A fungal chitin derived from Hericium erinaceus residue: Dissolution, gelation and characterization

Highlights

• Chitin extracted from Hericium erinaceus residue was dissolved in NaOH/urea.

• Chitin hydrogels were regenerated from chitin solution in NaOH/urea.

• Characterization of the chitin hydrogels was carried out.

Abstract

In this work, a low-cost and sustainable non-animal chitin from Hericium erinaceus residue was used to prepare hydrogels through a facile procedure. The chitin was firstly dissolved in NaOH/Urea aqueous solution by freeze thawing, and then the obtained chitin solution was directly regenerated to form hydrogels. The results showed that the extracted chitin could dissolve in NaOH/urea solvent system. The chitin solutions with concentrations from 1% to 4% showed no gelation behavior, while those with concentrations from 5% to 7% could form hydrogels. The obtained hydrogels showed elastic characteristic and desirable transmittance. In addition, the hydrogels with higher chitin concentration exhibited more compact gel network and improved gel strength, but decreased swelling degree and cumulative drug release. Compared with neat chitin, the thermal stability of the regenerated chitin hydrogels was also improved. The 5-Fu could be gradually released from the hydrogels, which followed Fickian diffusion model.

Introduction

Development of natural polymer-based hydrogels is of great significance in the field of biomedicine due to the biocompatibility, biodegradability and nontoxicity. Until now, some quintessential natural polymers such as cellulose, chitin, starch, gelatin and alginate have been used for hydrogel preparation [[1], [2], [3], [4]]. These prepared hydrogels hold great potential due to the 3D porous structures and abundant hydrophilic functional groups especially hydroxyl in their molecules.

Chitin, a β-(1,4)-linked homopolymer of N-acetylglucosamine, is one of the most abundant biopolymer [5,6], which can be found in crustaceans, insects and fungi. But the main commercial products of chitin are always derived from marine sources, such as shrimp, crabs, and lobsters [7]. It is undeniable that these biomass materials have been widely used, but there are still limitations. As an example, animal-derived chitin is not suitable for vegetarians when it is used as a drug delivery material. In addition, the chitin extracted from marine animals will increase the risk of allergy to seafood. Therefore, development of non-animal chitin is a crucial aspect in biomaterial field. Prior investigations have shown that fugal chitin can be isolated from mushrooms, and the properties of the extracted fugal chitin are comparable to the chitin from marine animals [[8], [9], [10]]. So mushroom deserves more attentions as a source of chitin.

Hericium erinaceus is a traditional edible mushroom, which can be consumed as fresh food or value-added products such as polysaccharides. Recently, Hericium erinaceus polysaccharides have attracted great attentions in the fields of functional foods due to the numerous health promoting functions [[11], [12], [13], [14]]. A large amount of Hericium erinaceus residue was produced after the extraction of water-soluble polysaccharides. The effective utilization of the Hericium erinaceus residue deserves special attention as it is meaningful for biological resource wastes and environmental pollutions. Actually, the dried Hericium erinaceus residue mainly consists of chitin, minerals, proteins, and pigments [15]. Therefore, the utilization of chitin from Hericium erinaceus residue to prepare hydrogel is available.

Most of the hydrogels are usually prepared by the dissolution of polymer followed by the gelation of polymer solution. However, chitin is insoluble in water or organic solvents due to its strong intra- and inter-molecular hydrogen bonding. To date, only a few solvents can be used to dissolve native chitin. These solvents include some conventional polar solvent systems such as lithium chloride/dimethylacetamide (LiCl/DMAc) [16], lithium chloride/N-methyl-2-pyrolidone (LiCl/NMP) [17], and calcium chloride dihydrate/methanol (CaCl₂·2H₂O/MeOH) [18]. Although the use of these polar solvents can effectively dissolve native chitin, the toxicity or corrosivity of these organic components can reduce the potential application range of the resulting hydrogels. More recently, some alternative green solvents such as ionic liquids [19], deep eutectic solvents [20], and alkali/urea aqueous systems [21], have been applied for dissolving native chitin. Among these solvent systems, the alkali/urea aqueous systems have been more studied for the dissolution of chitin, and a series of biocompatible chitin-based materials have been directly constructed [[22], [23], [24], [25]].

Nevertheless, the chitin-based materials were invariably prepared from animal resources. To the best of our knowledge, this work is the first time to report direct utilization of non-animal chitin extracted from Hericium erinaceus residue to prepare hydrogels in NaOH/Urea system. What's more, the comprehensive utilization of waste Hericium erinaceus residue was enriched. The structure of prepared hydrogels was characterized. The swelling kinetics and release kinetics with 5-fluorouracil (5-Fu) as the model drug were also investigated.