Development of nanotechnology for advancement and application in wound healing: a review

Abstract

Wound healing is a series of different dynamic and complex phenomena. Many studies have been carried out based on the type and severity of wounds. However, to recover wounds faster there are no suitable drugs available, which are highly stable, less expensive as well as has no side effects. Nanomaterials have been proven to be the most promising agent for faster wound healing among all the other wound healing materials. This review briefly discusses the recent developments of wound healing by nanotechnology, their applicability and advantages. Nanomaterials have unique physicochemical, optical, and biological properties. Some of them can be directly applied for wound healing or some of them can be incorporated into scaffolds to create hydrogel matrix or nanocomposites, which promote wound healing through their antimicrobial, as well as selective anti‐ and pro‐inflammatory, and proangiogenic properties. Owing to their high surface area to volume ratio, nanomaterials have not only been used for drug delivery vectors but also can affect wound healing by influencing collagen deposition and realignment and provide approaches for skin tissue regeneration.

1 Introduction

Wound or injury is known as the damage of the normal tissue [1]. Wound healing is a dynamic and complex phenomenon of replacing damaged tissues of the skin or body. Normal skin is composed of two layers; the epidermis (surface layer) and the dermis (deeper layer), collectively forming a protective barrier against the pathogens of the external environment. When the barrier is broken, a series of biochemical and physiological events quickly occur to repair the damage [2, 3]. The physiological process of wound healing has four temporarily overlapping phases: haemostasis, inflammation, proliferation (proliferation, granulation, and contraction), and remodelling phases (Fig. 1) [4, 5]. An improper wound‐healing process may lead to scar formation that fails to heal. Depending on the degree of a wound, whether acute or chronic, wound care is necessary to reduce bacterial infection that may cause stress and other health consequences [6, 7]. Many factors can impair wound healing; such as lifestyle (alcohol consumption, smoking and lack of exercise) and age of the subjects, therefore having a huge impact on the rate of wound closing (10–14 days). Furthermore, the health conditions of a patient, such as high cholesterol level, diabetes, peripheral arterial disease, cutis laxa, hypothyroidism, and homocystinuria are also responsible for delayed wound healing [8]. Over the years, wound dressing and healing have been problematic to clinicians due to the unavailability of ideal wound healing material [9]. A well‐designed wound dressing should maintain a moist environment to inhibit generation of surplus heat at the wound, allow for the normal debridement process, permit gaseous and fluid exchange, protect the wound from bacterial infection, absorb wound odour, non‐adherent to the wound and removable without any trauma, debridement, non‐toxic, non‐allergic, non‐sensitising, sterile, non‐scarring, biocompatible, and biodegradable in nature [10, 11].

Fig. 1.

Phases of cutaneous wound healing showing the cells responsible for wound healing

The most promising advancement in nanotechnology offers the facile synthesis of biocompatible nanomaterials (NMs) and opens an innovative approach to wound healing. This review provides a general overview of the current advances on nanotechnology such as metal‐based nanoparticles (NPs), solid lipid NPs (SLN), polymeric NPs, hydrogels, nanofibres etc. used for the chronic wound treatment particularly targeting different phases of wound repair (Fig. 2).

Fig. 2.

Schematic representation of the NM‐based therapies employed in different phases of wound healing

2 Nanoparticles (NPs)

NPs are tiny particles having a size range of 1–100 nm [12]. They have unique properties such as size, shape, large surface area to volume ratio etc. [13]. NPs due to their vast range of antimicrobial property and rapid effectiveness with minimal dose are one of the choices of researchers for wound healing. Conventional wound healing drugs have limited potential as they cannot penetrate the cell membrane, which a NP can [14]. The antimicrobial activity of NPs includes the destruction of the cell membrane, alteration at the gene level and blockage of enzymes and their synthesis, which cause cell death [15]. Other than this, NMs provide the soft, flexible, and biocompatible wound dressing material [16], which can have antimicrobial activity, gas barrier, absorbs extra exudates and protects from further trauma or heating of the wound [10]. There are two main categories of NMs used in wound healing: (i) NMs that exhibit intrinsic properties beneficial for wound treatment and (ii) NMs employed as delivery vehicles. Table 1 summarises the different NMs, which have been investigated for the management of wound care.

Table 1.

Different NMs studied for wound healing application

NMs	Activities	References
non‐mulberry (Antheraea mylitta) silk protein sericin‐based nanofibrous matrices	it is required for adhesion, proliferation, and cellular interconnection of human keratinocytes for tissue reconstruction	[17]
nanofibriallated cellulose hydrogel	act as a potential carrier of protein for wound healing dressing	[18]
dual‐layer aligned‐random silk fibroin (ARS) poly(l ‐lactic acid‐co‐ε‐caprolactone) nanofibrous scaffolds	ARS could effectively augment the tendon‐to‐bone integration and improve gradient microstructure in a rabbit extra‐articular model	[19]
magnetic/bacterial nanocellulose (BNC) (Fe3 O4 /BNC) nanocomposite	it promotes wound healing in human dermal fibroblast	[20]
ionic liquid assisted‐nanosilver combined with VEGF	AgNPs incorporated VEGF material is highly favourable to fracture healing and mainly as blood vessel repair	[21]
AgNPs/poly(gamma‐glutamic acid) hydrogel	it has antibacterial property, high water retention property and preventing contraction of wound, i.e. ideal for tissue regeneration	[22]
PEGylated graphene oxide‐mediated quercetin‐modified COL hybrid scaffold	this scaffold has multiple properties; a biocompatible, cell‐adhesive surface for accelerating MSC attachment and proliferation; high stability and adjustability for differentiation of MSCs into adipocytes and osteoblasts	[23]
sericin‐agarose polymeric hydrogel loaded with lysozyme	this composite has antimicrobial activity, cytocompatibility, high porosity, and excellent water absorption property	[24]
TiO2 NPs conjugated with human morphogenetic protein	it has a strong antibacterial property and effective bone fusion behaviour	[25]
TA‐modified AgNPs	it has good cytotoxicity and immunomodulatory properties	[26]
keratin‐CS/n‐ZnO nanocomposite	this nanocomposite has bactericidal activity, swelling property, and biocompatibility to human dermal fibroblast	[27]
porous poly(l ‐lactic acid) electrospun fibrous membranes containing dimethyloxalylglycine‐loaded mesoporous silica NPs	these NPs stimulate the proliferation, migration and angiogenesis‐related gene expression of human umbilical vein endothelial cells	[28]

elastic nanoliposomes	these nanoliposomes loaded with low molecular weight protamine‐conjugated growth factors enhances skin regeneration, permeation, and stability	[29]
sponge‐like macroporous PVA hydrogels	it can be used as wound dressing material as it is highly non‐toxic, non‐carcinogenic, and biocompatible	[30]
polypropylene nanofibre coated with broad‐spectrum MMP inhibitor GM6001	it reduces inflammation by inducing tissue vascularisation, increases COL deposition and improves mesh biointegration	[31]
nanocellulosic composites with fluorescent elastase peptide	this biosensor have a high affinity towards human neutrophil elastase	[32]
Ag/Ag@AgCl/ZnO Nanostructures embedded in a hydrogel	these composites have high‐antibacterial efficacy and accelerates wound healing activity under visible light	[33]
2, 2, 6, 6‐tetramethylpiperidine‐1‐oxyl‐oxidised cellulose nanofibre‐silk fibroin scaffold	it proliferates human skin fibroblast cells and promotes wound healing markers in a rat wound model	[34]
nano‐cellulose loaded CS film with AgNPs/curcumin	it has strong antibacterial activity and acts as wound dressing material	[35]
GEL/HA hydrogel	sustained release of recombinant thrombomodulin from cross‐linked hydrogels potentiate wound healing in diabetic mice	[36]
graphene oxide‐natural biopolymer nanofibres	this nanohybrid has enhanced antibacterial activity. It proliferates human skin fibroblast and promotes vascular epithelialisation and accelerates faster wound healing.	[37]
tourmaline/CS nanocomposite	it promotes faster regeneration of dermis, cell adhesion, and faster proliferation	[38]
silicon‐dioxide‐polyvinylpyrrolidone nanocomposite	the nanocomposite is stable, non‐toxic, accelerates wound healing by faster re‐epithelialisation	[39]

2.1 Non‐polymeric

Some of the non‐polymeric NPs such as metal and metallic NPs, lipid NPs, and ceramic NPs are known to be used for wound dressings, which are discussed as follows:

2.1.1 Metal and metallic NPs

Metal‐based NPs that have been reported for antimicrobial activity and wound healing activity are silver NPs (AgNPs), gold NPs (AuNPs), copper NPs (CuNPs), zinc oxide NPs (ZnO NPs), titanium dioxide (TiO₂), cerium oxide (CeO₂), yttrium oxide (Y₂ O₃) etc.

From ancient time, silver has been used as medicine. In recent years, various forms of silver, i.e. metallic silver, silver nitrate, silver sulphadiazine, have been used for the treatment of open wounds, burns and chronically infected wounds [40]. Recently, due to the increase in multi‐drug‐resistant pathogenic bacteria and limited use of antibiotics, the use of AgNPs has gained attention, mainly in the treatment of open wounds. Among all the NPs, AgNPs are the most studied NMs due to their remarkable antimicrobial activity [40]. AgNPs have a large surface area to volume ratio, which provides better contact with microorganisms, makes them more efficient for showing higher antimicrobial activity as compared to the conventional drugs. AgNPs have the ability to interact with sulphur‐containing proteins embedded in bacterial membranes; they can also interact with the phosphorus‐containing compounds in the microbial cell, such as DNA. One of the important mechanisms of their antimicrobial activity is to attack the respiratory chain and the cell division mechanism, which is the most fundamental pathway for survival and finally leading to cell death. Additionally, it can also discharge Ag+ ions leading to enhanced bactericidal activity [39]. Consequently, AgNPs are efficient even at a very low concentration, which reduces the dose‐induced tissue toxicity due to silver delivery. In vitro, AgNPs exhibit strong antibacterial activity against Bacillus subtilis, Escherichia coli, Staphylococcus aureus etc. [41]. Orlowski et al. have reported that tannic acid (TA)‐modified AgNPs exhibited effective antibacterial activity against Pseudomonas aeruginosa, S. aureus, and E. coli and stimulated migration of keratinocytes in vitro [42]. TA‐modified AgNPs promoted better wound closure, epithelialisation, angiogenesis and formation of the granulation tissue in a mouse splint wound model. Additionally, this TA‐AgNPs elicited expression of vascular endothelial growth factor (VEGF)‐α, platelet‐derive growth factor‐β and transforming growth factor‐β1 cytokines involved in wound healing more efficiently [42].

A frequent dressing of wounds leads to recurrence of infection and inflammation due to miss handling of wounds and can be solved by replacing traditional medicines with AgNPs mediated wound dressing. This wound dressing NM releases AgNPs in a slow manner, which can permit the covering of the wound to be changed less frequently. This method is highly useful and efficient in wound healing with a minimum chance of getting antimicrobial resistance [43].

AgNPs suspended in a hydrogel matrix proven to be more effective for the treatment of skin infections as compared to bare AgNPs in water [44]. A variety of biochemical has enhanced the quality of gelling properties for superior viscosity, spreadability, in vitro release profile and their antibacterial activity. This biochemical includes sodium carboxymethylcellulose, sodium alginate (Na‐alginate), hydroxypropyl methylcellulose, pluronic F‐127 and chitosan (CS) [45, 46, 47].

In a recent study, Abdel et al. have observed the antibacterial effect of hyaluronan/silver nanocomposite fibres against Gram‐negative bacteria E. coli K12. The fibres were non‐toxic to the human keratinocyte cell line (HaCaT). Further study in animal models showed that this nanocomposite fibre has strong wound healing efficacy (non‐diabetic/diabetic rat model) than the plain hyaluron fabrics [48].

Peng et al. have reported that low molecular weight CS‐coated AgNPs have better biocompatibility, lower body absorption characteristics and highly effective against methicillin‐resistant S. aureus compared to polyvinyl coated AgNPs [49]. In a recent study, AgNPs containing CS polyethylene glycol (PEG) hydrogel has been developed for the dressing of diabetic chronic wounds [50].

Other than this, AuNPs [51], ZnO NPs [52], CuNPs, CuO NPs [53] and many more NPs are known for their antimicrobial activity and wound healing properties. For decades, many kinds of research are done on these metallic NPs. A few of the recent developments in the field of metal NPs and wound healing are discussed here.

Pan et al. have discussed the topical application of keratinocyte growth factor (KGF)‐conjugated AuNPs and their effect on wound healing [51]. KGF stimulates multiplication and migration of keratinocytes but is not stable enough to promote faster wound healing. When KGF is cross‐linked with AuNPs, its stability increased. AuNP–KGF accelerates the proliferation of keratinocytes in vitro. In vivo study of an animal model has shown enhanced wound healing activity as compared to bare KGF or AuNPs [51]. This NP has superior biocompatibility, good colloidal dispersion, and effective biological property.

