Here, we report on studies on the influence of different crosslinking methods (ionic and chemical) on the physicochemical (swelling ability and degradation in simulated body fluids), structural (FT-IR spectra analysis) and morphological (SEM analysis) properties of SA/PVA hydrogels containing active substances of natural origin. First, an aqueous extract of Echinacea purpurea was prepared using a Soxhlet apparatus. Next, a series of modified SA/PVA-based hydrogels were obtained through the chemical crosslinking method using poly(ethylene glycol) diacrylate (PEGDA, M n = 700 g/mol) as a crosslinking agent and, additionally, the ionic reaction in the presence of a 5% w/v calcium chloride solution. The compositions of SA/PVA/E. purpurea-based hydrogels contained a polymer of natural origin—sodium alginate (SA, 1.5% solution)—and a synthetic polymer—poly(vinyl alcohol) (PVA, M n = 72,000 g/mol, 10% solution)—in the ratio 2:1, and different amounts of the aqueous extract of E. purpurea—5, 10, 15 or 20% (v/v). Additionally, the release behavior of echinacoside from the polymeric matrix was evaluated in phosphate-buffered saline (PBS) at 37 °C. The results indicate that the type of the crosslinking method has a direct impact on the release profile. Consequently, it is possible to design a system that delivers an active substance in a way that depends on the application.
From a medical, pharmaceutical and biomedical point of view, hydrogels are a very important group of polymeric materials. They constitute a three-dimensional hydrophilic network of crosslinked polymeric chains [1,2,3]. Their main feature is the ability to absorb (swell) and retain water inside the macromolecule without dissolving it. Polymer chains in a dry state are in the form of tightly closed “bundles”, while under the influence of water molecules, the functional groups present in the chains are solvated and dissociated [1,4,5].
Furthermore, hydrogel matrices are an indispensable class of materials for biomedical applications due to their high biocompatibility, the ability to absorb water and other fluids in a reversible manner, as well as good mechanical properties within biological tissue [2,4,6]. The water-sorption capacity is attributed to the interaction between water molecules and polar groups, which are present in the polymer matrix. In addition, their chemical structure allows gradual drug release, which is associated with the prolongation of its action. As far as the role of hydrogel matrices in the medical field is concerned, an important application is wound dressings, which can be used in the treatment of burn wounds, pressure ulcers as well as post-operative wounds. Moreover, hydrogel matrices are used in the production of contact lenses, in tissue engineering and as a controlled drug release system [2,7,8,9,10]. Currently, hydrogels based on natural polymers, such as polysaccharides and proteins, are very popular and desirable. From the point of view of medical and pharmaceutical applications, alginates are very promising, because they belong to biocompatible and biodegradable components. In our research, we used alginates, which constitute linear polymers composed of (1→4)-α-L-guluronic acid blocks (GG), (1→4)-β-D-mannuronic acid blocks (MM) and, additionally, heteropolymeric sequences of M and G (MG blocks) [11,12]. Moreover, in the case of the preparation of hydrogel or composite materials, alginates are used together with some additional polymers, such as: poly acrylamide (PAAm) [13], poly(acrylic acid) (PAA) [14], gelatin [15], and chitosan [16], as well as PVA [17]. Poly(vinyl alcohol) is very important and interesting, because it characterized by biocompatibility, biodegradability and non-toxicity due to it is commonly used in the medical and pharmaceutical, such as: artificial organs, drug delivery as well as wound dressings. Moreover, PVA hydrogels exhibit not only good biocompatibility, but also good physicochemical and especially outstanding bio-tribological properties [18,19,20,21].
It is worth noting that in recent years there has been a trend showing the interest of researchers in modifying hydrogels intended to wound healing also with medicinal substances of natural origin, mainly extracted from plants [22]. For example, it has been shown that Cryphaea heteromalla aqueous extract in the hydrogel film may prevent excess oxidative stress generation during wound healing [23]. In turn, Ciolacu et al. incorporated lignin into the hydrogel and produced hydrogel structures with a higher drug release rate; however, Wang and Chen successfully tested the influence of the addition of cellulose nanowhiskers to cellulose physical gels resulting in the more steady release of the protein. [24,25].
