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Abstract
Cartilage is a structural tissue with unique mechanical properties deriving from its electrically-charged porous structure. Traditional three-dimensional environments for the culture of cells fail to display the complex physical response displayed by the natural tissue. In this work, the reproduction of the charged environment found in cartilage is achieved using polyelectrolyte hydrogels based on polyvinyl alcohol and polyacrylic acid. The mechanical response and morphology of microporous physically-crosslinked cryogels are compared to those of heat-treated chemical gels made from the same polymers, as a result of pH-dependent swelling. In contrast to the heat-treated chemically-crosslinked gels, the elastic modulus of the physical cryogels was found to increase with charge activation and swelling, explained by the occurrence of electrostatic stiffening of the polymer chains at large charge densities. At the same time, the permeability of both materials to fluid flow was impaired by the presence of electric charges. This cartilage-like mechanical behavior displayed by responsive cryogels can be reproduced in other polyelectrolyte hydrogel systems to fabricate biomimetic cellular scaffolds for the repair of the tissue.
Highlights
Cartilage is the biological tissue responsible for the correct functioning of the joints, covering the surface of long bones and ensuring smooth sliding and load-bearing capacity
The macroscale mechanical behavior of the polyelectrolyte hydrogels as a function of charged state varied notably depending on the type of crosslinking (Fig. 1)
The load relaxation profiles with time were analyzed in a poroelastic framework, which considers the pressurized fluid flow away from the region of indentation contact when evaluating the elastic modulus of the materials
Summary
Cartilage is the biological tissue responsible for the correct functioning of the joints, covering the surface of long bones and ensuring smooth sliding and load-bearing capacity. The mechanical behavior of uncharged biological polymers has been reproduced by mimicking their network structure[8] Despite all these advances, the scaffolds produced are still often too simplistic when compared to the complex extracellular environment of natural cartilage. Synthetic polyelectrolytes are used to fabricate microporous hydrogel scaffolds with enhanced mechanical behavior similar to that of natural cartilage These responsive materials change their conformation as a result of changes in environmental pH10. The results demonstrate that electrostatic effects are fundamental in understanding and enhancing the mechanical behavior of the materials Such results are not exclusive of the particular system here studied, but are mimetic of natural cartilage and applicable to any charged polymer system to produce functional tissue engineering scaffolds
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