Image: NIST scientists combined a glass slide, plastic sheets, and double-sided tape to create an inexpensive and simple-to-build microfluidic device for exposing an array of cells to different concentrations of a chemical (Photo courtesy of Cooksey/NIST).
A glass slide, plastic sheets, and double-sided tape were combined to create a diffusion-based gradient generator, a tool that rapidly assesses how changing concentrations of specific chemicals affect living cells.
Exposing an array of cultured cells to a chemical gradient provides a solution where the chemical concentration changes gradually and predictably across the array. Such gradients are a rapid, high-throughput way to evaluate the effect on cell growth or toxicity.
Created by National Institute of Standards and Technology (NIST; Gaithersburg, MD, USA) engineer Javier Atencia the gradient generator is built in layers, with each section precisely positioned with an alignment tab. The base is a glass slide, upon which is attached a strip of double-sided tape cut to have a row of four micrometer-sized channels. On top of this is placed a polystyrene strip cut to have two lines each of four tiny circular "wells" where each pair lines up with the ends of the channel below it. The next layer is another strip of double-sided tape, this time with a Y-shaped canal cut into it to serve as the flow path for the chemical gradient. Finally, a Mylar strip cut to have an identical Y-canal serves as the cover.
The hinged cover allows access to the wells for adding test cells. Once done, the cover is lowered and affixed, sealing the gradient generator. Fluid flow in and out of the system is accomplished using magnetic connectors. Under constant pressure, the flow assures a steady-state stream through the device and creates a diffusion gradient in each buried channel. Cells in the channels are simultaneously exposed to a range of chemical concentrations from high to low.
Conventional microfluidic systems usually mix fluids by pumping them in a circular motion or by twisting and folding them together. The new NIST system's gradient is created by diffusion––the gentle movement of matter from one point to another by random molecular motion. This greatly reduces the risk of cells being swept away or damaged by shearing forces in the test fluid.
The device was tested by loading it with cells genetically engineered to produce large amounts of green fluorescent protein (GFP) and then introduced cycloheximide (CHX), a chemical that shuts down ribosomes, the cell's protein factories. Cells exposed to the toxin quickly stop synthesizing GFP, decreasing fluorescence by an amount directly related to the concentration of CHX.
This is what the scientists observed in the gradient generator assays. The cells were exposed three times to CHX, and each time, the level of GFP fluorescence increased as the concentration of CHX in the gradient decreased, and vice versa.
The new device was described in the 2012 edition of the journal Lab-on-a-Chip.
National Institute of Standards and Technology