2.1. Design and fabrication of the centrifugal
microfluidic disc
The serial dilution disc was designed using computer-aided design
software (AutoCAD, Autodesk, Inc., Sausalito, CA, USA). The disc was
fabricated by assembling three separate layers: a 5 mm polycarbonate
bottom plate (IDEL Corp., Chilgok, Korea), a 1 mm polycarbonate top
plate (IDEL Corp., Chilgok, Korea), and a 0.1 mm middle layer of
double-sided polyimide tape (930H, Coretech, Gimpo, Korea). Figure 1(a)
shows an expanded view of this assembly, while its assembled form is
shown in Figure 1(b). The top and bottom layers were milled by a
computer numerical control machining center (HI-M1300, Hwacheon, Seoul,
Korea). The bottom layer was further treated with a diluted chloroform
solution to obtain a smooth and transparent milled-surface, and both the
top and bottom layers were cleaned with 70% ethanol and distilled water
(DW). The layers were additionally autoclaved when they were used to
dilute the Escherichia Coli O157:H7 (ShigatoxigenicEscherichia coli (STEC)) samples. The bottom layer was mainly
composed of six identical sections containing two diluent chambers, one
dilution chamber, seven product chambers, and associated deep channels,
while the top layer was designed to consist of the appropriately
positioned ferrowax grooves, venting holes, inlet holes, and outlet
holes. A double-sided polyimide tape for the middle layer was fabricated
using a cutting plotter (Silhouette Cameo 3, Silhouette
America® Inc., Orem, UT, USA). The middle layer was
used to bind the top and bottom layers after the areas corresponding to
the chambers and channels are peel off.
To facilitate the convenient and accurate assembly of the three layers,
we employed 3D printed jig fabricated by 3D printing (3DWOX 2X, Sindoh
Co., Ltd., Seoul, Korea) of a polylactic acid filament (Figure S1),
which enabled the accurate alignment of the three layers having a
perforated structure of the central axis and six alignment holes. Prior
to the assembly, the ferrowax prepared by mixing the same volumes of
paraffin wax (Tm = 50–52°C, Fluka Chemie GmbH, Buchs,
Switzerland) and ferrofluid (APG 314, Ferrotec Inc., Santa Clara, CA,
USA) was loaded onto the ferrowax grooves of the top layer. The
assembled disc was finally laminated using a steel roller to eliminate
any gaps or air bubbles between the tape and polycarbonate plates. A
pressure-sensitive adhesive tape (HJ-Bioanalytik GmbH, Mönchengladbach,
Germany) was used to seal the inlet and outlet holes during the dilution
operation.