Abstract
The methods available for the disposal of synthetic polymers are not advanced in an environment-friendly way. Consequently, their waste persists as a non-degradable pollutant that discharges toxic substances, which have now reached the deepest parts of the ocean. As an alternative, biopolymers such as polylactic acid, polyhydroxyalkanoates, and poly(butylene succinate), synthesized from natural sources such as plants, animals, and microbes, are an eco-friendly option, as they are biodegradable and a better option to shift from synthetic polymer dependency. The fabrication of electrospun nanofibers (NFs) using biopolymers is a novel approach, by which new ideas have been proposed in various fields, such as agriculture, biomedical, food packing, textiles, adsorption, drug delivery, three-dimensional printing, etc. Electrospun NFs are receiving increasing attention due to their diverse properties, including flexibility. This review provides a perception of the novel biopolymers that are currently utilized by the electrospun technique and their various applications.
Owing to the excessive usage of fossil fuel over recent decades, many economic issues such as expensive renewable energy increasing the injudicious use of fossil fuels and environmental issues such as global warming have put the planet in danger. Hence, humans are now in dire need of different types of renewable energy sources and eco-friendly goods to replace fossil fuel. For example, the energy sectors are working hard on generating wind power, photovoltaics, and biofuels, which are renewable, sustainable, and eco-friendly resources. 1 The petrochemical industrial sectors are also trying to replace environmentally harmful goods with biodegradable materials to save the planet. Polymeric goods are used for either long-term or short-term applications, such as synthetic textile fabrics, wood, jute, polyethylene (PE) cups, nylon bearings, epoxy glue, paints with polymers, tires, balloons, and gloves. 2 Nevertheless, polymer disposal into the environment can become harmful for living beings and the ecosystem. Failure to implement protocols to carry out polymer recycling and the slow degradation of fossil fuel-based polymers gives rise to a huge environmental concern. 3 Hence, replacing the current polymers, which are made mainly from fossil fuel, with biodegradable polymers is an essential goal for many countries to help in saving the planet. This encourages researchers all around the world to find novel, renewable, and eco-friendly polymeric products made from environment-friendly substances, such as plants, microbes, and animals. Thus, the idea of creating bio-based polymers from renewable resources has been proven to be quite promising in recent years. 3
Biopolymers come from two sources: plants and animals. Plant-based biopolymers come from leaves, latex, stems, litter, wood, and food crops, while animal-based biopolymers are prepared from chitosan (CTS), protein, DNA, and RNA. 4
Apart from these, bacterial polymers are increasingly garnering attention because of their easy availability, cost-effectiveness in terms of preparation, easy handling and storability, biodegradability, and different industrial and therapeutic applications. Polysaccharides, such as alginate, cellulose, CTS, curdlan, dextran, gellan, and xanthan, and proteins such as glycoprotein, polyglutamate, peptidoglycan, tolin, protein polysaccharides, and lipopolysaccharides, are also used for biopolymer production. These biodegradable polymers have wide applications in the biomedical, agricultural, and food industries.5,6
A sharp growth in bio-based polymer production indicates a positive sign for the environment. Recent biopolymers, such as PE, polyethylene terephthalate (PET), bio-polyethylene (bio PE), polyhydroxybutyrate (PHB), polyethylene furanoate (PEF), polylactic acid (PLA), polyhydroxyalkanoates (PHAs), and poly(butylene succinate) (PBS), have proven to be the most promising biopolymers. Moreover, carbohydrates, proteins, DNA, and RNA are also used as composites for various biomedical procedures. 7 Carbohydrate-based nanofibers (NFs) are used in tissue engineering, antibacterial treatment, drug delivery systems, filters and adsorbents, sensing applications, and enzyme immobilization. 8 DNA NFs are used for gene transfer in mammalian cells 8 and the manufacturing of semiconducting materials, fullerene DNA-hybrid materials, DNA-nanowires, and DNA-guided conducting materials. 9 The RNA molecules incorporated in NFs are used for different applications, such as gene silencing, 10 wound healing, 11 and cancer cell lines. 12 Protein NFs also play a vital role in biomedical applications, such as neural regeneration, 13 bone regeneration, 14 vascular regeneration, 15 and drug delivery. 16
Biopolymers can be shaped into different forms, such as nanotubes, nanowires, and NFs. The fabrication of biopolymers in nanofibrous form makes them promising materials with unique properties, such as a high surface area, high porosity, and high flexibility. For example, biopolymer NFs have the ability to act as nanocarriers for the delivery of different classes of drugs, such as antibiotics, anticancer drugs, microbial peptides, and DNA and RNA peptides to target sites in the human body.17,18 DNA is adsorbed into NFs as plasmid DNA for gene delivery and RNA is adsorbed into NFs for gene silencing, and proteins such as hormones, growth factors, and enzymes are incorporated for various applications. 19
NFs are fabricated using various techniques, such as electrospinning (ES),20–23 phase separation,24,25 solution blowing, 7 self-assembly, 25 and template synthesis. 24 Among these, ES is the most promising.
ES is the most popular method used for polymeric NF fabrication because of its simplicity and capacity to produce micro and nanofibers of sizes 1 µm–100 nm. 7 This process was first proposed by William Gilbert in 1600, who did so by observing the electrostatic force of liquids. ES has occurred over the past 500 years; in the 19th century, it reached its prime. In 1938, the first electrospun fiber was produced by Rozenblum, which is now used as a filter material. In 1990, Reneker and his team popularized ES and demonstrated how plant-based organic polymers could be converted to NFs using this process. 26 Following this, ES has been used to prepare numerous types of biopolymer mats with various applications.
Green ES is a new way of defining eco-friendly materials used for NF production. Raw materials used in this process are eco-friendly and biodegradable along with being non-toxic, pollution-free, and solvent-free. Green ES includes three categories: green and degradable material, green solution ES, and solvent-free ES. The ES polymer materials are of three different types: natural polymer, chemical synthetic polymer, and bio-synthetic polymer materials. The green and degradable materials are derived from natural sources, which is finally dedicated to nature. Materials synthesized using microorganisms and chemicals are known as bio-synthetic polymers and chemical synthetic polymers, as their name denotes. These natural and bio-synthetic materials are biodegradable and, thus, eco-friendly. Synthetic polymers exhibit an anticorrosion ability, which renders them resistant to decomposition causing serious pollution. Regularly practiced methods of plastic waste disposal, such as burial, incineration, and landfilling, are impractical in densely populated regions. To overcome this, eco-friendly materials are most welcomed in recent years; the recycling of biopolymers can solve the issues of resource shortage and environmental pollution. However, the primary shortcoming of manufacturing biopolymers is the technology required and the high polymer cost. 27
The scope of this review covers the properties of biopolymer NFs, their economic importance, the recent developments in NFs fabricated using ES, their relevance in different industries, biomedical implications and other biotechnological applications, along with current and future perspectives.
Need for a sustainable source
Previously, polymers were made from natural sources, such as casein, shellac, linoleum, and rubber. However, in the mid-20th century, a strong call for the replacement of petroleum-based synthetic polymer products was sounded. The major issue with synthetic polymers is that they are produced using non-renewable energy sources and negatively impact the environment. Nearly 1.6 million people are directly employed by around 60,000 plastic production companies in Europe alone, with a turnover of 360 billion euros.
Plastic production globally has grown gradually from two million tons in 1950 to 380 million tons in 2015; of this, only 18% was recycled. 28 It was estimated that around 12.5 million tons of plastic reached the ocean in 2010, mostly from coastal countries. Every year, nearly 8% of crude oil and natural gas is used for plastic production globally. 29 Globally, 93 million barrels of crude oil are produced every day, of which around 25% is estimated to be used for plastic production by the end of the year 2100, as shown in Figure 1. 30 In 2018, 360 million tons of plastics were produced around the world as compared to the 348 million tons produced in 2017. Of this, 51% came from Asia, 30% of which was produced by China, 3% came from the Commonwealth of Independent States, 17% came from Europe, 7% came from the Middle East and Africa, 18% came from North America, and 4% came from Latin America, as shown Figure 2 31,32; nearly 50% of this production comprises single-use plastics in the form of food and beverage packages, and daily use products such as grocery bags, straws, tea cups, and water bottles.
Oil- and bio-based polymer degradation and the fate of synthetic polymer in the environment (Rajmohan et al., 2019).
Global production of plastics based on petroleum products. CIS: Commonwealth of Independent States.
Around 25% of the total plastic produced is used for making permeant structures, such as shelters, and construction materials, such as pipes and electrical wires, and the remaining 25% is used for home appliances and electronics. 31 These plastic materials are toxic to the environment. There are a few ways to dispose of plastic, such as using it for landfills and incineration, while tertiary recycling includes chemical recycling, thermochemical recycling, and pyrolysis.
