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The direct on-disk wireless temperature measurement system [1,2] presented at μTAS 2010 was further improved in its
robustness. We apply it to an IR thermocycler as part of a centrifugal microfluidic analyzer for polymerase chain
reactions (PCR). This IR thermocycler allows the very efficient direct heating of aqueous liquids in microfluidic cavities
by an IR radiation source. The efficiency factor of this IR heating system depends on several parameters. First there is
the efficiency of the IR radiator considering the transformation of electrical energy into radiation energy. This radiation
energy needs to be focused by a reflector to the center of the cavity. Both, the reflectors shape and the quality of the
reflecting layer affect the efficiency. On the way to the center of the cavity the radiation energy will be diminished by
absorption in the surrounding air/humidity and especially in the cavity lid of the microfluidic disk. The transmission
spectrum of the lid material and its thickness is of significant impact. We chose a COC polymer film with a thickness of
150 μm. At a peak frequency of the IR radiator of ~2 μm approximately 85 % of the incoming radiation energy passes
the lid and is absorbed within the first 1.5 mm depth of liquid in the cavity. As we perform the thermocycling for a PCR,
after heating to the denaturation temperature of ~ 92 °C we need to cool down rapidly to the primer annealing
temperature of ~ 55 °C. Cooling is realized by 3 ventilators venting air of room temperature into the disk chamber. Due
to the air flow itself and an additional rotation of the centrifugal microfluidic disk the PCR reagents in the cavities are
cooled by forced air convection. Simulation studies based upon analogous electrical models enable to optimize the disk
geometry and the optical path. Both the IR heater and the ventilators are controlled by the digital PID controller HAPRO
0135 [3]. The sampling frequency is set to 2 Hz. It could be further increased up to a maximum value being permitted by
the wireless temperature data transmission system. As we are controlling a significantly non-linear process the controller
parameters need to be optimized for all temperatures relevant for the PCR thermocycling process. Such we get a
dynamic system for both, the heating and the cooling process. Heating rates up to 5 K/s with our IR heater (100 W
electrical power) could be achieved. Cooling rates of instantly 1.3 K/s at 20 Hz rotation frequency could be even further
increased by higher rotation frequencies, faster air circulation, optimization of the controller parameters or an active air
cooling unit.
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