1. Introduction
Nanofibers with different physical properties can be produced from a range of synthetic and natural polymers to suit different application requirements. Nanofibers produced from natural polymers/fibers have a range of biomedical applications, such as scaffolds for tissue engineering, cardiovascular implants, repair of articular cartilage, urethral catheters, mammary prostheses, vascular grafts, penile prostheses, artificial skin, and adhesion barriers. Natural fibers are generally derived from three sources: plants, minerals, and animals. They are characterized by low density, moderate tensile strength (200–1000 MPa), stiffness in the range of 20–60 GPa, and low cost [
1,
2]. Around 2000 classes of plants are used as a source for obtaining these fibers [
3]. Plant fibers can be classified into nonwood and wood fiber. The recycling process of plant fibers is easier than that of mineral fibers [
4]. Overall, plant fibers have superior stiffness and strength compared with animal fibers, with the exception of silk fibers. These characteristics make natural fibers the most suitable fibers for the manufacturing of bioproducts [
5,
6].
Plant fibers and plant fiber composites have drawn the attention of research in the last two decades [
6]. The nanofibers used in the biomedical and healthcare sector are particularly made of biodegradable or biocompatible materials [
7]. Whey protein nanofibers have been used in regenerative medicine applications. For the development of new therapeutic wound dressing, electrospun hydrophilic nanofiber mats are used [
8]. Gelatin nanofiber mats from the
Centella asiatica (L.) plant are used for its healing ability [
9]. Nanofiber from chitosan/polyethene and oxide/green is used to decrease inflammation and increase the speed of recovery of wound healing [
10]. Nanofibers from sorghum and zein nanofiber are used for practical uses in medical applications and controlling bacterial growth [
11]. Nanofibers from basil seed mucilage are used for different applications, such as packaging film production and bioactive encapsulation [
12]. Deep eutectic solvent-zein nanofibers developed in the range of 350 ± 50 nm through the electrospinning process show exceptional hydrophilic properties [
13]. Yue Jioa et al. produced self-healing hydrogels using chemical and physical functionalization. The polymerization process consisted of 2-2-6-6-tetramethylpiperidin-1-yl)oxyl, polyacrylic acid. The research claimed high mechanical properties, viscoelasticity, and increased self-healing. Different techniques, such as self-assembly, template-based synthesis, polymerization, sonochemical synthesis, and electrospinning, are employed to produce natural micro-/nanofibers [
14]. Electrospinning is, however, one of the more preferred techniques that is employed to easily produce natural nanofibers. Among various advantages, electrospinning allows controlled porosity of the electrospun material, making of 3D structures, ease in the functionalization of fabricated nanofibers, and ease of fabrication of very thin fibers with a bigger surface area.
A typical electrospinning process, shown in
Figure 1, consists of a syringe pump set up to provide high voltage (1 to 30 kV) to induce charge on the drops of the polymer solution and a needle to release the polymer solution in the form of a fiber jet with a fastidious feeding rate to be collected on a collector. Due to Rayleigh’s uncertainty, the structure of a fiber can be pretentious. When the jet initially emits (a very short duration) from the tip, it follows a straight path, and then the looping, curling, winding, and bending of the jet happen [
15], forming a nanofiber over the collector. Collectors can be different types, rotating or fixed, placed with different ranges 5–30 cm away from the core electrode. This distance is not fixed; it can be varied, depending on the spinning condition. Generally, stationary collectors are used to collect randomly oriented nanofibers.
Electrospinning can be used to produce nanofibers from a number of natural polymers. Kebede, T.G. et al. fabricated good-quality blended nanofibers, with an average diameter of 232.87 ± 59.35 nm, from water-soluble proteins extracted from Moringa stenopetala seeds using electrospinning. The concentrations and parameters used include 10% (
w/
v) protein/polyvinyl alcohol (PVA) solution in 3% formic acid, a voltage of 15 kV, 12.5 cm tip-to-collector distance, and a flow rate of 5 μL/min [
16]. F. Kurd et al. fabricated nanofibers with an average diameter of 179–390 nm using electrospinning from basil seed mucilage (BSM) [
12]. N. Angel et al. produced nanofibers, with an average diameter of 404–1346 nm, using electrospinning from cellulose acetate using acetone solvent. The electrospinning process parameter included a needle (22 gauge), flow rate (2 mL/h), and voltage (9 to 15 kV) [
17]. Sailing Zhu et al. manufactured an elastomer composite from carbon-nanotube-doped silylated cellulose nanocrystal. The resulting product showed high electrical conductivity along with high strength and was tested as a strain sensor [
18]. S.O. Han et al. electrospun nanofibers from cellulose and studied the deacetylation of CA through different solvent systems and showed that by changing the composition of the mixed solvent, the average diameters of the CA nanofibers could be controlled from 160 to 1280 nm [
19]. Silvestri et al., using the electrospinning method, produced nanofibers from graphene oxide (GO), gum arabic (GA), and polyvinyl alcohol (PVA) [
20]. S.T. Sullivan et al. produced nanofibers from whey proteins. Aqueous whey protein solutions, whey protein isolate (WPI), and beta-lactoglobulin (BLG) were electrospun into nanofibers, with an average diameter of 312 to 690 nm, using a polymer, polyethylene oxide (PEO) [
21]. Sofia El-Ghazali et al. successfully developed artificial blood vein using electrospinning nanofibers from the solution of poly (ethyleneglycol-co-1,4-cyclohexane di-methylene-co-isosorbide terephthalate) and poly (1,4 cyclohexane dimethylene-co-isosorbide terephthalate) [
22].
M. Kowalczyk et al. showed that a composite formed using cellulose possesses has higher storage modulus as compared with composites formed using a PLA matrix [
23]. R. Panneerdhass et al. fabricated epoxy polymer hybrid composites from luffa and found the range of mechanical properties to be: a compressive strength of 26.66 to 52.22 MPa, a tensile strength of 10.35 to 19.31 MPa, a flexural strength of 35.75 to 58.95 MPa, and an impact energy of 0.6 to 1.3 joules [
24]. S. Ochi et al. developed biodegradable “green” composites from Manila hemp fiber bundles and a starch-based emulsion-type biodegradable resin. The tensile and flexural strengths of the composites increased with increasing fiber content up to 70%. The tensile and flexural strengths of the composites were found to be 365 and 223 MPa, respectively. Fabrication with emulsion-type biodegradable resin contributed to the reduction in voids in the composites [
25,
26]. S.H. Teng et al. electrospun uniform composite fibers, with a diameter of 60 nm, from collagen −30 wt. % HA composite solutions in an organic solvent [
27,
28].
The literature review shows that the use of natural nanofibers is generally limited to the medical engineering field because of the low strength. It is therefore important to see how the strength of natural nanofiber composites can be improved. The paper investigates the effect of adding hemp seed (HS) on the mechanical properties of a hybrid nanocomposite of “cellulose acetate (CA)”. A nanocomposite in the form of nanofibers was developed through the electrospinning process from CA (matrix) and a mixture of acetone and acetic acid (solvent). HS was initially used for reinforcement. Hybrid composites were later developed using a glass fiber and epoxy polymer matrix with different volume fractions to carry out the tensile testing. The electrospun nanofiber composites were collected over glass fiber mats, and a hybrid composite was manufactured using VARTM. After the hybrid composite fabrication, five rectangular plates having dimensions of 165 mm × 19.5 mm for tensile testing were machined by CNC milling to investigate the mechanical properties of the hybrid composites.