Overview of some relevant techniques for production of nanometal-based antimicrobial wound dressings.
The progress in nanotechnology and the medical application of novel generations of nanomaterials have opened new horizons in the definition of non-conventional approaches against multiple diseases. Biomaterials coated with antimicrobial metal nanoparticles, along with the topical applications of zinc, silver or copper-based formulations have demonstrated huge potential in prevention from infections associated with implantable medical devices and in biofilm eradication. In wound healing, in particular, the increasing healthcare costs and the antibiotic resistance demonstrated by several microorganisms have encouraged researchers and companies in the development of innovative wound dressings with antibacterial properties and capability to promote and enhance the healing process. Supported by scientific evidence, many formulations have been proposed and a large number of works involves the use of hybrid metal nanoparticles/polymer products, which have demonstrated encouraging results both in vitro and in vivo. In this chapter, recent progress in the development of novel wound dressings based on antibacterial metal nanoparticles is presented, along with the most interesting results achieved by the authors, mainly devoted to the application of silver nanocoatings in wound management.
- wound dressings
- metal nanoparticles
The Wound Healing Society has defined the wound as the result of ‘disruption of normal anatomic structure and function’. A wound can be also described as a defect or break in the skin, which results from physical or thermal damage, or from medical and physiological conditions . The wound healing is a dynamic process consisting of four continuous and precisely programmed phases, namely haemostasis, inflammation, proliferation and remodelling. Multiple factors, such as infections, stress, diabetes, smoking and obesity, can lead to impaired wound healing by interfering with one or more of these phases . Once injured, the skin loses many of the protective defence mechanism of the intact skin and is colonized by the microorganisms on its surface. According to the replication status of the microorganisms, a wound can be classified as contaminated, colonized, locally infected and/or with spreading invasive infection . The main bacterial mode of living in an infected wound is biofilm, which can be defined as a confluent community of adherent bacteria characterized by high cell densities and encased in an extracellular polymeric matrix that acts as physical barrier for biological and pharmaceutical antimicrobials [4, 5]. The presence of bacterial biofilm is associated with impaired epithelialization and granulation tissue formation and promotes a low-grade inflammatory response that interferes with wound healing . The biofilm matrix plays an important role in the increased antibiotic resistance and has an enormous impact on medicine in terms of both therapeutic options and costs. Biofilm has been estimated to be associated with 65% of nosocomial infections, and the treatment costs associated with biofilm infection and chronic wounds have been estimated to be more than 1 billion USD annually in the United States [4, 7].
The increasing resistance of bacteria to antibiotics represents a huge concern, so that the World Health Organization recently has described the problem as ‘so serious that it threatens the achievements of modern medicine’ [8, 9]. Moreover, the large number of wound dressings and the limited guidelines available have induced an undesirable inconsistency in wound-care practice . The local treatment of wounds is crucial for preventing infections, controlling exudates and providing the moist environment necessary for wound healing. At this purpose, efforts have been made by many research groups in the development of bioactive dressings, which can play an active role in wound protection and healing, and/or are able to release biomolecules and antimicrobials for prevention and treatment of wound infections [8, 11, 12]. A strategy for the treatment of infected wounds with increased resistance to traditional antibiotic therapy is the use of specific antibacterial agents immobilized on the surface of a material, thus providing a wide spectrum of activity in terms of bacterial toxicity and destructuration of the bacterial biofilm matrix .
This chapter aims to provide the reader with an overview of the most promising routes to develop advanced biomaterials with antimicrobial properties for the management of wound infections through nanotechnology approaches. The new generation and application of nanomaterials with novel properties are one of the century’s key technology developments, which offer extraordinary opportunities in the pharmaceutical and medical field . The great potential of nanometals such as zinc, copper and silver in wound dressing formulations and their use as antimicrobial agent in wound infections will be presented, along with the most recent efforts and results achieved by several research group in the definition of effective strategies for prevention of wound infection and for enhanced wound healing. Moreover, the most relevant results obtained by the authors of this chapter in the field of silver-based antibacterial treatments for wound-healing application will be presented and discussed.
