A
microfluidic embedded calorimeter for picoliter scale biochemical
reactions
Introduction
Calorimetry is a well known technique for complete characterization of thermodynamics of molecular interactions in biology and chemistry. It is a unique technique because of its universal applicability: No specific labeling or immobilization of reactants is needed for a wide range of chemical reactions and biological systems. However, the application of calorimetry in today’s molecular and cellular biology research is limited. This is because traditional calorimetric measurement is slow and requires large amounts of reagents. As a result, an up-rising trend in the field of calorimetry is the miniaturization of calorimetric devices. [1~3]
The CIT Biocalorimeter
We are developing the micro-fabricated calorimeter combined with microfluidics. Generally, sensitivity of calorimeter is determined by its thermometry and thermal insulation. Small scale calorimeters utilize very sensitive thin film thermopile or thermistor (typically 0.1mK sensitivity). However, the level of the insulation is far worse than large scale commercial calorimeter, which has been main reason for the poor sensitivity. We used parylene microfluidic system [4] with
very low gas permeability and high mechanical strength, which enable us to build thin fluidic channel wall supporting vacuum structure.
Picoliter scale injections can be made into the few nanoliter volume reaction chamber using PDMS microfluidics. The PDMS microfluidic part can be combined to other lab-on-a-chip system for pre- or post-process the sample after calorimetric measurement.
Au-Ni thermopile thermometry is used to measure temperature change up to ~0.5mK difference. Au heater is used to calibrate thermal conductance. Current device thermal conductance is ~15μW/K. We expect to have ~200nW/K with further device optimization..
A major advantage of having small sample volume comes from short measurement time. Moreover, a micro-fabricated calorimeter can be built in array form, which increases the throughput by many orders of magnitude.
The device concept can be applied to different type of calorimetry (such as ITC, DSC and flow calorimeter) or biochemical detector.
Future work
We are currently working on developing calorimeter for single cell metabolism study. It is anticipated that the application of calorimetry to single cell would yield important information regarding the metabolism of single cell. Single cell study is extremely important to the field of biology because the response of a population of cells is not identical. An experiment involving a population of cells is just an average response of all the cells. Therefore, calorimetry at single cell level would allow clear identification of unique signature of a particular process inside a cell.
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