**3. Antimicrobial wound dressings**

### **3.1. Need for antimicrobial wound dressing**

The major need for antimicrobial dressing is drug resistance to bacteria. Zubair et al. [47] isolated bacteria from diabetic foot ulcer patients and their resistance to different classes of drugs with the penicillins showing highest susceptibility to resistance followed by cephalosporins (54%), quinolones and fluoroquinolones (52.8%), aminoglycosides (38.5%), beta lactams (32.2%), and carbapenems (18.4%). Further, most chronic wound sufferers such as older patients and diabetics with leg and foot ulcers suffer from complications of poor circulation at the lower extremities, which makes oral and IV antibiotics ineffective. In addition, topical dressings are able to avoid the adverse effects of systemic administration (oral and IV) of high antibiotic doses including nausea, vomiting, diarrhea, allergic reactions, leukocyturia, insomnia, headache, and vaginosis, when only small doses above the minimum inhibitory concentration are required at the infected wound site. Finally, production costs of most dressings are less than those of IV or oral products.

### **3.2. Advanced medicated antimicrobial wound dressings**

Hydrogels consist of hydrated polymers which make them hydrophilic in nature. Water content is higher than 95%, and as a result they cannot absorb much exudate and cause maceration. But, this dressing is very useful in dry wound which can maintain moisture within wounds [36]. A Cochrane Review [38] of hydrogel dressings for healing diabetic foot ulcers suggests that hydrogel dressings are more effective than basic wound contact dressing. Hydrogels have advantages of autolytic debridement of slough and necrotic tissue and do not support bacterial growth [39, 40]. Hydrocolloid dressings are occlusive and can absorb wound exudate into the matrix to help improve healing. It can work for a sustained period of time, thus reducing the frequency of dressing changes. It also assists autolysis of necrotic materials [40]. Due to its extra absorbent nature, it is widely used in the treatment of cavity wounds [41]. A Cochrane Review [42] reported that any type of hydrocolloid and other dressings have no difference in efficacy. Foam dressings are highly absorptive, protective, and comfortable to the body surface. They promote thermal insulation, angiogenesis, and autolysis [43]. Film dressings are adhesive, transparent, durable, comfortable, and cost effective. Due to their transparency, the wound bed can be monitored without removing the dressing. However, films are suitable for superficial pressure wounds. The disadvantage of film dressing is maceration of wound exudate [36]. Lyophilized wafers are one of the most recent moist dressings proposed for wound care. Due to their highly porous nature, they can absorb high amounts of exudate rapidly which improves wound healing. Wafers can carry both antibacterial and anti-inflammatory drugs at the same time which give dual effects of inhibiting bacteria and reducing inflammation [44]. Wafers have good adhesion and diffusion properties [45] while Labovitiadi et al. [46] reported that wafers are a compatible delivery system for both insoluble and soluble antimicrobial drugs that exhibit better antimicrobial

The major need for antimicrobial dressing is drug resistance to bacteria. Zubair et al. [47] isolated bacteria from diabetic foot ulcer patients and their resistance to different classes of drugs with the penicillins showing highest susceptibility to resistance followed by cephalosporins (54%), quinolones and fluoroquinolones (52.8%), aminoglycosides (38.5%), beta lactams (32.2%), and carbapenems (18.4%). Further, most chronic wound sufferers such as older patients and diabetics with leg and foot ulcers suffer from complications of poor circulation at the lower extremities, which makes oral and IV antibiotics ineffective. In addition, topical dressings are able to avoid the adverse effects of systemic administration (oral and IV) of high antibiotic doses including nausea, vomiting, diarrhea, allergic reactions, leukocyturia, insomnia, headache, and vaginosis, when only small doses above the minimum inhibitory concentration are required at the infected wound site. Finally, production costs of most

activity.

**3. Antimicrobial wound dressings**

378 Wound Healing - New insights into Ancient Challenges

**3.1. Need for antimicrobial wound dressing**

dressings are less than those of IV or oral products.

