Monolithic polymers improve microfluidic devices

Berkeley Lab scientists have dramatically increased the efficiency of microfluidic devices in extracting compounds such as biological and chemical toxins from air, soil, and water samples.

Their innovation lies in pairing two existing technologies: microfluidic chips used to isolate compounds from samples, and monolithic porous polymers. This coupling will enable researchers to determine if samples contain extremely dilute molecules, proteins, and microorganisms, including dangerous toxins and biological weapons.

	Microfluidic chips are typically rectangular plates made of glass, silica, or plastic, trenched with narrow channels.

"We will soon be able to collect and prepare a sample, and determine if it contains a substance, in the field using a single hand-held device containing a microfluidic chip," says Frantisek Svec of Berkeley Lab's Materials Sciences Division. "We'll take the lab to the sample rather than the sample to the lab."

A typical microfluidic chip is a small rectangular plate about three inches long and one inch wide. It's made of glass, silica, or plastic and is trenched with narrow channels. A sample that needs to be analyzed is injected into one of these channels as a tiny stream.

Most microfluidic chips rely on open-channel architecture, so named because only the channel's walls are coated with a substance that extracts the desired compound from the stream. Because only the channel's walls contain this substance, and not the interior volume of the channel, only a small portion of toxin in a sample is absorbed. The remainder flows through uncollected.

To greatly increase the surface-to-volume ratio and therefore the channel's loading capacity, Frantisek Svec and Jean Fréchet, also of Berkeley Lab's Materials Sciences Division, filled the entire cross section of the channel with a monolithic polymer-based material they invented.

Click for larger image
To increase the efficiency of microfluidic devices, Berkeley Lab scientists filled a portion of the chip's channel with a macroporous polymer monolith.

Porous monolithic polymers are a new category of materials developed during the last decade. In contrast to polymers composed of very small beads, a monolith is a single, continuous piece of a polymer prepared using a simple molding process. In this case, the microfluidic chip's channel serves as the mold.

And when this monolithic polymer fills the channel's cross section, the dilute sample is exposed to much more active surface as it courses through the channel. This increases the opportunity for the chemical interactions that capture the desired compound from the sample. In early tests, for example, Svec and Fréchet increased the concentration of proteins extracted from a dilute solution by a factor of 1000.

To prepare the polymer in the microfluidic chip, the channel is first filled with a liquid mixture of monomers and porogens.

Next, a mask that is opaque to ultraviolet light is placed over the chip. This mask has a small slit that exposes a small portion of the channel. Finally, the mixture is irradiated with ultraviolet light through this tiny opening. This triggers a polymerization process that produces a solid but porous monolithic material, which completely fills the cross section of the channel.

A sample is then injected through this channel, and in a process called solid phase extraction, the porous polymer material absorbs the target compound while the remainder of the sample flows through. This absorbed material is later released using a solvent, allowing researchers to collect and analyze it.

"The porous monolithic material allows us to substantially increase the available surface, which in turn helps to extract a larger amount of a substance," Svec says. "And increasing the concentration is extremely important because it enables the detection of compounds even in dilute air and soil samples."

This is especially true when it comes to detecting biological and chemical warfare agents. Although deadly, such compounds tend to disperse widely in the air and water, meaning if such a dangerous compound is in the environment, it's in a very dilute form. To detect the compound, its concentration must be increased so that the ratio of the compound to the background environment is quite pronounced. This results in fewer false positives -- instances in which the supposed detection of a compound turns out to be erroneous.

"This is a nightmare. If you have two or three false alarms, people will become skeptical," Svec says. "Now, we can increase the concentration of the compound and improve the reliability of the detection."

In addition to increasing the concentration of biological and chemical warfare agents, the technique can be used to prepare samples for DNA sequencing, protein mapping, enzyme assays, chromatographic analysis, as well as to evaluate the environment for pollutants.

Svec and Fréchet have made additional refinements to enhance the chip's usefulness in the field. Their chip is made of plastic, making it cheap and disposable as opposed to other chips made of glass, silica, and even quartz.

And they can fine-tune the porosity of the monolithic polymer using a carefully formulated combination of porogenic solvents and reaction conditions. This enables them to create a polymer porous enough to allow a sample to flow through the channel with minimal resistance, but not too porous so that much of the sample isn't exposed to the polymer. They've used this technique to achieve flow rates of up to 10 microliters per minute, which represents a flow velocity of 50 mm per second -- a speed that far exceeds the velocities typically used in analytic microfluidic chips.

One step the current microfluidic chip-polymer combination can't undertake yet is separating a compound from the polymer after it is absorbed. Their chip can only harvest the compound from a sample. However, in cooperation with the Berkeley Lab team, Sandia National Lab scientists are developing a more complex system called MicroChem Lab. It will enable the collection of a compound using solid phase extraction, separation from the polymer using electrophoretic techniques, and, ultimately, the detection of specific compounds.

Svec and Fréchet will soon add a separation component to their monolithic polymer-based microfluidic chip. But unlike the Sandia system, they are developing more sophisticated separation methods using chromatographic modes.

Ultimately, they will amass a lab's worth of collection, separation, and detection capabilities in a single chip. In analyzing a solution for specific proteins, for example, their chip will perform a multistep process: collect the sample via solid phase extraction, prepare the sample using enzymatic digestion, separate the peptides, label them, and finally, detect their presence. And it will do this in the field, far from the lab.

"Rather than collect a sample and bring it to the lab, it's much easier to conduct the complete analysis on site, use the chip once, and throw it away," Svec says.

Additional information:

• More about Frantisek Svec
• More about Jean Fréchet