DNA Extraction by Isotachophoresis in a Microfluidic Channel
Biological assays have many applications. For example, forensics personnel and medical professionals use these tests to diagnose diseases and track their progression or identify pathogens and the host response to them. One limitation of these tests, however, is that most of them target only one piece of the sample - such as bacterial DNA - and other components (e.g. host genomic DNA) get in the way, even though they may be useful for different tests. To address this problem, it would be useful to extract several different substances from a complex biological sample - such as blood - in an inexpensive and efficient manner. This summer, I worked with Maxim Shusteff at Lawrence Livermore National Lab on the Rapid Automated Sample Prep project. The goal of the project is to solve the aforementioned problem by creating a system that uses a series of different extraction methods to extract cells, bacteria, and DNA from a complex biological sample. Biological assays can then be run on purified output samples. In this device, an operator could input a complex sample such as blood or saliva, and would receive separate outputs of cells, bacteria, viruses, and DNA. I had the opportunity to work this summer with isotachophoresis (ITP), a technique that can be used to extract nucleic acids from a sample. This technique is intended to be the last stage of the purification device. Isotachophoresis separates particles based on different electrophoretic mobilities. This technique is convenient for out application because free solution DNA mobility is approximately equal for DNA longer than 300 base pairs in length. The sample of interest - in our case DNA - is fed into the chip with streams of leading electrolyte (LE) and trailing electrolyte (TE). When an electric field is applied, the species migrate based on their electrophoretic mobilities. Because the ions in the leading electrolyte have a high electrophoretic mobility, they race ahead of the slower sample and trailing electrolyte ions. Conversely, the trailing electrolyte ions have a slow electrophoretic mobility, so they lag behind the sample, thus trapping the species of interest between the LE and TE streams. In a typical isotachophoresis configuration, the electric field is applied in a direction parallel to the direction of flow. The species then form bands that stretch across the width of the channel. A major limitation of that approach is that only a finite amount of sample can be processed at once, and the sample must be processed in batches. For our purposes, a form of free-flow isotachophoresis is more convenient, where the DNA forms a band parallel to the edges of the channel. To achieve this, in our chip, the electric field is applied transversely. This creates a force perpendicular to the direction of flow, which causes the different ions to migrate across the flow direction. Because the mobility of the DNA is between the mobility of the leading and the trailing electrolyte, the DNA is focused in a tight band near the center of the channel. The stream of DNA can then be directed to a different output to produce a highly concentrated outlet stream without batch processing. One hurdle that must be overcome for successful ITP is isolating the electrochemical reactions that result from the application of high voltage for the actual process of isotachophoresis. The electrochemical reactions that occur around metal electrodes produce bubbles and pH changes that are detrimental to successful ITP. The design of the chips we use incorporates polyacrylamide gels to serve as electrodes along the central channel. For our design, the metal electrodes are located away from the chip, and high conductivity buffer streams carry the potential to the chip, functioning as a 'liquid electrode.' The stream then runs alongside a gel barrier. The gel electrode permits ion transfer while simultaneously isolating the separation chamber from any contaminants in the outer, 'liquid electrode' streams. The difference in potential from one side of the chip to the other creates an electric field. This field traverses the inner, separation channel, containing the leading electrolyte, the trailing electrolyte, and the sample of interest (DNA). To increase the ease of use of the chips, a newer chip design has been fabricated. This design has wire electrodes integrated on the chip, rather than elsewhere. To keep the pH changes and bubbling isolated from the separation channel, the chip contains deeper wells near the electrodes so that the flowing buffer can wash away any gases that form around the electrode. This design is significantly more compact because it eliminates the cumbersome electrode boxes. Eliminating the electrode boxes also decreases the required voltage, making the experiments safer. This happens because when the 'liquid electrode' streams travel through small diameter tubing, they lose much of their voltage due to the electrical resistance of the fluid in the tubing.