Antimicrobial dressings can be broadly classified into two groups as antiseptic or antibiotic dressings. Antiseptic dressings have broad spectrum activity which can kill or inhibit bacteria, fungus, protozoa, viruses, and prions [48]; however, some antiseptic dressings often show dose-dependent cytotoxicity to the host cells including keratinocytes, fibroblasts, and leukocytes [49, 50]. The concentration of povidone iodine greater than 0.004 and 0.05% is completely toxic to keratinocytes and fibroblasts, respectively [51]. Cadexomer iodine is reported to be nontoxic to fibroblasts *in vitro* at concentrations of up to 0.45% [52]. Chlorhexidine also shows dose-dependent toxicity to fibroblasts at concentrations between 0.2 and 0.001% [53, 54]. Moreover, silver-impregnated dressings have been reported to be more cytotoxic to epidermal keratinocytes and dermal fibroblasts than honey-based dressings [55]. On the other hand,


**Table 1.** Summary of antibiotic dressings reported in the literature.

antibiotic dressings (**Table 1**) are nontoxic and can work effectively on the target sites without damaging host tissues [49]. The ideal antimicrobial dressing should have broad spectrum activity against all major microorganisms, be nonallergic and nontoxic to host cells, have the ability to drain exudate and maintain a moist wound environment, should release drugs rapidly in a sustained manner, should reduce malodor, and be cost effective [56, 57].

### **3.3. Silver-based dressings**

Silver is a natural broad spectrum antibiotic, and its dressings have not yet shown any bacterial resistance. Silver exists in different forms such as silver oxide, silver nitrate, silver sulfate, silver salt, silver zeolite, silver sulfadiazine (SSD), and silver nanoparticles (AgNPs). Before the eighteenth century, silver nitrate was used for leg ulcers, epilepsy, acne, and venereal infections [86]. Currently different forms of silver are widely used in acute wound (burns, partialthickness burns, freshly grafted burns, second-degree burns, surgical/traumatic wounds, colorectal surgical wounds, pilonidal sinus, and donor site), and chronic wound (pressure ulcers, leg ulcers, and diabetic foot ulcers) healing [87].

### *3.3.1. Antimicrobial activity of silver dressings*

Antimicrobial activity of silver dressings depends on the amount and rate of silver release and its toxicity to bacterial, fungal, and algal cells. Silver works by interacting with thiol groups present in bacterial cells thus stop their respiration process. In the case of *E. coli*, silver prevents phosphate uptake and catalysation of disulfide bonds with silver tending to change the nature of protein structure in *E. coli.* The degenerative changes in cytosolic protein cause cell death [86, 88]. Feng et al. [89] reported antibacterial mechanism of action of silver ions on *E. coli* and *S. aureus* and showed that silver ions penetrate into bacterial cells and condense DNA molecules which inhibit their replication capabilities leading to cell death. Matsumura et al. [90] introduced two bactericidal mechanism actions of silver zeolite on *E. coli*. Firstly, silver ions released from silver zeolite come into contact with cells and penetrate into cells, altering the cellular functions that cause cell death. Secondly, silver ions inhibit respiration process through the generation of reactive oxygen molecules. Silver zeolite has also been reported against oral microorganisms (*Streptococcus mutans, Lactobacillus casei, Candida albicans, and S. aureus*) [91].

Silver nanoparticles show the most efficient antimicrobial activity amongst all forms of silver. The bactericidal effects of AgNPs depend on the size, shape, surface characteristics, and their dose [88, 92–101]. It has been reported that 75 μg ml−1 of AgNPs having 1–100 nm particle size inhibits all bacterial strains (specifically, *E. coli, Vibrio cholerae, Salmonella typhi, and Pseudomonas aeruginosa).* It has also been reported nanoparticles having particle size ∼1–10 nm have higher affinity of attaching to the surface of the cell membrane as compared to larger nanoparticles. Because of this nature, AgNPs can attach to the larger surface area of bacterial cell membrane and cause native membrane porations which cause cell damage [92]. Ivask et al. [93] examined toxicity of silver nanoparticles to bacteria (*E. coli*), yeast (*Saccharomyces cerevisiae*), algae (*Pseudokirchneriella subcapitata*), crustacean (*Daphnia magna*), and mammalian cells (murine fibroblast) according to their particle sizes ranging from 10 to 80 nm. They confirmed that the smaller-sized nanoparticles showed highly toxic effect. The review of Rai et al. [88] and Rizzello et al. [92] explained that truncated triangular nanoparticles are the strongest biocidal active products compared to spherical- and rod-shaped nanoparticles. 1 μg of truncated triangular nanoparticles shows greater activity than 12.5 μg of spherical-shaped nanoparticles and 50– 100 μg of rod-shaped nanoparticles due to the enhancement of electrostatic interaction with bacterial cells (**Table 2**).

