Rat collagen 1

Polyacrylamide (PA) hydrogels were manufactured as described before [17] (link) [18] (link). Briefly, the solutions of 1 kPa PA gel was prepared by mixing 6% acrylamide (Sangon Biotech, China) and 0.05% bis-acrylamide (Sangon Biotech, China), and the solutions of 20 kPa PA gel was prepared by mixing 8% acrylamide and 0.264% bis-acrylamide. Polymerization was initiated with 0.1% tetramethylethylenediamine (TEMED, Klamar, China) and 0.01% ammonium persulfate solution. To make a substrate suitable for each well in a 6-well culture dish, 200 µl of the final solution was dropped onto a glass slide pre-treated with gel slick (Lonza, Switzerland), and a silanized coverslip 30 mm in diameter was placed on the top of the solution. The PA gel yielded was 30 mm in diameter and 200 μm in thickness. After 10 min of polymerization, the glass slide was removed and the coverslip with the gel attached was treated with sulfo-SANPAH (ProteoChem, USA) under UV for 15 min and coated with 0.1 mg/ml rat collagen I (Corning, USA). Before cell seeding, the gels were sterilized by UV for 30 minutes and 75% ethanol.