Details of device fabrication and cell culture can be found in the Electronic Supporting Information (S1 and S2) . The Visisens handheld microscope was mounted vertically in a variable incubator and the microfluidic devices (bonded to sensor foils and filled with collagen gel and cells) were placed on top. The oxygen partial pressure in the device was monitored in the central 5 by 2.5 mm2 area including the gel and adjoining media channels. All oxygen measurements were evaluated using the Visisens software (AnalytiCal 1, VA 1.11 prototype version) after calibration against two reference solutions. Oxygen-free solution was prepared by dissolving 1 g of sodium sulfite in 100 ml water with 50 ul of Co(NO3)2 standard solution (1g l−1 in 0.5 M nitric acid). Water bubbled with air for 30 min was used as a second calibration point (air-saturated). Due to the opaqueness of the oxygen-sensitive coating, control devices with a glass coverslip bottom were fabricated to facilitate visualization of cell morphology and distribution (Fig. S1 ). Excellent viability over 2 days of culture in all devices was observed, confirming earlier results with human endothelial cells.25 (link) Phase-contrast images were taken with a CCD color digital camera (Olympus DP72, Japan) connected to an inverted Olympus IX71 microscope. Images were captured with DP2 BSW Software (version 2.2) and analyzed by ImageJ software (Scion Corp., USA).
The oxygen consumption in the devices during EC or HEP culture was simulated using a commercial finite element software (COMSOL Multiphysics v4.2). Transport of diluted species equation (Fick’s law) was adopted as the governing equation, considering the molar concentration of oxygen c as dependent variable. The computational mesh consisted of ~1.8×106 tetrahedral elements with a total volume of 410 mm3. To reduce the calculation time, only a quarter of the device was considered by imposing a symmetry condition on the two cutting planes. The cells were represented as a thin (10 μm) layer on top of the PDMS glue layer. A no-flux condition was implemented to represent the impermeable oxygen sensor foil. A constant concentration of oxygen (17%) was considered as boundary condition on the surfaces exposed to the external environment (fluid channel and gel injection inlet). A time dependent simulation was solved for a period of 4 h at 10 min intervals. The diffusion coefficients and initial oxygen concentration in the different materials are summarized below.
The oxygen consumption in the devices during EC or HEP culture was simulated using a commercial finite element software (COMSOL Multiphysics v4.2). Transport of diluted species equation (Fick’s law) was adopted as the governing equation, considering the molar concentration of oxygen c as dependent variable. The computational mesh consisted of ~1.8×106 tetrahedral elements with a total volume of 410 mm3. To reduce the calculation time, only a quarter of the device was considered by imposing a symmetry condition on the two cutting planes. The cells were represented as a thin (10 μm) layer on top of the PDMS glue layer. A no-flux condition was implemented to represent the impermeable oxygen sensor foil. A constant concentration of oxygen (17%) was considered as boundary condition on the surfaces exposed to the external environment (fluid channel and gel injection inlet). A time dependent simulation was solved for a period of 4 h at 10 min intervals. The diffusion coefficients and initial oxygen concentration in the different materials are summarized below.
