intoand dilution with the surroundings."

Chiu has three methods for forming droplets. In themost widely used method, the aqueous solution is in one arm of a T-shaped intersection between twomicrochannels and the oil phase is in a second arm. With a laser beam, the particle being trappedis moved and held at the interface between the two phases. The pressure applied to the aqueousphase is slowly increased until a droplet forms and shears off, trapping the cell or organelleinside. Chiu's team has trapped both single cells and mitochondria this way.

The disadvantage of this method, Chiu says, is thatit requires a precisely controlled flow. "You have to be able to start it and stop it, andthat turns out to be challenging." Other methods for forming droplets that Chiu is starting touse include sending the aqueous solution through a nozzle and using an electric field to assist indroplet formation.

"We might use a shear method if we want togenerate a continuous stream of droplets," he says. "For single droplets on demand, thenozzle and electric field methods work much better."