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Abstract
Fabrication of microfluidic devices by soft lithography is by far the most popular approach due to its simplicity and low cost. The approach relies on casting of elastomers, such as polydimethylsiloxane (PDMS), on masters fabricated from photoresists on silicon substrates. These masters, however, can be expensive, complicated to fabricate, and fragile. Here we describe an optimized replica molding approach to preserve the original masters by heat molding of polycarbonate (PC) sheets on PDMS molds. The process is faster and simpler than previously reported methods and does not result in a loss of resolution or aspect ratio for the features. The generated PC masters were used to successfully replicate a wide range of microfluidic devices, including rectangular channels with aspect ratios from 0.025 to 7.3, large area spiral channels, and micropost arrays with 5 µm spacing. Moreover, fabrication of rounded features, such as semi-spherical microwells, was possible and easy. Quantitative analysis of the replicated features showed variability of <2%. The approach is low cost, does not require cleanroom setting or hazardous chemicals, and is rapid and simple. The fabricated masters are rigid and survive numerous replication cycles. Moreover, damaged or missing masters can be easily replaced by reproduction from previously cast PDMS replicas. All of these advantages make the PC masters highly desirable for long-term preservation of soft lithography masters for microfluidic devices.
Highlights
Soft lithography relies on casting of elastomers, such as polydimethylsiloxane (PDMS), on master molds fabricated from photoresists on silicon substrates [1,2]
The elastomeric replicas of silicon-photoresists (SPM) masters were realized by standard soft lithography using PDMS (Sylgard 184, Dow Corning): monomer was mixed in a 10:1 mass ratio with its curing agent, degassed, poured on the silicon-photoresist masters (SPMs), degassed again, and cured at 75 ◦ C for 2 h
The PC masters (PCMs) fabrication process starts with making a PDMS replica of the original master
Summary
Soft lithography relies on casting of elastomers, such as polydimethylsiloxane (PDMS), on master molds fabricated from photoresists on silicon substrates [1,2]. These silicon-photoresist masters (SPMs) offer excellent feature resolution and are conventionally fabricated by photolithography in a cleanroom using SU-8 negative photoresists. Photoresists generally perform poorly as structural materials due to delamination at the photoresist-silicon interface after a number of heating-cooling cycles due to repeated PDMS casting. This issue is more prominent for thicker resists and higher aspect ratio structures [5]. High roughness of the generated surfaces and the limitations in feature resolution, coupled with high cost of required equipment, limit their use
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