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Few would contest that the field of microfluidics has been built upon a foundation of soft lithography and the use of polydimethylsiloxane (PDMS) devices. PDMS microfluidic chips are easy, fast and cheap to fabricate and possess advantageous physical properties, such as high transparency, elasticity and biocompatibility. Unfortunately, due to its popularity, the inherent drawbacks and limitations of PDMS are normally overlooked. Its poor solvent compatibility excludes the use of PDMS-based devices in a range of chemical synthesis applications, while the permeability of PDMS to small molecules leads to droplet shrinking during prolonged incubation “on-chip”. Indeed, the most overlooked drawback of PDMS is its tendency to (bio-)foul, i.e. adsorb and absorb (hydrophobic) molecules. The extent of biofouling is molecule-specific, posing a challenge for surface modifications that are often employed to “prevent” fouling. This is particularly true for proteins with their diverse surface properties. To address the inherent drawbacks of PDMS we have developed microfluidic platforms made from Teflon™ fluoropolymers. Although reported over a decade ago, applications of Teflon™-based microfluidic platforms have remained surprisingly scarce. In the current work, Teflon™ is used in applications inaccessible to PDMS-based microfluidics. In the field of biochemistry, a Teflon™-based microfluidic platform was developed to study complex enzymatic reaction networks, such as the eukaryotic glycosylation machinery. This platform is used to perform sequential glycosylation reactions in microfluidically-generated droplets and mimic the early steps of protein glycosylation along the secretory pathway. Next, we leveraged the properties of Teflon™ to rethink how state-of-the-art microfluidic platforms are designed. Specifically, a microfluidic platform to investigate homogeneous and heterogeneous ice nucleation was designed, fabricated and tested. Here, the combination of efficient heat transfer on the microscale along with the solvent resistance of Teflon™ materials allowed immersion of the microfluidic platform in a cooling solvent. Instead of using conventional cold-stage techniques to cool droplets from below, this approach affords isotropic droplet cooling, which in turn minimizes heat gradients and temperature uncertainties. The microfluidic platform was subsequently used to better understand the homogeneous nucleation rate of water with high temperature accuracy and over a wide temperature range. Significantly, data indicate that the homogeneous nucleation rates at higher temperatures are underappreciated in the currently recommended parametrization schemes, with possible impacts in atmospheric science, food preservation and pharmaceutical production. |
