Recent Advances in the Use of Algal Polysaccharides for Skin Wound Healing

Abstract

Background:

Chronic skin wounds and pressure ulcers represent major health care problems in diabetic individuals, as well as patients who suffered a spinal cord injury. Current treatment methods are only partially effective and such wounds exhibit a high recurrence rate. Open wounds are at high risk of invasive wound infections, which can lead to amputation and further disability. An interdisciplinary approach is needed to develop new and more effective therapies.

Methods:

The purpose of this work is to review recent studies focusing on the use of algal polysaccharides in commercially available as well as experimental wound dressings. Studies that discuss wound dressings based on algal polysaccharides, some of which also contain growth factors and even living cells, were identified and included in this review.

Results and Conclusion:

Algal polysaccharides possess mechanical and physical properties, along with excellent biocompatibility and biodegradability that make them suitable for a variety of applications as wound dressings. Furthermore, algal polysaccharides have been used for a dual purpose, namely as wound covering, but also as a vehicle for drug delivery to the wound site.

1. INTRODUCTION

1.1. Skin Wounds/Ulcers/Sores

Chronic and slow-healing skin wounds are a major medical concern due to an aging population and the increasing prevalence of chronic systemic diseases, such as obesity, diabetes and spinal cord injury (SCI) [1–5]. In the United States, chronic wounds affect approximately 2% of the population, and are mainly seen in people over 60 years of age [6]. The incidence of pressure ulcers in the SCI population ranges from 22–66%, with a reoccurrence rate of 60–85%, which is attributed to the weak scar tissue and lack of protective sensation [4–5]. SCI patients that bear the above-mentioned comorbidities are more susceptible to develop chronic wounds. Wound healing normally progresses through sequential phases, namely hemostasis, inflammation, proliferation/migration, and remodeling/maturation (Fig. 1), each requiring inter-related biological and molecular processes [7]. Chronic wounds are thought to remain stalled at the inflammation stage and fail to progress to the next phases of wound healing. The treatment of such wounds may require an assortment of methods which overall take weeks to months to affect wound closure [8].

Fig. (1).

Schematic presentation of skin injury and its stages of healing. Below, each phase shows relevant commercially available alginate wound dressings (brown seaweed, Phaeophyceae).

1.2. Current Wound Dressings for Clinical Care

Current wound dressings come in many forms (transparent films, hydrogel, hydrophilic foams, hydrocolloids) and act as a pathogens barrier while absorbing excess fluid discharged from the wound site [9]. The type of dressing used depends on the wound location, shape, etiology, and patient characteristics (Fig. 1) [8]. An optimal wound dressing keeps a moist wound environment, which is generally considered to facilitate wound healing [10] and may also help regulate local pH, temperature and gas exchange. It must also be non-allergenic, non-toxic, easy to sterilize, and wound edge compatible [8, 11]. Wound dressings may also provide long-lasting delivery of bioactive molecules, unlike thin coatings of creams or ointments. These bioactive compounds may be incorporated into the dressing, and/or the dressing material may also have the biological activity of its own [1].

1.3. Why Use Algal Derived Polysaccharides?

Due to the constant demand in the biomedical field for new innovative biomaterials, some researchers have concentrated their attention on natural polymers because of their biocompatibility and biodegradability [12]. Algae, a highly abundant form of biomass that has been underexplored and underused, represent a promising source for the extraction of new bioactive materials such as polysaccharides. Algal polysaccharides have a plethora of chemical structures which determine their physicochemical and biochemical properties, physical behavior, and biological activities [12]. They have been shown to exert a wide range of beneficial biological effects in vitro and in vivo, including the ability to: 1) provide antiviral, anti-bacterial and anti-fungal activities, 2) modulate coagulation, hemostasis, and thrombosis, 3) locally modulate oxidative stress, inflammatory and immune responses, as well as pain signals, 4) provide anti-proliferative, tumor inhibitory, apoptotic and cytotoxic activities in some cases, and pro-angiogenic activities in others. There are several other areas where these polysaccharides are widely used , such as in the food industry, cosmetics, and in the biosensor field due to their properties of water absorption, as bulking agents, thickeners, emulsifiers, and preservatives [13–15]. They are also used as tablet agents and dental impression materials, engineered cartilage scaffolds, encapsulating cell materials to prevent the host cell immune response [16–19], and as immobilization agents for enzymes and multiple biological molecules [15, 20]. In the food industries, polysaccharides are used to improve shelf-life by acting as gelling/thickening and preservative agents [21]. They are very common ingredients in toothpaste, air freshener gels, and other gelling agents (Danagel, Viscarin, Genugel) [22, 23]. Fucoidan has antiulcer and anti-aging properties besides other above-mentioned properties which can be applied in the medical field [24, 25]. Ulvan has exhibited strong anticoagulant/antithrombotic, lipid-lowering, hyperplasia prevention and gastrointestinal prevention [26–29]. Laminarin acts as a good dietary fiber source, metabolic modulator in addition to antiviral, antitumor, anti-inflammatory activity, etc. [30, 31]. Agarose is a good dietary fiber source, thickening agent in jellies and bakery products, and medium for electrophoresis and protein purification [32]. Thus, these materials may find a wide range of applications in therapeutics and regenerative medicine. In the context of wound healing, they may have beneficial properties for drug delivery [12]. In the following sections, we showcase studies using the major algal-derived polysaccharides, namely alginate, agaroid, laminarin, carrageenan, fucoidan, and ulvan (Fig. 2) to manufacture wound dressings and drug delivery systems for skin wound healing applications.

Fig. (2).

Chemical structure of algal polysaccharides.

