Biomedical Engineering Reference
In-Depth Information
Figure 8.21
Separation of a surface from the AFM tip in the presence of an adhesive interaction.
In this particular case, adhesion proteins are grafted on the tip and on the surface. When the force
acting on the cantilever is too high, it snaps back to its equilibrium position. The nonlinear part of
the curve represents the stretching of a single PEG linker.
An extension of this direct force measurement technique is the biomembrane
force probe (BFP) in which the cantilever is replaced with a very soft red blood cell
(RBC). The RCB is aspired in a micropipette. The aspiration pressure in the pipette
controls the stiffness of the RBC over a wide range. Similarly to the AFM, the inter-
action is measured between a bead glued at the apex of the RBC and a solid surface.
The extension of the RBC spring is measured optically and the force deduced from
it [48]. The very low adjustable stiffness of the force sensor makes it possible to use
(8.18) over a wide range and to access to x
β
and k
off
very accurately.
8.4.2 Optical Tweezers
Optical tweezers (OTs) use a highly focused optical beam to trap particles at the
focus. When a laser is focused through a high numerical aperture microscope objec-
tive, it defines a well-defined light “cage” in which not only the intensity is maximal
but where the gradients in light intensity are also extremely strong [40]. We detail in
Chapter 10 how a spatial gradient of electric field can be used to trap particles of a
different polarizability than the one of the surrounding medium (dielectrophoresis).
The physical principle is the same in the present situation: the light intensity within
a laser beam is not uniform but is maximum at its center. In fact the light distribu-
tion is Gaussian. The light intensity gradients then naturally drive particles of index
of refraction higher than the solution toward the center of the beam. If the beam is
tightly focused, the same effect drives the particles transversally toward the focus.
The net effect is a trap localized close to the focal point. Close but not exactly at
this point, because there is another force: the scattering force that tends to “push”
the particle away.
These traps can immobilize and transport particles with forces in the range of
a few tens of piconewtons, which is well suited to many practical situations. The
lasers used for applications in biology are usually in the infrared spectrum in a
range for which there is no absorption of energy by the water molecules (and thus
no heating).
OTs have been used with many different objects: viruses, bacteria, and organ-
elles within cells but the most well-known application is the trapping of micron-sized