Usage of discarded plastic for landfilling depends on the location, size of the landfill, environmental conditions, climate, and the type of discarded material. 33 Apart from this, incineration is also a major form of plastic recycling process; however, 20% of it turns into ash and over 1.5% chlorine, which is extremely harmful to the environment. The separation of polyvinyl chloride (PVC) may help to reduce chlorine emission, but it is a tough task, as it requires a great deal of labor and is expensive. 34
Every form of plastic produced using oil-based products has its own level of environmental toxicity and recycling barriers. Thus, researchers are now focusing on biodegradable, eco-friendly, and easily available materials as alternatives to synthetic polymers. Biopolymers have been used as bio-composite materials in recent years because of their easy availability, cost-effectiveness, biocompatibility, and lower toxicity. 35
Global status of biopolymers
Biopolymers are replacing synthetic polymers gradually. Biotechnology is being used to produce many biodegradable compounds, such as biopolymers, vitamins, and organic acids, from natural sources. The European Union (EU) is leading in bio-based plastic production; around 3.35 million tons (1%) of the total 335 million tons of plastic produced globally is bioplastic. 36
The investment made in biopolymers is increasing every year. France has invested around €20 million over the past 10 years to produce novel biological gears, biopolymers, and biomaterials from microorganisms that can be used as renewable carbon sources, which would further boost the bio-based economy. A recent survey has shown that nearly 335 million tons of plastic are produced globally every year, of which 1% comprises biopolymer. Around 2.11 million tons of biopolymer were manufactured in 2018, and an estimated 2.62 million tons are projected to be manufactured in 2023. PHA is the most preferred type of biopolymer, and experts opine that PHA production will be doubled by 2025. PLA is another biodegradable thermoplastic polyester obtained from renewable plant sources. It is a good replacement for polystyrene, polypropylene, and acrylonitrile butadiene styrene. 36
Therefore, the demand for PLA production is estimated to increase two-fold by 2023. 37 Bio-based products, such as PE, PET, and polyamides (PAs), are produced globally and account for 48% of the total bioplastic produced, which is nearly one million tons of bioplastic. 37 Moreover, PEF is also attracting attention in the market and is likely to be rolled out in commercial products in 2023. PEF is made of biomaterials and is suitable for packaging beverages, food, and commercial products. This is a 100% renewable material as compared to oil-based products, such as PET. 38
The EU concentrates more on developing renewable polymers from different sources; their production of bio-based polymers is accounts for a fifth of total global production. By the end of 2023, the EU bioplastic companies’ shares are expected to rise by 27% because of the recent initiatives taken by the Italian and French governments. In 2018, 55% of bioplastics came from Asia, 16% from North America, 9% from South America, and 1% from Australia (https://ect-center.com/blog/biopolymers-market-2019).
Global bioplastic consumption has increased in recent years, resulting in a higher demand for bio-based products. Around 2.11 million tons of bioplastic was produced in 2018, and 2.62 million tons are expected to be produced by the year 2023. Pioneering materials, such as PLA and PHA, have shown tremendous growth in the biopolymer field. These polyesters are 100% biodegradable and are made from bio-based products, with a wide range of properties. Bio-based non-biodegradable plastics, such as PE, PET and PA, together account for around 48% of the total biopolymers produced globally. Naturally degradable biopolymers, such as PBAT (poly(butylene adipate-co-terephthalate)), PBS, PLA, PHA, and starch blends, comprise 43.2% of the total bioplastic produced, as shown in Figure 3. 36
Global production of different types of bioplastics produced in 2018. PET: polyethylene terephthalate; PA: polyamide; PE: polyethylene; PTT: Poly(trimentylene terephthalate); PLA: polylactic acid; PBAT: poly(butylene adipate-co-terephthalate); PBS: poly(butylene succinate); PHA: polyhydroxyalkanoate.
Electrospinning
NFs produced through ES have become quite popular in different fields for a wide range of applications because of their unique physical and chemical properties, such as easy adaptability, high surface area, non-toxicity, and sensitivity to environmental changes. 39
NFs are produced using various techniques, such as drawing,40,41 template synthesis,42,43 phase separation,44,45 self-assembly,24,46,47 and ES.20,48–54 Although all these techniques have certain drawbacks, ES is the most advantageous in terms of successful fabrication of numerous NF mats for various applications, such as water treatment, 55 oil–water separation, 56 desalination, 57 and making solid oxide fuel cells. 58
Polymer fibers with a size below microns are gaining more attention than those with larger diameters. ES can produce lengthy single fibers of diameter less than 1–100 nm, and the ES process parameters can be controlled to alter the fiber mats’ diameter, porosity, surface area, and charges. The rapid growth of ES in the past couple of decades has resulted in the production of remarkable products that are being used in various fields due to their simplicity and cost-effectiveness, 48 as shown in Figure 4.
The electrospinning types and biopolymer applications in various fields. PHB: polyhydroxybutyrate; PEF: polyethylene furanoate; PLA: polylactic acid; PHA: polyhydroxyalkanoate; PBS: poly(butylene succinate).
ES has a simple setup that comprises a high-voltage power supply, a syringe pump to transport the polymer solution, and a rotating collector connected to the power supply. The pump gently forces the polymer solution through the charged needle endpoint, and the electric charge in the needle persuades the charged ions present in the polymer solution toward the oppositely charged electrode. This results in the formation of a jet of solution that moves to the oppositely charged collector where the spinning of polymer solution occurs. The solvent evaporates during the process, and the hardened fibers are caught by the collector.53,59,60
Voltage has a crucial impact on the diameter of NFs; an increased voltage, along with other optimal conditions, such as the flowrate and spinning distance, decrease the diameter.
However, the temperature and humidity must be considered as well. 39 ES is classified into six types: blend ES, melt ES, emulsion ES, basic ES, gas jet ES, and co-axial ES, as shown in Figure 5. 61 A brief explanation about each ES technique is given in the following sections for more information; it is recommended to read the following references where useful reviews about the ES types are presented.61–63
Various types of electrospinning (ES) discussed in this review.
Blend electrospinning
Blend ES is a process of mixing various polymers or a drug with polymer in different ratios before ES them under favorable conditions. This usually results in the encapsulation of the drug within a polymer matrix. Blend ES has been effectively tested with various biological substances, such as antibiotics,64–66 probiotics, 67 anti-inflammatory agents,68,69 and proteins. 70
This method is excellent to produce material for delivering small biological molecules. However, due to the nature of the organic solution, the bioactive compounds may lose their activeness. A clear understanding of the nature of the polymer and biomolecule is more important for a controlled release of molecules using electrospun fibers.
Co-axial electrospinning
This is the most well-established and most preferred ES method for producing polymers to be used for drug release purposes. It is a process of co-spinning two different polymer solutions simultaneously to prepare NF composites with core–shell structures. Co-axial spinning helps to produce synthetic and biopolymers with enhanced physical, chemical, and biological properties.63,70
At the target site, the electrospun composite initially bursts and releases its outer shell and then releases the drug from the core over a sustained duration. 71 Usually, the co-axial NFs displays an even morphology of NFs with a protein dispersion that is consistent in the co-axial NFs. A sustained release of proteins from the co-axial NFs was achieved when poly(ethylene glycol) (PEG) was added, which eases the release from the NF mat. Subsequently, the process reduces the bioactivity of the protein, with a maximum recovery of up to 75%. 70
This method is also used to promote wound healing in plants, using NFs made from cellulose acetate (CA) and polyurethane (PU) that carry 6-benzylaminopurine for sustained release. CA–PU NFs show remarkable properties, such as non-toxicity, increased mechanical strength, and high elasticity, which help with plant wound healing and graft survival. 72
Emulsion electrospinning
Emulsion ES is more advantageous than the previously mentioned methods, as it has a combination of blend and co-axial ES methods, including emulsification. Using this technique, biomolecules and surfactants are mixed to generate a water-in-oil (W/O) emulsion that is spun to form core–shell NFs using a single nozzle. This core–shell formation shields the cargo inserted inside. The emulsion is based on a couple of or multiple phases usually not mixed in the process of ES, so the polymer solutions are of a dissimilar liquid phase. In ES, a continuous phase forms the fiber shell, and the droplet phase forms the fiber core.63,73
Melt electrospinning
Melt ES has gained popularity for being a solvent-free process. Instead of the regular polymer solution, a polymer melt is electrospun and later hardened through cooling. A melt condition and controlled flow rate of the polymer melt is the key to form a superior fiber with even morphology and a large diameter. 74
This process of micro and nanofiber production does not involve solvents; this is an advantage because solvents have a lethal impact on the environment. The melt temperature can possibly affect the nature of the incorporated drugs and other biomolecules, such as proteins.
The viscosity and level of flow determine the nature of the fiber output. 75 A limitation related to the Taylor cone size directly correlates with the needle size; the initial jet diameter is also directly related to the final fiber diameter. Thus, if a thin fiber must be produced, the cone size should be limited. 76 Fibers of this category can carry out slow drug discharge, as opposed to the rapid release found in polymer solution-based NFs. 77
Gas jet electrospinning
Gas jet ES is an updated version of the melt ES technique; an additional setup of a gas jet device is used in this. Melt ES lacks a uniform temperature control appliance that would ensure that the polymer is melted evenly throughout its path to the needle. Even though few researchers have used infrared heaters in the spinneret region, it is always difficult to maintain uniform heating in the nozzle while maintaining the temperature in the polymer jet toward the spinneret.
Gas-assisted melt ES (GAME) is a unique method that can overcome this issue. PLA was used to test the efficacy of GAME; for this, a gas stream was made to pass via the nozzle that delayed polymer solidification, and the active gas flow helped the polymer to move quickly and produce a thinner fiber with added production of a polymer mat. Also, it produced a minimum micron scale of 0.18 µm, much lower than the previous micron scale of 3.5 µm produced without the GAME technique. 78
Recently, the processing condition of a multi-nozzle system for the bulk production of fibers through GAME was carried out. The shear viscous stress of the jet surface (30 Pa) was calculated, followed by the strength of the electric field (1.5 kV/mm) at the Taylor cone tip; this resulted in the development of shear viscous stress. Based on these conditions, a spinnability diagram was proposed for the successful handling of multi-nozzle GAME for industrial-scale production. 79
Electrospun biopolymer characterizations
Four different categories of characterizations, geometrical, chemical, physical, and mechanical, are usually performed to fully characterize NFs.
Geometrical characterization
Geometrical characterization is the most important type of characterization, since it gives information about fiber diameter, fiber diameter distribution, fiber orientation, cross-section shape, surface roughness, and porosity. 80 Advanced characterization using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) is utilized to obtain the geometrical properties. The roughness and porosity of the fibrous mat are obtained using atomic force microscopy (AFM) and a gas pycnometer, respectively.