2. USE OF ALGAL POLYSACCHARIDES FOR WOUND DRESSINGS

2.1. Types of algal polysaccharides used in wound dressings

2.1.1. Alginate

Alginate, extracted from brown seaweed (Phaeophyceae), is probably the most widely used and characterized algal polysaccharide for biomedicine. It is relatively inexpensive, non-toxic, biocompatible, and albeit slowly biodegradable. This anionic and hydrophilic copolymer is comprised of α-L-guluronic acid (G) and β-D-mannuronic acid (M) (Fig. 2A) and forms gels by complexation with divalent cations, usually Ca²⁺ [10, 33]. The seaweed origin and extraction method impact on the overall G and M contents, as well as on the length of GG and MM block domains. Gel rigidity increases with an increasing proportion of G and GG content [34]. This, in turn, it allows for tuning the biomaterial’s mechanical properties in wound management products.

The benefits of alginate in wound healing have been extensively described in the literature. Lee et al. [35] showed that alginate dressings and scaffolds decreased wound closure time, decreased the expression of fibrogenic factors (fibronectin; transforming growth factor-β, TGF-β; vascular endothelial growth factor, VEGF) and increased the expression of collagen type I in rat skin wound [35]. These findings were also confirmed in a diabetic wound healing rat model where alginate treatment boosted collagen-I expression and promoted faster reepithelization [36]. Wound practitioners currently have over ten different alginate-based wound dressing models to choose from to treat acute and chronic wounds (Table 1) [11].

Table 1.

Alginate skin wound dressings.

Alginate Wound Dressing	Composition	Applications
Algicell TM	Sodium alginate (SA), Silver (1.4%)	Ulcers, traumatic and surgical wounds, donor sites
AlgiSite M TM	Calcium alginate (CaA)	Ulcers, surgical wounds
Comfeel Plus TM	CaA, Sodium carboxymethylcellu-lose (SCMC)	Ulcers, burns, skin graft donor sites, and postoperative wounds
Kaltostat TM	SA	Ulcers, donor sites, traumatic wounds
Sorban TM	CaA	Ulcers, donor sites, graft sites, post-operative, and traumatic wounds
Tegagen TM	SA	Diabetic and infected wounds
Guardix-SG	SA, poloxamer	Thyroid and spine surgeries
SeaSorb	CaA	Ulcers
Algivon	CaA, Manuka honey	Necrotic and wounds with odors
Fibracol TM Plus	CaA, Collagen	Full and partial thickness wounds, ulcers, and second-degree burns
Hyalogran	Hyaluronic acid ester (HAE), SA	Ulcers, diabetic, ischemic, and necrotic wounds
Tromboguard	SA, CaA, Chitosan, polyurethane, silver cations	Post-operative and traumatic wounds, gunshots, and skin graft donor sites

Table adapted from Aderibigbe and Buyana [11].

Many of these dressings use alginate alone, but newer ones are seen to incorporate additional natural or synthetic polymers with the intention of tailoring their mechanical properties and degradation rate. Some also include antimicrobial agents to promote infection resolution [11]. As a wound dressing material, alginate has several advantages over traditional (i.e. gauze) wound dressings: 1) it is non-toxic, non-adherent, and safe to use, thus making it easy to remove without disturbing the fragile granulation tissue; 2) the ability to exchange Ca²⁺ ions with the Na⁺ ions in the wound, promoting coagulation and hemostasis; 3) its hydrophilicity helps maintain a moist environment while conferring a high capacity for fluid uptake, thus minimizing wound exudate leakage; and 4) it has mild anti-bacterial properties [12, 34, 35]. Alginate-based wound dressings are available in different forms (film, foam, hydrogel, nanofiber, membrane, sponge, etc). Their use and therapeutic efficacy in wound healing are well documented in a review article by Aderibigbe and Buyana [11], which emphasizes the role of the ratio and type of supplementary polymers/biological molecules used in combination with alginate, such as the addition of antimicrobial agents or nanoparticles, the type of cross-linker and cross-linking time used, and the type of excipient used. For the purpose of this review, we have summarized these studies in the table below (Table 2).

Table 2.

Alginate based wound dressing used in experimental studies.

