1 Introduction
Nanostructured aerogels have shown great promise in various pharmaceutical applications during the last few decades. They are considered potential candidates for the encapsulation, adsorption, and delivery of various compounds owing to their large surface area, large pores, and tunable structures achieved by controlling the composition and synthesis conditions [
1]. Additionally, the wide variety of materials available for synthesis
(e.g., polymers, ceramics, and others), ease of fabrication, and ability to produce a variety of shapes have increased their popularity in different fields [
2,
3]. The use of biocompatible aerogels in medicine was first suggested and patented in 1996 [
4]. Since then, aerogels have gained increasing attention in the literature as drug delivery vehicles due to their structure and high porosity which enable high loadings of pharmaceutical agents. Pharmaceutical agents can be loaded into the pores of aerogels in different stages during their synthesis such as during gel formation, solvent exchange, or supercritical drying [
5]. Additionally, aerogels offer the opportunity to control drug release, show protection against biodegradation, and increase the bioavailability of poorly soluble drugs [
6].
Over the years, various materials have been investigated to prepare aerogels [
7,
8]. Amongst many, silica aerogels have been widely evaluated as drug carriers and have shown great promise [
9‐
11]. However, they are non-biodegradable which limits their use in medical applications [
12]. On the other hand, the use of organic materials, polysaccharides [
5,
13‐
17], can be preferred for pharmaceutical applications particularly due to both their biocompatibility and biodegradability [
18].
Upon delivery, hydrophilic polysaccharides can swell and quickly degrade in biological environments which can cause the early release of drugs, especially when highly hydrophilic polysaccharides are used [
19]. A physical or chemical crosslinker can be added to alter the physical and mechanical properties of the porous network [
20]. Cross-linking of polysaccharide aerogels can enhance the three-dimensional structural integrity and reduce swelling by tightly bounding the network, which can overcome the issue of early drug liberation and obtain sustained drug release [
21,
22]. Nevertheless, the use of cross-linking agents in pharmaceutical applications is limited due to their possible drug deactivation or cytotoxic effect [
23]. To avoid toxicity and compatibility issues, the physical properties of polysaccharide aerogels can be modified by incorporating another material to prepare composite aerogels [
24]. Composite aerogels of two or more materials possess different properties than their constituents alone, and these properties can be utilized to control the release of drug(s) loaded [
25].
Among different polysaccharides, tragacanth gum is an anionic natural polysaccharide derived from the exudate of some trees obtained from the Astragalus family. It has gained interest as it is derived from renewable sources and is already used in various biomedical applications for the preparation of drug carriers and wound healing purposes. It has the benefits of being biocompatible, non-toxic, and relatively inexpensive [
26]. Tragacanth is composed of two parts. Once tragacanth is placed in water, the water-soluble part (tragacanthic acid) dissolves, whereas the water-insoluble part (bassorin) swells forming a gel [
27]. Tragacanth/acrylic acid hydrogels were developed for oral delivery of amphotericin B to increase the drug’s bioavailability while reducing the adverse effects associated with standard i.v. administration [
28]. Results indicated that this novel formulation showed low toxicity and enhanced therapeutic index of the drug. Recently, a tragacanth-polyvinyl alcohol aerogel was vacuum-impregnated with silymarin as an oral delivery system [
29]. The high porosity of the produced aerogels facilitated efficient loading and release of silymarin which presents the possibility of using these systems as an oral drug vehicle. Tragacanth has also been investigated for the development of mucoadhesive gel by providing the proper matrix needed for drug loading and delivery [
30]. Thus, the feasibility of using tragacanth as a source to synthesize efficient drug delivery systems makes it an attractive polymer that should be further investigated.