Chemical characterization
The chemical molecular structure of NFs is obtained using Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) techniques. FTIR and NMR show the structure of a single polymer or two electrospun blended polymers with their inter-molecular interaction. NF surface chemical property X-ray photoelectron spectroscopy (XPS), drop shape analysis (DSA), and Fourier transform infrared - attenuated total reflectance (FTIR-ATR) machines are used to study the chemical properties at the surface of the fiber mat. 80 The NF macromolecule configuration can be also obtained using wide-angle X-ray diffraction (WAXD), optical birefringence, small-angle X-ray scattering (SAXC), and differential scanning calorimetry (DSC). 80
Physical characterization
A setup called a dynamic moisture vapor permeation cell (DMPC) has been successfully used to measure the air and vapor transport properties of electrospun nanofibrous mats. The moisture vapor transport and the air permeability of electrospun fiber mats can be measured using this device. 80
Mechanical characterization
Mechanical conventional testing techniques are used to test the mechanical properties of nanofibrous nonwoven membranes. Tensile strength, elasticity, and other mechanical properties are obtained from conventional mechanical testing machines. The tensile strength of a nanofibrous mat was found to be similar to that of natural skin. 80
Recent trends in biopolymers and their electrospinning
Natural polymers can be used to create biopolymers using a direct method or a basic chemical reaction to separate them from their core source. Biopolymers can be obtained from various sources (Figure 6). These sources are further processed to form different products, such as PHB, PEF, PLA, PBS, and PHA, which are biodegradable and non-toxic to the environment. The production of these biopolymers, their chemical structures (Table 1), and their ES are discussed in detail in the following sections.
Different sources of biopolymers.
Various polymers and their structure discussed in this review
PLA: polylactic acid; PHB: polyhydroxybutyrate; PBS: poly(butylene succinate); PHA: polyhydroxyalkanoate; PEF: polyethylene furanoate.
Why they are biodegradable
The biodegradation of degradable biopolymer materials occurs due to microbial intervention with enzymes in the active position of polymers; this induces the polymer molecules to go through the hydrolysis process and break down into small molecules, as a result of which the polymers become degradable. 27
Polyhydroxybutyrate
PHB production
PHB belongs to the PHA family, which is polyester produced for various environmental applications, owing to its extreme melting temperature of 175°C. 81 It is a short-chain thermoplastic with a high crystalline and brittle polyester polymer with the potential for biodegradability and biocompatibility. It is synthesized using different microbes as a stored energy resource as cell inclusions.
PHB was first isolated from the bacteria Alcaligenes eutrophus in the late 1980s and gene-transferred to Arabidopsis thaliana, the best model plant to produce polymer.82–84 PHB is produced in different ratios depending on the plants used; for instance, Arabidopsis cytosol produces 0.01% and plastids produce 14%.
The highest level of PHB was produced using plastids present in Arabidopsis, that is, 40% of dry weight or 4% of fresh weight.84–86 Apart from Arabidopsis, up to now, tobacco plant plastids have produced 18.8% of PHB and Brassica napus plastids have produced 7.7%.87–89
Apart from this, maize stover, sugarcane leaves, and switchgrass can also be used to produce PHB in different proportions. Notably, the highest amount of PHB is known to be produced using switchgrass because of the fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase) genes.90–93
Even though PHB is produced from different sources, microbial sources are more likely to be chosen because of their easy handling and cost-effectiveness. Properties of PHB are identical to those of polypropylene, but it is biodegradable in natural environmental conditions. The bacteria Ralstonia eutropha store up to 80% of PHB inside their cells in the form of inclusion bodies and also dry weight.
The greatest limitation to the mass production of PHB is the costly purification procedures involved. Nevertheless, using genetic engineering as a tool in microbes for PHB production in vast recombinant bacteria can prove to be more cost-effective. 94 Recent research has shown the production of PHB using synthetic bacterial consortia to be another successful method. The natural bacterial consortium reveals sturdiness and defensiveness against invasive microbes or contamination and yields several byproducts by sharing metabolites. 95
A similar bacterial consortium, Synechococcus sp. PCC 7002, has been used in recent studies to produce different products that were previously thought to be impossible to produce by bacteria. 95 Weiss et al. 96 attempted to produce PHB from a microbial consortium consisting of cyanobacteria Synechococcus elongates CscB and Halomonas boliviensis. The S. elongates support H. boliviensis by supplying sucrose in the absence of carbon. H. boliviensis stocks PHB as a carbon source to be utilized in nitrogen exhaustion. These consortiums work under phototrophic conditions and last for greater than five months, which is effective for light-based bio-production strategies.
Any product that is produced using cheap raw material and incurs low cost is highly welcomed by the industrial sector. Agriculture waste/plant waste is used for the production of PHB using Bacillus sp. The optimal conditions under which this bacterium can produce PHB include a pH of 7, a temperature of 37°C, and a time of 48 h. Glucose and peptone can be used as carbon and nitrogen sources to produce PHB in the range of 25–55.6%. Agro-waste, such as sugarcane bagasse, teff straw, banana peel, and corn cobs, are used as carbon sources as well. This method of PHB production is more useful, as it helps to minimize the problem of agro-waste disposal; moreover, the production of eco-friendly biopolymers will reduce the pollution caused by synthetic non-degraded oil-based polymers. 97 Due to its distinctive properties, such as biocompatibility and biodegradability, it is used for various applications, such as food packing, wound dressing, retinal tissue engineering, cartilage tissue engineering, and many other biomedical applications.
PHB electrospinning
PHB was enhanced by blending with synthetic CA to form an efficient electrospun fibrous mat using chloroform (Chl) and N,N-dimethylformamide (DMF) (70:30 v/v) solvents. The components were used in different ratios—0:100, 60:40, 70:30, 80:20; 90:10, 0:100 w/w—to identify the right combination. The resultant fiber exhibited a submicron size diameter of 80–680 nm, with cylindrical, bead-free, and uniform structures. Owing to the absence of a chemical reaction, the structural stability between PHB and CA was maintained through ES. A biodegradation rate of 15 ± 0.38% of its initial weight was observed; this can be tuned with various blend ratios, which can be later controlled by the crystallinity, fiber diameter, and surface area. A cell adhesion study was conducted using PHB/CA blended for 48 h and 3T3 fibroblast cells to prove its worth in biomedical applications. As a result, the NFs showed cell adhesion and proliferation comparatively better than that shown by raw PHB films. This PHB/CA blend scaffold can be used in wound dressings and tissue engineering as a biocompatible material. 98
Burkholderia xenovorans LB400 is an excellent bacterial model for studying the many aromatic compounds and industrial products produced by it. A recent research work suggested growing LB400 on glucose to produce bioplastic PHB. Apart from glucose, xylose and mannitol can also be used as a carbon source to produce PHB. A fiber diameter (Yd) of 2.6 µm and morphology index (Ym) of 0.9625 by using xylose as the carbon source and Yd of 2.2 µm and Ym of 0.9875 by using mannitol as the carbon source are achieved. Even though both NFs are produced from the same bacterial polymer source, the morphology differs due to the variation in carbon sources. 99
Apart from bacteria, microalgae are also a suitable biological choice for PHB production. The phytoplankton Spirulina is used for the large-scale production of PHB using a suitable carbon source along with a nitrogen supply, which favors the production. Polymer waste materials are used as the supplement source that provides 25% (v v−1) waste for the production of PHB by Spirulina sp. LEB 18, which gives an output of 10.6% (w w−1). 100 Similarly, an attempt was made by Kuntzler et al. 101 to produce PHB, with outstanding mechanical and thermal properties similar to those of synthetic polymers, using Spirulina sp. LEB 18. ES resulted in NFs of size 810 ± 85 nm exhibiting phenolic compound activity against S. aureus ATCC 25923. Properties such as better tensile strength and elongation along with hydrophobic and higher degradation temperature make it suitable for food packing, 102 dye removal, 103 and antibacterial activity. 102 Moreover, its antibacterial properties and non-toxic nature toward food items made it an excellent eco-friendly food packaging material. The blending of natural polymers with PHB increases its suitability for various biomedical applications. For instance, retinal degeneration causes blurred vision and even complete blindness in humans. Retinal cell transplantation is a viable approach to completely or partially overcome this disease. Synthetic polymers are mechanically stronger but weaker in biological activities. 104 Thus, a combined pectin–PHB NF was produced through ring-opening polymerization (ROP) of β-butyrolactone. This pectin–PHB was later blended with PHB to achieve the finest NFs through ES to improve the hydrophilicity and produce fiber diameters of 336–426 nm along with 39–335% enhanced elongation at the breakpoint to form pristine PHB NFs. These NFs exhibit improved mechanical properties compared with those of pristine PHB, and they are non-cytotoxic and more biocompatible. Human retinal pigmented epithelium (ARPE-19) cells are seeded on the surface, where they proliferate rapidly using the pectin–PHB NFs films as scaffolds. This vital property made pectin–PHB the potential scaffold material for tissue engineering. 105 The articular cartilage tissue expresses less regeneration capacity when it is weak or injured, which results in the need for whole joint replacement. Nevertheless, a novel approach of regenerating this tissue using cells and scaffold to improve the affected cartilage tissue has been suggested. PHB was blended with CTS using triflouroacetic acid (TFA) as the solvent; a fine fibrous scaffold was produced by ES it in various concentration of CTS. Sample P1 showed NFs with the size of 575.77 nm (minimum) and sample P5 showed NFs with the size of 1806.71 nm (maximum). CTS as a co-polymer at 15% and 20% increased the hydrophilicity, and SEM analysis revealed a tight hold, penetration, and spread of chondrocytes cells on the polymer scaffold mat. This shows that the PHB/CTS blend scaffold shows the excellent capability of bringing about cartilage tissue regeneration. 106
Polyethylene furanoate
PEF production
PEF is commercially synthesized using the polycondensation process, which is a crucial step to remove byproducts and to attain the vital polymer chain length required for specific material properties. PEF is known for its low defensive nature toward thermal impacts, a quality that is needed to produce thermo-tolerant commercial plastics. ROP is a method for PEF to achieve adequate molecular weight in a short period of time. During this process, ROP provides bottle-grade PEF within a minimum time scale of 30 min, with a material grade superior to that of the regular ones. A reduction in the emission of greenhouse gas during production is also observed. Because of these abilities, PEF obtained through ROP is used to produce the green bottle, which exhibits a reduced carbon emission.
