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Photograph of an assembled device. The discrete liquid drops controlled in this device are 100 to 240 nl, with fluidic channel dimensions of 200 to 600 m wide and 50 m deep. |
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Schematic representation of an integrated microfluidic device. There are three liquid entry channels (sample, PCR reagents and RD reagents), several metering channels, drop mixing intersections, a sealed PCR chamber, an open RD chamber, and an electrophoresis channel. Each valve is individually and electronically addressable. |
The device, called the Genotyper, integrates fluidic and thermal components such as heaters, temperature sensors, and addressable valves to perform two independent serial biochemical reactions, followed by an electrophoretic separation. Integration of multiple steps of biological assays on a single device provides significant advantages in terms of sample/reagent consumption, process automation, analysis speed and efficiency, and contamination reduction.
The key components (phase change valves, thermally isolated reaction chambers, gel electrophoresis, and pulsed drop motion) of this device are electronically addressable and simple to operate, properties that could eventually lead to autonomous operation.
The device's compact design and mass-production technology make it an attractive platform for a variety of genetic analyses.
Ultimately, the Genotyper will be fully portable, even connecting wirelessly, and thus able to track the spread of existing or emerging flu strains around the world. "We are still working with experimental prototypes, not devices ready for mass manufacture," says Ron Larson, Professor of Chemical Engineering at the U of M and one of the device's developers.
Genotyper identifies flu type through a process resembling the genetic fingerprinting used in DNA identification. The process uses standard biochemical assays, only miniaturized.
First, the influenza RNA (its genetic material) is converted to DNA using the same biological processes that the HIV virus uses to convert RNA to DNA, thereby replicating itself and eluding the immune system. Then, a segment of the DNA is amplified thousands or millions of times by reverse transcription polymerase chain reaction, and enzymes are released that "digest" or cut that DNA in a sequence-specific manner.
The DNA is stained and pushed through a polymer gel matrix by an electric field. The DNA pieces move at rates controlled by their size. One type of flu, for example, would have DNA that is not cut by the enzyme, while another is cut. So, DNA bands are formed whose locations tell if the DNA is cut or not. This provides a "fingerprint" that distinguishes the type of flu.
Different types of DNA strands can be tested and identified using the same device by simply using different reagents. To demonstrate the Genotyper's versatility, researchers tested DNA from a human, a mouse, and from a strain of influenza. The entire device would be about the size of a TV remote control.
Presently, there are different versions of the device measuring between 1 and 2 in. in length and width. The chip does not yet include a purification unit, and there is still a need for external "readers" to see the DNA bands. Some versions require external air pressure while others generate pressure on the chip. The devices are made by bonding silicon and glass-substrate components.
The Genotyper will remain in the research and development phase until its reliability can be established.