Simultaneous production of microchannels with
parallel, electrically conducting metal wires.
Magnetic components that can be controlled by the
application of an external electric field are useful in many different
applications. They can serve as microfluidic pumps, mixers, or valves
in miniature lab-on-chip systems, or they can help in sorting and
arranging magnetic particles. Biochemistry and cellular biology in
particular benefit from many possible uses: for example, antibodies or
other ligands that bind to individual biomolecules or to surface
structures of cells can be coupled to magnetic beads in order to
recognize and bind to their specific bonding partner even in complex
mixtures. They can subsequently be fished out of the mixture with an
electromagnet. Electromagnets have an additional advantage over
permanent magnets: they can easily be switched on and off with an
electric current. Also, the field strength can be adjusted to the
desired value and can be changed as required. However, electromagnets
do have the disadvantage of generating weaker magnetic fields, meaning
that they must be very close to the place where they are to be used.
G. M. Whitesides and his co-workers at Harvard
University in Cambridge, USA, have now developed an uncomplicated
method for producing a microfluidic channel along with two metal
cables parallel to it and only 10 µm away. First, a structure
consisting of a 40-µm-wide and 40-µm-deep inner channel between two
120-µm-wide and 40-µm-deep outer channels was lithographically
engraved into a polydimethylsiloxane resin. Treatment with
3-mercaptopropyltrimethoxysilane silanized the surfaces of the outer
channels. This allowed them to be coated with molten solder that was
poured into the heated forms in the next step. Upon cooling, the
liquid metal solidified, forming two stable metal cables to the left
and right of the inner channel. Application of an electrical field to
these two wires generates magnetic fields of up to 2.8 mT within the
central channel.
It was also possible to steer magnetic spheres
through the channel: the scientists again made a channel with parallel
wires on either side, but this time the channel forked after a few
millimeters. A suspension of magnetic spheres flowed through the
channel. If current was allowed to flow through wire on the right, the
spheres flowed to the right as they reached the fork, and vice versa.
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