Wound healing and antimicrobial effect of active secondary metabolites in chitosan-based wound dressings: A review

Highlights

• Chitosan is one of the most promising biopolymers applied in wound dressings.

• Different methods are used for production chitosan-based wound dressing systems.

• Plants are the primary source of active secondary metabolites.

• The bioactive metabolites can effect in at least one phase of the healing process.

Abstract

Wound healing can lead to complex clinical problems, hence finding an efficient approach to enhance the healing process is necessary. An ideal wound dressing should treat wounds at reasonable costs, with minimal inconveniences for the patient. Chitosan is one of the most investigated biopolymers for wound healing applications due to its biocompatibility, biodegradability, non-toxicity, and antimicrobial activity. Moreover, chitosan and its derivative have attracted numerous attentions because of the accelerating wound healing, and easy processability into different forms (gels, foams, membranes, and beads). All these properties make chitosan-based materials particularly versatile and promising for wound dressings. Besides, secondary natural metabolites could potentially act like the antimicrobial and anti-inflammatory agents and accelerate the healing process. This review collected almost all studies regarding natural compounds applications in wound healing by focusing on the chitosan-based bioactive wound dressing systems. An accurate analysis of different chitosan formulations and the influence of bioactive compounds on their wound healing properties are reported.

Introduction

Skin with about 15 % of body weight is the largest organ of the integumentary system and has several essential functions. The most important is the prevention of organism desiccation and the protection of inner body structures from the environment (Chua et al., 2016; Ojeh, Pastar, Tomic-Canic, & Stojadinovic, 2015; Wikramanayake, Stojadinovic, & Tomic-Canic, 2014). The skin has a multilayer structure and any component has specific properties. The epidermis or outer layer of skin, mainly consisting of keratinocytes, it represents the main barrier against external environmental factors such as ultraviolet radiation (UV), pathogenic microbial agents, and mechanical disturbances. The immediately adjacent layer is dermis or middle layer, a connective tissue rich in collagen; it is a thicker layer, which consists of extracellular matrix (ECM), living cells, nerve endings and blood vessels, providing energy and nutrition to the epidermis. Fibroblasts represent the main cell population of the dermis. They release collagen and elastin which are responsible for mechanical strength and elasticity of the skin. Dermis and epidermis are in constant communication to maintain and eventually restore the hemostasis (Paul & Sharma, 2004). The deepest layer is hypodermis or subcutaneous layer. It is constituted by fat tissue and it is responsible for thermal isolation and mechanical protection of the body (Gurtner, Werner, Barrandon, & Longaker, 2008; Kamoun, Kenawy, & Chen, 2017).

A wound is described as damage and any disorder in the healthy structure and function of the skin. The wound can be due to injury, genetic disorders, acute trauma, thermal trauma, and even surgical interventions (MacNeil, 2007). In general, wounds are classified as open and close wounds, depending upon their nature. Close wounds include contusion, hematoma, and abrasions such as damage of soft tissues, small blood vessels or deep tissue layers (Mutsaers, Bishop, McGrouther, & Laurent, 1997), while open wounds generally consist of lacerations, cutting-pricking tool wounds, surgical wounds, insect bites, stings, radionecrosis, vascular neurological, and metabolic wounds (Bhardwaj, Chouhan, & Mandal, 2017; Kapoor & Appleton, 2005; Moura, Dias, Carvalho, & de Sousa, 2013).

Furthermore, the wounds can be clinically divided into acute and chronic wounds. In acute wounds, the injury could be induced by different factors, such as radiation, extreme temperature changes, or contact with chemicals. This kind of wound spontaneously heals in 8–12 weeks. On the other hand, chronic wounds generally require a longer healing time (even some months), due to the prolonged inflammation. They can occur for several causes, including tumors, infection or physical agents.

Dermal wound repair is a very dynamic process, consisting of four overlying stages (Fig. 1). The first immediate response to the injury is hemostasis, the process in which the blood loss held at the wound site (Sinno & Prakash, 2013). The second stage occurs right after the injury and includes inflammation that takes from 24 h to 4–6 days. This stage is called the inflammatory phase and starts with the emitting of proteolytic enzymes and pro-inflammatory cytokines over invaded immune cells to the wound zone. These inflammatory cells generate reactive oxygen species (ROS). The quantity of ROS correlated with the kind of wound, but it is usually higher in burns and chronic wounds (Latha & Babu, 2001). Indeed, ROS preserve the organism from bacteria and infection (Ojha et al., 2008). In spite of having a positive impact on wound healing in low concentrations, ROS in high concentrations could be destructive for wound healing through its contribution to the chronic pathogenesis, and chronic wounds, which increases oxidative stress, lipid peroxidation, and severe cell damage (Bryan et al., 2012). Besides, the most common type of wound infection is Skin and Soft Tissue Infections (SSTIs) and up to the kind of microbial invasion can be even life-threatening (Hospital, Medicine, Training, & Centre, 2008). Indeed, gram-positive bacteria such as Staphylococcus aureus (S.aureus) and Streptococcus pyogenes (S.pyogenes) play the principal role in the first stage of the infection, and gram-negative bacteria such as Escherichia coli (E.coli) and Pseudomonas aeruginosa (P.aeruginosa) can be found when the wound is already developed (Cardona & Wilson, 2015).