Another polysaccharide that is utilized to prepare novel drug delivery systems is alginate. It has been extensively investigated owing to its excellent properties including biodegradability, biocompatibility, and gel-forming ability [
14,
31]. Alginate can be extracted from brown seaweed, including
Laminaria hyperborea, Laminaria digitata, Laminaria japonica, Ascophyllum nodosum, and Macrocystis pyrifera [
32]. It is comprised of blocks of (1,4)-linked β-d-mannuronate (M) and α-l-guluronate (G). The ratio of these blocks varies depending on the source of extraction. Many alginate-based aerogels have been already reported as effective drug-delivery systems for various applications [
33]. Sodium alginate aerogel microspheres were investigated as a potential drug carrier for several drugs including loratadine and ibuprofen [
34]. In another study, the feasibility of utilizing alginate-chitosan aerogels for the topical delivery of levomycetin was evaluated [
35]. These aerogels were found to be mechanically stable with a high sorption capacity which can be utilized to absorb wound exudate, while enabling controlled release of levomycetin up to 95% in 4 h.
In this research, novel organic polysaccharide aerogels were synthesized using tragacanth and alginate. The purpose was to evaluate the use of tragacanth and tragacanth-alginate composite aerogels as potential oral drug delivery systems. Sodium alginate is a widely used polymer in pharmaceutical formulation, including commercially available drug products. It is easily accessible and affordable which can reduce the cost of the product. Despite its unique physicochemical properties, when used as drug delivery system, generally it requires the addition of crosslinking agents to obtain a stable network structure. Tragacanth is non-toxic, non-allergenic, abundant and has low production cost, and good thermal stability. Herein, we aimed to combine the advantages of each polymer and synthesize composite aerogels without additional crosslinker. Different compositions of aerogels were prepared by altering the amounts of tragacanth and alginate to manipulate the physical properties of the resulting aerogels. Various characterization methods were used to understand the physiochemical properties of the resulting samples and to confirm the presence of the model drug in the aerogels. The effect of the different compositions of composite aerogels on the release rate of the model drug, paracetamol, was investigated. To the best of our knowledge, this is the first study focusing on the use of tragacanth-alginate composite aerogels as novel drug delivery systems. This system presented a novel and completely biodegradable approach to drug delivery which can show great promise for future applications.
4 Conclusion
There is an increasing number of research studies investigating the potential application of aerogels as drug delivery systems. Different release profiles of the pharmaceutical compounds have been already achieved with diverse aerogel types. In this study, a natural polysaccharide, tragacanth, was used to synthesize aerogels. Alginate, another polysaccharide that is widely used in health sciences was also incorporated in the structure to form composite aerogels. No chemical crosslinker was used. Tragacanth and tragacanth-alginate composite aerogels were successfully synthesized by sol-gel method followed by supercritical drying. Aerogels were found to have mesoporous and macroporous structures and showed very low densities ranging from 0.03 g/cm3 to 0.06 g/cm3.
Tragacanth aerogels showed loading as high as 99 wt. % (mg drug/mg aerogel) of the model drug paracetamol whereas 114 wt. % (mg drug/mg aerogel) loading was obtained for composite aerogels. It was shown that high drug loadings can be achieved inside the pores via this method while maintaining the monolithic structure and the porous characteristics intact. When alginate was added, the release rate of the drug was decreased by strengthening the porous network which decreased swelling and therefore, provided controlled drug release. Furthermore, it was observed that drug release was governed by the Korsmeyer Peppas model indicating that the release was controlled by diffusion. Thereby, it can be concluded that as an alternative to the various approaches used to tailor drug release from aerogels, alternations in the composition of aerogels can be utilized as a practical, simple way to modify drug release rate. As presented in this study, tunability of the release profile of tragacanth can be achieved through the addition of another polymer, alginate, as it enhances the structural integrity by entangling with tragacanth, producing a more complex network. This approach not only eliminates the need for a crosslinker alleviating impurities from the system but also offers the opportunity to develop novel biodegradable delivery systems for controlled drug release.
The novel composite aerogels present several favorable properties such as very low density, high porosity, high loading efficacy, prolonged release, and relative ease of synthesis and could be suggested as potential biodegradable drug delivery systems.
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