INTRODUCTION:

Nanofibers are fibers which having diameter in nanometer ranges from 50-1000 nm. It can be prepared from different polymers which having different physical properties and application potential. The diameter of nanofibers are depends on the type of polymers used in the preparation method. Nanofibers have various properties due to which it is used mostly, as compare to other such properties are,

· Large surface area

· High porosity

· Small pore size

· Diameter range

· Mechanical strength

· Flexibility

There are various methods are used to prepare the nanofibers such as electrospinning, self assembly, template synthesis etc. from which electrospinning method is mostly used due to its straightforward setup, produce thin fibers of same diameter, high efficiency, low cost. This method was first developed in 1934. By this method we can fabricate the nanofibers in various shapes such as hollow, cone, flat, ribbon.¹

HISTORY:

· Nanofibers were first produced by English Physician Willium Gilbert in 1544-1603 by electrospinning method. He was documented the electrostatic attraction between liquid in which he observed spherical drops of liquid warp in cone shape when it was held in high electrically charged field.

· In 1872-1919, English Physicist Lord Rayleigh analysed the unstable state of liquid drop and stats that liquid is ejected in tiny jet.

· In 1987, British Physicist Charles Vernon Boys publish manuscript about nanofibers development.

· In 1934-1944, Auton Formhals was attempt nanofibers production.

· In 1996, Simons published a device that can be produced thin and light weight nanofibers.

· During 20th century the electrospinning and nanofibers are the words become common language for Scientist and Researchers.

POLYMERS USED:

Various types of polymers are used to prepare nanofibers like natural polymers and synthetic polymers as follows:

A] NATURAL POLYMERS:

1) Collagen

2) Cellulose

3) Silk fibroin

4) Chitosan

5) Gelatin etc.

1. COLLAGEN:

The name collagen comes from the Greek colla (kólla), meaning "glue", and suffix -gen, denoting "producing". Collagenis the main structural protein in the extracellular space in the various connective tissues in the body. As the main constituent of connective tissue, it is the most abundant protein in mammals,² creating 25% to 35% of the whole-body protein content. Collagen consists of amino acids wound together to form triple-helices of elongated fibrils.³ It is mostly found in fibrous tissues such as tendons, ligaments, and skin. Plasma treatment of collagen coated poly(L-lactic acid)-co-poly (caprolactone) developed into a mesh type nanofiber with an indication to prevent hyperplasia of small diameter vessels. The surface distribution of collagen on these fibers enhances the spreading, attachment, viability and proliferation of human coronary artery endothelial cells, necessitates the use of this biomaterial in vascular tissue grafting. The mechanical properties of collagen have been improved by grafting of poly (caprolactone) to form a graft co-polymer. The combination resulted in improvement of both biological and mechanical properties. The grafting was achieved by dissolving poly (caprolactone) in a mixture of dimethylformamide /dichloromethane and reacted with collagen by means of carbodiimide coupling chemistry. The grafted co-polymer was then spun in the form of scaffolds by using the electrospinningtechnique.⁴

2. CELLULOSE:

Cellulose is associate organic polysaccharide located inside the fiber walls of plants. This natural polymer is the most significant and interesting biopolymer. Cellulose is insoluble in water and most common solvents, used in a broad range of applications involving composites, netting, upholstery, coatings, packing, paper, etc⁵. Cellulose is the most abundant biomaterial on earth that possesses excellent biocompatibility and biodegradability. These versatile properties make cellulose nanofibers useful in many fields like tissue engineering, filtration and as protective clothing^6,7. The fabrication of pristine cellulose nanofibers is often a complex process because of its low solubility in most of the common solvents and its inability to melt, this often demands high voltage intensity⁸. Moreover, this often requires fabrication of cellulose nanofibers from its derivatives and then regenerating the cellulose by different chemical reactions. Recently, cellulose nanofibers have been directly electrospunusing solvents like N-methylmorpholineoxide/water and lithium chloride/N,N-dimethyl acetamide systems.^9–11.

