Page 43 - Optofluidics Fundamentals, Devices, and Applications

24 Cha pte r T w o

Photoresist
100 μm
1. Silanize wafer
Si wafer
2. Mold in PDMS
PDMS 3. Cure
1. Punch inlet/outlet holes
2. Plasma oxidize PDMS
3. Seal to glass or PDMS base
700 μm
Left Right
Base electromagnet electromagnet Channel 50 μm
1 mm Solder
40 μm
40 μm 5 mm
10 μm Microfluidic 500 μm
Silanize channels for solder channel
(b)
Silane 1. Fill channel with liquid solder
2. Insert wires in inlet/outlet holes I 1 mm
3. Cool to 25°C
4. Apply UV epoxy and cure
Epoxy Q
Wire
Solder
(a) (c)
FIGURE 2-8 (a) A schematic diagram depicting the fabrication of electromagnets in
PDMS. The completed device has three microfl uidic channels: two outer channels fi lled
with solder (length = 1.5 cm, width = 120 μm, height = 40 μm), and a central channel
for fl uids (length = 1.5 cm, width = 40 μm, height = 40 μm). (b) Electromagnet:
photographs of the three channels as viewed from above at low magnifi cation (left), high
magnifi cation (upper right) and the cross-section of the three channels (lower right). The
photograph of the cross-section was obtained by sectioning the channels with a razor
blade (shown as the dashed line in the upper right image); the dark line in the left
electromagnet is the result of imperfect sectioning; the light areas at the bottom of the
image are refl ections of the metal. In the photograph at low magnifi cation, lines were
drawn to outline the location of the microfl uidic channel. (c) Microheater: photograph of
a solder coil positioned axisymmetrically around a central microfl uidic channel; the
channel was fi lled with dyed deionized water to make it easier to see; the exterior walls
of the central microfl uidic channel were highlighted for clarity. (A. C. Siegel, D. A.
Bruzewicz, D. B. Weibel, and G. M. Whitesides, “Microsolidics: fabrication of three-
dimensional metallic microstructures in poly(dimethylsiloxane),”Adv. Mater., 19, (2007),
727–733. A. C. Siegel, S. S. Shevkoplyas, D. B. Weibel, D. A. Bruzewicz, A. W.
Martinez, and G. M. Whitesides, “Cofabrication of electromagnets and microfluidic
systems in poly(dimethylsiloxane),” Angew. Chem., Int. Ed., 45, (2006), 6877–6882.
Copyright Wiley-VCH Verlag Gmbh & Co. KGaA. Reproduced with permission.)

Electromagnet
The channels for electromagnets [64] were fabricated in close proximity
(~10-μm separation), and in the same plane as the channels used to
transport fluid (Fig. 2-8b). By passing electrical current through the wires,
−1
magnetic fields up to 2.8 mT and field gradients up to 40 Tm have been
generated inside microfluidic channels. This electromagnet has been
demonstrated to be capable of sorting super-paramagnetic beads.
Microheater
Figure 2-8c shows a photograph of a coil microheater [63] fabricated by
the injection of solder into a PDMS channel. The device consists of a