Collagen is the most abundant structural protein in the extracellular matrix of the various connective tissues in the body (i.e., skin, bones, ligaments, tendons, and cartilage) (Figure 1a) [1,2]. The primary biomedical applications of collagen were in biomaterials, especially as drug and gene carriers, tissue engineering, absorbable surgical suture, osteogenic and bone filling materials, hemostatic agents, immobilization of therapeutic enzymes, and burn/wound cover dressings [3–8]. Collagen is a crucial component of the wound-healing process; it acts as a natural structural scaffold or substrate for new tissue growth and plays an essential role in all phases of wound healing, including hemostasis, inflammation, proliferation, and remodeling [8]. The collagen produced by organisms is defined as endogenous, which consists of three long helicoidally shaped chains of amino acids [2]. Chains of polypeptide constituted by the repeated sequence (Gly-X-Y)n form collagens, where X and Y can be occupied by any amino acid, although these positions are commonly occupied by proline and 4-hydroxyproline (Hyp, Figure 1b) [9,10]. Collagen breaks down due to aging, exposure to ultraviolet light, and tobacco. The degradation of collagen results in wrinkles, sagging skin, stiff joints, and dry skin, and therefore, it is essential to identify new resources of collagen for regenerative tissue applications [11,12]. Polymers 2020, 12, x FOR PEER REVIEW 2 of 38 exposure to ultraviolet light, and tobacco. The degradation of collagen results in wrinkles, sagging skin, stiff joints, and dry skin, and therefore, it is essential to identify new resources of collagen for regenerative tissue.

  Marine collagens such as fish skin, bone, cartilage, and scales, including both marine vertebrates and invertebrates sources, are more bioavailable compared to bovine or porcine collagen and have a higher absorption capability (up to 1.5 times more efficiently into the body) [33] and more rapid bloodstream circulation due to their low molecular weight and small particle size [34]. In addition, marine-based collagens are similar to conventional bovine and porcine collagen in terms of amino acid composition and biocompatibility [35]. Fish collagen can be obtained from various fish by-products such as fishbone, scales, and skins (Figure 2a) that are consumed daily in different parts of the world and result in a large amount of waste—50% to 70% of original raw materials, which is generated from fish shops and processing factories [36].