Natural and Synthetic Polymers for Biomedical ...

Natural and synthetic polymers are a versatile platform for developing biomaterials in the biomedical and environmental fields. Natural polymers are organic compounds that are found in nature. The most common natural polymers include polysaccharides, such as alginate, hyaluronic acid, and starch, proteins, e.g., collagen, silk, and fibrin, and bacterial polyesters. Natural polymers have already been applied in numerous sectors, such as carriers for drug delivery, tissue engineering, stem cell morphogenesis, wound healing, regenerative medicine, food packaging, etc. Various synthetic polymers, including poly(lactic acid), poly(acrylic acid), poly(vinyl alcohol), polyethylene glycol, etc., are biocompatible and biodegradable; therefore, they are studied and applied in controlled drug release systems, nano-carriers, tissue engineering, dispersion of bacterial biofilms, gene delivery systems, bio-ink in 3D-printing, textiles in medicine, agriculture, heavy metals removal, and food packaging. In the following review, recent advancements in polymer chemistry, which enable the imparting of specific biomedical functions of polymers, will be discussed in detail, including antiviral, anticancer, and antimicrobial activities. This work contains the authors&#; experimental contributions to biomedical and environmental polymer applications. This review is a vast overview of natural and synthetic polymers used in biomedical and environmental fields, polymer synthesis, and isolation methods, critically assessessing their advantages, limitations, and prospects.

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Natural and synthetic polymers have been widely discussed over the years, and interest in the topic has increased significantly in the last ten years. shows the increase in publications on natural and synthetic polymers and their use in biomedical and environmental applications.

Synthetic polymers are omnipresent in society as textiles and packaging materials, in construction, and in medicine, among many other essential applications. Synthetic polymers are a highly versatile and diverse group of substances, many of which have been explicitly applied in drug delivery, for example, solubilizing agents, nanoparticulate formation, surface modification, drug carriers, diagnostic imaging agents, and implants [ 11 ]. In addition, some of these polymers show many biological activities in their own right (e.g., antitumor, antibiotic, antiviral, and antithrombotic activities, as well as inhibition of efflux pumps such as P-glycoprotein) [ 12 ].

Synthetic polymers are defined as polymers that are artificially produced in laboratories, also known as manufactured polymers [ 8 ]. They are classified as thermoplastic and thermoset polymers and elastomers. Some examples of synthetic polymers are polyethylene (PE), polystyrene (PS), polyamides (PA), poly (vinyl chloride) (PVC), polytetrafluoroethylene (PTFE), polyisoprene (PI), phenol formaldehyde resins, and many others. Polymers made from synthetic substances (monomers) derived from petroleum oil are often created in a controlled environment, and their backbone usually comprises carbon&#;carbon bonds [ 9 ]. Specific initiators and catalysts are used to initiate and accelerate the chemical reactions between monomers. compares some of the properties and features of natural and synthetic polymers [ 9 , 10 ].

In recent years, natural polymers from marine resources have increasingly attracted attention, as they are more abundant and biologically active than polymers from other resources [ 7 ]. Marine sources, for instance, crustaceans, seaweeds, and algae, are rich in polysaccharides such as agar, chitin/chitosan, alginate, and glycosaminoglycans and thus exhibit exciting features and properties. For instance, chitin is a structural material in the exoskeletons of crustaceans and insects. Such marine-derived biopolymers constitute a platform for developing valuable advancements with environmental and economic advantages [ 7 ]. Marine polymers are becoming popular in the biomedical field due to their abundance and inherent features such as biocompatibility, biodegradability, and biological activity. However, some of these polysaccharides have limitations regarding solubility in water and organic solvents due to strong intra- and intermolecular hydrogen-bonded polymer chains [ 2 ]. As a result, this restricts their ability to be processed and converted into value-added matrices, including fibers, membranes, scaffolds, and nanomaterials. Therefore, searching for effective, eco-friendly, and feasible solvents is essential [ 2 , 7 ]. Polysaccharides are made up of sugars, called monosaccharides, that are linked together by O-glycosidic linkages. Some of their properties, such as solubility, viscosity, gelling potential, and surface and interfacial properties, are determined by differences in the composition of monosaccharides, types and patterns of linkages, shapes of chains, and molecular weight. Additionally, polysaccharides have various physiological functions, making them highly valuable for applications in tissue engineering and regenerative medicine [ 2 ].