Several recent studies on PEF have mostly talked about its production and characterization. Solid-state polymerization (SSP) is a unique industrial process implemented in PEF production, which results in polyester with high molecular mass. To date, only a handful of research has been conducted on PEF produced using SSP. 107 To produce PEF, minimal pressure was applied in the presence of a catalyst, Ti(IV)-isopropoxide. The prolonged duration of polymerization can cause an increase in the molecular weight of PEF. For instance, a molecular weight of 25.000 g . mol–1 after 24 h of SSP can be increased to 83,000 g . mol–1 by increasing the duration of SSP to 72 h at a temperature of 180°C. Apart from this, several other methods are followed to produce more stable PEF. PEFs with high molecular weight are preferred for food packaging and storing applications. For this purpose, PEF is subjected to a 6-h vacuum at 205°C and further subjected to melt polycondensation in the presence of a tributyltin (TBT) catalyst. This is followed by a re-melting process for 15 min at 250°C. After this, the PEF is powdered using a grinding process and exposed to a temperature of 170°C for 6 h. Following this, the polyester undergoes SSP at time intervals of 1, 2, 3.5, and 5 h at temperatures of 190°C, 200°C, and 205°C in vacuum conditions. The expected viscosity is obtained after SSP at 205°C for 5 h, with a high molecular weight of above 1 dl g–1, which is suitable for the manufacturing process, along with the additional benefit of a decreased carboxyl-end group. 108
PEF has obtained a special place within the group of polymers because of its potential to replace PET, a polymer that has been used for decades around the world.109,110 To produce a stable polymer for commercial use, researchers are currently employing different protocols. 111 They have attempted different catalysis processes and observed their effects during melt polycondensation and esterification on PEF and observed the coloration. TBT and titanium isopropoxide (TIS) are the two catalysts that are more active than tin(II)2-ethylhexanoate (TEH) and dibutyltin(IV) oxide (DBTO) in reaction conditions. The polyester’s color changed during this reaction, from light yellow to brown, in which the titanium catalyst showed a highly intensive color. The duration of polycondensation also affects the coloration due to the degradation of byproducts. A white-colored polyester is obtained through dissolution by trifluoroacetic acid followed by a Chl mixture. The coloration of polymer is important for the future production of other polymer products.
Polyethylene furanoate electrospinning
PEF is used in industry for bottle making, electronics, food packaging, and in the textile industry because of its property of reduced permeability toward oxygen, carbon dioxide, and H2O. 112 The global demand for PEF is estimated to rise to 17,000 tons in 2022. The properties of PEF are more attractive than those of PET. PET has been used for decades around the globe to produce bottles; however, the industry is shifting gradually toward PEF-based bioplastics (Grand View Research, Inc., 2020).113 PEF is known for its excellent properties and 100% biodegradable nature. However, its applications as electrospun NFs—which can be excellent for packing, biomedical, and agricultural uses—have not yet been studied in detail.
Polylactic acid
PLA production
PLA is the most popular biodegradable biopolymer material extracted from renewable natural feedstock (starch). Lactic acid (LA) is extracted from the sugar produced during the fermentation of corn and molasses of sugarcane. Its main features, such as biocompatibility, easy degradability, bioabsorbability, mechanical strength, high melting point (175°C), excellent degree of transparency, non-toxicity, and easy adoptability in the human body, are the reasons it is chosen for an array of applications. The remarkable mechanical properties, biocompatibility, and biodegradability make it an excellent thermoplastic for various applications in the health industry. 114 PLA is acquired from LA, which in turn is degraded to form a monomer of LA, mostly via hydrolysis. The condensation process of LA to form lactide, which is an α-hydroxy acid, is the primary step. The ring-opening condensation process results in the polymerization of lactide, which generates PLA. 115 The type of lactide and its quantity decide the final strength and optical clarity of PLA plastics L- or D-LA.116,117 Since it contains a carboxyl group along with a hydroxyl group, it assumes the state of a monomer during chemical transformations. 118 Figure 7 shows the chemical structure of PLA, its properties, and its applications.
Chemical structure of polylactic acid (PLA) and its properties and applications. NFs: nanofibers.
The precise depolymerization of the LA monomer by the fermentation of corn, sugars, starch, etc., results in the production of PLA, which is a dependable biopolymer with properties such as biodegradability, compostability, water solubility resistance, and excellent mechanical property. 119
The PLA obtained from renewable sources can be recycled. A combined approach of biological and chemical methods to obtain pure LA is mandatory for PLA production. PLA is exposed to temperature-controlled hydrolysis and converted to LA.
The rate of hydrolysis increases under alkali conditions, with a reduced lag period when matched with H2O; using this, a remarkable output of 0.95 g g−1 of PLA can be obtained. The hydrolysis of PLA generates D-LA as a byproduct, which is later removed by Escherichia coli to harvest L-LA with the optical clarity of 99% or more; this LA can be reused to produce PLA. 120 Microbial production of LA is a cost-effective process as it uses residues of agriculture waste as a substrate and is fermented using bacteria, such as Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Pediococcus, and Tetragenococcus, and several fungi, such as Mucor, Monilia, Rhizopus, and Yeast, which are capable of producing LA. 121 There are several reports supporting LA production using microbes that use natural substances; for instance, a mixed consortium of B. coagulans LA1507 and L. rhamnosus uses sweet sorghum juice as a substrate, 122 L. paracasei A-22 uses potato stillage, 123 L. bulgaricus uses dairy waste, 124 R. oryzae uses tobacco waste, 125 L. delbrueckii ssp. Lactis uses Kodo millet bran residue, 126 and Pediococcus acidilactici uses wheat straw. 127 Supportive material in the form of blends is introduced into the PLA matrix for stability and improvement.
Another remarkable food packing material with biodegradable properties is produced using composites of PLA and polycaprolactone (PCL) along with starch. These are processed under different parameters, and the final composites are toughened by subjecting them to microwave treatment. This results in a strong linkage between the chemicals, as opposed to the regular physical linkage generated by heat treatments. 128 A similar method called for melt blending PLA and PBA (poly(1,4-butylene adipate)). Normally, PLA exhibits 7.9% elongation at the breakpoint; however, when it was blended with PBA, a huge increase of up to 74.4% in the value was seen. This happens due to the modification of properties, which changes the PLA into a ductile material and causes an increase in the elongation. 129 Because the properties of PLA are suitable for commercial purposes, it is widely used in the field of medicine, such as for drug delivery, orthopedics, packaging materials, and sutures.
PLA electrospinning
LA is the building block of PLA, an eco-friendly biodegradable polymer, which is widely used for the production of packing materials, vessels, and stationery things. PLA is derived from LA synthesized from the fermentation of sugarcane or corn by microbes. 130 ES was carried out to produce NF mats using a high-voltage electrical field and a collector to produce the desired polymers from either synthetic or plant NFs. 131 Nevertheless, the solubility and processability of PLA produced by ES need much improvement, Thus, to overcome this problem, PLA can be blended with other composites. 132 With a 9:1 ratio of Chl and DMF solvent prepared with a 10% PLA polymer solution and 2.5% PHB, the combined NFs are bio-based polymeric membranes that show a network of yarns, three-dimensional (3D) structures, and intercalated pores of 20 µm, and these NFs are used in filtering heavy metals. 133
A mixed polymer composite was produced using corn straw waste and PLA; alkali, silane, and acid treatment were used on waste corn straw to produce treated corn straw fiber (TCSF). Later, this TCSF was mixed with modified PLA (MPLA), and PLA/CSF was also mixed to produce a composite NF mat. A NF mat of a MPLA/TCSF composite of 100–500 nm was produced with greater tensile strength. A strong adhesion was also observed between these two fibers, with an additional property of higher water resistance than that seen in the PLA/SCF composite. This composite NF is biocompatible when used for treating with HaCaT cells. Corn straw can be used as a reinforcement material to improve the NF mat for cosmetic masks and air filtration units. 134 Plant-based ES NF mats are preferred over those made with synthetic NFs, since they consist of terpenoids, tannins, flavonoids, and phenylpropanoids with antimicrobial, anti-inflammation, and antifungal properties. 135 Clary sage and black pepper essential oils (EOs) are used to modify PLA NF mats. Thermodynamic fluxes occur due to EOs during ES, which leads to the formation of wrinkled and nano-textured surfaces. NFs in the range of 1.4 ± 0.2 µm are produced through ES, which exhibit antibacterial properties; when subjected to viability testing against E. coli and S. epidermidis, they showed 100% defense against microbial degradation. The exhibited properties deem the material fit for biomedical applications, such as for wound dressings, as an antibacterial agent, and for skin rejuvenation mechanisms. 136 PLA was fabricated with CTS to produce NFs that could be applied for cardiac tissue engineering for the rejuvenation of myocardia. The NF mat showed an abundance of PLA; reduced bead formation and improved smoothness of the fibers were seen when used in ratios of PLA:CTS of 3:1, 7:1, and 10:1. Moreover, a greater concentration of CTS decreased the fiber diameter but improved the hydrophilicity. A fiber diameter of 210–350 µm was obtained with lower PLA concentration, and higher concentration produced a reduced fiber size of 120–200 µm. The 7:1 ratio of PLA–CTS was observed to help in the regeneration of myocardia. 137 Apart from this, PLA is highly useful in agriculture; it can be used to control the release of fertilizers (CRFs). Key minerals, such as nitrogen, phosphorous, and potassium (NPK), are encapsulated with the core–shell fiber technology using co-axial ES to make a safe delivery schedule in plants. For this purpose, poly(vinyl alcohol) (PVA) is used in its core phase and PLA is used in its shell phase. The fiber size initially ranged from 0.1 to 0.3 µm, and after loading it with fertilizers, a minor increase of 0.2–0.5 µm was observed. A combination of PVA and PLA fibers exhibits a better encapsulation than PVA fibers alone; the core–shell fibers display a release of NPK fertilizers over a longer duration rather than a sudden release in the chosen green and red cos lettuce plants. These CRF-based polymers are eco-friendly and biodegradable. The core–shell polymer can be used in horticulture and crops of greater importance. 138
In addition, technology to separate oil from water is the need of the hour. To this end, PLA NFs were produced through ES, and a size of around 700–800 nm was observed. Then this mat was subjected to titanium dioxide (TiO2) nanoparticle (NP) coating through the sol–gel method. Methyl trichlorosilane (MTS) was later layered on the PLA surface using polycondensation to improve the roughness of the surface to minimize surface energy. As a result, superhydrophobic NFs were obtained. Using a simple filtration technique, the n-hexane made contact with the PLA/TiO2/MTS NF mat and swiftly penetrated to the outer beaker, since the density of oil is lower than that of water, and the water was blocked by the membrane. The membrane exhibited excellent hydrophobicity of 157.4 ± 0.9° water contact angle in various pH conditions with minimum water adhesion. Several oils were tested to analyze the efficiency of the membrane; they exhibited excellent separation results, with efficacy greater than 95%. 139
