University of Twente Student Theses


Microfluidic pump based on arrays of rotating magnetic microspheres

Beld, W. van den (2012) Microfluidic pump based on arrays of rotating magnetic microspheres.

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Abstract:We demonstrate a novel, flexible and biocompatible method to pump liquid through microchannels without the use of an external pump. The pumping principle is based on the rotation of superparamagnetic microspheres around permalloy disks, driven by an external in-plane rotating magnetic field. By placing the permalloy disks close to the edge of the channel, a netflow of 9 μm/s was generated in the middle of the channel. A possible use of this pumping principle could be to recirculate the medium of a cell culture chamber, which will create new possibilities for closed cell culturing systems on chip with controllable flow rates [23]. The principle of controllable movement and positioning of magnetic particles using permalloy patterns has been demonstrated by Gunnarsson et al [20]. We apply this technique to rotate magnetic microspheres around an array of permalloy disks, positioned close to the edge of a circular micro fluidic channel. Due to their position near the edge of the channel, the drag caused by the channel wall will cause an asymmetry in liquid displacement, resulting in a net pumping motion (Figure 1). The effect is multiplied by using an array of magnetic disks. To fabricate the structure a 480 nm permalloy film was sputtered on a silicon wafer and patterned using conventional lithography and etching, yielding an array of disks with a diameter of 25 μm. The 37 μm deep microchannels were wet etched in a borofloat glass wafer. To get tightly sealed channels, both wafers are bonded anodically at 425 oC. The disadvantage of this heating step is that it increases the coercivity of the permalloy (Figure 2). However the hysteresis loop of the permalloy film after heating (Figure 2, left), shows that the increase in coercivity is limited to 6 kA/m , which can still be easily reached with conventional electromagnets. Pumping experiments were performed using biocompatible, 30 μm superparamagnetic microspheres [33]. Their hysteresis loop (Figure 2, right) shows that the remanent magnetization of the microspheres is very small, which will prevent permanent sticking of the particles to the disk or each other. The external magnetic field was generated by a magnetic quadrupole electromagnet, with a field of 95 kA m at rotating frequencies of up to 10 Hz. Red polystyrene microspheres of 3 μm [38] were added to visualize the flow in a microscope. The rotation frequency of the micropheres could be controlled well up to 6 Hz, resulting in a maximum microsphere velocity of 470 μm/s . Above this frequency the microspheres do not perform exclusively a circular motion but also spin around their own axis or stop moving completely. Even though coverage of the disks by beads was only partial in our setup, the pumping principle works well. Figure 3 shows an analysis of the recorded paths of the indicator micropheres. There is a net ow in the middle of the channel with an average velocity of 9 μm/s. We expect that further optimization of the geometry and microsphere coverage can lead to a significant increase of the flow rate.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:53 electrotechnology
Programme:Electrical Engineering MSc (60353)
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