Natural polymers are components of biological systems responsible for performing various essential functions [ 3 ]. For instance, specific natural polymers, such as cellulose and chitin, play a vital role in maintaining the structural integrity of cells in plants and animals. In contrast, others, such as lysozymes, offer biological protection against surrounding environments [ 4 ]. The diversity in their origin and composition provides these natural polymers with distinct physicochemical and biological properties and are of interest in various fields, e.g., in the manufacture of paper goods and textiles, as additives in food products, in the formulation of nutraceuticals and functional foods, and in the biomedical field (e.g., in cosmetic treatments and drug delivery) [ 5 ]. Their exploitation is favorable due to the natural abundance, renewability, and intrinsically low carbon footprint of polymers derived from renewable resources. Such properties are pivotal in developing advanced materials for films, membranes, coatings, hydrogels, and micro- and nanoparticle systems [ 6 ]. Natural polymers are essential for supporting life and enabling organisms to adjust to their surroundings through vital biological processes like molecular identification and genetic information transfer. Examples of natural polymers and their structures can be seen in .

Natural polymers extracted from organic sources such as microorganisms, algae, plants, or animals have been widely used for decades in biomedical applications such as pharmaceuticals, tissue regeneration scaffolds, drug delivery, and imaging [ 1 ]. Polysaccharides, proteins, and polyesters derived from plant and animal kingdoms are part of the family of natural polymers. Several of these polymers comprise our diet and have been used in various human applications [ 2 ]. These polymers are recognized by the biological environment and directed into metabolic degradation. Natural polymers are similar to extracellular matrix (ECM) components, enabling them to avoid chronic immunological reactions and toxicity, which are frequently observed with synthetic polymers [ 2 ].

2. Natural Polymers for Biomedical Use

2.1. Antibacterial

Antimicrobial medication coatings, antimicrobial gauze or dressings, and medical tapes containing antimicrobial agents are a few examples of antimicrobial wound healing techniques. Chi et al. created a patch called the biomass-energetic chitosan microneedle array (CSMNA) to aid in healing wounds [46]. Due to its exceptional qualities and inherent antibacterial capabilities, chitosan is often utilized for wound healing [47]. The microneedle&#;s microstructure also helps to prevent excessive skin and patch adherence while delivering the drug-carrying agent to the target location. Meanwhile, a temperature-sensitive hydrogel wraps vascular endothelial growth factor (VEGF) in the CSMNA micropore [46]. As a consequence, the temperature rise brought on by the inflammatory response of the wound may be exploited to regulate the release of smart drugs. Biomass CSMNA patches have been proven in studies to support tissue regeneration, angiogenesis, collagen synthesis, and inflammatory control during wound healing [46]. Therefore, this multifunctional CSMNA patch may be helpful in clinical applications such as wound healing. A detailed scheme and explanation of the microneedle patch can be seen in .

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Zhang et al. suggested a novel class of controlled responsive particles for the release of drugs and the healing of wounds [48]. These hybrid particles comprised black phosphorus quantum dots (BPQDs) loaded with growth stimulants and antimicrobial peptides, gelatin, agarose, and filipin protein. The BPQDs absorb near-infrared (NIR) light and elevate the temperature of the particles to gelatin&#;s melting point when exposed to NIR radiation. The reversible phase transition (melting of gelatin) causes the enclosed medications to liberate gradually. BPQD-loaded particles with NIR-responsive characteristics have shown in vitro and in vivo investigations that they may accomplish the necessary regulated release of growth factors, hence encouraging neovascularization [48]. The particles were also antibacterial throughout storage and usage because the antimicrobial peptide was combined with a secondary hydrogel and enclosed in the scaffold. Due to these characteristics, BPQD-loaded natural protein hybrid particles are excellent for medication delivery and wound healing.

Silver nanoparticles (AgNPs) are often employed when making medical products like wound dressings. However, there is no agreement regarding the efficacy and safety of AgNPs. To establish the antibacterial impact of nanosilver in vivo and to assess the wound-healing capacity of silver-doped chitosan membranes, Shao et al. clarified the effects of proteins and inorganic ions on the antimicrobial characteristics of nanosilver [49]. Their antibacterial qualities and silver ion release patterns were assessed through in vitro interactions with a phosphate buffer or serum. In vivo tests were conducted to evaluate the antibacterial efficacy and wound-healing capacity of the systems. The findings demonstrated that the biological environment significantly impacts silver ions release: proteins are a barrier to prevent silver release, whereas inorganic ions cause delayed silver release. To achieve the in vivo antibacterial action, a high quantity of silver nanoparticles must be included. Additionally, embedding silver nanoparticles had no impact on the pace of tissue response or wound healing. It can be concluded that AgNP incorporation enhances the antimicrobial effect of biomaterials without modifying the wound-healing capacity of chitosan-based membranes [14].

