Abstract
This review presents an overview of the different techniques developed over the last decade to regulate the temperature within microfluidic systems. A variety of different approaches has been adopted, from external heating sources to Joule heating, microwaves or the use of lasers to cite just a few examples. The scope of the technical solutions developed to date is impressive and encompasses for instance temperature ramp rates ranging from 0.1 to 2,000 °C/s leading to homogeneous temperatures from −3 °C to 120 °C, and constant gradients from 6 to 40 °C/mm with a fair degree of accuracy. We also examine some recent strategies developed for applications such as digital microfluidics, where integration of a heating source to generate a temperature gradient offers control of a key parameter, without necessarily requiring great accuracy. Conversely, Temperature Gradient Focusing requires high accuracy in order to control both the concentration and separation of charged species. In addition, the Polymerase Chain Reaction requires both accuracy (homogeneous temperature) and integration to carry out demanding heating cycles. The spectrum of applications requiring temperature regulation is growing rapidly with increasingly important implications for the physical, chemical and biotechnological sectors, depending on the relevant heating technique.
Highlights
The development of lab-on-a-chip requires the integration of multiple functions within a compact platform, which is readily transportable and can deliver rapid data output
Reaction (PCR) [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15], Temperature Gradient Focusing for Electrophoresis (TGF) [16,17], digital microfluidics [18,19,20,21,22,23,24,25,26,27,28,29,30,31], mixing [32,33,34], and protein crystallization [35]
The growing demand for handling picoliter to nanoliter volumes of biological samples has driven the development of droplet techniques where a variety of processes, including mixing, splitting and heating are efficiently controlled: these approaches have been designated as digital microfluidics
Summary
The development of lab-on-a-chip requires the integration of multiple functions within a compact platform, which is readily transportable and can deliver rapid data output. The vast majority of studies, involving heating/cooling technologies exploit external approaches such as the use of macroscopic Peltier or pre-heated liquids [1,2,3,4,5,36,37,38,39,40] flowing through the microsystem These technologies facilitate both homogeneous temperature regulation within the whole microsystem, and linear temperature profiles often with a high degree of accuracy; the control is not integrated and may limit the potential applications. As well as the conditions reported (heating method, level of integration, range of temperature, spatial distribution and power needed), are summarized in a table at the beginning of the paper It seems that there is currently no consensus on any given technique that would satisfy all the requirements specified by the complete range of applications; each of the techniques described here has successfully demonstrated the integration of temperature control for specific applications. Note that temperature mapping techniques are beyond the scope of the current paper, for this we refer the reader to the recent review by Gosse, Bergaud and Löw [71]
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