PLA biodegradability
Polymer degradation occurs generally through the detachment of either the polymeric main or side chain. The degradation mechanisms may involve chemical, biological, or a combination of chemical and biological reactions. Polymeric biodegradation is affected by different factors: (1) first-order structure properties, such as the chemical structure and molecular weight; (2) higher order structure properties, such as melting and glass transition temperatures, crystallinity, and the modulus of elasticity; (3) surface properties, such as surface area, porosity, hydrophobicity, and hydrophilicity. 140
PLA biodegradability is affected by the environment as well. PLA is believed to be initially degraded by hydrolysis in human and animal bodies. When PLA is exposed to the environment, it is first hydrolyzed into low molecular weight oligomers, following which microorganisms mineralize these oligomers to water and carbon dioxide. 141
Polyhydroxyalkanoates
PHA production
PHA has been researched thoroughly ever since its development in the 1980s. It comes from the family of biopolyesters but with a different structure, as it is wholly synthesized by microbes. Notably, around 30% of the microbes found in soil can produce PHA. 142 Microbial PHA has several properties that make it suitable for industrial applications, such as bioplastics, industrial fermentation, food and feeds, medicine, biofuels, drugs, and chemicals. 143 PHA is a bio-based polyester produced naturally by microbes in the form of cell inclusion bodies; it does not cause any toxic effect on the host and has advantages over petroleum-based polymers.144,145 Algae are also used as living cell factories for PHA production because of their benefits, such as high yield, easy maintenance, and adaptability to different environments; moreover, they also help by consuming the atmospheric carbon and regulating greenhouse gases. Approaches such as these minimize disadvantages of the previous methods of using agricultural feedstock for polymer production. 146 The microalgae Cyanobacteria are prokaryotes that can be used for the production of PHA because of their property of biomass production using sunlight and CO2 as the sole energy source, which is shown in detail in Figure 8.147–150 Modifications in the cultivation strategies improve the quantity of biopolymer produced by altering the parameters of nutrients, such as phosphorous and nitrogen; changing the salinity induces stress along with dynamic strain, which is an important factor for a high yield of PHA, that is, up to 70% in dry weight. Many studies have reported microalgae that produce a high quantity of PHA, ranging from 30% to 70%, such as Synechococcus sp., 151 Nostoc muscorum, 152 microalgae consortium, 153 Spirulina sp. LEB18, 154 Nostoc muscorum, 155 the microalgae consortium, 156 and Aulosira fertilissima 157 ; these studies were conducted under different growth conditions. Apart from the production of PHA from microbial sources, alternative methods include PHA recovery done through municipal wastewater sludge and sugarcane molasses.158,159
Polyhydroxyalkanoate production from micro algae and transformation to products.
Many methodologies have been proposed for the extraction of PHA from waste material. One such method is the solvent extraction method; although it is not eco-friendly, it is acceptable as a cost-effective method. This method uses wastewater as a substrate and microbial consortium as the biocatalyst. A high concentration of PHA is produced when stress is imposed on the consortium—supplying inadequate nitrogen gives 45% PHA of dry cell weight and an inadequate phosphorous supply gives 54% PHA. The bacteria most effective in PHA production are firmicutes and proteobacteria. 160 A similar study was conducted to extract PHA by optimizing the time, temperature, and substrate concentration, which resulted in a production of 0.605 g of PHA; this is considerably higher than the amount produced under conditions with unoptimized parameters. 161 The flexibility of PHA makes it suitable for use in different fields. When compared with PLA, PHA has gained popularity in a short period. Moreover, owing to its cheaper production cost, it will become a vital product in the market.
PHA electrospinning
PHA has properties such as microporosity, biodegradability, and biocompatibility, which make it suitable for biomedical applications. Bacterial infections and interference during wound healing are serious concerns; moreover, continuous heavy doses of antibiotics trigger antibiotic resistance in bacteria. Hence, a programmed release of biocide is needed to tackle this issue. A core–shell NF mat was prepared with polyvinylpyrrolidone (PVP) and dodecyl trimethyl ammonium chloride (DTAC) as the core layer, which is water-soluble, and a shell layer consisting of PHAs and poly(ether sulfone) (PES), which is insoluble in water. The SEM-observed diameter of the core–shell structure was 672.9 ± 87.5 nm and the pore size was 286.4 ± 98.4 nm. A reduced level of biocide release was noted from the core–shell NFs compared with the conventional fibers. Pseudomonas aeruginosa suppression was observed at 97.4% and 86.9% in single NFs and core–shell NFs at 2 h and 98.9% and 4 h and 98.0%, respectively. Owing to this property, PHA-based NF mats can be used effectively to treat wound infections. 162 Bio-based PHA NFs were prepared with chitosan powder (CSP) obtained from the pupa of the Black Soldier Fly (BSF). It was produced by homogenizing the BSF pupa shell using H2O, an acid, and an alkali. The combined PHA/CSP was electrospun using a bi-axial feed method to obtain antibacterial NFs of size 50–500 nm. Later, to improve the compatibility and function, acrylic acid (AA) was grafted onto the surface of PHA. The PHA-g-AA/CSP NFs were observed to be uniform, and excellent adhesion was observed between the PHA-g-AA and CSP mat, along with improved tensile strength. The PHA-g-AA/CSP NFs expressed improved water resistance, more than the PHA/CSP NFs. The antibacterial activity was tested by subjecting them to E. coli and S. aureus strains; both fibers exhibited strong inhibition results. A cytocompatibility analysis showed improved proliferation of fibroblasts when the CSP was increased in NFs. These results are promising for the successful development of PHA-g-AA/CSP NFs for tissue engineering and manufacturing air filtration membranes and bio-protective materials with an antibacterial nature. 163 The ES coating methodology allows for a unique and new packing material to be used in the food industries. Annealed AgNPs were coated with PHA to produce effective antimicrobial NFs of size 0.57 ± 0.23 µm. A low quantity silver load of 0.002 ± 0.0005 wt% was used to fabricate PHA NFs to exhibit bactericidal activity toward Salmonella enterica. This work paved the way for the production of a biodegradable material to prevent microbial proliferation in food packages. 164 Cellulose- and lignocellulose-based NFs have also recently garnered attention because of their mechanical strength and ability to create an oxygen barrier in dry conditions. However, these NFs are hydrophilic and show poor moisture resistance, which is important for use in the food packaging industry. A recent study showed how PHA can be used to produce multilayers to create moisture-resistant nanopapers. A double-sided coating was done using ES to strengthen the mechanical properties and control the hydrophilicity. The poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) electrospun coating increased the oxygen barrier of nanopaper under various humid conditions. The PHA coating on nanopapers created by ES shed light on new technology for packing food and a new way to add antimicrobial compounds. 165
Poly(butylene succinate)
PBS production
Aliphatic polyesters are unique because of their different structural properties when reinforced with other materials. This plastic possesses high biodegradability when compared with fossil fuel-based polymers. One such product in this category is PBS, an emerging polyester material praised for its high performance and degradability. 166 Recent research works have suggested PBS reinforcement to make it more thermostable than PLA.167–172 PBS is degradable by microbes in different environments, such as soil, sludge, and water, both in aerobic or anaerobic conditions; the degradation is caused mainly by ester bond hydrolysis. 173 PBS is an aliphatic polyester that consists of succinic acid as monomers, which are acquired from renewable or fossil fuels. Mostly it is produced as a result of condensation polymerization of succinic acid. 174 Lignin is another natural polymer made from phenyl propane monomers found abundantly in plants, with unique characters of antimicrobial properties along with an antioxidant (AO) nature.175–177 Domínguez-Robles et al. 178 attempted to use lignin and PBS composites to form a biodegradable and biocompatible polymer with excellent antimicrobial properties coupled with AO properties, which act as a barrier against microbial colonization on the material’s surface. The combination of these two polymers consists of 15% lignin (W/W), when produced through hot-melt extrusion. The measured value of stiffness of this material is quite similar to that of PBS. It showed a strong interaction between both the materials when analyzed using FTIR and differential scanning calorimetry, and it consists of a melt flow index of 6.9–27.7 g per 10 min. Such remarkable properties of PBS make it suitable for use in the field of agriculture, as it is non-toxic to the environment and is biodegradable. The chemical structure of PBS is altered with a suitable co-polymer to attain the desired property. 179 Han et al. 180 made a poly(butylene succinate-co-butylene 2-methylsuccinate) (P(BS-BMS)), a novel biodegradable co-polymer combination, with the capacity to act as a pesticide carrier. The prepared co-polymer microparticles are used to encapsulate avermectin using the premix membrane emulsification method. The eco-friendly nature of P(BS-BMS) along with its drug-carrier properties make it favorable for use to administer pesticides in the agricultural field. PBS degrades under natural conditions under the action of bacteria and fungi.181,182 Biopolymers are more preferred for their ability to prevent environmental contaminations. To understand the range of degradation, a composite of PBS and sugarcane rind fiber (SRF) was prepared through melt blending, and a burial experiment was carried out for 100 days in a natural soil environment. The results were compared with those of degraded pure PBS using different evaluation methods. Observations such as surface erosion indicated that the composites contain numerous pores and cracks. A 5% SRF has a better impact on the composite surface than PBS alone. 183 The diverse nature of PBS has found another novel place in environment management practices, that is, for cleaning oil spill in oceans. PBS had not previously been used as a cleaning material for performing oil spill cleaning or recovery. Maghemite was fused with PBS to produce a PBS/maghemite nanocomposite. Due to the high crystallinity of PBS, it acts as an insoluble material to help in managing petroleum spills. The way to recover it is by using this composite: 1 g of this composite can remove 11 g of petroleum from water. Moreover, PBS is becoming cheaper every year, which makes it even more advantageous. 171 In addition, numerous other materials are being used to prepare composites with PBS to improve the stability, durability, and cost efficiency. PBS is also used as a packing material in the food industries, since it possesses better properties than PE and polypropylene. In the food packaging sector, limiting microbial and fungal pathogenic growth is a challenging process. The fungal metabolite ‘cavoxin’ extracted from Phoma cava is an effective fungicide against Aspergillus niger and Penicillium roqueforti, which are commonly found in the food industry. Cavoxin and PBS polymer matrix blend films with an antifungal nature are prepared to protect food from contaminations. The cavoxin blended with PBS is a vital step toward dealing with Penicillium roqueforti, which is highly tolerant against regular food grade preservatives. 184 The high thermostability of the PBS–cavoxin blend and nucleating nature of both PBS and cavoxin result in a strong physical interaction, which helps increase the life of packed foods.
