What is PDMS used for in microfluid models?
PDMS in fact is established to be a reliable material for cell culture in many microfluidic devices. 
Here are several papers (Titles) including reviews on microfluidic devices using PDMS:


Microfluidic devices for cell cultivation and proliferation Here
Adhesion patterns in the microvasculature are dependent on bifurcation angle Here (Microvascular Research)
Advantages and challenges of microfluidic cell culture in polydimethylsiloxane devices Here
A physiological model of the tumor microenvironment for screening drug delivery systems Here (Cancer Research Proceedings)
A Novel Dynamic Neonatal Blood-Brain Barrier on a Chip Here (Plos One)
A novel microfluidic assay reveals a key role for protein kinase C δ in regulating human neutrophil–endothelium interaction Here(JLB - Journal of Leukocyte Biology)


My research at Temple University is actually based on a microfluidic device to study cell-cell interaction in a real-time fashion. 
Majority of established protocols are using fibronectin or collagen, and in the case of tumor cells culture, matrigel for extracellular matrix. Devices are being coated with the ECM protein and then cells can be cultured in the device. 

We have shown that in our microfluidic device, human umbilical vein endothelial cells (a.k.a. HUVECs) are forming a complete 3D lumen in the channels. 

It is important to note that due to the elasticity of the PDMS, cells are needed to be under physiological flow conditions. Otherwise, it was shown that the cells start to detach from the PDMS. Under such physiological conditions (flow, media, gas exchange), cells can be alive and functional up to a week or two, depending on the design. In our system, the last two publications, cells are functional up to 2 weeks and 1 week respectively.