Xylan-Based Hydrogels: A Polymeric Carrier for Sustained and Targeted Delivery of Drugs

Abstract

In spite of good advancement for diagnosis and treatment, cancer is the second most common disease after cardiovascular disorders, may be responsible for maximum deaths in the world. Cancer is a leading cause of death worldwide, accounting for nearly 10 million deaths in 2020. Among cancers, colon or colorectal cancer is the second most common form of cancer globally with 916,000 deaths reported annually. Colon is the largest part of large intestine extending from ileocecal junction to anus. The delivery of drugs to the targeted site such as colon requires protection to the drug. As the most of the drugs are unstable in the gastric environment of the stomach and are susceptible to absorb in the upper gastrointestinal tract (GIT). This causes poor drug bioavailability and diminishes their efficacy against inflammatory bowel diseases (IBD). Thus, to deliver a drug to the targeted site such as colon via GIT requires protection from an undesirable release in the upper GIT to achieve maximal pharmacological effect, while administered orally. As a consequence, protection of drugs can be achieved by xylan-based hydrogel polymeric carriers, which are of non-toxic and biocompatible nature, and which can also undergo in-vivo biodegradation easily.

1. Introduction

Cancer is a leading cause of death worldwide, accounting for about 10 million in 2020. After lung cancer, colorectal cancer is the second most common form of cancer globally with 916,000 deaths reported annually [1]. Colorectal is a part of large intestine, extending from ileocecal junction to anus. The delivering of a drug to the targeted site for instance colon is a shattering problem as the most of the drugs have been reported to be unstable in the gastric environment of stomach and is more susceptible to absorb in the upper GIT. This causes diminished drug bioavailability and reduced their efficacy against inflammatory bowel disease (IBD). Thus drug delivery to the targeted site through GIT requires protection from an undesirable release in the upper GIT to achieve maximal pharmacological effect, while administered orally [2]. The protection of drugs can be achieved by using natural polymeric carrier-based hydrogels, such as xylan-based hydrogels, which are of non-cytotoxic [3], biocompatible nature, and which can also undergo in-vivo biodegradation easily. Hydrogels can provide spatial and sequential control over the release of various therapeutic agents, including small-molecule drugs, macromolecular drugs, and cells [4]. On account of their tunable physical properties, controllable biodegradability and capability to protect labile drugs from degradation, hydrogels serve as a molecule in which various physiochemical interactions with the encapsulated drugs control their release. Therefore, by imparting designed functionality and providing appropriate treatments of xylan-rich hemicellulose by-products can effectively be used for targeted drug delivery via hydrogel preparation. Thus, the aim of the present research is to focus on the isolation of xylans, their derivatizations, synthesis of hydrogels, and their applications.

2. Methods of isolation of xylan-rich hemicelluloses

Hemicelluloses are the second most abundant non-crystalline linear or branched polysaccharides after cellulosic one, which can be isolated from plant resources (Figure 1) or some bio-based industrial processes. It is a predominant byproduct of chemical and mechanical pulps [5, 6] in pulp industries and is severely wasted without an appropriate treatment. However, they could be used as a renewable matrix material with tunable functionality and biocompatibility in the field of pharmacy, cosmetic, food, etc. Hemicelluloses have lower molar masses than cellulose. Xylan is the principal structural hemicellulosic polysaccharides, present as 15–30% and 7–10% in hardwood and softwood, respectively. The major chain of xylan is composed of D-xylopyranosyl residues as backbone, which is linked by β-(1 → 4) glycosidic linkages, is similar to that of cellulose with a missing C-6 group. However, in marine algae, xylan additionally contains β-(1 → 3) linkages. The β-(1 → 4)-D-xylopyranose unit of xylan is randomly substituted by glycosyl and acetyl groups depending on the source of the feedstock and extraction method [7, 8]. Depending upon the extraction method such as non-alkaline and alkaline treatment, xylan can be classified as acetylated (xylopyranose unit is substituted with acetyl groups) and non-acetylated (without substitution). Thus isolation of xylan can either be done by alkaline treatment or non-alkaline treatment.

Figure 1.

Working plan for delignification and xylan isolation.

The isolation of xylan from agro-waste raw materials is depicted in Figures 1 and 2. To isolate the xylan, the raw materials were milled into powder with uniform size of narrow distribution by mechanical agitation. The powdered raw materials were dewaxed with a mixture of toluene and ethanol in a ratio of 2:1 (v/v) in Soxhlet extractors for 12 hours. The solvent-soluble non-volatile materials were removed during the extraction known as dewaxing or extractive free process. The dewaxed/extractive free raw materials were delignified either with acidified NaClO₂ or with per acetic acid (PAA).

