Biology Reference
In-Depth Information
A
B
C
FIGURE 6.8
Design and application of portable magnetic tweezers. (A) Photo of aligned portable magnetic
tweezers showing the NdFeB magnets placed directly over the objective and centered
with respect to the optical axis; this enables sideways pulling of embedded beads. A sample
holder plate is machined to elevate the sample and minimize the distance between the
magnets and imaged field of view. (B) Schematic showing direct visualization of cytoskeletal
deformation under localized force; magnetic bead (blue) is being pulled in the direction of
the red arrow. (C) Dual color image showing force-induced deformation of a 25
M entangled
MT network. Image is an overlay of an image collected under no force (red) and under
the application of
m
25 pN force (green). In this entangled MT network, the motion of the
network is limited to the area just surrounding the bead (4.5
m
m diameter), and highly bent
filaments can be observed.
Adapted from
Yang et al. (2012)
. Reproduced by permission of The Royal Society of Chemistry.
than viscosity in the flow regime. Two types of time-dependent loading schemes are
used to obtain network response to the sudden application of force (creep response),
or to oscillatory loading.
6.1.4.1
Creep analysis
In a creep measurement, an external force is suddenly applied and then maintained at
a constant value for a period of time before the load is reduced to zero. We dissect the
bead motion into three distinct regimes—a short-time elastic jump, a relaxation tran-
sition, and a long-time creep regime (
Fig. 6.9
A;
Yang et al., 2012
). Creep velocity
v
is determined from the slope of a linear fit to bead displacement
x
versus time
t
for
several (typically
10) seconds prior to the retraction of the magnet pair. We define
the start time of the elastic regime by the motion of the magnets toward the sample
<
