Biomedical Engineering Reference
In-Depth Information
beads that are used as handles on biopolymers or on molecular force-generating
systems such as biological motors [41]. Indeed, not only can OTs trap particles,
they can also measure forces applied to them: as the object is pulled away by an
external force, its position in the trap varies. Recent developments in optical mi-
croscopy have made it possible to track displacements of a few nanometers (see
Section 8.3.1.1). The exact position of the bead then witnesses the force applied to
it. In another configuration, the light intensity can be cranked up to balance for this
external force so that the position of the bead remains unchanged. Both approaches
need a calibration of the trap usually by moving the particle in the fluid (or vice
versa) at a known velocity and using the Stockes Einstein friction to measure the
applied force.
We have discussed so far the use of these optical tweezers as a mean to handle
a single particle. An interesting development is the possibility of creating arrays
of traps in which many particles can be manipulated at will. The simplest way to
perform this task is by defining two or more positions for the focus and having
the beam rapidly switch between these positions. Another approach is to define a
holographic array where the energy landscape the particles are submitted to (for
instance an array of traps) is defined in the Fourier plane [42].
8.4.3  Flow-Based Techniques
Flows can be used not only to transport particles or molecules but also to manipu-
late them. For instance, elongational flows where the velocity increases linearly in
Figure 8.22  Fluorescently labeled DNA stuck on a solid surface after “molecular combing.” (From
[45]).
 
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