PDMS leaching and its implications for on-chip studies focusing on bone regeneration applications

Polydimethylsiloxane (PDMS) is among the most widely used materials for organ-on-chip systems. Despite its multiple beneficial characteristics from an engineering point of view, there is a concern about the effect of PDMS on the cells cultured in such devices. The aim of this study was to enhance the understanding of the effect of PDMS on cellular behavior in a context relevant for on-chip studies. The focus was put on an indirect effect of PDMS, namely leaching of uncrosslinked oligomers, particularly for bone regeneration applications. PDMS-based chips were prepared and analyzed for the potential release of PDMS oligomers within the microfluidic channel when kept at different flow rates. Leaching of uncrosslinked oligomers from PDMS was quantified as silicon concentration by inductively coupled plasma - optical emission spectrometry and further confirmed by mass spectrometry. Subsequently, PDMS-leached media, with a silicon concentration matching the on-chip experiment, were prepared to study cell proliferation and osteogenic differentiation of MC3T3-E1 pre-osteoblasts and human mesenchymal stem cells. The silicon concentration initially detected in the media was inversely proportional to the tested flow rates and decreased to control levels within 52 h. In addition, by curing the material overnight instead of 2 h, regardless of the curing temperature (65 and 80 °C), a large reduction in silicon concentration was found, indicating the importance of the PDMS curing parameters. Furthermore, it was shown that PDMS oligomers enhanced the differentiation of MC3T3-E1 pre-osteoblasts, this being a cell type dependent effect as no changes in cell differentiation were observed for human mesenchymal stem cells. Overall, this study illustrates the importance of optimization steps when using PDMS devices for biological studies, in particular PDMS curing conditions and extensive washing steps prior to an experiment.