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Polydimethylsiloxane (PDMS) belongs to a group of polymeric organo-silicon compounds that are commonly referred to as silicones. PDMS is non-toxic, optically clear, bio compatible, flexible and chemically stable. The PDMS chemical structure can be represented as:
PDMS can be synthesized with a variety of end groups suitable for a wide variety of applications. The PDMS used in soft lithography, in particular, is methyl group terminated. In addition the base polymer is available in different molecular weights and viscosities. LNF stocks Sylgard 184 (from Dupont Corp.) which has a viscosity in the 4000 - 6500 cSt.
Typically PDMS is etched with a mixture of O2 and a fluorine-containing gas such as SF6 or CF4.
- SiO2 Conversion - The surface of PDMS can be converted into an SiO2 like material by selectively etching the hydrocarbon with ozone.
- PDMS-PDMS and PDMS-glass adhesion
This treatment, commonly used in the asembly of microfluidics, is done to modify the surface of the PDMS to facilitate adhesion. This is done in the Glen 1000P Plasma Cleaner which is part of the microfluidic fabrication suite.
- PDMS is usually patterned by molding or soft lithography. Its chemical innertness, makes it difficult to wet etch and dry etch. Usually the dry etch of PDMS yields rough surfaces and in most of the cases residue. Several pares have been written about a combination of dry and wet etching 
- PDMS is permeable to gases and solvents, this is why it used as a permeable membrane in some microfluidic applications, but when un vacuum it looses volume and expands when brough back into air. This is a problem when it is needed as a substrate for metal deposition. PDMS swells when immersed in a solvent such as acetone, this volume change should be kept in mind when patterning PDMS with photoresist.
- In general PDMS is low surface energy material, plasma surface modification can selectively change the surface energy 
- Uncured PDMS is often used as an adhesion layer between PDMS layers
PDMS in Soft Lithography
For acomplete description of Soft Lithography in LNF and particular instructions how to mix and work with PDMS go to
Technical Data Sheet
- Applied Surface Science 258 (2012) 9427– 9431