Slabs were made by end-milling (Minitech CNC; ~1-μm XYZ resolution) a circular pocket with a diameter of 2.5 cm into acrylic squares with dimensions of 3 cm by 3 cm to a depth ranging from 140 to 400 μm. These pockets were filled with a PDMS prepolymer. The PDMS prepolymer was mixed and degassed before pouring. Excess PDMS was squeezed out by placing a silicon wafer flush against the acrylic square. Before casting, the silicon wafer was spin-coated with a thin (≪1 μm) layer of 5% (w/w) PMMA/anisole at 2000 rpm to form a release layer. This acrylic-PDMS prepolymer–silicon wafer construct was then cured at 60°C for 1 hour in an oven to cross-link the elastomer. Micropatterned features were fabricated into PDMS by replica molding. PDMS slabs were cured against micropatterned wafers (see the “Micropatterning silicon wafers” section for procedure for micropatterning). Slabs that did not require micropatterning were molded against an unpatterned (smooth) silicon wafer to ensure a consistent level of roughness between all slabs. The Young’s modulus of the PDMS was controlled by mixing different prepolymer base:cross-linker ratios. We fabricated slabs using PDMS mixed at ratios of 5:1, 12:1, and 30:1 (base to cross-linker) and measured the Young’s moduli of 3.12 ± 0.11 MPa, 0.75 ± 0.05 MPa, and 0.11 ± 0.01 MPa respectively, using a uniaxial tensile test (Mark-10, Instron). These moduli are referred to as 3, 0.8, and 0.1 MPa, respectively. Slabs were then exposed to UV/ozone (NovaScan) for 4 hours to permanently remove any adhesive tack from PDMS (36).

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