Typical extractions in the laboratory require a significant amount of sample material, large-scale equipment, and hands-on participation of a scientist. Such a setup is impractical (or impossible), heavy, and expensive to be carried out on spaceflight instrumentation. While great strides are being made to improve spaceflight instrumentation, the front-end sampling and extraction steps are often neglected. Only a few examples discuss the development of extraction systems for spaceflight, and they are immature in development. Additionally, several other microfluidic methods in development at GSFC will eventually require front-end sampling and extraction instrumentation.
Thermal Micro-Extraction Laboratory (TherMEL) is a microfluidic microchip whose capabilities are to extract organic and inorganic analyte by the use of heat and induced turbulence in the flow of the liquid in order to suspend analytes trapped within ground-up solid samples from planets, satellites, or other primitive objects into aqueous form. TherMEL addresses the problem of sample preparation for downstream microanalytical instruments, but does not address the problem of sample acquisition.
This work fabricated a miniaturized labscale extraction system to be coupled to other GSFC-developed microfluidic instruments. To achieve this, the design involved development of a long and serpentine microchannel to maximize retention time within TherMEL. Simultaneously, those microchannels contained specially designed obstacles to change direction of fluid and induce some turbulence into the fluid, thereby enhancing mixing. Moreover, heaters were incorporated into the chip with the capability to heat above 200 °C. The purpose of these heaters is to enhance extraction. Design was followed by microfabrication with the use of semiconductor fabrication processes in GSFC’s Detector Development Laboratory.
Microfabrication involved processing of silicon wafers, where the channels and platinum heaters were created. Additionally, Pyrex wafers were machined to match the silicon wafer designs. Lastly, the silicon and Pyrex wafers were bonded together via anodic bonding to allow formation of the device. The bonded wafers were then diced to release individual TherMEL microchips. To conduct testing, a fixture was developed to house TherMEL and allow for the macroscale introduction of fluid into the chip as well as electrical connections for the control of heaters.