## 3 Fn Application and Involvement in Biomaterials and Medical Device Engineering
### Fn in Volume
#### 3.3.2 Fn Covalent Cross-Linking
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of Binding** & **Mechanism of Fn Addition** & **Polymer** & **Biological Activity** & **References** \\ \hline \multirow{8}{*}{Physical entrapment} & \multirow{8}{*}{Dispersed into a hydrogel} & \multirow{8}{*}{Alginate} & Improving cell proliferation and metabolic activity in rat Schwann cells and olfactory enschaeting cells; enhancing nerve separation in vivo & [104, 105] \\  & & & Increasing cell viability for the long-term culture of human induced pluripotent stem cells (inMSCs) and cell differentiation & [102] \\ \cline{2-5}  & & Alginate/gelatin/hyaluronic acid & Enhancing the proliferation, migration, and chondrogenic differentiation of rat endrogenic progenitor cells & [106] \\ \cline{2-5}  & & & Improving nerve regeneration in adult rats & [107] \\ \cline{2-5}  & & Collagen & Co-localizing and reorganizing Fn in fibrils after the seeding of fibronectin-null mouse embryonic fibroblasts & [108] \\ \cline{2-5}  & & & Improving the adhesion, spreading, and viability and migration of human apical peppilla cells; gene expression of \(\sigma\) integrin, \(\sigma\)V integrin, and type I & [109] \\ \cline{2-5}  & & & and type III collagens & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \hline \multirow{8}{*}{Covalent cross-linking} & \multirow{8}{*}{Fibrin} & \multirow{8}{*}{Increasing cell viability and tubular organization of HUVECs} & \multirow{8}{*}{[101]} \\  & & & Improving the cell adhesion and proliferation of MSCs expressing VAS & \\ \cline{2-5}  & & & protein in encapsulated cells & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \hline \multirow{8}{*}{Covalent cross-linking} & \multirow{8}{*}{Fibrin} & \multirow{8}{*}{Increasing cell adhesion and proliferation in human cardiovascular cell types} & \multirow{8}{*}{[114]} \\  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \hline Enzymatic cross-linking & \multirow{8}{*}{Fibrin} & \multirow{8}{*}{Increasing the cell adhesion and proliferation of MNSCs and HUVECs} & \multirow{8}{*}{[100, 115]} \\  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & & & & \\ \hline Cross-linking by radicals & Plasma immersion PA & Improving the cell adhesion and proliferation of human dermal fibroblast & [116] \\ \hline \end{tabular}
\end{table} 
Table 2: Fn in a three-dimensional (3D) matrix using different strategies and related biological activities. Moreover, Fn can also be covalently linked to synthetic-material-based hydrogels. For example, maleimide-functionalized PEG can react with cysteine residues of Fn and form covalent bonds with sulfhydryl groups [114; 118]. The resulting PEG-Fn, associated with PEG-heparin, formed a hydrogel that significantly enhanced the adhesion of cardiovascular cells to a greater extent than when PEG-RGD was used in place of PEG-Fn [114]. Other studies report Fn-covalent incorporation in Tetronic(r)-tyramide polymer via a one-step enzymatic reaction with H2O2 and horseradish peroxidase (HRP) [115; 10]. The cell adhesion and proliferation of hMSCs were remarkably influenced by the Fn concentration and covalent immobilization; at the highest concentration tested (50 mg/mL), mature F-actin fibers and higher cell proliferation were observed, whereas no improvement was observed when Fn was physically adsorbed on hydrogels [115]. Similar results were obtained in a PA hydrogel obtained by embedding radicals from plasma immersion ion implantation [116]. Fn was mixed into an acrylamide precursor prior to polymerization or coated on the surface of the formed hydrogel. The presence of radicals inside the hydrogels generated by plasma immersion is believed to covalently cross-link Fn to the hydrogel matrix. The researchers demonstrated significantly higher human dermal fibroblast adhesion and proliferation on both hydrogels functionalized with Fn compared to those on a bare hydrogel.