Influence of diisocyanate reactivity and water solubility on the formation and the mechanical properties of gelatin-base
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Influence of diisocyanate reactivity and water solubility on the formation and the mechanical properties of gelatin-based networks in water Tim Gebauer1, 2, 3, Axel T. Neffe1, 2, 3, and Andreas Lendlein1, 2, 3 1 Institute of Biomaterial Science and Berlin-Brandenburg Centre for Regenerative Therapies, Helmholtz-Zentrum Geesthacht, Kantstraße 55, 14513 Teltow, Germany 2 Institute of Chemistry, University of Potsdam, 14476 Potsdam-Golm, Germany 3 Helmholtz Virtual Institute í Multifunctional Biomaterials for Medicine, Teltow and Berlin, Germany ABSTRACT Gelatin can be covalently crosslinked in aqueous solution by application of diisocyanates like L-lysine diisocyanate ethyl ester in order to form hydrogels. Reaction of isocyanate groups with water is however a limiting factor in hydrogel network formation and can strongly influence the outcome of the crosslinking process. Here, diisocyanates with different water solubility and reactivity were applied for the formation of gelatin-based hydrogel networks and the mechanical properties of the hydrogels were investigated to gain a better understanding of starting material/ hydrogel property relations. L-Lysin diisocyanate ethyl ester (LDI), 2,4-toluene diisocyanate (TDI), 1,4-butane diisocyanate (BDI), and isophorone diisocyanate (IPDI) were selected, having different solubility in water ranging from 10-4 to 10-2 mol·L-1. BDI and LDI were estimated to have average reactive isocyanates groups, whereas TDI is highly reactive and IPDI has low reactivity. Formed hydrogels showed different morphologies and were partially very inhomogeneous. Gelation time (1 to 50 minutes), water uptake (300 to 900 wt.-%), and mechanical properties determined by tensile tests (E-moduli 35 to 370 kPa) and rheology (Shear moduli 4.5 to 19.5 kPa) showed that high water solubility as well as high reactivity leads to the formation of poorly crosslinked or inhomogeneous materials. Nevertheless, diisocyanates with lower solubility in water and low reactivity are able to form stable, homogeneous hydrogel networks with gelatin in water. INTRODUCTION Mechanical properties of hydrogels are generally influenced by the degree of crosslinking of the polymer network [1]. In biomaterials research, crosslinking of biopolymers with isocyanates is a common approach to form covalently crosslinked multifunctional hydrogel systems with tailorable mechanical properties [2, 3, 4, 5]. Unfortunately these crosslinking reactions are typically carried out in organic solvents, which can remain entrapped inside the formed polymer networks and thus increase their potential toxicity. In order to exclude toxic solvents gelatin-based hydrogels were established, which were covalently crosslinked with diisocyanates in water [6, 7]. These reactions provide hydrogels with tailorable mechanical properties and degradation behavior. Crosslinking reactions were performed using L-lysine diisocyanate ethyl ester (LDI) in water with the application of a surfactant that confers improved solubility for the rather hydrophobic diisocyanate. Al
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