Controlling Material Properties of Ionically Cross-Linked Alginate Hydrogels by Varying Molecular Weight Distribution
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Controlling Material Properties of Ionically Cross-Linked Alginate Hydrogels by Varying Molecular Weight Distribution Hyun-Joon Kong1, David J. Mooney1-3 1 Department of Biologic & Materials Sciences, 2Biomedical Engineering, and 3Chemical Engineering, University of Michigan, Ann Arbor, MI 48109 ABSTRACT The mechanical properties of alginate hydrogels are critical to their successful use in tissue engineering. We hypothesized that combining alginates of differing molecular weights would allow one to readily control and decouple the viscosity of the pre-gel solution from the post-gel stiffness. To test this, binary hydrogels were prepared by incorporating low molecular weight (MW) alginate (Mn ~ 3.3 x 104 g/mol) into 1 %(w/w) alginate solutions composed of high MW alginate (Mn ~ 2.2 x 105 g/mol). The addition of low MW alginate had minimal effects on the solution viscosity, allowing high solids loading in the solution while maintaining a workable solution. However, the increased mass loading resulted in much higher stiffness gels following ionic cross-linking with calcium. These data may be interpreted in relation to the microstructure of the solution versus hydrogel. INTRODUCTION Alginate hydrogels have been utilized in various biomedical applications, due to their advantageous properties [1, 2]. However, the mechanical properties (e.g., stiffness) of these gels can only be varied over a limited range, and these properties may be critical to the utility of these materials [3]. In general, the stiffness of alginate hydrogels can be enhanced by simply increasing the alginate concentration [4, 5]. However, the viscosity of the pre-gelled solution is also increased by this process, leading to difficulty in processing and irreproducible material properties. The coupling typically observed between mass loading and solution viscosity limits the practical mass loading to form gels. We hypothesized that solution viscosity and gel strength could be decoupled by introducing low molecular weight (MW) alginate into the system and preparing binary hydrogels. Low MW alginate polymer chains should be capable of participating in gel formation, while having minimal effects on solution viscosity. The rheological properties of alginate solutions and mechanical properties of hydrogels were examined to infer the interactions between alginate chains in the solution and corresponding cross-linked structure of hydrogels. EXPERIMENTAL DETAILS Sodium alginate with high G content (LF 20/40, Mn ~ 2.2 x 105 g/mol, Mw/Mn = 1.3), obtained from FMC Technologies, was used as the high MW component of binary hydrogels. Low MW alginate (Mn ~ 3.3 x 104 g/mol, Mw/Mn = 1.6) was prepared by irradiating LF 20/40 at a gamma dose of 5.0 Mrad for 4 hours. Hydrogels were prepared GG5.7.1
by mixing alginate solutions with calcium sulfate slurries, keeping the molar ratio between carboxylic groups on the alginate chains and calcium constant at 1:1. Total alginate concentrations of the gels were varied from 1 %(w/w) to 5 %(w/w). On preparing binary gels, the mas
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