Physical Form	Wound Dressing Components	Experimental System	Study Highlights	References
Hydrogel	Polyvinyl alcohol (PVA)/SA hydrogel (SA= 0, 5, 10, 20, 30%), Nitrofurazone (NFZ)	In vitro: Physicochemical characterization In vivo: Skin wound (rat)	Increasing SA concentrations decreased gelation (%) and an increased break elongation, elasticity, swelling ability, and thermal stability. Increasing SA content was proportional to wound size reduction. Application of PVA/SA led to epithelialization. Wound dressed with PVA/SA dressing resulted in proliferative tissue with epithelium cells with no sign of inflammatory cells.	[37]
Hydrogel sheet	Alginate Chitin/Chitosan fucoidan (ACF-HS) (60:20:2:4 w/w)	In vitro: Cells cytotoxicity In vivo: Full-thickness skin wound (rat)	ACF-HS studies showed improved absorption and non-cytotoxic nature of the developed sheets. Animal studies showed that the sheet is easy to apply and remove from wounds. Improved wound healing was observed with ACF-HS versus commercial dressing.	[38]
Hydrogel	SA gelatin (G-Type B) SA: G- 20:80, 30:70, 60:40, 50:50, 40:60, 30:70, 80:20	In vitro: Physicochemical characterization	Hydrogel SA/G ratio, cross-linker concentration and cross-linking time affected the mechanical strength and swelling capacity of the hydrogel. Higher cross-linker concentration increased mechanical strength but decreased water uptake. Shortened cross-linking time resulted in worse mechanical strength and increased swelling capacity, while increased cross-linking time resulted in decreased swelling capacity due to higher cross-linking density. Hydrogel with 50:50 SA/G ratios had appropriate characteristics for wound dressing.	[39]
Hydrogel film	Alginate, Aloe Vera Gel (95:5, 85:15, 75:25, v/v)	In vitro: Physicochemical characterization	The combination of alginate and Aloe Vera resulted in thin malleable films fit for skin wound application. Aloe Vera addition to alginate increased the water absorption property of the films and improved their transparency and thermal stability.	[40]
Hydrogel	SA, Gelatin (G) (Type B) SA: G 70:30, 60:40, 50:50, 40:60, 30:70	In vitro: Physicochemical characterization	SA: G ratio combination influenced the morphology of hydrogel and water holding capacity. Hydrogel with higher SA content exhibited droplet morphologies while hydrogel with higher gelatin content exhibited fibrous morphology. The water uptake was influenced by the polymer’s composition. Hydrogel with higher G content had up to 90.1% water uptake capability.	[41]
Hydrogel	Alginate Chitosan	In vitro: Physicochemical characterization Toxicity studies	Higher concentrations of alginate decreased swelling capacity. Biocompatible	[42]
Hydrogel	SA (0.2% w/v) Fibrin (4% w/v) Chitosan (0.1% w/v)	In vitro: Physicochemical characterization	The resulting hydrogel had a fibrous structure (pore size: 100–400 mm). Good mechanical properties such as 0.35 mm thickness, 4.44% elongation, and 23.34 MPa tensile strength.	[43]
Hydrogel film	Alginate Aloe Vera Gel (5, 15, 25%)	In vitro: Physicochemical characterization	Increased transparency of alginate film (dry/wet) with different % of Aloe Vera. In vitro slow degradation of the film thereby maintains the structural integrity. Aloe Vera contents influenced the thermal and mechanical properties of the film.	[44]
Hydrogel	G, SA, HA 1:8:1; 3:6:1;4.5:4.5:1; 6:3:1;8:1:1	In vitro: Physicochemical characterization	The G-SA-HA at different ratios did not change the surface and structural morphology of hydrogel. Addition of SA increased the moisture vapor transmission property of hydrogel.	[45]
Hydrogel	Alginate Chitosan (4:1 v/v) Lauric acid (LA, 0–5% w/v)	In vitro: Physicochemical characterization	Addition of varying concentrations of LA as a plasticizer can influence hydrogel’s characteristics. Linear correlation of LA concentration (1–4%) with water absorption, elongation, and thickness of hydrogel. In contrary, it decreases hydrogel tensile strength. Overall, hydrogel was recommended for wound dressing applications.	[46]
Nanofibers	SA, Poly(ethylene oxide) (PEO) (1:1; 2:1; 3:1; 1: 2; 2:2; 3:2, w/w) Lecithin (0.3 wt %)	In vitro: Physicochemical characterization	Increased concentration of PEO decreased the conductivity of the fibers; however, it resulted in smooth fibers. Reduced content of PEO, such as 2:1 ratio, resulted in beaded fibers. SA/PEO nanofibers are biocompatible and it showed promise for exuding wounds.	[47]
Nanofibrous mat	Electrospun PVA, SA/Zinc oxide (ZnO) composite nanofibers (1:1:0.5, 1, 2, 5 %)	In vitro: Physicochemical characterization Antibacterial Biocompatibility	ZnO concentration is directly proportional to an anti-bacterial property of nanofibrous mat. Higher concentrations of ZnO were cytotoxic in vitro. SA/PVA/ZnO nanofibers could be an ideal wound dressing biomaterial once the ZnO concentration is optimized.	[33]
Nanofibrous mat	SA PVA	In vivo: Splitthickness skin defect (rabbit)	In vivo evaluation demonstrated the good healing properties of the mat. Mat promoted epithelization, angiogenesis, and growth of hair follicles.	[48]
Nanofibers	SA PEO (1:3, 1:1, 3:1, 0:1)	In vitro: Physicochemical characterization	The presence of PEO facilitated the formation of alginate nanofibers by Electrospinning. Different component ratios affected the smoothness and diameter of the nanofibers. Fibers with a 1: 1 ratio exhibited smooth fibers with a diameter of 105 mm; while nanofibers with a 3:1 ratio had decreased diameter and spindle-like defects.	[49]

2.1.2. Agaroid and Carrageenan

Agaroid (agar/agarose) and carrageenan are sulfated polysaccharides, mostly galactans, extracted from red macro-algae (Rhodophyta) [12]. Their chemical composition includes repetitive units of 1,3-α-galactose and 1,4-β-D-galactose that alternate with each other, and/or 3,6-anhydrogalactose with mannose and xylose as monosaccharide substitutions (Fig. 2) [12]. These two major polysaccharide groups differ in their rotation of galactose in 1,3-linked residues, which differentiate agaroids (L-isomer) from carrageenan (D-isomer) [12]. Agaroids are biocompatible and highly hydrophilic polymers which are especially apt at forming films. They are mechanically strong albeit with moderate water resistance [50]. They have been used extensively in the food industry, cell culture, and electrophoresis. More recently, agar/agarose has been used as potential scaffold material for tissue regeneration or as drug carrier [51]; however, agaroids degrade slowly, lack cell-recognition motifs and need to undergo chemical cross-linking to be used as biomaterials. Agaroids can form various blends with different biopolymers to alter the physiochemical properties including water resistance, as well as improve the functional properties of the film [51]. For wound dressing development, agaroids are composed of collagen or gelatin to increase degradation rate as well as promote cellular adhesion [51].

Bao et al. [51] described the development of a skin wound dressing by mixing 1% or 2% agar and 1% collagen type I at different ratios to make membranes or scaffolds. The composite material was then cross-linked using glutaraldehyde. Through mechanical analysis, the agar membrane’s modulus value (640–1060 MPa) decreased (340–820 MPa) after being composited with collagen-I [51]. Furthermore, the new composite material had interconnected pores, which was unexpected given the vastly different structures of collagen-I scaffolds. Collagen scaffolds have honeycomb-like pores, and agar scaffolds have laminated gaps. Additionally, the cross-linked composites showed no toxicity in mice, and when tested as wound dressing over rabbit skin lesion, they promoted good tissue repair with minimal scarring, fluid exudation, and no infection [51].

Miguel et al. [52] produced a thermoresponsive hydrogel made of chitosan and agarose (3% w/v) for skin regeneration. Evaluation of the hydrogel’s physicochemical characteristics revealed a pore size of 90–400 μm, suitable for doing fibroblasts migration and proliferation studies in vitro [52]. Anti-microbial activity against Staphylococcus aureus was observed at >188 μg/mL of chitosan concentration. In addition, the wound healing performance of the hydrogel was assessed in full-thickness skin wounds in rats; the results suggested the group treated with chitosan-agarose hydrogel had better healing and the absence of an inflammatory reaction, indicating its suitability as a wound dressing [52].

