**5. Nanoparticles used as therapeutic materials for wound repair**

A range of metallic NPs, polymeric NPs, peptide-loaded nanostructures, and carbon-based nanomaterials have been investigated for applicability in wound repair due to the astonishing physical, chemical, and biological characteristics, such as their ease of fabrication, biodegradability, and biocompatibility [79, 80]. The aim of any injury due to burns or accidents is to achieve healing and epithelialization as soon as possible without any side effects [81]. A variety of nanomaterials used for the treatment of wounds and their brief mechanistic action are given in detail in the following sections (**Figures 2** and **3**).

#### **5.1 Metallic nanoparticles**

In the past literature, different varieties of metal or metal oxide NPs were reported for their wound-healing application. Metal NPs, such as silver, gold, and zinc, are frequently used for dermal wound treatment due to their ease of use and large surface-area-to-volume ratio. These metal NPs provide a moist wound environment and possess strong antimicrobial activity [82]. Silver NPs (AgNPs) are among the ones used in most of the dressings available in the market. AgNPs as well as silver are known to show great antimicrobial action against wide spectra of microbes that include bacteria, fungi, yeast, and even viruses [83]. A nanogel prepared from biologically synthesized AgNPs from cell-free extract of *Saccharomyces boulardii* resulted in superior healing of burn wounds in rats as compared to marketed formulations [81]. AgNPs successfully interrupt the quorum sensing mechanism, resulting in decreased biofilm formation and detoxification of the bacterial toxins [84]. AgNPs carry the silver ions (Ag<sup>+</sup> ) that are solely responsible for antimicrobial activity by interfering with the respiratory chains at the cytochrome, damaging cell wall, binding with DNA, *Nanotechnological Interventions and Mechanistic Insights into Wound-Healing Events DOI: http://dx.doi.org/10.5772/intechopen.106481*

**Figure 2.**

*Schematic representation of the main biological phases and events in the wound healing cascade along with the most abundant cells and soluble factors present in each phase, which is responsible for wound repair. The arrows at the top indicate the timeline of healing phases suggesting the overlapping nature of the wound healing cascade.*

#### **Figure 3.**

*Pictorial representation of the various types of nanotechnology-driven wound repair materials currently under research/available in the market.*

and inhibiting its replication [85, 86]. In a recent *in vitro* study, it was demonstrated that the use of AgNPs led to a significant decrease in levels of inflammatory cytokines, oxidative stress in human keratinocytes and dermal fibroblasts that ultimately accelerated the rate of healing [87]. In a burn wound model, topical application of AgNPs tends to reduce the neutrophil count and IL-3 levels along with an increase in levels of IL-10, TGF-β, VEGF, and interferon-gamma (IFN-γ) [88].

Gold NPs (AuNPs) are way more biocompatible than other metallic NPs. It is very exciting to describe that AuNPs alone or along with other drugs have also been examined for their wound-healing efficacy [71]. The proteasome inhibitory activity, antibacterial, and antioxidant potential of AuNPs synthesized using aqueous extract of the rind of *Citrullus lanatus* may serve as potential candidates for wound healing [89]. Electrospun scaffold containing pharmaceutical intermediate-capped AuNPs provided a remedy for the treatment of full-thickness wounds infected by multi-drug resistant (MDR) bacteria [90]. The antibacterial mechanism of action of AuNPs illustrates that AuNPs alter the membrane potential and inhibit the ATP synthase enzyme that ultimately causes a collapse in the energy metabolism of the cell and cell death [91].