3. SILK FIBROIN:

Silk fibroin is a structural protein encapsulating the sericin extracted from cocoons of Bombyxmori and is characterized by a distinct amino acid composition^12,13. The protein has quite a significant resemblance to the extracellular matrix of the bone and is, therefore, moulded into scaffolds for bone tissue regeneration¹⁴. The polydimethylsiloxane films have been known for use in the development of electronic skin^15,16. The material is highly sensitive towards pressure and this is a key factor that skin can easily react to pressure sensors. However, this material is known for biotoxicity and is redundant for the fabrication process, thus limiting its use in pressure based sensors. Fabricating this material with a biocompatible polymer like silk fibroin has been demonstrated in one of the studies conducted by Wang et al. The fabrication process involves the carbonization of silk nanofiber membranes prepared by the process of electrospinning. The carbonized silk fibroin nanofibers were then integrated with polydimethylsiloxane films to construct flexible sensors through a cost-effective and large-scale production approach. The sensors demonstrated superior performance in terms of high sensitivity, ultralow detectable pressure limit (0.8 Pa) and a response time (30 to 50 ms) faster than the human fingertips. The sensor material due to its excellent flexibility can be attached conformally onto an uneven surface such as the human skin. Furthermore, it can be used to monitor human physiological signals like expiration, inhalation, jugular venous pulses, subtle touches and spatial distribution of pressure. Since the developed sensor material can be scaled up, therefore, it possesses huge potential applications in next-generation wearable electronics¹⁷

4. CHITOSAN:

Chitosan is a linear polysaccharide made by randomly distributed β-(1→4)-linked D-glucosamine and N-acetyl-D-glucosamine . It is created by treating the chitin shells of shrimp and different crustaceans with an alkaline substance, like sodium hydroxide. Chitosan is the de-acetylated derivative of chitin and is known to function as collagen in higher vertebrates¹⁸. The chitosan nanofibers have been fabricated by electrospinning and other different techniques for efficient applications in drug delivery and tissue engineering due to their excellent biocompatibility and biodegradability^19,20. However, this polymer is difficult to process due to solubility in acidic medium and often requires blend in with synthetic polymers and/or surface functionalization²¹. Other problems that hinder its use in biological applications is its antigenic nature and this issue has also been addressed by functionalizing the amine moiety with different ligands^22,23. The most common problem associated with chitosan that limits its utility in guided bone tissue regeneration is the uncontrolled swelling and mechanical instability in the aqueous environment. In one of the studies, this problem has been overcome by surface butyrylation of the chitosan nanofibers. This surface butyrylation was obtained by immersing the chitosan nanofibers in a mixture of pyridine and butyrylanhydride.²⁴

5. GELATIN:

Gelatin is a biomaterial based on animal proteins, it is a pure protein food ingredient prepared by the thermal denaturation of collagen. It appears slightly yellow and commercially solid as transparent, odorless, brittle and tasteless granule, sheets, flakes or powder, soluble in hot water, glycerol and acetic acid, and insoluble in organic solvents²⁵. This natural polymer is derived from collagen and mostly found in fibrous tissues such as tendon, ligament, and skin. It is also abundant in cornea, cartilage, bone, blood vessels, gut, and intervertebral disc²⁶. This natural polymer has been used for long years ago in pharmaceutical formulation, cell culture and tissue engineering due to its excellent biocompatibility, ease of processing and availability at low cost.²⁷ Surface modified nanofibers of poly(caprolactone) fabricated by electrospinning have been proposed for application in making artificial blood vessels. The grafting of both aligned and non-aligned nanofibers of poly(caprolactone) by gelatin was achieved by treating them with air plasma to induce –COOH groups on the surface, followed by covalent grafting of gelatin molecules on the COOH groups using carbodiimidevas the coupling agent. The grafting of the gelatin was found to enhance the spreading, adhesion, alignment and proliferation of endothelial cells compared to pristine nanofibers. Furthermore, the immunostaining micrographs showed expression of platelet-endothelial vascular cell adhesion molecule 1 on these nanofibrous scaffolds, suggesting their potential role in blood vessel fabrication²⁸.

