Synthesis and biodegradation of arabinogalactan sponges prepared by reductive amination
Introduction
Porous, absorbable matrices made from natural [1], [2], [3] and synthetic [4], [5], [6] polymers are currently being investigated as scaffolds for cell and tissue transplantation. Polymer scaffolds for use in cell transplantation must be highly porous with large surface-to-volume ratio to accommodate a large number of cells, and to allow proper blood supply and removal of toxicants [7]. The chemistry of the scaffolds as well as their pore characteristics should allow the homogenous distribution of cells within the matrix, their adherence, and the retention of the differentiated function of attached cells. In addition, they should be biocompatible, and must be completely absorbable and eliminated quickly when they are no longer needed as supports.
Since the range of potential tissue engineered systems is broad and the ongoing research interests are towards the search for materials that may have broad applicability and can be tailored to several tissues, we have selected the polysaccharide arabinogalactan as main component in the design of new scaffold systems. Arabinogalactan (AG) is a highly branched natural polysaccharide with high water solubility (70% in water). It is extracted from the Larix tree and it is available in a 99.9% pure form with reproducible molecular weight and physicochemical properties [8], [9]. The high solubility in water, biocompatibility, biodegradability, and the ease of chemical modification in aqueous media, makes it an attractive polymer for the synthesis of scaffolds for possible use in tissue engineering.
In the current research, we explored the ability of dextran and AG to form insoluble three-dimensional sponges by covalent crosslinking of the oxidized polysaccharide with diamines or polyamines such as alkanamines or chitosan. The degree of oxidation of the polysaccharide can be varied to a large extent to allow various degrees of pore size and degradation time. One possible attractive polyamine for crosslinking of AG to form three-dimensional sponges is chitosan, a partially deacetylated derivative of chitin [10], [11], [12]. Chitosan is a biodegradable cationic polysaccharide comprised of glucosamine residues, and is known to accelerate wound healing and bone formation [13], [14], [15]. Other suitable molecules for this purpose are natural or synthetic oligoamines such as spermine and spermidine.
We now report on the preparation and characterization of arabinogalactan-based sponges in comparison with dextran-based sponges as scaffolds for the possible use in tissue engineering.
Section snippets
Materials
AG was a gift from Larex International, St. Paul, MN, USA. Dextran (40 kDa), spermidine, and butanediamine were purchased from Sigma Chemical Inc., St Louis, MO, USA. Chitosan (average MW: 400 kDa, degree of deacetylation: 75–85%), potassium periodate, and sodium borohydride were purchased from Aldrich Chemical Inc., Milwaukee, USA. Spermine was purchased from Fluka, Buchs, Switzerland. All other reagents and solvents used were of analytical grade.
Synthesis of polysaccharide–lysine conjugates
In the first step, dextran and AG were oxidized
Synthesis of AG and dextran-based sponges
Sponges were synthesized by reacting oxidized AG or dextran with diamines or polyamines as described in Fig. 1. In these experiments, the amino acid lysine was used as a coupling reagent to increase the molecular weight of the polysaccharide or to form a crosslinked sponge. Solutions of oxidized AG or dextran (10% w/v) were reacted with increasing amounts of lysine hydrochloride in carbonate buffer, and the imine bond was then reduced to the more stable amine form with borohydride. The
Conclusions
In this study several types of polysaccharide-based sponges were synthesized by using different molecules or polymers as coupling agents with the polysaccharides AG or dextran. Three kinds of AG-based sponges were synthesized which differ in their side chain moieties and therefore in their properties and behavior in aqueous media. The synthesized sponges exhibited high swelling in aqueous media and suitable pore size for cell growth. AG–chitosan sponges were selected for further evaluation
Acknowledgements
We thank the Canadian Faye Kaufman foundation for their generous support. We thank Dr. Ezra Rahamim from the Bletherman Center for Macromolecules at the Hebrew University of Jerusalem for his help with the scanning electron microscope. We thank Dr. Yoram Zilberman for his help with the MRI experiments. This work was supported in part by the US–Israel binational fund (BSF).
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The author is affiliated with the David R. Bloom Center for Pharmacy, and the Alex Grass Center for Drug Design and Synthesis at the Hebrew University.