antibiotic dressings (**Table 1**) are nontoxic and can work effectively on the target sites without damaging host tissues [49]. The ideal antimicrobial dressing should have broad spectrum activity against all major microorganisms, be nonallergic and nontoxic to host cells, have the ability to drain exudate and maintain a moist wound environment, should release drugs

Silver is a natural broad spectrum antibiotic, and its dressings have not yet shown any bacterial resistance. Silver exists in different forms such as silver oxide, silver nitrate, silver sulfate, silver salt, silver zeolite, silver sulfadiazine (SSD), and silver nanoparticles (AgNPs). Before the eighteenth century, silver nitrate was used for leg ulcers, epilepsy, acne, and venereal infections [86]. Currently different forms of silver are widely used in acute wound (burns, partialthickness burns, freshly grafted burns, second-degree burns, surgical/traumatic wounds, colorectal surgical wounds, pilonidal sinus, and donor site), and chronic wound (pressure

Antimicrobial activity of silver dressings depends on the amount and rate of silver release and its toxicity to bacterial, fungal, and algal cells. Silver works by interacting with thiol groups present in bacterial cells thus stop their respiration process. In the case of *E. coli*, silver prevents phosphate uptake and catalysation of disulfide bonds with silver tending to change the nature of protein structure in *E. coli.* The degenerative changes in cytosolic protein cause cell death [86, 88]. Feng et al. [89] reported antibacterial mechanism of action of silver ions on *E. coli* and *S. aureus* and showed that silver ions penetrate into bacterial cells and condense DNA molecules which inhibit their replication capabilities leading to cell death. Matsumura et al. [90] introduced two bactericidal mechanism actions of silver zeolite on *E. coli*. Firstly, silver ions released from silver zeolite come into contact with cells and penetrate into cells, altering the cellular functions that cause cell death. Secondly, silver ions inhibit respiration process through the generation of reactive oxygen molecules. Silver zeolite has also been reported against oral microorganisms (*Streptococcus mutans, Lactobacillus casei, Candida albicans, and S. aureus*) [91].

Silver nanoparticles show the most efficient antimicrobial activity amongst all forms of silver. The bactericidal effects of AgNPs depend on the size, shape, surface characteristics, and their dose [88, 92–101]. It has been reported that 75 μg ml−1 of AgNPs having 1–100 nm particle size inhibits all bacterial strains (specifically, *E. coli, Vibrio cholerae, Salmonella typhi, and Pseudomonas aeruginosa).* It has also been reported nanoparticles having particle size ∼1–10 nm have higher affinity of attaching to the surface of the cell membrane as compared to larger nanoparticles. Because of this nature, AgNPs can attach to the larger surface area of bacterial cell membrane and cause native membrane porations which cause cell damage [92]. Ivask et al. [93] examined toxicity of silver nanoparticles to bacteria (*E. coli*), yeast (*Saccharomyces cerevisiae*), algae (*Pseudokirchneriella subcapitata*), crustacean (*Daphnia magna*), and mammalian cells (murine fibroblast) according to their particle sizes ranging from 10 to 80 nm. They confirmed that the smaller-sized nanoparticles showed highly toxic effect. The review of Rai et al. [88] and Rizzello

rapidly in a sustained manner, should reduce malodor, and be cost effective [56, 57].

**3.3. Silver-based dressings**

380 Wound Healing - New insights into Ancient Challenges

ulcers, leg ulcers, and diabetic foot ulcers) healing [87].

