Figure 3.

Thickness gradient membrane fabrication using a custom-made PMMA device. A) Illustration of the PMMA device designed with an adjustable top opening as the only path for parylene deposition on the enclosed silicon wafer. The restricted diffusion path for parylene monomers before polymerization at the wafer surface leads to a linear decrease in thickness from the top of the wafer down. B) Increases in the PMMA device opening results in a nearly linear decrease in thickness gradient across the membrane. C) Thickness difference across a frame-supported UPP membrane with a length of 2 cm. SEM images of different sites of frame supported membrane reveal a consistent pore size and distribution across different thicknesses (scale bar=1 μm).