2.3.4.2 Keratin
Keratin derived materials have shown potential to transform the world of bio-based materials because of their intrinsic biocompatibility, biodegradability, mechanical durability, and natural abundance [58]. keratin is the most abundant structural protein in epithelial cells [122] and a most important biopolymer in animals along with collagen. Keratin is a polypeptide consisting of amino acids having intermolecular bonding of cysteine and few intramolecular bonding of polar and non-polar groups. The cysteine residues have thiol groups which produce strong disulfide bonds leading to the cross-linking of the matrix molecule. Keratins can be exited as delicate keratins (such as stratum corneum) generally poorly united and with a lower measure of sulfur and lipids, and hard keratins found in hair [123], nails, paws, noses, feathers [124], plumes, which have a more rational structure and a higher measure of sulfur.
2.3.4.3 Worm and Spider Silk
Silk is a polymer made from the fine threads produced by certain insect larvae. Silk was traditionally known to be produced from the silk worm [125]. Silk is, by nature, a protein biopolymer produced by polymer producer organisms. Examples of silk producers are spiders, silkworms [126], mites [127], scorpions and flies [128]. There is a rise in interest in the silk produced by spiders [129]. Spider silk is an interesting biomaterial, elastic and strong that is comparable to the best fibers synthetized by new technology in terms of mechanical properties. It is also a biodegradable material and environmentally safe. Because of the limited amount of spider silk, silk fibroin as a natural polymer produced by silkworms is a good alternative. Sericin and fibroin are the major components of it. Fibroin, a fibrous protein creating the silk core, is composed of a fibroin light chain, fibroin heavy chain and fibrohexamerin. Excellent mechanical properties, biocompatibility and slow degradability make this material interesting.
2.3.4.4 Collagen, Gelatin, Elastin, Albumine and Fibrin
Collagen and gelatine are animal polymers found in skin and connective tissues. Collagen degraded to high molecular weight polypeptide, called gelatine, can be obtained by thermal denaturing of collagen. Gelatin is a water-soluble proteinaceous substance [130]. Gelatin is an important high molecular weight polypeptide hydrocolloid. It is commonly used in a wide range of food, medicinal, pharmaceutical, and polymeric materials. Most hydrocolloids are polysaccharide, whereas gelatin is a protein containing all the amino acids except tryptophan [131]. It was fabricated to different forms to match different applications [132]. It is essential in drug caps, X-rays, photographic film development and food processing. Gelatin grades used in drug delivery and tissue engineering are also available in a wide range of viscosities. It does not show antigenity and is resorbable in vivo. Its physico-chemical properties can be suitably modulated. Gelatine can be plasticized thanks to the addition of water or of glycerol. There is, however, a limit to the use of this interesting material because there is a risk of viral animal contamination. Blends of polyvinyl alcohol and gelatine are the object of studies and research. Elastin, albumine and fibrin are other proteins from animal sources. They have been investigated especially for various biomedical applications. Elastin is used as a biopolymer in enhancing cellular uptake in the tumor cells [82, 83, 133–136].
2.3.4.5 Wheat Gluten
Wheat gluten is a protein by-product of the starch fabrication. In addition to wheat, grain sources of gluten are barley, rye, triticale, spelt, einkorn, emmer and kumut. It is available in high quantity and at low cost [137]. Gluten is a part of our food and is contained in pasta, bread, cereals, soups, deserts, soy sauce, hydrolyzed wheat proteins, wheat bran hydro lysate, wheat protein isolate, wheat starch, glucose syrups, wheat maltodextrin, sorbitol, lactitol, maltitol, caramel, glucan, alcohol/ ethanol, vinegar, wheat germ oil, medications, and so on. They are relatively impermeable to oxygen and to CO2 but are sensitive to humidity. Potential applications are producing soluble receptacles for the controlled release of a chemical product (such as toilet detergent). Wheat gluten contains two main groups of proteins, gliadin and glutenin [138]. Gliadins are protein molecules with disulphide bonds. They have low molecular weight and a low level of amino acids with charged side groups. Gliadin has antimicrobial activity and is used in food packaging and coating applications [130, 139].
The molecular weight of glutenins is at least ten times higher than that of gliadins. Wheat gluten materials have the fastest degradation rates. Gluten is fully biodegradable and the products obtained are non-toxic. Wheat gluten has proved to be an excellent film-forming agent [18, 140]. In practice, the term “gluten” refers to the proteins, because they play a key role in determining the unique baking quality of wheat by conferring water absorption capacity, cohesivity, viscosity and elasticity to dough. It is also used for improving solubility, emulsification, and film-forming properties [22, 141, 142]. The amino acid compositions of glutenins are similar to those of gliadins, with high levels of glutamine and proline and low levels of charged amino acids. Glutenins can be broadly classified into two groups, the high molecular weight (HMW) and the low molecular weight (LMW) subunits.
2.3.4.6 Soy Protein
Soy protein has been used since 1959 as an ingredient in a variety of foods for its functional properties, which include emulsification and texturizing. Soy protein is used in many applications [143]. Recently the popularity of soy protein has risen, mainly because of its health benefits. It has been proved that soy protein can help to prevent heart problems. Soy protein films do not have as good mechanical and barrier properties as most protein films, due to their hydrophilic nature. They are used to produce flexible and edible films.
2.4 Biopolymer Type Number 3: Polysaccharides
Polysaccharides are among the most widespread organic compounds in the plant kingdom and used in many applications [144]. Polysaccharides play essential roles in the life processes of all plants. They can be divided into several broad groups according their functions, i.e., structural polymers (cellulose), protective polysaccharides (pectin and hemicelluloses) and reserve polysaccharides (starch). Further, polysaccharides can form glycoconjugates with proteins and lipids resulting in biological macromolecules in the cell wall and cell membranes, and play important roles in many physiological and biochemical processes.
2.4.1 Starch
Starch is a complex carbohydrate and one of the largest molecules in nature found chiefly in seeds, fruits, tubers, roots and stem pith of plants, notably in corn, potatoes, wheat, and rice. Starch is a polymer of D-glucose organized in two major constituents of huge molecular weights. Amylose contains amorphous and crystalline regions [145, 146]. Amylopectin is used in high-performance flocculents and as non-ionic surfactant [145]. To improve their resistance to shock and moisture, polyolefins were added in small quantities (about 10–15%) or in large proportions up to 85–95% to starch. Those polymer mixtures disappeared during the biodegradation process leaving small fragments whose degradation time was a function of their carbon chain length. Supol (Supol, Germany) Potato flour is submitted to a thermal treatment under pressure. Pellets can be injected to produce single-use dishes which are microwavable and which are compostable or can be added to animal food. Evercom (Comstarch, Japan) plasticized maize starch can be injected to make small parts for catering or for horticultural applications. This product is compatible with other biopolymers such as PHBV, PLA, PCL polyesters. Native starches differ in the amylose/amylopectin ratio depending on their botanic source, such as native starches are composed of 20–30% amylose and an additional amount of amylopectin, amyloseenriched starch may contain up to 84% amylose while waxy starches consist of nearly pure amylopectin. The main applications are for producing mulch films, shopping bags, food packaging (yogurts), nappies, medicinal and personal hygiene products [147, 148].
2.4.2
