Liquid-infused surface in digital microfluidics and its application in genetic engineering

Abstract

Digital microfluidics (DMF) chip has attracted high attention in biomedical and biochemical engineering as a novel tool in droplet manipulation. The droplet controllability fairly depends on the interface between the droplet and the substrate, thus the property of the contact surface becomes increasingly important. Liquid-infused surface (LIS) provides a liquid-liquid contact rather than the conventional liquid-solid contact when a droplet slides on the surface, hence reducing the flow resistance while at the same time possessing self-healing and anti-biofouling properties. Accordingly, many biochemical reactions can be done preferably when LIS is integrated with the DMF chip. To fabricate a versatile LIS, a layer of porous polytetrafluoroethylene (p-PTFE) film is grown on the substrate by thermal evaporation of PTFE powders. The fluorine-groups of the p-PTFE film provide chemical affinity with fluorinated oil. Droplets with different surface tensions, 22 to 72 mN/m, on the LIS all present a fairly low contact angle hysteresis (CAH), even on those LIS with 10000 rpm lubricant spin-coating speed. In addition, the LIS can be easily patterned into hydrophilic-hydrophobic areas. LIS conformally-coated 3D-printed channels are used to demonstrate the movement of droplets on rough surfaces. To solve biofouling problem of the DMF chip, LIS is further integrated with DMF chips on both top and bottom plates. Electrowetting and liquid-dielectrophoresis behaviors perform well on the LIS. Series passive droplet dispensing and a single polystyrene bead isolation is demonstrated by hydrophilic spots on the LIS. The thermal stability of the droplets on the LIS is evaluated by heating and cooling experiments. The outstanding anti-biofouling performance is assessed with DNA and serum droplets, compared with the conventional Teflon-based DMF chip. Finally, plasmid extraction without the use of surfactants is successfully carried out on the DMF chip. To accomplish multiple biochemical reactions in a single LIS-coated DMF chip, a full procedure of genetic engineering is demonstrated. A DMF chip with paths, reservoirs, functional zones and a hydrophilic spot is fabricated. The DMF chip is further integrated with Peltiers and a magnet to achieve more functions. Plasmid extraction is carried out firstly, following with gene sequence amplification by polymerase chain reaction (PCR) and purification in the hydrophilic spot area. Digestion and ligation are performed afterward to constitute a new plasmid. Transformation step is finally executed with the help of Peltier by cooling and heat shock. An enhanced fluorescence intensity is observed in the cultured bacteria colony with the modified plasmids. The integration of LIS with the DMF chip fully demonstrates the advantages of the DMF chip in dealing with biomolecular droplets. Genetic engineering and other biochemical reactions can be done with the click of a button, instead of tedious repeated pipetting manually in the laboratory. DMF chips with LIS have the potential to eventually enable a fully automated droplet manipulation system on a massive scale for handling complex biomedical and biochemical analysis and synthesis tasks, which could serve as a revolutionary tool for a wide range of practical bio-related applications.