Fig. 3. Synthetic base materials for the construction of hydrogels used in fibroblast culture. (A) Polyacrylamide hydrogels (right) are commonly formed from acrylamide and N,N′-methylenebisacrylamide and often used in 2D fibroblast culture. For example, human pulmonary fibroblasts (left) have been observed to proliferate on a collagen-coated polyacrylamide substrate with a stiffness gradient. As the modulus of the polyacrylamide substrate increased, fibroblasts proliferated to a greater degree and had highly organized cytoskeletons (red f-actin).¹⁶ Adapted from references 16 and 147. (B) Polyethylene glycol (PEG) has been used in 2D and 3D fibroblast culture. PEG hydrogels, like the one shown on the right, can be formed by functionalizing PEG with acrylate end groups and then polymerizing with an acrylate-functionalized peptide.¹⁴⁷ Proteinase-degradable peptides often have been used in PEG hydrogels for 3D cell culture because they allow the matrix to be degraded and invaded by the encapsulated fibroblasts. Here, 3T3 fibroblasts are observed spreading in degradable PEGDA hydrogel containing basic fibroblast growth factor (bFGF) (time lapse images). bFGF was found to induce cell spreading within the gel over the course of 15 days.⁸¹ Adapted from references 81 and 147.