## 3 Fn Application and Involvement in Biomaterials and Medical Device Engineering
### Fn 2D Molecular Coatings
#### 3.1.1 Fn Covalent Binding
This Fn-covalent-grafting strategy permits the obtention of a normal cell morphology, higher cell adhesion, and the proliferation of NIH3T3 fibroblast cells [30], primary human foreskin fibroblasts [31], chondrocytes [32], and porcine radial arterial and endothelial cells [33]. Other strategies for Fn covalent immobilization on hydrogel surfaces using anvidin-biotin-binding system have also been described in the literature [34], as have those involving surface activation with hydrazine hydrate [35] or via a polydopamine-coating process [36]. Recently, [PERSON] et al. reported the biofunctionalization of bacterial nanocellulose-hydrogel-based vascular grafts (BNCs) with human albumin, Fn, or heparin-chitosan [37]. Briefly, BCN surfaces were activated with 1-cyano-4-dimethylamino pyridinium tetrafluoroborate (CDAP). Bioconjugation with proteins was induced by the formation of isoura bonds with amino acids. The Fn bioactivity regarding human saphenous vein endothelial cell (VEC) and endothelial progenitor cell (EPC) adhesion and proliferation was remarkable compared to that of albumin or heparin coatings [37]. The grafting processes described above remain difficult to implement, as they involve many complex steps. Therefore, [PERSON] et al. developed a simple and low-cost method for the immobilization of Fn on chitosan using N-(3-dimethylaminopropyl)-N\({}^{\prime}\)-ethylcarbodiimide (EDC) as a cross-linker [38]. They showed that human-osteoblast-like cell adhesion is promoted on chitosan functionalized with immobilized Fn and that the long-term proliferation of cells is enhanced compared to that on surfaces with adsorbed Fn. On the contrary, adsorbed Fn has been shown to desorb over time, as only weak and unspecific forces are involved in the protein's immobilization on the surface (hydrogen bonds, Van der Waals forces, and electrostatic or hydrophobic interactions). This simple technique was successfully used to functionalize different hydrogel surfaces such as fibrinogen [39], polyethylene glycol diacrylate (PEGDA)/polyacrylamideamide (PA) [40], poly(N-isopropylacrylamide) [41], and PVA [42]. A homogeneous distribution of Fn on all surfaces and a good cell-hydrogel interaction via improved cell adhesion and proliferation has been highlighted. Another similar and simple approach using N-hydroxy succinimide (NHS) to covalently attach Fn has also been described. The cross-linking between the NHS-ester of modified PEG-NHS and free amino groups on Fn has also been reported [43, 44]. [PERSON] et al. demonstrated a better cell adhesion and proliferation of human fetal lung fibroblasts, human aortic smooth muscle cells (HASMCs), and human aortic endothelial cells (HAECs) in a PEG-Fn hydrogel compared to that in a bare PEG hydrogel or PEG-gelatin hydrogel [43]. Recently, the modification of a hydrogel surface with sulfo-SANPAH (a molecule containing an NHS-ester and a photoactivatablenitrophenyl azide) to cross-link Fn has been reported [45, 46, 47]. [PERSON] et al. showed the biofunctionalization of a PA hydrogel surface with ECM proteins such as Fn, type I collagen, type IV collagen, and laminin using sulfo-SANPAH. They reported that Fn and laminin biofunctionalization results in a more homogeneous distribution than collagen type I or IV, using immunostaining. Moreover, the quantity of Fn immobilized at the surface is higher than those for other ECM proteins [46]. The covalent binding of Fn to the surface can affect Fn's conformation and, thus, bioactivity. [PERSON] et al. showed that Fn can be anchored via its lysyl or cysteyl moieties on a PTFE or silica treated with ammonia plasma via two conjugation strategies using glutaratic anhydride (GA) and sulfosuccinidyl 4-(p-maleimidophenyl) butyrate (SMPB) cross linking agents. They showed that Fn adopts different conformations and that the covalent binding of Fn via its lysyl group is more favorable for cell adhesion [48; 49].