1 General overview of biopolymers: structure and properties

Abstract

Biopolymers are synthesized from a biological origin under natural phenomenon especially during their growth cycle, in the form of polymeric substances that portrays excellent properties such as flexibility, tensile strength, steadiness, reusability, and so on. The amalgamated form of two or more biopolymers leads to the formation of “biocomposites” with novel applications. Several mechanisms were identified for the effective production of biopolymers from diverse life forms such as microbial origin plant and animal origin. Based on their origin, biopolymer differs in their structure and functions. Biopolymers are preferred over chemically synthesized polymers due to their biodegradability and their impact on the environment. Biopolymers play a pivotal role in pharmaceutical industries. The biopolymers could be employed for, the administration of medicine as well as regenerative medicine to reach minimal immunogenicity and maximum pharmacological expressivity in a treated individual. Based on their properties biopolymers were exclusively used in medical devices, cosmaceuticals, and confectionaries, it is also used as additives in food industries, bio-sensors, textile industries, and wastewater treatment plants. Ecological support is of utmost concern nowadays due to the ever-expanding ramification over the planet by usage of plastic as packaging material, turning up scientists and researchers to focus on biodegradable biopolymer utilization. The miscibility-structural-property relation between every biopolymer must be focused on to improve the better environment. Specific biopolymers are designed for the betterment of agrarian and commoners of society. Advanced structural modifications, properties of biopolymers, and applications of biopolymers to achieve a greener environment were discussed in this chapter.

1.1 Introduction

Biopolymer is an organic polymer derived from natural living organisms such as plants, animals, and microbial sources. Biopolymers can degrade completely at accelerated rates. They break down into simple substances such as CO2, methane or H2O that are easily found in the environment by synthetic actions of microbes [1]. Structural units of biopolymers are amino acids, sugars, polysaccharides, proteins, and nucleotides [2]. Biopolymers are appearing to hold great business prosperity because of their elasticity, employability, and tough nature. The chemical and pharmaceutical industries share a major advantage in using these biopolymers [3]. Plant source includes cellulose, pectin, and xylan. The animal source includes chitosan, collagen, keratin, and gelatin [4]. Polymers are now derived from microbial sources as well which include polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), and bacterial cellulose (BC). Chemically synthesized polymer is polylactide [PLA]. The most predominant and useful biopolymers are starch, cellulose, chitin, collagen, keratin, PHA, PHB, BC, and PLA. Biopolymers production rate estimated to become a 14.92 billion dollar industry by the financial year 2023. The biocomposites hike up the mechanical properties of biopolymers by 60% by their reactive structural modification.

Starch is an environment-friendly material. Starch can replace the use of synthetic polymers in the plastic industry. Starch is a heterogeneous material comprised of two types of polymers namely amylase and amylopectin. Naturally occurring starch exists in granulated form. Amylose is a polysaccharide made of d-glucose monomers. 1, 4 alpha glycoside bonds connect the amylase whereas 1, 6 alpha glycoside bonds links the amylopectin. Due to their structural difference their properties also differ greatly. Amylose is more crystalline while amylopectin is readily digestible because of its branching. Amylopectin has a branched structure such that most short chains of 1, 4-linked anhydrous units as in amylase and 4–5% branching point with 1, 6 linkages frequently occurring at every 20–30 glucose units, which forms a major component of starch.

1.2 Classes of biopolymers

Cellulose is the most abundantly occurring biopolymer which is found in the cells of plants. Cellulose has been widely used as a biocomposite in many fields. Cellulose has the most unique structure compared with others. Cellulose is also made up of d-glucose subunits. Cellulose has beta orientation while others have alpha orientation. The hydroxyl group of cellulose is placed overhead the level of sugar (glucose ring). Cellulose is crystalline and it has a white powdery appearance. Cellulose in plant cells is in the form of cellulose micro fibrils. Chitin is an ample biopolymer next to cellulose. Deacetylation of chitin gives chitosan. It is a positively charged polysaccharide. Arthropods like shrimps, crabs, lobsters, and insects have chitin in their exoskeleton. The fibrous crystalline state is the native form of chitin. It is extensively used in drug delivery, antimicrobial agents and also in food industry.

Collagen is extensively used in the biomedical industry. It is the major structural compound of the skins and hides of animals. Collagen exists in fibrous form thus it provides mechanical assistance and organizational structure to the connective tissues of our body. Native collagen structure is triple helical alpha domains. These chains have L-handed amino acid sequence polyproline. Approximately there are more than 30 types of collagens. Type I collagen consists of an N-terminal propeptide, middle collagen domain, and C-end pro-peptide. It appears in bones, tendons, and organs. Type II collagen forms a basis of hyaline cartilage, it includes articular cartilages. It forms a homotrimer with alpha 1 chains and it occupies 50% of protein in cartilage region. They are articulated into fibrils. Type III collagen is a major structural component of large blood vessels, uterus etc. Interaction with platelets of blood for clotting the blood and acting as an important signaling molecule during wound healing are the two major functions of type III collagen. Type IV collagen is an important component of cell adhesion, migration, proliferation, and cell differentiation. It provides mechanical stability for cells. Type V collagen is also called as fibrilar collagen. It is usually found in hair and nails. Type V collagen constitutes bone matrix, interstitial cell of muscles lungs and placenta [5].

Keratin is a member of the fibrous protein family. Its intricate structure like a nanofibre lattice structure acts as a barrier in our epithelial cells. Based on order keratin is classified into alpha and beta keratin. α-keratin is profoundly found in nails and α/β-keratin is found in the carapace of turtles. Keratin is a natural source of nitrogen and is used as fertilizer. Keratin structure is dependent on the amino acid constitution. Keratin has a strong and stable structure, so it is insoluble in water, acids, alkali, and solvents. Keratin protects the epithelial cells and strengthens skin and internal organs [6]. PHA is an agreeable polymeric substance that is derived from many microorganisms that act as a renewable carbon resource. A wide range of bacteria can hold these polymer substances in their body as a storehouse of energy and carbon sources in cases of nutrition deficiency. PHAs are used as fabrication material for absorbable medical supplies such as meshes, implants, sutures, etc. [2]. PHB is a family of PHA biopolymers that is built up by a variety of microorganisms [7]. PHB can be used as a conventional product for many petroleum-derived plastics. PHB is biocompatible in nature [8]. Bacterial cellulose (BC) which is similar to plant cellulose and has the same molecular formula is a sustainable natural nanomaterial....