l a b fa b
Closing the loop in 3D fabrication A new method creates closed loops in 3D microscale structures.
C
soft lithography because the walls on eimaster structures that contained closed urrent fabrication techniques don’t ther side pinch off,” Nuzzo explains. loops interrupted by thin membranes lend themselves well to producing closed loops in 3D microscale structures passing through their centers. The mem- But the researchers “are actually using that in defining 3D topology.” branes were replicated in the PDMS on a mass scale. Because lithographic The investigators estimated that ≥50 procedures create features one at a time, mold. When molding material was molds could be made from a sinthey aren’t suitable for mass progle master structure and that duction. Soft lithographic tech(a) (b) each mold could be used ≥20 niques build 3D structures layer before it degraded. The memby layer. Now, John Fourkas and brane can be reproduced in the colleagues at the University of mold or left out at will. Fourkas Maryland, College Park, have and colleagues used pressure to developed a technique that crecontrol the membrane’s replicaates many arbitrarily shaped mition. When they compressed the croscale structures with closed (c) (d) PDMS mold by applying a slight loops in one shot (Proc. Natl. vacuum, the membranes would Acad. Sci. U.S.A., 2006, 103, not replicate in the molding ma8589–8594). terial. If the mold was expanded The investigators had previby the application of some presously demonstrated that microsure, then the membranes repliscale structures could be procated. duced en masse by multiphoton “You can start with one masabsorption polymerization (MAP) Scanning electron micrographs of (a and c) master structures ter and go for many generations combined with a soft lithographic with membranes and (b and d) their corresponding replicate before you decide to get rid of technique called microtransfer structures without the membranes. Scale bars = 10 µm. (Adaptthat membrane,” Fourkas says. molding ( J. Phys. Chem. B 2004, ed with permission. Copyright 2006 National Academy of Sci“You can make as many exact 108, 11,256 –11,258). They used ences, U.S.A.) copies of the master structure as MAP to create a master structure you need to and mold all of those. You poured into the mold, it was unable to in an acrylic resin. Then, by microtranscan multiply up to huge numbers of wet the PDMS in the membrane region fer molding, a PDMS mold was made structures.” from the master. The final structure was because PDMS tends to stick to itself. Ugaz and Nuzzo wonder if the The membrane wasn’t incorporated into produced when molding material was method can create hollow 3D microfluthe final structure, and thus a complete poured into the mold, cured, and then closed loop was created. Now, structures idic channels. “Directly stamping out a extracted. microfluidic network is not going to such as interlocking rings and five-turn “One reviewer of that paper comwork,” admits Fourkas, but he adds that coils can be fabricated in a single step. mented on the fact that we couldn’t his group is working on it. They are The investigators “have come up do structures that were truly 3D. We currently developing devices based on a with a way to make a mold for 3D showed we could [make] structures previous demonstration that metals can structures that can be released from the with very high aspect ratio [and] things be incorporated into the molding matemold in a straightforward fashion,” says that had big overhangs, but closed rials. “Something we’re very interested Victor Ugaz at Texas A&M University. loops were still a major obstacle,” says in right now is using [MA-µTM] to Ralph Nuzzo of the University of IlliFourkas. If this technique is to be really stamp out inductors,” explains Fourkas. nois at Urbana–Champaign agrees. He useful for the mass production of deWith a structure like the five-turn coil says that the clever part is that the revices, “you want to replicate things that “that we’ve shown we can replicate searchers have turned the mechanical have closed loops, hopefully in a single quite easily, you can imagine putting instabilities of PDMS—which normally shot,” he says. metal on it and making it into an induclimit the features that can be fabricated So, Fourkas and colleagues tackled tor. Suddenly, you’re very close to with soft lithography—into an advanthe problem and came up with mem[building] microfluidic NMR probes.” a tage. “If you were trying to make the brane-assisted microtransfer molding —Rajendrani Mukhopadhyay membrane, it would be hard to do by (MA-µTM). With MAP, they created © 2006 AMERICAN CHEMICAL SOCIETY
S E P T E M B E R 1 , 2 0 0 6 / A N A LY T I C A L C H E M I S T R Y
5951