PBS electrospinning
PBS is a biodegradable and eco-friendly polymer suitable for various applications in modern science. The electrospun PBS mat is of great interest due to its nano and micro structures with excellent biocompatibility; nevertheless, identifying the right NF mat is the key to success of its application. Bio-based PBS was electrospun with various substances, such as Chl, Chl/dimethyl sulfoxide (DMSO), and Chl/DMF, at ambient temperatures to produce NFs of micron size. The NF morphology depends on various parameters, such as the PBS concentration, quality of the PBS, and the nature of the solvent. A 10% CHCL3/DMSO solvent mix has less bead formation when compared with a DMF/CHCl3 system. A 15% w/v PBS concentration produces smooth, bead-free fibers with high porosity and small pore size. Smaller beads have improved tensile strength and mechanical properties, which make them suitable materials for wound healing and tissue regeneration. 185 Keratin is a type of protein found in the human body in the hair, internal organs, and nails and in other things, such as wool and feathers. It is a natural polymer suitable for tissue engineering and medication; even though it has bioactivity and drug delivery capacity, it lacks the mechanical property mandatory for these applications. In order to compensate for keratin’s defects, PBS is also fabricated with keratin to improve its mechanical strength. Different concentrations of PBS/keratin polymer blends have been tested, but a 50–50 combination was found to be most suitable for producing electrospun NF mats of pore size 290 nm. The blend composition produces excellent thermal stability and swelling capability and increases drug release ability. The swelling ability of the NF mat is vital to absorb wound fluids and dry the wound surroundings to minimize infections. It also improves the diffusion ability of diclofenac sodium to help it mix into the wound environment for better healing. Keratin’s polar nature helps it to interact with water molecules to form a firm complex. 186 Employing the vast range of applications of PBS, a natural filler composite is made to improve its usage as material for packing, construction, and biomedical purposes. A scaffold of PEG and PBS was used to produce a co-axial microfiber to use as a drug carrier. The chemical composition, thermotolerance, and molecular interaction were studied to observe the nature of NFs. Later, the PEG scaffold was immersed in an aqueous media, which resulted in high porosity and a hydrophobic nature. In this process, PBS was dissolved in the solution, but a trace amount of PEG remained stuck to the PBS matrix, which shows a strong interaction between both polymers. The surface texture differs based on which polymer is in the shell, indicating the success of co-axial ES; usually, PEG is smooth and PBS is rough by nature. The PBS matrix is loaded with triclosan and curcumin, which are antibacterial drugs. A slow release is observed when the matrix is treated with phosphate buffer saline, and a higher release is observed when it is treated with ethanol (30:70 v/v). Triclosan release depends on the distribution of the fiber and the composition. Curcumin is effective for treating various diseases, such as myeloma, colon, and pancreatic cancers, and Alzheimer’s.187,188 Bacteria have no impact on core–shell NFs, but co-axial fibers are much more susceptible. Thus, these co-axial fibers will be much more useful for carrying out a controlled drug release process. 189
Electrospun biopolymer applications
Biopolymers are biomolecules obtained from natural sources, such as plants, animals, and microorganisms. They are monomeric units covalently attached to form larger molecules. Different materials can be synthesized from biological resources, such as vegetable oils, proteins, amino acids, and fats. These polymers are produced through direct fermentation and by chemical polymerization. 190 Synthetic polymers have been used to fabricate NFs for almost half a century now, but the latest development in the form of biopolymer production is more promising, as synthetic polymers have been found to be capable of being converting to NFs by ES in recent researches. However, their limited mechanical properties and easy degradability are becoming a hurdle at some points when compared to synthetic NFs. To minimize these barriers, many co-polymers are now being used to fabricate combined NFs, which show excellent durability, as shown in Table 2. 191
Biopolymers used for electrospinning with co-polymers, their setup parameters, nanofiber diameter, and applications
PLA: polylactic acid; PHB: polyhydroxybutyrate; PBS: poly(butylene succinate); PHA: polyhydroxyalkanoate; DMF: N,N-dimethylformamide; NPs: nanoparticles.
Superhydrophobic coatings
Superhydrophobicity is exhibited by surfaces with a static water contact angle of more than 150° and a roll of angle below 10°. Paper manufacturing industries use alkyl ketene dimer (AKD) as a sizing agent, as it is inexpensive. The glass and paper are dipped in molten AKD at 40°C for 3 min and later subjected to solidification treatment with ethanol. This creates superhydrophobicity on the surface of paper with advancing and receding contact angles of 158.7 ± 1.4° and 156.8 ± 0.9°, respectively.
However, if the melt temperature of AKD and heating period are increased to 70°C and 6 h, respectively, followed by ethanol treatment, advancing and receding contact angles of up to 163.7 ± 1.3° and 162.6 ± 1.2°, respectively, can be achieved. Successful superhydrophobicity is achieved due to porous formation and irregular micro/nano textures, which act as a surface air cushion. 192 The polystyrene and PU fibrous surface adhesion force and liquid bridge among the fibers have been studied in detail. A water droplet containing 15% glycerol was kept between the fibrous surface of a plate coated with polystyrene or PU; the plates were then subjected to compressing, stretching, and shearing. A force balance analysis simulation was carried out to determine the tangential adhesion force in the liquid bridge. The observations show that a symmetric profile was maintained between the liquid bridge and NF mat despite the anisotropic roughness of the nanofibrous structure. The simulation analysis also shows that the surface energy of the system is inversely proportional to the magnitude of the attraction force between the liquid bridge and the surface. The shear force increased with relative displacement between the plates when it was shearing the liquid bridge and reaches the plateau. Also, the findings show that an increase in the liquid volume and decrease in the plate spacing lead to a surge in shear force in the plates. These findings will help to improve the properties of biopolymers and other nonwoven fibrous media in terms of their fluid adsorption applications and fluid release capacity. 193
Three-dimensional printingThree-dimensional printing is a recent approach of fabricating products directly on the surface of raw materials using a 3D digital model by the multiple layer method. 194 This technique usually employs materials such as polymers, metals, ceramics, wax, and composites.195–199 Bio-based 3D printing has become an attractive option in recent times, as it benefits the medical field immensely. Cellulose hydrogel is one of the materials of choice when using 3D printing for tissue engineering, wound healing, and drug delivery systems.197,200,201
Three-dimensional printing with cellulose materials
A new method, 3D additive batch ES (3D-abES), has been proposed for preparing layered hydrogel gelatin microfibers, which can be tethered to various 3D printed thermoplastic designs. Layer-by-layer print paths are maintained to reduce sample-to-sample variation; dry fibers display a uniform pattern of 2–4 µm diameter, but crosslinked gelatin fibers uptake water and swell, which results in a deviation from the original dry fiber print paths. A thermoplastic design was used to maintain structural rigidity and stability, and the liquid absorbed fiber integrity showed an optimized fiber drawing speed of 4500 mm/min. The crosslinking time and concentration of polymer are crucial parameters that help control curly, wavy patterns. This hydrogel microfiber, because of its excellent consistency, can be used in cell culture, bio-sensing, mechatronics, robotics, etc. Further microfiber devices can be used as in situ force probes in tissue engineering. 202
A Fused deposition modeling (FDM)-ES method is adopted to produce a nanocomposite material for bone tissue engineering. The PLA micro struts and gelatin–forsterite fibrous composite are prepared using a combination of FDM and ES. PLA scaffolds belonging to various groups with pore diameters of 230–390 µm were fabricated using the FDM technique. After this, ES was performed to make a gelatin–forsterite nanofibrous layer on the FDM-made PLA. SEM, FTIR, and infrared spectroscopy was performed to analyze the composite microfibers. The reduced pore size in the PLA/gelatin–forsterite scaffold material compared with the raw PLA fiber is the chief property of this material. This material was immersed further in simulated body fluid solution for 28 days, after which it showed the formation of bone-like apatite over its surface on struts. This material is thus recommended for bioactive bone in maxillofacial applications. 203 Chen et al. 204 made a novel approach with the cartilage decellularized matrix (CDM), a dependable biomaterial for making cartilage tissue engineering scaffolds. A supreme CDM-based material has properties such as enhancing cell growth, tissue regeneration, and a suitable pore size for cell infiltration. Three-dimensional printing has become more popular in preparing customized structures with a suitable pore size, and for this process the CDM was processed in to a semifluid ink suitable for 3D printing. However, still the mechanical properties need to be improved, so a gelatin/poly(lactic-co-glycolic acid) was electrospun into the fiber and it was further properly dispersed by homogenizing and evaporation drying, and was added with CDM ink 50% for enhancing toughness and stiffness with excellent biocompatibility, which repairs articular cartilage defects in rabbits. This novel study is a new approach in ES fiber–CDM scaffolds with 3D printing technology.
