Spectroscopic imaging of microfluidic devices

PDMS microfluidic deviceFTIR spectroscopic imaging is particularly suited to studying microfluidics due to its high chemical specificity and ability to study dynamic systems.  Microfluidics facilitates a high degree of control of chemical reactions, heat transfer and mass transfer processes with only small quantities of reagents.  This is particularly useful when studying compounds of limited availability such as pharmaceutical drugs or biological materials.  We  pioneered a new approach using a microdrop system to produce microfluidic devices, which is able to print designs onto transmission windows or ATR crystals using small wax droplets to form microfluidic channels [1,3].  This versatile approach allows new devices to be prototyped quickly and has been used extensively in our research.

Microfluidic devices can be studied in transmission mode or in attenuated total reflection (ATR) mode [2,3]  depending on the device layout.  ATR mode in particular allows devices to be placed on the crystal itself where the crystal surface forms one side of the microfluidic device [3,4].  This is advantageous for studying aqueous samples where absorption of water is high but for heterogeneous systems such as oil and water, boundary wall effects become apparent.  Under flow, Microfluidic device on silicon ATR crystalparticles or dispersed phases can move away from the ATR crystal surface, giving the appearance of a decrease in concentration whereas in transmission this can be avoided as the entire height of the channel is probed [4,5].  However, in transmission, scattering and chromatic aberration can occur with the use of thick infrared windows.  We have pioneered the use of lenses which form a pseudo-hemisphere around the microfluidic device, correcting for chromatic aberration and reducing scattering [6.7].

We introduced a novel and versatile approach for the rapid prototyping of microfluidic devices suitable for use with FTIR spectroscopic imaging in both transmission and ATR modes. [3,5] The device is based on the direct 3D printing of paraffin droplets to create the walls for the microfluidic channels on the window surface of a standard infrared transmission liquid cell or on the surface of the ATR crystal. The advantage of this approach includes: the fact that the device can be printed in minutes, the design of the device can be altered easily and new devices can be printed immediately. The suitability of this new type of microfluidic devices for FTIR spectroscopic imaging analysis has been demonstrated by analysis of the laminar flow of water and low molecular weight polymer in microchannels. The use of this novel device combined with inherent chemical specificity FTIR imaging enhances applicability of FTIR spectroscopic detection method of flows in microfluidic devices.[3,5] 

Microfluidics is often conducted in a steady-state mode, where reagents are introduced into the device and a continuous stream of products are formed, for example, the reaction of H2O and D2O and the establishment of steady-state conditions allows for mapping a large device [3].  An alternative is the use of immiscible phases to form small self-contained reactors, where droplets contain reagents or even live cells [6], but there is no diffusion or transfer of material from one droplet to another.  These small moving droplets can be imaged using fast scanning modes to produce chemical movies [8]. Such chemical movies of microfluidics enabled us to see the processes in micro-channels in greater detail, for example chemical reactions in moving droplets [9], thus assisting in increasing efficiency of production of certain chemicals. Our most recent advances demonstarted a possibility of studying pharmaceutical microformulations under flows in microfluidics channels [10], a novel approach of using microchip for in-column FTIR spectroscopy to monitor affinity chromatography purification of monoclonal antibodies [11], chemical maging of live cells in microfluidic devices without optical aberrations [7, 12] and most recently introduced a combined  thermo-spectrosocpic imaging approach for measurements of heat and components concentrations for polymerisation reactions studied in microfluidic devices. [13] 

Schematic of microfluidic mixer on ATR crystal

Key References

  1. Kazarian, S. G. Enhancing high-throughput technology and microfluidics with FTIR spectroscopic imaging, Anal Bioanal Chem (2007) Vol: 388 , Pages: 529 - 532 , ISSN: 1618-2650 (doi)
  2. Chan, K. L. A., Kazarian, S. G., ATR-FTIR spectroscopic imaging with expanded field of view to study formulations and dissolution, Lab on a Chip (2006) Vol: 6 , Pages: 864 - 870
  3. Chan, K.L.A.,Niu X. de Mello A. J., Kazarian S, G., Rapid prototyping of microfluidic devices for integrating with FT-IR spectroscopic imaging, Lab on a Chip, 2010. 10(16): p. 2170-2174 (doi)
  4. Chan, K.L.A., Gulati S., Edel J.B., deMello A. J, Kazarian S. G. Chemical imaging of microfluidic flows using ATR-FTIR spectroscopy, Lab on a Chip, 2009. 9(20): p. 2909-2913 (doi)
  5. Chan, K.L.A. and S.G. Kazarian, Label-Free Chemical Detection in Micro-Fabricated Devices Using FT-IR Spectroscopic Imaging, Spectroscopy, 2012. 27(10): p. 22-30.
  6. Chan, K.L.A. and S.G. Kazarian, Correcting the Effect of Refraction and Dispersion of Light in FT-IR Spectroscopic Imaging in transmission through thick infrared windows, Analytical Chemistry, 2013 85(2): p. 1029-1036 (doi)
  7. Chan, K.L.A. and S.G. Kazarian, Aberration-free FTIR spectroscopic imaging of live cells in microfluidic devices, Analyst, 2013. 138(14): p. 4040-4047 (doi)
  8. Chan, K.L.A., Niu, X., de Mello, A. J., Kazarian, S. G. Generation of Chemical Movies: FT-IR Spectroscopic Imaging of Segmented Flows, Analytical Chemistry, 2011. 83(9): p. 3606-3609. (doi)
  9. Chan K. L. A., Kazarian S. G. FTIR spectroscopic imaging of reactions in multiphase flow in microfluidic channels, Analytical Chemistry 84 (2012) 4052-4056. (doi)
  10. Ewing A. V., Clarke G. S., Kazarian S. G.   ATR-FTIR spectroscopic imaging of pharmaceuticals in microfluidic devices, Biomicrofluidics (2016) 10(2), 024125 (doi)
  11. Boulet-Audet M., Byrne B., Kazarian S. G. In-column ATR-FTIR spectroscopy to monitor affinity chromatography purification of monoclonal antibodies. SCIENTIFIC REPORTS (2016) 6, 30526  (doi)
  12. Kimber J. A., Foreman L.,  Turner B., Rich P., Kazarian S. G. FTIR spectroscopic imaging and mapping with correcting lenses for studies of biological cells and tissues, Faraday Discussions (2016) 187, 69-85 (doi)
  13. Ryu M., Kimber J. A. et al. Infrared thermo-spectroscopic imaging of styrene radical polymerization in microfluidics Chemical Engineering Journal, accepted (2017) (doi).