15 Smart Platforms for Biomedical Applications
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however, will continuously mechanically rotate to align its magnetization with the
applied field transducing torque over the entire field duration. Over even the 60 s
field duration this proved to be enough to result in a 50% difference in cell killing
capability between the two sets of particles. This study clearly highlighted the need
to engineer magnetic particles to their application to ensure maximum efficiency.
Lithographic techniques offer great precision in this regard and are expected to be
of significant interest for the fabrication of magnetic micro- and nanoparticles for
biotechnology applications.
15.3.4 Magnetically Driven Labs-On-Chips
The transport of magnetic beads using either domain walls [140] or periodic magnetic
elements [141] is an interesting prospect technologically and may offer a new take on
lab-on-chip processes where the analyte of interest is not transported by fluid flow as
in microfluidics, but via the motion of magnetic beads. The study here [142] demonstrates an architecture of patterned permalloy nanotracks on a Si chip that creates a
magnetic domain wall routing network capable of transporting magnetic beads along
paths selected by the application of external fields. This has exciting implications for
more complex operations such as dynamic sorting of magnetic particles allowing for
intricate downstream processing and sensing.
Magnetic domain walls are localized sources of stray field capable of trapping
superparamagnetic beads with forces in the range of hundreds of pN [143–146].
Domain walls, and hence the particles they trap, may be moved in micro- and
nanowire structures with an applied field at speeds approaching 1 mm/s [140]. A
permalloy curvilinear nanotrack was fabricated consisting of semi-circular links,
with track width of 800 nm, thickness of 40 nm and an outer diameter of 20 μm for
the semi-circular links. A junction was created that spits the track into two paths. This
study focussed on vortex-type domain walls [147], a magnetic texture analogous to
the vortex flux closure state in a patterned microdisc. A vortex domain wall exists
in thin magnetic strips where two in-plane domains of the opposite magnetization
direction meet. At this intersection between the two domains, the domain wall forms
with a magnetic vortex core in the centre of the wall, and a chirality given by the
direction that the magnetization rotates about the core of the wall [148]. The domain
wall can be formed in a head-to-head or tail-to-tail configuration according to the
direction of the domains in the magnetic strip, and these two configurations may be
thought of as having opposite signs, and their stray fields (pointing out of the plane
of the track) are of opposite sign as well.
The vortex domain walls in this study are moved in the permalloy nanotrack by
a rotating magnetic field of the appropriate rotational direction in the plane of the
track. As the domain wall is moved through the junction, it splits and creates two
domain walls in each of the new paths, one of the same sign and one of the opposite
sign. The crucial point of this work shows that since the domain wall splits into two
domain walls of the opposite sign, a field pulse applied out of the plane of the system
367
however, will continuously mechanically rotate to align its magnetization with the
applied field transducing torque over the entire field duration. Over even the 60 s
field duration this proved to be enough to result in a 50% difference in cell killing
capability between the two sets of particles. This study clearly highlighted the need
to engineer magnetic particles to their application to ensure maximum efficiency.
Lithographic techniques offer great precision in this regard and are expected to be
of significant interest for the fabrication of magnetic micro- and nanoparticles for
biotechnology applications.
15.3.4 Magnetically Driven Labs-On-Chips
The transport of magnetic beads using either domain walls [140] or periodic magnetic
elements [141] is an interesting prospect technologically and may offer a new take on
lab-on-chip processes where the analyte of interest is not transported by fluid flow as
in microfluidics, but via the motion of magnetic beads. The study here [142] demonstrates an architecture of patterned permalloy nanotracks on a Si chip that creates a
magnetic domain wall routing network capable of transporting magnetic beads along
paths selected by the application of external fields. This has exciting implications for
more complex operations such as dynamic sorting of magnetic particles allowing for
intricate downstream processing and sensing.
Magnetic domain walls are localized sources of stray field capable of trapping
superparamagnetic beads with forces in the range of hundreds of pN [143–146].
Domain walls, and hence the particles they trap, may be moved in micro- and
nanowire structures with an applied field at speeds approaching 1 mm/s [140]. A
permalloy curvilinear nanotrack was fabricated consisting of semi-circular links,
with track width of 800 nm, thickness of 40 nm and an outer diameter of 20 μm for
the semi-circular links. A junction was created that spits the track into two paths. This
study focussed on vortex-type domain walls [147], a magnetic texture analogous to
the vortex flux closure state in a patterned microdisc. A vortex domain wall exists
in thin magnetic strips where two in-plane domains of the opposite magnetization
direction meet. At this intersection between the two domains, the domain wall forms
with a magnetic vortex core in the centre of the wall, and a chirality given by the
direction that the magnetization rotates about the core of the wall [148]. The domain
wall can be formed in a head-to-head or tail-to-tail configuration according to the
direction of the domains in the magnetic strip, and these two configurations may be
thought of as having opposite signs, and their stray fields (pointing out of the plane
of the track) are of opposite sign as well.
The vortex domain walls in this study are moved in the permalloy nanotrack by
a rotating magnetic field of the appropriate rotational direction in the plane of the
track. As the domain wall is moved through the junction, it splits and creates two
domain walls in each of the new paths, one of the same sign and one of the opposite
sign. The crucial point of this work shows that since the domain wall splits into two
domain walls of the opposite sign, a field pulse applied out of the plane of the system