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Ultrasonic particle switching in microfluidic channels

Publiceringsår: 2004
Språk: Engelska
Sidor: 5-7
Dokumenttyp: Konferensbidrag

Sammanfattning

Introduction

It is known that suspended particles are affected by radiation forces when exposed to ultrasound. Suspended particles are gathered in the pressure nodal or pressure anti-nodal planes of an ultrasonic standing wave field depending on the density and compressibility of the particles and the medium. By combining this effect with laminar flow in a silicon micro channel different particle types can be separated from each other and/or their medium. The channel must have vertical side walls and a width chosen to correspond to half the ultrasonic wavelength. When ultrasonically actuated this results in an acoustic standing wave between the walls, perpendicular to the direction of flow. A pressure node is generated along the middle of the channel and one pressure anti-node along each channel wall. The suspended particles will be forced into the nodal or anti-nodal planes as they flow along the channel. If the end of the channel is split into three outlets the particles in the nodal plane will exit through the centre outlet while the particles in the anti-nodal planes exit through the side outlets. A large portion of the medium can be removed from one particle type by letting the main part of the medium exit through the outlets with no such particles. However, in some applications of particle separation, for example blood washing, the medium (blood plasma) can be heavily contaminated and must be totally separated from the particles (red blood cells). This can be achieved by using multiple inlets. If the inlet is split in three, like the outlet, and the suspended particles in the contaminated medium enter through the side inlets and a clean medium enters through the centre inlet the particles can be switched from the contaminated medium to the clean by the acoustic forces.

Two major problems complicate blood recycling during and after surgery, fat particles and contaminations to the plasma by inflammatory components and activated coagulation factors. Fat particles are the cause of fat emboli in the brain, i.e. small brain damages that give rise to a cognitive decay. Luckily, the difference in density and compressibility between red blood cells, fat particles and blood plasma cause the red blood cells to gather in the pressure nodes and the fat particles in the pressure antinodes of the acoustic standing wave. It is therefore possible to separate the red blood cells from the fat using the method described above. Furthermore, the contaminated plasma can be replaced by clean plasma using the split inlet.

Materials and methods

The separation channel (350 µm wide, 125 µm deep and 20 mm long) was fabricated in <100>-silicon by double sided photolithography and anisotropic wet etch. The beginning/end of the channel had three inlet/outlet channels, one leading straight forward and one on each side at an angle of 45º to the prolongation of the separation channel. A glass lid, attached by anodic bonding, covered the channel. Rubber silicon tubings were glued to the inlets and outlets on the rear side of the chip. A piezoelectric element (2 MHz) was attached to the rear side with ultrasonic gel in between. A contaminated fluid with suspended particles was simulated by a mixture of latex spheres (5 µm in diameter), blue pigment (Evans blue) and distilled water and a clean medium with distilled water only. The same experiment was repeated using blood contaminated with “Evans blue” and non-contaminated blood plasma. A sonicated emulsion of tritium-labelled triolein (1% concentration) was used to measure the fat removal efficiency.

Results and discussion

More than 95% percent of the particles can be shifted from the contaminated medium to the clean and exited through the centre outlet and more than 95% of the fat particles can be removed when conditions are ideal. Less than 5% of the contaminant exited the system through the centre outlet when latex particles were shifted from contaminated distilled water to clean distilled water. The corresponding measurements on contaminated blood showed that less than 15% of the contamination followed the red blood cells out through the centre outlet. The experiments have shown that this method is a powerful particle separation tool and can be used in several blood wash applications.

Disputation

Nyckelord

  • Chemistry

Övriga

Micro Structure Workshop 2004
2004-03-30/2004-03-31
Ystads Saltsjöbad, Sverige
Published
Yes

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