The inherent antibacterial nature of zinc oxide (ZnO) NPs promotes the applicability of such nanomaterials in numerous hydrogel-based wound dressings. In a study, cotton wound dressings impregnated with AgNPs, ZnO NPs, and mixed Ag/ZnO NPs resulted in high antibacterial action of wound dressings (**Figure 3**). Bandages impregnated with a liquid solution of AgNPs showed more antibacterial activity as compared to ZnO and mixed Ag/ZnO NPs [92]. ZnO-NPs successfully prepared from aqueous leaf extract of the plant *Barleria gibsoni* exhibited a remarkable wound-healing potential in rats and acted as an effective and better topical antimicrobial formulation to treat burn wounds [93]. In another study, the authors explored the healing potential of Ag-ZnO composite NPs in Wistar rats and showed comparatively faster healing in 10 days as compared to pure AgNPs and dermazin (the standard drug) [94]. Topical administration of antibacterial ZnO NPs also decreased bacterial skin infections in mice model by the induction of disintegration of the cell membrane and oxidative stress response in macrophages [95]. Iron oxide NPs were also evaluated for wound healing purposes. Fe2O3 NPs conjugated with thrombin significantly stimulated incisional wound healing by improving the tensile strength of the skin and reducing scar formation [96].

#### **5.2 Nanoparticles containing polymers**

Wound dressing materials are often based on polymeric nanostructures that include either synthetic or natural polymers. Mainly, poly(lactic-co-glycolic acid) (PLGA), poly(ethylene glycol) (PEG), poly(lactic acid) (PLA), and polycaprolactone (PCL) are the mostly used synthetic polymers to engineer biomaterials for wound care applications [97]. Natural biodegradable polymers are chitosan, cellulose, alginates, and hyaluronic acids, which have played a well-versed role in the healing of wounds. In a study, electrospun nanofibers prepared from the blend of PLA and hyperbranched polyglycerol and loaded with curcumin showed wound-healing potential. *In vitro* scratch assay results indicated that the curcumin-loaded electrospun nanofibers were able to completely cover the wound within 36 h [98]. In another study, curcumin-PLGA nanostructures resulted in two-fold increase in woundhealing potential when compared to either PLGA or curcumin. Curcumin-loaded PLGA NPs reduced reactive oxidative species (ROS) and downregulated expression of

#### *Nanotechnological Interventions and Mechanistic Insights into Wound-Healing Events DOI: http://dx.doi.org/10.5772/intechopen.106481*

anti-oxidative molecules (glutathione peroxidase and NFκβ) that are responsible for reducing the inflammatory responses (**Figures 2** and **3**) [99]. A hybrid alginate hydrogel cross-linked by calcium gluconate crystals deposited in PCL-PEG-PCL was shown to promote wound regeneration in a full-thickness skin defect model of rats that suggested their great potential in skin tissue engineering [100].

Another important biopolymer, collagen is structural component of the extracellular matrix and is known for providing excellent strength to tissues [101]. Collagen nanofibers mat incorporated with AgNPs resulted in accelerated re-epithelialization, collagen production, and better wound contraction compared with plain collagen nanofibers. Due to its excellent biocompatibility and bioadhesive nature, collagen nanofibers mat promotes cell adhesion and interacts with cells and regulates cell migration, proliferation, and survival. Collagen dressings also result in accelerated fibroblast production and promote wound healing [102]. Another collagen-derived biopolymer, that is, gelatin has been used mainly in the establishment of biocompatible and biodegradable wound dressings. The porosity and interfiber distance of gelatin structure tend to promote healing. In a report, topical administration of gelatin wound scaffolds resulted in rapid wound closure and faster wound repair in a rat model [103]. Chitosan is another natural polymer that acts as an optimum woundhealing material as it bears film-forming capacity, gel-forming characteristics, positive charge, and a strong tissue adhesive trait in response to increased coagulation of blood [104]. Its analogous structure with glycosaminoglycan (main component of extracellular matrix) plays a great role in its utility in tissue engineering biomaterials [105]. In a study, chitosan-Ag-ZnO nanocomposite dressing enhanced the wound healing and promoted re-epithelialization and collagen deposition [106]. Similarly, a spongy bilayer wound dressing material composed of chitosan-Ag NPs and chitosan-*Bletilla striata* polysaccharide also showed hastened healing of skin wounds of mice, and the bilayer displayed improved mature epidermization and less inflammation on day 7 [107]. In another report, insulin-delivering chitosan NPs coated onto the electrospun PCL/collagen demonstrated nearly full wound closure when compared to the sterile gauze, which showed approximately 45% of wound closure [108].

