by Caitlin Smith
Imagine yourself suddenly shrunk to micrometer scale, walking around inside a “lab-on-a-chip” microfluidic device. You would experience a series of interconnected tunnels that were sometimes awash in microliter, nanoliter, or even picoliter volumes of moving liquids. Strangely, these fluids would behave differently than you are used to in your real “macroworld.” For example, when two different kinds of fluids meet, the lack of turbulence in the microworld means that the flow remains laminar, so they won’t mix except by diffusion. Other factors begin to have a big influence too, such as surface tension, capillary forces, and fluidic resistance.
Microfluidic products are now used in a range of applications, including lab-on-a-chip devices, DNA microarrays, protein microarrays, and biochips used in clinical pathology for immediate disease diagnosis. Besides the convenience of their small size and portability, microfluidic devices offer many advantages over clunky conventional methods. The small chip volumes mean low fluid volumes used in experiments, which also means you have more precise control over the heating, cooling, and mixing of the fluids. The fact that the microfluidic channels are arranged in parallel makes it possible to do high-throughput work.
The use of microfluidics as a research tool is a relatively new phenomenon, but its use is growing steadily. “A few years ago,” says Andrea Chow, vice president of microfluidics R&D at Caliper Life Sciences, “the issues that many of our customers wanted to understand are ‘does microfluidics really work’ and ‘how does it work,’ because microfluidics was a new technology. Now that microfluidic commercial products have proven to provide a great deal of valuable benefits in numerous applications, most of our present customers want to know if they are reliable in their use environment.” But what are these applications? How are we taking advantage of microfluidics technology today?
What do we use it for?
That most microfluidics products are in the bio-applications arena, notes Micralyne R&D Director John Crabtree, is not surprising given the nature of the technology. The ability to integrate sequential steps onto a microfluidic chip lends itself to bioassays, which tend to require multiple steps. In addition, says Crabtree, “the miniaturized environment on a chip allows small sample sizes to be analyzed both more effectively and more quickly than with conventional macro-scale analytical procedures; this miniaturized fluid handling ability is well tailored for many bio-samples that are minute to begin with, or which would require additional amplification steps for processing with conventional macro procedures.”
An example of the many bio-applications of microfluidics technology today is in quality control. According to Tony Owen, liquid phase separations platform marketing manager at Agilent Technologies, Agilent’s Bioanalyzer is used primarily for quality control applications. The microfluidics-based Bioanalyzer “has become the industry standard for RNA quality control, and is also used a great deal for quality control of protein-based drugs. Bioanalyzers are also used to determine the quality of PCR products prior to sequencing for mitochondrial DNA based forensic identification.” The Bioanalyzer also uses DNA analysis to identify species of foods or bacteria.
Microfluidics has made its way quickly into drug discovery research. Agilent’s microfluidics-based HPLC-Chip/MS is used mainly “in proteomics identification and protein biomarker discovery,” says Owen. “It’s also popular in the growing field of metabolomics. HPLC-Chip capability was recently extended across the entire Agilent mass spectrometer portfolio, and this means that the technology can now be applied to small molecule analysis as well as life science experiments.” Chow, too, notes Caliper’s LabChip3000 being used in drug discovery, especially for screening and profiling compounds with the aim of finding drug targets, such as kinases, proteases, phosphatases, and lipid modifying enzymes.
Researchers are also using microfluidic devices for gel electrophoresis separations in genomics and proteomics research, for estimating the sizes and quantities of DNA, RNA, and protein. For example, Caliper’s high-throughput platform LabChip 90 uses “sipper chips” that “sip” samples from a microtiter plate automatically onto the chip. “Once the sample is loaded, a number of operations are integrated inside the chip without any user intervention,” explains Chow. “These steps include sample injection into the separation column, gel electrophoresis, detection, and automated data analysis, performed at a rate of tens of seconds per sample instead of tens of minutes compared to conventional methods such as slab gels, SDS-PAGE, and capillary electrophoresis.” One high-throughput LabChip 90 system can analyze hundreds of samples per day. By contrast, notes Chow, “the Agilent Bioanalyzer 2100 and the Bio-Rad Experion are gel electrophoresis platforms marketed by our partners for lower-throughput applications.” These systems use “planar” chips for users analyzing a few to tens of samples a day.
Challenges in microfluidics
One of the earliest challenges facing the designers of microfluidic devices continues to plague them today—that the samples inside behave differently on a microscale. “As the scale reduces, surface area increases compared to volume, and the surface properties have greater influence over the reactions. Knowledge gained in the macro-world doesn’t always apply to the micro-world,” says Owen. He believes that another challenge facing makers of microfluidic devices is encouraging researchers to embrace not only a new technique, but also a new way of thinking about how their samples are processed. “It must let them do something that can’t be done any other way, or reduce cost-per-experiment, or increase the number of samples that can be run per shift,” says Owen. “Ease-of-use is also important, and microfluidic devices like the Agilent Bioanalyzer and the HPLC-Chip/MS address this by integrating multiple steps into a single device. The Agilent Bioanalyzer eliminates the manual procedures of slab gel electrophoresis and the HPLC-Chip/MS eliminates the often troublesome plumbing of conventional nano-LC.”
Yet in a way, plumbing can still be problematic for microfluidic devices, too. Crabtree explains that, “often the most challenging aspects of microfluidic device design are centered around their interface with the real world.” Currently, most microfluidic components perform part of a procedure, perhaps several steps in an analytical assay. “When the sample processing moves on-chip,” says Crabtree, “there needs to be an efficient way to bring it on-chip, and in some cases, there needs to be a way to capture the sample after it leaves the chip. All of these interfaces are customized and are only possible with considerable design complexity and development effort.”
Looking forward
It is likely that microfluidics methods will gradually replace some traditional methods—such as sample preparation and extraction, nucleic acid amplification, and post-processing detection—as the new technology becomes more accepted. For the HPLC-Chip, there will be new small molecule applications such as drug metabolism and pharmacokinetics, according to Owen. Also, a new high-sensitivity protein assay will be coming up for Agilent’s Bioanalyzer. “As acceptance of the platform increases for routine testing, Agilent will develop complete workflow solutions,” says Owen. “Our recent acquisition of Stratagene, with their catalog of bioreagents and PCR products, provides excellent opportunities to develop end-to-end solutions utilizing microfluidics.”
Crabtree predicts that in addition to continued miniaturization of bioassays, the future might also see other applications turning to microfluidics technology, such as industrial process control or environmental processing. “Many of the same fundamental attributes of miniaturization which are favorable for bio-assays will be similarly advantageous for these and other areas, and there are real economic or societal benefits to be reaped.”
