Silicon Wafer Preparation for Cell Culture

N-type [100]-orientation silicon wafers with 500 nm or 1.9 μm of silicon oxide were purchased from Addison Engineering. Wafers with 500 nm or greater silicon oxide were selected to maximize interference contrast (Supplementary Fig. 12). Wafers were cut into approximately 1 cm × 1 cm chips by scoring with a diamond-tip pen. They were cleaned by sonicating in acetone for 20 minutes, rinsing with water, sonicating for 20 minutes in 1 M potassium hydroxide, and rinsing with water. The wafers were then chemically activated to enable conjugation of extracellular matrix (ECM) proteins for cell adhesion. The wafers were incubated for one hour in 0.5% (3-aminopropyl)trimethoxysilane (APS) in water, sonicated five times in water for five minutes to remove excess APS, incubated for one hour in 0.5% gluteraldeyde in phosphate buffered saline (PBS; pH = 7.4), sonicated five times in water for five minutes, and dried under nitrogen gas. The wafers were sterilized under a UV lamp and incubated overnight at 4 °C in 10 μg ml⁻¹ human plasma fibronectin (Millipore).