**6.2 Nanoparticles for nitric oxide delivery**

Nitric oxide is an important diatomic molecule that is synthesized by three different isoforms of enzyme nitric oxide synthases by the conversion of amino acid Larginine. In the skin, different forms of nitric oxide are produced and released by various cells involved, such as macrophages, keratinocytes, melanocytes, and fibroblasts. In wound healing, nitric oxide is synthesized in the early inflammatory phase by inflammatory macrophage cells, whereas many cells secrete nitric oxide in the proliferative phase during wound healing. Nitric oxide being lipophilic in nature possesses the tendency to interact with various biomolecules. It can cross various biological barriers to reach the target site and diffuse along a concentration gradient to rapidly move from cell to cell. Nitric oxide is antibacterial in nature, modulates the immune response, maintains homeostasis and regulates wound healing, and helps in collagen deposition, cell proliferation, and wound contraction [140]. In this context, topical application of nitric oxide for acute and chronic wound healing always remained a challenge. To meet this challenge, engineering NPs mediated nitric oxide release to the wound bed served as a novel approach to allow its free radicals to exert antibacterial action [141].

In this respect for nitric oxide delivery, nitric oxide released from NPs system constituted of composite of polymer/glass hydrogel was tested for its efficacy in methicillin-resistant *Staphylococcus aureus* (MRSA) abscesses in mice. The study results documented that antibacterial nitric oxide-NPs treatment of abscesses reduced the involved area and bacterial load, and ultimately improved the skin architecture [142]. In a recent study, a novel wound-healing material was formulated by combining chitosan with electrospun PCL nonwoven mat for the loading of nitric oxide. Nitric oxide was released in a sustained fashion from the developed wound dressing under the physiological conditions. *In vivo* wound healing evaluated in full-thickness cutaneous wounds of mice resulted in accelerated wound healing through reepithelialization and granulation formation due to immunomodulation and enhanced collagen synthesis provided by the sustained release of nitric oxide [143]. Nitric oxide released from silica NPs has been demonstrated to exert a bactericidal activity against *P. aeruginosa*, which is one of the important pathogens causing wound infections in hospitals [144]. In a different report, nitric oxide donor precursor glutathione was encapsulated chitosan NPs with an encapsulation efficiency of 99.60% (**Figure 2**). Small size and positive zeta of chitosan NPs led to the delivery of nitric oxide through the skin for topical applications due to the affinity of positively charged chitosan NPs with negatively charged phospholipids that further result in changing the permeability of the skin membrane and reducing the skin barriers [145].

#### **6.3 Antibiotics-loaded nanoparticles for wound repair**

There are many types of wounds that fail to heal and turn into chronic wounds [146]. The major cause of delayed healing of such wounds is the persistence of infectious agents or microbial growth around the wound bed [147]. The major goal of any wound-healing treatment is to control the microbial infections to allow normal healing to proceed. Conventional therapies to treat microbial infections are based on either the systemic administration of antibiotics or the topical application of antibiotic formulations [146]. The systemic administration of antibiotics causes toxicity along with kidney and liver complications, but topical application of antibiotics provides high local concentration with a short residence time on the wound surface [148].

Therefore, an appropriate antimicrobial therapy of the wound to control microbial colonization is still required for optimum wound care. The delivery of antibiotic therapy *via* NPs offers great potential advantages, such as slow and sustained delivery, targeted delivery, and decrease in toxicity and improvement in antimicrobial and pharmacokinetic properties (**Figure 2**) [149].

In this respect, a novel wound dressing based on the Spanish Broom fibers impregnated with vancomycin-loaded chitosan NPs was designed, which showed an increased antibacterial action against *S. aureus* and was not toxic to HaCaT keratinocytes as compared to the fibers containing vancomycin without NPs [150]. In another study, chitosan nanofibers mat was functionalized with thiol groups, and gentamicin-loaded liposomes (17% loading efficiency) were immobilized covalently. Liposomes showed sustained and controlled release of gentamicin during 16 h, achieving a steady state at 24 h [151]. Nanofibers of small diameter exhibit unique properties such as excellent mechanical properties, flexibility in surface functionalities, and high specific surface area for wound healing [152]. Chitosan is used in such dressings because its biodegradable, cell adhesive, and possesses hemostatic activity and high mechanic strength [153]. In another study, ZnO NPs were coated with gentamicin and integrated into the chitosan matrix to yield a ZnO/gentamicinchitosan gel (**Figure 2**). The resulting gel showed 91% of gentamicin release after 8 h and evidenced a four-fold minimum inhibitory concentration (MIC) reduction for *S. aureus* and 2-fold reduction of MIC for *P. aeruginosa*. The resulting antibacterial gels could serve as potential candidates for wound healing [154]. Few other reports welldocumented the loading of antibiotics in NPs or nanofibers matrix for the sustained release of antibiotic drugs to prepare wound dressings. Ampicillin-incorporated electrospun polyurethane scaffolds [155], PVA films containing tetracycline hydrochloride-loaded quaternized chitosan NPs [156], tetracycline hydrochlorideloaded electrospun nanofibers mats based on chitosan, and PVA NPs [157] wound dressings have shown biocompatibility and strong antibacterial activity against different strains of bacteria.
