Development of contaminant-free and effective micro-mixing methods based on non-contact dispensing system Sung Jea Park, Moonwoo La, Kyoung Je Cha, Dong Sung Kim ⇑ Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang 790-784, South Korea article info Article history: Available online 4 April 2013 Keywords: Dispensing system High-speed-solenoid valve Mixing Non-contamination abstract This paper presents the development and characterization of two different types of effective micro-mix- ing 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 alter- nating 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 liq- uids 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-tar- get liquids to quantitatively characterize the mixing performances. The mixing performances were eval- uated 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-mix- ing system for clinical applications. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Micro-mixing method to handle small liquid volumes has been widely used in fields of life science, analytical chemistry, and food engineering [1–3]. The mixing of bio-samples is generally per- formed manually so that the process becomes tedious and time- consuming. The risk of contamination is also raised when consider- ing the crosstalk in biochemical assays for clinical applications. In this regard, it has drawn attention to develop contaminant-free and effective micro-mixing methods in biological areas. To meet these demands, automated micro-mixing systems have been sug- gested based on various mixing methods [4–6]. The representative micro-mixing methods used in the auto- mated systems can be micro-stirrers (agitator), micro-well shak- ing, and microfluidic mixers using microchannels. The mixing method using the micro-stirrers is expensive and unsuitable for high throughput works. The contamination also frequently occurs due to the residual bio-samples since an agitator should be im- mersed into the samples to mix [7,8]. Though the micro-well shak- ing method can prevent the contamination of bio-samples from happening by removing physical contact, it is generally ineffective to control and mix small sample volumes of liquids [9,10]. The microfluidic mixing method enable to control and mix very small sample volumes precisely but it is not suitable for biological areas requiring the use of various bio-samples due to limitation of han- dling volume and continuous process characteristics [11,12]. The scale-up of microfluidic devices and surface treatment suitable for biochemical assays are still open issues. The objective of this study is to develop new mixing methods applicable in the automated mixing system using the small volume of miscible bio-samples. To achieve the objective, we developed a non-contact dispensing system based on a high-speed-solenoid valve and pneumatic systems, which can generate sample droplets with a controlled volume ranging from several ten nanoliters to 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.140 ⇑ Corresponding author. Address: Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang, Gyeongbuk 790-784, South Korea. Tel.: +82 54 279 2183; fax: +82 54 279 2957. E-mail address: smkds@postech.ac.kr (D.S. Kim). Microelectronic Engineering 111 (2013) 175–179 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee