Burns are a major threat to public health and the economy due to their costly and laborious treatment and high susceptibility to infection. Efforts have been made recently to investigate natural bioactive compounds with potential use in wound healing.
1. Background
The importance lies in the capacities that these compounds could possess both in infection control by common and resistant microorganisms, as well as in the regeneration of the affected tissues, having in both cases low adverse effects. However, some bioactive molecules are chemically unstable, poorly soluble, and susceptible to oxidative degradation or have low bioavailability. Therefore, developing new technologies for an efficient treatment of wound healing poses a real challenge. In this context, electrospun nanofibers have gained increasing research interest because bioactive molecules can be easily loaded within the nanofiber, resulting in optimal burst control and enhanced drug stability. Additionally, the nanofibers can mimic the extracellular collagen matrix, providing a suitable highly porous structural support for growing cells that facilitate and accelerate skin burns healing.
2. Categorization and Characterization of Electrospun Fibers
The electrospinning method consists of pumping a simple or complex polymer solution through a capillary subjected to a high-voltage electric field [1][2][3]. Due to electrostatic repulsions from the Taylor cone formed at the tip of the capillary, the polymer solution travels to a collector that may have an opposite electrical charge or be grounded [4][5]. The jet is stretched and churned as it travels to the collector, the solvent evaporates during this process, and a solid, non-woven, fibrous matrix is deposited on the collector [6]. The alignment of the fibers in the collector is random; however, methods such as rotational, magnetic, gap, or post-drawing are being studied to induce a more ordered alignment in order to expand the mechanical properties and improve a variety of physical properties [7]. Many parameters affect the electrospinning process, such as operational variables (electric field intensity, fluid flow, distance to the collector plate), properties of the solution (concentration, viscosity, electrical conductivity, voltage surface, dielectric properties), and environmental parameters such as humidity and temperature [1].
Analyzing the publications reviewed, it was found that—despite the differing conditions such as voltage, flow ratio and distance between ejector and collector—the fibers obtained presented diameters on the nanometer scale in all cases except in the studies of Kadakia et al.
[8], Li et al.
[9], and Ilomuanya et al.
[10] (
Table 1).
Table 1. Characteristics of electrospun fibers with intended biological effects from 24 articles reviewed.
Matrix |
Encapsulated Bioactive Compound |
Electrospinning Parameters |
Diameter of Fibers |
Fiber Type |
Biological Effects of Electrospun Fibers |
Reference |
Polyurethane |
Badger (Meles meles) oil |
Voltage: 20 kV Flow Rate:—L/h Distance:15 cm |
375–518 nm |
Blend-composite |
Antibacterial |
[11] |
Polyurethane/Silver nanoparticles (10/3% w/w) |
Olive Oil (Olea europaea L.) |
Voltage: 15 kV Flow Rate:—mL/h Distance: 10 cm |
250–550 nm |
Blend-composite |
Antibacterial |
[12] |
Silk fibroin/Gelatin (1:3 w/w) |
Astragaloside IV |
Voltage: 15 kV Flow Rate: 0.1 mL/h Distance:—cm |
_ |
Blend-composite |
Accelerate the process of wound healing |
[13] |
Chitosan-Deacetylated |
Chitosan/L-arginine |
Voltage: 28 kV Flow Rate: 1.2 mL/h Distance: 10 cm |
50–500 nm |
Blend-composite |
Antibacterial |
[14] |
Polycaprolactone/Gelatin (Core) (8/4% w/w)
Gelatin (Shell) |
Minocycline hydrochloride
G. sylvestre extracts |
Voltage: 13 kV Flow Rate: 1.2 & 1 mL/h Distance: 12 cm |
300–450 nm |
Core/Shell |
Antibacterial Nanofibers |
[15] |
Polylactide/Collagen (20/4% w/v) |
Fermented rooibos A. linearis extracts |
Voltage: 25 kV Flow Rate: 0.1 mL/min Distance: 22 cm |
13–23 µm |
Blend-composite |
Antibacterial Nanofibers; Accelerate the process of wound healing |
[10] |
