Fig. 3. Range of natural and synthetic polymer building blocks. Molecular structures of typical polymer repeat units used for preparation of cell compatible hydrogels: (A) hyaluronic acid, (B) chitosan, (C) heparin, (D) alginate, (E) linear poly(ethylene glycol) (PEG), (F) four-arm PEG, (G) poly(ethylene glycol)-b-poly(propylene oxide)-b-poly(ethylene glycol) (PEG-PPO-PEG), (H) poly(lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly(lactic acid-co-glycolic acid) (PLGA-PEG-PLGA), and (I) poly(vinyl alcohol).