Figure 2.

Represents the steps involved in extraction of xylan-rich hemicellulose.

2.1 Delignification of biomass with per acetic acid

Seeing as, PAA is decomposed with metal ions thereby, the extractive free raw materials was initially treated with 0.2% (w/v) EDTA at 90°C for 1 hour to remove the metal ions prior to delignification [9]. The delignification of the extractive free materials was performed with 500 mL of 10% (v/v) PAA at pH 3.5 (adjusted with NaOH solution), at 85°C for 30 minutes with constant stirring. The system was cooled in an ice bath followed by dilution twice with distilled water. The delignified biomass (holocellulose) was collected by filtration and washing with warm distilled water and finally with acetone. The holocellulose was dried at 60°C in vacuum oven.

2.2 Delignification of biomass with acidified sodium chlorite

The delignification of biomass can also be done by acidified NaClO₂ at 70°C for 2 hours. The biomass obtained after delignification contains cellulose as well as hemicellulose and thus, it is collectively known as holocellulose [10].

2.3 Isolation of acetylated xylan

A sample of 6 g of delignified biomass (holocellulose) which was done either by PAA or by NaClO₂ was treated with 130 mL of DMSO, at room temperature for 24 hours, under inert atmosphere with constant stirring. Subsequent to the treatment, the suspension was filtered through a polystyrene membrane (porosity 60 μm) followed by washing with ∼20 mL of distilled water. The filtrate was added to 600 mL of ethanol at pH 3.5 (adjusted with formic acid) and left for 12 hours at 4°C. The precipitated xylan-rich hemicelluloses were isolated by centrifugation and washing properly with methanol. The xylan-rich hemicelluloses were dried in a vacuum oven at 50°C for 24 hours. The total yield was calculated gravimetrically on oven dried basis of extractive-free biomass.

2.4 Isolation of non-acetylated xylan

The non-cellulosic product such as xylan-rich hemicelluloses was extracted from delignified biomass (holocellulose) which was obtained from agro-waste raw materials [11, 12]. The holocellulose was treated with 10% (2.5 N) NaOH solution with a solid to liquid ratio of 1:20 for 6 hours at room temperature. The NaOH treatment to holocellulose dissolves the xylan-rich hemicelluloses properly. The dissolved hemicellulose was filtered through Whatman No. 44 filter paper to separate it in the form of extract. The extract was further neutralized with acetic acid to get a precipitate of xylan-rich hemicellulose. The precipitate was separated by settling down after ethanol addition. Subsequently, several washing steps were performed using ethanol and filtered and dried at 60°C in vacuum oven to get xylan-rich hemicellulose. The percentage yield was calculated on oven dried basis of extractive-free raw materials. The schematic diagram of steps involved in preparation of xylan-rich hemicelluloses is represented (Figures 2 and 3).

Figure 3.

Represents the chemical reactions involved.

3. Modification of xylan-rich hemicellulose

The extracted xylan-rich hemicellulose can easily be functionalized because of the presence of abundance of the hydroxyl moiety in its structure. The chemical modification of xylan provides more possibilities to tailor its properties, which aims to improve its applicability. Various xylan derivatives with better properties are successfully obtained after chemical modifications recently. Various methods to prepare xylan derivatives are discussed below.

3.1 Derivatization of xylan as carboxymethyl xylan (CMX)

The carboxymethyl xylan (CMX) synthesis is carried out by using sodium chloroacetate as a carboxylate group donor to xylan. On dissolution of xylan-rich hemicelluloses in NaOH solution (i.e., under alkali treatment), NaOH reacts with the hydroxyl group of xylan and generates a strong nucleophile as alkoxide ion. The alkoxide ion from the alkali xylan attacks the chloroacetate via SN2 reaction resulting in the carboxymethylation of xylan as a product. The product obtained is neutralized with acetic acid and washed with ethanol. The resulting product is filtered, centrifuged, and dried in vacuum oven [13, 14]. The degree of carboxymethylation depends on the number of hydroxyl group substituted with carboxymethyl groups. Furthermore, the product of glycolic acid and NaCl could be generated as by-products at high concentrations of sodium chloroacetate and NaOH [15] removed in the form of extract. The involved chemical reactions are represented as Figure 4.

Figure 4.

