"Our lab on a chip offers several advantages," Ramsey says. "It saves money because it uses a much smaller amount of materials that are expensive or hard to get, such as DNA and certain enzymes for chopping it up. It uses liquid samples that are 10,000 times smaller than samples used in conventional analytical instruments. In addition, it requires less labor because it is computer controlled. And it's faster. It can perform a DNA analysis using enzymes in only 5 minutes instead of an hour, as required by the conventional analytical technique."
DNA chips, Ramsey says, could be built for use in genetic diagnosis, DNA fingerprinting, and drug research. "The MicroBioLab," he notes, "could be used to screen for people carrying genes that predispose them to getting breast cancer, becoming obese, or having future children with cystic fibrosis. To make such an analysis, we eventually hope to require only a few white blood cells or skin cells in a tiny liquid sample. A finger prick blood sample is a huge volume of material for us."
The ORNL technology could also have forensic uses such as DNA fingerprinting - a technique for comparing molecular characteristics of blood at the crime scene with those of the blood from victims and suspects. "One person could collect blood samples at the scene of the crime and do an analysis then," Ramsey says. "This technology would simplify chain of custody and reduce the chance of contamination by other people's blood, which was seen as a problem in the 1995 trial of O. J. Simpson."
Another possible use is in drug research. "One large drug company is interested in using labs on a chip to test a variety of possible candidate drug compounds," Ramsey says. "These chemicals are hard to come by, so the company wants to use the smallest samples possible for the research to identify economically the most effective ones."
As thin as a microscope slide, ORNL's glass microchip has been etched to form interconnected chambers and channels, just beneath the surface. Charged molecules in the tiny liquid sample are mixed in a chamber and "pumped" through the hairlike channels by an electrical field applied by electrodes near the chip. This process is called electrokinetic transport.
The molecules glide around the hairpin turns of a winding capillary channel, covering an area the size of a dime. Before entering a separation channel, where they are separated by size and electric charge, they react with a fluorescent dye.
The dye causes the molecules to give off light when a laser beam is shone on them just below the separation channel; the larger the separated fragment, the stronger the fluorescence. The various light intensities are detected, and the information is fed to a computer, which sorts through the signals from separated molecules to provide a sample analysis.
ORNL's MicroBioLab has been used for the common DNA analysis technique called restriction fragment length polymorphism. Here, liquids containing DNA and a restriction enzyme are injected into different chambers. Electric fields pump the liquids into a reaction chamber, where the enzyme cuts the DNA into pieces of different lengths specific to the DNA sample. Electric fields force the DNA snippets to the separation channel, where they are sorted by size and tagged with fluorescent dyes for detection.
"DNA fragments of various sizes are sorted in the liquid, which contains fibrous strands of a polymer," Ramsey says. "The DNA fragments get tangled with the polymer strands, slowing them down as they pass through. Small chunks of DNA find their way through the tangled web faster than the larger ones, so separation results."
The research was supported by DOE through ORNL's Laboratory Director's Research and Development Fund. ORNL, one of DOE's multiprogram research laboratories, is managed by Lockheed Martin Energy Research Corp.