The introduction of additional active substances of natural origin, such as the Echinacea purpurea extract or Aloe vera, into the hydrogel composition allows the preparation of materials with improved healing properties. Echinacea purpurea contains many active substances, such as polysaccharides, caffeic acid derivatives (including cichoric acid), alkylamides, and glycoproteins [10,11]. The most active compounds of Echinacea purpurea are polyphenols—derivatives of caffeic acid: caftaric acid, chlorogenic acid, cynarin and silicic acid [12]. They play a significant role in the therapeutic effect of this plant, especially in activating the immune system by triggering the alternating complement pathway, as well as increasing the number of white blood cells distribution, stimulating phagocytosis, T-cell production, lymphocytic activity, cytokine production, cell respiration and enzyme secretion [26,27]. These bioactive ingredients are characterized by pharmacological antiviral, anti-inflammatory, bacteriostatic and immunoregulatory effects [28].
Generally, bio-based hydrogels can be prepared using several chemical and ionic crosslinking methods and also in different ways, e.g., photopolymerization and exposure to radiation (e.g., gamma rays or microwaves) [29]. The chemical method is based on the formation of covalent bridges during the reaction between the polymer or monomer with the crosslinking agent (N, N’-methylenebisacrylamine (NMBA) and poly(ethylene glycol) diacrylate (PEGDA) in the presence of an initiator (ammonium, potassium or sodium persulfates [30,31,32]. Meanwhile, the method of crosslinking by ionic interaction takes place in the presence of bivalent or multivalent ions, such as Ca2+, Cu2+, Fe2+ or Al3+. One of the best-known polymers that can be crosslinked by ionic interaction is alginates [33]. This group of polysaccharides can form a three-dimensional structure under the influence of bivalent ions at room temperature and in a physiological environment (pH = 5.2–5.5). This kind of gelation process is quite interesting, because the ionically crosslinked alginates make it possible to form a three-dimensional structure called the “egg box” model. Divalent ions are attached to guluronic acid by ionic bonding, and then the guluron block of one polymer chain connects with adjacent G blocks [33,34,35]. The chemical structure of alginate has a great influence on the properties of alginate hydrogels obtained by this method. Hydrogels made from M-block-rich alginate are very soft and brittle, and may also be less porous. On the other hand, gels obtained from alginates with a high content of G blocks show higher stiffness and porosity [35,36,37]. Comparing both methods, crosslinking of alginates by chemical method leads to obtaining hydrogels that are characterized by higher mechanical strength. The obtained hydrogels have an ordered, complex three-dimensional structure and a higher crosslinking density. In addition, they show greater stability in physiological solutions compared to hydrogels obtained by the ionic method. Ion-crosslinked alginate hydrogels lose more than 60% of their initial mechanical strength within 15 h of exposure to physiological buffers. The reason for this may be the ion exchange between divalent ions inside the hydrogel and monovalent ions contained in the surrounding fluid [9,38,39].
The basic principle of crosslinking with ultraviolet radiation is the conversion of UV radiation into chemical energy and can occur through the radical or ionic mechanism [40]. In the case of the radical mechanism, the unsaturated bonds present in a polymer chain are polymerized. The presence of photoinitiators, i.e., benzoin, dimethoxyacetophenone, acyloximeester or benzoyl ketals, is necessary for the initiation of the crosslinking reaction in the radical mechanism. On the other hand, crosslinking via the cationic mechanism consists in the opening of epoxy rings or the reaction of vinyl groups, which are present in the polymer chain [40,41,42]. Another method of obtaining hydrogel matrices is the application of radiation, which connects polymer chains by means of covalent bonds. They are not broken down and, as a result, the entire macromolecule is not destroyed. The initiator of the crosslinking process is gamma radiation coming from the isotope of cobalt 60Co or cesium 137Cs, and it can also be an electron beam from an accelerator. Radicals are formed on polymer chains following symmetric breaking of the C-H bond. This decay is caused by gamma radiation, which strikes polymer solutions and initiates the formation of radicals. Additionally, hydroxyl radicals that have an unpaired electron are formed [43,44].
In summary, in this article, the influence of the type of crosslinking agent on the properties of modified hydrogels is investigated. It turns out that different crosslinkers yield totally different changes in the properties of obtained final products. The method of crosslinking definitely affects the morphology of hydrogels as well as the physicochemical properties of the materials obtained for future dressings. In-depth knowledge about the behavior and properties allow to select the parameters of the production process and the method of crosslinking to design the release of active substances from the matrix in accordance with the expectations, and thus predict the direction of wound treatment. This means that the research presented in this study is up-to-date and allows to enrich the general knowledge about the possibilities of using hydrogel substances in medicine.