Modification of xylan as carboxymethyl xylan (CMX).

3.2 Derivatization of xylan as dialdehyde xylan (DAX)

Oxidation is one of the prominently used pathways for derivatization. Oxidation of xylan is an easy and practically available method on the lab-scale now. The final products from the oxidation of xylan called modified xylan have been proved an important role in material enhancement and functionalization.

A very well-known oxidation such as 2,2,6,6-tetramethylpiperidine 1-oxyl radical (TEMPO) oxidation used in cellulosics’ material chemistry but is not pertinent to xylan, as xylan have D-xylopyranosyl residue as their building blocks, are lacking of primary hydroxyl group, which are the primary target of the TEMPO oxidation. Therefore, in oxidative xylan modification primarily sodium periodate is used, as it is easily and practically applied on the lab scale. Periodate oxidative cleavage of vicinal diol (i.e., cleavage of C2–C3 bond) is resulting in “dialdehyde polymers”, which have a number of interesting applications in tissue engineering, drug delivery and as flocculating agents, and for ion-exchange separation. The resulting dialdehyde polymers exist as 2,3-hemialdal forms, which can be used for further functionalization [6, 16] particularly to prepare hydrogels. The preparation of DAX has been shown in Figure 5.

Figure 5.

Chemical reactions for the formation of DAX.

3.3 Derivatization of xylan as xylan methacrylate (XMA)

Another method for xylan modification is developed by Lin et al. [17] where xylan was dissolved in dimethyl sulfoxide (DMSO) at 95°C followed by addition of the catalyst 4-dimethylaminopyridine (DMAP) and cooling the mixture at room temperature. The reaction mixture was stirred for 30 minutes at room temperature. Later on, glycidyl methacrylate was added and the final reaction mixture was stirred for 24 hours. The reaction mixture was precipitated with anhydrous ethanol and filtered. The obtained precipitate was washed with anhydrous ethanol three times. In this way modified xylan (MX) was obtained as xylan methacrylate as light brown in color. The schematic diagram is represented as Figure 6.

Figure 6.

Chemical reactions for the formation of xylan methacrylate.

4. Preparation of hydrogel using modified xylan

Hydrogels are physical or chemical cross-linked three-dimensional polymer networks which swell upon by absorbing water without dissolving in it. The shape and the volume of hydrogels can be reversibly changed by various external stimuli such as pH, temperature, light, and electric and magnetic fields. Xylan is a renewable natural polymer bearing the advantages over other polymers in terms of non-toxicity, biocompatibility, biodegradability, and natural abundance. The ease of functionalization or chemical cross-linking of the hydroxyl moieties present in the backbone of xylan make it an attractive precursor for hydrogel preparation. Recently, xylan-based hydrogels have gained attention because of the multi-responsive behavior toward pH, organic solvents, and ions.

4.1 Preparation of hydrogel using carboxymethyl xylan as a precursor

Hydrogel using CMX, a modified xylan is prepared by dissolving it in distilled water and stirring it at 60°C in a water bath followed by addition of ammonium persulfate (APS) solution, acrylic acid, and methylene-bis-acrylamide (MBA). A series of chemical reaction takes place in the mixture which is continued for about 4 hours and results as hydrogels which is taken out and washed with water properly and cut into small pieces [17]. The chemical reactions for the preparation of hydrogel are presented in Figure 7.

Figure 7.

Reactions involved in the preparation of hydrogel from CMX.

4.2 Preparation of hydrogel using dialdehyde xylan as a precursor

In this method, prepared dialdehyde xylan (DAX) as shown in Figure 5, a biocompatible gel material employed as a biopolymer-based crosslinker to enable the formation of 3D gel network. The transparent, clean, and non-toxic DAX-crosslinked hydrogel could be obtained from the Schiff base reaction between aldehyde groups of DAX and amino groups of gelatin (G) [16, 18]. The demonstrated xylan-based hydrogel through a simple approach opened a new door for skin care products from natural and renewable biomass (Figure 8).

Figure 8.

Reaction involved in the preparation of hydrogel from DAX.

4.3 Preparation of hydrogel using carboxymethyl cellulose (CMC) as a precursor

Carboxymethyl cellulose (CMC) is used as a precursor for preparation of stimuli-responsive hydrogels. The carboxymethyl group adds a negative charge to the pyranose backbone of xylan, and it significantly increases the cross-linking points and reactive sites. Thus, thermal radical reactions are often employed with the cross-linker to prepare CMC-based homopolymer and copolymer hydrogels. Overall, the CMC-based hydrogels are used as carriers for drugs and biological macromolecules.

