Biomedical Engineering Reference
In-Depth Information
the direction of flow can be induced in microfluidic chambers. Single DNA mol-
ecules stretched in this type of flow can be observed by fluorescence microscopy and
their shape compared with existing theoretical models [43].
Flows have also been used to align molecules: Elongated rigid objects such as
microtubules orient naturally in the direction of the flow and a receding meniscus
can be used to perfectly align DNA molecules in the direction of drying. This “mo-
lecular combing” is performed on a modified surface and in the right pH conditions
in order to have one end of the DNA stick to the surface [44] (Figure 8.22).
Once the DNA molecules are all aligned and stretched, their analysis is much
easier. In principle, one should be able to analyze genomic DNA by analyzing the
images and to identify genes or particular sequences after hybridization.
Flow chambers are used to quantify the interactions of beads or cells with a
surface. In the laminar flow conditions imposed by the geometry, the established
Poiseuille flow imposes a constant shear rate near the solid surface. The force acting
on the flowing object next to the wall can then be computed and is found to be [46]:
∼
2
F
a Q
(8.19)
Where Q is the flow rate, a the radius of the particle, and
η
the viscosity. The pro-
portionality constant is imposed by the channel geometry and can be analytically
calculated. In the framework of the theory of Bell mentioned above (8.18), the
duration of arrests of particles interacting with a protein bound on the solid surface
is related to the dynamic characteristics of the bond and to a characteristic length
[46, 47].
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