## 3 Fn Application and Involvement in Biomaterials and Medical Device Engineering
### Fn in Volume
#### 3.3.1 Fn Physical Dispersion  
Some remarkable studies dealing with Fn physical dispersion have been reported for alginate-based hydrogels [98; 100; 101; 113]. For instance, [PERSON] et al. investigated the addition of Fn in an alginate solution before physical gelation with Ca\({}^{2+}\) and its impact on nerve regeneration. Schwann cells (SCs) were encapsulated in an alginate or alginate-Fn hydrogel. The alginate-Fn hydrogel appeared to clearly improve cell proliferation and metabolic activity. In vivo studies in a peripheral-nerve-injury model of SCs encapsulated in an alginate-Fn hydrogel demonstrated a steady progression of axons up to six weeks [100]. Interestingly, similar results were obtained by [PERSON] et al., with an increase in the metabolic activity of olfactory ensheathing cells and the improvement of neurite sprouting from dorsal root ganglion in an alginate-Fn hydrogel compared to that in a bare alginate hydrogel [101]. The differentiation of stem cells in alginate-Fn physical hydrogel beads has also been evaluated. [PERSON] et al. described the differentiation of human-induced pluripotent stem cells (hiPSCs) to dopaminergic (DA) neurons. They showed higher cell viability for the long-term culture (20 days) of encapsulated hiPSCs in alginate-Fn hydrogel beads compared to alginate alone and an increase in the transcription-factor (i.e., LMX1A and FOX2) expression of DA neurons over time [98]. The advantage of using this 3D culture system was demonstrated by the presence of synaptic connections formed by 3D neurons. An interpenetrating polymer network hydrogel composed of alginate/gelatin/hyaluronic acid polymers was fabricated through dual cross-linking with a temperature change and the addition of Ca\({}^{2+}\) for a 3D impression [102]. Fn was added to the mixture before gelation, and chondrogenic progenitor cells (CPCs) were encapsulated into the hydrogel. Although the Fn was released from the hydrogels (a burst release within 8 days was observed, followed by a sustained release up to 14 days), the cells encapsulated in the Fn-functionalized hydrogel showed an increase in chondrogenic marker (COL2, PRG4, and SOX9) expression compared to those in a bare hydrogel [102]. The potential of using a collagen-based system was also explored. Several studies report Fn entrapped within collagen hydrogels [103; 104; 114; 115]. For example, [PERSON] et al. investigated the regeneration of a critical sciatic nerve gap (15 mm) in rats. In this case, they compared the effect of the functionalization of laminin or Fn in a collagen type I hydrogel within chitosan conduits and showed an improvement in nerve regeneration in the collagen-Fn hydrogel [103]. Recently, [PERSON] et al. evaluated the protein organization and biological activity of a collagen-Fn hydrogel using an ultrasound methodology. Fn was added to a neutralized collagen solution before ultrasound exposure at a constant frequency. A pattern of radially aligned collagen fibers with co-localized Fn was observed in the ultrasound-exposed samples. Fn-null mouse embryonic fibroblastswere seeded on top of these hydrogels, and the reorganization of Fn in fibrils was only observed in the ultrasound-exposed samples [104]. A composite collagen/gelatin hydrogel was biofunctionalized with Fn in order to improve the chemotactic and bioactive action on human apical papilla cells for dental-pulp regeneration [105]. The study showed that the entrapment of Fn at a concentration of 10 \(\upmu\)g/mL promoted cell viability and cell migration through a membrane compared to the collagen/gelatin hydrogel alone. The higher gene expression of \(\alpha\)5 integrin (ITGA5), \(\alpha\)V integrin (ITGV), type I collagen (COL1A1), and type III collagen (COL3A1) was also observed. In other studies, a PEG-based system was considered. PEG-diacrylate (PEGDA) hydrogels were combined with natural polymers, such as collagen [106] and fibrinogen [107]. In these cases, Fn was physically entrapped within hydrogels to provide an appropriate 3D environment for stem cells. [PERSON] et al.