2.2. Hydrogel Preparation and Application

Hydrogels prepared from natural polymers have attracted extensive attention in many biomedical fields, such as their use for drug delivery, wound healing, and regenerative medicine due to their excellent biocompatibility, degradability, and flexibility [15]. Hydrogels are three-dimensional networks formed by hydrophilic polymers through chemical cross-linking (covalent or ionic bonds) or physical cross-linking (hydrogen bonds, van der Waals forces, and physical entanglement) swollen in water [50,51].

Hydrogels based on natural polymers such as alginate, starch, cellulose derivatives, chitosan, gelatin, collagen, hyaluronic acid, pectin, and so on show good degradability, biocompatibility, nontoxic degradation products, good flexibility similar to natural tissue, and have natural abundance, which endows them with widespread applications in medicinal fields, for instance, as drug carriers, wound dressing for wound healing, substrates for cell culture, cell delivery systems, scaffolds for tissue regeneration, and so on ( ) [15,16].

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Hydrogels based on natural polymers have emerged as promising alternatives for the ECM in biomedical applications due to their unique integration of biodegradability, biocompatibility, mechanical property tunability, biomimicry, and responsiveness, which could provide microenvironments with the preservation of cellular functions, promotion of cell health, and encouragement of tissue formation [53].

2.3. Drug Delivery

Extensive research has been conducted on the use of natural polymers as carriers for drugs and other bioactive substances, which has garnered a great deal of interest. Natural polymers have inherent advantages such as marvelous biocompatibility, controlled enzyme degradation, specific interactions with some biomolecules [54], and easy modification, which make them versatile for drug delivery applications. In this context, drug delivery systems (DDSs) constructed using representative natural polymers such as polysaccharides (chitosan and hyaluronic acid) and proteins (silk fibroin and collagen) are generally summarized. These DDS systems are used to deliver payloads, which mainly include low molecular weight drugs, proteins, and DNA, and are employed for various applications such as tissue engineering, wound healing, or anticancer therapy [55,56]. Moreover, a DDS constructed by modified biopolymers has also been presented, focusing on the chemical and morphological modifications, the additions of smart stimuli-triggered or targeted motifs, and so on, which promoted delivery and therapy efficiency.

Polysaccharides and protein-based materials show more similarities to the extracellular matrix, thus endowing natural polymer-based drug carriers with minimally invasive properties [17]. Moreover, polymer chains are abundant in some groups with accessibility to modification, including amino groups, carboxyl groups, hydroxyl groups, and so on, enabling easy accessibility for further modifications [17]. Finally, with more profound research into life science, more and more specific interaction behaviors between native polymers and organs or cells have been pointed out. Some natural polymers have shown a higher affinity to the receptors of cells and regulate cellular processes, including adhesion, proliferation, and migration, which provides promising potential for designing more specific target usages of high efficiency [57]. Moreover, their degradation behavior in the presence of enzymes in vivo also ensures their ability to construct stimuli-responsive systems for the delivery of drugs in the local sites. This perspective sheds light on the following two kinds of polysaccharides (chitosan and hyaluronic acid) and the other two types of proteins (silk fibroin and collagen) involving the delivery systems constructed by the original polymers and their derivatives [57].

Polymer/metal-organic frameworks (MOFs) are a class of crystalline materials possessing structures formed from the coordination of metal ions to multidentate organic groups. The main characteristics of MOFs are the high degree of porosity and the tunable architecture of the structure obtained by selecting appropriate metal ions and linkers. Furthermore, the surface of MOFs can be modified additionally, thereby increasing their functionality [58]. The high surface areas and large pore sizes of MOFs favor the encapsulation of high drug loadings. In contrast, MOFs&#; high structural and functional flexibility allow their adaption to the drug molecules&#; shape, size, and functionality. When a MOF is combined with a polymer, its colloidal stability is enhanced without loss of crystallinity. However, a recurrent issue is the decrease of porosity due to the polymer obstructing the entrance to the pores or the penetration of the polymer chains inside the MOF cavities [58]. In addition to increasing the stability, the polymer coating offers the possibility of adding targeting functionalities or introducing a stimuli-responsive release, allowing for the preparation of improved drug delivery or imaging devices. Some of the natural polymers used are hyaluronic acid, gelatin, chitosan, and alginate. Hyaluronic acid, for example, has been used to increase the binding affinity of nanoparticles selectively for the surface of cancer cells and was found to mediate the targeting recognition of CD44 over-expressing cancer cells [58]. Hyaluronic acid has incomparable chemical&#;physical properties, and numerous biological functions characterize it [18,19]. Also, hyaluronic acid has excellent antioxidants, good viscoelastic properties, excellent gelling properties, anti-inflammatory properties, wound-healing activity, excellent cosmetic properties, and drug carrier ability. Therefore, it is widely used in pharmaceutical, cosmetic, and biomaterials production industries. Hyaluronic acid has also recently been explored as a drug-delivery agent via different methods, such as nasal, oral, pulmonary, ophthalmic, topical, and parenteral [18,19].

