IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 21 (2011) 054013 (9pp) doi:10.1088/0960-1317/21/5/054013 Thermal fracture of oxidized polydimethylsiloxane during soft lithography of nanopost arrays Wes W Tooley, Shirin Feghhi, Sangyoon J Han, Junlan Wang and Nathan J Sniadecki 1 Department of Mechanical Engineering, Universityof Washington, Seattle, WA 98195-2600, USA E-mail: nsniadec@uw.edu Received 31 October 2010, in final form 29 January 2011 Published 28 April 2011 Online at stacks.iop.org/JMM/21/054013 Abstract During the fabrication of nanopost arrays for measuring cellular forces, we have observed surface cracks in the negative molds used to replicate the arrays from a silicon master. These cracks become more numerous and severe with each replication such that repeated castings lead to arrays with missing or broken posts. This loss in pattern fidelity from the silicon master undermines the spatial resolution of the nanopost arrays in measuring cellular forces. We hypothesized that these cracks are formed because of a mismatch in the coefficient of thermal expansion (CTE) of polydimethylsiloxane (PDMS) and its oxidized surface layer. To study the fracture of PDMS due to thermal effects, we treated circular test samples of PDMS with oxidizing plasma and then heated them to cause surface cracks. These cracks were found to be more abundant at 180 ◦ C than at lower temperatures. Finite element analysis of a bilayer material with a CTE mismatch was used to validate that thermal stresses are sufficient to overcome the fracture toughness of oxidized PDMS. As a consequence, we have ascertained that elevated temperatures are a significant detriment to the reproducibility of nanoscale features in PDMS during replica molding. S Online supplementary data available from stacks.iop.org/JMM/21/054013/mmedia (Some figures in this article are in colour only in the electronic version) Introduction Soft lithography is a set of manufacturing techniques for the fabrication of micro- and nanoscale structures [1]. It complements silicon-based fabrication techniques because it uses an elastic material, polydimethylsiloxane (PDMS), to replicate intricate features made in silicon or photoresist in a rapid and inexpensive manner [2]. Soft lithography is quite versatile in biological applications for it can be used to form fluid channels for biosensor platforms [3–5], flexible stamps for patterning biomolecules and cells [6], and topological surfaces for mimicking the native cellular environment [7]. PDMS is biocompatible and its surface can be functionalized with extracellular matrix proteins that facilitate the adhesion 1 Author to whom any correspondence should be addressed. of cells and allow for their long-term culture on PDMS devices [8, 9]. Since PDMS is also mechanically flexible, it can be used to replicate arrays of posts for measuring cellular forces [10–13]. Recently, nanopost arrays have been produced, which provide better spatial resolution of cellular forces than micropost arrays due to the higher density of flexible PDMS post sensors underneath a cell (figure 1)[12]. PDMS is an organosilicon polymer that is synthesized from dimethylsiloxane oligomers with vinyl-terminated end groups, dimethylvinylated and trimethylated silica filler, a platinum catalyst, a cross-linking agent, dimethylmethylhydrogen siloxane, and an inhibitor, tetramethyltetravinyl cyclotetrasiloxane [8]. This mixture undergoes a hydrosilylation reaction which results in silicon- carbon bonding between the siloxane oligomers and silica filler to create a solid, elastic material. Heat is typically added 0960-1317/11/054013+09$33.00 1 © 2011 IOP Publishing Ltd Printed in the UK & the USA