Biopolymers are extracted from natural sources and represent a possible shift from plastics that can help our reliance on fossil fuels.
Introduction
Biopolymers, which are used to produce a wide variety of biodegradable packaging, are natural polymers that are produced by living organisms or derived from renewable resources. They have various forms and structures, depending on the type of monomers (building blocks) and the way they are linked together. Biopolymers have many advantages over synthetic polymers, such as biodegradability, biocompatibility, sustainability, and functionality. Biopolymers have various applications in medicine, food, packaging, and petroleum industries.
This review article is focused on various aspects of biopolymers, such as their classification, synthesis, characterization, and applications.
Classification of biopolymers
There are three main classes of biopolymers, classified according to the monomers used and the structure of the biopolymer formed: polynucleotides, polypeptides, and polysaccharides
Polynucleotides
Polynucleotides are long chains of nucleotides, which are composed of a nitrogenous base, a sugar, and a phosphate group. The most common examples of polynucleotides are DNA and RNA, which store and transmit genetic information in living cells. DNA has a double helix structure, where two strands of nucleotides are held together by hydrogen bonds between complementary bases. RNA has a single-stranded structure, but it can fold into complex shapes and perform various functions.
Polypeptides
Polypeptides are long chains of amino acids, which are organic molecules that contain an amino group and a carboxyl group. 20 different amino acids can be combined in various sequences to form polypeptides. Polypeptides are also known as proteins, which are essential for the structure and function of living organisms. Proteins have different levels of organization, from the primary structure (the sequence of amino acids) to the quaternary structure (the arrangement of multiple polypeptide chains). Proteins can have various shapes and functions, such as enzymes, hormones, antibodies, and receptors.
Polysaccharide
Polysaccharides are long chains of sugar molecules, which are composed of carbon, hydrogen, and oxygen atoms. Many types of sugars can form polysaccharides, such as glucose, fructose, galactose, and mannose. Polysaccharides can have linear or branched structures, and they can form bonds with other molecules, such as proteins or lipids. Polysaccharides are important for the storage and transport of energy, as well as the structural support and protection of living organisms. Examples of polysaccharides are starch, cellulose, glycogen, and chitin.
Synthesis of biopolymers
Biopolymers can be synthesized by two main methods: biological synthesis and chemical synthesis.
Biological synthesis
Biological synthesis is the process of producing biopolymers by living organisms, such as plants, animals, or microorganisms. Biological synthesis involves the use of enzymes, which are biological catalysts that speed up the reactions of monomers to form biopolymers. Biological synthesis can be divided into two types: intracellular synthesis and extracellular synthesis.
Intracellular synthesis
Intracellular synthesis is the process of producing biopolymers inside the cells of living organisms. For example, DNA and RNA are synthesized by the enzymes DNA polymerase and RNA polymerase, respectively, inside the nucleus of eukaryotic cells or the cytoplasm of prokaryotic cells. Proteins are synthesized by the enzyme ribosome, which translates the genetic code from mRNA to amino acids, inside the cytoplasm of all cells. Glycogen is synthesized by the enzyme glycogen synthase, which links glucose molecules together, inside the cytoplasm of animal cells.
Extracellular synthesis
Extracellular synthesis is the process of producing biopolymers outside the cells of living organisms. For example, cellulose and chitin are synthesized by the enzyme cellulose synthase and chitin synthase, respectively, outside the plasma membrane of plant and fungal cells. Starch is synthesized by the enzyme starch synthase, which links glucose molecules together, outside the chloroplast of plant cells. Polyhydroxyalkanoates (PHAs) are synthesized by the enzyme PHA synthase, which links hydroxy alkanoic acids together, outside the cytoplasm of bacterial cells
Chemical synthesis
Chemical synthesis is the process of producing biopolymers by chemical reactions, such as polymerization, condensation, or cross-linking. Chemical synthesis involves the use of catalysts, which are substances that speed up the reactions of monomers to form biopolymers. Chemical synthesis can be divided into two types: natural monomer-based synthesis and synthetic monomer-based synthesis.
Natural monomer-based synthesis
Natural monomer-based synthesis is the process of producing biopolymers by using natural monomers, such as sugars, amino acids, or nucleotides, as the starting materials. For example, polylactic acid (PLA) is synthesized by the polymerization of lactic acid, which is derived from the fermentation of corn or sugar cane. Polyethylene glycol (PEG) is synthesized by the condensation of ethylene glycol, which is derived from the hydration of ethylene. Chitosan is synthesized by the deacetylation of chitin, which is derived from the exoskeleton of crustaceans or insects.