Antioxidants
AO compounds have the property of inhibiting the damaging effect of free radicals and reactive oxygen species (ROS) by scavenging them. 205 The chief problem that the food packing industries face is oxygen permeability in packed foods, as it affects food freshness. Compounds such as ascorbic acid, catechol, ferrous acid, sulfites, and unsaturated hydrocarbons are used as oxygen scavenging agents. 206 The addition of AOs or antimicrobial peptides in food packing materials is highly beneficial in preserving food and increasing shelf life. Food and animal-derived antioxidant peptides (AOPs) have been described in many recent studies for their AO properties.205–209 AOPs consist of 2–30 amino acid residues with a molecular weight ranging from 500 to 1800 Da. 210 Recent studies have discussed ES of AOPs loaded with CTS/PVA for food packing. AOP extracted from fish and encapsulated with CTS/PVA is fabricated using ES. The resultant fibrous mats exhibit a uniform, bead-free nanostructure, with a diameter from 157.9 ± 28.8 to 195.5 ± 34.4 nm. Under AFM, the mats showed a surface roughness of 180.6–279.1 nm. FTIR and X-ray diffraction (XRD) analysis show that molecular interactions among functional groups enriched the mechanical properties and thermal stability; moreover, as compared to raw PVA, the tensile strength shows a three-fold improvement. Upon the addition of AOP, NF mats exhibit an increased level of hydrophobicity of 109.45 ± 3.88° at θ t = 0. CTS/PVA/AOP fiber mats showed high levels of encapsulation and retained AO activity. 208
Apart from this, curcumin/hydroxypropyl-beta-cyclodextrin (HP-β-CyD) and curcumin/hydroxypropyl-gamma-cyclodextrin (HP-γ-CyD) combined complex ES NF webs are produced as an AO food supplement for orally dissolving medicines. NFs encapsulating curcumin exhibit no loss during the ES process, resulting in a diameter of 200–900 nm of curcumin/HP-β-CyD and curcumin/HP-γ-CyD. Curcumin exhibits poor water solubility, but when added with cyclodextrin, it exhibits improved water solubility. 205
Food-borne pathogens possesses a serious challenge for the food industry, because of their rapid growth and effect of reduced shelf life. Fonseca et al. 211 attempted to prepare carvacrol encapsulated in NFs from soluble potato starch. It was added in different concentrations to starch solution: 0, 20, 30, and 40% v/v. The electrospun NF mat showed an average diameter of 73–95 mm, and carvacrol loaded in the NFs showed improved thermal stability as compared to free carvacrol. The AO activity of carvacrol increased by up to 83% in the 40% (v/v) carvacrol-loaded nonwoven mats. The 30% (v/v) carvacrol-loaded NFs inhibited the growth of L. monocytogenes, S. typhimurium, E. coli, and S. aureus by 89.0%, 68.0%, 62.0%, and 49.0%, respectively. Prolonged antibacterial activity for up to 30 days was observed in nonwoven mats. This method of preparing nonwoven mats is useful in food packaging systems, as it improves shelf life and food safety. A similar bioactive compound of vitamin E (α-tocopherol) AO extracted from plants fabricated into PCL solution and electrospun fiber was prepared and analyzed for AO activity with a 2,2-diphenyl-1-pi-crylhydrazyl (DPPH) radical assay. The results were found to be quiet promising, and this ES mat can be used as a food packing material. 212
Adsorbents
A change in the surface roughness of NFs makes them superhydrophilic, which becomes superhydrophilic, superhydrophobic, superoleophilic, and superoleophobic. The tough task of separating oil from water can be achieved by using NFs because of their superwettability properties. Various NF membranes are used to separate water from oil, depending on the density. 213 A two-nozzle ES was carried out using a starting material of amidoxized polyacrylonitrile for uranium extraction. The polyamideoxime (PAO) NF mat exhibits high porosity, Upon further blending with polyvinylidene fluoride (PVDF) NFs, the mechanical property, hydrophilicity, and porosity of PAO/PVDF mats are enhanced. The adsorption test conducted in simulated seawater shows a satisfactory result of adsorbing uranyl ions onto the PAO/PVDF composite mat. The AO groups present in the PVDF blend mat adsorb uranyl ions at a higher rate than regular PAO NF mats. The desorption test showed the successful separation of two elements, uranyl and vanadium ions. This two-nozzle ES can help in uranium mining from seawater. 214 Aris et al. 215 demonstrated the adsorption of organophosphorus pesticides (OPPs) using superhydrophilic graphene oxide (GO)/electrospun cellulose nanofibers (CNFs). The formation of GO/CNF was confirmed by FTIR, and this GO/CNF-10 wt% can be used for the adsorption of OPPs (methyl parathion, chlorpyrifos, ethoprophos, sulfotepp). The highest adsorption capacity was found at a pH of 12, with OPP adsorbency of 5 mg and 15 min reaction time. An average of 71.14–99.95% of OPPs was adsorbed from the river, lake water, and food samples. This synthesized GO/CNF-10 wt% is an economical, efficient, and promising material for the adsorption of OPPs from water and food samples. The preparation of electrospun NFs from plant protein, zein, soy protein, and wheat gluten for heavy metal adsorption has been previously studied. 216 Hordein, a byproduct of beer brewing, is used to produce NFs for the removal of heavy metal, Cd(II), from wastewater. The addition of N,N′-methylenebisacrylamide (MBA) to ES NFs increased the Cd(II) adsorption capacity. Hordein/MBA NFs have been found to turn bead-free upon addition of 30% MBA (W/W); this membrane has an ideal adsorption capacity of 160 mg L–1 of Cd(II) at pH 5–6 in 10 h of contact time, with a maximum adsorption capacity of 48.78 mg g–1. This NF mat remained highly efficient even after being recycled five times, and this membrane was a very promising adsorbent material for the removal of Cd(II) present in wastewater. 217 Another alginate-based NF mat can be prepared by ES along with the incorporation of zinc oxide NPs. This mat reveals a uniform texture, with the NFs having a diameter of 100 and 140 nm of interconnected voids. The membrane adsorption and release capacity were tested with methylene blue (MB) and Congo red (CR). The membrane shows a fine adsorbing capacity toward MB and CR. The membrane can be reused by washing in simple deionized water. The affinity for desorption was higher toward MB than CR, because of the ionic interactions. This is a low-cost, innovative product that can be used for drug delivery and purification methods. 218 Certain pollutants in drinking water can be removed by adsorption using the PLA and PE glycol-based ES NFs, especially because of the hydrophobicity of PLA NFs, which can help with efficient oil–water separation. 219
Drug delivery
A large specific surface area, porosity, biocompatibility, and biodegradability are key features of electrospun NFs, because of which they can be chosen as drug delivery systems. Electrospun NFs are used in for tissue engineering, wound dressings, and controlled release of drugs. 62 The rapid dissolving of gelatin fibers carrying lycopene in the digestive system is a negative impact. To minimize this, a lycopene-loaded gelatin NF layer was sandwiched between hydrophobic Zein NFs. The encapsulation of lycopene in gelatin was observed between 83.97% and 90.51%, with an acceptable level of encapsulation efficiency. The presence of lycopene in the sandwich structure and a shift in the amid I band were observed by FTIR, suggesting that a physicochemical interaction occurred between the lycopene and protein. DSC analysis also showed an increase in thermal stability in the encapsulated lycopene. The lycopene release speed was higher in the small intestine than the gastrointestinal track, with a bioaccessibility rate of 16.44% with a lycopene load of 0.075% (w/v). These NF mats provide a rapid and safe system for the delivery of lycopene to their target cells in the small intestine. 220 Similarly, Qin et al. 221 prepared fast delivering oral films from ES CTS and pullulan for successful drug delivery. The CTS/pullulan ratio plays a vital role in a NF mat and, additionally, CTS increases the viscosity and conductivity of the solution. FTIR analysis shows hydrogen bond interaction within CTS and pullulan molecules with a reduced crystallinity of materials, which is observed by XRD. These oral films dissolve in water within 60 s, so aspirin fabricated in oral films can be used as a model drug. Fazli-Abukheyli et al. 222 attempted to choose an alternative method of drug delivery, nanoporous anodic alumina (NAA). PVDF and PEG mixed NFs were coated over the top surface of NAA for controlled drug release.
The NAA was incorporated with the model drug indole-3-acetic acid (IAA) and it was closely monitored to identify, whether NAA sample efficiency screened with or without NFs is more efficient; a better efficiency in IAA release was found with the NF coating. This coating exhibited a controlled and prolonged slow release of drugs from its nanoporous surface. Yan et al. 223 fabricated a pH-sensitive PVA/PCL core–shell NF through co-axial ES in which PVA formed the core structure and PCL formed the shell structure. TEM analysis showed the successful formation of a core–shell structure, the feed ratio during the spinning process in PVA/PCL changed the surface morphology of the NF mat, and FTIR observation showed a negative result of PVA/PCL chemical interaction. These NFs can be used as anticancer agents to carry doxorubicin (DOX) owing to their property of constant and pH-dependent release of drugs, which is effective in preventing HeLa cell proliferation. This core–shell NF efficiently degrades in an acidic and neutral environment, which shows it is a promising eco-friendly drug carrier.