2.4. Stem Cell Morphogenesis

Polymeric materials have great potential in tissue engineering thanks to their biodegradability, processing, and property design flexibility [59]. Moreover, polymers may be used to regulate cell function. Stem cells are a promising option for tissue engineering since they uniquely self-renew and differentiate into various lineages, such as neurogenic, osteogenic, chondrogenic, and myogenic, under proper stimulation from extracellular components [59]. Due to their properties, stem cells and polymeric materials are critical design choices. Stem cells can self-renew and commit to specific cell lineages under appropriate stimuli. Polymeric materials are biocompatible, degradable, and flexible in processing and property design. Therefore, a significant focus of tissue engineering is to utilize polymers, or soft materials, to control stem cell function via physical, chemical, mechanical, and biological cues &#;communicated&#; from the polymer to the cells [60]. Examples of natural polymers include collagen, fibrin, and polysaccharides, such as hyaluronic acid and alginate [60]. Polymers found in nature consist of diverse biological cues that include sequences for cell adhesion. Consequently, they are capable of being identified by cells. However, natural polymers are subject to batch-to-batch variation due to their structure and chemical composition complexity, leading to variations in tissue engineering outcomes [60].

There are at least two advantages of using biopolymeric materials for tissue regeneration. First, the structure and composition of polymers can be easily tailored to give rise to various physical and chemical properties that can promote certain cellular functions, including proliferation and differentiation, in a controlled manner [60]. Second, many polymers are biodegradable through either hydrolysis or enzymes secreted by cells. Therefore, over a prescribed time, the scaffold can be replaced by newly formed tissue. Thus, with degrading polymers, secondary surgery is unnecessary to remove the scaffold after implantation [60]. Polymeric materials are usually encapsulated by a layer of fibroblasts, collagen, and inflammatory cells in vivo, which is suboptimal for tissue formation. However, the biocompatibility of polymer materials can be improved by engineering the functionality of these materials. The behavior of stem cells can be controlled by engineering functionality into a biomaterial, such as via immobilization of adhesion peptides, modification of surface chemistry, and mineralizing polymer surfaces [60].

2.5. Wound Healing

Natural polymers play significant roles in different skin wound healing processes, contributing to the overall effectiveness of wound management and tissue repair. Natural polymers such as chitosan and hyaluronic acid can help reduce inflammation in the early inflammation phase of wound healing. With its anti-inflammatory properties, chitosan can minimize the inflammatory response, while hyaluronic acid contributes to a balanced immune response, potentially reducing excessive inflammation [14].

Natural polymers like collagen, chitosan, and keratin provide a scaffold for cell migration and proliferation. Collagen-based dressings act as a structural framework for cells to move into the wound area and stimulate cell division, promoting granulation tissue formation [14]. As a primary component of the extracellular matrix, collagen facilitates the formation of this supportive network. It guides fibroblasts to synthesize new collagen, helping reestablish tissue integrity. Hyaluronic acid and alginate maintain a moist wound environment conducive to cell proliferation and migration. This wet environment also helps prevent the formation of scabs, promoting faster healing. Collagen and gelatin contribute to collagen deposition and organization during the remodeling phase [14,46]. Collagen-based dressings and scaffolds help ensure the proper alignment and bundling of collagen fibers, improving the tensile strength of the healing tissue. Specific natural polymers, such as keratin, have been found to minimize scarring and promote a more natural appearance of healed tissue [14,15]. This is particularly valuable in aesthetic areas or where scar formation could impair function. Chitosan has inherent antimicrobial properties, helping prevent infections in the wound area. Chitosan dressings can inhibit the growth of bacteria, making them suitable for wounds at risk of infection. Alginate dressings, composed of seaweed-derived alginic acid, absorb excess exudate from the wound. This maintains a moist environment and helps prevent bacterial proliferation in overly damp conditions [61].

Some natural polymers can enhance their bioavailability and activity when used as carriers for growth factors. This can further stimulate cell proliferation and tissue regeneration. For instance, hyaluronic acid can be a carrier for growth factors like epidermal growth factor (EGF). Natural polymers can stimulate angiogenesis (forming new blood vessels) by influencing growth factors and cell behavior. This is vital for ensuring adequate blood supply to the healing tissue. Polymers like pectin and chitosan, which can be used in wound dressings, create a protective barrier over the wound, allowing for oxygen and nutrient exchange ( and ) [61].