Synthetic monomer-based synthesis
Synthetic monomer-based synthesis is the process of producing biopolymers by using synthetic monomers, such as acrylates, vinyl, or epoxides, as the starting materials. For example, polyacrylamide (PAM) is synthesized by the polymerization of acrylamide, which is derived from the hydration of acrylonitrile. Polyvinyl alcohol (PVA) is synthesized by the hydrolysis of polyvinyl acetate, which is derived from the polymerization of vinyl acetate.
Characterization of biopolymers
Biopolymers can be characterized by various methods, depending on their properties, such as molecular weight, molecular structure, thermal behaviour, mechanical behaviour, and biodegradability
Molecular weight
Molecular weight is the mass of one molecule of a biopolymer, expressed in atomic mass units (amu) or daltons (Da). Molecular weight can be measured by methods such as gel permeation chromatography (GPC), size exclusion chromatography (SEC), or mass spectrometry (MS). Molecular weight can affect the physical and chemical properties of biopolymers, such as solubility, viscosity, melting point, and crystallinity
Molecular structure
Molecular structure is the arrangement of atoms and bonds in a molecule of a biopolymer. The molecular structure can be determined by methods such as nuclear magnetic resonance (NMR), infrared spectroscopy (IR), ultraviolet-visible spectroscopy (UV-Vis), or X-ray diffraction (XRD). The molecular structure can influence the functional groups, stereochemistry, and conformation of biopolymers, which can affect their reactivity, stability, and compatibility
Thermal behavior
Thermal behaviour is the response of biopolymers to changes in temperature, such as melting, crystallization, degradation, or transition. Thermal behaviour can be analyzed by methods such as differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), or dynamic mechanical analysis (DMA). Thermal behaviour can reflect the thermal stability, thermal history, and phase transitions of biopolymers, which can affect their processing, performance, and application
Mechanical behavior
Mechanical behaviour is the response of biopolymers to external forces, such as stress, strain, or deformation. Mechanical behaviour can be evaluated by methods such as tensile testing, compression testing, bending testing, or impact testing. Mechanical behaviour can indicate the strength, stiffness, elasticity, toughness, and fracture resistance of biopolymers, which can affect their durability, reliability, and functionality
Biodegradability
Biodegradability is the ability of biopolymers to be broken down by microorganisms into harmless substances, such as water, carbon dioxide, and biomass. Biodegradability can be assessed by methods such as respirometry, gravimetry, or spectroscopy. Biodegradability can reveal the environmental impact, life cycle, and disposal options of biopolymers, which can affect their sustainability, eco-friendliness, and social responsibility
Applications of biopolymers
Biopolymers have various applications in different fields, such as medicine, food, packaging, and petroleum industries. Some of the examples of biopolymers and their applications are
Medicine
- DNA and RNA are used for gene therapy, diagnosis, and vaccination.
- Proteins are used for drug delivery, tissue engineering, and biosensors.
Advantages of Biopolymers
Biodegradable packaging is a great alternative to common plastic, foam and paper packaging. They can help alleviate the long-term issues presented by extra plastic usage. These kinds of packaging that are based on biopolymers are made from renewable sources, as compared to plastics that are made of oil.
Furthermore, biopolymers are synthesized in a relatively energy-efficient process, requiring much less energy than the production of plastic polymers. Another major advantage of biodegradable packaging is that they are not toxic to natural environments or humans. This makes them much easier to dispose of and they do not build up over time as plastics do.
Lastly, biopolymers can reduce our dependence on oil and decrease carbon dioxide emissions. This is debatably the most important advantage of biodegradable packaging, as it works to reduce climate change, which is a global issue.
Disadvantages of Biopolymers
1- High cost of research and development
Finding the right formula for producing biopolymers requires a lot of time and money for R&D. The investment in this field is usually borne by universities and governments and goes to one or more teams of PhD students, university professors, and experienced researchers. They are also equipped with state-of-the-art laboratories to perform numerous experiments to test biopolymers.
2- Need for expensive equipment for processing
The disadvantage of many biopolymers is that they require expensive processors and industrial composters. Especially those that require high temperatures to decompose. Also, the equipment needs to be available, and sometimes this cannot happen.
3- Need for more arable lands to produce raw materials
Because biopolymers are sourced from plants and renewable sources, their production depends on the use of arable land to produce the raw materials. Though using these renewable resources, such as corn, will boost agriculture and reduce the unemployment rate, more land is required to produce corn. Considering the facts above still, corn is the most widely used source for making these types of biopolymers all over the world.
4- Biopolymers are more expensive
The cost of producing bioplastics is 20 to 50 per cent higher than the cost of producing common plastics. However, with improved technologies and greater access to materials, this cost can be significantly reduced.
The starched-based biopolymers help reduce environmental damage while having a significant impact on the economy. They help businesses to be green and become eco-friendly brands. Customers are also supportive of green businesses that are switching to technologies or products that are harmless to the environment. Eco Clicky helps green businesses to be well recognized by customers through their products that significantly reduce their negative impact on the environment.