Cellulose, another important biopolymer occurring abundantly, has been used widely in wound dressing applications due to its biodegradability, biocompatibility, and high tensile strength. Methylcellulose-containing AgNPs showed excellent antibacterial action and burn wound healing [109]. Nanocomposites containing cellulose nanocrystals (CNCs) incorporated with silver NPs have also been used for acute and diabetic wound healing in mice. The nanocellulose in these nanocomposites possessed good water-absorbing capacity and porous nature that assisted in the rapid healing of acute and diabetic wounds [110–112].

#### **5.3 Peptide encapsulated nanostructures**

Peptides of various types possess astonishing functions for wound-healing applications. But the controlled and prolonged delivery of peptides to the wound site is quite challenging. The use of NPs for encapsulation of peptides serves as a platform to provide sustained and controlled delivery of peptides and protect the peptides from degradation, thus promoting rapid healing of wounds [113]. In a study, solid lipid nanoparticle (SLN) encapsulating simultaneously LL37 and serpin A1 was used to deliver the agent at specific ratios. The developed nanostructures resulted in faster wound repair by promoting wound closure in fibroblasts and keratinocytes and increasing antibacterial action against bacteria *S. aureus* and *E. coli* as compared to

bare LL37 or serpin A1 alone. LL37 is well-known endogenous host defense peptide possessing the antimicrobial trait and takes part in the regulation of the healing process. Further, Serpin A1, an elastase inhibitor has been reported to demonstrate healing properties [114]. In another report, a recombinant fusion protein comprised of stromal cell-derived growth factor-1 (SDF1) and elastin-like peptide (ELPs) was developed, which possess the tendency to self-assemble into NPs. SDF1 is known to promote neovascularization for early re-epithelialization of cutaneous wounds in diabetic mice. ELPs are non-immunogenic, non-pyrogenic, and biologically compatible derivatives of tropoelastin. The topical application of wounds treated with SDF1-ELP NPs resulted in 95% closure of full-thickness wounds by day 21, and complete closure by day 28. On the other hand, only 80% of wound closure was achieved by treatment with free SDF1, ELP alone, or vehicle control by day 21, and the wounds took 42 days for complete closure [115]. In another study, heparin mimetic peptide nanofibers angiogenic scaffolds were developed for slow release of growth factors and protection from degradation. Heparin mimetic peptide nanofibers have the potential to bind and enhance the activity and production of major angiogenic growth factors, such as VEGF, and thus provided a therapeutic way to accelerate the healing of diabetic wounds [116]. Another similar study demonstrated the use of heparin-mimetic peptide nanofiber gel for increasing the rate of healing of burn injuries [117].

A different study reported the simple one-step cross-linking strategy for the preparation of collagen peptide with recombinant human collagen (RHC)-chitosan nanofibers for wound healing. The results showed rapid epidermalization and angiogenesis in Sprague–Dawley (SD) rat scalding model after treatment with *in situ* crosslinked nanofibers (**Figure 3**). The *in situ* cross-linked nanofibers behaved well as a scaffold showing better cell attachment and proliferation. The breakdown products of RHC played a role as chemotactic agents for the faster synthesis of granulation tissue for showing better healing performance [118]. A different type of hybrid multifunctional nanofibrous matrix composed of poly(citrate)-*ε*-poly-lysine and PCL was designed to inhibit MDR bacteria and enhance full-thickness wound healing [119].

#### **5.4 Carbon-based nanomaterials**

Carbon nanomaterials, such as graphene oxide (GO)-NPs, carbon dots, and fullerenes, possess the potential to be used as skin repair agents (**Figure 3**). The application of carbon nanotubes (CNTs) to wound healing provides enhanced functionality for dressing, delivery of antiseptics in a controlled manner, and real-time monitoring of healing events. Polyvinyl alcohol (PVA) functionalized multi-walled CNTs further conjugated with glucose oxidase enzyme showed antibacterial activity against bacterial pathogens due to the generation of hydrogen peroxide. The antibacterial activity of the developed nanomaterials opened innovative ways for the potential of such materials in wound-healing applications [120]. In a study, GO nanosheets incorporated in ultrafine biopolymer fibers were tested for skin wound-healing potential in adult male rats. From the *in vivo* studies, it was found that a large open wound (1.5 1.5 cm2 ) was completely regenerated after 14-day of injury. Pathological studies confirmed the formation of thick dermal tissue and complete epithelialization in the presence of 1.5wt% GO nanosheets [121]. In a different study, a novel 3D collagen scaffold containing carbon-based 2D layered material, GO was characterized for periodontal healing of dogs. GO scaffold was implanted into dog class II furcation defects, and periodontal healing was examined after 4 weeks of surgery. The outcomes suggested that GO scaffold was biocompatible and possessed excellent bone and

