Fabrication of complex multilevel microchannels in PDMS by using three- dimensional photoresist masters Kwang-Seok Yun* a and Euisik Yoon b Received 22nd August 2007, Accepted 21st November 2007 First published as an Advance Article on the web 3rd December 2007 DOI: 10.1039/b712932g This paper demonstrates a new method of implementing complex microchannels in PDMS, which is simply constructed using three-dimensional photoresist structures as a master mold for the PDMS replica process. The process utilizes UV-insensitive LOR resist as a sacrificial layer to levitate the structural photoresist. In addition, the thickness of photoresist structures can be controlled by multi-step UV exposure. By using these techniques, various three-dimensional photoresist structures were successfully implemented, including the recessed cantilevers, suspended bridges, and the complex plates with micro-pits or micro-villi. We demonstrate that the three-dimensional photoresist structures are applicable to implementing complex multiple microchannels in PDMS by using the PDMS replica method. 1. Introduction In the last few years, polydimethysiloxane (PDMS) has been largely employed for implementation of microfluidic devices. 1–6 The advantages of PDMS are many: 7 optical transparency in a broad range of spectrum, bio and chemical compatibility with safe use, 8,9 low cost, easy and superior bonding property, 10 low water permeability 11,12 and simple processing using the micro molding technique. 7,13 The typical PDMS molding is a process of casting PDMS prepolymer into structured PDMS in solid state, which is composed of three steps: (1) pouring of PDMS prepolymer onto the structured master, (2) thermal curing, and (3) peeling-off of cured PDMS from the master. The structured PDMS can be utilized in a further process through bonding with other plates, such as silicon, glass, plain PDMS, and other structured PDMS. 7,13,14 For casting of PDMS prepolymer, a variety of masters have been employed, including structured silicon or glass plate, electroplated metal on substrate, and most frequently, a patterned photoresist on substrate because of its easy forma- tion on substrate by simple photolithography. Although a simple single-level microchannel in PDMS can be easily formed by those masters, there are growing demands for more complex multilayer structures, such as multilevel micro- channels or buried microchannels which are indispensable to implement integrated microfluidic components, 15 micro analytical systems, 16 and integrated microfluidic networks. 17 To date, the buried PDMS channel or multilevel PDMS channel structures have been formed by stacking and bonding of patterned PDMS plates. 5,6,14,15,17–19 These methods require an alignment between PDMS plates, which is not compatible with conventional alignment tools and is easily susceptible to large misalignment. In addition, the via structures, which connect microchannels in different levels, are formed by phy- sical clamping 5,14,18 or spin coating of PDMS prepolymer. 20 This requires a labor-intensive PDMS process, resulting in poor reproducibility. The other method to obtain multilevel microchannels is to utilize the three-dimensional suspended photoresist structures as masters for PDMS replica processes. It has been reported that a negative photoresist, such as SU-8, can easily be made into three-dimensional structures, such as suspended cantilevers or buried channels using photolithographic tech- nology. 21–25 However, such a negative photoresist is hard to remove, and this difficulty restricts its application to sacrificial master structures for PDMS structures formation. Although there are several methods of obtaining three-dimensional masters through three-dimensional MEMS techniques, most of them have the same problem that the negative photoresist has. To address these issues, our research group has previously developed and reported a technique of forming suspended three-dimensional positive photoresist structures on a glass wafer using backside UV exposure. 26 However, this method can be adopted only on transparent substrate, and it requires additional metal deposition/patterning for the backside UV mask. Another problem is a long photoresist development time. The fabrication technique for similar three-dimensional photoresist structures on an opaque substrate like silicon has also been reported on. 27 We adopted LOR resist (Microchem. Co.), which is used for the lift-off process as a sacrificial photoresist. We have fully utilized the properties of LOR resist: insensitivity to UV exposure and selective development in some specific developers. This paper reports on the details of a fabrication technique for the formation of various three-dimensional positive photoresist structures. This technique can be generally applied without limitation in substrate types. Also as an application of this technique, the formation of three-dimensional PDMS microchannel structures will be demonstrated. a Department of Electronic Engineering, Sogang University, 1 Shinsoo- dong, Mapo-gu, Seoul, Korea. E-mail: ksyun@sogang.ac.kr b Department of Electrical and Computer Engineering, University of Minnesota, 5-125 EE/CSci Bldg, 200 Union Street S.E., Minneapolis, MN 55455, USA PAPER www.rsc.org/loc | Lab on a Chip This journal is ß The Royal Society of Chemistry 2008 Lab Chip, 2008, 8, 245–250 | 245