Biology Reference
In-Depth Information
square pixels), which enables a maximum frame rate of 280 fps for full-frame col-
lection. An 8-tap Camera Link frame grabber card (NI PCIe-1433, National Instru-
ments) is used to capture images and relay them to the image processing computer.
To maximize the number of beads that can be simultaneously tracked, we select a
Dell workstation with a single-chip quad-core processor (Intel
®
Xeon
®
Processor
X5687) with 3.60 GHz clock speed, 6.4 GT/s QuickPath Interconnect, and a
12 MB Smart Cache. With this data acquisition system, we are able to track
6 beads
at 280 fps in real time using the full 2048
1024 pixel array. Tracking more beads is
possible but requires a reduction in image size by cropping or binning, decreased
frame rate, use of a smaller calibrated image stack size, and/or elimination of other
computer demands (such as real-time data display).
6.1.3.5
High-force implementation
Simple magnetic tweezers devices employ an antiparallel pair of NdFeB magnets
above the sample stage. This arrangement can produce forces of
<
100 pN on
m
m commercial magnetic beads (
Lin & Valentine, 2012a
). For higher force ap-
plications, it is necessary to redesign the magnet assembly to focus and confine the
magnetic fields. To optimize the design, FE modeling of the magnetic fields is useful
and can provide quantitative predictions of field strengths and gradients for various
magnet types, orientations, and shapes. Using this approach, we have found an as-
sembly incorporating two pairs of strong N50 0.5-in. cubic NdFeB magnets and
shaped iron “pole pieces” are able to confine and direct the magnetic fields in a man-
ner that produces
4.5
m magnetic beads
(
Fig. 6.6
). This arrangement not only improves field strength but also increases the
field gradient, which, in the limit of saturation of the bead's magnetic moment, is
directly proportional to the force applied to the magnetic bead. In practice, we find
low-carbon steel to be a good alternative to iron, as it has similar magnetic properties
and is easier to machine.
1-2 nN forces at the sample plane on
4.5
m
LCS bars
LCS tips
NdFeB
magnets
Bottom view
Side view
FIGURE 6.6
Magnet assembly optimized for high-force applications. An aluminum housing holds four
cubes of NdFeB N50 magnets between two low-carbon steel (LCS) backing bars. Below the
magnet array, two LCS tips are press fit into the housing to direct and focus themagnetic fields.