B. SYNTHETIC POLYMERS:

1. Polyvinyl alcohol (PVA )

2. Polystyrene

3. Polylactic acid (PLA )

4. Polyglycolic acid (PGA )

5. Polyethylene Glycol (PEG) etc.

1. POLYVINYL ALCOHOL:

Poly(vinyl alcohol) (PVOH, PVA, or PVA) is a water-soluble synthetic polymer. It has the idealized formula [CH₂CH(OH)]n. It is used in papermaking, textiles, and a variety of coatings. It is white (colourless) and odorless. It is sometimes supplied as beads or as solutions in water.²⁹Polyvinyl alcohol has excellent film forming, emulsifying and adhesive properties. It is also resistant to oil, grease and solvents. It has high tensile strength and flexibility, as well as high oxygen and aroma barrier properties. However these properties are dependent on humidity, in other words, with higher humidity more water is absorbed. The water, which acts as a plasticiser, will then reduce its tensile strength, but increase its elongation and tear strength.³⁰

2. POLYSTYRENE:

Polystyrene (PS) is synthetic aromatic hydrocarbon polymer made from the monomer styrene.³¹ Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and rather brittle. It is an inexpensive resin per unit weight. It is a rather poor barrier to oxygen and water vapour and incorporates comparatively low freezing point.³² Polystyrene is one of the foremost widely used plastics, the scale of its production being many million tonnes annualy. Polystyrene can be naturally transparent, but can be coloured with colourants. Uses include protective packaging such as containers, lids, bottles, trays, tumblers, disposable cutlery and within the creating of models.³³

3. POLYLACTIC ACID:

It is a thermoplastic polymer fabricated by the polymerization of lactic acid. It exists in two isomers: poly (L-lactic acid) and poly (D-lactic acid). Polyesters are most ordinarily used polymer in bone tissue engineering. There are several polyesters that are food and drug administration (FDA) approved. Usually, polyesters is copolymerized with completely different proportions to tune the degradation rate. PGA and PLA are shown to promote healing and osseointegration. These are slow delivery carrier for growth factors. Having a low modulus value, PLA must be either copolymerized with polymer having higher modulus or created into a composite with appropriate material. PGA and PLA are copolymerized to manage over the degradation rate for a specific application in bone tissue engineering.³⁴

4. POLYGLYCOLIC ACID (PGA):

Polyglycolic acid (PGA) is simple linear aliphatic the rmoplastic polyester with high crystallinity (46–50%). Its melting point is 225^◦C and glass transition temperature of 36^◦C. PGA has high modulus (7 GPa) and degrades almost completely within 4–6 months under in-vivo condition. It is good material used for bone tissue engineering application.

5. POLYETHYLENE GLYCOL (PEG):

Polyethylene glycol (PEG, Molecular weight less than 20,000g/mol), polyethylene oxide (PEO, Molecular weight above 20,000g/mol), and polyoxyethylene (POE, Molecular weight less than 20,000g/mol) are chemically similar, but only differ by their molecular weight. These are oligomer or polymer of ethylene oxide (-CH2-CH2-O). Due to the chain length effect, their physical properties are different but exhibit nearly identical chemical properties⁹. Polyethylene glycol is a hydrophilic polyether with a wide range of percentage crystallinity, molecular weight, glass transition temperature, melting point, and degradation rate based on the synthesis technique. Co-polymerization of PEG with other hydrophobic polymers, such as PLA³⁵, PCL, and PGA have been practiced to enhance the degradation rate and neutralize the acidic products that were obtained by physiological degradation. PEGylation is a process of covalently coupling a PEG structure to another large molecule³⁶.

METHODS OF PREPARATION:

1. Electrospinning

2. Thermal- induced phase separation

3. Drawing

4. Template synthesis

5. Self assembly

1. ElectrospinningMethod:^36,37,38

Electrospinning is that the most widely used methodology to fabricate nanofibers. A typical electrospinning set-up, consists of mainly three components:

1. A capillary with pipet or needle of tiny diameter

2. A high voltage supplier

3. A metal collecting screen

Electrospinning is administrated at temperature with atmosphere condition.

There square measure primarily 2 electrospinning setups; vertical and horizontal.

· A high voltage is applied to form an electrically charged jet of chemical compound resolution or soften.

· The jet undergoes stretching before it reaches the collector and it solidifies on the collector within the kind of nanofibers by the evaporation of the solvent.

· In electrospinning, most of the polymers square measure dissolved in a very solvent, forming a compound solution. The polymer fluid is then fed to the capillary for electrospinning. For chemical compound melts, the electrospinning method should be conducted at vacuum condition.

· In these process principle involves subjecting chemical compound solution or soften control at its own physical phenomenon at the top of a capillary to an electrical field. As the intensity of the electrical field is magnified, the subfigure surface of the solution at the tip of the capillary elongates and forms a cone-shaped shape called the Taylor cone.

