Acrylamide gel substrate preparation

Variable stiffness polyacrylamide gels were prepared as previously described (34, Pelham and Wang, 1997). Gels were tuned to specific elasticities by varying the concentration of Bis-acrylamide added. Gels were coated with 50mM sulfosuccinimidyl 6 (4’-azido-2’-nitrophenyl-amino) hexanoate (Sulfo-SANPAH, Pierce) in HEPES, pH 8.5 buffer and exposed to UV light for 10 min to photoactivate the crosslinker. Sulfo-SANPAH was removed, washed with HEPES buffer, and incubated with 0.2mg/mL type I collagen (Sigma) solution overnight at 4°C. Prior to use, gels were washed with 1xPBS.