text: Polymerization time of fibrinogen and thrombin is affected primarily by concentration of thrombin and temperature, while fibrinogen concentration has a minor effect. Fibrin gel characterization by scanning electron microscopy reveals that thick fibers make up a dense structure at lower fibrinogen concentrations (5 mg/ml) and thinner fibers and looser gel can be obtained as fibrinogen concentration (20 mg/ml) increases whereas increase in thrombin concentration (from 0.5 U/ml to 5 U/ml) has no such significant result although the fibers steadily get thinner.
  Fibrin gels can be enriched by addition of other extracellular matrix (ECM) components such as fibronectin, vitronectin, laminin and collagen. These can be linked covalently to fibrin scaffold by reactions catalyzed by transglutaminase. Laminin originated substrate amino acid sequences for transglutaminase can be IKVAV, YIGSR or RNIAEIIKDI. Collagen originated sequence is DGEA and many other ECM protein originated RGD sequence can be given as other examples. Heparin binding sequences KβAFAKLAARLYRKA, RβAFARLAARLYRRA, KHKGRDVILKKDVR, YKKIIKKL are from antithrombin III, modified antithrombin III, neural cell adhesion molecule and platelet factor 4, respectively. Heparin-binding growth factors can be attached to heparin binding domains via heparin. As a result, a reservoir can be provided instead of passive diffusion by liberation of growth factors in extended time. Acidic and basic fibroblast growth factor, neurotrophin 3, transforming growth factor beta 1, transforming growth factor beta 2, nerve growth factor, brain derived neurotrophic factor can be given as examples for such growth factors.
  For some tissues like cartilage, highly dense polymeric scaffolds such as polyethylene glycol (PEG) are essential due to mechanical stress and that can be achieved by combining them with natural biodegradable cell-adhesive scaffolds since cells can not attach to synthetic polymers and take proper signals for normal cell function. Various scaffold combinations with PEG-based hydrogels are studied to assess the chondrogenic response to dynamic strain stimulation in a recent study. PEG-Proteoglycan, PEG-Fibrinogen, PEG-Albumin conjugates and only PEG including hydrogels are used to evaluate the mechanical effect on bovine chondrocytes by using a pneumatic reactor system. The most substantial increase in stiffness is observed in PEG-Fibrinogen conjugated hydrogel after 28 days of mechanical stimulation.