ZnO NPs applied in wound dressing accelerates rapid healing of acute and chronic wounds due to their chemical stability [54], antibacterial and anti‐inflammatory properties [55], and releasing Zn²⁺ ions next to the wound that leads to the increased migration of keratinocytes [56]. Moreno‐Eutimio et al. have illustrated the enhanced wound healing and anti‐inflammatory effect of carbohydrate‐based polymer with ZnO NPs in chronic venous leg ulcers [57].

As Cu has the ability to enhance angiogenesis, Xiao et al. have shown Cu‐based metal‐organic framework NPs stabilised with folic acid and can efficiently cure non‐curing wounds in diabetic wounds [58]. In general, the application of Cu salts repeatedly can cure wounds, but it increases the probability of toxicity. To solve this problem, the modified NP has been developed, which can release Cu²⁺ in a controlled and slow manner [59]. This controlled release feature of this NP is rendered by folic acid cross‐linking, which provides stability and hydrophobicity needed for wound healing application. In vitro and in vivo studies have shown excellent wound healing results. The in vivo study was done on splinted excisional dermal wound model in diabetic mice showed induced angiogenesis, stimulated collagen (COL) deposition and re‐epithelialisation. This study also revealed a safe and relevant wound healing material [58]. Biosynthesised CuNPs (BNCPs) by P. aeruginosa were evaluated for its wound healing activity by excision wound in rats. The wound healing was enhanced by BNCPs compared with Cu in a native form [59]. CuNPs conjugated with complex drug based on CS and starch rapidly healed wound and eliminate wound contaminating agent in white male rats with an experimental purulent wound infected with clinical poly‐antibiotic resistant strains of S. aureus [60]. Not only that, green synthesis of Cu oxide NPs (CuO NPs) by Ficus reigiosa leaf extract has strong antibacterial activity against Klebsiella pneumoniae, Shigella dysenteriae, S. aureus, Salmonella typhimurium, and E. coli. Moreover, these CuO NPs have enhanced wound healing activity in Wistar albino rats [61].

2.1.2 Lipid NP

In recent years, lipid NPs have drawn the attention of pharmaceutical researchers for their applications as a drug delivery system [62]. Other than this, lipid nanostructures can be tailored according to the need of disease conditions and method of administration. These formulations are safe and cost‐effective and even protect the drug from unnecessary biological degradation. Thus, lipid NPs are used as drug delivery carriers in sensitive organs due to its low toxicity and high efficiency [62]. The types of lipid NPs are SLN, nanostructured lipid carriers (NLC) and lipid drug conjugate.

Essential oils have the property of wound healing, soothingness, and antimicrobial property, which solves the problem of discomfort and infection and also help in the rapid healing of wounds [63]. Eucalyptus leaf extracts and rosemary essential oils have the property of wound healing, but as being essential oils, they are volatile in nature and are easily degraded by oxidation, heating, light, volatilisation [64, 65, 66, 67] etc. To prevent this, a carrier is needed, which can encapsulate the essential oils and preserve its viable activity [68]. SLN and NLC are one of the reliable vectors for such kind of drug administration [69]. Saporito et al. have prepared an NLC loaded with eucalyptus and rosemary essential oils for enhanced wound healing [63]. NPs consist of coco‐butter as solid lipid and olive oil as liquid lipid are highly stable in nature, biocompatible, cytocompatible and have rapid wound healing property in normal human dermal fibroblasts. NLC with eucalyptus essential oil and olive oil showed the best physicochemical properties. Olive oil has oleic acid, which is known to have a synergic effect with eucalyptus and enhanced wound healing property and antimicrobial property [63]. Another important natural drug Propolis is very well known for its antimicrobial activity, tissue regeneration, antioxidant, antiulcer [70], fungicidal, antiviral, immunostimulating [71] etc. which makes it as an excellent wound healing product. Propolis can be extracted efficiently by the ethanol extraction process, but ethanol present in the extract may be harmful to the wounds [70]. Thus, to overcome this problem NLC and SLN are the most appropriate carriers. Rosseto et al. have formulated a lipid‐based NP of Propolis [72]. The in vitro and in vivo study of Propolis NP as a topical delivery system showed excellent wound healing property. Other than this, lipid NPs are also used to deliver siRNA to the target site of the body. Rabbani et al. have developed cationic lipid NP conjugated with siRNA to produce a stable lipoproteoplex NP [73]. This lipoproteoplex NP has high‐transfection efficiency and exerts minimum toxicity. An in vivo study of diabetic wound healing in the humanised murine model showed efficient transfection of siRNA of Keap1 and Nrf2 and helps in accelerated tissue regeneration and reduces oxidative stress in the wound. In a recent study, another group has developed lipid NPs with siRNA conjugate to examine how a reduction in the inflammatory cytokine, tumour necrosis factor α, influences the wound healing process [74].

2.1.3 Ceramic NPs

Among the inorganic NPs, ceramic NPs are one of the most widely used NPs for wound healing. Ceramic NPs are known for antimicrobial activity, tissue regeneration and wound healing [75]. Ceramic NPs such as nanohydroxyapatite and calcium triphosphate [76] are known for its wide application in drug delivery and wound healing. Preparations of ceramic NPs are generally done by sol‐gel technique [77]. One of the extensive uses of ceramic NPs is in bone tissue engineering. Deepthi et al. have reported the application of ceramic‐based NPs with CS for bone regeneration [78]. Hydroxyapatite, bioglass ceramic, silicon dioxide, TiO₂, and zirconium oxide in a combination of CS forms nano‐scaffolds, which are highly efficient in bone regeneration. In conventional therapies, surgical methods were used to implant artificial materials to treat bone defects, which have lots of side effects [78]. To overcome these drawbacks ceramic NPs are used to pave the pathway of tissue regeneration. Hydroxyapatite with fibrillar COL and inorganic components such as Ca₁₀ (PO₄)₆ (OH)₂ mimic the native bone, which helps in the formation of artificial bones in case of bone replacement therapy. The blend of these biodegradable polymers and bioactive ceramic provides an extracellular matrix for bone tissue regeneration [78]. Khajuria and colleagues reported a biodegradable polymer composed of CS‐based risedronate/zinc‐hydroxyapatite intra‐pocket dental film, which is beneficial for the treatment of alveolar bone loss in the animal model of periodontitis [79].

2.2 Polymeric NPs

In recent years, efforts have been made for the controlled release of drug delivery in biocompatible polymeric NPs such as polylactic‐co ‐glycolic acid (PLGA), alginate, gelatin (GEL), CS, as well as other polycaprolactones (PCLs), PEG for chronic wound therapy. Polymeric NPs allow controlled drug release over time, reduce drug degradation by wound proteases, avoid frequent administration and prolong treatment effectiveness [80]. Additionally, a very small dose will be required to avoid dose‐induced side effects.

2.2.1 Synthetic

Among all the polymeric NPs, PLGA is one of the most widely used polymers for growth factor entrapment in chronic wound therapy because it is biocompatible, biodegradable, and less hydrophilic than other polymers. It absorbs less water and thus is slowly degraded allowing sustained drug release [80]. In addition, degradation of PLGA produces lactate that accelerates angiogenesis, activates pro‐COL factors and recruits endothelial progenitor cells to the wound site. One of the studies demonstrates that PLGA‐curcumin NPs shows increase wound‐healing capability by two‐fold in comparison with that of PLGA or curcumin. Curcumin is known for its anti‐inflammatory, antioxidant and anti‐infective properties. PLGA NPs loaded with curcumin quenched reactive oxygen species, inhibited myeloperoxidase, down‐regulated the expression of anti‐oxidative enzymes such as glutathione peroxidase and nuclear factor‐κβ that minimised the inflammatory responses, expedited re‐epithelialisation and improved granulation tissue formation [81]. Another study reported the novel synthesis of a new PLGA‐curcumin microparticle embedded CS scaffold, which also has the biocompatibility and wound‐healing properties [82].

PCL is biodegradable polyester synthesised from polyurethane. PCL is known for its selective cell response through controlled intracellular re‐absorption pathways. In comparison with other synthetic polymers, this polymer can be easily manipulated into a wide range of nanostructures such as porous scaffolds, micro and nano‐carriers [83]. In many studies, PCL can be combined with any metal or metal‐based NPs to enhance the antibacterial and wound‐healing activity. ZnO NPs are incorporated in a membrane consisting of electrospun PCL as skin substitution, which enhances fibroblast proliferation and wound healing without any scar formation. This membrane also did not show any significant inflammation when studied in vivo [84]. Similarly, in another report, PCL nanofibre combined with nanosilver acts as a bacterial inhibitor and wound healing agent. The combination of this polymer helps in safe, sustained, silver release in the wound area and accelerates wound healing [85].

2.2.2 Natural

CS is widely obtained from the shells of crustaceans composed of randomly distributed β‐linked d ‐glucosamine and N ‐acetyl‐d ‐glucosamine. CS is a promising material for wound healing applications owing to its attractive biodegradability, biocompatibility, mucoadhesive, cellular‐binding capability, wound‐healing effect and antimicrobial activity [44]. In a recent study, CS NPs were used as protein vehicles for loading into electrospun PCL nano‐scaffold. Piran et al. have studied the effects of topography and drug release of these nano‐scaffolds on skin fibroblast [86]. In another study, silver sulphadiazine nanocrystals were encapsulated in ginipin cross‐linked CS hydrogels for wound dressing formulation, which showed strong antibacterial effects against a broad range of Gram‐positive and Gram‐negative bacteria. It was also found that CS‐encapsulated silver sulphadiazine is non‐toxic to human fibroblast cells. Furthermore, AgSD nanocrystal hydrogels distinctly reduced the level of inflammatory cytokine interleukin‐6 and amplified the expression of growth factors VEGF‐A and transforming growth factor‐β1 simultaneously [87].

Another important natural polymer is alginate, which is extracted from the cell wall of brown algae. It has a high scaffold formation capability. Liao et al. have investigated a porous microsphere/alginate hydrogel. This hydrogel consists of PCL, PEG cross‐linked by calcium gluconate crystals, which are deposited in the pores of microspheres [88]. The porous structure of the hydrogel provides a better attachment and growth of fibroblast. Thus, this finding provides new hope for skin tissue engineering.

GEL is the protein substance derived from COL. It is also naturally present in mammalian tendons, ligaments, and tissues. The gelling nature of GEL is widely used for scaffold formation and drug delivery. Pankongadisak et al. have synthesised a novel GEL scaffold with AgNPs containing calcium alginate [89]. This scaffold releases Ag+ ions by burst release with a controlled prolonged release system. Owing to its high antibacterial activity and low cytotoxicity, this scaffold is applicable in a chronic wound dressing.

2.3 Hydrogels

Hydrogel dressings are the most suitable wound healing agent because hydrogels are designed to hold moisture at the wound surface, providing an ideal environment for healing, maintaining skin hydration and allowing the body to rid itself of necrotic tissue. Moreover, they are easy to prepare and allow sustained release of drugs. Hydrogels can be prepared using natural or synthetic polymers such as CS, fibrin, hyaluronic acid (HA), COL, GEL or synthetic polymers, e.g. cellulose derivatives or poly(ethylene oxide) poly(propylene oxide) copolymers, commonly known as poloxamers.

The unique property of hydrogel is the creation of a moist and cool environment for wound healing and providing high water vapour permeability along with preventing penetration of microbes into the wound surface. Khorasani et al. prepared a heparinised polyvinyl alcohol (PVA)/CS/nano ZnO (nZnO) hydrogel that has good antibacterial activity with no cellular toxicity [90].

A novel hydrogel consisting of coumestrol/hydroxypropyl‐β‐cyclodextrin incorporated into hydroxypropyl methylcellulose was developed by Bianchi et al. [91]. Coumestrol protects from photo‐aging, improves skin elasticity during post‐menopause. As coumestrol is insoluble in water, it was solubilised and incorporated in hydroxypropyl‐β ‐cyclodextrin forming a hydrogel. This hydrogel accelerated wound healing faster by good proliferation and migration of cells as well as it has good cytocompatibility and cell adhesion property when studied in Wistar rats.

Hsu et al. have developed a GEL‐based hydrogel with adipose derived stem cells (ADSCs) as a wound healing agent. ADSC was harvested from both mouse and porcine. An in vivo study on both mouse and porcine models showed enhanced wound healing [92]. Nitric oxide plays a pivotal role in different mechanisms of wound healing. Its topical delivery may improve healing in acute or chronic wounds. It has been reported that an antibacterial peptide has been designed, which has the attribute of self‐assembling upon pH alteration that in turn accentuates hydrogel formation. The resulting compound showed bactericidal activity against E. coli and significantly produced COL by human dermal fibroblast [93]. Nowadays, the hydrogel is developed for better wound healing. In a recent article, polyvinyl (alcohol)/CS/nZnO nanocomposite hydrogels were formed that have strong antibacterial activity, biocompatibility, faster wound healing property and non‐toxic [94].

