Advances in microchannel heating and cooling based on fine scale heterogeneity of thermal conductivity; Theory, Optimization, and Experimental Validation

A major advance in microchannel heating and cooling technology is described and has the potential to change the way thermal processing of microscale fluidic samples is achieved. The microchannel is one of the key building blocks of micro total analysis systems (muTAS), which are already being released to market in the form of disposable test kits for biometrics and disease identification. However, significant cost reductions and other advances in microchannel heating and cooling technology are still needed and can be expected to improve the accuracy with which quantitative analysis can be performed. In part, this is because the fidelity of many existing and potential microfluidic applications is critically dependent on precise temperature control.;The scientific objective of the study was to understand and then manipulate the physics of thermal transport within fine scale composite materials containing an embedded microchannel. In essence, it was necessary that the composition of the composite materials be manipulated in a precise way that established a particular nonlinear pattern of temperature along the length of the microchannel. In practice, this required the development of a systematic inverse solution process that could identify an acceptable composition to exactly match the desired pattern of temperature variation. As a demonstration of capability, fully operational prototypes comprised of patterned copper laminated within polyimide and acrylic polymers were designed, fabricated, and tested. In terms of conceptual advances, this is the first time that composite materials of this type have been used for microchannel heating AND cooling. From an analytical perspective, the critical advance was the development of a robust simulation framework consisting of an outer optimization loop and an inner finite element analysis (FEA) loop, which was used to iteratively identify the optimal copper pattern for each of several applications. In terms of experimental advances, the key fabrication steps included UV laser micromachining, acid etching, microchannel formation, and hot vacuum lamination. To avoid any ambiguity or artifact, microchannel temperatures were painstakingly measured at each of several flow rates using fine thermocouples embedded in the microchannel wall. For the most part, the measured temperatures were within 1 °C of the expected values based on simulation, with spatial temperature gradients of up to +/-94 °C/mm (or equivalently +/-188 °C/s at 2 l/min flow rate) within the range of 16 °C to 92 °C, using a resistively heated source at 100 °C and a thermoelectrically cooled sink at 2 °C. The temperature profiles studied here included two highly nonlinear cyclical profiles applicable to 1) continuous-flow polymerase chain reaction (PCR) and 2) thermally mediated counter-flow separation in conjunction with electrophoresis.