Agar has also been mixed with synthetic polymers to modify its mechanical properties and other natural products such as honey to make bioactive wound dressings. Zohdi et al. [53] fabricated a dressing made of a cross-linked hydrogel comprised of 15% (w/v) polyvinylpyrrolidone (PVP), 1% (w/v) polyethylene glycol (PEG), 1% (w/v) agar, and 6% (w/v) honey. After mixing all components and gelling them at room temperature, the gel was cross-linked by an electron beam at 25 kGy. The resulting hydrogel consisted of transparent sheets with a thickness of 3–4 mm. Property analysis resulted in a slightly more acidic pH for the honey-containing hydrogel (pH 4.3) compared to hydrogel control (no honey, pH 5.3) and significantly higher water absorbing capability than control [53]. The increased water absorption may be attributed to the high osmolarity resulting due to the incorporated honey, which would be expected to increase the potential to absorb wound exudates. The therapeutic efficacy of the hydrogel was tested on deep partial thickness burns in a rat model. In this model, the honey-laden hydrogel performed significantly better than hydrogel control as shown by the acceleration of granulation tissue formation, dermal repair (including collagen synthesis and capillary formation), re-epithelialization, and attenuation of inflammation. The honey hydrogel also significantly enhanced wound healing (day 21versus day 28) [53].

Carrageenans are mostly used in the food industry, with a market worth of $525 million [9]. They exist in various types depending on the number and position of the sulfate groups and the presence of 3–6 anhydro-galactose bridge, which are called kappa (κ), iota (ι), and lambda (λ) [9]. Carrageenans are water-soluble due to their high content of hydroxyl and sulfate groups. The λ-carrageenan is a non-gelling polysaccharide. Conversely, κ-carrageenan (KC) can form hard and fragile thermo-reversible gels through hydrogen bonds and ionic interactions, especially K⁺. Food and non-food industries both use KC as thickening and gelling agent. The ι-Carrageenan forms soft and elastic gels that are suitable for use in cosmetic emulsions [54]. Gelled carrageenans readily solubilize in water, and therefore must be chemically modified to generate stable hydrogel.

Carrageenans have been shown to have a variety of biological functions such as anti-viral, anti-bacterial, inflammatory, and immunomodulatory, which are highly dependent on the specific algae of origin, type (kappa, iota or lambda), and molecular weight. While some carrageenans stimulate T cell and macrophage activity, others may inhibit it [12]. Carrageenans can also be used to make fibers, membranes and porous structures [12].

Sen and Avci [54] investigated the use of poly(N-vinyl-2-pyrrolidone)– KC hydrogels (PVP-KC) in wound dressings. To make the hydrogel, they combined PVP (3, 5, 10% w/w), KC (1, 2, 3% w/w), KCl (0.1, 0.3, 0.5% w/w), and PEG (1.5% w/w) solutions followed by gamma irradiation. The resulting hydrogel had the following describable properties: flexibility and transparency, good mechanical strength and elasticity, effective at fluid absorption, and the ability to be painlessly removed [54]. Another PVP/KC/PEG hydrogel wound dressing was developed by Awanthi et al. using 7% (w/v) PVP, 1% (w/v) KC and 1.5% (w/v) PEG by gamma irradiation, which also acted as a sterilization step [55]. The hydrogel demonstrated absorption behavior, tensile properties, adherence to human skin, and sterility/shelf-life comparable to commercial hydrogels.

In an effort to diversify the uses of KC for tissue engineering applications, Mihaila et al. [56] introduced photo-cross-linkable methacrylate moieties onto KC to create physically (by using K⁺ ions) and chemically (by using ultraviolet (UV light) cross-linkable hydrogels (MA-KC) [56]. By chemically functionalizing KC with methacrylate groups, Mihalila et al. were able to overcome the drawbacks of KC hydrogels formed by ionic crosslinking.. By varying the extent of methacrylation, it was possible to control the elastic moduli, pore size distribution, and swelling ratio of the hydrogel. In addition to testing the potential of this hydrogel as a cell carrier, in vitro studies were performed with NIH-3T3 fibroblasts, MC3T3 E1–4 pre-osteoblasts, and human mesenchymal stromal cells (hMSCs) encapsulated in MA-KC hydrogels. Cell-laden hydrogels showed high viability and no significant change of reactive species production (superoxide anion or nitrogen species) for short (hours-days) and long time periods (up to 21 days) for most cross-linking conditions, except when high degrees of methacrylation were used. Based upon the above-mentioned results, dual cross-linked MA-KC based hydrogels show potential for wound dressing [56].

2.1.3. Fucoidan

Fucoidans are sulfated polysaccharides extracted from brown seaweeds (Phaeophyceae) that contain a backbone of (1 →3)-linked α-l-fucopyranosyl or alternating (1→3)- and (1→4)-linked α-l-fucopyranosyl residues. Also included are sulfated galactofucans on the backbone of (1 → 6)-β-d-galacto- and/or (1→2)-β-d-mannopyranosyl units (Fig. 2). Attached to the backbone, one finds fucose or fuco-oligosaccharide branching, with glucuronic acid, xylose or glucose substitutions [57]. In general, fucoidans exhibit anti-viral, anti-bacterial, anti-inflammatory, immuno-stimulatory, anti-metastatic, angiogenic, and cardioprotective activities [12]; however, the structural diversity and complexity of these polysaccharides make it difficult to predict biological activity from polysaccharide composition. Fucoidans are also known for enhancing the activity of heparin-binding factors, for example, fibroblast growth factor (FGF), and to modulate the effects of TGF-β1 on skin wound healing [38, 58].

