Simultaneous fabrication of PDMS through-holes for three-dimensional microfluidic applications Bobak Mosadegh, a Mayank Agarwal, a Yu-suke Torisawa a and Shuichi Takayama * ab Received 24th February 2010, Accepted 27th April 2010 DOI: 10.1039/c003590d Here we describe a simple yet efficient approach to making through-holes in a bound polydimethylsiloxane membrane for use in 3D microfluidic applications. Localized tearing of an elastomeric membrane is achieved by ripping an elastomeric stamp that is bound to the membrane by posts at desired regions. The tears in the membrane are confined by the underlying channel architecture of the substrate to which the membrane is bound. By varying the membrane thickness and channel dimensions, holes of different sizes can be obtained. This high-throughput method of generating through-holes will enable the design of complex microfluidic devices that require crossing of channel networks. Introduction Development of efficient fabrication techniques is an important area of research since it enables widespread use of microfluidic devices for many different applications. 1–3 The added versatility of three-dimensional microfluidics has been hindered by the inability to efficiently integrate through-holes into poly- dimethylsiloxane (PDMS) membranes used in multi-layer soft lithography. Through-holes provide passages between layers of a device enabling complex paths to bypass each other, which allows for the design of intricate devices. This type of 3D plumbing has been shown to be useful for a myriad of applica- tions including combinatorial mixing of fluids, 4 integrated pneumatic valve devices, 5 and passive auto-regulating components. 6–8 There are currently several low-cost methods to generate through-holes in thin PDMS membranes. One of the simplest is to manually punch out each hole using a small gauge needle or biopsy punch as done for access holes for most microfluidic devices. 9 Although simple and effective, this method is tedious and time-consuming, particularly for devices requiring many holes and is limited to through-hole sizes on the order of hundreds of microns. As an alternative, perforated membranes can be fabricated using an SU-8 mold pressed against a hard- surface during curing of the membranes or spinning PDMS lower than the tallest feature height. 6,10 However, often thin films of PDMS are cured over the SU-8 feature requiring manual removal, making this approach inefficient for high-throughput fabrication, particularly for thick membranes (>30 mm). 5 Recently there have been simple methods to eliminate the pres- ence of the thin film by blowing with an air gun, but this requires manual aiming on each hole and provides additional design restrictions on the spacing between holes, which limits the density able to be integrated into a single device. 11 Selective ripping of PDMS substrates has been previously shown as a simple way to fabricate features on a flat substrate at both the microscale and nanoscale. 12,13 Use of decal transfer microlithography also provides a means to form through-holes by selective release of a micropatterned membrane, however the membrane is more effectively used for stencils than for 3D microfluidic applications since the top surface of the membrane is coated with non-stick material that inhibits proper bonding to another PDMS layer, and there is formation of a meniscus around each feature that will result in bonding gaps between layers. 12 Using this concept of selective ripping of PDMS, we have developed an efficient single- step method to simultaneously fabricate a large number of through-holes in a bound PDMS membrane using conventional microfluidic tools. Experimental All PDMS materials were made from PDMS prepolymer and curing agent (Sylgard 184, Dow Corning Co., Midland, MI) at a 10 : 1 ratio. PDMS stamps were molded against master molds made by standard photolithography using the negative-photo- resist SU-8 (MicroChem Co., Newton, MA). The master molds were silanized in a desiccator for two hours (United Chemical Tech., Bristol, PA). The heights of all features are 80 mm. The PDMS molds were cured in a 120  C oven for at least 30 min. PDMS membranes were made by spin-coating a PDMS layer on a silanized glass slide and then curing in a 120  C oven for at least 30 min. For the characterization experiments, PDMS layers were oxidized/bonded together using a plasma etcher in air (SPI Plasma-Prep II, Structure Probe, Inc., West Chester, PA) for 30 s. An unnecessary but useful step prior to bonding of the post stamp is microcontact printing of uncured PDMS oligomers on areas between post regions to render the surface hydrophobic. 14 This step has been seen to minimize non-specific binding of the post stamp to undesired regions of the membrane. For the 3D microfluidic device demo, the stamp was bonded to the membrane using PDMS glue (mixture of 1 part toluene to 2 parts PDMS) spin coated on a glass slide at 1500 rpm for 30 s. The glue was cured in a 120  C oven for 20 min. Shrinkage of PDMS was noticed but did not cause any alignment issues since the stamp a Department of Biomedical Engineering, University of Michigan, 2200 Bonisteel Blvd, Ann Arbor, Michigan, 48109-2099, USA. E-mail: takayama@umich.edu b Macromolecular Science and Engineering Center, University of Michigan, 2300 Hayward St., Ann Arbor, Michigan, 48109, USA This journal is ª The Royal Society of Chemistry 2010 Lab Chip, 2010, 10, 1983–1986 | 1983 TECHNICAL NOTE www.rsc.org/loc | Lab on a Chip Downloaded by University of Michigan Library on 20 May 2011 Published on 26 May 2010 on http://pubs.rsc.org | doi:10.1039/C003590D View Online