Controlling Fracture Behavior of Polymeric Hydrogels
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Controlling Fracture Behavior of Polymeric Hydrogels Hyun Joon Kong, David J. Mooney Division of Engineering and Applied Science Harvard University, Cambridge, MA 02138, USA
ABSTRACT Refined control over the mechanical properties of hydrogels formed from cross-linking of polymers is increasingly regarded as critical for their successful application. In general, increasing the cross-linking density (ρ) of polymer gels raises the mechanical rigidity, but makes the gels more brittle. We proposed that controlling properties of the cross-linking junction and the cross-linking type would mediate the fracture response of the gels, and allow one to decouple the dependency of the mechanical stiffness and toughness from ρ of the gel. This possibility was investigated with alginate hydrogels, because alginate can be gelled via ionic or covalent crosslinking. Increasing ρ of the gels formed using covalent cross-linking with adipic acid dihydrazide or poly (acrylamide-co-hydrazide) raised the elastic modulus (E), but led to a reduction in the toughness of the gels. In contrast, increasing the number of calcium cross-links slowed the crack opening of the gels, and subsequently raised both E and work to fracture (W). From the results of this study, we could demonstrate a novel approach to regulate different mechanical properties of gels in an independent manner. This study provides a valuable guideline to the design of a broad array of polymer hydrogels.
INTRODUCTION Hydrogel-based materials have been increasingly utilized in various biomedical applications including drug delivery, plastic surgery, cell therapy, and tissue engineering [1-2]. A refined control of gel mechanical properties is critical for their successful use in many of these applications. Many studies demonstrate one can modulate the rigidity of various types of hydrogels via control of cross-linking density, cross-linking molecules, and the structure of the polymers. In contrast, little attention has been paid to the fracture behavior of the gels, although hydrogels formed from covalent cross-linking generally become more brittle with increases of cross-linking density [3]. The increase in the brittleness of the gels may be disadvantageous to their use as a mechanical support in an environment subjected to continuous mechanical loading.
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A natural organic-inorganic composite such as the abalone shell exhibits a fracture resistant behavior via the stepwise elongation of polymer adhesive fibers between inorganic layers [4]. In a similar context, we hypothesized that the properties of the cross-linked junction responsible for the mechanical response of a gel, such as the type of cross-linking, can modulate the fracture behavior of the gels. We tested this hypothesis with alginate molecules consisting of guluronic acid blocks, mannuronic acid blocks, and alternating guluronic and mannuronic acid blocks [5]. Alginate molecules readily form gels via ionic cross-linking with divalent cations, or covalent cross-linking with molecules having di- or multi
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