Hydrogel is known as a hydrophilic polymer with three-dimensional chemically or/and physically cross-linked network [1], which is able to absorb and hold large amounts of water or biological fluids to a thousand times their own weight [2]. Hydrogels are able to be prepared by using natural or synthetic polymers, or their combination [3], which has the opposite yet complementary mechanical responses to each other. In this case, double networking has been demonstrated to be an effective approach to enhancing the strength and/or toughness of hydrogels [4]. One of the possibilities to form a double network is that the first network needs to be a strong polyelectrolyte, however this leads to limits in using different polymers. Then the second monomer is polymerised in the first network thereby the presence of initiators-activators is inevitable in these networks that adversely affect performance in the biological environment [5]. Thankfully their properties such as biocompatibility, biodegradability, great swelling capacity and microporous structure [6] make them the ideal candidate for various applications such as tissue engineering, wound healing, drug delivery systems, biosensors and bioelectronics, wastewater remediation, daily life products and many more [7]. Thanks to their softness, flexibility, and chemical composition they are able to simulate and mimic biological systems such as the extracellular matrix (ECM) [3]. Thus, this is a structural support network composed of diverse proteins, sugars, and other components that is why ECM regulates cellular processes including survival, growth, proliferation, migration and differentiation. However, in view of the fact that every single application has specific needs, it is necessary to take care of their properties and porous microstructure [2].
Polyvinyl alcohol (PVA) is a synthetic polymer, containing hydroxyl groups synthesised commonly from polyvinyl acetate. It shows excellent properties, such as non-toxicity, biocompatibility, good biodegradability, high hydrophilicity, fiber/film forming ability and convenient mechanical properties [1], also it is a water-soluble polymer, which is able to easily dissolve in the solution without using any organic solvents [7]. Hydrogels based on PVA can be prepared by using chemical or physical methods, one of them is the freezing-thawing process [8], which is an advantageous method that does not require other crosslinking agents [3], that may cause toxicity [9]. Repeated freezing-thawing cycles of a polyvinyl alcohol aqueous solution lead to the formation of crystallites which act as cross-linking sites, and a hydrogel with a high swelling capacity is produced [3]. PVA-based hydrogels, are a well-known soft and elastic materials [7] with high degree of swelling in water and hydrophilicity [10], which makes them a promising biomaterial and candidate for biomedical area [1] and/or for polymer matrix in pharmaceutical fields [10].
Another widely employed polymer in the development of the polymer-based hydrogels is sodium alginate (SA) [1]. SA is a biopolymer with polyanionic linear polysaccharide comprising of 1,4-linked-α-L-guluronic acid and β-D-mannuronic acid which are able to be found in brown seaweeds [9]. Thanks to their biocompatibility, biodegradability, immunogenicity and non-toxicity, SA belongs among the promising materials for drug delivery systems [1]. The crosslinking reaction, ionic gelation, with the presence of divalent ions, such as calcium, barium and/or strontium ions, enable the formation of gels [8], which represent a group of materials, which found their application to the skin in case of difficult-healing wounds, including bedsores, venous ulcers, and diabetic wounds [11]. The ionic gelation is an effective and easy technique that enables the formation of spherical beads with regular shape, size and smooth surface as well as the preparation of ideal release retarding membrane [9]. However, limitations caused by poor mechanical properties and shortage of processing helped to find a combination of sodium alginate (SA) with a synthetic polymer, which could produce the optimal properties [12]. Thus, PVA is a perfect partner for biopolymers like sodium alginate [7], both components are non-toxic and biocompatible [11].
Incorporation of hydrophobic or micellar domains into the gel structure is one of the possibilities of how to modify, tailor or upgrade hydrogel’s properties [2]. These self-assembled supramolecular assemblies are composed of reverse micellar structures as the basic building blocks, which are formed due to the weak van der Waals forces, hydrogen bonding and electrostatic interactions [13]. For example, Lecithin is a naturally occurring mixture of phospholipids, which generally consist of fatty acids and a polar or charged headgroup esterified to a glycerol backbone, with the key difference being the properties of the headgroup as well as the fatty acid type. Typical fatty acids associated with soy phospholipids are palmitic, stearic, oleic and linoleic acids [14]. With more water, spatial reorganisation from spheroidal to tubular reverse micelles takes place and at a critical water concentration. These tubular reverse micelles overlap and entangle thereby entrapping solvent molecules. The three-dimensional network of the tubular reverse micelles is held together by hydrogen bonds between the molecules of lecithin and water, and hydrophobic interactions between the organic and aqueous phases. Lecithin is extensively used in applications as a structuring agent in animal and human foods, pharmaceuticals, and cosmetics [2].
Hybrid hydrogel based on PVA and SA is not completely new, but still, nowadays the interest in their study is constantly growing. In this work, a series of experiments were performed in order to understand the properties of hybrid networks of hydrogels based on PVA and SA. Knowledge of the components used, their behaviour and the possibility of cross-linking with other substances (e.g. drugs, organic dyes, internal structure modifiers, etc.) is essential here. The prepared hybrid hydrogel represents a hydrogel material that can not only protect the wound, but also support the healing process with the help of any built-in drugs, therefore such a material could potentially be used to cover wounds. Mechanical, transport and swelling capabilities were determined using physicochemical methods. Furthermore, the internal structure of the prepared hybrid hydrogels was characterised using scanning electron microscopy. The addition of a natural component during the preparation of gel systems is a simple way of changing the internal structure and modifying the properties, which is suitable for their biocompatibility and the possibility of biodegradation.