As discussed earlier, skin wounds are complicated and need proper care for faster recovery. Many hydrogels are already discussed, which helps in skin repair, but Zhu et al. have formulated a novel hydrogel, which mimics skin [95]. They have designed a hydrogel consisting of HA and carboxylated CS to human‐like COL cross‐linked by trans ‐glutaminase, which mimics skin as well as has enough mechanical strength [95]. The hydrogel was studied in vitro on L929 cells and showed good biocompatibility, enhanced cell adhesion and proliferation. An in vivo study revealed that this skin mimicking hydrogel showed good protection from infiltration of an outside pathogen and also accelerated wound healing. The subcutaneous implantation study also revealed the degradation rate of the hydrogel, which is ideal for healing cycle and wound recovery. Thus, this hydrogel can be used as a good wound healing agent [95].

2.4 Nanofibres

Nanofibres are a type of NM with unique properties and characteristics. Nanofibres have a diameter of nearly 10–100 nm with a high specific surface area [96]. They are constructed by electrospinning, where the electric force is used to prepare nanometric fibres from the different polymer solution. This process is cost effective, simple and versatile [97, 98]. These fibres have the capability of forming a highly porous mesh with fine interconnectivity and network formation capability. It also has an extraordinarily high surface area to volume ratio and hardy structures. These features of nanofibres make it suitable for use as a scaffold. Nanofibres can be synthesised from natural polymers, synthetic polymers, carbon‐based materials, semiconductor materials, composite materials etc. It has multiple applications in environmental remediation, energy, healthcare, biomedical application and much more. These fibres are also famous for tissue engineering and regenerative medicines, wound dressing, drug delivery agents, biological sensing etc. [98, 99].

Some of the nanofibres mimicking skin extracellular matrix composites are COL/CS [100], COL/Zein [101], PCL/GEL [102, 103], polyurethane/GEL [104], silk fibroin/polyethylene oxide [105] and many more. One such nanofibre scaffold, synthesised and fabricated by COL‐coated ostholamide (OSA) was formulated by Kandhasamy et al. for wound healing [106]. OSA combined with polyhydroxybutyrate and GEL formed an electrospun nanofibre. These nanofibres were then coated with COL forming a nanofibre scaffold, which is similar to the extracellular matrix. This nanofibre film was found to have an excellent mechanical stability, which is a basic requirement for wound healing. It also has strong antimicrobial activity against P. aeruginosa and S. aureus, respectively. This nanofibrous scaffold has the capability to release OSA in a controlled manner and also are stable against enzymatic degradation. Thus, this scaffold has the property of sustained drug release, antimicrobial activity, promotes cell proliferation and contributes to wound healing. This scaffold has excellent cytocompatibility on mouse 3T3 fibroblast cell. An in vivo study of this scaffold on Wistar rats also exhibited strong wound repairing ability [106].

Another example of nanofibre possessing wound healing property is described by Goins et al. [107]. A nanofibre scaffold made of poly(1,8‐octanediol‐co ‐citrate) and poly(acrylic acid) has strong wound healing property, antimicrobial activity, hydrogel‐like water uptake capability and the capability to deliver physiologically relevant growth factors on the site of the wound. This electrospun nanofibre has the capability of efficient cell adhesion property, low cytotoxicity and high cell proliferation of skin fibroblast cells.

The wound healing property of Aloe vera was combined with recombinant human epidermal growth factor formed a scaffold, which was prepared by PLGA by electrospinning technology. The in vivo test on db/db mice showed excellent wound recovery and reduced inflammation [108].

2.5 Nanocomposites

As discussed earlier, there are many ‘types of’ nanotechnologies that are used for wound dressing materials. The combination of these nanotechnologies with each other can lead to the formation of more sophisticated and enhanced wound healing technology. Some of these are discussed here.