In this phase, all foreign particles and tissue debris are removed from the wound bed by neutrophils and macrophages, thus preventing infections (Broughton, Janis, & Attinger, 2006). In addition, fibroblasts and myofibroblasts are stimulated by the release of cytokines and enzymes, and the moisture essential for the healing is guaranteed by wound exudate (Das & Baker, 2016; Suarato, Bertorelli, & Athanassiou, 2018).

The third stage is a proliferation in which by phase epithelialization, new granulation tissue is formed and start to grow on the wound area, making new extracellular matrix (ECM). Finally, the last stage of healing is remodeling. During this phase, the composition of matrix changes and type III collagen is replaced with type I which leads to an increase in the new tissue tensile strength (Bielefeld, Amini-Nik, & Alman, 2013; Das & Baker, 2016; Gurtner et al., 2008; Sinno & Prakash, 2013).

Therefore, wound healing is a multi-factorial physiological process, but its complexity can result in several irregularities. In these cases, a wound treatment must promote and guide the healing process. A general care procedure includes swabbing for infection, cleaning of the wound bed from tissue debris, even if it needs to the split-thickness skin autografts or allografts, and finally applying the wound dress (Dreifke, Jayasuriya, & Jayasuriya, 2015). Due to the complexity of healing mechanisms, in which several factors are involved and due to the big variety of wound types, the choice of an adequate wound dress is crucial. Successful wound management relies on an accurate assessment of the wound type and the patient, combined with know-how on the features of potential dressing materials. An ideal wound dressing should promote rapid healing, with minimal inconveniences for the patient. In particular, it should be able to remove excessive exudate, improve autolytic debridement, and keep the moisture adequate for healing. Along with these properties, the wound dress should be flexible, adhesive and easy to remove (Suarato et al., 2018).

According to Sezer and Cevher, potential wound dresses are classified into three categories (Sezer & Cevher, 2011):

• Traditional dressings: they are still the most employed materials for wound and burn treatment. The traditional dressings are generally applied in the first stage of therapy to stop bleeding and avoid any contact between wound and environment (Stashak, Farstvedt, & Othic, 2004).

• Biomaterial-based dressings: they are biological dressings with collagen-type structures used as an alternative to autografts (Ambrosio, 2014).

• Artificial dressings: include cheaper and more effective materials and have a longer shelf-life (Freyman, Yannas, & Gibson, 2001; Stashak et al., 2004).

Lack of stability and risk of infection are the main drawbacks of traditional and biomaterial-based dressings. It seems undeniable to develop active wound dressings, capable to protect the wound from microorganism to face this drawback. For achieving this goal, there are various pathways which involve formulation of the active agents on the surface or into the wound dressing materials to have active wound dressings.

Both synthetic and natural polymers can be applied for the preparation of artificial dressing materials. However, the biocompatibility and the biodegradability along with the high level of biomimicry and the physicochemical features of natural biopolymers make them particularly interesting for wound dressing application (Suarato et al., 2018). In particular, in the term of biopolymer-based wound dress, the degradation rate has to be followed by the dynamics of the wound healing process and guarantied the physiological healing evaluation and releasing active material (Suarato et al., 2018).

Several biopolymers, such as collagen, alginate, hyaluronic acid, chitosan, fucoidan, and Poly-N-acetyl glucosamine, are investigated as wound dressings (Gokarneshan, 2017; Helary et al., 2015; Park et al., 2017; Prosdocimi & Bevilacqua, 2012; Straccia, D'A yala, Romano, Oliva, & Laurienzo, 2015; Tran et al., 2015). Among them, chitin and chitosan are widely used for the treatment of wounds. They are marine polysaccharides, chitin obtain from exoskeletons of crustaceans, such as shrimps and clamps while chitosan is a chitin derivative.

Due to their interesting characteristics, such as low toxicity, biodegradability, and haemostatic properties, chitosan and their derivatives can be considered as the most promising materials for wound dressing applications (D’Ayala, Malinconico, & Laurienzo, 2008; Hu, Zhang, Lu, Li, & Li, 2018; Moeini et al., 2018; Periayah et al., 2017; Yang & Lei, 2015). Recently, the attention has focused on active wound dressings, obtained by the addition of antimicrobial and antibiotic metal nanoparticles in the different wound dressing systems. However, as far as our knowledge, there is not any review regarding bioactive natural metabolites formulation in chitosan-based wound dressings. Therefore, in this review, we focused on the most prominent active natural agents incorporated in chitosan along evaluating their influence on the wound healing process.