Biomedical Engineering Reference
In-Depth Information
Fig. 14.16.
A schematic illustration of the measurement of the dynamics of DNA-
protein interaction by stretching the DNA between two beads, trapping one of the
bead, and tracking its Brownian motion with a quadrant-photodiode
any polarization sensitive detection scheme. Instead of a bead with optical
birefringence, any bead lacking axial symmetry illuminated by a laser beam is
also expected to generate an optical scattering signal modulated (in intensity)
with a frequency component at that of the rotational frequency of the bead.
The basic principle of this approach is to detect (by analyzing the transla-
tional and the rotational motion of the trapped bead) the dynamic of the
conformational change of a stretched double-strand DNA molecule interact-
ing with RecA-ssDNA filaments injected into the surrounding buffer solution.
The generic goal of the experiments, such as the one outlined above, is to un-
derstand the relationship between the physical properties and the biochemical
(or functional) properties of DNA at a fundamental level.
14.4.3 Optical Trapping and Stretching of Red Blood Cells
As mentioned earlier, a red blood cell (RBC) can be trapped and stretched
with the aid of a fiber optical dual-beam trap-and-stretch to measure its visco-
elastic property. A schematic diagram to illustrate the application concept of
a fiber-optical dual-beam trap-and-stretch in conjunction with a microfluidic
flow chamber, fabricated with poly dimthylsiloxane (PDMS), to inject the
RBC one at a time for such measurement is shown in Fig. 14.17. Photographs
of an experimental set up in our laboratory are depicted in Fig. 14.18. Pre-
liminary experimental results illustrating the morphological change of human
RBC samples, osmotically swollen into spherical shape, as a function of the
optical power are shown in Fig. 14.19 along with the experimental data on
the fractional change in length of the major axis and the minor axis of a
