Engineering
Different types of polymers are used in tissue engineering [45]. The existence of the genetic elements in the cells is an essential part of their viability and productivity. Cells aggregate to form tissue: growing free cells in a particular space made of a biodegradable polymer enables their injection or cultivation in a tissue. In a successful process the cells then will differentiate to form or to fill in the target tissue. Upon degrading the polymer and replicating the cells, the new cells become a part of the oriignal repaired tissue, a new technology named “tissue engineering” which offers the possibility to help in regenerating tissues damaged by disease or trauma and, in some cases, to create new tissues. Usually this is achieved through using degradable biomaterials to either induce the surrounding tissue and cell to grow or to serve as temporary scaffolds for transplanted cells to attach, grow, and maintain differentiated functions. In some trials the polymer improves the growth of the cultivated cells. Leucocytes show improvement growth on PHA polymer surface [28].
2.2.2 Gene Therapy and Delivery
Biopolymers were designed in many formulations to either react by themselves or to be used in gene therapy [46–48]. They are used widely in tissue engineering and in genetic cell engineering [49]. They enable the cells to provide biochemical signals [50] which direct cell proliferation and differentiation [51]. The unique criteria in the polymers used in gene therapy, is their biocompatibility, mucoadhesive character, and biodegradability. The biocompatibility of natural polymers allows cells to infiltrate the matrix and transfection can occur as these cells come into contact with the embedded DNA. The biodegradability of the matrices obtained from natural polymers may also assist the release of gene transfer agents into the surrounding environment and thus affect nearby cells [48, 52, 53].
2.2.3 As Biosensor
A biosensor is a molecular device that converts a biological response into a detectable measurable signal. A biosensor is formed from a sensor which could use different types of materials including the polymer and the bioreceptor, such as antibodies, enzymes, nucleic acid, etc., [54], is coupled with the sensor through different immobilizing techniques [54]. Different polymers are used to produce biosensors such as in antibacterial electrospun, dual fluorescence “turn-on” sensors of cysteine and silver ions, chitin nanofiber paper toward optical (bio)sensing applications, click off colorimetric detection of peroxide and glucose etc. [55].
2.3 Biopolymer Type Number 2: Polyamides
2.3.1 Protein (πρώτειος)
The word protein comes from the Greek word πρώτειος (proteios) “primary”. Proteins were first described and named by the Swedish chemist Jöns Jakob Berzelius in 1838. However, the involvement of proteins in living system organisms was correctly understood in 1926, when James B. Sumner showed that urease was a protein defined by the sequence of its related nucleotides and amino acids [56]. The genetic code can include selenocysteine and in certain cases (such as in archaea) pyrrolysine. The residues in a protein are often observed to be chemically modified by post-translational modification, which can happen either before the protein is used in the cell, or as a part of control mechanisms. Proteins can also work together to achieve a particular function, and they often associate to form stable complex functions, such as actin and myosin in muscles and proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle [57].
Proteins are natural chains of amino acids joined by amide linkages. They are degraded by enzymes (proteases). For thousands of years people used natural proteins such as wool [58], silk [45] and hair (keratin) for clothes, decoration or to display their wealth. After understanding their composition, they were reformulated and their properties changed to match certain demands. Numerous proteins were studied for developing natural bio-based materials such as keratin, collagen, albumin, gelatin, and fibroin [59]. They are degraded by enzymes. Nowaday some old techniques are still used to produce products from some types protein–polymer such as wool [60], silk [61–64], and hair [65–67]. However, some modern applications have been invented such as the use of Keratin [68–70] in hydrogel. Silk is used in tissue engineering, in drug delivery for musculoskeletal therapeutics. The first industrial applications of protein as polymer were in the early 1930s and 1940s with casein and with soy protein. Protein biopolymers can be classified with animal proteins (e.g. casein [71–75], whey [76–79], keratin [58, 68, 80, 81], collagen [82–85] and gelatine [86], polyglutamic) and in plant proteins (wheat, corn, soy, pea and potato proteins) and microbial protein such as polyglutamic [87–89], cyanophycin [90–93] protein biopolymers remained present in some niche markets such as encapsulates (pharmaceutical), coatings (food industry), adhesives or surfactants. They are used in the packaging industry for breweries, wineries and essential oil composite film for refrigerated products; microcapsules based on biodegradable polymers [72, 90, 93]. Protein biopolymers remain in some niche markets such as encapsulates (pharmaceutical), coatings (food industry), adhesives or surfactants. They are used in the packaging industry for breweries, wineries and refrigerated products essential oil composite film, microcapsules based on biodegradable polymers [57, 72].
Mammalians are not able to produce all 20 amino acids (essential amino acids). They obtain those essential amino acids through their diet. The first protein to be fully sequenced was insulin, by Frederick Sanger, who won the Nobel Prize for this achievement in 1958. It is important to highlight that Sanger succeed in solving the insulin sequence using insulin itself rather using the related genes. It was a complicated process especially in the presence of the disulfide bond between A and B fragments. Sanger discovered early the importance of the 3D structure of the protein. The first protein structures to be solved using data obtained from x-ray diffraction analysis included hemoglobin and myoglobin, by Max Perutz and Sir John Cowdery Kendrew, respectively, in 1958 [94, 95]. The 3D structures of both proteins were determined; Perutz and Kendrew shared the 1962 Nobel Prize in Chemistry for these discoveries. Rapid advances in site-directed mutagenesis and total gene synthesis combined with new expression systems in prokaryotic and eukaryotic cells have provided the molecular biologist with tools for modification of existing proteins to improve catalytic activity, stability and selectivity, for construction of chimeric molecules and for synthesis of completely novel molecules that may be endowed with some useful activity. The results are used in the improvement of the design by using knowledge-based procedures that exploit facts, rules and observations about proteins of known 3D structure [96].
2.3.2 The Biology of the Protein
Most of organic compounds have applications that were successfully synthesized chemically after solving their structures. Even though the basic concept of the protein structure and composition was solved, proteins resist chemical synthesis, this is due mainly to its long variant polymeric chain. Proteins cannot be synthesized by organic chemists in large quantities and they cannot be manipulated in vitro to modify single amino acids in a protein and leave all other amino acids of that variety unchanged. However, the majority of proteins can be manipulated using in vivo genetic engineering. Once a gene coding for a protein has been cloned from the original wild-type genome into a vector it can be manipulated by using synthetic oligonucleotides to produce site specific mutations in the cloned material. This is specific and can alter any side chain of a particular amino acid to any other of the 20 naturally occurring amino acids. The technique of site-directed mutagenesis can alter any number of amino acids in a protein and can be used to build proteins from scratch. The position of the amino acid is decided by inspection of the tertiary structure of the primary structure (using conserved amino acids and site directed mutagenesis experiments) and the tertiary structure of the protein (using protein modeling), and the interaction of the amino acid with the substrate or with other parts of the main protein is evaluated mathematically. Conserved amino acids can be determined from the protein primary structure using alignment. Conserved amino acids are usually responsible for important functions. It is, of course, necessary to have a reasonable idea of what property one is trying to enhance in the target protein. Wrong manipulation of protein could lead to fatal problems. Proteins