# Routes towards Novel Collagen-Like Biomaterials
## 2 Collagen as a Biomaterial
### Towards Spun Collagen Fibers  
Surgical sutures, historically known as catgut [29], have been a major utilization for collagen fibers in biomedical applications, since they get readily resorbed into the surrounding tissue over time and thereby promote wound healing and tissue regeneration. While catgut collagen is prepared from the decellularized small intestine of sheep and is therefore a fully matured and cross-linked collagen network that has been reshaped into fibers by cutting and stripping, its mechanical properties, although poor compared to that of silks and synthetic polymers in terms of toughness and ultimate stress, are still well suited for its purpose: Given that the catgut suture closely resembles the original structure of the intestinal endothelium, the elastic modulus is very similar to that of most internal soft-tissue ECMs and therefore will show comparable deformation when stressed, which in turn reduces the strain on the suture itself.  
Nevertheless, the biomechanical properties of living tissue in the human body span several orders of magnitude both in elasticity and toughness [30], with, for instance, an aorta valve having a low ultimate stress of 0.3-0.8 MPa, compared to that of skin (1-20 MPa) or tendon (50-100 MPa). ruptured cartilage, tendon and bone, even though they mostly consist of type I collagen, cannot typically be sutured with catgut, since the material does not have the required tensile strength and toughness, and instead require the use of synthetic materials with lower biocompatibility and no biodegradability [31].  
Researchers have therefore begun to investigate processing methods in order to improve the mechanical properties of collagen-based fibers and thereby tailor the material to the requirements of the tissue it is intended to be used in. One major improvement was the wet-spinning of extracted or recombinantly produced, soluble collagen [10; 12; 32], which demonstrated the possibility of producing fibers from this source material outside of a biological system. Since the mechanical properties were still poor, these materials were mostly used to demonstrate their applicability for in vitro tissue engineering. However, due to the high cost of soluble full-length collagen, no commercial products have been made available so far.  
Another recent approach [11] used a microfluidic system to produce mechanically highly stable fibers from acid-soluble type 1 collagen (Figure 1). The authors suggest that the small diameter and the directional laminar shear forces in the spinning channel help in axially aligning the collagen fibrils in a fashion comparable to the fibril formation from tropocollagen during biosynthesis, thereby yielding a product with similar mechanical characteristics as tendon.  
All of these approaches use the intrinsic property of collagen triple helices to be soluble but still folded in dilute acetic acid and therefore require an independent first step inducing the folding of the collagen triplehelix from the \(\alpha\)-chain precursors. As a result of this limitation, no conditions such as high temperature or denaturing solvents, which would melt the triple helix, can be used during production, processing and sterilization. It is, therefore, highly desirable to find collagen-like proteins that intrinsically possess the ability to form soluble triple helices without requiring complex biological systems and mild conditions for their manufacture.