*3.3.1. Antimicrobial activity of silver dressings*




**Table 2.** List of selected commercially available antimicrobial silver-containing dressings [22, 102, 103].

### *3.3.2. Silver dressings in wound healing*

**Dressing type Brand name Silver form**

382 Wound Healing - New insights into Ancient Challenges

Films/meshes Avance Silver

Alginate based Algidex Ag Ionic silver

McKesson Calcium Alginate with Antimicrobial Silver

**with Silver**

**Restore Calcium Alginate Dressing**

Gentell Calcium Alginate Ag Silver Silverlon Calcium Alginate Silver

Vliwaktiv Ag Silver

Arglaes film Silver

Algicell Ag Silver

Biatain Alginate Ag Silver

DermaGinate/Ag Silver Dermanet Ag+ Silver Maxorb ES Ag+ Silver

Opticell Ag+ Ionic silver

Sofsorb Ag Silver Sorbalgon Ag Ionic silver Suprasorb A + Ag Calcium Alginate Silver

Melgisorb Ag Silver SeaSorb Ag Ionic silver Silvasorb Ionic silver Sorbsan Silver Silver Sorbsan Algidex Ag Ionic silver

Askina Calgitrol Ag Silver alginate

Urgotul SSD/S.Ag Silver sulfadiazine

Invacare Silver Alginate Silver sodium hydrogen zirconium

Simpurity Silver Alginate Pads Silver particles Urgotul SSD Silver sulfadiazine

Acticoat 7 Elemental silver

Acticoat Absorbent Elemental silver

CalciCare Silver zirconium

Maxorb Extra Ag+ Silver zirconium phosphate

Silver

Ionic silver

phosphate

AgNPs (∼11 to ∼12 nm) containing gelatin fiber mats were prepared by electrospinning process and inhibited major microorganisms present in wounds [104]. Lin et al. [105] compared silver-containing carbon-activated fibers with commercially available silver-containing dressings and showed the silver-containing carbon-activated fibers to exhibit antibacterial activity and biocompatibility and promoting granulation and collagen deposition. A novel chitosan–hyaluronic acid composite with nanosilver was reported as a potential antimicrobial wound healing dressing for diabetic foot ulcers possessing high porosity, swelling, water uptake abilities, and biodegradable and potential blood clotting ability. The authors proved the inhibitory effects on *S. aureus, E. coli*, MRSA, *P. aeruginosa,* and *Klebsiella pneumoniae* [106]. In a related study, chitosan incorporated with polyphosphate and AgNPs was studied. The polyphosphate acts as a procoagulant which boosts blood clotting, platelet adhesion, and thrombin generation [107]. A similar scaffold dressing was developed by incorporating silver nanoparticles with chitin and showed antibacterial and blood clotting activity [108]. In another study, AgNPs containing hydrogel without any cytotoxicity but with antibacterial activity were reported [109]. Various inorganic forms of silver including silver zeolite, silver zirconium phosphate silicate, and silver zirconium phosphate demonstrate antimicrobial activity against oral microorganisms [91]. Pant et al. [110] stated AgNPs containing nylon-6 nanofibers prepared by one-step electrospinning process could be an effective antimicrobial wound dressing to kill both Gram-negative *E. coli* and Gram-positive *S. aureus*. Archana et al. [111] evaluated chitosan-blended polyvinyl pyrrolidone (PVP)–nano silver oxide (CPS) as an effective wound dressing *in vitro* and *in vivo*.

Lansdown et al. [112] investigated two forms of silver-containing dressings (Contreet foam and Contreet hydrocolloid) and found these promoted healing in chronic venous leg ulcers and diabetic foot ulcers. Polyvinylpyrolidone and alginate-based hydrogel-containing nanosilver has been functionally evaluated for efficient fluid handling capacity and strong antimicrobial activity against all major microorganisms such as *Pseudomonas, Staphylococcus, Escherichia, and Candida* [113]. Jodar et al. [114] demonstrated silver sulfadiazine-impregnated hydrogel for antimicrobial topical application for wound healing. Silver sulfadiazine (SSD) impregnated hydrogel was prepared by polyvinyl alcohol (PVA) and dextran blending. Boateng et al. [115] formulated an ideal lyophilized wafer dressing composed of alginate and gelatin containing silver sulfadiazine for wound healing and showed the controlled release of SSD over 7 h and expected to diminish microbial load in the wound area. A novel SSD-loaded bilayer chitosan membrane was prepared with sustained release of silver which inhibits the growth of *P. aeruginosa and S. aureus* [116]. Shanmugasundaram et al. [117] formulated SSDimpregnated collagen-based scaffold with strong antibacterial activity *in vitro*. Ammons et al. [118] formulated dressings by combining commercial silver dressings (ActicoatTM Absorbent, Aquacel® Ag, and TegadermTMAg) with lactoferrin and xylitol and demonstrated greater efficacy against MRSA and *P. aeruginosa*.