Chitosan |
Bromelain |
Voltage: 10 kV Flow Rate: 0.5 mL/h Distance: 20 cm. |
140–360 nm |
Blend-composite |
Accelerate the process of wound healing |
[16] |
Polylactide/Poly(ethylene glycol) (Core) (1:1 w/w)
Polylactide/Poly(vinyl pyrrolidone) (Shell) (5:5, 7:3, 8:2, 9:1 w/w) |
Peptides HHC36
Curcumin |
Voltage: 20 kV Flow Rate:—mL/h Distance: 15 cm |
3.2–4.6 μm |
Core/Shell |
Antibacterial |
[9] |
Gelatin |
ε-Polylysine |
Voltage: 12 kV Flow Rate: 0.8 mL/h Distance: 12 cm |
425 ± 33 nm |
Blend-composite |
Antibacterial |
[17] |
Poly(vinyl alco-hol) |
Chitosan |
Voltage: 18 kV Flow Rate: 0.8 mL/h Distance: 12 cm |
130–170 nm |
Blend-composite |
Antibacterial |
[18] |
Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) |
_ |
Voltage: 8 kV Flow Rate: 0.002 mL/min Distance: 12 cm |
510–670 nm |
Simple Fibers (Mono-polymer) |
Accelerate the process of wound healing |
[19] |
Silk fibroin/Poloxamer 407 (P407) (1:0, 3:1, 1:1 w/w) |
Manuka Honey |
Voltage: 25–23 kV Flow Rate: 3–4 mL/h Distance: 16.5–18 cm |
2.4–5.9 μm |
Blend-composite |
Antibacterial; Accelerate the process of wound healing |
[8] |
Sodium Alginate-Poly(ethylene glycol)/Pluronic F127 (surfactant) (8:2 w/w—1.5% w/v) |
Lavender essential oil (Lavandula angustifolia) |
Voltage: 25 kV Flow Rate: 0.5 mL/h Distance: 20 cm |
50–125 nm |
Blend-Emulsion Electrospinning |
Antibacterial Nanofibers; Accelerate the process of wound healing |
[20] |
Polycaprolactone/Chitosan (10, 15, 20/15% w/w) |
Quercetin/Rutin |
Voltage: 24–32 kV Flow Rate: 0.77 mL/h Distance: 15 cm |
90–120 nm |
Blend-composite |
Antibacterial Nanofibers; Accelerate the process of wound healing |
[21] |
Chitosan/Poly(ethylene oxide) (2/0.5% w/w) |
Actinidin |
Voltage:—kV Flow Rate: 0.5–1.5 mL/h Distance: 7–9 cm |
100–200 nm |
Blend-composite + Actinidin enzyme immobilization |
Antibacterial Nanofibers; Accelerate the process of wound healing |
[22] |
Gelatin (layer 1)
Poly(vinyl alcohol)/Sodium Alginate (layer 2) (13/2.5% w/v)
Chitosan/Poly(vinyl alcohol) (layer 3) (2/15% w/v) |
Fibrin |
Voltage: 25–30 kV Flow Rate: 0.8–1.1 mL/h Distance:—cm |
150–350 nm |
Blend-composite |
Antibacterial Nanofibers; Accelerate the process of wound healing |
[23] |
Polycaprolactone |
α-Lactalbumin |
Voltage: 9–18 kV Flow Rate: 0.3–0.6 mL/min Distance: 15 cm |
183–344 nm |
Blend-composite |
Accelerate the process of wound healing |
[24] |
Poly(vinyl pyrrolidone)/Keratin (3:1, 2:1, 1:1 w/w) |
Cinnamon essential oil |
Voltage: 24 kV Flow Rate: 350–850 μL/h Distance: 25 cm |
315–466 nm |
Blend-composite |
Antibacterial Nanofibers; Accelerate the process of wound healing |
[25] |
Polycaprolactone/Gelatin (6:4 w/w) |
Plant extracts: I. aspalathoides A. indica M. edule M. andamanica |
Voltage: 15 kV Flow Rate: 1 mL/h Distance: 12 cm |
266–601 nm |
Blend-composite |
Accelerate the process of wound healing |
[26] |
Poly-D,L-lactic acid |
Microalga Spirulina (Arthrospira platensis) |
Voltage: 15 kV Flow Rate: 2 mL/h Distance: 15 cm |
260–270 nm |
Blend-composite |
Accelerate the process of wound healing |
[27] |
Poly(L-lactic acid)/polyhedral oligomeric silsesquioxane nanoparticles (24:1 w/w) |
Plasmid DNA Encoding Angiopoietin-1 (pAng) |
Voltage: 13 kV Flow Rate: 0.8 mL/h Distance: 15 cm |
580–780 nm |
Blend-composite |
Accelerate the process of wound healing |
[28] |
Polycaprolactone/Collagen (55:25 w/v) |
_ |
Voltage: 13 kV Flow Rate: 3 mL/h Distance: 13 cm |
170–275 nm |
Blend-composite |
Accelerate the process of wound healing |
[29] |
Poly(lactic-co-glycolic acids)/Collagen (4:1 w/w) |
_ |
Voltage: 28 kV Flow Rate: 1 mL/h Distance: 17 cm |
100–300 nm |
Blend-composite |
Accelerate the process of wound healing |
[30] |
Polycaprolactone (12.5% w/v) Poly(vinyl al-co-hol) (8% w/v) |
Curcumin |
Voltage: 12, 18, 24 kV Flow Rate: 1, 2, 3 mL/h Distance: 16 cm |
_ |
Blend-composite |
Antibacterial |
[31] |
Another characteristic of electrospun fibers that must be identified is whether they are of the “blend-composite” or “core/shell” type, as these present differences in structure and in the controlled release of bioactive compounds (Figure 1).
