3.1. Design and operational principle of the centrifugal
microfluidic system
The centrifugal microfluidic disc designed in this work is composed of
six identical sections to allow six simultaneous dilution processes.
Each section mainly consists of two diluent chambers, a single dilution
chamber, seven product chambers, and connecting channels between them,
as shown in Figure 2(a). The diluent needs to be loaded to the two
identical diluent chambers prior to the operation, and the user needs to
apply only the target analyte solution to the dilution chamber to start
the serial dilution process. After the diluent and analyte solutions are
loaded through the green-colored inlet holes above the respective
chambers, the inlet and outlet holes are all sealed using a
pressure-sensitive adhesive tape. Each diluent chamber has three outlet
channels at different levels, such that the diluent chamber is radially
occupied by three equivalent volumes of the diluent. The six outlet
channels were initially all closed by the ferrowax microvalves (DV1
~ DV6, diluent valve) to isolate the dilution chamber
from the diluent chambers. The main channel to elute the diluted sample
from the dilution chamber to the product chamber has seven initially
open ferrowax microvalves (EV1 ~ EV7, elution valve),
which is disconnected to the seven final product chambers by the closed
ferrowax microvalves (FV1 ~ FV7, final valve) located at
the entry of the seven respective branch channels.
The process begins by eluting the original 1 X analyte solution by
rotating the disc at 2,000 rpm after opening FV1. Then, predetermined
volume of 1 X solution was eluted to the first product chamber, while
the rest of the 1 X solution remained in the dilution chamber. For the
first dilution, EV1 was closed to isolate the 1 X product chamber from
the main channel, and DV1 was opened, followed by centrifugation at
2,000 rpm. Then, one third the initial volume of diluent was eluted to
the dilution chamber. For the complete mixing of the analyte solution
and the diluent, the disc was shaken by repeatedly rotating at 60
degrees clockwise and counterclockwise. After shaking, the first diluted
solution was eluted to the second product chamber by opening the FV2 and
rotating the disc at 2,000 rpm. The second and more consecutive
dilutions can be accomplished by repeating the same dilution procedure
but appropriately changing the microvalves to be opened and closed
(Figure 2(b)).
As illustrated in Figure S4, the ferrowax was loaded in the groove of
the top layer, which was then integrated with the bottom layer having
discontinuous deep channel and the middle layer having the channel part
peeled off. The ferrowax microvalves within the fabricated channels on
the disc were initially all in an open state, but the DVs and FVs were
made to be closed prior to the dilution process. The valve-closing
process was automatically executed by irradiating the laser to the
groove part to melt the ferrowax. Due to the capillary force, the melted
ferrowax would be guided to the nearby shallow channel, where the
ferrowax was solidified, consequently blocking the channel. Whenever
needed, the closed channels were reopened by irradiating the laser to
the shallow channel blocked by the ferrowax. The melted ferrowax was
removed away by flowing in the same direction with the solution.