# Nano-Fibrous Networks from Co-Assembly of Amphiphilic Peptide and Polyelectrolyte
## 1 Introduction  
Electrostatic interactions are known to provide a strong driving force towards stable co-assemblies of molecules, polyelectrolytes, or particles. If the presence of electrostatic charges ensures the solubility or the dispersibility of the objects, then charge compensation will often lead to an anarchical precipitation of the ensembles. Therefore, an additional strategy is required to build a material in order to control and predict its structural organization. For example, in the layer-by-layer technique (LbL), a polyelectrolyte is first coated onto a substrate, the excess is rinsed off and then a second complimentary charged polyelectrolyte is coated to grow the film [1, 2, 3]. In other cases, interesting complexes were scaled up forming nanomaterials made from oppositely charge precursors through charge compensation after mixing or diffusion of their aqueous solutions [4, 5, 6]. A particularly suitable molecule is peptide amphiphiles (PA) since they can self-assemble in a robust and predictable way. They can form gels and can have a charged sequence in its structure. PA are a family of self-assembling molecules consisting of a hydrophobic alkyl chain attached to a peptide sequence [7, 8]. Interestingly, if a \(\beta\)-sheet forming peptide sequence is inserted, then the assembly is driven towards mono dimensional nanostructures such as cylindrical nanofibers [9]. A peptide sequence carrying charges could be incorporated as well in order to improve the amphiphilicity and possibly to bring bioactivity in the frame of biomedical applications [10]. The presence of charges has been used to trigger a higher level of assembly and obtain for example nanofibrous materials through controlled charge neutralization or throughelectrostatic interactions. As an example, [PERSON] et al. have reported Lauryl-VVAGK PAL oligonucleotide (ODN) hydrogel thanks to a charge screening for the cellular delivery of ODN in the scope of gene therapy [11]. In another work, a complex between heparin and PA has been studied by [PERSON] et al. as a binding gel substrate for signaling molecules such as angiogenic growth factor. Here, it is the rigidity of an amphiphilic peptide network to be compared to that of a hydrogel formed from a polymer which was sought so as to better orient the growth factors and thus maximize their activity [12]. The combination of two oppositely charged PAs was found to self-assemble into nanofibers, enabling the bringing of different bioactive epitopes into a gel network [13]. The same strategy was later used in conjunction of building an independent poly(ethylene glycol) hydrogel as a versatile synthetic mimics of natural extracellular matrix for tissue engineering and regenerative medicine applications [14]. A very large contribution in this field can be attributed to the team of [PERSON]. [PERSON], with in particular the preparation of nanofibrous networks formed from a high molecular weight polyanion (hyaluronic acid, HA) and a positively charged PA C\({}_{16}\)V\({}_{3}\)A\({}_{3}\)K\({}_{3}\)[15]. Each component in a separated aqueous solution was put into contact and a well-ordered solid assembly was formed in milliseconds at the interface to yield hierarchical membranes with tunable water permeability [16]. The intrinsic structure of these membranes depends directly on the nature of the interactions between HA and PA [17] and can be altered on demand by an electric field [18]. Other polysaccharides were more recently tested such as alginate which self-assemble with Pro-K-(Phe-K)\({}_{5}\)-Pro into nanofiber network [19]. A LbL technique between alginate and lauryl-V\({}_{2}\)AGK\({}_{3}\)-Am PA has amplified the preparation scale towards multilayered biomaterials by repeating the alternate deposition of both components [20]. Self-assembling peptides-alginate composite hydrogel has demonstrated improved mechanical properties to serve as injectable biomaterial for bone regeneration [21].