2. Wound infections
The skin represents a complex and effective barrier between the organism and the environment, preventing invasion of pathogens, chemical and physical insults and unregulated loss of water and solutes . From a microbiological point of view, the primary function of normal and intact skin is to control the microbial populations that live on the skin surface and to prevent the underlying tissues from invasion and colonization by potential pathogens . A wound, which represents the loss of skin integrity and following exposure of subcutaneous tissue, provides a moist, warm and nutritious environment for microbial colonization and proliferation. The abundance and diversity of microorganisms in any wound depend on different factors such as wound type, depth, location and quality, the level of tissue perfusion and the antimicrobial efficacy of the host immune response . As all open wounds lack the protective covering of skin, microorganisms from endogenous or exogenous sources can be introduced onto the wound surface and can lead to colonization [17, 18]. Colonization is defined as the presence of proliferating bacteria on the surface of a wound, without a noticeable host response and without clinical signs and symptoms. Differently, wound infection depends on the pathogenicity of the microorganisms and on the immune competency of the host, and it is characterized by the presence of the clinical signs of infection such as erythema, pain, tenderness, heat, oedema, cellulites and abscess or pus [19, 20]. Within an infected wound, the main bacterial mode of living is a biofilm . Bacterial biofilm consists of a complex microenvironment of single or mixed bacterial species encased within an extracellular polymeric substance (EPS) produced by bacteria. The moist, adhesive and proteinaceous wound surface represents the ideal environment for biofilm development . If microbes attach to the wound surface and proliferate, the biofilm begins to develop and, when it is well established, it exhibits resistance to the host immune system and antimicrobials. At this stage, the biofilm is considered mature and difficult to eradicate, thus requiring specialized management practices and increasing the risk of non-healing and clinically infected wound (i.e. showing signs of inflammation or purulence) . Biofilm infections compromise wound closure and contribute to wound chronicity. Persistent infections may arrest the growth of the repairing tissue and significantly  impairing the key healing processes such as the inflammatory immune response, granulation tissue formation and epithelialization. Although a moist environment is necessary for optimal wound healing, poor moisture/exudate control within a wound environment promotes the development of biofilm. Consequently, moisture balance is essential to optimize the wound environment for healing and minimize the opportunity for biofilms to develop [23–25]. Preventing biofilm is fundamental for faster and more effective treatment of chronic wounds . However, despite the evidence for the presence of biofilm in wounds, research studies are required to detect biofilm and to determine the exact role played by multispecies biofilms in terms of delayed wound-healing process . Different biofilms can be identified within a wound environment, such as aggregates of cells dispersed within the wound exudate, in slough or on necrotic tissue or on the wound dressing . The microbial community presents multiple difficulties for clinicians in attempting to heal a chronic wound. Biofilms are resistant to many biocides, antibiotics and wound-care products. So, managing biofilm often involves its physical removal from the wound surface with sharp or surgical debridement .
The control of biofilm is a key part of chronic wound management, but the use of antiseptic dressings for preventing and managing biofilm and infection still needs further research involving well-designed, randomized controlled trials . The concept of a bacterial contamination, colonization and biofilm-related infection is now widely accepted in wound care, and the recognition of the biofilm and the evolution of topical antiseptics to control bioburden in wounds are considered strictly related to the concept of TIME (tissue, infection/inflammation, moisture balance and edge of wound) and to its relation with the current best practice . In healthcare, infections lead to longer hospital stays for patients, specifically wound dressings and increased hospital costs . Also worsened by an ageing population and the incidence of diabetes and obesity, the huge economic and social impact of wounds requires higher level of attention and resources to understand biological mechanisms underlying cutaneous wound complications .
Infections of the dermis, including burns, surgical site infections and non-healing diabetic foot ulcers affect over a million people. Individuals with diabetes represent a particularly vulnerable category because many of them develop foot ulceration during the course of their disease and undergo amputation. In addition to diabetics, several other groups of immune-compromised patient populations are plagued by slow-healing and non-healing wounds, such as trauma and burn victims, cancer patients and pressure ulcers in the elderly . The incidence, morbidity, mortality and costs associated with non-healing of chronic skin wounds are dramatic. Chronic wounds cost millions of dollars annually in the healthcare industry of the United States, and biofilm significantly contributes many billions of dollars to the global cost of chronic wounds because of its role in delaying the wound-healing process and extending the inflammatory phase of repair [19, 33–35].
Along with the direct medical costs borne by the hospital or insurer, also indirect costs including lost patient productivity and diminished functional status should be considered . The control of bioburden is recognized as an important aspect of wound management, which requires new solutions against microbes and their biofilms. Octenidine dihydrochloride and polyhexanide are effective and tolerated antiseptics used in wound management today, but antiseptics alone may not be able to achieve wound healing without addressing other factors such as the general health of patients or the wound’s physical environment [37, 38]. Next generation of wound treatment strategies for non-healing chronic wounds can be achieved by adopting a biofilm-based management approach to wound care, in order to kill and prevent reattachment of microorganisms .
