2.3. Polyacrylamide Hydrogels (PA gels) Preparation

Hydrogel manufacturing was performed as previously described [34,35]. Briefly, acrylamide and bis-acrylamide were mixed to their desired concentrations in distilled H₂O. 1 % v/v ammonium persulfate (APS, Sigma-Aldrich, St. Louis, MO, USA), and 0.1% v/v methylethylenediamine (TEMED, Sigma-Aldrich, St. Louis, MO, USA) were added to gel solution and vortexed quickly. Twenty-five μL gel solution was quickly pipetted onto the chloro-silanated side of the glass slides, and the amino-silanated coverslips were covered. The gel was allowed to polymerize for 5 to 30 min. The PA gels were coated with 0.2 mg/mL rat-tail collagen type I (Sigma-Aldrich, St. Louis, MO, USA) via sulfosuccinimidyl 6-(4′-azido-2′-nitrophenylamino)hexanoate (sulfo-SANPAH; Sigma-Aldrich, St. Louis, MO, USA) crosslinker. Collagen-coated PA gels were sterilized with UV for 15 min and then soaked in the full medium for at least 30 min immediately prior to use.