Figure 1. Types of electrospun fiber: (a) simple; (b) blend; (c) core-shell.
Simple fibers: these are the most basic type of electrospun fiber. The fiber consists of a single type of material, which allows the bioactive compounds to be loaded due to its intrinsic characteristics. Only one publication
[20] using this type of fiber was identified in this research.
Blend-composite fibers: It is important to establish the differences between blend fibers and composite fibers. The blend fibers are produced by mixing two or more materials, polymer–polymer or polymer–small molecule (drug, protein, antioxidant, etc.) until a homogeneous solution is formed, which is subsequently electrospun
[32][33]. On the other hand, composite fibers can be developed by different types of mixtures, such as polymer–polymer (organic), polymer–inorganic, and inorganic–inorganic
[34]. These types of fibers are characterized as having at least two different phases, which can be developed in situ, by film stacking or coating by rotation, or impregnation in solution
[35][36]. In this research, researchers identified 21 publications that developed these types of fibers. This simple and effective method of preparing devices loaded with bioactive compounds for therapeutic applications such as burn treatment allows for the controlled release of the drug and has adequate physical-mechanical characteristics for such an application. A variant of the fiber blend is called “emulsion/fiber blend”, in which the materials are mixed with an emulsion containing the bioactive compound. Hajiali et al.
[20] used this technique to encapsulate an essential oil that had been added to the oil/water (O/W) emulsion.
Core/shell fibers: Core/shell electrospinning, also called coaxial electrospinning, is a modification of conventional electrospinning, characterized by the use of sample ejection capillaries arranged for the injection of a solution into the other solution. The core/shell fibers have two clearly different sections, a central core formed by a solution, and a shell or outer layer formed by another solution
[37]. Core and shell can encapsulate drugs independently
[1][9][15]. The main advantage of this structure is that the shell polymer helps to protect the drug from direct exposure to the deleterious external biological and ambient environments. The core–shell structure is also useful for reducing the burst-release phenomenon. Fibers developed by this method allow controlled release in two different phases: the first occurs in the shell and can be used to treat acute inflammatory responses in primary wound healing because it occurs first; the second phase, release of the core compounds, occurs later, so it can be used to deliver compounds required in later stages of the burn healing process. This was the strategy proposed by Li et al.
[9]: rapid release of antimicrobial peptides loaded in the shell, followed by controlled release of curcumin from the core. It was also used by Ramalingam et al.
[15] to generate phased release of the drug minocycline hydrochloride and natural extract of
G. sylvestre, thus obtaining a synergistic effect of infection control and accelerated burn healing.
3. Materials Used for Production of Electrospun Fibers
The electrospun fibers with biomedical applications for skin burn treatment observed in the publications reviewed were produced using either a single polymer type or a polymer blend. Some of these polymers, such as poly(vinyl alcohol), polyurethane, or gelatin, fulfil a purely structural function in the fiber, while others, such as chitosan, alginate, or poly(3-hydroxybutyrate-co-3-hydroxyvalerate), provide structural functions and/or bioactivity. The most used natural materials in the development of electrospun fibers were chitosan and gelatin, while the most widely used synthetic materials were poly(vinyl alcohol) and polycaprolactone. The polymers reviewed are described below (Table 2).
