**5. Alginate-Based Beads and Microcapsules for Wound Healing**

The microcapsules (around 200 µm) and beads are obtained either by using CaCl<sup>2</sup> as a cross-linking agent, or they can be obtained by dripping a liquid polysaccharide solution in an acidic (pH < 4) gelling solvent [15]. The bead size is also influenced by the gravity force and the resisting interfacial tension force when the droplet is falling in the liquid [45]. Furthermore, alginate beads [79], obtained through emulsion or extrusion, have the ability to entrap drugs, proteins, growth factors such as the platelet-derived growth factor (PDGF) and/or other wound healing promotors. One example is the alginatechitosan polyelectrolyte membranes, with or without silver sulfadiazine (AgSD), and the chitosan–fibrin–sodium alginate hydrogel that displayed wound healing properties [11], as seen in Table 2.


**Table 2.** Alginate-based beads and microcapsules used for wound healing.

#### **6. Alginate-Based Nanofibers and Fibers for Wound Healing**

When discussing the average diameter of alginate-based nanofibers obtained after electro-spinning authors mention a myriad of dimensions, ranging from 70 to almost 200 nm, as follows: 93 ± 22 nm after lavender oil was added to a Na ALG-Polyvinyl alcohol (PVA) blend; 100.35 ± 12.79 nm for a Na ALG-PVA; 105 nm for Na ALG—polyethylene oxide/glycol (PEO) blend, 175 ± 75 nm Na ALG-PVA-moxifloxacin hydrochloride [85]; 151 ± 19 nm for Na ALG-PEO [86]; 190–240 nm Na Alg-PVA [87]; 196.4 nm for a collagenalginate [88]. The alginate-based fibers can be obtained either by spinning in an aqueous media or by extrusion. The average diameter of these fibers depends on the gauge of the used extrusion device, ranging from 70 µm up to 0.1 mm for the extruded ones, whereas fibers obtained in a coagulation bath had a diameter of 6 mm [89]. Liao et al. [90] mentions fibers with an average diameter of 10–20 µm.

A bio-polymeric system, effective in chronic wound therapy, remains a challenge. Bioactive functionalized bio-polymeric supports based on nanofibers, with integrated antibacterial components, is an area of extremely high interest, both in chemical and biopharmaceutical terms. This is because the changes in nanofibers diameters affect the rate of controlled release of the active agent within the nanofibers network [91].

When the alginate fiber dressings make contact with a wound, the space in-between its fibers will close and the bacteria will be trapped in this wound dressing because of the water intake and thus the fiber swelling [34].

Alginate-based nanofibers are obtained through electrospinning. This process takes place after high voltage electrical current passes through a liquid drop that becomes charged, and because of the antagonistic tension surface and electrostatic forces, the drop will elongate until it reaches the nanofiber state [92] (Figure 3).

The molecular weight and viscosity of the ingredients influence the resulting nanofibers [23]. Sodium alginate (Na ALG) cannot form electrospun nanofibers on its own [86,87] but the possibilities regarding the blends are numerous (Table 3). If Na ALG is blended with PEO or PVA, then the resulting nanofibers are smooth [86,93,94] and ZnO nanoparticles can be cross-linked to the mat for an antibacterial effect. When a 1:1 ratio of PVA and Na ALG was used, under a 17 kV treatment at a 0.1 mL/h rate with the collector being 5 cm away from the needle, the resulting average diameter of the nanofibers was 190–240 nm [87]. Alginate fibers can also be treated with silver nanoparticles (AgNPs) either from a silver nitrate solution or Ag+/Ag<sup>0</sup> ions and have antimicrobial and antifungal properties [34].

**Figure 3.** Electrospun ALG nanofibers.

PVA-Na ALG nanofibers were also generated through electrospinning at 15 kV, a flow rate of 0.5 mL/h and 15 cm away from the syringe tip and ciprofloxacin was incorporated in the created patch through active loading. The diameter of the resulting PVA-Na ALG nanofibers was 200–300 nm and it increased after the drug was loaded. These drug-loaded nanofibers showed encouraging results in an in vivo wound healing study when applied as a composite nanofiber patch following the Higuchi and Korsmeyer–Peppas drug-releasing models [95].

During an in vitro study PEO and Na ALG were used to create nanofibers at a 1:1 ratio. By adding DMSO and Triton X-100 the surface tension and the viscosity of the solution lowered. The conditions for obtaining nanofibers with an average diameter of 151 ± 19 nm were 20 kV through an 18G needle, the collector being 20 cm away from the source in a 30% humidity area. If cross-linkers like 1M calcium nitrate tetrahydrate (Ca(NO3)2) and glutaraldehyde (C5H8O2) treat the fibers they become thinner (Ca(NO3)<sup>2</sup> average diameter of 149 ± 69 nm versus C5H8O<sup>2</sup> average diameter 130 ± 51 nm), have better tensile strength and degradation rate, while losing their flexibility. The addition of 0.1 vol % poly-L-lysine increased the fibroblast cell attachment and their proliferation, showing the feasibility of using this type of nanofibers in other wound healing studies [86,94].

In another study, performed by Kataria et al. [95], 0.5 cm deep and 4 cm<sup>2</sup> incisions were inflicted on male rabbits in a study that compared the wound healing ability of ciprofloxacin loaded- and non-loaded PVA with or without Na ALG electrospun composite nanofiber transdermal patches. The changes of the wound site were observed every 5 days for a total of 20 days and the best results were recorded the used transdermal patch was the drug-loaded PVA-Na ALG dressing. This result was proven after both histological and biochemical assays, when the complete healing of the wounded area was seen after 17 days, and the maximum amounts of collagen and hydroxyproline in the wound bed were measured after 20 days. Another advantage of the alginate-based dressing was its ability to be removed by dissolution because of its gel-forming property.


**Table 3.** Alginate-based nanofibers and fibers used for wound healing.

An uniform morphology of the nanofibers can also be obtained by adding lecithin as a natural surfactant [93], or arginine–glycine–aspartic acid (RGD) [103].

Nanofibers can also be obtained by mixing methacrylated alginate, RGD-modified methacrylated alginate and PEO at 10.4 kV, with a flow rate of 0.6 mL/h and the collector being placed at 15 cm away from the syringe tip. The interesting part about this blend was the UV treatment with 365 nm UV light at <1 mW/cm<sup>2</sup> that can or cannot be followed by the PEO extraction. The resulting photo-cross-linked nanofibers can also be coated with gold. Before the cross-linking the fiber diameters were between 185.5 ± 37 and 195.4 ± 23 nm, after cross-linking the fiber diameters were between 182.2 ± 36 and 190.4 ± 30 nm, but the PEO extraction lead to a diameter increase due to nanofiber swelling, ranging between 256.3 ± 43 and 297.9 ± 42 nm [96]. When this study used PEO/methacrylated heparin-, RGD-modified-, or unmodified methacrylated alginate-based nanofibers, it concluded that—by adding methacrylated heparin—the stress–strain curves are influenced, therefore making the elongation at break significantly lower and the tensile strength and Young's modulus significantly greater than those observed for the unmodified or RGD-modified methacrylated alginate-based nanofibers [96].