There are several clinical studies with silver-containing dressings in the treatment of infected wounds to enhance wound healing, and the reader is referred to these [119–125].

### **3.4. Iodine and other antiseptics**

Iodine is an old agent used in the treatment of chronic wounds and was used by soldiers during wars. The antibacterial activity of iodine was first investigated by Davaine in 1880 [126]. Iodine penetrates into the cell wall of microorganisms and damages the cell membrane by blocking hydrogen bond. This phenomenon alters the structure and function of cell proteins and enzymes, leading to cell death [127]. Iodine is active against a broad spectrum of microorganisms including *S. aureus, E. coli, Pseudomonas, Streptococcus, Salmonella, Candida, Enterobacter, Klebsiella, Clostridium, Corynebacterium, and Mycobacterium* [126]. Iodine dressings can be found in two preparations as povidone iodine and cadexomer iodine, and the various commercial formulations are summarized in **Table 3**.

Polyhexamethylene biguanide (PHMB) is an another antiseptic and widely used as antimicrobial dressing in wound healing. PHMB is known to be effective against *E. coli. S. aureus* and *S. epidermidis.* PHMB also works like iodine as it attaches to the bacterial cells and disrupts cell membrane resulting in leakage of potassium ions and cytosolic components that lead to cell death [128]. A study by Eberlein et al. [129] confirmed that PHMB containing biocellulose wound dressings were more effective than silver-containing dressing in retarding microbial loads present in locally infected wounds. Loke et al. [130] developed a two-layer dressing with sustained release of chlorhexidine which showed activity against *S. aureus* and *P. aeruginosa in vitro*.


**Table 3.** List of other commercially available antiseptics [36, 127].

### **3.5. Honey dressings**

In a related study, chitosan incorporated with polyphosphate and AgNPs was studied. The polyphosphate acts as a procoagulant which boosts blood clotting, platelet adhesion, and thrombin generation [107]. A similar scaffold dressing was developed by incorporating silver nanoparticles with chitin and showed antibacterial and blood clotting activity [108]. In another study, AgNPs containing hydrogel without any cytotoxicity but with antibacterial activity were reported [109]. Various inorganic forms of silver including silver zeolite, silver zirconium phosphate silicate, and silver zirconium phosphate demonstrate antimicrobial activity against oral microorganisms [91]. Pant et al. [110] stated AgNPs containing nylon-6 nanofibers prepared by one-step electrospinning process could be an effective antimicrobial wound dressing to kill both Gram-negative *E. coli* and Gram-positive *S. aureus*. Archana et al. [111] evaluated chitosan-blended polyvinyl pyrrolidone (PVP)–nano silver oxide (CPS) as an

Lansdown et al. [112] investigated two forms of silver-containing dressings (Contreet foam and Contreet hydrocolloid) and found these promoted healing in chronic venous leg ulcers and diabetic foot ulcers. Polyvinylpyrolidone and alginate-based hydrogel-containing nanosilver has been functionally evaluated for efficient fluid handling capacity and strong antimicrobial activity against all major microorganisms such as *Pseudomonas, Staphylococcus, Escherichia, and Candida* [113]. Jodar et al. [114] demonstrated silver sulfadiazine-impregnated hydrogel for antimicrobial topical application for wound healing. Silver sulfadiazine (SSD) impregnated hydrogel was prepared by polyvinyl alcohol (PVA) and dextran blending. Boateng et al. [115] formulated an ideal lyophilized wafer dressing composed of alginate and gelatin containing silver sulfadiazine for wound healing and showed the controlled release of SSD over 7 h and expected to diminish microbial load in the wound area. A novel SSD-loaded bilayer chitosan membrane was prepared with sustained release of silver which inhibits the growth of *P. aeruginosa and S. aureus* [116]. Shanmugasundaram et al. [117] formulated SSDimpregnated collagen-based scaffold with strong antibacterial activity *in vitro*. Ammons et al. [118] formulated dressings by combining commercial silver dressings (ActicoatTM Absorbent, Aquacel® Ag, and TegadermTMAg) with lactoferrin and xylitol and demonstrated greater

