DOE Human Genome Program Contractor-Grantee Workshop VIII
February 27-March 2, 2000  Santa Fe, NM

26. Micro-Fabricated Devices for Concentrating DNA by Induced-Dipole Trapping

Charles Asbury and Ger Van Den Engh

Department of Molecular Biotechnology, University of Washington, Seattle WA 98195-7730

engh@biotech.washington.edu

DNA molecules placed in a divergent electrical field experience an attractive force towards regions of higher field strength. This force is a result of a charge-dipole that is induced along the molecule's axis. The dipole interacts with the field gradient. Because induced dipoles always oppose the field, the attractive force is independent of the fields polarity. Migration due to dipole forces can be observed with both AC and DC fields. In contrast, electrophoretic forces, which are due to the native charge of DNA molecules, always move the molecules towards the positive electrode. Homogeneous oscillating fields with a 50% duty cycle do not cause a net displacement of DNA. In oscillating fields with steep gradients the molecules move towards the field's origin.

We are developing small chambers for manipulating DNA that allow independent application of both electrophoretic and induced-dipole forces. The devices consist of thin metal layers on a quartz substrate. By combining the two types of forces, cohorts of DNA molecules can be concentrated and moved with high precision. Dipole traps concentrate DNA out of a dilute solution. Electrophoretic forces can then be employed to move DNA cohorts between traps.

We are seeking the optimal conditions for dipole trapping of DNA. The ionic composition of the medium and the frequency, strength, and gradient, of the field are important. We current use a salt concentration below 10 mM and use fields oscillating between 30-100 Hz. By comparing the rate of Brownian movement and diffusion the trapping forces can be quantitated. We are using this information to develop devices in which DNA can be separated by size without the use of a sieving medium. We will present a gold-on-quartz device that consists of a capillary lined with dipole traps. Such capillaries can be combined with other modules to perform complex operation of small cohorts of DNA molecules.