Sezer et al. [59] developed wound dressing films made of mixtures of chitosan and fucoidan to treat skin burns. Chitosan and fucoidan were mixed at concentrations of 1.0–2.0% and 0.25–0.75%, respectively, and cross-linked to propylene glycol (2.5%) [59]. Material characterization showed porous films with water vapor permeability ranging from 3.3–16.6/0.1 g, water absorption capacity of 0.67–1.77 g/g, and tensile strength of 7.1–45.8 N. In vitro evaluation of bio-adhesion energy capacity ranged from 0.076–1.771 mJ/cm². In vivo studies were performed in a rabbit model of superficial burn wounds in which each rabbit received four wounds that were treated with: 1) chitosan film with fucoidan (CFF), 2) fucoidan in solution (FH), 3) chitosan film alone without fucoidan (FS), and 4) no treatment control. On post-injury days 14 and 21, wound contraction was found to be stimulated by CFF. Overall results suggested that wound closure was promoted best in the CFF group, followed by the CF, FS and control groups. In summary, fucoidan-chitosan films induced burn wound healing via wound contraction and is a candidate material for wound dressing [59].

In another study evaluating fucoidan- based materials, Sezer et al. [60] investigated the potential application of fucoidan-chitosan hydrogels as burn healing accelerator [60]. To make the hydrogels, chitosan (1.50–2.00%) and fucoidan (0.50–0.75%) were mixed in a 1% m/v lactic acid solution and left to swell overnight. Mechanical testing resulted in higher viscosity, water absorption capacity, and bio-adhesion with increased polymer concentration. As per the mechanical properties of the hydrogels, a formulation of 2% chitosan and 0.75% fucoidan was tested in an in vivo rabbit skin burn would healing model. The study was performed with the following treatments: 1) fucoidan-chitosan hydrogels (CFH), 2) fucoidan solution (FS), 3) chitosan hydrogel without fucoidan (CH) and 4) no treatment (control). After 7 days of treatment, groups treated with CFH and FS showed the fixed formation of fibrous tissue and scarring. Compared to other groups, the best regeneration and fastest wound closure were observed in the CFH treated groups after 14 days of treatment. CFH may, therefore, be suitable as components in wound dressings [60].

Webber et al. studied the formation and enzymatic breakdown of a polyelectrolyte multilayer (PEM) system made of poly-L-arginine and fucoidan. Poly-L-arginine (PLAR) was selected as a biopolymer for this construct because it can supply nitric oxide (NO) to the wound after degradation by nitric oxide synthase [61]. For the purpose of this study, the authors used trypsin, which has been shown to accelerate wound healing progression when topically applied, to degrade the PEM and breakdown of PLAR into arginine. The authors hypothesized that topical trypsin application can be used in combination with a wound dressing coated with fucoidan and PLAR to accelerate NO release into the wound area. The complete film (8 bilayers) had low hydration (30% water) and released both polymers over time, with complete film degradation reaching after approximately 24 hours [61].

2.1.4. Ulvans

Ulvans are anionic sulfated hetero-polysaccharides extracted from green algae (especially from Ulva sp.) with a composition that is highly variable, and which depends on a multitude of factors, such as the time period of collection, the growth conditions of the algae, and the polysaccharide extraction method [27]. The main sugar molecules in ulvans are rhamnose, xylose, glucuronic and iduronic acid, which are highly sulfated (Fig. 2D). Ulvans bear a striking resemblance with glycosaminoglycans (GAGs) found in animal and human systems and have shown beneficial effects both in vitro and in vivo [12]. This relatively underexplored class of polysaccharides could potentially form the basis of new and effective biomaterial for biomedical applications.

Toskas et al. [62] developed an electrospinning method to manufacture ulvan nanofibers comprised of ulvan with PEO and PVA at different mass ratios (5:50–90:10) [62]. PEO and PVA were added to decrease the viscosity of the ulvan solution to enable electrospinning. The ulvan-PVA mixture resulted in nanofibers with highly ordered crystalline fibers of less than 100 nm in diameter. Conversely, ulvan-PEO mixtures only yielded beaded nets. Decreasing the ratio of PVA to ulvan also led to bead-like structures. Fiber diameters tended to decrease as ulvan concentration increased, while bead formation decreased with increasing total polymer concentration (ulvan + PVA) [62]. These studies highlight the possibility of using ulvan-based nanofibers as wound dressing’s materials, or other tissue engineering applications, albeit they may require that the mechanical integrity of the fibers be reinforced.

Alves et al. [63] described the production of 3D porous constructs based on 1,4-Butanediol diglycidyl ether (BDDE)-cross-linked ulvan and investigated their feasibility for tissue engineering applications [63]. To make the constructs, various ulvan and cross-linker concentrations were used (4–8%; w/v) of ulvan and 0.20–2.00 (w/w) of BDDE and their mechanical performance, water uptake, degradation time, and biocompatibility were evaluated. Overall results indicated that ulvan-based structure’s characteristics are highly dependable on the cross-linker and ulvan concentrations. The studied constructs had a highly porous structure with a compressive modulus varying from 0.23 to 3.60 MPa, high-water absorption capacity ranging from 8–20 times their dry weight, and could be dissolved in aqueous media immediately or have a dissolution time between 3–7 days. In vitro biocompatibility studies on L929 cells seeded on the constructs for 1, 3 or 7 days showed high cell viability and proliferation, suggesting that these materials may be suitable as components in wound dressings [63].

Kikionis et al. [64] also prepared electrospun nanofibers composed of ulvan and PEO or polycaprolactone (PCL). Ulvan was blended at ratios of 1:1 and 2:1 or 1:8, 3:8, and 3:2 with PEO or PCL, respectively [64]. Fiber diameter decreased with increasing ulvan content. In these mixtures, ulvan, as a heparin analog of algal origin, may provide anti-thrombogenic properties while PEO and PCL govern the degradation rate of the composite material. The long biodegradation period of PCL, in particular, may be lead to ulvan-containing composites that are especially well suited as biomaterial scaffolds [64].