There are several clinical studies with silver-containing dressings in the treatment of infected

Iodine is an old agent used in the treatment of chronic wounds and was used by soldiers during wars. The antibacterial activity of iodine was first investigated by Davaine in 1880 [126]. Iodine penetrates into the cell wall of microorganisms and damages the cell membrane by blocking hydrogen bond. This phenomenon alters the structure and function of cell proteins and enzymes, leading to cell death [127]. Iodine is active against a broad spectrum of microorganisms including *S. aureus, E. coli, Pseudomonas, Streptococcus, Salmonella, Candida, Enterobacter, Klebsiella, Clostridium, Corynebacterium, and Mycobacterium* [126]. Iodine dressings can be found in two preparations as povidone iodine and cadexomer iodine, and the various commercial

wounds to enhance wound healing, and the reader is referred to these [119–125].

effective wound dressing *in vitro* and *in vivo*.

384 Wound Healing - New insights into Ancient Challenges

efficacy against MRSA and *P. aeruginosa*.

formulations are summarized in **Table 3**.

**3.4. Iodine and other antiseptics**

Honey has been used as wound dressing over centuries [131]. Honey has been reported in several clinical studies for treating chronic diabetic foot ulcers [132–135] and has antimicrobial and anti-inflammatory activity [136–138]. It is reported that honey can inhibit around 60 species of bacteria including *Alcaligenes faecalis, Citrobacter freundii, E. coli, Enterobacter aerogenes, Klebsiella pneumoniae, Mycobacterium phlei, Salmonella california, Salmonella enteritidis, Salmonella typhimurium, Shigella sonnei, S. aureus, and Staphylococcus epidermidis* [139]. In addition, it is reported Manuka honey and Cameroonian honey have an effect on *Pseudomonas aeruginosa*, methicillin-resistant *S. aureus* (MRSA), and vancomycin-resistant *Enterococcus* species [137, 140]. The antimicrobial properties of honey are ascribed to its low pH, hygroscopic nature, and peroxide-containing compounds [141]. The rich contents of sugar in honey generate high osmotic pressure and present an unsuitable environment to bacterial growth and cell proliferation [139]. Van den Berg et al. [142] investigated the anti-inflammatory properties of different types of honey *in vitro* by testing reactive oxygen species (ROS) inhibition capability and found American buckwheat honey exhibits high ROS inhibition ability. Many clinical studies have been performed on the basis of the antimicrobial effect of honey [143–145]. Clinical studies and bioactivity demonstrate the efficiency of honey in wound healing, maintaining a moist environment, promoting drainage of wound exudate and autolytic debridement [144]. It has been reported in minimizing malodour and scar formation of the wound [145] as well as angiogenic activity [146].

Sasikala et al. [147] developed a chitosan-based film dressing loaded with Manuka honey. They identified chitosan–lactic acid with 6% honey showed ideal dressing properties in terms of water vapor transmission rate, water absorption, tensile strength, elongation, and antibacterial activity against *E. coli* and *S. aureus.* **Table 4** summarizes the commercially available honeybased dressings currently sold on the market.


**Table 4.** List of selected commercially available honey dressings used in wound healing [22, 148, 149].

### **3.6. Polymer-based antimicrobial dressings**

Natural and synthetic polymers are widely used in acute and chronic wound healing due to their biodegradability, biocompatibility, and wound exudate handling capacity. However, some polymers themselves have an antimicrobial activity [150]. The combination of polymers and antimicrobial drugs provides effective dressings to improve wound healing. Biazar et al. [151] evaluated a synthetic polymer-based hydrogel dressing that exhibits biocompatible and antimicrobials activity. In another study, synthetic polyvinyl alcohol was blended with calcium alginate to produce nano fiber matrix by electrospinning technique. *In vitro* antibacterial test showed the rate of inhibition of *S. aureus* depends on the concentration of calcium alginate [152]. Chitosan is a cationic polymer whose positive charge interacts with a negative charge of the microbial cell membrane, resulting in disruption and agglutination [153]. Carboxymethyl chitosan has been reported as a broad spectrum antibiofilm agent which can prevent biofilm formation for *E. coli and S. aureus* by 81.6 and 74.6%, respectively [154].