CMC-based hydrogel is prepared by dissolving the carboxymethyl cellulose to distilled water followed by the addition of potassium persulfate (KPS) at 70°C for half an hour. The neutralized acrylic acid (AA) is added into the reaction mixture and subsequently the continuing stirred for about 2 hours. Thereafter, the pH of the reaction mixture is raised upto 8 by the addition of NaOH solution. The resulting outcome is precipitated by the addition of acetone. The precipitates of CMC-g-PNaA are dried in a vacuum oven for 24 hours at 60°C.

The CMC-g-PNaA (2% w/v) is further dissolved in distilled water by using a mechanical stirrer at 300 rpm for 6 hours. Then, 50 mL of CMC-g-PNaA solution is added dropwise in the 100 mL aqueous solution of FeCl₃ (0.02 mol). The mixture converts into spherical hydrogel beads which are filtered, washed with distilled water, and dried in a vacuum oven for 24 hours [19]. The chemical reactions involved in preparation of hydrogel from CMC are shown in Figures 9 and 10.

Figure 9.

Chemical reactions of CMC react with radical-anion to form CMC-free radicals.

Figure 10.

Chemical reactions of CMC-free radicals with AA and finally with ferric ions.

5. Optimization of physico-chemical properties of hydrogels

The hydrogels had been prepared using different ratios of precursors and cross-linkers. The prepared hydrogels had been optimized on the basis of biocompatibility, biodegradability, swelling ratio, mechanical strength, and pore size. The pore-size of hydrogels could be improved by using different inorganic or organic pore-forming agents. The chain length and the molecular weight of organic pore-forming agents such as polyethylene glycol 2000, carbamide, and polyvinyl pyrrolidone had an important influence on the pore size, the compressive strength, and the swelling ratio. However, among inorganic pore-forming agents such as NaCl, CaCO₃, and NaHCO₃, hydrogels with NaHCO₃ displayed a great performance in terms of the pore size of the hydrogels, mechanical properties, and drug release. Moreover, the pore-forming agents had little influence on the thermal stability of the hydrogels. The strength of hydrogels could also be enhanced by the formation of coordination bonds between Zn²⁺ and anionic groups such as –COO⁻ when hydrogel is immersed in the solution of ZnCl₂ [20].

6. Applications of xylan-based hydrogels

Xylan-rich-hemicellulose-based hydrogels have many promising applications for the researchers and scientists as:

6.1 Super adsorbent

Colored effluents of industries can be highly toxic and carcinogenic, causing a great danger to living organisms. As organic dye effluents have complex chemical structure, high hydrophilicity and stability, resistant to their removal poses difficulties for wastewater treatment [21]. Technologies such as coagulation and flocculation, oxidation, adsorption, membrane separation, and electro-coagulation methods [22, 23] have been used to treat colored wastewater. Among these, the adsorption method is regarded as one of the most attractive processes because of its easy operation and high removal efficiency. The xylan-based hydrogels showed outstanding adsorption capacity for organic dyes removal instead of its excellent properties of biodegradability and renewability [24, 25, 26]. Sun and co-workers were prepared a novel adsorbent composite hydrogel of acylated xylan and silanized graphene oxide via free radical polymerization reactions. The composite hydrogel based-adsorbent showed excellent removal capacity of Cu²⁺ ions from an aqueous solution. Study of adsorption thermodynamics showed the adsorption of Cu²⁺ ions was endothermal and spontaneous, and the adsorption amount rose with an increase in temperature. In addition, higher desorption percentages of Cu²⁺ ions from the used hydrogel were also achieved up to 77.3% after recycling for six times. Thus, all obtained results were indicated that the prepared hydrogel is promising for water treatment and collection of metal ions [27].

6.2 Sensitive response to H₂O₂ detection

Acetylated xylan-based magnetic Fe₃O₄ nano-composite hydrogels were prepared by fabricating Fe₃O₄ particles within a hydrogel matrix. The hydrogel matrix was synthesized by graft copolymerization reaction of acetylated xylan (porous nature of hydrogels were increased by introducing the acetyl functional group) with acrylamide and N-isopropylacrylamide under ultraviolet irradiation. The magnetic hydrogels were accessible to excellent catalytic activity and provided a sensitive response to H₂O₂ detection even at low concentration. The approach to prepare magnetic hydrogels endows with promising applications in the field of environmental chemistry [28].

