Although separation of polymers based on the combination of dielectrophoretic trapping and electrophoretic forces was proposed 15 years ago, experimental proof has not yet been reported. Here, we address this problem for long DNA fragments in a simple and easy-to-fabricate microfluidic device, in which the DNA is manipulated by electrophoresis and by electrodeless dielectrophoresis. By slowly increasing the strength of the dielectrophoretic traps in the course of the separation experiments, we are able to perform efficient and fast DNA separation according to length for two different DNA conformations: linear DNA (lambda (48.5-kbp) and T2 (164-kbp) DNA) and supercoiled covalently closed circular plasmid DNA (7 and 14 kbp). The underlying migration mechanism-thermally induced escape processes out of the dielectrophoretic traps in the direction of the electrophoretic force-is sensitive to different DNA fragments because of length-dependent DNA polarizabilities. This is analyzed in a second series of experiments, where the migration mechanism is exploited for the quantitative measurement of the DNA polarizabilities. This new and simple technique allows for the systematic characterization of the polarizability not only for DNA but also for other biomolecules like proteins. Furthermore, our results have direct implications to future biotechnological applications such as gene therapy and DNA vaccination.

Control of surface properties in microfluidic systems is an indispensable prerequisite for successful bioanalytical applications. Poly(dimethylsiloxane) (PDMS) microfluidic devices are hampered from unwanted adsorption of biomolecules and lack of methods to control electroosmotic flow (EOF). In this paper, we propose different strategies to coat PDMS surfaces with poly(oxyethylene) (POE) molecules of varying chain lengths. The native PDMS surface is pretreated by exposure to UV irradiation or to an oxygen plasma, and the covalent linkage of POE-silanes as well as physical adsorption of a triblock-copolymer (F108) are studied. Contact angle measurements and atomic force microscopy (AFM) imaging revealed homogeneous attachment of POE-silanes and F108 to the PDMS surfaces. In the case of F108, different adsorption mechanisms to hydrophilic and hydrophobic PDMS are discussed. Determination of the electroosmotic mobilities of these coatings in PDMS microchannels prove their use for electrokinetic applications in which EOF reduction is inevitable and protein adsorption has to be suppressed.

tions, adapted quantitatively to the microstructure geometry and the experimentally determined free diffusion and mobility of the microbeads (see supplementary information). The key feature of the resulting response curve (Fig. 1) is the negative slope that is symmetrical around the origin -a distinct and unambiguous signature of absolute negative mobility. We also measured the average bead velocity by tracking the beads using real-time video microscopy for different values of U DC (see supplementary information) and found that the measured experimental response was in good agreement with theory ( Fig. 1).To explain how absolute negative mobility occurs, we consider the small gaps in the microstructure as 'traps' -electrical field lines can pass through them but the beads cannot. For 0|ǁU 0 +U DC |, the bead becomes trapped after moving forwards a short distance when +U 0 +U DC >0, whereas it travels backwards for a longer distance when ǁU 0 +U DC <0, resulting in absolute negative mobility.We find that this response by a particle is very sensitive to particle size, even to the extent of allowing particles of different sizes to be steered in opposite directions (our unpublished results). Absolute negative mobility therefore holds promise for bioanalytical applications, for example in the sorting of cells, organelles and biochemical compounds.

Using Brownian dynamics simulations, we study the migration of long charged chains in an electrophoretic microchannel device consisting of an array of microscopic entropic traps with alternating deep regions and narrow constrictions. Such a device has been designed and fabricated recently by Han et al. for the separation of DNA molecules (Science, 2000). Our simulation reproduces the experimental observation that the mobility increases with the length of the DNA. A detailed data analysis allows to identify the reasons for this behavior. Two distinct mechanisms contribute to slowing down shorter chains. One has been described earlier by Han et al.: the chains are delayed at the entrance of the constriction and escape with a rate that increases with chain length. The other, actually dominating mechanism is here reported for the first time: Some chains diffuse out of their main path into the corners of the box, where they remain trapped for a long time. The probability that this happens increases with the diffusion constant, i. e., the inverse chain length.

A new concept based on microstructuring techniques is presented to achieve size-dependent electrophoretic migration of DNA in free solution. Topographically structured microfluidic channels with periodical cavities in the range of the radius of gyration of the tested DNA molecules (, 3 mm) are produced by moulding of polydimethylsiloxane on a master wafer fabricated by contact lithography of SU-8 photoresist. The electrophoretic migration of single DNA molecules stained with the intercalator YOYO was investigated by real-time fluorescence video microscopy. It could be demonstrated for large DNA molecules that l-DNA (48 kbp) always migrated significantly faster than T2-DNA (164 kbp) in the two tested microchannel layouts. Integration into a microdevice for DNA separation in free solution is now in progress. 