3. The antibacterial activities of metals
The antibiotic resistance of microorganisms determines serious complications like infection, and delayed wound healing and great concerns are related to the numbers and types of residing microorganisms and the ability of the host’s immune system to control their proliferation [39–41]. Along with the emergence of microorganisms’ resistance to multiple antibiotics, the increased healthcare costs and the huge social and economic impact of wound care have increased attention towards the biological mechanisms underlying cutaneous wound complications and have encouraged the researchers towards the development of new bactericide agents [31, 42]. The new frontier in clinical medicine and disease burden is represented by the medical applications of nanotechnology. Antimicrobial nanoparticles (NPs) offer an effective approach against numerous microorganisms where conventional antimicrobial agents fail [43, 44] and, compared with micron-sized particulate matter, have greater potential to enter cells and be more biologically active due to their small size and large surface area [X3]. Endowing ordinary products with new functionalities, consumer products containing engineered nanoparticles, are growing tremendously, and the global nanotechnology industry is becoming a major economic force of the twenty-first century [45, 46]. Some natural antibacterial materials such as zinc, silver and copper possess great antibacterial properties at nanometric size and their way of interaction with bacteria provides unique bactericidal mechanisms [43, 47].
Zinc is a transitional metal known since ancient time and widely distributed in the human environment. Today, many zinc-containing products are available for topical application in wound management due to the demonstrated improved re-epithelialization, reduced inflammation and bacterial growth. [48, 49]. ZnO has demonstrated to possess both antibacterial and anti-inflammatory properties and to accelerate the healing of both acute and chronic wounds. ZnO-NPs have exhibited antimicrobial capability and effectiveness against Gram-positive and Gram-negative bacteria, including pathogens such as
The bactericidal effect of metal nanoparticles has been attributed to their small size and high surface to volume ratio, and it is not merely due to the release of metal ions in solutions . Copper ions released subsequently may bind with bacterial DNA molecules and disrupt biochemical processes inside bacterial cells. The exact mechanism behind bactericidal effect of copper nanoparticles is not fully elucidated; however, Cu-NPs were found to cause multiple toxic effects such as generation of reactive oxygen species, lipid peroxidation, protein oxidation and DNA degradation in
In combination with silver, copper nanoparticles may give rise to more complete bactericidal effect against a mixed bacterial population . The broad-spectrum antimicrobial activity of silver has been demonstrated against a wide range of microorganisms, including methicillin resistant bacteria, fungi and viruses . Although the exact antimicrobial mechanism still represents a debated topic, many theories on the action of silver nanoparticles on microbes have been proposed. One of them involves the anchorage and penetration of the nanoparticles into the bacterial cell wall, which cause structural changes in the cell membrane such as permeability and respiration [59–62].
Nanosilver products safety data available in EPA’s formal incident reporting database indicates that nanosilver products are safe. Silver nanoparticles can be easily incorporated into matrix materials and have demonstrated a great potential in applications of huge interest in nanotechnology . When incorporated into wound treatment systems, silver nanoparticles can provide clinically relevance in the development of ideal environment for rapid and effective healing. These systems may significantly reduce the time required for the homeostatic equilibrium, while reducing the risk of complications and improving the physical appearance of the scar . Silver nanoparticles induce rapid healing and improved cosmetic appearance in a dose-dependent manner and exert positive effects through their antimicrobial properties, reduction in wound inflammation and modulation of fibrogenic cytokines .
4. Metal nanoantimicrobials for wound dressing applications
Wound healing still represents a clinical challenge, which requires efficient wound management strategies . Indeed, a crucial component of wound care is the choice of dressing. Many modern wound dressings have been developed to promote wound healing, such as dressings designed to absorb exudate, to provide an ideal moisture balance at the wound surface, to prevent maceration of surrounding tissue and infections and to reduce the bacterial load [70, 71]. Biomaterials, such as chitosan, alginate and collagen, play an important role as wound dressing materials by accelerating the healing of wounds and also because they can embed many nanoparticles for the development of metal nanoparticles-based wound dressings [69, 72]. Hydrogel-based wound dressings provide a cooling sensation and a moisture environment . Several systems based on the combination of hydrogel and metal nanoparticles, such as zinc, copper and silver, have been recently proposed by many authors, aiming to develop wound dressings with antibacterial and enhanced wound-healing properties. For example, an alginate hydrogel/zinc oxide nanoparticles composite bandage was developed by Mohandas
Among the recent trends against burn infections involving the use of noble metal antimicrobials, the most prevalent is represented by silver . For nearly 50 years, silver-containing compounds have been the mainstay of burn wound care and silver sulfadiazine (SSD) has been the standard topical antimicrobial for burn wounds for decades [64, 81].