6.3 Photodynamic antimicrobial chemotherapy (PACT)

The use of most of the existing photosensitizers has been harshly hampered because of their significant self-quenching effect, poor water solubility, lack of selectivity against bacterial cells, and possible damage to the surrounding tissues. Therefore, to overcome the limitations, the PS encapsulated hydrogels were prepared recently. Recently, hydrophilic photosensitizer (PS) for instance 5, 10, 15, 20-tetrakis (1-methylpyridinium-4-yl)porphyrin tetraiodide (TMPyP) incorporated xylan-based hydrogels were synthesized which showed prolonged release of PS up to 24 hours with a cumulative release of 100%. TMPyP-loaded hydrogel were effective against Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus strains, and Bacillus cereus, while ineffective in the dark. This PACT showed to be promising antimicrobial treatment to overcome the challenges of multidrug resistant bacteria [29].

6.4 Skin care

The bio-compatibility [11] and cyto-compatibility [6] of xylan based hydrogels were confirmed through experiments and thereby the fabricated hydrogels may be used as a potential material in skin. Fu and co-workers were prepared attractive hydrogels by using dialdehyde xylan (DAX) as a crosslinker for substrate gelatine (G). The properties of hydrogels were further improved in terms of texture, antibacterial, and cyto-compatibility by the introduction of glycerol (Gly) and nicotinamide (NCA) and the prepared DAX-G-Gly-NCA hydrogel showed highly fascinating materials in the application of skin care [6].

6.5 Targeted drug carriers

The delivery of drugs to the colorectal is the major problem in the treatment of colorectal cancer because of the instability of drugs in the gastric environment of upper GIT. This problem can be solved if the release time of drugs extended or drugs are binded strongly with the matrix material. Hore and Kohne [30] showed delayed drug release by enhancing the binding of a loaded drug to the hydrogel matrix. The multi-responsive (temperature, pH, and magnetic) xylan-based hydrogels encapsulate the magnetic nanoparticles which would facilitate drug release in specific region and realize drug controlled delivery remotely [11]. In a study, the drug loading to the hydrogel is done by immersing the dried hydrogel to the phosphate buffer saline (PBS) solution having pH 7.4. The drug loading and cumulative drug release properties of a hydrogel could be improved significantly using pore-forming agents. Suitable pore-forming agents gave rise to the enhancement of the drug release properties of the hydrogels due to the introduction of desirable pores within the network of the hydrogels. Study showed that the cumulative drug release has been substantially improved up to 71.05%, when hydrogel prepared with NaHCO₃ pore-forming agent [31]. Gao and co-workers [11] prepared a pH susceptible xylan-based hydrogels with N-isopropylacrylamide (NIPAm) and acrylic acid (AA) using N,N′-methylene-bis-acrylamide (MBA) as a cross-linker and 2,2-dimethoxy-2-phenylacetophenone as a photoinitiator by using ultraviolet irradiation. The prepared hydrogels showed efficient encapsulation efficiency of acetylsalicylic acid (upto 97.6%) and cumulative release of 26.35 and 90.12% in the gastric and intestinal fluid, respectively. The porous xylan-β-cyclodextrin based hydrogels were synthesized by using glycol diglycidyl ether as a crosslinker in alkaline medium. The model drugs of curcumin and 5-fluorouracil (5-FU) were loaded 26 and 56%, respectively. Furthermore, the hydrogels were showed the highest cumulative release of 37% curcumin and 56% 5-FU and after 24 hours [32].

7. Conclusions

The most of the drugs have been reported to be unstable in the gastric environment and are susceptible to absorption in the upper gastrointestinal tract (GIT) therefore; delivery of a drug to the targeted site via GIT requires protection. The protection of drugs can be achieved by encapsulation within polymeric network of xylan based-hydrogels. The drug-loading and drug-release profile can further be enhanced by pore-forming agent. The non-cytotoxic and biocompatible nature of hydrogels endow to skin care application. The photosensitizers encapsulated hydrogels showed promising antimicrobial treatment to overcome the challenges of multidrug resistant bacteria. The acetylated xylan-based polymeric network of hydrogels encapsulates the magnetic nanoparticles which endow the excellent catalytic activity and provide a sensitive response to H₂O₂ detection even at low concentration.

Acknowledgments

Authors would like to acknowledge the AKS University for the financial support and facilities in carrying out this research work.

Conflicts of interest

The authors declare no conflict of interest.