Using pattern homogenization of binary grayscale masks to fabricate microfluidic structures with 3D topography{ Javier Atencia, a Susan Barnes, b Jack Douglas, b Mark Meacham a and Laurie E. Locascio* a Received 20th June 2007, Accepted 28th August 2007 First published as an Advance Article on the web 31st August 2007 DOI: 10.1039/b709369a Because fluids at the microscale form three dimensional interfaces and are subject to three dimensional forces, the ability to create microstructures with modulated topography over large areas could greatly improve control over microfluidic phenomena (e.g., capillarity and mass transport) and enable exciting novel microfluidic applications. Here, we report a method for the fabrication of three-dimensional relief microstructures, based on the emergence of smooth features when a photopolymer is exposed to UV light through a transparency mask with binary motifs. We show that homogeneous features emerge under certain critical conditions that are also common to other, apparently unrelated, phenomena such as the emergence of macroscopic continuum properties of composite materials and the rates of ligand binding to cell membrane receptors. This fabrication method is simple and inexpensive, and yet it allows for the fabrication of microstructures over large areas (centimetres) with topographic modulation of features with characteristic dimensions smaller than 100 micrometres. Introduction Microfluidic systems are widely used across many fields such as analytical chemistry, 1 single molecule detection 2 and biological cell studies, 3 and in many applications that take advantage of the unique physical properties of microscale interfaces. 4 Planar lithography, the most common fabrication technology used in microfluidics, has been optimized for microelectronics, a binary world where it is critical to create high resolution two- dimensional (2D) patterns with vertical sidewalls (‘‘all-or-none’’ patterning). This invariably results in flat topographies; however, the ability to contour microscale topography in three dimensions (3D) is crucial for the microscale manipulation of liquids, which are inherently subject to 3D forces and form 3D interfaces. Although it is possible to generate multilevel features using planar lithography, the approach is time consuming (mask alignment and UV exposure needs to be repeated for each level), and the method does not allow generation of smooth transitions between levels. There are several technologies that can be used to create 3D topographies on a photoresist, such as e-beam lithography 5 and two-photon lithography 6 among others. 7–10 However these methods require special equipment, and in some cases, are time consuming for the fabrication of microfluidic devices where a compromise in feature resolution is often acceptable, 11 but the ability to pattern large areas (centimetres) is usually a must. Of special interest are the ‘‘grayscale technologies’’ where a photoresist is exposed to UV light through a mask that can be designed with several levels of light transmission. These masks can be continuous 12 (with excellent resolution and high cost), or discrete. In the last case (conventional grayscale lithography for MEMs), a mask having transparent and opaque pixels is used to expose a conventional photoresist to UV light. 13 The mask pixels are designed to have size and pitch (distance between pixels) below the optical resolution of the system (minimum feature that can be resolved), thereby, producing a smoothing effect. Different levels of light transmission are achieved by increasing or decreasing the size of a pixel within its pitch. This method has been shown to be extremely useful for MEMs fabrication, but it is still costly (requires expensive chrome masks) and is de facto not practical for patterning large areas because of the amount of data needed to define all of the pixels (area scales linearly with number of pixels squared). An alternative to chrome masks for grayscale lithography is to use inexpensive transparency masks and reduce the designs using an array of micro lenses. 14 Although this approach is cost-effective, it is only valid for features of #100 mm in diameter and can only generate arrays of repetitive features. Several fabrication technologies have been recently proposed for microfluidic fabrication due to the inadequacy of conven- tional grayscale technology to pattern large areas. These methods, which can be considered unconventional grayscale techniques, include (1) using microfluidic photomasks, 15 (2) overexposure of a photoresist to create non-linear polymeriza- tion 16 and (3) using colored masks to block UV light. 17 In the first case, a PDMS layer with microchannels is placed directly in contact with the photoresist that is going to be exposed to UV light. Dyes with different concentrations are flowed through the channels. Light transmission is a function of the dye concentration that is flowing through the channel at that particular moment. Since the flow is laminar, one can introduce gradients of different concentrations in a a Biochemical Science Division, NIST, Gaithersburg, USA. E-mail: laurie.locascio@nist.gov; Tel: +1 (301) 975-3130 b Polymers Division, NIST, Gaithersburg, USA { Electronic supplementary information (ESI) available: Figure S1– Determination of homogeneous/discrete patterns for the optical adhesive Norland R-81; and Figure S2–Fabrication of ejector plate with an array of horns. See DOI: 10.1039/b709369a PAPER www.rsc.org/loc | Lab on a Chip This journal is ß The Royal Society of Chemistry 2007 Lab Chip, 2007, 7, 1567–1573 | 1567 Downloaded by National Institutes of Standards & Technology on 24 April 2012 Published on 31 August 2007 on http://pubs.rsc.org | doi:10.1039/B709369A View Online / Journal Homepage / Table of Contents for this issue