Textiles
NF-coated textile material applications
Antibacterial textile fabrics are used to prevent infectious diseases, but the lack of durability reduces their applications. A strong supportive additive like NFs is required to enhance their properties. Qiu et al. 224 proposed a method of producing NF embedded textiles with long-lasting antibacterial activity, by incorporating antibacterial electrospun NFs within the fiber mesh of cotton fibers and fabricating a regular textile material. These materials exhibit strong antibacterial activity of 99.99% against E. coli and S. aureus. In addition, the cytotoxicity of the antibacterial fabrics was reduced upon treatment with mouse embryonic 3T3 fibroblast cells and the results showed a low cytotoxic response. This material can be used to prepare sportswear because of its prolonged antibacterial durability of 95% even after 35 washing cycles. A similar study conducted by Yu et al. 225 showed that this antibacterial textile fabric could have broad applications in the fields of medicine and biomedical engineering, but also that this antibacterial textile has a chances of becoming contaminated during the production. A new type of antibacterial yarn is produced by ES highly efficient antibacterial loaded NFs. GO and solubilizing material, polyethyleneimine (PEI), can be used to prepare GO-PEI composites, and to this AgNPs can be added to produce GO-PEI/Ag using microwave heating. The GO-PEI/Ag composite can be added to cotton yarn or core yarn to produce NF core spun yarn, where the antibacterial agents are tightly packed with the fibers to facilitate easy removal, compared to the previous preparation of textile, such as dipping or coating.226,227 A 99.99% antibacterial potential was observed against E. coli and S. aureus and was maintained even after 10 washes. 228 Mosquito repellent textiles are in high demand, since they can prevent mosquito-borne diseases such as dengue, malaria, and yellow fever. A repellent-containing material, such as an ethylcellulose (EC) electrospun NF mat, can be prepared against mosquitos. Citriodiol (CD), a natural bio repellent used against Aedes aegypti, was combined with textiles to produce CD-loaded EC mats with various proportions. A fabulous result was obtained from the CD-loaded nanofibrous mats, which exhibit a prolonged repellent property due to the CD (CD/EC 7:10) present in the core as the active ingredient; the outer sheath reduces the rate of release and sustains it for a longer duration of up to 34 days with 100% repellency. 228
Natural polymers
Chitin
Chitin is a polysaccharide derivative consisting of amino and acetyl groups. It is found in abundance in the skeletal materials of invertebrates, such as crabs, shrimps, prawns, and insects, and in the cell wall of mushrooms. Annually around 10 10 –10 11 tons of chitin is produced. Chitin is insoluble in common solvents because it is a neutrally charged polymer, which is a disadvantage.4,6 Chitin nanofibril (CN) from fisheries and nanolignin (NL) from plants have antimicrobial and anti-inflammatory properties at the nanoscale. The CN and NL are mixed into micro complexes loaded with glycyrrhetinic acid (GA) and CN-NL/GA (CLA) complexes, which are used to fabricate polymer surfaces with electrospray on the fiber meshes of P(3HB)/P(3HO-co-3HD). This produces strong anti-inflammatory activity that down-regulates cytokines and induces HBD-2 human keratinocytes. This helps to cure complex skin wounds that have irritation. 229 The oil–water filtration can be done using advanced functional materials, such as electrospun membranes. PVDF membranes are electrospun and further reinforced with 0.5 and 1 wt% chitin nanowhisker (CNW) to increase their thermal stability, mechanical properties, and oil–water separation efficiency. The mechanical properties, such as elongation at break and tensile strength, are also improved at 1 wt% CNW nanowhisker. An improved water filtration capability was observed upon PVDF/CNW reinforced with CNW, as it increased the pure water flux, which is a remarkable challenge in the filtration process. An increased level of 99.1% oil rejection was observed. 56
Chitosan
CTS is a natural polymer usually extracted from crustaceans, with advanced properties such as biocompatibility, biodegradability, and antimicrobial and antifungal activity. Moreover, it is insoluble in water and alkaline solutions due to its rigid and crystalline structure; it is soluble only in acidic solutions with a pH of around 6.5. 230 Natural hydrogel scaffolds are poor in mechanical strength, which is why they are not helpful in bone tissue engineering. This drawback was overcome by adding CTS hydrogel into regenerated cellulose (rCL) NFs. This scaffold exhibits a uniform porous morphology after rCL NFs adsorb the CTS matrix. This NF scaffold displayed tunable water adsorption, swelling, and degradation capacity based on the rCL NF quantity. It also shows enhanced compressive strength of up to 30.19 kPa and increased bioactivity of Ca/P ratio = 1.83. Further, it facilitates MC3T3-E1 cell attachment and regeneration. 231 Similarly, Sedghi et al. 232 presented a novel approach in the form of composite NFs of PCL and CTS through a copper I-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Magnesium-doped hydroxyapatite (Mg-HA) was incorporated into the scaffold to improve bioactivity. SEM observation showed a smooth, defect-free electrospun NF with 419–495 nm diameter; in vitro cell viability test (MTT assay) revealed no cytotoxicity of the NFs. Improved antibacterial activity, mechanical strength, and cell attachment are also observed, which make this material promising for cancellous bone repair and other similar applications. CTS NFs fabricated with PEG hydrogel show improved scaffolding; increased mineralization ability is seen with a high CTS content. 233 Chitin/CTS NFs are extremely helpful in improving the mechanical properties of composite biomaterials. Raw chitin/CTS may lack the expected mechanical strength in applications, but this can be solved by blending it with other biopolymers or synthetic polymers. 234
Superhydrophobic coatings
Superhydrophobicity is a surface characteristic exhibited when the static water contact angle is above 150° and the roll of the angle is below 10°. The paper industries use AKD as a sizing agent, as it is inexpensive. The glass and paper are dipped in molten AKD at 40°C for 3 min and later subjected to solidification treatment with ethanol. This leads to superhydrophobicity on the surface, with advancing and receding contact angles of 158.7 ± 1.4° and 156.8 ± 0.9°, respectively. However an increase in the melt temperature and heating duration of AKD of 70°C for 6 h, respectively, followed by ethanol treatment, increases the advancing and receding contact angles up to 163.7 ± 1.3° and 162.6 ± 1.2°, respectively. A successful superhydrophobicity was achieved due to porous formation and irregular micro/nano textures, which forms a surface air cushion. 192 The polystyrene and PU fibrous surface adhesion force and liquid bridge among the fibers have been studied in detail. A water droplet containing 15% glycerol was kept between the fibrous surface coated plate with polystyrene or PU, and compressing, stretching, or shearing was done between the plates. A force balance analysis simulation was carried out to find the tangential adhesion force that occurs in the liquid bridge. The observations show that a symmetric profile was maintained in the liquid bridge even though there is anisotropic roughness of the nanofibrous structure. The simulation analysis shows that the surface energy of the system contrasts inversely with the magnitude of the attraction force between the liquid bridge and the surface. The shear force increased relative displacement between the plate when it was shearing the liquid bridge and reached the plateau. These findings show an increasing liquid volume and decreasing plate spacing, which lead to a surge in shear force in the plates. Thus, the outcome will be of great help in fluid adsorption applications, fluid release, and other nonwoven fibrous media. 193
Recommendations
Biopolymer-based products are novel and found to be better than synthetic polymers. Biopolymers extracted from environmental sources have a lower impact on the environment when recycled than petrochemical products. However, the gap between the production and demand in the market for biopolymers is significant. Thus, alternative approaches to the production of biopolymers should be considered, that is, from plants, feedstock waste, agricultural feedstocks, and biopolymers from engineered microorganisms. 235 Although the use of degradable polymers over regular plastics is preferred, it is still avoided because of the high production cost. Therefore, efforts must be made to produce cost-effective biopolymers. The increasing demand for biopolymers has resulted in the enlargement of cultivable land for the production of biopolymer feedstock; nearly 0.82 million hectares of land were globally used in 2017 and a rise up to 1.03 million hectares by 2022 is projected (IfBB, 2018).236 This will negatively impact food and livestock production. Thus, a balance should be maintained in the land utilized for agriculture production and feedstock production for biopolymers. 112 Electrospun NFs have gained attention in various fields, such as tissue engineering, food packing, textile engineering, agriculture, drug delivery, biomedicine, and reinforcing material production. NFs are custom-made depending on orders in various fields based on need and application. Thus, the large-scale production of NFs is always a challenging task. To this end, a few advancements have been made. For instance, a portable in situ wound dressing ES device is used to treat individual patients directly on the wound site. 237 PHB and PLA NFs that are suitable for use as wound dressing materials have been used in recent years, and these could be incorporated in portable ES devices as well. The development of portable ES apparatus that can produce biopolymer NF mats for various applications is much encouraged for future development rather than the current fixed conventional machines. PHA-based NFs are also quite interesting because they can be obtained from various biological sources such as bacteria, molasses, and sludge, rather than depending on a single source. Promoting PHA-based fibers are very useful for the controlled release of antibiotics. Although biopolymers have some drawbacks, when fabricated with synthetic polymers through ES, they become more stable and, thus, successful.
Conclusion
Recent research on electrospun NFs has focused on their rapid advancement in various fields. Among the several synthetic polymers on the market, biopolymer composites have grabbed everybody’s attention. In the near future, these biopolymer composites are expected to be used widely in oil recovery, antibacterial agent manufacturing, food packing, plant grafting, fertilizer manufacturing, tissue engineering, lithium ion manufacturing, and orthopedics, because of their sustainability. The techniques used for ES PHB, PEF, PLA, PHA, and PBS biopolymer composites are mostly carried out in the form of blend, co-axial, emulsion, melt, and gas jet ES. This review has discussed a wide range of information regarding biopolymers, and their production and application over the last few years. This will pave the way for researchers to explore the various applications of biopolymer-based products and their ability to create an eco-friendly environment, thereby encouraging the use of bio-based polymers over conventional oil-based polymers.
Footnotes
Author note
Vimalanathan ArunPrasanna is now affiliated to Masa Pest Management, Riyadh, Kingdom of Saudi Arabia.
Declaration of conflicting interests
The authors have no conflicts of interest to declare.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors extend their appreciation to the deputyship for Research and innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number ISP20-23.