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This can help prevent external contaminants from entering the wound. Natural polymers play multifaceted roles in different phases of skin wound healing. They support and optimize the processes of inflammation, proliferation, and tissue remodeling, promote a favorable wound environment, reduce inflammation, prevent infections, and enhance tissue regeneration. Their biocompatibility and biodegradability make them valuable components of wound care strategies, with applications in various types of skin wounds, ranging from acute injuries to chronic ulcers and surgical incisions [14,61].

2.6. Skin Tissue Engineering

Due to their excellent biocompatibility, biodegradability, and low cytotoxicity, as compared to synthetic polymers, natural polymers find extensive application in skin tissue engineering [62]. Polysaccharides and protein-based materials are the two primary categories of natural polymers employed in hydrogels for this purpose. Dermal substitutes comprising collagen or hyaluronic acid serve as scaffolds for cellular growth. On the other hand, epidermal substitutes, consisting of keratinocytes and fibroblasts, replace the outermost layer of the skin [21].

2.7. Bone Tissue Engineering

Natural polymers, including alginates, collagens, hyaluronic acid, and gelatin, are commonly used in bone tissue engineering. These polymers are employed in three primary forms: nanofibrous scaffolds, hydrogels, and microspheres. Biocomposites have also been developed for bone tissue engineering by combining natural polymers with hydroxyapatite. Bone scaffolds serve as a crucial application of natural polymers and provide a supportive structure for cellular growth [21]. Osteogenic differentiation, involving transforming mesenchymal stem cells into bone-forming osteoblasts, is another essential aspect of natural polymer utilization. Additionally, natural polymers are used in bone regeneration strategies, acting as scaffolds or carriers for growth factors to promote the restoration of damaged or lost bone tissue [21,22].

2.8. Cartilage Tissue Engineering

Cartilage is composed of thick proteoglycans and collagen. This thick and lubricated structure presents particular challenges for adhesives and bonding strategies. Furthermore, cartilage defects lack a regenerative capacity, as they lack blood vessels/neural tubes. Natural polymers, such as collagen, chitosan, gelatin alginate, silk fibroin, and hyaluronan, have extensive applications in cartilage tissue engineering [21]. Cartilage scaffolds serve as primary natural polymers in cartilage tissue engineering, providing a supportive structure for cellular growth. Various materials can fabricate these scaffolds, including chitosan, collagen, alginate, silk fibroin, hyaluronan, and gelatin. Chondrogenic differentiation is another significant application of natural polymers involving transforming mesenchymal stem cells into chondrocytes, contributing to cartilage formation. Furthermore, natural polymers are being investigated for repair and regeneration techniques to promote the restoration of damaged or lost cartilage tissue. These techniques often employ natural polymers as scaffolds or carriers for growth factors [21].

2.9. Heart Valve Tissue Engineering

Polysaccharides are the most abundant biomaterials in nature and meet several criteria for eligibility for tissue engineering, which include biocompatibility, biodegradation, and the ability to support cell development. Due to their biological properties and structural and functional similarities to ECM, it is reasonable to use them in tissue engineering [23]. Polysaccharides become essential to promote heart valve tissue regeneration in combination with appropriate cells or bioactive molecules. Their applications for heart valve tissue engineering are vast and varied, and approximately 70% of all studies in this field focus on chitosan, alginate, hyaluronic acid, and cellulose, respectively [23].

2.10. Cell Encapsulation

Cell encapsulation instead of therapeutic product encapsulation leads to longer delivery times, as cells continuously release encapsulated products. Moreover, cell encapsulation allows for the transplantation of non-human cells, which may be considered an alternative to the limited supply of donor tissues. In addition, genetically modified cells could also be immobilized to express any desired protein in vivo without host genome modifications [63]. Cell immobilization displays a significant advantage compared to protein encapsulation, allowing for the sustained and controlled delivery of de novo-produced therapeutic products at constant rates, leading to physiological concentrations. The versatility of this approach has adapted its use in treating diabetes, cancer, central nervous system diseases, heart diseases, and endocrinological disorders, among others. Hydrogels are among the most promising biomaterials for recreating native extracellular matrix (ECM) properties due to their high water content, biological compatibility, and moldability ( ) [24].