Table 2. Materials used for the production of electrospun fibers in 24 articles reviewed.
Material Type |
Material Name |
Reference |
Natural |
Chitosan |
[14][16][21][22][23] |
Collagen |
[10][29][30] |
Gelatin |
[15][23][26][38][39] |
Keratin |
[25] |
Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) |
[19] |
Silk Fibroin |
[13] |
Sodium Alginate |
[20][23] |
Synthetic |
Poly(ethylene glycol)/Poly(ethylene oxide)/Polyoxyethylene |
[9][22] |
Poly(lactic-co-glycolic acids) |
[30] |
Poly(L-lactic acid) |
[28] |
Poly(vinyl alcohol) |
[18][23][31] |
Poly(vinyl pyrrolidone) |
[9][25] |
Polycaprolactone |
[15][21][24][26][29][31] |
Poly-D,L-lactic acid |
[27] |
Polylactide |
[9][10] |
Polyurethane |
[11] |
3.1. Natural Materials
3.1.1. Chitosan (CH)
Chitosan are random copolymers of N-acetyl d-glucosamine and 2-amino-2-deoxy-β-d-glucosamine residues, achieved by deacetylation of chitin (linear polysaccharide mainly composed of β (1→4) units linked to N-acetyl-2-amino-2-deoxy-d-glucose residues)
[40]. This substance was discovered in 1859 and is the next most abundant compound in nature after cellulose. It has valuable properties such as biocompatibility, biodegradability, hydrophilicity, non-toxicity, high bioavailability, simplicity of modification, favorable moisture permeability, excellent chemical resistance, ability to form films, gels, nanoparticles, microparticles, and beads, as well as affinity for metals, proteins, and colorants
[41]. Chitosan was used by Bayat et al.
[16] to develop electrospun nanofibers loaded with bromelain, evaluating the recovery of burned skin in an animal model as one of the responses. The study concluded that chitosan-bromelain nanofibers at 2%
w/
v have a higher wound healing activity than chitosan-only nanofibers in the animal model tested. This may be due to chitosan having an antimicrobial activity that prevents infection in the burn; however, it does not promote healing or stimulate earlier or faster regeneration of the tissues, activities attributed to bromelain
[42]. Chitosan was also used by Antunes et al.
[14] in the development of an electrospun membrane composed of arginine-modified deacetylated chitosan (CH-A) for use as a wound dressing. The results showed improved tissue regeneration and faster wound closure when the modified membranes were used, compared to the unmodified membranes. These studies illustrate that this polymer has good biocompatibility and positive effects on the healing of skin wounds caused by burns. Talukder et al.
[23] developed three-layer electrospun fiber structures. The inner layer was composed of PVA/CH/Fibrin; CH was used to enhance antibacterial inhibition in wounded skin—which was demonstrated against
E. coli and
S. aureus in antibacterial activity measurement assays.
3.1.2. Collagen (COL)
Collagen is the most abundant protein in the animal body, representing approximately 30% of the total. It is the main component of the ECM and is vital for the mechanical protection of tissues and organs, and the physiological regulation of the cellular environment; it is widely used for biomedical and pharmaceutical applications
[38]. Collagen is a biodegradable, non-toxic protein, with higher biocompatibility than other natural polymers, and is only weakly antigenic; it is also a surfactant and can penetrate a lipid-free interface
[43]. Collagen molecules are made up of three polypeptide chains. These chains, aligned in parallel and wound to the left in a polyproline type II (PPII) helix, wrap around each other to form a triple helix to the right that is stabilized by hydrogen bonds between chains and within the n chain → π * interactions
[44]. Venugopal et al.
[29] used collagen mixed with polycaprolactone (PCL) to develop an electrospun nanofiber membrane. The study concluded that the collagen nanofibrous membrane mixed with PCL promotes greater cell adhesion, proliferation, and dissemination of the dermal fibroblast for wound healing compared to PCL membranes only; this better cellular response to the COL/PCL membranes may be due to the increased porosity and improved mechanical properties that collagen provides in electrospun nanofiber membranes developed with this mixture. Sadeghi-Avalshahr et al.
