Strong tough gels for 3D tissue constructs
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Strong tough gels for 3D tissue constructs Paul Calvert1, Marc in het Panhuis2, Geoffrey Spinks2, Robert Gorkin III2, Leo Stevens2, Shannon E. Bakarich2, Paul Balding2, Damian M. Kirchmajer2 1
Department of Bioengineering, University of Massachusetts Dartmouth, North Dartmouth, MA, 02747, U.S.A. 2 Intelligent Polymer Research Institute, ARC Centre of Excellence for Electromaterials Science, AIIM Facility and Soft Materials Group, School of Chemistry, University of Wollongong, Wollongong, NSW, 2522, Australia. ABSTRACT The mechanical characteristics of ionic-covalent entanglement hydrogels consisting of combinations of the biopolymers gellan gum and kappa-carrageenan, and the synthetic polymers polyacrylamide and an epoxy amine were investigated. Compression testing showed that these gels exhibited “double network” behavior, i.e. strong tough gels. INTRODUCTION For some tissue engineering objectives (such as cartilage tissue scaffolds), tougher and more robust hydrogels, which have the capacity to recover from mechanical deformation are required. Conventional hydrogels have poor mechanical properties because there are very few energy dissipation mechanisms to impede crack initiation and crack propagation. When under stress, forces cannot be evenly distributed over all polymer chains equally as the distance between the cross-linking points varies. When the shortest polymer chain fractures, all the load is immediately transferred to the next shortest chain, which may subsequently break. Hydrogels can be made substantially tougher using a variety of modern hydrogel science strategies including topological hydrogels, tetra-PEG hydrogels, double network (DN) hydrogels, and microgel reinforced hydrogels [1-4]. The DN approach has been proven to be one of the most successful approaches to preparation of tough hydrogels and has been explored extensively by J. P. Gong and co-workers using poly(2-acrylamido,2-methyl,1-propanesulfonic acid) and poly(acrylamide) (PAMPS/PAAm) systems [3]. Here, gels are prepared in a two-step scheme where a PAMPS hydrogel network is swollen in acrylamide (monomer), which is subsequently photo crosslinked. They have determined that “the rigid and brittle PAMPS network serves as a sacrificial bond that fractures at a relatively low stress, while the soft, ductile PAAm network serves as hidden length that sustains stress by large extension afterwards” [3]. J.P. Gong has also hypothesised that “the introduction of any sacrificial bonds that yield and dissipate energy upon deformation will toughen the materials” [5] and that “the same DN gel concepts can, in principle, be applied to other self-healing types of materials, if the covalent bonds are replaced by reversible bonds” [3]. This hypothesis has been tested and found to be true by preparing gels with different types reversible sacrificial bonds including the hydrophobic interactions of molecules forming lamellar bilayers and ionic cross-links [6-11].
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In this paper, we provide the mechanical characteristics (in compression) of strong tough
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