Microfluidic Tools for DNA Analysis - DNA Nanotechnology: From Structure to Function - page 127

Indeed, how to simplify the valve control for an integrated microchip attracted
widespread attention. Considering the heating characteristics of the amplification
devices, the thermo-responsive valves were proposed. Bau and coworkers modified
composition of the phase-change materials used for the valve to enhance the thermal
conductivity [ 65 ]. The valve opened when the temperature was below the phase
transition temperature, and the LAMP reactants can be introduced at that time. The
valve closed when the composite material was heated above the phase transition
temperature and thus expanded irreversibly with a 60-fold increase in volume. This
strategy provides an intelligent valve control, where the sealing can be automatically
launched only if the amplification reaction initiates. Convenient sealing effectively
reduced the complexity of the system, and more importantly, the sealing could
prevent evaporation which is an apparent problem under small scale and leakage
of amplicons into the outside environment possibly resulting in contamination.
Similar to the dried sample spot assay above (Fig. 7.10 ), paper-based microflu-
idic devices share many desired characteristics of a suitable POCT. It is rapid,
inexpensive, portable, and simple to operate, making them especially appropriate
for resource-poor settings. Besides proteins, glucose, cholesterol, lactate, alcohol,
metal ions, and gases [ 66 - 69 ], recently, DNA as a key biomarker has also become
a candidate for this device. As one of a few attempts for DNA analysis on paper-
based devices, Bohringer and coworkers devised a foldable paper chip for extraction
of DNA from raw viscous samples [ 70 ]. But this device still lacks the function of
amplification and detection which have to be accomplished off the chip. Rohrman
et al. presented another foldable chip made of plastic and paper that performs RPA
reaction for HIV DNA with a limit of detection of 10 copies in 15 min [ 71 ]. RPA
is an isothermal amplification technique competitive with LAMP. It mimics the in
vivo amplification mechanism, which makes RPA capable of proceeding at lower
temperature. It requires two enzymes but allows them to mix together. This method
employed a recombinase to realize the targeting of primers with template, and the
polymerase employed enables RPA to be performed at 37 ı C. The user operates
the device by pipetting reagents on the appropriate pads, dipping the wick into the
sample, and mixing reaction components by folding the device in half (Fig. 7.12 ).
The device stores lyophilized enzymes, suggesting that the device could be shipped
to remote areas for use at the point of care. However, the other reagents necessary
for RPA must be stored separately and dispensed onto the device with a pipette.
And, the device must be peeled apart to extract the reaction products for readout
using a separate lateral flow strip. It may take more effort to make the paper-based
amplification devices more practical.
In short, based on the advanced microfabrication technologies (for micro-pillars,
micro-valves, micro-wells, and so on), the traditional laboratory pipeline has been
miniaturized dramatically. By combining temperature control and fluidic control,
many sophisticated PCR chips were developed in the past decades. Users are
interested in the performance evaluation against traditional tube PCR. As basic
research, these technologies are all very interesting. But on the other hand, if we
consider the original goal (i.e., the general DNA amplification), many miniaturized