Silver has been used as an antimicrobial agent for a long time in the form of metal silver and silver sulfadiazine ointments , and today, there is scientific evidence supporting the use of silver-based wound dressings highlighting antimicrobial efficacy on biofilms within the
Hydrogels with polyvinyl pyrrolidone (PVP) and alginate were synthesized by Singh
The application of both silver dressings and antibiotic therapy can have a synergistic effect in improving wound healing, since the interaction of silver released from the dressings significantly increases the susceptibility of bacterial cells within biofilms to antibiotics. Moreover, the reduction of the silver particle size to nanoscale level provides better penetration and accumulation of silver within biofilms, thus contributing to the effectiveness of the silver based product . As silver is the most widely used substance to obtain antimicrobial effects, different formulations involving the use of silver-containing solution or silver nanoparticles have been developed. Among the most widespread antimicrobial dressings, silver foam dressings and silver alginate dressings are applied to exuding wounds and demonstrate improved performances than the traditional gauze dressings . Silver alginate wound dressings have demonstrated beneficial effects on wound healing, in terms of wound exudates levels and prevention from wound infections [95–97]. Silver alginate dressings are particularly known for the prolonged antimicrobial efficacy, which indicates sustained availability of ionic silver and suggests the necessity of reduced dressings changes . Excellent and sustainable controllability of Ag+ release were obtained by the AgNP-bacterial cellulose hybrid nanostructure developed by Wu
The use of cellulose/nanosilver sponge materials was strongly encouraged in case of serious wound infection and
|Zinc oxide (ZnO)||Hydrogel/zinc oxide nanoparticles (nZnO) composite wound dressings|
developed by freeze-dry method from the mixture of nZnO and alginate or
|Copper oxide (CuO)||Cu2+ cross-linked alginate hydrogels by a two-step cross-linking technique. (i) Preparation of solid alginate films through solvent-casting method from soft gels of alginate solutions lightly cross-linked using a Cu2+ sulphate solution;|
(ii) further cross-linking of the films in Cu2+ sulphate solution using a
|Silver nanoparticles incorporated in hydrogels network using microwave/electro-spinning, gamma radiation, self-assembling. Composite sponges and films obtained by freeze drying and solvent casting.||[89–91, 101, 102]|
The widespread use of silver-based dressings in surgery is promising, inexpensive and well tolerated. The placement of silver-nylon dressings over incision sites in colorectal, neurological, spinal, cardiovascular and orthopaedic procedures at the time of primary closure has been described by Abboud
|Polyester textile mesh impregnated with hydrocolloid particles, vaseline and silver sulphate.||Low to moderate exuding acute and chronic wound at risk of infection.||[112, 113]|
|Sodium carboxymethylcellulose hydrofibres combined with ionic silver.||Acute and chronic wounds at risk of infection,|
with moderate and abundant exudate.
|Flexible polyethylene cloth coated with nanocrystalline Ag particles.||Infected ulcers, surgical wounds and burns.||[87, 112]|
|Silver nylon cloth/activated charcoal.||Most type of chronic wounds and infected|
wounds and ulcers.
Although the impregnating silver solution was prepared by using a percentage of silver lower than 0.5 wt/v%, the antibacterial test clearly demonstrated that the presence of the silver coating successfully inhibited the bacterial growth beneath and around the sample, thus indicating a good potential of product as antibacterial wound dressing. Also flax substrates have been treated with silver by adopting the same technology and the microbiological activity was still confirmed after industrial washing, thus suggesting the excellent stability of the coating on the surface of the textile material [111, 112]. In order to provide flax substrates with a moist environment and antibacterial capability at same time, Paladini
5. Conclusion and future perspectives
Nanotechnology is gaining huge impetus in the present century due to the drastic changes of chemical, physical and optical properties of metals at nanoscale size . The cutting-edge combination of nanotechnology with medicine offers unprecedented opportunities to revolutionize currently available macro-scale therapeutics. Nanoparticles-based delivery systems can be highly beneficial to improve the therapeutic power of biological and synthetic molecules . Due to the knowledge of cellular and molecular processes underlying wound healing, the new therapeutic approaches act directly on cellular and subcellular events during the healing process .
In recent years, metal nanoparticles/polymer composites have created lot of attraction due to their wide range of applications . The interest in broad-spectrum antimicrobial agents is particularly increasing for medicated wound dressings, in order to control colonization of wounds by opportunistic pathogens. Medicated wound dressings have demonstrated efficacy
Despite its recognized importance, there have not been systemic studies that probe the targeting efficiency of nanoparticles nor international standards on their toxicology and biocompatibility . Despite their promise, further studies are needed to elucidate the pharmacokinetics of nanoparticles and potential for