· A critical value is attained upon further increase in the electric field in which the repulsive electrostatic force overcomes the surface tension and the charged jet of fluid is ejected from the tip of the Taylor cone.

^·The discharged polymer solution jet is unstable and elongates as a result, allowing the jet to become very long and thin Charged chemical compound fibers solidifies with solvent evaporation Randomly-oriented nanofibers area unit collected on the collector.^39,40

2. Thermal Induced – Phase Separation:

Thermal-induced phase separation separates a homogenous polymer solution into a multi-phase system via thermodynamic changes.

The procedure involves 5 steps:

compound dissolution, liquid-liquid or liquid-solid phase separation, polymer gelation, extraction of solvent from the gel with water, and freezing and freeze-drying under vacuum. Thermal-induced phase separation method is widely used to generate scaffolds for tissue regeneration. The homogenized compound solution within the beginning is thermodynamically unstable and tends to separate into polymerrich and polymer lean phases below acceptable temperature. Eventually when solvent removal, the polymer-rich section solidifies to make the matrix and therefore the polymer-lean section develops into pores. Liquid-liquid separation is usually used to form bicontinueous phase structures while solid-liquid phase separation is used to form crystal structures. The gelation step is important in order of controlling the pore size of nanofibers.^41,42

Polymer concentration affects fiber properties:

· Arise in compound concentration decreases consistence and will increase mechanical properties like enduringness.

· Solvent properties influence morphology of the scaffolds.

· After gelation, gel is placed in water for solvent exchange.

· Afterwards, the gel is removed from the water and goes through freezing and freeze-drying. It is then stored in a desiccator.^38,43

3. Drawing:

The drawing technique makes long single strands of nanofibers one at a time.

The pulling process is accompanied by solidification that converts the dissolved spinning material into a solid fiber.

A cooling step is important within the case of soften spinning and evaporation of solvent within the case of dry spinning. A limitation however, is that solely a elastic material which will bear intensive deformations whereas possessing decent cohesion to survive the stresses developed throughout actuation is created into nanofibers through this process.^44,45,46

4. Template synthesis:

The template synthesis method uses a nano- porous membrane template composed of cylindrical pores of uniform diameter to make fibrils (solid nanofiber) and tubules (hollow nanofiber).

This method can be used to prepare fibrils and tubules of the many styles of materials, including metals, semiconductors and electronically conductive polymers.

The uniform pore give management of the scale of the fibers therefore fibers with terribly little diameter are often created through this technique. However, a drawback of this method is that it cannot make continuous nanofibers one at a time.^47,48

5 Self assembly:

The self-assembly technique is employed to come up with amide nanofibers and amide amphiphilic. The method was impressed by the natural folding method of aminoalkanoic acid residues to make proteins with distinctive three-dimensional structures.⁴⁹ The self-assembly method of amide nanofibers involves varied driving forces such as hydrophobic interactions, electricity forces, chemical element bonding and van der Waals forces and is influenced by external conditions like ionic strength and hydrogen ion concentration.⁵⁰

APPLICATIONS:

1. Tissue engineering

2. Drug delivery

3. Air filtration

4. Oil – water separation

5. Wound dressing

6. Catalyst and enzyme

7. Sensor

8. Carriers

9. Sound absorbtive material etc.

1. Tissue Engineering:

A basic principle or purpose of the tissue engineering is to repair, replace, maintain, or enhance the function of a particular tissue or organ. It involves the use of living cells, manipulated through their extracellular environment or genetically to develop bio-logical substitutes for implantation into the body. First, a scaffold should possess a high porosity, with an appropriate pore size distribution. Second, a high surface area is needed. Third, biodegradability is often required, with the degradation rate matching the rate of new tissue formation.

Fourth, the scaffold should possess the specified structural integrity to stop the pores of the scaffold from collapsing throughout neo-tissue formation, with the acceptable mechanical properties.