[30] developed electrospun PLGA nanofiber scaffolds to which they added collagen by two methods: the first was to make a PLGA/COL mixture that was later electrospun; in the second, collagen was added to the surface of the nanofibers after electrospinning, using chemical methods. The authors then compared the mechanical and biocompatibility properties of the scaffolds produced by the two methods. For the first method, they prepared a 20% (
w/
v) solution of PLGA/COL with a weight ratio of 4:1 in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), obtaining smooth fibers with small pore size. These PLGA/COL nanofiber scaffolds showed lower mechanical properties than pure PLGA nanofibers and those obtained by the second method. The PLGA/COL scaffolds showed cytotoxicity against keratinocyte cells. When the degradation rate was evaluated, the PLGA/COL scaffolds degraded approximately five times faster than the controls and the scaffolds created by the second method. Ilomuanya et al.
[10] used collagen to modify the mechanical properties of PLA. It should also be noted that the biochemical interaction between cells and collagen resulted from the binding of collagen I to cell membrane receptors, mediated by fibronectin, an ECM glycoprotein. The improved interaction between keratinocytes and scaffolds would invariably facilitate wound healing through fibronectin mediation, especially in situations where the healing process has been impaired.
3.1.3. Gelatin (GE)
Gelatin is a heterogeneous mixture of peptides derived from collagen proteins obtained by procedures that involve the destruction of cross-links between polypeptide chains, together with some breaking of polypeptide bonds
[45]. This polymer is widely used in activities related to the food, pharmaceutical, and cosmetic industries due to its excellent biocompatibility, easy biodegradability, and weak antigenicity; in addition, it is easily obtained by extraction from animal tissue such as skin, muscles, and bones
[46]. Jin et al.
[26] used gelatin as a biodegradable polymer mixed with PCL in the development of nanofibers that allowed fibroblast cell proliferation; they used this as a positive control in the study. Zhang et al.
[13] used a mixture of gelatin with silk fibroin to develop nanofiber dressings loaded with Astragaloside IV, evaluating the therapeutic effects on wounds such as acute burn trauma. Gelatin was selected in this research due to its physical and mechanical properties, as it has high tensile strength, low ductility, and good air permeability; in addition, gelatin has a morphology similar to that of the dermis, as well as low antigenicity and rapid tissue degradation and absorption. Mayandi et al.
[17] developed gelatin nanofibers loaded with ε-polylysine, which were cross-linked using polydopamine (pDa); this method was selected because the electrospun fibers of this polymer lack adequate mechanical stability and show a high degree of swelling, which limits their biomedical applications
[47]. Talukder et al.
[23] developed three-layer electrospun fiber structures using gelatin for the outer layer to absorb exudates from wounded or burned skin.
3.1.4. Keratin
Keratin is a fibrous protein present in mammal hair, wool, quills, and horns (α-keratins) and in the feathers, claws, and beaks of birds and reptiles (β-keratins). Due to its abundance in nature and its ability to enhance cell proliferation, it is an ideal material for a variety of biomedical applications, ranging from scaffolds for cell growth to drug delivery. Given keratin’s rather poor mechanical properties and its low molecular weight (ranging from 10 to 60 kDa), it is often combined with synthetic polymers, such as PEO, PCL, or PVA, which serve as adjuvants to improve processability. Kossyvaki et al.
[25] showed that keratin has high antioxidant activity (uptake of the 2, 2-diphenyl-1-picryl-hydrazyl-hydrate free-radical (DPPH)), which has been scarcely reported in the literature. The antimicrobial activity determination tests showed that the PVP/keratin fibers had a bactericidal effect limited to
S. aureus. The authors also established that keratin is not cytotoxic, nor does it inhibit the growth and proliferation of primary human dermal fibroblasts (α-HDF cells). Finally, an important finding was that the keratin-based patches with and without cinnamon essential oil were able to reduce in vivo the expression of pro-inflammatory factors (IL-1b y IL-6) by 5–7 fold.
3.1.5. Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate) (PHBV)
Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) is a natural polymer with thermoplastic properties that can be produced by bacterial fermentation, but the process is not economically competitive with the production of polymers from petrochemical products
[48]. PHBV is a biocompatible and biodegradable polymer investigated for various tissue engineering applications. Sundaramurthi et al.
[19] studied the adhesion, proliferation, and epidermal differentiation of mesenchymal stem cells (BM-MSCs) in PHBV nanofibers. The results obtained demonstrated that this polymer provides a medium permitting the cellular differentiation of BM-MSCs; it can be used in a device based on electrospun nanofibers as immediate cover for third degree burns, traumatic ulcers, and diabetic wounds.