#### *Nanotechnological Interventions and Mechanistic Insights into Wound-Healing Events DOI: http://dx.doi.org/10.5772/intechopen.106481*

periodontal tissue formation ability [122]. Onion-derived carbon nanodots that comprised hydrophilic group-decorated amorphous nanodots exhibited accelerated healing in a full-thickness wound model of rat model attributed to its radical scavenging action [123]. Carbon C60 fullerenes exhibited fascinating properties that balance several pathological mechanisms accountable for hampering the wound repair pathway [124]. In a different study, C60 fullerene functionalized with cationic three dimethyl pyrrolidinium groups was examined to rescue mice from fatal wound infections of Gram-negative species, *Proteus mirabilis* and *Pseudomonas aeruginosa.* The study results successfully revealed that mice infected with *P. mirabilis* showed 82% survival due to the photodynamic therapy of fullerenes as compared to only 8% survival of mice without treatment [125].

#### **5.5 Solid lipid nanomaterials**

Various kinds of lipid nanomaterials, such as SLN, liposomes, micelles, nanostructured lipid carriers (NLC), or vesicles, have been used as therapies for wound healing (**Figure 3**). SLNPs encapsulated with morphine were reported to increase keratinocyte migration, proliferation, and differentiation responsible for accelerated wound repair [126]. In this context, liposome with silk fibroin hydrogel core was designed to stabilize bFGF. The study indicated that the skin permeability of bFGF was significantly enhanced by the developed liposomal system, and a major part of the encapsulated growth factor penetrated the skin dermis. Application of bFGF encapsulated liposomes resulted in improvement in the morphology of hair follicles at the wound site with hair regrowth shown on a deep second scald mice model. The healing action was mainly found to be associated with inhibiting scar formation and promoting vascular growth in dermis, which may serve as a potential candidate to improve wound healing [127]. In another study, liposomes were loaded with dexamethasonephosphate. The liposomes were further surface modified with either PEG or phosphatidylserine. Both formulations resulted in decreased IL-6 and TNF-α release and increased efferocytosis activity. A faster uptake and a higher potency were induced by phosphatidylserine-modified liposomes as compared to PEG-modified liposomes. Liposomes after shell modification with phosphatidylserine promoted dexamethasone delivery to macrophages and induced a phenotype favorable for chronic wound healing [128]. The topical administration of recombinant human EGF loaded into lipid nanocarriers showed accelerated healing of full-thickness cutaneous wounds in a porcine model. The administration of 20 μg of nanoencapsulated lipid carriers twice a week increased the wound closure rate, as well as improved the wound quality in *in vivo* experiments [129].

Micelles were also proposed as suitable candidates for the delivery of hydrophobic molecules to the wound site for the healing of chronic wounds. In this respect, a biodegradable hydrogel system cutaneous wound dressing was developed containing curcumin encapsulated in micelles. Curcumin suffers from problems such as low water solubility, poor oral bioavailability, and rapid first-pass metabolism. The application of developed micelles in both incision and excision wound models showed higher collagen level, better granulation, remarkable reduction in superoxide dismutase content, and small increase in catalase activity causing an enhancement in the healing of cutaneous wounds [130]. Another study demonstrated that clarithromycin-loaded micelles were prepared *via* self-assembly of chitosan with a mixture of linoleic and oleic acids. These micelles exhibited good biocompatibility, induced cell proliferation, and showed 20-times greater clarithromycin loading

capacity in comparison to its water-saturated solution, suggesting the potential of micelles in wound-healing applications [131].