Finally the scaffold should be non-toxic to cells and biocompatible, positively interacting with the cells to promote cell adhesion, proliferation, migration, and differentiated cell function.^39,40

2. Wound Dressing:

Wound healing may be a native method of create dermal and cuticular tissues. nanofiber membrane may be a sensible wound dressing candidate attributable to its distinctive properties: the extremely porous membrane structure and well interconnected pores ar significantly necessary for exuding fluid from the wound; the small pores and very high specific surface area not only inhibit the exogenous microorganism invasions, but also assist the control of fluid drain- age; in addition, the electrospinning process provides a simple way to add drugs into the nanofibers for any possible medical treatment and antibacterial purposes.^1,39,40,51

3. Filtration:

Filters are widely used in each households and business for removing substances from air or liquid. Filters for setting protection square measure wont to take away pollutants from air or water. Similar perform is additionally required for a few materials utilized in the medical space. Large particles square measure blocked on the filter surface thanks to the sieve impact. Particles that square measure smaller than the surface-pores can penetrate into the filter. The filtration efficiency is normally affected by the filter physical structure such as fiber fineness, matrix structure, thickness, pore size, etc. Electrospinning nanofibers for filtration application have a long history. A company in US (Donaldson) has produced electrospinning nanofiber based filter products for industry, consumer and defense applications for more than 20 years. Recently another company (AMSOIL) has conjointly developed a nanofiber based mostly filter for automobile applications. Electrospinning nanofiber membrane provides dramatic increases in filtration efficiency at relatively small decreases in permeability. In comparison with typical filter fibers at constant pressure drop, nanofibers with a diameter finer than half a micron have a much higher capability to collect the fine particles, as a result of the slip flow round the nanofibers will increase the diffusion, interception and inertial impaction efficiencies. nanofiber membrane had a rather higher filtration potency (99.993%) than the HEPA filter (99.97%). Besides solid particles, tiny liquid droplets within a liquid-liquid immiscible system could also be removed by a nanofiber membrane.^36,38

4. Drug Delivery:

Controlled release is an efficient process of delivering drugs in medical treatment. It can balance the delivery kinetics, reduce the toxicity and side effects, and improve patient compliance.⁵² In a controlled release system, the active ingredient is loaded into a carrier or device first, and then releases at appreciable rate in vivo when administered by an injected or non-injected route. The drug loading is very easy to implement through a electrospinning process, and the high applied voltage used in the electrospinning process had small effect on the drug activity. The release profile can be controlled by modulation of nanofiber morphology, porosity and composition. Many factors may influence the release performance, such as the type of polymers used, hydrophilicity and hydrophobicity of drugs and polymers, solubility, drug polymer comparability, additives, and the existence of enzyme in the buffer solution for a long-lasting release process, it would be important to maintain the release in an even and stable pace, and any early burst release should be avoided. The early burst release can be reduced when the drug is encapsulated within the nanofibermatrix.^53,54,55,56

5. Sound Absorbtive Material:

Nanofibers showed excellent efficiency to absorb sound. A leading nanofiber technology industry, Elmarco recently patented an electrospinning nanofiber material that had unique sound absorption features, with only about one-third of the weight of traditional sound absorption materials. It was able to absorb sounds across a extended range of frequencies, particularly low frequency sounds below 1000 Hz.⁵³

6. Sensor:

Sensors have been extended used to identify chemicals for environment protection, industrial process control, medical diagnosis, safety, security and shielding applications. A good sensor should have a small dimension, low fabrication cost and multiple functions, excluding the high sensitivity, selectivity and reliability. High sensitivity and quick response necessary to the sensor device to have a large specific surface area and extremely porous structure. The characteristics dominate by electrospinning nanofibers match well with these requirements.⁵⁴

7. Carriers:

Enzymes are normally immobilized with a carrier. The immobilization capability mainly depends on the porous structure and enzyme-matrix interaction. Nanostructured materials are recently used as enzyme carriers because of their large specific surface area and the high loading capability. To immobilize enzyme on electrospinning nanofibers, many approaches have been used, including grafting enzyme on fiber surface, physical adsorption, and incorporating enzyme into nanofiber through electrospinning followed by crosslinking reaction.⁵⁶

CONCLUSION:

Electrospinning is a promising and effective method which offers several possibilities for the creation of high added value products for various applications. From the above applications it is clear that nanofibers are the best for the use as compare to other fibers and it gives very promising effects. Hence due to its pore size, surface area, mechanical strength now a days it is popular.

AKNOWLEDGEMENT:

Author would like to thanks our Principle Dr. M. J. Patil and Dr. Avinash R. Tekade, Dean and HOD of Pharmaceutics.

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Received on 18.12.2019 Modified on 16.02.2020

Research J. Pharm. and Tech. 2021; 14(4):2321-2327.

DOI: 10.52711/0974-360X.2021.00410