Fig. 4. Approaches to biological functionalization of hydrogel culture systems. (A) Heparan sulfate is found throughout the ECM and is known to be an important mediator of cytokine binding and presentation; to mimic this, heparin has been incorporated within hydrogel-based culture systems for the purpose of biological functionalization. For example, four-arm PEG has been functionalized with an RGD adhesive peptide sequence (PEG(RGD)) or fibronectin (PEG(FN)) and crosslinked with low molecular weight heparin (LMWH) or LMWH functionalized with RGD (LMWH(RGD)). Hydrogels were formed from these materials using thiol–maleimide click chemistry (top). The resulting hydrogels were used to culture fibroblasts, where fibroblasts were observed to spread preferentially on hydrogels containing fibronectin, as determined by immunostaining for f-actin (green). The addition of LMWH to PEG(RGD) gels also increased fibroblast spreading, but the addition of LMWH(RGD) did not, indicating that the process of conjugating RGD to LMWH may have inhibited protein function.94 Adapted from reference 94. (B) Cytokines and growth factors are secreted by resident cells, as well as recruited immune cells, during healing. To capture this within model systems, these soluble proteins have been added to the culture medium or functionalized to the scaffold. Here, rat subcutaneous fibroblasts (green f-actin) were cultured on collagen lattices (inset A) and TGFβ1 was added (inset B), significantly increasing the number of activated cells (red αSMA).6 Adapted from reference 6.