3. ALGAL POLYSACCHARIDES AS DRUG DELIVERY PLATFORM FOR WOUND HEALING

While the wound dressing material may intrinsically have beneficial effects on wound healing, there is an impetus to incorporate bioactive compounds that target different and specific features of wound healing dynamics; thus, in this context, the potential of algal polysaccharides, including alginate, agaroid, carrageenan, fucoidan, and ulvan, as platform for controlled drug delivery has been explored [12]. Some of the bioactive compounds tested so far have focused on decreasing the adverse effects of high protease activity, promoting oxygen permeability, and stimulating endogenous growth factor and cytokine production [65]. These more advanced wound dressings can improve treatment safety and patient comfort by reducing the dose and treatment frequency, while improving the therapeutic efficacy by lengthening drug effect duration [65].

3.1. Alginate

Alginate has been extensively used in drug delivery for skin wound healing [65]. Alginate can be manufactured in various forms, such as wide applications include nanofibers, hydrogels, foams, and wafers to deliver a wide range of compounds, including anti-inflammatory drugs, growth factors, antibiotics, micro- and nano-particles to the wound site. For the purpose of this review, we summarize the studies recently compiled by Aderibigbe and Buyana [11] in Table 3 below.

Table 3.

Drug delivery system using alginate for skin wound healing.

Physical Form	Wound Dressing Components	Experimental System	Study Highlights	References
Bilayer hydrocolloid film	SA G-Ibuprofen	In vitro: Physicochemical characterization In vivo: Full-thickness skin wound (rat)	Bilayer construct had slower lower drug flux and hydration rate that was an indication of superior mechanical and rheological properties. Provided comparatively faster healing in vivo. The wound histology supported the healing phenomena (epidermis and granulated tissue). This system could be used for drug delivery.	[66]
Hydrogel	Polyvinylpyrrolidone (10% and 15%) Alginate (0.5% & 1%) Silver nanoparticles	In vitro: Antimicrobial test	Hydrogel exhibited a fluid uptake capability of ~2000% which is a desired characteristic to prevent exudates accumulation. It acted as anti-microbial hydrogel at a concentration of 70 ppm (nanosilver)	[67]
Hydrogel film	SA Essential oils: Cinnamon, Chamomile Blue, Eucalyptus, Lavender, Lemongrass Lemon, Peppermint, and Tea Tree	In vitro: Antimicrobial test	Antibacterial films comprised of essential oils with alginate. The anti-microbial property of these hydrogels was dependent on the combination and concentration of essential oils.	[68]
Hydrogel	Alginate Honey	In vivo: Full-thickness skin wound (rat)	Transparent. Faster wound healing was observed with the honey-alginate based hydrogel (7 versus 8 days). The honey-alginate hydrogel enhanced angiogenesis, re-epithelialization, and granulation process. The presence of alginate and honey had a synergistic effect on wound healing.	[69]
Hydrogel	PVA (10% w/v) SA (0–75% w/w) Ampicillin	In vitro: Physicochemical characterization, Antibacterial test	Hydrogel membranes containing high alginate content (~75%) exhibited increased water uptake, enhanced hydrolytic degradation, increased pore size, and distribution, while reduced elongation and tensile strength. The hydrogel had anti-bacterial properties regardless of the SA concentration.	[70]
Hydrogel/Nanoparticle composite bandage	Alginate, ZnO nanoparticles (0.05% −1% w/w)	In vitro: Physicochemical characterization Antimicrobial test Toxicity studies Ex-vivo: Porcine skin re-epithelialization model	Bandage wound dressing comprised of alginate hydrogel and ZnO nanoparticles with 60–70% porosity and 200–400 μm pore size. The presence of ZnO nanoparticles affected the swelling ratio, degradation nature, and antibacterial activity of the bandage. Increase the concentration of ZnO nanoparticles decrease the swelling ratio but improve the antibacterial activity. Toxicity evaluation showed a slight decrease in the viability of cells at high ZnO nanoparticles concentration. In the ex vivo model, ZnO nanoparticles enhance proliferation and migration of keratinocytes to the wound area which resulted in re-epithelialization after 48 hours.	[71]
Hydrogel	Alginate Chitosan hydrochloride	In vitro: Physicochemical characterization, Antimicrobial test Cytotoxicity test	The alginate-chitosan hydrogel showed high water uptake and reduced release kinetics of rhodamine B resulting in sustained drug release. Antibacterial activity against Escherichia coli was observed at different alginate concentrations. Cytotoxicity tests showed no adverse toxic effects from chitosan.	[72]
Hydrogel microspheres	Alginate Hydroxyapatite Simvastatin	In vitro: Angiogenesis assay In vivo: Rat full-thickness wound	The wound dressing demonstrated angiogenic properties via induction of hypoxia-inducible factor-1α and vascular endothelial growth factor. In vivo studies exhibited enhanced angiogenesis and wound re-epithelialization.	[73]
Nanofibers	SA (1%) PVA (9%) (2:1)	In vitro: Physicochemical characterization In vivo: A rabbit model of full-thickness wound	Nanofibers were composed of uniform and continuous fibers (diameter ~100 nm). The air permeability of the nanofibers was poor due to process parameters. In vivo studies demonstrated faster (98 versus 99 %) wound closure via contraction versus commercially available Suprasorb-A.	[74]
Nanofibers	SA (4% w/v) PEO (4%w/v) Pluronic-127 (1.5% w/v) Lavender essential oil (5% v/v)	In vitro: Bioactivity Antimicrobial test In vivo: Burn skin injury	Dressing was bioactive and exhibited anti-microbial and anti-inflammatory activities in vitro and in vivo. Faster recovery from burn injury in animals. Comparatively (versus Tegaderm™) better in terms of downregulating inflammatory cytokines.	[75]
Nanofibers	SA (2%) PVA (12%) (8:2, 7:3, 6:4, 5:5, 4:6) Moxifloxacin hydrochloride	In vitro: Antimicrobial test In vivo: Full-thickness wounds (rat)	This nanofibrous membrane exhibited anti-bacterial activity. Direct correlation of moxifloxacin hydrochloride concentration with the antibacterial activity of the membrane. In vivo studies showed accelerated wound closure in the treatment group.	[76]
Foam	Alginate Curcumin	In vitro: Physicochemical characterization Antimicrobial test	Alginate curcumin foam provided burst release after hydration. Both loaded and unloaded foams rapidly (12–16 times) absorbed their dry weight when hydrated. Curcumin exhibited anti-bacterial effects in vitro.	[77]
Foam	Alginate 5,10,15,20-tetrakis(4-hydroxyphenyl) porphyrin (THPP) Pluronic-127	In vitro: Physicochemical characterization	Foam was thin, flexible, and easy to handle. THPP exhibited loading between 0.12–0.13 % (w/w). Drug release influenced by β-cyclodextrin derivatives. Alginate foams could be viable delivery vehicles for photosensitizers in antibacterial photodynamic therapy of wounds.	[78]
Wafer	SA G (0:100, 75:25, 28 50:50, 25:75, 0:100] Silver sulfadiazine (0.1% w/w)	In vitro: Physicochemical characterization	The presence of different concentrations of alginate affected the morphology, water uptake capability, and the drug release profile of the wafer. Formulations containing high alginate content exhibited porous morphology, high water uptake, and faster drug release when compared to other formulations.	[79]
Wafer	SA Xanthan gum Methylcellulose Sodium fluorescein	In vitro: Physicochemical characterization	Wafers underwent rapid hydration forming highly viscous gels with good surface adhesion. The presence of SA enhanced the swelling ability in the formulations. Addition of sodium fluorescein emphasized the gels as wound dressing and drug delivery tools.	[80]
Wafer	Polyox-carrageenan (75:25 w/w) SA (50:50 w/w) Streptomycin Diclofenac	In vitro: Physicochemical characterization	Wafers were soft, flexible, and provided controlled drug release (Streptomycin and Diclofenac). Non-loaded wafers had higher swelling ratios and adhesion capability. Strength and flexibility of the wafers highlight their ability to protect and preventing damage of newly formed skin tissue.	[81]
Wafer	SA Guar gum (1:1) Neomycin	In vitro: Antimicrobial test In vivo: Full-thickness skin wound (rat)	Showed anti-microbial activity. Wafers had a sponge-like structure with an interconnecting porous network. Antimicrobial tests showed growth inhibition for both bacterial strains. The wafer (including neomycin) enhanced the skin wound healing in rats.	[82]

3.2. Agaroids and Carrageenans

Rivadeneira et al. [50] developed films with different proportions of soy protein and agar (SPI-AG-3:1, 1:1, and 1:3 weight ratio) loaded with the antibiotic ciprofloxacin hydrochloride (Cipro), and cross-linked with glycerol (4.28 % w/v). They then characterized their physicochemical properties, drug release profile and antimicrobial activity [50]. Increasing the AG content increased water absorption capacity and also caused the film surfaces to become more irregular. The SPI-AG ratio also impacted the Cipro release kinetics, with higher AG content resulting in faster initial burst release. After 15 days (corresponding to maximum release), lower AG content resulted in decreased Cipro release (by as much as 80%). SPI-AG films loaded with Cipro inhibited Staphylococcus aureus and Pseudomonas aeruginosa strains. Thus, the SPI/AG ratio is an adjustable parameter that can be optimized to obtain the desired drug delivery profile; burn wound and ulcers were suggested as potential applications for the SPI-AG films.

Da Silva et al. [83] embedded magnetite (Fe₃O₄) nanoparticles into KC hydrogels, and the mechanical properties and release kinetics were studied using the model drug compound, methylene blue [83]. Increasing Fe₃O₄ nanoparticle content increased mechanical strength and swelling ratio, but at the same time, slowed down the release rate of methylene blue. When the nanoparticles were incorporated as a dispersed phase, the nano-composite showed an unusual transport behavior controlled by the polymer relaxation [83].

Boateng et al. [84] developed a film wound dressing consisting of PEO (polyox) and KC laden with the antibiotic streptomycin (STP) and the anti-inflammatory diclofenac sodium (DLF) [84]. They reported that the drug combination had synergistic antimicrobial activity and that DLP relieved injury-associated pain and swelling. The gels were made by blending 1% (w/w) polyox and KC at a 75:25 weight ratio to yield 1% (w/w) of total polymer, cross-linked with 0–50% glycerol, and loaded with DLF and STP. The drug concentration in the end-product ranged from 10 to 30% (w/w). Drug-loaded films were transparent and smooth and exhibited a range of desirable mechanical properties, including high elasticity (1000%), moderate elastic modulus and tensile strength (both ~1 MPa). The films also showed a high absorption capacity and substantial mucoadhesion force, which allows for adherence to and protection of the wound. Continuous release of SPT and DLF was observed over 72 hours, which resulted in significant inhibition of S. aureus, P. aeruginosa, and E. coli growth. In summary, the drug-loaded polyox-KC film exhibited good characteristics for a wound dressing with possible dual anti-inflammatory and anti-microbial properties [84].

Singh et al. [67] prepared anti-microbial hydrogels blends containing PVP (15%), KC (0.25%), and silver nanoparticles (0, 30. 50, 70, and 100 ppm) that were sterilized by gamma irradiation [67]. Incorporation of silver NPs did not have an impact on the fluid absorption capacity or vapor barrier properties of the material. Anti-microbial properties against several common wound pathogens (P. aeruginosa, S. aureus, E. coli, and C. albicans) was demonstrated showing that silver nanoparticles effectively inhibited microbial growth in the material [67].

Padhi et al. [85] created gelatin and ι-carrageenan (IK) hydrogels in different ratios (10:0, 9:1, 8:2, 7:3, 6:4, 5:5) and loaded them with ciprofloxacin (Cipro), a broad spectrum antibiotic [85]. The hydrogels exhibited desirable properties of biocompatibility, adhesion, and swelling. At physiological pH, the 7:3 hydrogel exhibited a linear release profile of Cipro and the product exhibited measurable anti-microbial activity against a range of bacteria, including Bacillus sp, Vibrio sp, Pseudomonas sp, and E. coli, while no cytotoxicity towards two human cell lines (HaCaT and HEK293) [85]. Thus, this antibiotic hydrogel may be further explored for wound healing applications.