Chen et al. have designed konjac glucomannan (KGM)/AgNP composite sponge with excellent antimicrobial activity and wound healing property [109]. KGM is a water‐soluble polysaccharide with β ‐1,4‐linked d ‐mannose, and d ‐glucose. KGM has a high water absorption capability and film formation property. These properties help in good cell adhesion, biodegradability, biocompatibility, and non‐toxicity. KGM is also known for its enhanced cell proliferation activity and metabolic activity, which help in promoting wound healing. The AgNPs are well known for their antimicrobial activity. They were first synthesised using egg white as a natural deoxidiser and then KGM powder was mixed with the AgNPs and the KGM/AgNP composite sponge was obtained by lyophilisation. This nanocomposite has high‐antimicrobial activity and good cytocompatibility on L929 cells [109]. In addition, the in vivo study also showed good fibroblast growth and enhanced epithelialisation. These properties of this scaffold make it a good wound healing nanocomposite.

Another spongy composite for wound healing was designed by Ding et al., which consists of CS, AgNPs, and CS–Bletilla striata polysaccharide [110]. B. striata are well known for its wound healing by promoting the proliferation of human vascular endothelial cells and enhanced expression of VEGFs [111]. Genipin is a natural cross‐linking agent derived from Gardenia jasminoides Ellis, which cross‐links the amino group of CS. As most of the cross‐linkers have some of the side effects, thus, genipin being a natural cross‐linker has no toxicity and therefore is ideal to be used in wound dressing material [112]. However, the problem is an amino group of CS, which is cross‐linked and loses its antimicrobial activity. However, this can be solved by the use of AgNPs, which have widely accepted antimicrobial activity [113]. However, AgNPs also have cell toxicity [114], and therefore to stop penetration of microbial community in the wound region, a bilayer spongy scaffold was prepared, where AgNPs and CS comprise the top layer, which restricts microbial entry and CS with Bletilla striata and genipin as cross‐linker in the second layer, thus enhancing the curing of wound [110]. CS with B. striata and genipin as a cross‐linker also provides demanded mechanical strength with high water retention capability and more L929 cell proliferation as compared to CS or genipin scaffold alone. An in vivo study of this scaffold revealed accelerated healing of cutaneous wounds in mice and enhanced epidermisation with less inflammation within 7 days. Thus, this nanocomposite has a good wound healing capability [110].

Other than this, many more NPs with a combination of different scaffolds are also discovered. One such versatile nano scaffold was developed by Yang et al. [115]. The novel nanoscaffold consists of six‐amino penicillanic acid coated AuNP‐doped in electrospun nanofibre constructed by poly(ɛ‐caprolactone) (PCL)/GEL polymers, and have strong antimicrobial activity against multidrug‐resistant bacteria and also enhanced rapid wound healing. These properties of this scaffold have proved it to be good wound dressing material especially for infected wounds [115].

Thanusha et al. have formulated a hydrogel that can cure second‐degree burn wounds [116]. This hydrogel consists of GEL and glycosaminoglycan's (HA and chondroitin sulphate) as a base material of the scaffold incorporated with asiatic acid, which is a triterpenoid and Zn and CuNPs [116]. This hydrogel showed strong antimicrobial activity against E. coli and S. aureus. An In vitro cell viability test was conducted on L929 fibroblast cells and was found to be cytocompatible. An in vivo study of second‐degree burn wound healing was studied on Wistar rats for 28 days and found to be excellent wound healing in comparison with the control. This scaffold also showed decreased tumour necrosis factor‐α and increased matrix metalloproteinase (MMP)‐2 expression within 7 days of application leading to faster wound healing. Histopathology studies have also revealed the enhanced re‐epithelisation, COL fibre arrangement, and angiogenesis as compared to control.

3 Conclusion and perspective

Wounds are one of the most sensitive tissue damage to be dealt with. Wound healing involves many factors. Many wound healing agents are available but their application and effectiveness depend on the type of wound. Cell proliferation, cytocompatibility, cell adherence, and anti‐microbial activities are the factors, which highly affect wound healing. Many nanotechnologies such as NPs, hydrogels, and nanocomposites are excellent wound healing materials as they have all these properties. Among all the NMs, hydrogel is one of the most promising ameliorative agents for wound healing due to its moist and cool environment. Recently, researchers have provided new insights into the nanotechnology of wound healing by incorporation of mesenchymal stem cells (MSCs) into the hydrogel matrix. The recent researches have delved into the matter of skin mimicking which could effectively reduce the cellular toxicity of NPs by means of controlled drug release and accelerated wound healing without any scar formation and inflammation. More novel natural polymer composites with better mechanical strength and antimicrobial activity with enhanced wound healing technology can also be analysed.