Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
Load with
pipette
5 mm
500 µm
(d)
(e)
(f )
Slip
(g) 100 pg/µL, cpw = 8 × 10
1
(h) 1 pg/µL, cpw = 8 × 10
-1
(i) 100 fg/µL, cpw = 8 × 10
-2
(j) 10 fg/µL, cpw = 8 × 10
-3
(k) 1 fg/µL, cpw = 8 × 10
-4
(l) Control, cpw = 0
5 mm
FIGURE 4.24
Digital. PCR. using. the. SlipChip.. (From. Feng. Shen,. Wenbin. Du,. Jason. E.. Kreutz,.
Alice.Fok,.and.Rustem.F..Ismagilov,.“Digital.PCR.on.a.SlipChip,”.
Lab Chip
.10,.2666-2672,.2010..
Reproduced.with.permission.from.The.Royal.Society.of.Chemistry.)
4.6.5.3 PCR Reaction Volume, Temperature Control, and Speed
Miniaturization of the reaction volume is not always desirable, as efects such as loss of sample/
solution on or into the channel walls become relatively more severe. he smaller the thermal mass
(including the reaction volume), the faster the cycling. (World record of stationary PCR, as of 2006, is
40 cycles in less than 6 minutes using a silicon chip with integrated heater.) Fabricating the heater as
a separate reusable unit makes it cheaper to assemble, optimize, and reuse but increases cycling time.
Continuous-low systems are faster because the heat inertia is limited mostly to the sample thermal
mass; a 500 base pair DNA fragment was ampliied in 1.7 minutes in 2005. Also, note that the time
dependence of luorescence acquisition typical of real-time PCR is eliminated because the cycles and
temperatures are spread over space instead of over time. Incorporating microfabricated tempera-
ture sensors is feasible but increases device thermal mass and, anyways, reads only temperature at
a few points; a better option is to add a thermochromic liquid crystal or luorescence dye indicator
or monitor thermal radiation in the infrared (IR) spectrum (“IR hermography”) or by laser-based
Raman spectroscopy so temperature can be read optically. Integration of several platforms into one
presents intellectual property challenges that hinder commercialization.
Although there are other ampliication schemes possible, PCR remains the standby. One of the
earliest biochemical procedures to be embodied in silicon was PCR ampliication. Clearly, one
would like to speed up the ampliication process as much as possible. his can be accomplished
by increasing the speed of thermal cycling. However, there is a minimum time required for the
creation of the new copies of DNA that depends on the rate at which the enzyme turns over.
hermal cycling faster than that cannot improve the process, and can even make it worse. It was
thought that the excellent thermal conductivity of Si would allow rapid temperatures changes.