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Alginate is the most studied material for cell encapsulation and has been adopted for many biomedical applications. Alginate has historically been used as a protective barrier to enhance cell therapies for immunoprotection of pancreatic islets, treatment of brain tumors, treatment of anemia, and cryopreservation [64]. Current treatments include surgery, immunotherapy, chemotherapy, targeted therapy, hormone therapy, and radiation therapy and result in numerous adverse effects affecting the patient&#;s health, making the search for alternative therapies an emerging need [24,64]. Another critical study reported that liposomes, vesicles formed by phospholipids encapsulated in an alginate matrix, were transported directly and could release drugs directly to the colon cancer target, reaching higher drug concentrations in the tumor. Thus, in addition to comprising an alternative to cancer treatment, one of the most common diseases worldwide with high mortality rates, the alginate biopolymer may also be used as a carrier agent for targeted drugs [24].

2.11. Biofabrication

Recent advancements in biofabrication techniques allow the production of a polymer matrix biophysically and structurally similar to the ECM. Combined with different cell lines, this matrix can proliferate and differentiate into the desired tissue. Moreover, incorporating different growth factors or other biomolecules can improve the cells&#; migration, growth, and differentiation [1,15].

Numerous research studies for polymer matrix biofabrication follow two different strategies for cell incorporation: (i) cell implantation on a previously formed polymer matrix and (ii) fabrication of a polymer matrix with encapsulated cells.

The first strategy has been used for the last decade and is restricted to cell implantation. Typically, these techniques do not enable effective assimilation between cells and the polymer matrix. The success of these methods in regenerating tissues depends on the polymer matrix&#;s physical characteristics, such as its degradation rate, hydrophobicity, and stiffness. The most used techniques are layer-by-layer melt molding, photolithography, and self-assembly [1].

The second strategy has been implemented in recent years as it allows the fabrication of advanced cell-laden structures with complex cellular microenvironments. Recently, advanced techniques, such as microfluidics, electrospinning, and 3D bioprinting, have permitted the integration of cells directly into the polymer matrix with accurate physical and biological properties to match the ECM of the desired tissue [1].

2.12. Bio-Based Monomers

Itaconic acid, a promising bio-based monomer material, can be obtained using fermentation. Baup discovered it in while conducting the pyrolysis of citric acid. However, it was only in that it was reported as a biological product synthesized by Aspergillus itaconicus [18]. Due to its non-toxicity, biocompatibility, biodegradability, chemical reactivity, and microbe resistance, it has excellent potential for various scientific uses in biomedical, food, agricultural, pharmaceutical, and other industries [18,25].

Georgius Agricola discovered succinic acid in [18]. Succinic acid is a C-4 dicarboxylic acid, considered one of the most promising bio-based monomers for producing microbial fermentation. It has been used in the food industry and is derived from various microorganisms and agricultural carbohydrates; it is non-toxic, biocompatible, and biodegradable. Thus, it is widely used in developing biomedical products, food additives, pharmaceutical products, surfactants, detergents, microbe-resistant products, green solvents, and biodegradable plastics [18,65].

Citric acid production on an industrial scale began in , thanks to the Italian citrus fruit industry. In , James Currie, an American food chemist, discovered a way to produce citric acid using Aspergillus niger [18]. Two years later, Pfizer, a pharmaceutical company, started using this technique for industrial production [18]. Citric acid is a natural organic compound involved in the Krebs cycle. It is multifunctional, nontoxic, biocompatible, and biodegradable. It finds widespread use in the chemical, food, cleaning, and biomaterials production industries [18,66]. While the industrial applications of citric acid are well known, the biomedical applications of chemically and physically modified citric acid or cross-linked polymer biomaterials have not been thoroughly reviewed.

Microbial fermentation produces glutamic acid, a biodegradable natural bio-based amino acid monomer. In , Karl Heinrich Ritthausen, a German chemist, discovered and identified glutamic acid by treating wheat gluten (the substance&#;s namesake) with sulfuric acid [18]. Glutamic acid plays a crucial role in the body&#;s disposal of excess or waste nitrogen and undergoes oxidative deamination catalyzed by glutamate dehydrogenase. Because of its non-toxicity, biodegradability, biocompatibility, and excellent cation chelating ability, glutamic acid has various applications in various industries, such as pharmaceuticals, cosmetics, food, water treatment, and agriculture [18,67]. Poly (glutamic acid) (PGA) is a natural linear polymer synthesized by bacilli like Bacillus subtilis, formed by the peptide bonds between the α-amino group and the γ-carboxyl group at the end of the glutamic acid side chain. Biomaterial development has extensively utilized glutamic acid due to its excellent bioactive properties, which can be achieved by chemical and physical modification or cross-linking with natural and synthetic polymers [18,67]. summarizes the characteristics of the natural bio-based monomers mentioned.

