Search

The authors investigate the elongation and orientation of different-sized deoxyribose nucleic acid (DNA) molecules, tethered onto gold electrodes via a terminal thiol, under the influence of high frequency ac electric fields. The DNA molecules are elongated from a random coil into an extended conformation and orientated along the electric field lines as a result of the forces acting on the molecules during the application of the ac electric fields. Elongation was observed in the frequency range 100 kHz–1 MHz, with field strengths of 0.06–1.0 MV/m. Maximum elongation for all DNA fragments tested, irrespective of size, was found for frequencies between 200 and 300 kHz. The torque acting on the induced dipole in the DNA molecules, complemented by a directional bias force, opposite in direction to the dielectrophoretic force, provides the main contribution to the elongation process. The length of elongation is limited to either half the distance between opposing electrodes or to the contour length of the DNA, whichever is shorter. Further, the authors show that the normalized length of the elongated DNA molecules is independent of the contour length of the DNA. [ABSTRACT FROM AUTHOR]

We have designed and characterized a simple Rayleigh-surface acoustic wave-based micropump, integrated directly with a fully enclosed 3D microfluidic system, which improves significantly the pumping efficiency within a coupled fluid whilst maintaining planar integration of the micropump and microfluidics. We achieve this by exploiting the Rayleigh-scattering angle of surface acoustic waves into pressure waves on contact with overlaid fluids, by designing a microfluidic channel aligned almost co-linearly with the launched pressure waves and by minimizing energy losses by reflections from, or absorption within, the channel walls. This allows the microfluidic system to remain fully enclosed--a pre-requisite for point-of-care applications--removing sources of possible contamination, whilst achieving pump efficiencies up to several orders of magnitude higher than previously reported, at low operating powers of 0.5 W. [ABSTRACT FROM AUTHOR]

The two-dimensional concentration and manipulation of micron-scale particles by orthogonal, surface acoustic, standing waves is demonstrated. The particles are organized by liquid pressure waves in a microfluidic system over a piezoelectric substrate and form a uniform two-dimensional array with a spacing governed by the mechanical nodes of the two orthogonal, surface acoustic, standing waves. The nodal spacing can be controlled in each orthogonal direction independently by adjustment of the radio frequency applied to the separate acoustic wave transducers. This technique could be used to enhance the particle concentrations at sensing locations in DNA or protein array detectors. [ABSTRACT FROM AUTHOR]

We report on the use of standing surface acoustic waves, formed on a single-crystal piezoelectric substrate, to organize micron-scale latex particles into an array comprising a series of lines in an adjacent microfluidic system. The lines of particles are formed parallel to the substrate surface and perpendicular to the surface acoustic wave vector. They extend across the width of the acoustic beam aperture, with a periodicity of one-half the surface acoustic wavelength. The position and spacing of the particle arrays can be altered by adjusting the acoustic wave frequency within the device passband. We discuss the mechanism responsible for the formation of the lines, which could be widely applicable to the alignment of microscopic objects held in suspension. [ABSTRACT FROM AUTHOR]

We report three-dimensional imaging measurements using confocal microscopy of fluorescently labelled deoxyribonucleic acid (DNA) strands subjected to strong ac electric fields. The DNA molecules are covalently tethered by one end to gold microelectrodes and the observed elongation patterns are compared with the electric field lines obtained from numerical simulations and with previously determined fluid flow patterns. We demonstrate that the major contribution to the elongation stems from the ac electrokinetic torque, supplemented by a small bias force provided by the electric-field-induced fluid flow, and we provide evidence that the observed restricted elongation owing to the geometries of the electrodes results from a sign change in the bias force. [ABSTRACT FROM AUTHOR]

We demonstrate that aqueous suspensions of microbubbles, formed into arrays using standing surface acoustic waves (SSAWs), can be transported by controlled modulation of the SSAW frequency. The array is repeatedly captured at a sequence of spatial positions along the acoustic beam path and long-range transportation is achieved by periodic cycling of the applied frequency across the transducer bandwidth. We also demonstrate that controllable alignment and transport can be achieved in a detachable microfluidic device, where the microfluidic channel, in which particle transport occurs, is separated from the piezoelectric substrate by an acoustic coupling gel. Proof-of-concept transport is first discussed using a test system of latex particles before the non-invasive manipulation technique is applied to arrays of microbubbles. We explore the role of acoustic radiation forces in the spatial control of particles by analysing the dynamics of particle manipulation by SSAWs. Our results highlight the exquisite control which we have over the position and transport of particles and we anticipate that this technique could find wide applications for the accurate and programmable, non-invasive ordering and transport of biological samples in microfluidic systems. [ABSTRACT FROM AUTHOR]