1. Introduction
Serial dilution is one of the most essential unit operations in numerous biological and chemical experiments, which currently normally involves tedious and time-consuming manual pipetting. The serial dilution by the labor-intensive manual pipetting, however, is quite influenced by the user’s expertise and the accurate pipetting is not ensured, particularly when the sample volume is very small down to microliter scale. Therefore, there is a great incentive existing for the development of an automated serial dilution technique to minimize the manual efforts and eliminate the user influence.
Currently, two strategies are being mainly exploited to achieve automated serial dilutions, which basically rely on automated dispensing using pipetting machines [1-5] and microfluidic systems [6-12]. The pipetting machine can eliminate user influence and easily control the number, factor, and volume of dilutions. However, it is relatively expensive and many pipetting errors are still inevitable as in the conventional manual pipetting, particularly when handling highly viscous samples such as general biological specimens or PCR master mixes.
As an alternative approach, the microfluidic systems have been extensively exploited to achieve an automatic serial dilution by employing various elaborately designed microchannels and metering structures. A major advantage of the microfluidic systems is the ability to precisely manipulate small volume samples, thus reducing reagent and/or sample loss. The microfluidic systems, however, normally require device integration with the macroscale environment and essentially involve external pumps and connecting tubes between the channels and pumps for fluid control, which would make the system expensive and complicated. Furthermore, the control of the sample volume or dilution ratio in the microfluidic systems is quite intricated due to the continuous propensity of the fluidic flow within the indiscrete structure [9-12]. In this regard, the centrifugal microfluidic system could greatly simplify the microfluidic process by manipulating liquid flow by rotating the disc at various speeds and in different spinning directions, and eliminating the requirements for the pumps, tubings, and associated connectors [13-20].
Based on this background, centrifugal microfluidic devices have been intensively developed to achieve automatic serial dilutions. Most representatively, Kim et al. [21] developed a centrifugal microfluidic device capable of automatically conducting arbitrary serial dilution by utilizing reversible diaphragm valves and specially designed two metering zones. They demonstrated that the device was able to accomplish wide dynamic range serial dilutions with excellent accuracy (R2 > 0.97). Juelg et al. [22] also reported the centrifugal microfluidic system for automated serial dilutions by utilizing fill-level-coupled valving as a key element. They successfully accomplished the automated serial dilutions required for the on-disc construction of qPCR standard curve, verifying the capability of the system for the serial dilution in a wide dynamic range with high accuracy and precision.
In this paper, we present another advanced centrifugal microfluidic system equipped with individually addressable laser-irradiated ferrowax microvalves (LIFM) [23], which enables fully automated serial dilutions in an accurate, ultrafast, and loss-free manner. To this end, the capability of the system was successfully verified by accomplishing six consecutive two-fold and ten-fold dilutions with excellent accuracy.