Biology Reference
In-Depth Information
LED
L
1
M
otorized
stage
Δ
z
1
Magnet assembly
xy
Sample
plane
obj.
Δ
z
2
Piezo
stage
L
3
Camera
M
FIGURE 6.4
Schematic of a typical magnetic tweezers device. The sample stage is a manual x-y
positioning stage with excellent mechanical stability mounted on a heavy-duty platform to
minimize the vibrations. Above the sample stage, the magnet assembly is mounted onto a
long-distance travel motorized stage which allows the assembly to be moved at a height
D
100 mm/s. Above the magnet, the
illumination optics consist of the LED housing with built-in collimating lens and a set of lenses
(L
1
,L
2
) that direct light onto the sample. Below the sample stage, the 100 objective
lens is mounted on a precision piezoelectric one-axis translation stage. A turning mirror M
directs the image to the camera, which is formed by L
3
, a simple 150-mm relay lens. The image
is captured by a CMOS camera, which is connected to a computer workstation (not shown).
Adapted from
Lin and Valentine (2012a)
. Reprinted with permission from the American
Institute of Physics, Copyright 2012.
z
1
(typically
D
z
1
<
30 mm) at speeds of up to
objective lens is mounted onto a piezoelectric stage (such as P-725; Physik Instru-
mente) to enable nanopositioning of the focal plane. A 650-nm light-emitting diode
(Roithner Lasertechnik) provides semicoherent illumination, and a camera captures
brightfield images of the magnetic beads. The sample is placed on a custom-built
heavy-duty platform to minimize vibrations. A manual
x
-
y
positioning stage (i.e.,
MT25; Marzhauser) with excellent mechanical stability is used to scan through
the sample during imaging.
In the simplest case, the applied magnetic field is generated by placing a pair of
permanent rare-earth NdFeB magnets (NS-505050; Applied Magnets) oriented such
that the alignment of their magnetic moments is antiparallel above the sample stage.