Table 2

Bio-Based MonomersSourceCharacteristicsItaconic acid Aspergillus itaconicus Antimicrobial activity, non-toxic, biocompatible, biodegradable, chemical reactivity, surfactant forming ability, hydrophilic activity, wound-healing activity, coating forming ability, water uptake ability, drug carrier ability, and hydrogel-forming abilitySuccinic acidActinobacillus succinogenes, Anaerobiospirillum, and Mannheimia succiniciproducensBiocompatible, biodegradable, non-toxic, chemical reactivity, food additives ability, food flavoring ability, surfactant/detergent extender/foaming ability, drug carrier ability, pH control ability, antimicrobial activity, and corrosion prevention abilityCitric acidCitrus fruits and Aspergillus nigerBiocompatible, biodegradable, non-toxic, excellent chelating property, anti-odor property, chemical reactivity, pH control ability, food additives ability, food flavoring/preservative ability, and drug carrier abilityGlutamic acidBacillus subtilis and Bacillus licheniformisBiodegradable, biocompatible, non-toxic, excellent chelating property, heavy metal removal ability, cosmetic property, drug carrier ability, hydrophilic activity, anionic property, thickener property, aging inhibitor ability, and use as an additiveOpen in a separate window

Polyolefins and related polymers

By far the most important industrial polymers (for example, virtually all the commodity plastics) are polymerized olefins. Olefins are hydrocarbons (compounds containing hydrogen [H] and carbon [C]) whose molecules contain a pair of carbon atoms linked together by a double bond. Most often derived from natural gas or from low-molecular-weight constituents of petroleum, they include ethylene, propylene, and butene (butylene).

Olefin molecules are commonly represented by the chemical formula CH2=CHR, with R representing an atom or pendant molecular group of varying composition. As the repeating unit of a polymeric molecule, their chemical structure can be represented as:

The composition and structure of R determines which of the huge array of possible properties will be demonstrated by the polymer.

Polypropylene (PP)

This highly crystalline thermoplastic resin is built up by the chain-growth polymerization of propylene (CH2=CHCH3), a gaseous compound obtained by the thermal cracking of ethane, propane, butane, or the naphtha fraction of petroleum. The polymer repeating unit has the following structure:

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Only the isotactic form of polypropylene is marketed in significant quantities. (In isotactic polypropylene, all the methyl [CH3] groups are arranged along the same side of the polymer chain.) It is produced at low temperatures and pressures using Ziegler-Natta catalysts.

Polypropylene shares some of the properties of polyethylene, but it is stiffer, has a higher melting temperature, and is slightly more oxidation-sensitive. A large proportion goes into fibres, where it is a major constituent in fabrics for home furnishings such as upholstery and indoor-outdoor carpets. Numerous industrial end uses exist for polypropylene fibre as well, including rope and cordage, disposable nonwoven fabrics for diapers and medical applications, and nonwoven fabrics for ground stabilization and reinforcement in construction and road paving. However, because of its very low moisture absorption, limited dyeability, and low softening point (an important factor when ironing clothing), polypropylene is not an important apparel fibre.

As a plastic, polypropylene is blow-molded into bottles for foods, shampoos, and other household liquids. It is also injection-molded into many products, such as appliance housings, dishwasher-proof food containers, toys, automobile battery casings, and outdoor furniture. When a thin section of molded polypropylene is flexed repeatedly, a molecular structure is formed that is capable of withstanding much additional flexing without failing. This fatigue resistance has led to the design of polypropylene boxes and other containers with self-hinged covers.

It is generally accepted that isotactic polypropylene was discovered in by the Italian chemist Giulio Natta and his assistant Paolo Chini, working in association with Montecatini (now Montedison SpA) and employing catalysts of the type recently invented by Karl Ziegler for synthesizing polyethylene. (Partly in recognition of this achievement, Natta was awarded the Nobel Prize for Chemistry in along with Ziegler.) Commercial production of polypropylene by Hercules Incorporated, Montecatini, and the German Farbwerke Hoechst AG began in . Since the early s production and consumption have increased significantly, owing to the invention of more efficient catalyst systems by Montedison and the Japanese Mitsui & Co. Ltd.

Polystyrene (PS)

This rigid, relatively brittle thermoplastic resin is polymerized from styrene (CH2=CHC6H5). Styrene, also known as phenylethylene, is obtained by reacting ethylene with benzene in the presence of aluminum chloride to yield ethylbenzene, which is then dehydrogenated to yield clear, liquid styrene. The styrene monomer is polymerized using free-radical initiators primarily in bulk and suspension processes, although solution and emulsion methods are also employed. The structure of the polymer repeating unit can be represented as:

The presence of the pendant phenyl (C6H5) groups is key to the properties of polystyrene. These large, ring-shaped groups prevent the polymer chains from packing into close, crystalline arrangements, so that solid polystyrene is transparent. In addition, the phenyl rings restrict rotation of the chains around the carbon-carbon bonds, thus lending the polymer its noted rigidity.

