A portable battery-operated chip thermocycler based on induction heating

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Abstract

A microchip thermocycler, fabricated from silicon and Pyrex #7740 glass, is described. Usual resistive heating has been replaced by induction heating, leading to much simpler fabrication steps. Heating and cooling rates of 6.5 and 4.2 °C/s, respectively have been achieved, by optimising the heater dimensions and heating frequency (∼200 kHz). Four devices are mounted on a heater, resulting in low power consumption (∼1.4 W per device on the average). Using simple on–off electronic temperature control, a temperature stability within ∼0.2 °C is achieved. Features such as induction heating, good temperature control, battery operation, and low power consumption make the device suitable for portable applications, particularly in polymerase chain reaction (PCR) systems.

Introduction

Recently there has been a lot of interest in performing chemical and biochemical reactions on a chip. Of these reactions, the polymerase chain reaction (PCR), which amplifies strands of DNA, is of extreme importance. PCR involves thermally cycling DNA a large number of times (∼30 cycles) at three temperatures (∼95, ∼52, and ∼70 °C). Each cycle doubles the amount of DNA, through the steps of denaturation, annealing and extension, typically giving a billion copies after 30–40 cycles. PCR reactions have been performed on microfabricated chips made of silicon [1] or glass [2].

As a material for microfabrication, glass has the advantage of being electrically non-conducting, optically transparent, and having low native fluorescence, thereby enabling optical detection by fluorescence. However, silicon is preferred due to its large thermal conductivity (157 W m−1 K−1) and well-controlled etching properties. These miniaturised devices [1], [2] have the advantages of low cost (due to batch fabrication technology and reduced reagent volume), low power consumption, on-line control (by having sensor and microfluidic components on the same chip), speed (due to small reagent volume), and efficiency (by efficient heat transfer) [3].

Most of earlier microreactor designs had heating resistors, formed by depositing and patterning a thin film of metal [4] or polysilicon [5]. Although this led to a more efficient reaction when compared to non-miniaturised thermocyclers, it involved cumbersome patterning steps and needed exact positioning of the sample with respect to the heater for making electrical contact. Some recent designs have considered the possibility of non-contact heating, which led to the development of a hot-air cycler. Oda et al. [6] have reported the use of a tungsten lamp, instead of the heating coil, as an inexpensive radiation source. This configuration needs lenses and filters, and also requires positioning of the reaction mixture at the appropriate focal distance for the reaction to occur. Moreover lamp heating is not very efficient in terms of power transfer (e.g. Upadhyay reported the use of a 500 W halogen lamp) [7]. Power efficiency is very crucial in the design of portable microreactors. In this work, we report the design and fabrication of a thermocycler using induction heating as an efficient and non-contact method of thermocyling.

Section snippets

Induction heating arrangement

The induction heating arrangement consists of a primary coil, with the PCR chips fixed to a metal ring acting as the secondary. The primary coil is made of 2 mm diameter enamelled copper wire, with eight turns wound on a 10 mm diameter ferrite core. The coil is 10 mm long and has an inductance of 2.3 μH at 100 kHz. Fig. 1 shows the driver circuit for the coil. It is driven by one n-channel and four paralleled p-channel power MOSFETs (IRFP150 and IRF9540N, respectively) from a 12 V lead-acid

Results

Experiments were performed to optimise the device performance in terms of heating and cooling rates, temperature stability at set points, power dissipation, and the reproducibility of the cycles over long periods of time. The metal heater dimensions were first optimised, with the inner and outer diameters as 5 and 14 mm (matching with the primary coil diameter for maximum heating efficiency), respectively. Effects of varying heater thickness and frequency on heating and cooling rates were

Theoretical modelling of data

The dependence of heating and cooling rates on heater thickness can be qualitatively explained as follows: as the heater is thinned down its thermal mass decreases. For a fixed input power and under open-loop conditions, this leads to an increase in heating and cooling rates.

The observed power dissipation at various frequencies (from 100 to 250 kHz) under open-loop conditions is explained by lumped circuit modelling. The driver circuit for induction heating is modelled as a transformer with a

Discussions

The performance of our device compares well with others in terms of temperature stability, power dissipation, and cycle reproducibility. We report a typical temperature variation of ±0.2 °C at all set points, obtained by on–off control only. This is a better value compared to those reported (±0.5 °C) using PID control [2]. On the average, ∼1.4 W per device (with a reaction volume of ∼1 μl) is dissipated during a cycle in our device.

In the experiments reported here, heating and cooling rates of 6.5

Conclusions

In summary, we have demonstrated a microchip thermocycler using induction heating for the first time. This method has the advantage of being non-contact, and therefore, obviating the requirement of any accurate positioning of the sample with respect to the heater. It also eliminates the thin-film deposition steps needed to pattern heaters on the chip during resistive heating. It consumes low power, works well with on–off electronic control (±0.2 °C), and does not require any elaborate PID

Acknowledgements

The authors thank BRN and LSRB, Govt. Of India, for partial financial support.

Debjani Pal is a graduate student at the Department of Physics in Indian Institute of Science, Bangalore, India. She works in application of micromachining technology to biology.

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Debjani Pal is a graduate student at the Department of Physics in Indian Institute of Science, Bangalore, India. She works in application of micromachining technology to biology.

V. Venkataraman received his PhD degree from Princeton University in electronic engineering in 1994. Since 1997, he has been an assistant professor at the Department of Physics in Indian Institute of Science, Bangalore, India. His research interests include study of transport and optical properties in modulation-doped semiconductor heterostructures, and silicon micromachining technology.

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