Biomedical Engineering Reference
In-Depth Information
array of pillars [20]. The photolithography-derived techniques enable the fabrica-
tion of micron-size posts separated by a few microns, dimensions close to the gel's
pore size used for long DNA molecules. Combined with the ability to visualize single
DNA molecules, these experiments have pioneered a whole school of experiments
where the behavior of the chains could be unambiguously correlated to their exten-
sion, hooking and releasing from the posts, an invaluable help in the understanding
of the phenomena. Indeed the first experiments of migration of DNA subjected to a
low electric field in such arrays of posts have shown a lot of common features with
their behavior in gels. Although this strategy uses well-known techniques derived
from the microelectronics industry for the fabrication of the substrates, it has not
yet come to the point where it is used for routine analysis. Similar to gels where
the range of accessible molecular weights is dictated by the gel's pore size and thus
its degree of cross-linking, the efficiency of these devices is conditioned by the size
and spacing of the posts, parameters that can be tuned during the fabrication step
but not as readily as for a gel. However, for research purposes, this approach has
been very successful and will be with no doubt a tool that can and will be used in
particular situations.
Indeed, after these proofs-of-concept experiments, pulsed-field electrophoresis
has been performed in 2-D hexagonal arrays of pillars with extremely good results.
Because of the high regularity of the array, DNA chains move more regularly than
in a gel and sequences of electric field turned by 120° enable extremely fast separa-
tion of long molecules (up to a few 100 kbp in a few 10s which is 100 to 1,000
faster than pulsed field gel electrophoresis) [21]. Again, it seems relatively straight-
forward to integrate such structures in on-chip separation devices. More recently,
arrays of columns made of magnetic beads that align in a regular hexagonal array
upon the application of a magnetic field, have been used with good results. This
strategy has some obvious advantages (no microfabrication and easy replacement
of the medium), it does not allow however a range of variation in the spacing be-
tween posts as large as would be desirable [22].
Nanopores and Nanochannels
Not only DNA or other biopolymer chains can be forced through a micro-channel
or a micro-aperture, it can be threaded through a nanopore. In the present version
Figure 10.7
Pulsed field electrophoresis in an hexagonal array of posts (diameter and spacing
around 1
m
m). Picture taken after a 120° change in orientation of the electric field. from [21].
