Low-Cost Microdevices for Point-of-Care Testing - Point-of-Care Diagnostics on a Chip - page 12

to integrate cell capture, cell lysis, and nucleic acid purification into a single
microfluidic device [ 66 ]. Integration of sample pretreatment with analysis could
lead to improvements in sensitivity (as less sample is lost in between steps) and con-
venience [ 5 ]. One of the first microfluidics-based DNA purification procedures has
been demonstrated using silicon microdevices, taking advantage of high-aspect ratio
features to increase the capture surface area and hence the loading capacity [ 67 ].
Other work has focused on single-cell mRNA extraction and analysis via cDNA
synthesis on a microfluidic chip [ 66 ]. The Cepheid's GeneXpert has employed a
macrofluidic approach integrated with a miniaturized sonicator [ 68 ]; this system
has shown success in detecting drug-resistant tuberculosis in the developing world
[ 69 ]. In another approach, Claremont BioSolutions has developed a miniaturized
bead blender for sample preparation.

1.4.3.2 Signal Amplification

Since the amount of nucleic acid acquired from either the preparation step or from
the raw sample is usually low for immediate identification and quantification [ 60 ],
a method of amplification is needed to obtain a sufficiently nucleic acid detection
signal. The most common technique is the polymerase chain reaction (PCR), for
which miniaturization promotes the ability to reduce the reagent consumption,
reduce the cycle time, and automate the process [ 5 ]. PCR requires thermal cycling
for the reaction. Miniaturization of PCR provides many advantages, such as
decreased cost of fabrication and operation, decreased reaction time for DNA
amplification, reduced cross talk of the PCR reaction, and ability to perform large
numbers of parallel amplification analyses on a single PCR microfluidic chip. Also,
microfluidics allows for increased portability and integration of the PCR device.
One of the first silicon-based stationary PCR chip was described several years
after the introduction of PCR itself [ 70 ]. Since then, many research groups began
to develop microdevice-based PCR devices. Most of these devices are based on
silicon and glass, but more recently, polymer materials such as PDMS [ 71 ], PMMA,
polycarbonate, SU-8, polyimide, poly(cyclic olefin), and epoxy are being used.
HandyLab (HandyLab, Inc., is now part of Becton Dickinson) has also developed
a disposable microfluidic chip that implements heat and pressure gradients to move
microliter-sized plugs via valves and gates through different temperature zones
within the chip [ 49 , 72 ]. Flow-through designs also decrease the possibility of
cross-contamination between samples as well as allow for the incorporation of
many other functions, which is appealing as it leads toward the development of a
micro total analysis system. As PCR is a temperature-controlled, enzyme catalyzed
biochemical reaction system, the method in which the different temperature zones
are generated and maintained is crucial to the design of PCR microfluidics. Various
heating methods have been employed, and they can be broadly categorized into
contact and noncontact heating methods. Contact heating has been implemented in
PCR microfluidics using integrated thin-film platinum resistors as both the heating
and sensing elements on these chips [ 73 ] and noncontact methods include hot-air