Modeling heat losses in microfluidic devices: The case of static chamber devices for DNA amplification

Abstract

Most of the functions integrated in lab-on-a-chip (LoC) systems require the heating and cooling of samples in order to activate specific reactions. A common example in LoC systems for molecular diagnostics of pathogens is DNA amplification by the polymerase chain reaction (PCR). The practical use of such LoC systems is strictly related to the speed of the operations and the power and energy consumption, especially in case of point-of-care (PoC) applications, the latter influenced by the heat losses to the ambient. This study investigates the effect of the heat loss mechanisms and parameters on the heating and cooling rates, and the power and energy requirements through a systematic computational analysis. The case study is a static chamber microfluidic device for DNA amplification based on PCR and the means is a detailed 3d computational framework taking into account the air flow around the microfluidic device. An evaluation of existing simplified models for heat losses is performed, in terms of the accuracy provided and the computational cost. Furthermore, a new effective model for the convective heat losses is proposed: Beyond the constant values of the heat transfer coefficient (h) used in the literature and overcoming the applicability problems of empirical equations for h to the small-sized microfluidic devices, a function of h versus the surface temperature is derived from a parametric study in a 2d axisymmetric geometry resembling the real geometry of the microfluidic device. The results of the new model not only compare satisfactorily with those of the detailed 3d framework, but also entail a 10-fold lower computational cost. Measurements of the temperature profile in a thermal cycle are also performed and are in good agreement with the computational results.