Development of contaminant-free and effective micro-mixing methods based on non-contact dispensing system

• We develop a non-contact dispensing system using a high-speed-solenoid valve.
• Micro-mixing methods are proposed based on the dispensing system.
• The principle is to generate droplets of small volume for high diffusive surface area.
• Mixing performances are evaluated according to the different dispensing conditions.
• Mixing performances are enhanced by kinetic energy and collision of droplets.

This paper presents the development and characterization of two different types of effective micro-mixing methods with small volumes of liquids, both of which are based on simple non-contact dispensing systems to avoid the contamination between mixing-target liquids, thereby preventing undesirable crosstalk in biochemical assays. The principle of micro-mixing methods is to control the trajectory and volume of dispensing droplets. The first micro-mixing method induces mixing by smashing two different droplets in the air by dispensing the droplets simultaneously (called, a simultaneous micro-mixing method, SMM). The second method is to dispense two different droplets into a well in a controlled alternating manner (namely, an alternating micro-mixing method, AMM). In the dispensing system developed in this study for the micro-mixing, pressurized air transfers a liquid from a reservoir to a nozzle via a high-speed-solenoid valve, thus injecting liquid droplets with volume ranging from several ten nanoliters to several microliters, depending on the viscosity of the liquid. The droplet volumes of mixing-target liquids dispensed from the dispensing system were measured under various dispensing conditions by changing operating pressure and opening time of high-speed-solenoid valve. Sodium hydroxide (NaOH) ethanol solution and phenolphthalein ethanol solution added with glycerol were used as the mixing-target liquids to quantitatively characterize the mixing performances. The mixing performances were evaluated according to the different volumes of dispensing droplets and micro-mixing methods. The SMM exhibited a higher mixing performance compared to the AMM. However, both micro-mixing methods achieved the improved mixing performances compared with conventional mixing methods, such as micro-stirrer and micro-well shaking. The present methods could be useful in the automated micro-mixing system for clinical applications.

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