# Routes towards Novel Collagen-Like Biomaterials
## 2 Collagen as a Biomaterial
### Collagen-Analogues as Basis for Future Biomaterials
In addition, the trimerizing V-domain has been shown to be an effective collagen nucleation inducer, even for non-streptococcal collagens and collagen-mimetic peptides [20]. These bacterial collagen-like proteins can be recombinantly produced in high yields in common bacteria such as _E. coli_. When the immunogenic V-domain is proteolytically cleaved and the remaining collagen cross-linked to increase the thermal stability, they have been shown to be non-immunogenic when implanted into mice for up to 6 weeks [63]. However, it is unclear whether long-term presence and degradation of bacterial collagen-like proteins exposes the host to short, immunogenic peptides, since several examples are known that show different immunogenicity between triplehelical and denatured collagen [64; 65; 66; 67]. Nevertheless, since these proteins circumvent all the problems associated with the production of eukaryotic collagens in bacterial hosts mentioned earlier, they are strong candidates for future materials based thereon; however, their low melting point of 36-38 \({}^{\circ}\)C requires the presence of a non-collagenous folding domain which has been shown to be strongly immunogenic [63] and necessitates other means of stabilization, such as glutaraldehyde crosslinking, once this domain has been removed. Increasing the stability of the Scl2 protein by multimerization does not significantly increase its melting point [68]. Therefore, while the low temperature stability of the Scl proteins is of no consequence for the production of three-dimensional scaffolds [69; 70; 71], it hinders the spinning of stable collagen fibers. The three collagen-like proteins found in sawfly silk (SfC A-C) [2] have been recombinantly produced in a similarly successful fashion, but so far they have not been investigated as potential future biomaterials. Engineered collagens (eCols) are closely related to collagen mimetic peptides but are produced biotechnology in _E. coli_. A model collagen consisting of (GPP)\({}_{50}\) produced at high yields, was found to be sufficiently stable to be used in common applications even without containing hydroxproline, and, when an aminorinal nucleation site was introduced, demonstrated the ability to form collagen triple helices rather than gelatin networks [72]. The mentioned eCol has been successfully used as a matrix for the fixation of collagen-mimetic peptides without the use of crosslinking agents, and could be used as a substratum in cell culture, showing its applicability for tissue engineering. In addition, fully synthetic genes also allow the direct incorporation of integrin-specific cell adhesion sites, such as the RGD motif and more collagen-typical adhesion sites such as GFOGER-like sequences [73]. When processed into more tightly packed structures such as fibrils and fibers, direct integrin mediated cell adhesion has been shown to be impaired [74], and thus will possibly require more complex solutions to mediate the adhesion between eCol-fibrils and the cells within the ECM. One recently described group of proteins, the so-called COLIBRIs (COLIagen INtegrin BRIdging proteins) [75; 76], which form a bridge between natural fibrillar collagen and integrin including the well-known proteins fibronectin and von Willebrand factor, might be able to fulfill this role. Thus, despite the promising results, the development of eCols is still in its early stages. Synthetic collagen-based block copolymers are artificially designed constructs and combine some of the approaches mentioned so far. When short collagen-like (GPP)\({}_{n}\)-peptides are positioned around a hydrophilic unstructured core domain and expressed in _P. pastoris_, a stable gelatin-like hydrogel can be produced with high yields [77; 78]. Since these structures, like engineered collagens, are based on synthetic genes, the addition of short peptide sequences such as cell adhesion motifs or calcification sites is trivial. When the same authors replaced the randomized central sequence with a _B.