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.