This Communication describes the use of patterned elastomeric stamps to fabricate three-dimensional (3D) microstructures of hydrogels. Hydrogels are indispensable for many chemical and biological applications, including electrophoretic separation, chromatography, drug delivery, biosensing, and tissue engineering. 1 The ability to pattern the topologies of gels at the microscale is desirable in these applications, because microscale structures are often more sensitive and versatile than their macroscale counterparts are. To date, patterning of gels at the microscale has relied primarily on photopolymerization of liquid precursors; 2,3 this strategy cannot be used with many types of gels, such as those made of proteins or sugars. Here, we introduce a general strategy for microfabrication of gels: the use of poly(dimethylsiloxane) (PDMS) stamps to mold, release, and stack gels into 3D structures, and the use of surface modification to promote the release or adhesion of molded gels to a substrate.Our work builds upon the studies of Whitesides and others in soft lithography, which use patterned PDMS stamps to generate microscale structures in photoresist, polymers, and metals. 4,5 We first identified surface treatments for PDMS that allowed release of a molded gel from a stamp, because gels that were molded against untreated PDMS often adhered to the stamps and deformed irreversibly upon separation from them. Of the surface treatments that we tested, only modification of PDMS stamps by hexa(ethylene glycol)-terminated self-assembled monolayers (SAMs), or by adsorbed monolayers of bovine serum albumin (BSA), allowed distortion-free separation of stamps from gels molded against them. 6 These treatments greatly reduce nonspecific adsorption of protein; 7 we suspect that their ability to promote release of gels results from a decrease in adsorption of soluble gel precursors onto treated stamps.Modification of PDMS stamps with these treatments allowed the application of standard soft lithographic techniques, 8 such as replica molding, microtransfer molding (µTM), and micromolding in capillaries (MIMIC), to the microfabrication of gels. 9 Figure 1 shows images of microstructures molded in collagen, using modified PDMS stamps; we obtained similar results when molding other protein-based gels, such as gelatin and the tumor extract Matrigel, as well as sugar-based gels such as agarose. Replica molding, µTM, or MIMIC of liquid precursors generated monolithic gels (with surface relief), isolated microstructures, or interconnected networks, respectively, with a spatial resolution of e5 µm ( Figure 1A, inset). Microtransfer molding against substrates that promoted or resisted the adhesion of gels generated supported arrays of collagen gels and a suspension of free-standing gels, respectively ( Figure 1B). These gels were dimensionally stable for several days in saline at 4°C.Many potential applications of microstructured gels require the development of structures that are more complex than the uniform gels shown in Figure 1. For ...
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