The polymerization of styrene has been known since , when the German pharmacist Eduard Simon reported its conversion into solid styrol, later renamed metastyrol. As late as little commercial use was found for the polymer because of brittleness and crazing (minute cracking), which were caused by impurities that brought about cross-linking of the polymer chains. By Robert Dreisbach and others at the Dow Chemical Company&#;s physics laboratory purified the monomer and developed a pilot-plant process for the polymer, which by was being produced commercially.

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Foamed polystyrene is made into insulation, packaging, and food containers such as beverage cups, egg cartons, and disposable plates and trays. Solid polystyrene products include injection-molded eating utensils, audiocassette holders, and cases for packaging compact discs. Many foods are packaged in clear, vacuum-formed polystyrene trays, owing to the high gas permeability and good water-vapour transmission of the material.

More than half of all polystyrene produced is blended with 5 to 10 percent polybutadiene to reduce brittleness and improve impact strength. This blend is marketed as high-impact polystyrene.

Polyvinyl chloride (PVC)

Second only to PE in production and consumption, PVC is manufactured by bulk, solution, suspension, and emulsion polymerization of vinyl chloride monomer, using free-radical initiators. Vinyl chloride (CH2=CHCl) is most often obtained by reacting ethylene with oxygen and hydrogen chloride over a copper catalyst. It is a carcinogenic gas that must be handled with special protective procedures. As a polymer repeating unit, its chemical structure is:

homopolymer arrangement of polyvinyl chloride

Figure 3A: The homopolymer arrangement of polyvinyl chloride. Each coloured ball in the molecular structure diagram represents a vinyl chloride repeating unit as shown in the chemical structure formula.

The repeating units take on the linear homopolymer arrangement illustrated in Figure 3A.

PVC was first prepared by the German chemist Eugen Baumann in , but it was not patented until , when Friedrich Heinrich August Klatte used sunlight to initiate the polymerization of vinyl chloride. Commercial application of this plastic was limited by its extreme rigidity. In , while trying to dehydrohalogenate PVC in a high-boiling solvent in order to obtain an unsaturated polymer that might bond rubber to metal, Waldo Lonsbury Semon, working for the B.F. Goodrich Company in the United States, serendipitously obtained what is now called plasticized PVC. The discovery of this flexible, inert product was responsible for the commercial success of the polymer. Another route to a flexible product was copolymerization: in the Union Carbide Corporation introduced the trademarked polymer Vinylite, a copolymer of vinyl chloride and vinyl acetate that became the standard material of long-playing phonograph records.

Pure PVC finds application in the construction trades, where its rigidity and low flammability are useful in pipe, conduit, siding, window frames, and door frames. In combination with plasticizer (sometimes in concentrations as high as 50 percent), it is familiar to consumers as floor tile, garden hose, imitation leather upholstery, and shower curtains.

Polyvinylidene chloride (PVDC)

Vinylidene chloride (chemical formula CH2=CCl2, polymer repeating unit structure &#;[CH2&#;CCl2&#;]) can be made directly from ethylene and chlorine or by the further chlorination of vinyl chloride with subsequent removal of hydrogen chloride by alkali treatment. It is polymerized in suspension or emulsion processes, using free-radical initiators. The outstanding property of vinylidene chloride is its low permeability to water vapour and gases&#;a property that makes it ideal for food packaging. Copolymers of vinylidene chloride and other monomers are also marketed. The best known is saran, a trade name for a copolymer consisting of about 87 percent vinylidene chloride and 13 percent vinyl chloride. Saran is extruded into transparent films for use as a food wrap.

Polyvinyl acetate (PVAc)

The monomer vinyl acetate (CH2=CHO2CCH3) is prepared from ethylene by reaction with oxygen and acetic acid over a palladium catalyst. It is polymerized with free-radical initiators, primarily in emulsion processes, and forms the polymer phase in water-based paints. It is also polymerized in solution to give an adhesive with a very high degree of tack (stickiness).

Synthesis of three other industrial polymers begins with PVAc. Polyvinyl alcohol (PVA), a water-soluble polymer employed in textile and paper treatment, is made by hydrolyzing PVAc. Polyvinyl butyral (PVB) and polyvinyl formal (PVF) are manufactured from PVA by reaction with butyraldehyde (CH3CH2CH2CHO) and formaldehyde (CH2O), respectively. PVB is employed as a plastic film in laminated safety glass, primarily for automobiles. PVF is used in wire insulation.

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