While loading and releasing hydrophilic compounds from the largely hydrophilic algal-derived hydrogels have generally been successful, doing so with a hydrophobic drug has been much more difficult. Several strategies have been attempted to load hydrophobic drugs into such hydrogels; however, this often caused a deterioration of hydrogel properties, such as decreased swelling capacity and worsening biocompatibility. To address this issue, Nair et al. [86] came up with an approach to encapsulate hydrophobic compounds with cyclic β-(1/3) (1/6) glucans (CBG). The latter form include bodies, which can be dispersed inside IK hydrogels; this method was shown to work for a wide range of hydrophobic drugs [86]. Hydrogels were formed at weight ratios ranging from 0:100, 5:95, 25:75, to 50:50 by glutaraldehyde cross-linking. Drug-loaded hydrogels were formed with Cipro and CBG at a ratio of 1:4 (w/w) and blended into IK. The hydrogel drug loading and long-term release of Cipro were enhanced by the addition of the CBG complexes. The presence of CBG also increased the porosity of the hydrogel, which resulted in improved cell viability, proliferation, and nutrient diffusion. In a scratch assay wound healing model, the CBG-IK hydrogel showed faster gap closure with increased CBG concentration [86]. Furthermore, more rapid wound closure was observed in an in vivo excision wound healing model in the experimental group treated with CBG-IK hydrogels vs. commercial hydrogels, suggesting that these hydrogels may make promising wound dressing materials.

3.3. Fucoidans

Zeng et al. [87] fabricated a fucoidan-modified chitosan/alginate (F-CS/Alg) drug delivery scaffold for the controlled release and protection of basic FGF [87]. Because FGF is known to be a heparin-binding factor, the fucoidan moiety was over-sulfated (43% sulfation degree) to promote FGF binding and subsequent release from the matrix. To make the F-CS/Alg scaffolds, CS/Alg was mixed at 3:1, 1:1, and 1:3 weight ratios, cross-linked with calcium chloride, and lyophilized. Then, 1% fucoidan solution was added to the scaffolds and lyophilized. Finally, FGF (1000 ng/mL) was loaded into the F-CS/Alg scaffolds and lyophilized again. Material characterization showed that over-sulfated fucoidan protected FGF bioactivity from degradation by trypsin, showed good biocompatibility, and an ability to scavenge free radicals. The F-CS/Alg scaffold was tested for its bioactivity in a scratch wound assay, and the results showed that this material accelerated fibroblast migration [87]. Thus, FGF-loaded F-CS/Alg scaffold is an excellent carrier for drug delivery for skin wound healing.

3.4. Ulvans1

Morelli et al. [88] described the grafting of methacrylate groups on ulvan by treatment with methacrylic anhydride (MA) or glycidyl methacrylate (GMA), which results in UV photopolymerizable hydrogels (UMA and UGMA, respectively) [88]. UMA hydrogels had a higher degree of functionalization than their UGMA counterparts, which made them more resistant, stiffer, and coherent. In contrast, by having a lower degree of functionalization and consequently lower crosslinking density, the UGMA hydrogels swelled more easily and therefore had higher water uptake capacity than the UMA hydrogels.

Alves et al. [89] produced ulvan-based membranes as wound dressings which were also shown to serve as delivery vehicles for therapeutic agents. In order to make the membranes, ulvan was mixed with the cross-linker 1,4-butanediol diglycidyl ether (BDDE) in a 1:5 molar ratio [89]. After this process, cross-linked ulvan was dried into a powder that was then dispersed into water (1% w/v) to prepare the membranes by solvent casting. To make drug-loaded membranes, 15% (w/w) dexamethasone was added to the ulvan solution before the solvent casting step. The membranes had high water uptake capacity, increased mechanical performance (tensile modulus= 1769 kPa, tensile strength= 44 kPa, and tensile strain= 15.2%), and exhibited sustained drug release up to 14 days. These results warrant further studies to explore the potential of ulvan-based wound dressings [89].

Morelli et al. [90] modified ulvan to create an injectable hydrogel system for cell delivery. For this purpose, ulvan was modified with tyramine (UT) to prepare an in-situ gelling material using oxidative coupling [90]. A horseradish peroxidase-hydrogen peroxide combination was used to affect the cross-linking. UT-based hydrogels had favorable mechanical properties with the swelling ratio up to ~2000% initial dry weight. The rheological analysis resulted in complex and elastic moduli of 603 Pa, and a viscous loss modulus of 20 Pa. In addition, the viscosity of the pre-gelling mixture was low enough to enable delivery by injection, thus making the system suitable for in situ hydrogel generation [90]. The material was found to be biocompatible in vitro towards rat pheochromocytoma PC12 cells [90].

CONCLUSION

Recent advances in the basic science understanding of the wound healing dynamics suggest that preventing early infection, increasing blood flow, and providing nutrient/growth factors to the tissue can result in faster wound healing. However, the process of chronic skin wound healing is very complex and different etiologies may lead to different mechanisms of impaired healing. The management of these wounds (diabetic pressure ulcers, SCI-induced skin pressure ulcers, and burn wounds) remains difficult and costly. Algal polysaccharides provide a huge range of mechanical and physical properties along with excellent biocompatibility and biodegradability. Alginates, in particular, are widely available commercially and extensively used as wound dressings and as drug delivery systems. Some of the other algal polysaccharides exhibit several beneficial properties for skin wound healing, such as antiviral, anti-bacterial, and anti-fungal activities. They also possess the ability to modulate coagulation, hemostasis, and thrombosis, as well as the ability to modulate oxidative stress and protease activity, inflammatory and immune responses, and pain signals. They also exhibit favorable oxygen permeability, can stimulate local endogenous growth factors, cytokines, and pro-angiogenic factors. These properties suggest that algal polysaccharides should be explored and studied vigorously for the purpose of wound dressings or scaffolds for drug delivery.