Cross-linked hyaluronic acid hydrogel films: new biomaterials for drug delivery
Introduction
Hyaluronic acid (HA) is a naturally-occurring linear polysaccharide comprised of β-1,4-linked d-glucuronic acid (β-1,3) N-acetyl-d-glucosamine disaccharide units, and it is the only non-sulfated glycosaminoglycan (GAG) in the extracellular matrix (ECM) of all higher animals. This polyanionic polymer has a range of naturally-occurring molecular sizes from 1000 to 10,000,000 Da, and has unique physicochemical properties and distinctive biological functions [1]. HA has been implicated in water homeostasis of tissues, in the regulation of the permeability of other substances by steric exclusion phenomena, and in the lubrication of joints [2]. HA also binds specifically to proteins in the ECM, on the cell surface, and within the cell cytosol, thereby having a role in cartilage matrix stabilization [2], [3], cell motility [4], [5], growth factor action [6], morphogenesis and embryonic development [7], and inflammation [8].
Unmodified HA has many important applications in drug delivery and surgery. For example, HA is used as an adjuvant for ophthalmic drug delivery [9], and was found to enhance the absorption of drugs and proteins via mucosal tissues [10], [11], [12], [13]. In addition, HA has important applications in the fields of viscosurgery, viscosupplementation, and wound healing [14], [15], [16].
HA is also an attractive building block for new biocompatible and biodegradable polymers with applications in drug delivery, tissue engineering, and viscosupplementation [17], [18]. Chemical modification [19] allows the physicochemical properties and in vivo residence time of HA to be tailored to specific applications while retaining its natural biocompatibility, biodegradability, and lack of immunogenicity [18]. Composite materials containing HA [20], [21], [22], functional modification of carboxylic groups on HA, polymer–drug conjugates for controlled drug release [23], crosslinking into hydrogels and surface coating [18], and uses of HA-derived biochemical probes [24] have been recently summarized. Many HA-derived biomaterials have been evaluated in vitro and in vivo as potential biomedical devices [25]. For example, a new internally-esterified material, the HA autocrosslinked polymer hydrogel (ACP gel), was found to be effective in reducing postsurgical adhesions in laparoscopic surgery [26]. In addition, esterification of HA with various therapeutically-active and/or inactive alcohols produced mucoadhesive biopolymers with new physicochemical properties [27], [28] for use in controlled drug release and tissue engineering [29].
Obtaining HA-derived biomaterials with sufficient strength to withstand biomechanically stressful applications has been problematic. HA derivatives with the best mechanical properties have generally been prepared with highly alkaline crosslinking conditions and at elevated temperatures [30], [31], conditions that preclude inclusion of sensitive molecules or living cells during preparation of the polymer network hydrogel. Additionally, such reactions employed small molecular crosslinking reagents, often in large excess, which required considerable purification in order to obtain materials for physiological use.
We describe herein a new HA hydrogel well-suited to drug delivery, prepared using a macromolecular crosslinker under neutral aqueous conditions. This new biocompatible material crosslinks and gels in minutes and swells from a flexible dry film to a flexible porous hydrogel in seconds. The hydrogel can be used directly in any biological system without purification after gelling, since it only contains two biocompatible polymers. The swelling properties of the hydrogels, including equilibrium and swelling kinetics, were evaluated. Enzymatic degradation of the hydrogels by hyaluronidase (HAse), and in vitro release of anti-bacterial and anti-inflammatory drugs were examined.
Section snippets
Materials
Fermentation-derived hyaluronan (HA, sodium salt, Mr=1.5×106) was provided by Clear Solutions Biotechnology, Inc. (Stony Brook, NY). 1-Ethyl-3-[3-(dimethylamino)-propyl]carbodiimide (EDCI), adipic dihydrazide (ADH), and acridine orange were purchased from Aldrich Chemical Co. (Milwaukee, WI). Bovine testicular hyaluronidase (HAse, 880 U/mg), indomethacin, pilocarpine, diclofenac sodium, hydrocortisone, 6α-methyl-prednisolone, prednisolone, cortisone, corticosterone, dexamethasone, prednisone,
Results and discussion
Tissue-compatible hydrogels provide a natural, hydrophilic environment for controlled drug delivery [36], and the rate of drug release can be regulated by controlling gel-swelling and crosslinking density. Crosslinked HA hydrogels [37] have been prepared with diepoxybutane, ethyleneglycol diglycidylether (EGDGE), or polyglycerol polyglycidylether (PGPGE) in strongly alkaline conditions with ethanol as a co-solvent [30], [31]. These crosslinking methods produce HA hydrogel films with water
Conclusion
In conclusion, a novel fast-gelling and fast-swelling HA hydrogel was developed as a potential drug delivery system. This polymer network hydrogel was prepared from HA–ADH and a macromolecular crosslinker, PEG–diald. SEM images showed that the network structure of the hydrogel film is condensed in the dried state, and becomes porous after swelling. The thermal behavior of the HA hydrogel and its components were studied by DSC, which revealed that the hydrogel had a microstructure that differed
Acknowledgements
Financial support for this work was provided by Center for Biopolymers at Interfaces (The University of Utah (UUtah)), and by the start-up funds provided by UUtah to GDP. We thank C. Wang, Professor J. Kopecek, and Professor S.-W. Kim for valuable discussions and experimental assistance.
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