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Hard-soft microfluidic device bypasses drawbacks of PDMS them stuck together to produce a robust be hard and impermeable to gas and Now here’s a microfluidic device with a device. “We tried a lot of different ways water to make the device more cell-culhard head but a soft underbelly: reof bonding,” says Takayama, such as ture friendly. A widget made out of two searchers at the University of Michigan chemical grafting on the hard top, UV materials is not a novel idea; after all, Ann Arbor have paired hard materials polymerization between hard and soft laptops often have rubber padding atwith soft ones to make a device for cell components, and thermal sealing. None tached to the hard plastic casing to procultures and chemical analysis. The apof them worked. tect their innards from impacts. But, proach allows investigators to avoid Finally, they settled on an oxygenmuch to Takayama’s surprise, when he PDMS and the baggage it brings to and-argon plasma treatment. After the and his colleagues searched the literasome lab-on-a-chip applications (Anal. components were quickly exposed to the ture, they discovered that “there aren’t Chem. 2007, 79, 3248⫺3253). Shuichi plasma, they were stuck toTakayama, Jennifer Linderman, gether and heated overnight in and colleagues describe their a 60 °C oven with small weights device in a recent AC paper pressing on them. The resulting (DOI 10.1021/ac802178u). devices were stable at room Takayama’s group and othtemperature for up to several ers have long noted that PDMS months under dry conditions. is not the perfect material for “The method we described is microfluidic devices created to still not perfect,” notes study some biological systems Takayama. “But it’s, for the (see, for example, Anal. Chem. moment, one of the better 2007, 79, 1126⫺1134; Lab bonds we could get and still Chip 2006, 6, 1484⫺1486). have cell-happy channels.” Water escapes through the The investigators tested out PDMS, changing the osmolality their hard⫺soft devices with of cell-culture media and other hepatocytes (liver cells). Hepatosolutions, and oxygen sweeps cytes behave differently when through the polymer, disrupt(a) A hot-embossed COC device, (b) a schematic of the device with exposed to different levels of oxying the hypoxic conditions channel layer and membrane layer, (c) an image of the hard topgen. From computer simulations needed for some types of cells. soft bottom device with green food dye in the channels, and (d) the done by Linderman’s group and Small molecules also occasionhybrid device on the Braille array. hands-on experiments, Takayama ally float from solution into the says that the investigators found polymer’s crevices. very many papers describing devices that that the hybrid devices can produce an But PDMS does have some advancombine a soft piece with a hard piece oxygen gradient along the length of a tages that justify its popularity. For one, for microfluidics.” channel that mimics physiological condiit works beautifully for valving systems The investigators had to figure it out tions more accurately. His group is excited that require a flexible membrane. The about finding out how hepatocytes react Braille technique that Takayama’s group for themselves. Their approach began with a hard component made out of to high and low oxygen levels, he says. developed several years ago is such an polyethylene terephthalate glycol, cyclic “Gradients are great to mimic and induce example. In that method, pins push olefin copolymer (COC), or polystyrene. tissue zonation in livers.” against the soft membrane, pinch a Features such as channels were etched The fabrication methods for devices channel closed, and deflect liquids into the hard polymer by hot embosswith two different materials will need through another channel (Proc. Natl. ing, which, Takayama notes, lends itself more elbow grease from the microfluidics Acad. Sci. U.S.A. 2004, 101, well to mass production. These hard community to become better established, 15,861⫺15,866). Takayama says. His hope is that the paper Takayama and co-workers wanted the tops then had to be fixed onto soft elastomeric bottoms made of out polyure“will raise awareness of this not-so-flashy, best of PDMS without having to deal thane or PDMS-parylene C-PDMS. but understudied, topic.” with its worst. So they decided to build The attachment proved to be easier a microfluidic device made out of two This paper appears in the May 15, 2009, said than done; the investigators materials: one material would have the issue of AC. struggled to bring together two polysquishiness of PDMS required for the —Rajendrani Mukhopadhyay mers of very different stiffness and keep Braille technique, and the other would 5108
ANALYTICAL CHEMISTRY /
JULY 1, 2009
10.1021/AC900866N 2009 AMERICAN CHEMICAL SOCIETY
Published on Web 05/14/2009
