Mechanical Properties of Architectured Gelatin-Based Hydrogels on Different Hierarchical Levels
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Mechanical Properties of Architectured Gelatin-Based Hydrogels on Different Hierarchical Levels Radovan Vukićević,1 Axel T. Neffe,1 Tim Gebauer,1 Oliver Frank,1 Michael Schossig,1 Andreas Lendlein1 Institute of Biomaterial Science and Berlin-Brandenburg Center for Regenerative Therapies, Helmholtz-Zentrum Geesthacht, Kantstr. 55, 14513 Teltow ABSTRACT Preparation of three-dimensionally architectured porous biomaterials can be achieved in a one-step process by stabilizing gelatin with L-lysine diisocyanate ethyl ester (LDI) in water. The reaction of gelatin with LDI in presence of water leads to the formation of oligourea bridges between gelatin molecules and oligourea chains grafted on gelatin. The number and the length of the bridges, as well as of the grafted chains strongly depend on the concentration of the LDI used for the stabilization, and this has huge influence on the mechanical properties of the material on different hierarchical levels. Higher LDI concentrations yield materials with increased deformation resistance in tensile tests due to the higher number of covalent and physical netpoints in the material. However, mechanical properties determined on the micro-level by AFM indentation showed the opposite trend, i.e. a decrease of Young’s modulus with increasing LDI content. This was interpreted by a decreasing number of shorter oligourea bridges between gelatin chains with decreasing LDI content. INTRODUCTION 3D architectured gelatin-based hydrogels (ArcGels), prepared in a one-step procedure through the reaction of gelatin with L-lysine diisocyanate ethyl ester (LDI) in water, in presence of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) used as surfactant, have been shown to be very successful in inducing biomaterial-driven bone regeneration in critical size bone defects [1]. The stabilization of gelatin with LDI in the presence of water is very complex, due to occurrence of several competing reactions [2]. One of the reactions is between isocyanate groups from LDI and water resulting in formation of free amine groups, which can further react with other isocyanate groups and consequently LDI oligomers are formed (Scheme 1a). This leads to the formation of a complex 3D structure which comprises of protein chains (originating from gelatin), oligourea bridges, as well as grafted oligourea chains (both originating from LDI) (Scheme 1b). Differences in molecular structure in ArcGels prepared with different LDI amounts are related to changes in their mechanical properties. It should be noted that the pore size and porosity was constant for the ArcGels prepared with different LDI amounts [1]. Since mechanical properties of the material are crucial for the interaction with cells [3, 4], measuring the elasticity modulus of ArcGels on different length scales is of high importance. It was shown that ArcGels on the one hand showed similar behavior in compression tests [1], regardless of compositions, while in local indentation by an atomic force microscope (AFM) guided with an optic
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